Posts Tagged stellar parameters

Recent Postings from stellar parameters

The solar-stellar connection: Magnetic activity of seismic solar analogs

Finding solar-analog stars with fundamental properties as close as possible to the Sun and studying the characteristics of their surface magnetic activity is a very promising way to understand the solar variability and its associated dynamo process. However, the identification of solar-analog stars depends on the accuracy of the estimated stellar parameters. Thanks to the photometric CoROT and Kepler space missions, the addition of asteroseismic data was proven to provide the most accurate fundamental properties that can be derived from stellar modeling today. Here, we present our latest results on the solar-stellar connection by studying 18 solar analogs that we identified among the Kepler seismic sample (Salabert et al., 2016a). We measured their magnetic activity properties using the observations collected by the Kepler satellite and the ground-based, high-resolution HERMES spectrograph. The photospheric (Sph) and chromospheric (S) magnetic activity proxies of these seismic solar analogs are compared in relation to the solar activity. We show that the activity of the Sun is comparable to the activity of the seismic solar analogs, within the maximum-to-minimum temporal variations of the 11-year solar activity cycle. Furthermore, we report on the discovery of temporal variability in the acoustic frequencies of the young (1 Gyr-old) solar analog KIC10644253 with a modulation of about 1.5 years, which agrees with the derived photospheric activity Sph (Salabert et al, 2016b). It could be the signature of the short-period modulation, or quasi-biennal oscillation, of its magnetic activity as observed in the Sun and in the 1-Gyr-old solar analog HD30495. In addition, the lithium abundance and the chromospheric activity estimated from HERMES confirms that KIC10644253 is a young and more active star than the Sun.

Stellar Parameters of Main Sequence Turn-off Star Candidates Observed with the LAMOST and Kepler

Main sequence turn-off (MSTO) stars have advantages as indicators of Galactic evolution since their ages could be robustly estimated from atmospheric parameters. Hundreds of thousands of MSTO stars have been selected from the LAMOST Galactic sur- vey to study the evolution of the Galaxy, and it is vital to derive accurate stellar parameters. In this work, we select 150 MSTO star candidates from the MSTO stars sample of Xiang that have asteroseismic parameters and determine accurate stellar parameters for these stars combing the asteroseismic parameters deduced from the Kepler photometry and atmospheric parameters deduced from the LAMOST spectra.With this sample, we examine the age deter- mination as well as the contamination rate of the MSTO stars sample. A comparison of age between this work and Xiang shows a mean difference of 0.53 Gyr (7%) and a dispersion of 2.71 Gyr (28%). The results show that 79 of the candidates are MSTO stars, while the others are contaminations from either main sequence or sub-giant stars. The contamination rate for the oldest stars is much higher than that for the younger stars. The main cause for the high contamination rate is found to be the relatively large systematic bias in the LAMOST surface gravity estimates.

The CoRoT-GES Collaboration. Improving Red Giants spectroscopic surface gravity and abundances with asteroseismology

Nowadays large spectroscopic surveys, like the Gaia-ESO Survey (GES), provide unique stellar databases for better investigating the formation and evolution of our Galaxy. Great attention must be paid to the accuracy of the basic stellar properties derived: large uncertainties in stellar parameters lead to large uncertainties in abundances, distances and ages. Asteroseismology has a key role in this context: when seismic information is combined with information derived from spectroscopic analysis, highly precise constraints on distances, masses, extinction and ages of Red Giants can be obtained. In the light of this promising joint-action, we started the CoRoT-GES collaboration. We present a set of 1,111 CoRoT stars, observed by GES from December 2011 to July 2014, these stars belong to the CoRoT field LRc01, pointing at the inner Galactic Disk. Among these stars, 534 have reliable global seismic parameters. By combining seismic informations and spectroscopy, we derived precise stellar parameters, ages, kinematic and orbital parameters and detailed element abundances for this sample of stars. We also show that, thanks to asteroseismology, we are able to obtain a higher precision than what can be achieved by the standard spectroscopic means. This sample of CoRoT Red Giants, spanning Galactocentric distances from 5 to 8 kpc and a wide age interval (1-13 Gyrs), provides us a representative sample for the inner disk population.

The GALAH Survey: Observational Overview and Gaia DR1 companion

The Galactic Archaeology with HERMES (GALAH) Survey is a massive observational project to trace the Milky Way's history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R$\simeq$28,000) spectra taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo-Australian Telescope (AAT), GALAH will determine stellar parameters and abundances of up to 29 elements for up to one million stars. Selecting targets from a colour-unbiased catalogue built from 2MASS, APASS and UCAC4 data, we expect to observe dwarfs at 0.3 to 3 kpc and giants at 1 to 10 kpc. This enables a thorough local chemical inventory of the Galactic thin and thick disks, and also captures smaller samples of the bulge and halo. In this paper we present the plan, process and progress as of early 2016 for GALAH survey observations. In our first two years of survey observing we have accumulated the largest high-quality spectroscopic data set at this resolution, over 200,000 stars. We also present the first public GALAH data catalogue: stellar parameters (Teff, log(g), [Fe/H], [alpha/Fe]), radial velocity, distance modulus and reddening for 10680 observations of 9860 Tycho-2 stars that may be included in the first Gaia data release.

A critical reassessment of the fundamental properties for GJ 504: chemical composition and age

The recent development of brand-new observational techniques and theoretical models greatly advanced the exoplanet research field. Despite significant achievements, which have allowed to detect thousands extrasolar systems, a comprehensive understanding of planetary formation and evolution mechanisms is still desired. One relevant limitation is given by the accuracy in the measurements of planet-host star ages. The star GJ 504 has been found to host a sub-stellar companion whose nature is strongly debated. There has been a recent difference of opinion in the literature due to the uncertainty on the age of the system: a young age of $\sim$ 160 Myr would imply a giant planet as a companion, but a recent revision pointing to a solar age ($\sim$ 4 Gyr) suggests instead a brown dwarf. With the aim of shedding light on this debated topic, we have carried out a high-resolution spectroscopic study of GJ 504 in order to derive stellar parameters, metallicity and abundances of both light and heavy elements, providing a full chemical characterisation. The main objective is to infer clues on the evolutionary stage (hence the age) of this system. We performed a strictly differential (line-by-line) analysis of GJ 504 with respect to two reference stars, that is the planet-host dwarf $\iota$ Hor and the sub-giant HIP 84827. The former is crucial in this context because its stellar parameters (hence the evolutionary stage) is well constrained from asteroseismic observations. Regardless of the zero point offsets, our differential approach allows us to put tight constraints on the age of GJ 504 with respect to $\iota$ Hor, minimising the internal uncertainties. We found that the surface gravity of GJ 504 is 0.2 $\pm$ 0.07 dex lower than that of the main-sequence star $\iota$ Hor, suggesting a past turn-off evolution for our target [...]

Chemical abundances for A-and F-type supergiant stars

We present the stellar parameters and elemental abundances of a set of A--F-type supergiant stars HD\,45674, HD\,180028, HD\,194951 and HD\,224893 using high resolution ($R$\,$\sim$\,42,000) spectra taken from ELODIE library. We present the first results of the abundance analysis for HD\,45674 and HD\,224893. We reaffirm the abundances for HD\,180028 and HD\,194951 studied previously by Luck (2014) respectively. Alpha-elements indicates that objects belong to the thin disc population. From their abundances and its location on the Hertzsprung-Russell diagram seems point out that HD\,45675, HD\,194951 and HD\,224893 are in the post-first dredge-up (post-1DUP) phase and they are moving in the red-blue loop region. HD~180028, on the contary, shows typical abundances of the population I but its evolutionary status could not be satisfactorily defined.

Spectroscopy Made Easy: Evolution

Context. The Spectroscopy Made Easy (SME) package has become a popular tool for analyzing stellar spectra, often in connection with large surveys or exoplanet research. SME has evolved significantly since it was first described in 1996, but many of the original caveats and potholes still haunt users. The main drivers for this paper are complexity of the modeling task, the large user community, and the massive effort that has gone into SME. Aims. We do not intend to give a comprehensive introduction to stellar atmospheres, but will describe changes to key components of SME: the equation of state, opacities, and radiative transfer. We will describe the analysis and fitting procedure and investigate various error sources that affect inferred parameters. Methods. We review the current status of SME, emphasizing new algorithms and methods. We describe some best practices for using the package, based on lessons learned over two decades of SME usage. We present a new way to assess uncertainties in derived stellar parameters. Results. Improvements made to SME, better line data, and new model atmospheres yield more realistic stellar spectra, but in many cases systematic errors still dominate over measurement uncertainty. Future enhancements are outlined.

Abundance trend with condensation temperature for stars with different Galactic birth places

During the past decade, several studies reported a correlation between chemical abundances of stars and condensation temperature (also known as Tc trend). However, the real astrophysical nature of this correlation is still debated. The main goal of this work is to explore the possible dependence of the Tc trend on stellar Galactocentric distances, Rmean. We used high-quality spectra of about 40 stars observed with the HARPS and UVES spectrographs to derive precise stellar parameters, chemical abundances, and stellar ages. A differential line-by-line analysis was applied to achieve the highest possible precision in the chemical abundances. We confirm previous results that [X/Fe] abundance ratios depend on stellar age and that for a given age, some elements also show a dependence on Rmean. When using the whole sample of stars, we observe a weak hint that the Tc trend depends on Rmean. The observed dependence is very complex and disappears when only stars with similar ages are considered. To conclude on the possible dependence of the Tc trend on the formation place of stars, a larger sample of stars with very similar atmospheric parameters and stellar ages observed at different Galactocentric distances is needed

Gaia FGK Benchmark Stars: New Candidates At Low-Metallicities

We have entered an era of large spectroscopic surveys in which we can measure, through automated pipelines, the atmospheric parameters and chemical abundances for large numbers of stars. Calibrating these survey pipelines using a set of "benchmark stars" in order to evaluate the accuracy and precision of the provided parameters and abundances is of utmost importance. The recent proposed set of Gaia FGK benchmark stars of Heiter et al. (2015) has no recommended stars within the critical metallicity range of $-2.0 <$ [Fe/H] $< -1.0$ dex. In this paper, we aim to add candidate Gaia benchmark stars inside of this metal-poor gap. We began with a sample of 21 metal-poor stars which was reduced to 10 stars by requiring accurate photometry and parallaxes, and high-resolution archival spectra. The procedure used to determine the stellar parameters was similar to Heiter et al. (2015) and Jofre et al. (2014) for consistency. The effective temperature (T$_{\mathrm{eff}}$) of all candidate stars was determined using the Infrared Flux Method utilizing multi-band photometry. The surface gravity (log g) was determined through fitting stellar evolutionary tracks. The [Fe/H] was determined using four different spectroscopic methods fixing the T$_{\mathrm{eff}}$ and log g from the values determined independent of spectroscopy. We discuss, star-by-star, the quality of each parameter including how it compares to literature, how it compares to a spectroscopic run where all parameters are free, and whether Fe I ionisation-excitation balance is achieved. From the 10 stars, we recommend a sample of five new metal-poor benchmark candidate stars which have consistent T$_{\mathrm{eff}}$ , log g, and [Fe/H] determined through several means. These stars can be used for calibration and validation purpose of stellar parameter and abundance pipelines and should be of highest priority for future interferometric studies.

Asteroseismic modelling of the two F-type hybrid pulsators KIC10080943A and KIC10080943B

Pulsating binary stars are ideal targets for testing the theory of stellar structure and evolution. Fundamental parameters can be derived from binary modelling to high precision and provide crucial constraints for seismic modelling. High-order gravity modes are sensitive to the conditions near the convective core and therefore allow for a determination of parameters describing interior physics, especially the convective-core overshooting parameter. KIC 10080943 is a binary system, which contains two gravity- and pressure-mode hybrid pulsators. A detailed observational study has provided fundamental and seismic parameters for both components. We aim to find a model, which is able to predict the observed g-mode period spacings and stellar parameters of both components of KIC 10080943. By calculating model grids with the stellar evolution code MESA and the seismic code GYRE, we can compare theoretical properties to the observed mean period spacing and position in the Hertzsprung-Russell diagram. The masses of our best models are somewhat below the values estimated from binarity, which is a consequence of the low observed mean g-mode period spacing. We find that the amount of core overshooting and of diffusive mixing can be well constrained by the equal-age requirement for the two stars. However, we find no significant difference for different shapes of the core overshooting. The measured rotation rates are within the limit of validity for the first-order perturbation approximation. We can find a good fit by using the traditional approximation for the pulsations, when taking slightly younger models with a higher asymptotic period spacing. This is because the zonal modes experience a slight shift due to the Coriolis force, which the first-order perturbation approximation ignores.

Asteroseismic modelling of the two F-type hybrid pulsators KIC10080943A and KIC10080943B [Replacement]

Pulsating binary stars are ideal targets for testing the theory of stellar structure and evolution. Fundamental parameters can be derived to high precision from binary modelling and provide crucial constraints for seismic modelling. High-order gravity modes are sensitive to the conditions near the convective core and therefore allow for a determination of parameters describing interior physics, especially the convective-core overshooting parameter. KIC\,10080943 is a binary system that contains two gravity- and pressure-mode hybrid pulsators. A detailed observational study has provided fundamental and seismic parameters for both components. We aim to find a model that is able to predict the observed g-mode period spacings and stellar parameters of both components of KIC 10080943. By calculating model grids with the stellar evolution code MESA and the seismic code GYRE, we can compare theoretical properties to the observed mean period spacing and position in the Hertzsprung-Russell diagram. The masses of our best models are somewhat below the values estimated from binarity, which is a consequence of the low observed mean g-mode period spacing. We find that the amount of core overshooting and diffusive mixing can be well constrained by the equal-age requirement for the two stars, however, we find no significant difference for different shapes of core overshooting. The measured rotation rates are within the limit of validity for the first-order perturbation approximation. We can find a good fit by using the traditional approximation for the pulsations, when taking slightly younger models with a higher asymptotic period spacing. This is because the zonal modes experience a slight shift due to the Coriolis force, which the first-order perturbation approximation ignores.

$\zeta^2$ Ret, its debris disk, and its lonely stellar companion $\zeta^1$ Ret. Different $T_{\mathrm{c}}$ trends for different spectra

Several studies have reported a correlation between the chemical abundances of stars and condensation temperature (known as Tc trend). Very recently, a strong Tc trend was reported for the $\zeta$ Reticuli binary system, which consists of two solar analogs. The observed trend in $\zeta^2$ Ret relative to its companion was explained by the presence of a debris disk around $\zeta^2$ Ret. Our goal is to re-evaluate the presence and variability of the Tc trend in the $\zeta$ Reticuli system and to understand the impact of the presence of the debris disk on a star. We used very high-quality spectra of the two stars retrieved from the HARPS archive to derive very precise stellar parameters and chemical abundances. We derived the stellar parameters with the classical (nondifferential) method, while we applied a differential line-by-line analysis to achieve the highest possible precision in abundances, which are fundamental to explore for very tiny differences in the abundances between the stars. We confirm that the abundance difference between $\zeta^2$ Ret and $\zeta^1$ Ret shows a significant ($\sim$ 2 $\sigma$) correlation with Tc. However, we also find that the Tc trends depend on the individual spectrum used (even if always of very high quality). In particular, we find significant but varying differences in the abundances of the same star from different individual high-quality spectra. Our results for the $\zeta$ Reticuli system show, for example, that nonphysical factors, such as the quality of spectra employed and errors that are not accounted for, can be at the root of the Tc trends for the case of individual spectra.

Kea: a new tool to obtain stellar parameters from low to moderate signal/noise and high-resolution Echelle spectra

In this paper we describe Kea a new spectroscopic fitting method to derive stellar parameters from moderate to low signal/noise, high-resolution spectra. We developed this new tool to analyze the massive data set of the Kepler mission reconnaissance spectra that we have obtained at McDonald Observatory. We use Kea to determine effective temperatures (T_eff), metallicity ([Fe/H]), surface gravity (log g) and projected rotational velocity (v sin i). Kea compares the observations to a large library of synthetic spectra that covers a wide range of different T_eff, [Fe/H] and log g values. We calibrated Kea on observations of well-characterized standard stars (the Kepler field "platinum" sample) which range in T_eff from 5000 to 6500 K, in [Fe/H] from -0.5 to +0.4 dex and in log g from 3.2 to 4.6 dex. We then compared the Kea results from reconnaissance spectra of 45 KOIs (Kepler Object of Interest) to stellar parameters derived from higher signal/noise spectra obtained with Keck/HIRES. We find typical uncertainties of 100 K in T_eff, 0.12 dex in [Fe/H] and 0.18 dex in log g.

The Rotation-Activity Correlations in K and M dwarfs. I. Stellar parameters, compilations of $v\sin i$ and $P/\sin i$ for a large sample of late-K and M dwarfs

Reliable determination of Rotation-Activity Correlation (RAC's) depends on precise measurements of the following stellar parameters: $T_{eff}$, parallax, radius, metallicity, and rotational speed $v\sin i$. In this paper, our goal is to focus on the determination of these parameters for a sample of K and M dwarfs. In a future paper (Paper II), we will combine our rotational data with activity data in order to construct RAC's. Here, we report on a determination of effective temperatures based on the (R-I)$_C$ color from the calibrations of Mann et al. (2015) and Kenyon \& Hartmann (1995) for four samples of late-K, dM2, dM3 and dM4 stars. We also determine stellar parameters ($T_{eff}$, $log(g)$ and [M/H]) using the PCA-based inversion technique for a sample of 105 late-K dwarfs. We compile all effective temperatures from the literature for this sample. We determine empirical radius-[M/H] correlations in our stellar samples. This allows us to propose new effective temperatures, stellar radii, and metallicities for a large sample of 612 late-K and M dwarfs. Our mean radii agree well with those of Boyajian et al. (2012). We analyze HARPS and SOPHIE spectra of 105 late-K dwarfs, and have detected $v\sin i$ in 92 stars. In combination with our previous $v\sin i$ measurements in M and K dwarfs, we now derive $P/sin i$ measures for a sample of 418 K and M dwarfs. We investigate the distributions of $P/sin i$ and we show that they are different from one spectral sub-type to another at a 99.9\% confidence level.

Characterization of transiting exoplanets: analyzing the impact of the host star on the planet parameters

In this PhD dissertation, I discuss issues of the Radial Velocities (RV) and transit methods. These techniques allow us to derive the mass and radius of an exoplanet, necessary to model its bulk structure and to have insight on its formation. To do this, however, also the same parameters of its host star are needed. By using spectroscopy, I participated in TRANSITS, an RV follow-up program of Kepler Objects of Interest. I determined the parameters of nine host stars, enabling the characterization of their companions. With the same method, I participated in two studies which aim at exploring the mass-radius relationship of low-mass stars and at improving the statistics of star-planet interactions. I also inspected the behavior of SOPHIE/OHP spectra for instrumental effects which can affect the measure of the stellar parameters. From a different perspective, I studied Kepler-117, a multi-planetary system which presents Transit Timing Variations (TTV). A specific approach was developed in order to realize a simultaneous fit of transits, RV, and TTV, used to measure the mass of the lightest planet of the system, which was poorly constrained by RV alone. Finally, I focused on the impact of stellar activity in transit photometry and RV. This phenomenon affects the determination of the planet radius and mass, as well as other key parameters. I implemented two starspot modeling codes into a Markov Chain Monte Carlo software, and added spot evolution to one of them. I applied the codes to observations of the Sun, CoRoT-2, and CoRoT-7. In particular, I carried out an extensive study on the light curve of CoRoT-2, and explored the effects of the spots on the transit parameters. The planetary systems studied in this work provide further constraints on models of planet interior and formation. With the techniques here developed, they help in the preparation of future exoplanet surveys.

Constructing Polynomial Spectral Models for Stars [Replacement]

Stellar spectra depend on the stellar parameters and on dozens of photospheric elemental abundances. Simultaneous fitting of these $\mathcal{N}\sim 10-40$ model labels to observed spectra has been deemed unfeasible, because the number of ab initio spectral model grid calculations scales exponentially with $\mathcal{N}$. We suggest instead the construction of a polynomial spectral model (PSM) of order $\mathcal{O}$ for the model flux at each wavelength. Building this approximation requires a minimum of only ${\mathcal{N}+\mathcal{O}\choose\mathcal{O}}$ calculations: e.g. a quadratic spectral model ($\mathcal{O}=2$) to fit $\mathcal{N}=20$ labels simultaneously, can be constructed from as few as $231$ ab initio spectral model calculations; in practice, a somewhat larger number ($\sim 300-1000$) of randomly chosen models lead to a better performing PSM. Such a PSM can be a good approximation only over a portion of label space, which will vary case by case. Yet, taking the APOGEE survey as an example, a single quadratic PSM provides a remarkably good approximation to the exact ab initio spectral models across much of this survey: for random labels within that survey the PSM approximates the flux to within $10^{-3}$, and recovers the abundances to within $\sim 0.02$ dex rms of the exact models. This enormous speed-up enables the simultaneous many-label fitting of spectra with computationally expensive ab initio models for stellar spectra, such as non-LTE models. A PSM also enables the simultaneous fitting of observational parameters, such as the spectrum's continuum or line-spread function.

GU Mon, a high-mass eclipsing overcontact binary in the young open cluster Dolidze 25

Context. The eclipsing binary GU Mon is located in the star-forming cluster Dolidze 25, which has the lowest metallicity measured in a Milky Way young cluster. Aims. GU Mon has been identified as a short-period eclipsing binary with two early B-type components. We set out to derive its orbital and stellar parameters. Methods. We present a comprehensive analysis, including B and V light curves and 11 high-resolution spectra, to verify the orbital period and determine parameters. We use the stellar atmosphere code FASTWIND to obtain stellar parameters and create templates for cross-correlation. We obtain a model to fit the light and radial-velocity curves using the Wilson-Devinney code iteratively and simultaneously. Results. The two components of GU Mon are identical stars of spectral type B1 V, with the same mass and temperature. The lightcurves are typical of an EW-type binary. The spectroscopic and photometric analyses agree on a period of 0.896640 +- 0.000007 d. We determine a mass of 9.0 +- 0.6 Msun for each component and temperatures of 28 000 +- 2 000 K. Both values are consistent with the spectral type. The two stars are overfilling their respective Roche lobes, sharing a common envelope, and therefore the orbit is synchronised and circularised. Conclusions. The GU Mon system has a fill-out factor above 0.8, containing two dwarf B-type stars on the main sequence. The two stars are in a very advanced stage of interaction, with their extreme physical similarity likely due to the common envelope. The expected evolution of such a system will very probably lead to a merger while still on the main sequence.

Temperatures and metallicities of M giants in the galactic Bulge from low-resolution K-band spectra

With the existing and upcoming large multi-fibre low-resolution spectrographs, the question arises how precise stellar parameters such as Teff and [Fe/H] can be obtained from low-resolution K-band spectra with respect to traditional photometric temperature measurements. Until now, most of the effective temperatures in galactic Bulge studies come directly from photometric techniques. Uncertainties in interstellar reddening and in the assumed extinction law could lead to large systematic errors. We aim to obtain and calibrate the relation between Teff and the $\rm ^{12}CO$ first overtone bands for M giants in the galactic Bulge covering a wide range in metallicity. We use low-resolution spectra for 20 M giants with well-studied parameters from photometric measurements covering the temperature range 3200 < Teff < 4500 K and a metallicity range from 0.5 dex down to -1.2 dex and study the behaviour of Teff and [Fe/H] on the spectral indices. We find a tight relation between Teff and the $\rm ^{12}CO(2-0)$ band with a dispersion of 95 K as well as between Teff and the $\rm ^{12}CO(3-1)$ with a dispersion of 120 K. We do not find any dependence of these relations on the metallicity of the star, making them relation attractive for galactic Bulge studies. This relation is also not sensitive to the spectral resolution allowing to apply this relation in a more general way. We also found a correlation between the combination of the NaI, CaI and the $\rm ^{12}CO$ band with the metallicity of the star. However this relation is only valid for sub-solar metallicities. We show that low-resolution spectra provide a powerful tool to obtain effective temperatures of M giants. We show that this relation does not depend on the metallicity of the star within the investigated range and is also applicable to different spectral resolution.

Atomic diffusion and mixing in old stars VI: The lithium content of M30

The prediction of the PLANCK-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. NLTE abundances for Li, Ca, and Fe were derived where possible. Stellar parameters were derived by matching isochrones to the observed V vs V-I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-T_eff calibrations to the broadband photometry. Li abundances of the turnoff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including atomic diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by atomic diffusion and additional mixing. After applying a conservative correction for atomic diffusion, we find an initial Li abundance of A(Li) = $2.48\pm0.10$ for the globular cluster M30.

Parameterizing Stellar Spectra Using Deep Neural Networks

This work investigates the spectrum parameterization problem using deep neural networks (DNNs). The proposed scheme consists of the following procedures: first, the configuration of a DNN is initialized using a series of autoencoder neural networks; second, the DNN is fine-tuned using a gradient descent scheme; third, stellar parameters ($T_{eff}$, log$~g$, and [Fe/H]) are estimated using the obtained DNN. This scheme was evaluated on both real spectra from SDSS/SEGUE and synthetic spectra calculated from Kurucz's new opacity distribution function models. Test consistencies between our estimates and those provided by the spectroscopic parameter pipeline of SDSS show that the mean absolute errors (MAEs) are 0.0048, 0.1477, and 0.1129 dex for log$~T_{eff}$, log$~g$, and [Fe/H] (64.85 K for $T_{eff}$), respectively. For the synthetic spectra, the MAE test accuracies are 0.0011, 0.0182, and 0.0112 dex for log$~T_{eff}$, log$~g$, and [Fe/H] (14.90 K for $T_{eff}$), respectively.

High precision analysis of the solar twin HIP 100963

Methods. We used the HIRES spectrograph on the Keck I Telescope to acquire high resolution (R $\approx$ 70000) and high-signal-to-noise ratio spectra (S/N $\approx$ 400 - 650 per pixel) of HIP 100963 and the Sun, for a differential abundance analysis. We measured the equivalent widths (EWs) of iron lines to determine the stellar parameters employing the differential spectroscopic equilibrium. We determine through a differential abundance analysis with respect to the Sun, the composition of volatile, refractory and neutron-capture elements. Results. The stellar parameters that we found are: $T_{\rm{eff}}=5818 \pm 4$ K, log $g = 4.49 \pm 0.01$ dex, $v_{t} = 1.03 \pm 0.01 $ $\rm{km\ {s}}^{-1}$ and [Fe/H] $ = -\ 0.003 \pm 0.004$ dex. These low errors allow us to compute a precise mass ($1.03^{+0.02}_{-0.01}$ \sm) and age (2.0 $\pm$ 0.4 Gyr), obtained using Yonsei-Yale isochrones. Using our [Y/Mg] ratio, we have determined an age of $2.1 \pm 0.4$ Gyr, in agreement with the age computed using isochrones. Our isochronal age also agrees with the age determined from stellar activity (2.4 $\pm$ 0.3 Gyr). We study the abundance pattern with condensation temperature ($\rm{T_{cond}}$) taking into account corrections by GCE. We show that the enhancements of neutron-capture elements are explained by contributions from both the $s$-process and $r$-process. The lithium abundance follows the tight Li-age correlation seen in other solar twins. Conclusions. We confirm that HIP 100963 is a solar twin and demonstrate that after GCE corrections its abundance pattern is about solar. The star also shows enrichment in $s$ and $r$-process elements, as well as depletion in lithium caused by stellar evolution.

High precision analysis of the solar twin HIP 100963 [Replacement]

Methods. We used the HIRES spectrograph on the Keck I telescope to acquire high-resolution (R $\approx$ 70000) spectra with a high signal-to-noise ratio (S/N $\approx$ 400 - 650 per pixel) of HIP 100963 and the Sun for a differential abundance analysis. We measured the equivalent widths (EWs) of iron lines to determine the stellar parameters by employing the differential spectroscopic equilibrium. We determined the composition of volatile, refractory, and neutron-capture elements through a differential abundance analysis with respect to the Sun. Results. The stellar parameters we found are $T_{\rm{eff}}=5818 \pm 4$ K, log $g = 4.49 \pm 0.01$ dex, $v_{t} = 1.03 \pm 0.01 $ $\rm{km\ {s}}^{-1}$ , and [Fe/H] $ = -\ 0.003 \pm 0.004$ dex. These low errors allow us to compute a precise mass ($1.03^{+0.02}_{-0.01}$ M$_{\odot}$) and age (2.0 $\pm$ 0.4 Gyr), obtained using Yonsei-Yale isochrones. Using our [Y/Mg] ratio, we have determined an age of $2.1 \pm 0.4$ Gyr, in agreement with the age computed using isochrones. Our isochronal age also agrees with the age determined from stellar activity (2.4 $\pm$ 0.3 Gyr). We study the abundance pattern with condensation temperature ($\rm{T_{cond}}$) taking corrections by the GCE into account. We show that the enhancements of neutron-capture elements are explained by contributions from both the $s$- and $r$-process. The lithium abundance follows the tight Li-age correlation seen in other solar twins. Conclusions. We confirm that HIP 100963 is a solar twin and demonstrate that its abundance pattern is about solar after corrections for GCE. The star also shows enrichment in $s-$ and $r$-process elements, as well as depletion in lithium that is caused by stellar evolution.

Accelerated Fitting of Stellar Spectra

Stellar spectra are often modeled and fit by interpolating within a rectilinear grid of synthetic spectra to derive the stars' labels: stellar parameters and elemental abundances. However, the number of synthetic spectra needed for a rectilinear grid grows exponentially with the label space dimensions, precluding the simultaneous and self-consistent fitting of more than a few elemental abundances. Shortcuts such as fitting subsets of parameters separately can introduce unknown systematics and do not produce correct error covariances in the derived labels. In this paper we present a new approach -- CHAT (Convex Hull Adaptive Tessellation) -- which includes several new ideas for inexpensively generating a sufficient stellar synthetic library, using linear algebra and the concept of an adaptive, data-driven grid. A convex hull approximates the region where the data lie in the label space. A variety of tests with mock datasets demonstrate that CHAT can reduce the number of required synthetic model calculations by three orders of magnitude in an 8D label space. The reduction will be even larger for higher-dimensional label spaces. In CHAT the computational effort increases only linearly with the number of labels that are fit simultaneously. Around each of these grid points in label space an approximate synthetic spectrum can be generated through linear expansion using a set of "gradient spectra" that represent flux derivatives at every wavelength point with respect to all labels. On this basis, these techniques provide new opportunities to fit the full stellar spectra from large surveys with 15--30 parameters simultaneously, as it reduces the spectral fitting problem to a series of simple linear regressions.

Chemistry and Kinematics of Red Supergiant Stars in the Young Massive Cluster NGC 2100 [Replacement]

We have obtained K-band Multi-Object Spectrograph (KMOS) near-IR spectroscopy for 14 red supergiant stars (RSGs) in the young massive star cluster NGC 2100 in the Large Magellanic Cloud (LMC). Stellar parameters including metallicity are estimated using the J-band analysis technique, which has been rigorously tested in the Local Universe. We find an average metallicity for NGC 2100 of [Z]=$-$0.43$\pm$0.10 dex, in good agreement with estimates from the literature for the LMC. Comparing our results in NGC 2100 with those for a Galactic cluster (at Solar-like metallicity) with a similar mass and age we find no significant difference in the location of RSGs in the Hertzsprung--Russell diagram. We combine the observed KMOS spectra to form a simulated integrated-light cluster spectrum and show that, by analysing this spectrum as a single RSG, the results are consistent with the average properties of the cluster. Radial velocities are estimated for the targets and the dynamical properties are estimated for the first time within this cluster. The data are consistent with a flat velocity dispersion profile, and with an upper limit of 3.9 km/s, at the 95% confidence level, for the velocity dispersion of the cluster. However, the intrinsic velocity dispersion is unresolved and could, therefore, be significantly smaller than the upper limit reported here. An upper limit on the dynamical mass of the cluster is derived as $M_{dyn}$ $\le$ $15.2\times10^{4}M_{\odot}$ assuming virial equilibrium.

Chemistry and Kinematics of Red Supergiant Stars in the Young Massive Cluster NGC 2100

We have obtained K-band Multi-Object Spectrograph (KMOS) near-IR spectroscopy for 14 red supergiant stars (RSGs) in the young massive star cluster NGC 2100 in the Large Magellanic Cloud (LMC). Stellar parameters including metallicity are estimated using the J-band analysis technique, which has been rigorously tested in the Local Universe. We find an average metallicity for NGC 2100 of [Z]=$-$0.38$\pm$0.20 dex, in good agreement with estimates from the literature for the LMC. Comparing our results in NGC 2100 with those for a Galactic cluster (at Solar-like metallicity) with a similar mass and age we find no significant difference in the location of RSGs in the Hertzsprung--Russell diagram. We combine the observed KMOS spectra to form a simulated integrated-light cluster spectrum and show that, by analysing this spectrum as a single RSG, the results are consistent with the average properties of the cluster. Radial velocities are estimated for the targets and the dynamical properties are estimated for the first time within this cluster. The data are consistent with a flat velocity dispersion profile, and with an upper limit of 3.9 \kms, at the 95\% confidence level, for the velocity dispersion of the cluster. However, the intrinsic velocity dispersion is unresolved and could, therefore, be significantly smaller than the upper limit reported here. An upper limit on the dynamical mass of the cluster is derived as $M_{dyn}$ $\le$ $15.2\times10^{4}M_{\odot}$ assuming virial equilibrium.

Multi-Layered Neural Networks Infer Fundamental Stellar Parameters

The advent of space-based observatories such as CoRoT and Kepler has enabled the testing of our understanding of stellar evolution on thousands of stars. Evolutionary models typically require five input parameters, the mass, initial Helium abundance, initial metallicity, mixing-length (assumed to be constant over time) and the age to which the star must be evolved. These parameters are also very useful in characterizing the associated planets and in studying galactic archaeology. How to obtain the parameters from observations rapidly and accurately, specifically in the context of surveys of thousands of stars, is an outstanding question, one that has eluded straightforward resolution. For a given star, we typically measure the effective temperature and surface metallicity spectroscopically and low-degree oscillation frequencies through space observatories. Here we demonstrate that statistical learning, using multi-layered neural networks, is successful in determining the evolutionary parameters based on spectroscopic and seismic measurements. Our trained networks show robustness over a broad range of parameter space, and critically, are entirely computationally inexpensive. This method is both computationally cheap and inferentially accurate, paving the way for analyzing the vast quantities of stellar observations from past, current and future missions.

Label Transfer from APOGEE to LAMOST: Precise Stellar Parameters for 450,000 LAMOST Giants [Replacement]

To capitalize on a diverse set of large spectroscopic stellar surveys, measured stellar parameters must be precise and made consistent across surveys. Here, we demonstrate that this can be achieved by a data-driven approach to spectral modeling: we use The Cannon (Ness et al. 2015) to measure precise stellar parameters from the spectra of 450,000 LAMOST giants, using a model built from APOGEE spectra. The Cannon fits a predictive model for LAMOST spectra using a reference set of 9952 stars observed in common between the two surveys, taking five ASPCAP labels from APOGEE DR12 (Garcia Perez et al. 2015) as ground truth: Teff, logg, [Fe/H], [$\alpha$/M], and K-band extinction $A_k$. The model is then used to infer Teff, logg, [Fe/H], and [$\alpha$/M] for 454,180 giant stars in LAMOST DR2, roughly 20% of the total LAMOST DR2 stellar sample. By construction, these labels are on the APOGEE label scale. Thus, this "label transfer" enables us to tie low-resolution ($R \sim 1800$) LAMOST spectra to the label scale of a much higher-resolution (APOGEE $R \sim 22,500$) survey. We find that these new Cannon labels have an accuracy and precision comparable to the stated APOGEE DR12 values and uncertainties, dramatically reducing the existing systematic label inconsistencies resulting from the individual survey pipelines. By transferring [$\alpha$/M] labels from APOGEE, The Cannon produces the first [$\alpha$/M] values measured from LAMOST spectra, and the largest catalog of [alpha/M] for giant stars to date. This demonstrates that The Cannon can successfully bring different surveys onto the same label scale by transferring a label system from one survey to another.

The Hyades open cluster is chemically inhomogeneous [Replacement]

We present a high-precision differential abundance analysis of 16 solar-type stars in the Hyades open cluster based on high resolution, high signal-to-noise ratio (S/N ~ 350 - 400) spectra obtained from the McDonald 2.7m telescope. We derived stellar parameters and differential chemical abundances for 19 elements (C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ba) with uncertainties as low as ~ 0.01 - 0.02 dex. Our main results include: (1) there is no clear chemical signature of planet formation detected among the sample stars, i.e., no correlations in abundances versus condensation temperature; (2) the observed abundance dispersions are a factor of ~ 1.5 - 2 larger than the average measurement errors for most elements; (3) there are positive correlations, of high statistical significance, between the abundances of at least 90% of pairs of elements. We demonstrate that none of these findings can be explained by errors due to the stellar parameters. Our results reveal that the Hyades is chemically inhomogeneous at the 0.02 dex level. Possible explanations for the abundance variations include (1) inhomogeneous chemical evolution in the proto-cluster environment, (2) supernova ejection in the proto-cluster cloud, and (3) pollution of metal-poor gas before complete mixing of the proto-cluster cloud. Our results provide significant new constraints on the chemical composition of open clusters and a challenge to the current view of Galactic archeology.

The Hyades open cluster is chemically inhomogeneous

We present a high-precision differential abundance analysis of 16 solar-type stars in the Hyades open cluster based on high resolution, high signal-to-noise ratio (S/N ~ 350 - 400) spectra obtained from the McDonald 2.7m telescope. We derived stellar parameters and differential chemical abundances for 19 elements (C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ba) with uncertainties as low as ~ 0.01 - 0.02 dex. Our main results include: (1) There is no clear chemical signature of planet formation detected among the sample stars, i.e., no correlations in abundances versus condensation temperature. (2) The observed abundance dispersions are a factor of ~ 1.5 - 2 larger than the average measurement errors for most elements. (3) There are positive correlations, of high statistical significance, between the abundances of at least 90% of pairs of elements. We demonstrate that none of these findings can be explained by errors due to the stellar parameters. Our results reveal that the Hyades is chemically inhomogeneous at the 0.02 dex level. Possible explanations for the abundance variations include (1) inhomogeneous chemical evolution in the proto-cluster environment, (2) supernova ejection in the proto-cluster cloud, and (3) pollution of metal-poor gas before complete mixing of the proto-cluster cloud. Our results provide significant new constraints on the chemical composition of open clusters and a challenge to the current view of Galactic archeology.

The Gaia-ESO Survey: Dynamical Analysis of the L1688 region in Ophiuchus

The Gaia ESO Public Spectroscopic Survey (GES) is providing the astronomical community with high-precision measurements of many stellar parameters including radial velocities (RVs) of stars belonging to several young clusters and star-forming regions. One of the main goals of the young cluster observations is to study of their dynamical evolution and provide insight into their future, revealing if they will eventually disperse to populate the field, rather than evolve into bound open clusters. In this paper we report the analysis of the dynamical state of L1688 in the $\rho$~Ophiuchi molecular cloud using the dataset provided by the GES consortium. We performed the membership selection of the more than 300 objects observed. Using the presence of the lithium absorption and the location in the Hertzspung-Russell diagram, we identify 45 already known members and two new association members. We provide accurate RVs for all 47 confirmed members.A dynamical analysis, after accounting for unresolved binaries and errors, shows that the stellar surface population of L1688 has a velocity dispersion $\sigma \sim$1.14$\pm$0.35 km s$^{-1}$ that is consistent with being in virial equilibrium and is bound with a $\sim$80% probability. We also find a velocity gradient in the stellar surface population of $\sim$1.0 km s$^{-1}$pc$^{-1}$ in the northwest/southeast direction, which is consistent with that found for the pre-stellar dense cores, and we discuss the possibility of sequential and triggered star formation in L1688.

Lithium in Open Cluster Red Giants Hosting Substellar Companions

We have measured stellar parameters, [Fe/H], lithium abundances, rotation, and 12C/13C in a small sample of red giants in three open clusters that are each home to a red giant star that hosts a substellar companion (NGC2423 3, NGC4349 127, and BD+12 1917 in M67). Our goal is to explore whether the presence of substellar companions influences the Li content. Both 12C/13C and stellar rotation are measured as additional tracers of stellar mixing. One of the companion hosts, NGC2423 3, is found to be Li-rich with A(Li)_NLTE=1.56 dex, and this abundance is significantly higher than the A(Li) of the two comparison stars in NGC2423. All three substellar companion hosts have the highest A(Li) and 12C/13C when compared to the control red giants in their respective clusters; however, except for NGC2423 3, at least one control star has similarly high abundances within the uncertainties. Higher A(Li) could suggest that the formation or presence of planets plays a role in the degree of internal mixing on or before the red giant branch. However, a multitude of factors affect A(Li) during the red giant phase, and when the abundances of our sample are compared to abundances of red giants in other open clusters available in the literature, we find that they all fall well within a much larger distribution of A(Li) and 12C/13C. Thus, even the high Li in NGC2423 3 cannot be concretely tied to the presence of the substellar companion.

Seismic indices -- a deep look inside evolved stars

Independent of stellar modelling, global seismic parameters of red giants provide unique information on the individual stellar properties as well as on stellar evolution. They allow us to measure key stellar parameters, such as the stellar mass and radius, or to derive the distance of field stars. Furthermore, oscillations with a mixed character directly probe the physical conditions in the stellar core. Here, we explain how very precise seismic indices are obtained, and how they can be used for monitoring stellar evolution and performing Galactic archeology.

Gaia Benchmark stars and their twins in the Gaia-ESO Survey

The Gaia benchmark stars are stars with very precise stellar parameters that cover a wide range in the HR diagram at various metallicities. They are meant to be good representative of typical FGK stars in the Milky Way. Currently, they are used by several spectroscopic surveys to validate and calibrate the methods that analyse the data. I review our recent activities done for these stars. Additionally, by applying our new method to find stellar twins on the Gaia-ESO Survey, I discuss how good representatives of Milky Way stars the benchmark stars are and how they distribute in space.

New determination of abundances and stellar parameters for a set of weak G-band stars

Weak G-band (wGb) stars are very peculiar red giants almost devoided of carbon and often mildly enriched in lithium. Despite their very puzzling abundance patterns, very few detailed spectroscopic studies existed up to a few years ago, preventing any clear understanding of the wGb phenomenon. We recently proposed the first consistent analysis of published data for 28 wGb stars and identified them as descendants of early A-type to late B-type stars, without being able to conclude on their evolutionary status or the origin of their peculiar abundance pattern. We used newly obtained high-resolution and high SNR spectra for 19 wGb stars in the southern and northern hemisphere to homogeneously derive their fundamental parameters, metallicities, as well as the spectroscopic abundances for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution models that we used to determine the masses and to investigate the evolutionary status and chemical history of the stars in our sample. We confirm that the wGb stars are stars in the mass range 3.2 to 4.2 M$_\odot$. We suggest that a large fraction could be mildly evolved stars on the SGB currently undergoing the 1st DUP, while a smaller number of stars are more probably in the core He burning phase at the clump. After analysing their abundance pattern, we confirm their strong N enrichment anti-correlated with large C depletion, characteristic of material fully processed through the CNO cycle to an extent not known in other evolved intermediate-mass stars. However, we demonstrate here that such a pattern is very unlikely due to self-enrichment. In the light of the current observational constraints, no solid self-consistent pollution scenario can be presented either, leaving the wGb puzzle largely unsolved.

SP_Ace: a new code to derive stellar parameters and elemental abundances

Aims: We developed a new method of estimating the stellar parameters Teff, log g, [M/H], and elemental abundances. This method was implemented in a new code, SP_Ace (Stellar Parameters And Chemical abundances Estimator). This is a highly automated code suitable for analyzing the spectra of large spectroscopic surveys with low or medium spectral resolution (R=2,000-20,000). Methods: After the astrophysical calibration of the oscillator strengths of 4643 absorption lines covering the wavelength ranges 5212-6860\AA\ and 8400-8924\AA, we constructed a library that contains the equivalent widths (EW) of these lines for a grid of stellar parameters. The EWs of each line are fit by a polynomial function that describes the EW of the line as a function of the stellar parameters. The coefficients of these polynomial functions are stored in a library called the "$GCOG$ library". SP_Ace, a code written in FORTRAN95, uses the GCOG library to compute the EWs of the lines, constructs models of spectra as a function of the stellar parameters and abundances, and searches for the model that minimizes the $\chi^2$ deviation when compared to the observed spectrum. The code has been tested on synthetic and real spectra for a wide range of signal-to-noise and spectral resolutions. Results: SP_Ace derives stellar parameters such as Teff, log g, [M/H], and chemical abundances of up to ten elements for low to medium resolution spectra of FGK-type stars with precision comparable to the one usually obtained with spectra of higher resolution. Systematic errors in stellar parameters and chemical abundances are presented and identified with tests on synthetic and real spectra. Stochastic errors are automatically estimated by the code for all the parameters. A simple Web front end of SP_Ace can be found at http://dc.g-vo.org/SP_ACE, while the source code will be published soon.

Modelling the photosphere of active stars for planet detection and characterization

Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes. These effects are currently in the focus of many exoplanet search projects, since the lack of a well-defined characterization and correction strategy hampers the detection of the signals associated with small exoplanets. Accurate simulations of the stellar photosphere can provide synthetic time series data. These may help to investigate the relation between activity jitter and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales in order to design strategies to remove or minimize them. In this work we present the StarSim tool, which is based on a model for a spotted rotating photosphere built from the integration of the spectral contribution of a fine grid of surface elements. The model includes all significant effects affecting the flux intensities and the wavelength of spectral features produced by active regions and planets. A specific application for the characterization and modelling of the spectral signature of active regions is considered, showing that the chromatic effects of faculae are dominant for low temperature contrasts of spots. Synthetic time series are modelled for HD 189733. Our algorithm reproduces both the photometry and the RVs to good precision, generally better than the studies published to date. We evaluate the RV signature of the activity in HD 189733 by exploring a grid of solutions from the photometry. We find that the use of RV data in the inverse problem could break degeneracies and allow for a better determination of some stellar and activity parameters. In addition, the effects of spots are studied for a set of simulated transit photometry, showing that these can introduce variations which are very similar to the signal of an atmosphere dominated by dust.

IN-SYNC IV - The Young Stellar Population in the Orion A Molecular Cloud [Replacement]

We present the results of the SDSS APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC) survey of the Orion A molecular cloud. This survey obtained high resolution near infrared (NIR) spectroscopy of about 2700 young pre-main sequence stars throughout the region, acquired across five distinct fields spanning 6deg field of view (FOV). With these spectra, we have measured accurate stellar parameters (T_eff, log g, v sin i) and extinctions, and placed the sources in the Hertzsprung-Russel Diagram (HRD). We have also extracted radial velocities for the kinematic characterization of the population. We compare our measurements with literature results for a sub-sample of targets in order to assess the performances and accuracy of the survey. Source extinction shows evidence for dust grains that are larger than those in the diffuse interstellar medium (ISM): we estimate an average R_V=5.5 in the region. Importantly, we find a clear correlation between HRD inferred ages and spectroscopic surface-gravity inferred ages. This clearly indicates a real spread of stellar radii at fixed temperature, and together with additional correlations with extinction and with disk presence, strongly suggests a real spread of ages large than a few Myr. Focussing on the young population around NGC1980 iota Ori, which has previously been suggested to be a separate, foreground, older cluster, we confirm its older (5Myr) age and low A_V, but considering that its radial velocity distribution is indistinguishable from the Orion A's population, we suggest that NGC1980 is part of Orion A's star formation activity. Based on their stellar parameters and kinematic properties, we identify 383 new candidate members of Orion A, most of which are diskless sources in areas of the region poorly studied by previous works.

IN-SYNC IV - The Young Stellar Population in the Orion A Molecular Cloud

We present the results of the SDSS APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC) survey of the Orion A molecular cloud. This survey obtained high resolution near infrared (NIR) spectroscopy of about 2700 young pre-main sequence stars throughout the region, acquired across five distinct fields spanning 6deg field of view (FOV). With these spectra, we have measured accurate stellar parameters (T_eff, log g, v sin i) and extinctions, and placed the sources in the Hertzsprung-Russel Diagram (HRD). We have also extracted radial velocities for the kinematic characterization of the population. We compare our measurements with literature results for a sub-sample of targets in order to assess the performances and accuracy of the survey. Source extinction shows evidence for dust grains that are larger than those in the diffuse interstellar medium (ISM): we estimate an average R_V=5.5 in the region. Importantly, we find a clear correlation between HRD inferred ages and spectroscopic surface-gravity inferred ages. This clearly indicates a real spread of stellar radii at fixed temperature, and together with additional correlations with extinction and with disk presence, strongly suggests a real spread of ages large than a few Myr. Focussing on the young population around NGC1980 iota Ori, which has previously been suggested to be a separate, foreground, older cluster, we confirm its older (5Myr) age and low A_V, but considering that its radial velocity distribution is indistinguishable from the Orion A's population, we suggest that NGC1980 is part of Orion A's star formation activity. Based on their stellar parameters and kinematic properties, we identify 383 new candidate members of Orion A, most of which are diskless sources in areas of the region poorly studied by previous works.

Radii, masses, and ages of 18 bright stars using interferometry and new estimations of exoplanetary parameters [Replacement]

Accurate stellar parameters are needed in numerous domains of astrophysics. The position of stars on the H-R diagram is an important indication of their structure and evolution, and it helps improve stellar models. Furthermore, the age and mass of stars hosting planets are required elements for studying exoplanetary systems. We aim at determining accurate parameters of a set of 18 bright exoplanet host and potential host stars from interferometric measurements, photometry, and stellar models. Using the VEGA/CHARA interferometer, we measured the angular diameters of 18 stars, ten of which host exoplanets. We combined them with their distances to estimate their radii. We used photometry to derive their bolometric flux and, then, their effective temperature and luminosity to place them on the H-R diagram. We then used the PARSEC models to derive their best fit ages and masses, with error bars derived from MC calculations. Our interferometric measurements lead to an average of 1.9% uncertainty on angular diameters and 3% on stellar radii. There is good agreement between measured and indirect estimations of angular diameters (from SED fitting or SB relations) for MS stars, but not as good for more evolved stars. For each star, we provide a likelihood map in the mass-age plane; typically, two distinct sets of solutions appear (an old and a young age). The errors on the ages and masses that we provide account for the metallicity uncertainties, which are often neglected by other works. From measurements of its radius and density, we also provide the mass of 55 Cnc independently of models. From the stellar masses, we provide new estimates of semi-major axes and minimum masses of exoplanets with reliable uncertainties. We also derive the radius, density, and mass of 55 Cnc e, a super-Earth that transits its stellar host. Our exoplanetary parameters reflect the known population of exoplanets.

Radii, masses, and ages of 18 bright stars using interferometry. And new estimations of exoplanetary parameters

Accurate stellar parameters are needed in numerous domains of astrophysics. The position of stars on the H-R diagram is an important indication of their structure and evolution, and it helps improve stellar models. Furthermore, the age and mass of stars hosting planets are required elements for studying exoplanetary systems. We aim at determining accurate parameters of a set of 18 bright exoplanet host and potential host stars from interferometric measurements, photometry, and stellar models. Using the VEGA/CHARA interferometer, we measured the angular diameters of 18 stars, ten of which host exoplanets. We combined them with their distances to estimate their radii. We used photometry to derive their bolometric flux and, then, their effective temperature and luminosity to place them on the H-R diagram. We then used the PARSEC models to derive their best fit ages and masses, with error bars derived from MC calculations. Our interferometric measurements lead to an average of 1.9% uncertainty on angular diameters and 3% on stellar radii. There is good agreement between measured and indirect estimations of angular diameters (from SED fitting or SB relations) for MS stars, but not as good for more evolved stars. For each star, we provide a likelihood map in the mass-age plane; typically, two distinct sets of solutions appear (an old and a young age). The errors on the ages and masses that we provide account for the metallicity uncertainties, which are often neglected by other works. From measurements of its radius and density, we also provide the mass of 55 Cnc independently of models. From the stellar masses, we provide new estimates of semi-major axes and minimum masses of exoplanets with reliable uncertainties. We also derive the radius, density, and mass of 55 Cnc e, a super-Earth that transits its stellar host. Our exoplanetary parameters reflect the known population of exoplanets.

Observational $\Delta\nu$-$\bar\rho$ relation for $\delta$ Sct stars using eclipsing binaries and space photometry

Delta Scuti ($\delta$ Sct) stars are intermediate-mass pulsators, whose intrinsic oscillations have been studied for decades. However, modelling their pulsations remains a real theoretical challenge, thereby even hampering the precise determination of global stellar parameters. In this work, we used space photometry observations of eclipsing binaries with a $\delta$ Sct component to obtain reliable physical parameters and oscillation frequencies. Using that information, we derived an observational scaling relation between the stellar mean density and a frequency pattern in the oscillation spectrum. This pattern is analogous to the solar-like large separation but in the low order regime. We also show that this relation is independent of the rotation rate. These findings open the possibility of accurately characterizing this type of pulsator and validate the frequency pattern as a new observable for $\delta$ Sct stars.

Estimating stellar fundamental parameters using PCA: application to early type stars of GES data

This work addresses a procedure to estimate fundamental stellar parameters such as T eff , logg, [Fe/H], and v sin i using a dimensionality reduction technique called Principal Component Analysis (PCA), applied to a large database of synthetic spectra. This technique shows promising results for inverting stellar parameters of observed targets from Gaia ESO Survey.

Optically Visible Post-AGB Stars, Post-RGB Stars and Young Stellar Objects in the Large Magellanic Cloud

We have carried out a search for optically visible post-Asymptotic Giant Branch (post-AGB) stars in the Large Magellanic Cloud (LMC). First, we selected candidates with a mid-IR excess and then obtained their optical spectra. We disentangled contaminants with unique spectra such as M-stars, C-stars, planetary nebulae, quasi-stellar objects and background galaxies. Subsequently, we performed a detailed spectroscopic analysis of the remaining candidates to estimate their stellar parameters such as effective temperature, surface gravity (log g), metallicity ([Fe/H]), reddening and their luminosities. This resulted in a sample of 35 likely post-AGB candidates with late-G to late-A spectral types, low log g, and [Fe/H] < -0.5. Furthermore, our study confirmed the existence of the dusty post-Red Giant Branch (post-RGB) stars, discovered previously in our SMC survey, by revealing 119 such objects in the LMC. These objects have mid-IR excesses and stellar parameters (Teff, log g, [Fe/H]) similar to those of post-AGB stars except that their luminosities (< 2500 Lsun), and hence masses and radii, are lower. These post-RGB stars are likely to be products of binary interaction on the RGB. The post-AGB and post-RGB objects show SED properties similar to the Galactic post-AGB stars, where some have a surrounding circumstellar shell, while some others have a surrounding stable disc similar to the Galactic post-AGB binaries. This study also resulted in a new sample of 162 young stellar objects, identified based on a robust log g criterion. Other interesting outcomes include objects with an UV continuum and an emission line spectrum; luminous supergiants; hot main-sequence stars; and 15 B[e] star candidates, 12 of which are newly discovered in this study.

Stellar granulation and interferometry

Stars are not smooth. Their photosphere is covered by a granulation pattern associated with the heat transport by convection. The convection-related surface structures have different size, depth, and temporal variations with respect to the stellar type. The related activity (in addition to other phenomena such as magnetic spots, rotation, dust, etc.) potentially causes bias in stellar parameters determination, radial velocity, chemical abundances determinations, and exoplanet transit detections. The role of long-baseline interferometric observations in this astrophysical context is crucial to characterize the stellar surface dynamics and correct the potential biases. In this Chapter, we present how the granulation pattern is expected for different kind of stellar types ranging from main sequence to extremely evolved stars of different masses and how interferometric techniques help to study their photospheric dynamics.

A spectroscopic survey of Herbig Ae/Be stars with X-Shooter I: Stellar parameters and accretion rates

Herbig Ae/Be stars span a key mass range that links low and high mass stars, and thus provide an ideal window from which to explore their formation. This paper presents VLT/X-Shooter spectra of 91 Herbig Ae/Be stars, HAeBes; the largest spectroscopic study of HAeBe accretion to date. A homogeneous approach to determining stellar parameters is undertaken for the majority of the sample. Measurements of the ultra-violet (UV) are modelled within the context of magnetospheric accretion, allowing a direct determination of mass accretion rates. Multiple correlations are observed across the sample between accretion and stellar properties: the youngest and often most massive stars are the strongest accretors, and there is an almost 1:1 relationship between the accretion luminosity and stellar luminosity. Despite these overall trends of increased accretion rates in HAeBes when compared to classical T Tauri stars, we also find noticeable differences in correlations when considering the Herbig Ae and Herbig Be subsets. This, combined with the difficulty in applying a magnetospheric accretion model to some of the Herbig Be stars, could suggest that another form of accretion may be occurring within the Herbig Be mass range.

Bayesian Statistics as a New Tool for Spectral Analysis: I. Application for the Determination of Basic Parameters of Massive Stars

Spectral analysis is a powerful tool to investigate stellar properties and it has been widely used for decades now. However, the methods considered to perform this kind of analysis are mostly based on iteration among a few diagnostic lines to determine the stellar parameters. While these methods are often simple and fast, they can lead to errors and large uncertainties due to the required assumptions. Here we present a method based on Bayesian statistics to find simultaneously the best combination of effective temperature, surface gravity, projected rotational velocity, and microturbulence velocity, using all the available spectral lines. Different tests are discussed to demonstrate the strength of our method, which we apply to 54 mid-resolution spectra of field and cluster B stars obtained at the Observatoire du Mont-M\'egantic. We compare our results with those found in the literature. Differences are seen which are well explained by the different methods used. We conclude that the B-star microturbulence velocities are often underestimated. We also confirm the trend that B stars in clusters are on average faster rotators than field B stars.

Grid Search in Stellar Parameters: a software for spectrum analysis of single stars and binary systems

The currently operating space missions, as well as those that will be launched in the near future, (will) deliver high-quality data for millions of stellar objects. Since the majority of stellar astrophysical applications still (at least partly) rely on spectroscopic data, an efficient tool for the analysis of medium- to high-resolution spectroscopy is needed. We aim at developing an efficient software package for the analysis of medium- to high-resolution spectroscopy of single stars and those in binary systems. The major requirements are that the code has a high performance, represents the state-of-the-art analysis tool, and provides accurate determinations of atmospheric parameters and chemical compositions for different types of stars. We use the method of atmosphere models and spectrum synthesis, which is one of the most commonly used approaches for the analysis of stellar spectra. Our Grid Search in Stellar Parameters (GSSP) code makes use of the OpenMPI implementation, which makes it possible to run in parallel mode. The method is first tested on the simulated data and is then applied to the spectra of real stellar objects. The majority of test runs on the simulated data were successful in the sense that we could recover the initially assumed sets of atmospheric parameters. We experimentally find the limits in signal-to-noise ratios of the input spectra, below which the final set of parameters gets significantly affected by the noise. Application of the GSSP package to the spectra of three Kepler stars, KIC11285625, KIC6352430, and KIC4931738, was also largely successful. We found an overall agreement of the final sets of the fundamental parameters with the original studies. For KIC6352430, we found that dependence of the light dilution factor on wavelength cannot be ignored, as it has significant impact on the determination of the atmospheric parameters of this binary system.

Abundances in the Local Region I: G and K Giants

Parameters and abundances for 1133 stars of spectral types F, G, and K of luminosity class III have been derived. In terms of stellar parameters, the primary point of interest is the disagreement between gravities derived with masses determined from isochrones, and gravities determined from an ionization balance. This is not a new result per se; but the size of this sample emphasizes the severity of the problem. A variety of arguments lead to the selection of the ionization balance gravity as the working value. The derived abundances indicate that the giants in the solar region have Sun-like total abundances and abundance ratios. Stellar evolution indicators have also been investigated with the Li abundances and the [C/Fe] and C/O ratios indicating that standard processing has been operating in these stars. The more salient result for stellar evolution is that the [C/Fe] data across the red-giant clump indicates the presence of mass dependent mixing in accord with standard stellar evolution predictions. Keywords: stars: fundamental parameters - stars: abundances - stars: evolution - Galaxy: abundances

The nature of the KFR08 stellar stream

The origin of a new kinematically identified metal-poor stellar stream, the KFR08 stream, has not been established. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Ni, Zn, Sr, Y, Zr, Ba, La, and Eu for 16 KFR08 stream members based on analysis of high resolution spectra. Based on the abundance ratios of 14 elements, we use the chemical tagging method to identify the stars which have the same chemical composition, and thus, might have a common birthplace, such as a cluster. Although three stars were tagged with similar elemental abundances ratios, we find that, statistically, it is not certain that they originate from a dissolved star cluster. This conclusion is consistent with the large dispersion of [Fe/H] ($\sigma_{\rm{[Fe/H]}} = 0.29$) among the 16 stream members. We find that our stars are $\alpha$ enhanced and that the abundance patterns of the stream members are well matched to the thick disk. In addition, most of the stream stars have estimated stellar ages larger than 11 Gyr. These results, together with the hot kinematics of the stream stars, suggest that the KFR08 stream is originated from the thick disk population which was perturbed by a massive merger in the early universe.

Discovery of a strongly r-process enhanced extremely metal-poor star LAMOST J110901.22+075441.8

We report the discovery of an extremely metal-poor (EMP) giant, LAMOST J110901.22+075441.8, which exhibits large excess of r-process elements with [Eu/Fe] ~ +1.16. The star is one of the newly discovered EMP stars identified from LAMOST low-resolution spectroscopic survey and the high-resolution follow-up observation with the Subaru Telescope. Stellar parameters and elemental abundances have been determined from the Subaru spectrum. Accurate abundances for a total of 23 elements including 11 neutron-capture elements from Sr through Dy have been derived for LAMOST J110901.22+075441.8. The abundance pattern of LAMOST J110901.22+075441.8 in the range of C through Zn is in line with the "normal" population of EMP halo stars, except that it shows a notable underabundance in carbon. The heavy element abundance pattern of LAMOST J110901.22+075441.8 is in agreement with other well studied cool r-II metal-poor giants such as CS 22892-052 and CS 31082-001. The abundances of elements in the range from Ba through Dy well match the scaled Solar r-process pattern. LAMOST J110901.22+075441.8 provides the first detailed measurements of neutron-capture elements among r-II stars at such low metallicity with [Fe/H]<-3.4, and exhibits similar behavior in the abundance ratio of Zr/Eu as well as Sr/Eu and Ba/Eu as other r-II stars.

 

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