Posts Tagged red giant

Recent Postings from red giant

Outburst activity of symbiotic system AGDra

AG Dra is a well known bright symbiotic binary with a white dwarf and a pulsating red giant. The long-term photometry monitoring and a new behaviour of the system are presented. The detailed period analysis of photometry as well as spectroscopy was carried out. In the system of AG Dra, two periods of variability are detected. The longer one around 550 days is related to the orbital motion, and the shorter one around 355 days is interpreted as pulsations of the red giant in our older paper. In addition the active stages change distinctively, but the outbursts are repeated with the periods from 359 to 375 days.

Solar-like oscillations in subgiant and red-giant stars: mixed modes

Thanks to significant improvements in high-resolution spectrographs and the launch of dedicated space missions MOST, CoRoT and Kepler, the number of subgiants and red-giant stars with detected oscillations has increased significantly over the last decade. The amount of detail that can now be resolved in the oscillation patterns does allow for in-depth investigations of the internal structures of these stars. One phenomenon that plays an important role in such studies are mixed modes. These are modes that carry information of the inner radiative region as well as from the convective outer part of the star allowing to probe different depths of the stars. Here, we describe mixed modes and highlight some recent results obtained using mixed modes observed in subgiants and red-giant stars.

Why the globular cluster NGC 6752 contains no sodium-rich second-generation AGB star

(Abridged) Globular clusters host multiple stellar populations showing different sodium enrichments. These various populations can be observed along the main sequence, red giant and horizontal branch phases. Recently it was shown, however, that at least in the globular cluster NGC 6752, no sodium-rich stars are observed along the early asymptotic giant branch, posing an apparent problem for stellar evolution. We present an explanation for this lack of sodium-rich stars in this region of the colour-magnitude diagram.

The effect of common-envelope evolution on the visible population of post-common-envelope binaries

Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the {\alpha}-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)- main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.

The Hamburg/ESO R-process Enhanced Star survey (HERES) VIII. The r+s star HE 1405 0822

Aims.The aim of this study is a detailed abundance analysis of the newly discovered r-rich star HE 1405 0822, which has [Fe=H]=-2.40. This star shows enhancements of both r- and s-elements, [Ba/Fe]= +1.95 and [Eu/Fe]=1.54, for which reason it is called r+s star. Methods.Stellar parameters and element abundances were determined by analying high-quality VLT/UVES spectra. We used Fe I line excitation equilibria to derive the e?ective temperature. The surface gravity was calculated from the Fei/Feii and Ti I/Ti II equilibria. Results.We determined accurate abundances for 39 elements, including 19 neutron-capture elements. HE 1405-0822 is a red giant. Its strong enhancements of C, N, and s-elements are the consequence of enrichment by a former AGB companion with an initial mass of less than 3 M_Sun. The heavy n-capture element abundances (including Eu, Yb, and Hf) seen in HE 1405-0822 do not agree with the r-process pattern seen in strongly r-process-enhanced stars. We discuss possible enrichment scenarios for this star. The enhanced alpha elements can be explained as the result of enrichment by supernovae of type II. Na and Mg may have partly been synthesized in a former AGB companion, when the primary 22^Ne acted as a neutron poison in the 13^C-pocket.

Magellanic Cloud stars with TiO bands in emission: binary post-RGB/AGB stars or young stellar objects?

Fourteen stars from a sample of Magellanic Cloud objects selected to have a mid-infrared flux excess have been found to also show TiO bands in emission. The mid-infrared dust emission and the TiO band emission indicate that these stars have large amounts of hot circumstellar dust and gas in close proximity to the central star. The luminosities of the sources are typically several thousand L_sun while the effective temperatures are 4000-8000 K. Such stars could be post-AGB stars of mass 0.4-0.8 M_sun or pre-main-sequence stars (young stellar objects) with masses of 7-19 M_sun. If the stars are pre-main-sequence stars, they are substantially cooler and younger than stars at the birth line where Galactic protostars are first supposed to become optically visible out of their molecular clouds. They should therefore be hidden in their present evolutionary state. The second explanation for these stars is that they are post-AGB or post-RGB stars that have recently undergone a binary interaction when the red giant of the binary system filled its Roche lobe. Being oxygen-rich, they have gone through this process before becoming carbon stars. Most of the stars vary slowly on timescales of 1000 days or more suggesting a changing circumstellar environment. Apart from the slow variations, most stars also show variability with periods of tens to hundreds of days. One star shows a period that is rapidly decreasing and we speculate that this star may have accreted a large blob of gas and dust onto a disk whose orbital radius is shrinking rapidly. Another star has Cepheid-like pulsations of rapidly increasing amplitude suggesting a rapid rate of evolution. Seven stars show quasi-periodic variability and one star has a light curve similar to that of an eclipsing binary.

KIC 8410637: a 408-day period eclipsing binary containing a pulsating red giant

Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement of masses and radii of ther component stars. If at least one of the stars has evolved off the main sequence (MS), the masses and radii give a strict constraint on the age of the stars. Several dEBs containing a bright K giant and a fainter MS star have been discovered by the Kepler satellite. The mass and radius of a red giant (RG) star can also be derived from its asteroseismic signal. The parameters determined in this way depend on stellar models and may contain systematic errors. It is important to validate the asteroseismically determined mass and radius with independent methods. This can be done when stars are members of stellar clusters or members of dEBs. KIC 8410637 consists of an RG and an MS star. The aim is to derive accurate masses and radii for both components and provide the foundation for a strong test of the asteroseismic method and the accuracy of the deduced mass, radius and age. We analyse high-resolution spectra from three different spectrographs. We also calculate a fit to the Kepler light curve and use ground-based photometry to determine the flux ratios between the component stars in the BVRI passbands. We measured the masses and radii of the stars in the dEB, and the classical parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry. The RG component of KIC 8410637 is most likely in the core helium-burning red clump phase of evolution and has an age and composition very similar to the stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should therefore be similar to the mass of RGs in NGC 6819, thus lending support to the most up-to-date version of the asteroseismic scaling relations. This is the first direct measurement of both mass and radius for an RG to be compared with values for RGs from asteroseismic scaling relations.

Comparison of different nonlinear solvers for 2D time-implicit stellar hydrodynamics

Time-implicit schemes are attractive since they allow numerical time steps that are much larger than those permitted by the Courant-Friedrich-Lewy criterion characterizing time-explicit methods. This advantage comes, however, with a cost: the solution of a system of nonlinear equations is required at each time step. In this work, the nonlinear system results from the discretization of the hydrodynamical equations with the Crank-Nicholson scheme. We compare the cost of different methods, based on Newton-Raphson iterations, to solve this nonlinear system, and benchmark their performances against time-explicit schemes. Since our general scientific objective is to model stellar interiors, we use as test cases two realistic models for the convective envelope of a red giant and a young Sun. Focusing on 2D simulations, we show that the best performances are obtained with the quasi-Newton method proposed by Broyden. Another important concern is the accuracy of implicit calculations. Based on the study of an idealized problem, namely the advection of a single vortex by a uniform flow, we show that there are two aspects: i) the nonlinear solver has to be accurate enough to resolve the truncation error of the numerical discretization, and ii) the time step has be small enough to resolve the advection of eddies. We show that with these two conditions fulfilled, our implicit methods exhibit similar accuracy to time-explicit schemes, which have lower values for the time step and higher computational costs. Finally, we discuss in the conclusion the applicability of these methods to fully implicit 3D calculations.

Understanding angular momentum transport in red giants: the case of KIC 7341231

Context. Thanks to recent asteroseismic observations, it has been possible to infer the radial differential rotation profile of subgiants and red giants. Aims. We want to reproduce through modeling the observed rotation profile of the early red giant KIC 7341231 and constrain the physical mechanisms responsible for angular momentum transport in stellar interiors. Methods. We compute models of KIC 7341231 including a treatment of shellular rotation and we compare the rotation profiles obtained with the one derived by Deheuvels et al. (2012). We then modify some modeling parameters in order to quantify their effect on the obtained rotation profile. Moreover, we mimic a powerful angular momentum transport during the Main Sequence and study its effect on the evolution of the rotation profile during the subgiant and red giant phases. Results. We show that meridional circulation and shear mixing alone produce a rotation profile for KIC 7341231 too steep compared to the observed one. An additional mechanism is then needed to increase the internal transport of angular momentum. We find that this undetermined mechanism has to be efficient not only during the Main Sequence but also during the much quicker subgiant phase. Moreover, we point out the importance of studying the whole rotational history of a star in order to explain its rotation profile during the red giant evolution.

Star-Jet Interactions and Gamma-Ray Outbursts from 3c454.3

We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during November 2010 on top of a weeks-long plateau. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the star atmosphere by nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of the gamma-ray emitting clouds, $\delta\simeq 20$. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directly accelerated electrons.

Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3 [Replacement]

We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during November 2010 on top of a weeks-long plateau. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the star atmosphere by nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of the gamma-ray emitting clouds, $\delta\simeq 20$. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directly accelerated electrons.

Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3 [Replacement]

We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during November 2010 on top of a weeks-long plateau. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the star atmosphere by nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of the gamma-ray emitting clouds, $\delta\simeq 20$. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directly accelerated electrons.

The stellar IMF determined in early-type galaxies from a non-degenerate set of optical line indices

We investigate the optical spectral region of spectra of ~1000 stars searching for IMF-sensitive features to constrain the low-mass end of the initial mass function (IMF) slope in elliptical galaxies. We use the MILES stellar library in the wavelength range [3500-7500] A to select indices that are sensitive to cool dwarf stars and that only weakly depend on age and metallicity. We find several promising indices of molecular TiO and CaH lines. The use of these indicators bluer than NIR features (NaI, CaT, Wing-Ford FeH) is crucial if we want to compare our observations to optical simple stellar population models. In this wavelength range, the response of a change in the effective temperature of the cool red giant (RGB) population is similar to the response of a change in the number of dwarf stars in the galaxy. We show that it is possible to break the degeneracy between IMF variation and Teff,RGB with our new IMF indicators. We conclude that our new CaH1 index (6380A), the only indicator that comes purely from cool dwarfs, allows the determination of the low-mass end of the IMF from integrated-light measurements, when combined with different TiO lines and age- and metallicity-dependent features such as Hbeta, Mgb, Fe5270 and Fe5335. We measure line-index strengths of our new optical IMF-indicators in the Conroy & van Dokkum SSP models with ages= [3.0,13.5]Gyr, [a/Fe]=[+0.0,+0.4]dex and deltaTeff,RGB=[-200,+50] K. We compare these to index strengths of the same spectral features in a sample of stacked SDSS early-type galaxy spectra with varying velocity dispersions. We find a clear trend of steepening IMF with increasing velocity dispersion from 150 to 310 km/s described by the linear equation x =(2.26 +-0.08)*log(sigma200)+(2.13+-0.15),where x is the IMF slope and sigma200 is the central stellar velocity dispersion measured in units of 200 km/s.

Gamma-ray emission from nova outbursts

Classical novae produce radioactive nuclei which are emitters of gamma-rays in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and 22Na) and the positron-electron annihilation emission, with the 511 keV line and a continuum. Gamma-ray spectra and light curves are potential unique tools to trace the corresponding isotopes and to give insights on the properties of the expanding envelope. Another possible origin of gamma-rays is the acceleration of particles up to very high energies, so that either neutral pions or inverse Compton processes produce gamma-rays of energies larger than 100 MeV. MeV photons during nova explosions have not been detected yet, although several attempts have been made in the last decades; on the other hand, GeV photons from novae have been detected with the Fermi satellite in V407 Cyg, a nova in a symbiotic binary, where the companion is a red giant with a wind, instead of a main sequence star as in the cataclysmic variables hosting classical novae. Two more novae have been detected recently (summer 2012) by Fermi, apparently in non symbiotic binaries, thus challenging our understanding of the emission mechanism. Both scenarios (radioactivities and acceleration) of gamma-ray production in novae are discussed.

Spectral line asymmetries in the metal-poor red giant HD 122563: CO5BOLD predictions versus observations

We study the influence of convection on the asymmetries and Doppler shifts of Fe I spectral lines in the metal-poor red giant HD 122563. To this end, we compute theoretical Fe I line shifts and line bisectors using 3D hydrodynamical model atmosphere of HD 122563 calculated with the CO5BOLD code. We then make a detailed comparison of the theoretical line shifts and bisectors with those derived from the high quality HARPS spectrum of HD 122563 taken from the ESO Science Archive Facility (R = 115 000, average signal-to-noise ratio, S/N = 310). In general, we find a good agreement between the theoretically predicted and observed Doppler shifts of Fe I line cores, with somewhat larger discrepancies seen in the case of weaker (equivalent width W < 5 pm) and stronger lines (W > 11 pm). Both observed and theoretical coreshifts cover a range between 0 and -1 km/s, with increasingly stronger blueshifts for weaker lines and slight hints of a coreshift dependence on wavelength. Theoretical bisectors reproduce the observed ones reasonably well too, however, theoretical bisectors of the weak red (lambda > 600 nm) Fe I lines have blueshifts that are by up to 200 m/s larger than observed. The obtained results therefore suggest that the current CO5BOLD models are capable of reproducing the large-scale velocity fields in the atmosphere of HD 122563 suficiently well. Nevertheless, further efforts are needed in order to understand the physical reasons behind the discrepancies in theoretical predictions and observed properties of the weakest and strongest Fe I lines.

A Lithium-Rich Red Giant Below the Clump in the Kepler Cluster, NGC 6819

WIYN/HYDRA spectra in the Li 6708 Angstrom region have been obtained for 332 probable members of the old open cluster, NGC 6819. Preliminary analysis shows a pattern of Li depletion from the top of the turnoff to the base of the giant branch. Starting one magnitude below the level of the clump, all brighter giants have A(Li) below 1.0, with most having upper limits below 0.5. Star W007017, located BELOW the first-ascent red giant bump is Li-rich with A(Li) = 2.3. As a highly probable single-star astrometric and radial-velocity cluster member, its discrepant asteroseismic membership could be a by-product of the processes that triggered Li-enhancement. Its color-magnitude diagram location is consistent with only one proposed enhanced mixing process among first-ascent red giants.

Donors of Persistent Neutron-star Low-mass X-ray Binaries

Properties of X-ray luminosities in low-mass X-ray binaries (LMXBs) mainly depend on donors. We have carried out a detailed study of donors in persistent neutron-star LMXBs (PLMXBs) by means of a population synthesis code. PLMXBs with different donors have different formation channels. Our numerical simulations show that more than 90% of PLMXBs have main sequence (MS) donors, and PLMXBs with red giant (RG) donors via stellar wind (Wind) are negligible. In our model, most of neutron stars (NSs) in PLMXBs with hydrogen-rich donors form via core-collapse supernovae, while more than 90% of NSs in PLMXBs with naked helium star (He) donors or white dwarf (WD) donors form via an evolution-induced collapse via helium star ($1.4 \leq M_{\rm He}/M_\odot \leq 2.5$) or an accretion-induced collapses for an accreting ONeMg WD.

Asteroseismic classification of stellar populations among 13000 red giants observed by Kepler

Of the more than 150000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves were made public in late 2011. More than 13000 (over 85%) of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large scale asteroseismic investigations. We automatically extracted individual frequencies and measured the period spacings of the dipole modes in nearly every red giant. These measurements naturally classify the stars into various populations, such as the red giant branch, the low-mass (M/Msol < 1.8) helium-core-burning red clump, and the higher-mass (M/Msol > 1.8) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars will undoubtedly provide an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.

Novae in gamma-rays

Classical novae produce radioactive nuclei which are emitters of gamma-rays in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and 22Na) and the positron-electron annihilation emission (511 keV line and a continuum below this energy, with a cut-off at 20-30 keV). The analysis of gamma-ray spectra and light curves is a potential unique and powerful tool both to trace the corresponding isotopes and to give insights on the properties of the expanding envelope determining its transparency. Another possible origin of gamma-rays is the acceleration of particles up to very high energies, so that either neutral pions or inverse Compton processes produce gamma-rays of energies larger than 100 MeV. MeV photons during nova explosions have not been detected yet, although several attempts have been made in the last decades; on the other hand, GeV photons from novae have been detected in some particular novae, in symbiotic binaries, where the companion is a red giant with a wind, instead of a main sequence star as in the cataclysmic variables hosting classical novae. Both mechanisms of gamma-ray production in novae are reviewed, with more emphasis on the one related to radioactivities.

Internal rotation of red giants by asteroseismology

We present an asteroseismic approach to study the dynamics of the stellar interior in red-giant stars by asteroseismic inversion of the splittings induced by the stellar rotation on the oscillation frequencies. We show preliminary results obtained for the red giant KIC4448777 observed by the space mission Kepler.

Chemical Abundances in Field Red Giants from High-Resolution H-Band Spectra using the APOGEE Spectral Linelist

High-resolution H-band spectra of five bright field K, M, and MS giants, obtained from the archives of the Kitt Peak National Observatory (KPNO) Fourier Transform Spectrometer (FTS), are analyzed to determine chemical abundances of 16 elements. The abundances were derived via spectrum synthesis using the detailed linelist prepared for the SDSS III Apache Point Galactic Evolution Experiment (APOGEE), which is a high-resolution near-infrared spectroscopic survey to derive detailed chemical abundance distributions and precise radial velocities for 100,000 red giants sampling all Galactic stellar populations. Measured chemical abundances include the cosmochemically important isotopes 12C, 13C, 14N, and 16O, along with Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. A comparison of the abundances derived here with published values for these stars reveals consistent results to ~0.1 dex. The APOGEE spectral region and linelist is, thus, well-suited for probing both Galactic chemical evolution, as well as internal nucleosynthesis and mixing in populations of red giants using high-resolution spectroscopy.

Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present. The stellar winds of red giants may be tidally enhanced by companion stars if they are in binary systems. We investigate evolutionary consequences of red giants in binaries by including tidally enhanced stellar winds, and examine the effects on the HB morphology of GCs. We find that red, blue, and extreme horizontal branch stars are all produced under the effects of tidally enhanced stellar wind without any additional assumptions on the mass-loss dispersion. Furthermore, the horizontal branch morphology is found to be insensitive to the tidal enhancement parameter, Bw. We compare our theoretical results with the observed horizontal branch morphology of globular cluster NGC 2808, and find that the basic morphology of the horizontal branch can be well reproduced. The number of blue horizontal branch stars in our calculations, however, is lower than that of NGC 2808.

Asymptotic and measured large frequency separations

With the space-borne missions CoRoT and Kepler, a large amount of asteroseismic data is now available. So-called global oscillation parameters are inferred to characterize the large sets of stars, to perform ensemble asteroseismology, and to derive scaling relations. The mean large separation is such a key parameter. It is therefore crucial to measure it with the highest accuracy. As the conditions of measurement of the large separation do not coincide with its theoretical definition, we revisit the asymptotic expressions used for analysing the observed oscillation spectra. Then, we examine the consequence of the difference between the observed and asymptotic values of the mean large separation. The analysis is focused on radial modes. We use series of radial-mode frequencies to compare the asymptotic and observational values of the large separation. We propose a simple formulation to correct the observed value of the large separation and then derive its asymptotic counterpart. We prove that, apart from glitches due to stellar structure discontinuities, the asymptotic expansion is valid from main-sequence stars to red giants. Our model shows that the asymptotic offset is close to 1/4, as in the theoretical development. High-quality solar-like oscillation spectra derived from precise photometric measurements are definitely better described with the second-order asymptotic expansion. The second-order term is responsible for the curvature observed in the \’echelle diagrams used for analysing the oscillation spectra and this curvature is responsible for the difference between the observed and asymptotic values of the large separation. Taking it into account yields a revision of the scaling relations providing more accurate asteroseismic estimates of the stellar mass and radius.

ARGOS II: The Galactic Bulge Survey

We describe the motivation, field locations and stellar selection for the ARGOS spectroscopic survey of 28,000 stars in the bulge and inner disk of the Milky Way galaxy across latitudes of b = -5 deg to -10 deg. The primary goal of this survey is to constrain the formation processes of the bulge and establish whether it is predominantly a merger or instability remnant. From the spectra (R = 11,000), we have measured radial velocities and determined stellar parameters, including metallicities and [alpha/Fe] ratios. Distances were estimated from the derived stellar parameters and about 14,000 stars are red giants within 3.5 kpc of the Galactic centre. In this paper, we present the observations and analysis methods. Subsequent papers (III and IV) will discuss the stellar metallicity distribution and kinematics of the Galactic bulge and inner disk, and the implications for the formation of the bulge.

A Bayesian approach to scaling relations for amplitudes of solar-like oscillations in Kepler stars

We investigate different amplitude scaling relations adopted for the asteroseismology of stars that show solar-like oscillations. Amplitudes are among the most challenging asteroseismic quantities to handle because of the large uncertainties that arise in measuring the background level in the star’s power spectrum. We present results computed by means of a Bayesian inference on a sample of 1640 stars observed with \it{Kepler}, spanning from main sequence to red giant stars, for 12 models used for amplitude predictions and exploiting recently well-calibrated effective temperatures from SDSS photometry. We test the candidate amplitude scaling relations by means of a Bayesian model comparison. We find the model having a separate dependence upon the mass of the stars to be largely the most favored one. The differences among models and the differences seen in their free parameters from early to late phases of stellar evolution are also highlighted.

A Bayesian approach to scaling relations for amplitudes of solar-like oscillations in Kepler stars [Replacement]

We investigate different amplitude scaling relations adopted for the asteroseismology of stars that show solar-like oscillations. Amplitudes are among the most challenging asteroseismic quantities to handle because of the large uncertainties that arise in measuring the background level in the star’s power spectrum. We present results computed by means of a Bayesian inference on a sample of 1640 stars observed with Kepler, spanning from main sequence to red giant stars, for 12 models used for amplitude predictions and exploiting recently well-calibrated effective temperatures from SDSS photometry. We test the candidate amplitude scaling relations by means of a Bayesian model comparison. We find the model having a separate dependence upon the mass of the stars to be largely the most favored one. The differences among models and the differences seen in their free parameters from early to late phases of stellar evolution are also highlighted.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars II. Spectral line formation in the atmosphere of a giant located near the RGB tip

We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms, and molecules. We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (Teff=3660K, logg=1.0, [M/H]=0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO5BOLD and LHD stellar atmosphere codes, respectively. Both codes share the same atmospheric parameters, chemical composition, equation of state, and opacities, which allowed us to make a strictly differential comparison between the line formation properties predicted in 3D and 1D. The influence of convection on the spectral line formation was assessed with the aid of 3D-1D abundance corrections, which measure the difference between the abundances of chemical species derived with the 3D hydrodynamical and 1D classical model atmospheres. We find that convection plays a significant role in the spectral line formation in this particular red giant. The derived 3D-1D abundance corrections rarely exceed \pm0.1 dex when lines of neutral atoms and molecules are considered, which is in line with the previous findings for solar-metallicity red giants located on the lower RGB. The situation is different with lines that belong to ionized atoms, or to neutral atoms with high ionization potential. In both cases, the corrections for high-excitation lines (\chi>8 eV) may amount to \Delta_3D-1D ~ -0.4 dex. The 3D–1D abundance corrections generally show a significant wavelength dependence; in most cases they are smaller in the near-infrared, at 1600-2500 nm.

Hydrodynamic Studies of the Evolution of Recurrent, Symbiotic, and Dwarf Novae: The White Dwarf Components are Growing in Mass

Symbiotic binaries are systems containing white dwarfs (WDs) and red giants. Symbiotic novae are those systems in which thermonuclear eruptions occur on the WD components. These are to be distinguished from events driven by accretion disk instabilities analogous to dwarf novae eruptions in cataclysmic variable outbursts. Another class of symbiotic systems are those in which the WD is extremely luminous and it seems likely that quiescent nuclear burning is ongoing on the accreting WD. A fundamental question is the secular evolution of the WD. Do the repeated outbursts or quiescent burning in these accreting systems cause the WD to gain or lose mass? If it is gaining mass, can it eventually reach the Chandrasekhar Limit and become a supernova (a SN Ia if it can hide the hydrogen and helium in the system)? In order to better understand these systems, we have begun a new study of the evolution of Thermonuclear Runaways (TNRs) in the accreted envelopes of WDs using a variety of initial WD masses, luminosities and mass accretion rates. We use our 1-D hydro code, NOVA, which includes the new convective algorithm of Arnett, Meakin and Young, the Hix and Thielemann nuclear reaction solver, the Iliadis reaction rate library, the Timmes equation of state, and the OPAL opacities. We assume a solar composition (Lodders abundance distribution) and do not allow any mixing of accreted material with core material. This assumption strongly influences our results. We report here (1) that the WD grows in mass for all simulations so that canonical `steady burning’ does not occur, and (2) that only a small fraction of the accreted matter is ejected in some (but not all) simulations. We also find that the accreting systems, before thermonuclear runaway, are too cool to be seen in X-ray searches for SN Ia progenitors.

Symbiotic stars in X-rays

Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of 10 white dwarf symbiotics that were not previously known to be X-ray sources and one that was previously detected as a supersoft X-ray source. The 10 new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by 30%. Swift/XRT detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component, which we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component, which likely arises in a region where the winds from the two stars collide. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk are typically not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk.

An X-Ray and Optical Light Curv Model of the Eclipsing Symbiotic Binary SMC3

Some binary evolution scenarios to Type Ia supernovae include long-period binaries that evolve to symbiotic supersoft X-ray sources in their late stage of evolution. However, symbiotic stars with steady hydrogen burning on the white dwarf’s (WD) surface are very rare, and the X-ray characteristics are not well known. SMC3 is one such rare example and a key object for understanding the evolution of symbiotic stars to Type Ia supernovae. SMC3 is an eclipsing symbiotic binary, consisting of a massive WD and red giant (RG), with an orbital period of 4.5 years in the Small Magellanic Cloud. The long-term V light curve variations are reproduced as orbital variations in the irradiated RG, whose atmosphere fills its Roche lobe, thus supporting the idea that the RG supplies matter to the WD at rates high enough to maintain steady hydrogen burning on the WD. We also present an eclipse model in which an X-ray emitting region around the WD is almost totally occulted by the RG swelling over the Roche lobe on the trailing side, although it is always partly obscured by a long spiral tail of neutral hydrogen surrounding the binary in the orbital plane.

Numerical estimates of the accretion rate onto intermediate-mass black holes

The existence of intermediate-mass ($\sim 10^3 M_\odot$) black holes in the center of globular clusters has been suggested by different observations. Extended, central X-ray sources observed in NGC 6388 and in G1 in M31 could be interpreted as being powered by the accretion of matter onto such objects. In this work we explore a scenario in which the black hole accretes from the cluster interstellar medium, which is generated by the mass loss of the red giants in the cluster. We estimate the accretion rate onto the black hole and compare it to the values obtained via the traditional Bondi-Hoyle model. Our results show that the accretion rate is no longer solely defined by the black hole mass and the ambient parameters but also by the host cluster itself. Furthermore, we find that the more massive globular clusters with large stellar velocity dispersion are the best candidates in which accretion onto IMBHs could be detected.

Modeling and Analysis of a Spectrum of the Globular Cluster NGC 2419

NGC 2419 is the most distant massive globular cluster in the outer Galactic halo. It is unusual also due to the chemical peculiarities found in its red giants in recent years. We study the stellar population of this unusual object using spectra obtained at the 1.93-m telescope of the Haute-Provence Observatory. At variance with commonly used methods of high-resolution spectroscopy applicable only to bright stars, we employ spectroscopic information on the integrated light of the cluster. We carry out population synthesis modeling of medium-resolution spectra using synthetic stellar atmosphere models based on a theoretical isochrone corresponding accurately to the observed color-magnitude diagram. We study the influence of non-Local Thermodynamic Equilibrium for some chemical elements on our results. The derived age (12.6 Gyr), [Fe/H]=-2.25 dex, helium content Y=0.25, and abundances of 12 other chemical elements are in a good qualitative agreement with published high-resolution spectroscopy estimates for red giant members in the cluster. On the other hand, the derived element abundance, [alpha/Fe]=0.13 dex (the mean of [O/Fe], [Mg/Fe] and [Ca/Fe]), differs from the published one ([alpha/Fe] =0.4). We suggest that studies of the integrated light in NGC2419 using high-resolution spectrographs in different wavelength regions will help to understand the nature of these chemical anomalies.

Modeling and Analysis of a Spectrum of the Globular Cluster NGC 2419 [Replacement]

NGC 2419 is the most distant massive globular cluster in the outer Galactic halo. It is unusual also due to the chemical peculiarities found in its red giant stars in recent years. We study the stellar population of this unusual object using spectra obtained at the 1.93-m telescope of the Haute-Provence Observatory. At variance with commonly used methods of high-resolution spectroscopy applicable only to bright stars, we employ spectroscopic information on the integrated light of the cluster. We carry out population synthesis modeling of medium-resolution spectra using synthetic stellar atmosphere models based on a theoretical isochrone corresponding accurately to the observed color-magnitude diagram. We study the influence of non-Local Thermodynamic Equilibrium for some chemical elements on our results. The derived age (12.6 Gyr), [Fe/H]=-2.25 dex, helium content Y=0.25, and abundances of 12 other chemical elements are in a good qualitative agreement with published high-resolution spectroscopy estimates for red giant members in the cluster. On the other hand, the derived element abundance, [alpha/Fe]=0.13 dex (the mean of [O/Fe], [Mg/Fe] and [Ca/Fe]), differs from the published one ([alpha/Fe] =0.4 dex) for selected red giants in the cluster and may be explained by a large dispersion in the alpha-element abundances recently discovered in NGC2419. We suggest that studies of the {\it integrated} light in the cluster using high-resolution spectrographs in different wavelength regions will help to understand the nature of these chemical anomalies.

Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB

Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Our aim is to obtain seismic constraints on the internal transport and surface loss of angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of $1.3 M_{\odot}$ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.

Constraining the core-rotation rate in red-giant stars from Kepler space photometry

Rotation plays a key role in stellar structure and its evolution. Through transport processes which induce rotational mixing of chemical species and the redistribution of angular momentum, internal stellar rotation influences the evolutionary tracks in the Hertzsprung-Russell diagram. In turn, evolution influences the rotational properties. Therefore, information on the rotational properties of the deep interior would help to better understand the stellar evolution. However, as the internal rotational profile cannot be measured directly, it remains a major unknown leaving this important aspect of models unconstrained. We can test for nonrigid rotation inside the stars with asteroseismology. Through the effect of rotational splitting of non-radial oscillation modes, we investigate the internal rotation profile indirectly. Red giants have very slow rotation rates leading to a rotational splitting on the level of a few tenth of a \mu Hz. Only from more than 1.5 years of consecutive data from the NASA Kepler space telescope, these tiny variations could be resolved. A qualitative comparison to theoretical models allowed constraining the core-to-surface rotation rate for some of these evolved stars. In this paper, we report on the first results of a large sample study of splitting of individual dipole modes.

Galactic archaeology: mapping and dating stellar populations with asteroseismology of red-giant stars

Our understanding of how the Galaxy was formed and evolves is severely hampered by the lack of precise constraints on basic stellar properties such as distances, masses, and ages. Here, we show that solar-like pulsating red giants represent a well-populated class of accurate distance indicators, spanning a large age range, which can be used to map and date the Galactic disc in the regions probed by observations made by the CoRoT and Kepler space telescopes. When combined with photometric constraints, the pulsation spectra of such evolved stars not only reveal their radii, and hence distances, but also provide well-constrained estimates of their masses, which are reliable proxies for the ages of the stars. As a first application we consider red giants observed by CoRoT in two different parts of the Milky Way, and determine precise distances for ~2000 stars spread across nearly 15,000 pc of the Galactic disc, exploring regions which are a long way from the solar neighbourhood. We find significant differences in the mass distributions of these two samples which, by comparison with predictions of synthetic models of the Milky Way, we interpret as mainly due to the vertical gradient in the distribution of stellar masses (hence ages) in the disc. In the future, the availability of spectroscopic constraints for this sample of stars will not only improve the age determination, but also provide crucial constraints on age-velocity and age-metallicity relations at different Galactocentric radii and heights from the plane.

EG Andromedae: A Symbiotic System as an Insight into Red Giant Chromospheres

Symbiotic systems are interacting binary stars consisting of both hot and cool components. This results in a complex environment that is ideal for studying the latter stages of stellar evolution along with interactions within binary systems. As a star approaches the end of its life, in particular the red giant phase, it exhausts its supply of core hydrogen and begins burning its way through successively heavier elements. Red giants lose mass in the form of a dense wind that will replenish the interstellar medium with chemical elements that are formed through nuclear processes deep in the stellar interior. When these elements reach the interstellar medium they play a central role in both stellar and planetary evolution, as well as providing the essential constituents needed for life. The undoubted significance of these cool giants means the study of their atmospheres is necessary to help understand our place in the Universe. This thesis presents Hubble Space Telescope observations of the symbiotic system EG Andromedae as an insight into red giant stars. EG And is one of the brightest and closest symbiotic systems and consists of a red giant primary along with a white dwarf. The presence of the white dwarf in the system allows spatially resolved examination of the red giant primary. The benefits of using such a system to better understand the base of red giant chromospheres is shown. Along with the observations of EG And, new HST observations of an isolated red giant spectral standard HD148349 are described. The similarity between the isolated spectral standard and the red giant primary of EG And is demonstrated, showing that much of the information gleaned from a symbiotic system can be applied to the general red giant population. Using both ultraviolet and optical spectroscopy, the atmosphere of EG And and HD148349 are investigated and contrasted.

Circumstellar Shell Formation in Symbiotic Recurrent Novae

We present models of spherically symmetric recurrent nova shells interacting with circumstellar material in a symbiotic system composed of a red giant expelling a wind, and a white dwarf accreting from this material. Recurrent nova eruptions periodically eject material at high velocities ($\gtrsim 10^3$ km/s) into the red giant wind profile, creating a decelerating shock wave as circumstellar material is swept up. High circumstellar material densities cause the shocked wind and ejecta to have very short cooling times of days to weeks. Thus, the late time evolution of the shell is determined by momentum conservation instead of energy conservation. We compute and show evolutionary tracks of shell deceleration, as well as post-shock structure. After sweeping up all the red giant wind, the shell coasts at a velocity $\sim 100$ km/s, depending on system parameters. These velocities are similar to those measured in blue-shifted circumstellar material from the symbiotic nova RS Oph, as well as a few Type Ia supernovae that show evidence of circumstellar material, such as 2006X, 2007le, and PTF 11kx. Supernovae occurring in such systems may not show circumstellar material interaction until the inner nova shell gets hit by the supernova ejecta, days to months after the explosion.

Red giant seismology: observations

The CoRoT and Kepler missions provide us with thousands of red-giant light curves that allow a very precise asteroseismic study of these objects. Before CoRoT and Kepler, the red-giant oscillation patterns remained obscure. Now, these spectra are much more clear and unveil many crucial interior structure properties. For thousands of red giants, we can derive from the seismic data precise estimates of the stellar mass and radius, the evolutionary status of the giants (with a clear difference between clump and RGB stars), the internal differential rotation, the mass loss, the distance of the stars… Analysing this mass of information is made easy by the identification of the largely homologous red-giant oscillation patterns. For the first time, both pressure and mixed mode oscillation patterns can be precisely depicted. The mixed-mode analysis allows us, for instance, to probe directly the stellar core. Fine details completing the red-giant oscillation pattern then provide further information for a more detailed view on the interior structure, including differential rotation.

Attempts to reproduce the rotation profile of the red giant KIC 7341231 observed by Kepler

Thanks to the asteroseimic study of the red giant star KIC 7341231 observed by Kepler, it has been possible to infer its radial differential rotation profile (Deheuvels et al. 2012). This opens new ways to constrain the physical mechanisms responsible of the angular momentum transport in stellar interiors by directly comparing this radial rotation profile with the ones computed using stellar evolution codes including dynamical processes. In this preliminary work, we computed different models of KIC 7341231 with the Geneva stellar evolution code that includes transport mechanisms due to a shellular rotation and the associated large-scale meridional circulation and shear-induced turbulence. Once the global parameters of the star had been established, we modified some of the model’s input parameters in order to understand their effects on the predicted rotation profile of the modeled star. As a result, we find a discrepancy between the rotation profile deduced from asteroseismic measurements and the profiles predicted from models including shellular rotation and related meridional flows and turbulence. This indicates that a most powerful mechanism is in action to extract angular momentum from the core of this star.

Stellar transits in active galactic nuclei

Supermassive black holes (SMBH) are typically surrounded by a dense stellar population in galactic nuclei. Stars crossing the line of site in active galactic nuclei (AGN) produce a characteristic transit lightcurve, just like extrasolar planets do when they transit their host star. We examine the possibility of finding such AGN transits in deep optical, UV, and X-ray surveys. We calculate transit lightcurves using the Novikov–Thorne thin accretion disk model, including general relatistic effects. Based on the expected properties of stellar cusps, we find that around 10^6 solar mass SMBHs, transits of red giants are most common for stars on close orbits with transit durations of a few weeks and orbital periods of a few years. We find that detecting AGN transits requires repeated observations of thousands of low mass AGNs to 1% photometric accuracy in optical, or ~ 10% in UV bands or soft X-ray. It may be possible to identify stellar transits in the Pan-STARRS and LSST optical and the eROSITA X-ray surveys. Such observations could be used to constrain black hole mass, spin, inclination and accretion rate. Transit rates and durations could give valuable information on the circumnuclear stellar clusters as well. Transit lightcurves could be used to image accretion disks with unprecedented resolution, allowing to resolve the SMBH silhouette in distant AGNs.

An abundance study of the red giants in the seismology fields of the CoRoT satellite

A precise characterisation of the red giants in the seismology fields of the CoRoT satellite is a prerequisite for further in-depth seismic modelling. The optical spectra obtained for 19 targets have been used to accurately estimate their fundamental parameters and chemical composition. The extent of internal mixing is also investigated through the abundances of Li, CNO and Na (as well as 12C/13C in a few cases).

Chromospheric Thermal Continuum Millimetre Emission from non-dusty K and M Red Giants

We examine the thermal free-free millimetre fluxes expected from non-dusty and non-pulsating K through mid-M giant stars based on our limited understanding of their inhomogeneous chromospheres. We present a semi-analytic model that provides estimates of the radio fluxes for the mm wavelengths (e.g., CARMA, ALMA, JVLA Q-band) based on knowledge of the effective temperatures, angular diameters and chromospheric Mg II h & k emission fluxes. At 250 GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and sub-mm range will have a contribution from the chromospheric mate- rial that gives rise to the ultraviolet emission spectrum, as well as the cool molecular material known to exist above the photosphere. We predict a lower bound to the inferred brightness temperature of red giants based on heating at the basal-flux limit if the upper chromospheres have filling factor 1. Multi-frequency mm observations should provide important new information on the structuring of the inhomogeneous chromospheres, including the boundary layer, and allow tests of competing theoretical models for atmospheric heating. We comment on the suitability of these stars as mm flux calibrators.

Spin down of the core rotation in red giants

The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. We are now able to probe the rotational behaviour in their deep interiors using the observations of mixed modes. We aim to measure the rotational splittings in red giants and to derive scaling relations for rotation related to seismic and fundamental stellar parameters. We have developed a dedicated method for automated measurements of the rotational splittings in a large number of red giants. Ensemble asteroseismology, namely the examination of a large number of red giants at different stages of their evolution, allows us to derive global information on stellar evolution. We have measured rotational splittings in a sample of about 300 red giants. We have also shown that these splittings are dominated by the core rotation. Under the assumption that a linear analysis can provide the rotational splitting, we observe a small increase of the core rotation of stars ascending the red giant branch. Alternatively, an important slow down is observed for red-clump stars compared to the red giant branch. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass. Ensemble asteroseismology indicates what has been indirectly suspected for a while: our interpretation of the observed rotational splittings leads to the conclusion that the mean core rotation significantly slows down during the red giant phase. The slow-down occurs in the last stages of the red giant branch. This spinning down explains, for instance, the long rotation periods measured in white dwarfs

The Bizarre Chemical Inventory of NGC 2419, An Extreme Outer Halo Globular Cluster

We present new Keck/HIRES observations of six red giants in the globular cluster NGC 2419. Although the cluster is among the most distant and most luminous in the Milky Way, it was considered chemically ordinary until very recently. Our previous work showed that the near-infrared Ca II triplet line strength varied more than expected for a chemically homogeneous cluster, and that at least one star had unusual abundances of Mg and K. Here, we confirm that NGC 2419 harbors a population of stars, comprising about one third of its mass, that is depleted in Mg by a factor of 8 and enhanced in K by a factor of 6 with respect to the Mg-normal population. Although the majority, Mg-normal population appears to have a chemical abundance pattern indistinguishable from ordinary, inner halo globular clusters, the Mg-poor population exhibits dispersions of several elements. The abundances of K and Sc are strongly anti-correlated with Mg, and some other elements (Si and Ca among others) are weakly anti-correlated with Mg. These abundance patterns suggest that the different populations of NGC 2419 sample the ejecta of diverse supernovae in addition to AGB ejecta. However, the abundances of Fe-peak elements except Sc show no star-to-star variation. We find no nucleosynthetic source that satisfactorily explains all of the abundance variations in this cluster. Because NGC 2419 appears like no other globular cluster, we reiterate our previous suggestion that it is not a globular cluster at all, but rather the core of an accreted dwarf galaxy.

The Bizarre Chemical Inventory of NGC 2419, An Extreme Outer Halo Globular Cluster [Replacement]

We present new Keck/HIRES observations of six red giants in the globular cluster NGC 2419. Although the cluster is among the most distant and most luminous in the Milky Way, it was considered chemically ordinary until very recently. Our previous work showed that the near-infrared Ca II triplet line strength varied more than expected for a chemically homogeneous cluster, and that at least one star had unusual abundances of Mg and K. Here, we confirm that NGC 2419 harbors a population of stars, comprising about one third of its mass, that is depleted in Mg by a factor of 8 and enhanced in K by a factor of 6 with respect to the Mg-normal population. Although the majority, Mg-normal population appears to have a chemical abundance pattern indistinguishable from ordinary, inner halo globular clusters, the Mg-poor population exhibits dispersions of several elements. The abundances of K and Sc are strongly anti-correlated with Mg, and some other elements (Si and Ca among others) are weakly anti-correlated with Mg. These abundance patterns suggest that the different populations of NGC 2419 sample the ejecta of diverse supernovae in addition to AGB ejecta. However, the abundances of Fe-peak elements except Sc show no star-to-star variation. We find no nucleosynthetic source that satisfactorily explains all of the abundance variations in this cluster. Because NGC 2419 appears like no other globular cluster, we reiterate our previous suggestion that it is not a globular cluster at all, but rather the core of an accreted dwarf galaxy.

On the origin of the metallicity dependence in dynamically formed extragalactic low-mass X-ray binaries

Globular clusters (GCs) effectively produce dynamically-formed low-mass X-ray binaries (LMXBs). Observers detect ~100 times more LMXBs per stellar mass in GCs compared to stars in the fields of galaxies. It has also been observationally established that metal-rich GCs are about 3 times more likely to contain an X-ray source than their metal-poor counterparts. Recent observations have shown that this ratio holds in extragalactic GCs for all bright X-ray sources with Lx between 2X10^{37} and 5X10^{38} erg/s. In this Letter, we propose that the observed metallicity dependence of LMXBs in extragalactic GCs can be explained by the differences in the number densities and average masses of red giants in populations of different metallicities. Red giants serve as seeds for the dynamical production of bright LMXBs via two channels – binary exchanges and physical collisions – and the increase of the number densities and masses of red giants boost LMXB production, leading to the observed difference. We also discuss a possible effect of the age difference in stellar populations of different metallicities.

Absolute Magnitude Calibration for Red Giants based on the Colour-Magnitude Diagrams of Galactic Clusters. III-Calibration with 2MASS

We present two absolute magnitude calibrations, $M_{J}$ and $M_{K_s}$, for red giants with the colour magnitude diagrams of five Galactic clusters with different metallicities i.e. M92, M13, M71, M67, and NGC 6791. The combination of the absolute magnitudes of the red giant sequences with the corresponding metallicities provides calibration for absolute magnitude estimation for red giants for a given colour. The calibrations for $M_{J}$ and $M_{K_s}$ are defined in the colour intervals $1.3\leq(V-J)_{0}\leq2.8$ and $1.75 \leq (V-K_{s})_{0}\leq 3.80$ mag, respectively, and they cover the metallicity interval $-2.15 \leq \lbrack Fe/H \rbrack \leq +0.37$ dex. The absolute magnitude residuals obtained by the application of the procedure to another set of Galactic clusters lie in the intervals $-0.08<\Delta M_{J}\leq +0.34$ and $-0.10< \Delta M_{K_s}\leq +0.27$ mag for $M_{J}$ and $M_{K_s}$, respectively. The means and standard deviations of the residuals are $<\Delta M_{J}>= 0.137$ and $\sigma_{M_J}=0.080$, and $<\Delta M_{K_s}>=0.109$ and $\sigma_{M_{K_{s}}}=0.123$ mag. The derived relations are applicable to stars older than 4 Gyr, the age of the youngest calibrating cluster.

The Keck Aperture Masking Experiment: Dust Enshrouded Red Giants

While the importance of dusty asymptotic giant branch (AGB) stars to galactic chemical enrichment is widely recognised, a sophisticated understanding of the dust formation and wind-driving mechanisms has proven elusive due in part to the difficulty in spatially-resolving the dust formation regions themselves. We have observed twenty dust-enshrouded AGB stars as part of the Keck Aperture Masking Experiment, resolving all of them in multiple near-infrared bands between 1.5 microns and 3.1 microns. We find 45% of the targets to show measurable elongations that, when correcting for the greater distances of the targets, would correspond to significantly asymmetric dust shells on par with the well-known cases of IRC+10216 or CIT6. Using radiative transfer models, we find the sublimation temperature of 1130 +- 90 K and 1170 +- 60 K for silicates and amorphous carbon respectively, both somewhat lower than expected from laboratory measurements and vastly below temperatures inferred from the inner edge of YSO disks. The fact that O-rich and C-rich dust types showed the same sublimation temperature was surprising as well. For the most optically-thick shells (tau > 2 at 2.2 microns), the temperature profile of the inner dust shell is observed to change substantially, an effect we suggest could arise when individual dust clumps become optically-thick at the highest mass-loss rates.

The Optical Gravitational Lensing Experiment. Optical Counterparts to the X-ray Sources in the Galactic Bulge

We present a sample of 209 variable objects – very likely optical counterparts to the X-ray sources detected in the direction of the Galactic center by the Galactic Bulge Survey (GBS) carried out with the Chandra satellite. The variable sources were found in the databases of the OGLE long term survey monitoring regularly the Galactic bulge since 1992. The counterpart candidates were searched based on the X-ray source position in the radius of 3."9. Optical light curves of the candidates comprise a full variety of variability types: spotted stars, pulsating red giants (potentially secondary stars of symbiotic variables), cataclysmic variables, eclipsing binary systems, irregular non-periodic objects including an AGN (GRS 1734-292). Additionally, we find that positions of 19 non-variable stars brighter than 16.5 mag in the OGLE databases are so well aligned with the X-ray positions (<0."75) that these objects are also likely optical counterparts to the GBS X-ray sources. We provide the OGLE astrometric and photometric information for all selected objects and their preliminary classifications. Photometry of the candidates is available from the OGLE Internet archive, http://ogle.astrouw.edu.pl

 

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