Recent Postings from Solar and Stellar

The canonical Luminous Blue Variable AG Car and its neighbor Hen 3-519 are much closer than previously assumed

The strong mass loss of Luminous Blue Variables (LBVs) is thought to play a critical role in the evolution of massive stars, but the physics of their instability and their place in the evolutionary sequence remains uncertain and debated. A key to understanding their peculiar instability is their high observed luminosity, which for Galactic LBVs often depends on an uncertain distance estimate. Here we report direct distances and space motions of four canonical Milky Way LBVs---AG Car, HR Car, HD 168607, and (the LBV candidate) Hen 3-519---whose parallaxes and proper motions have been provided by the Gaia first data release. Whereas the distances of HR Car and HD 168607 are consistent with those previously adopted in the literature within the uncertainty, we find that the distances to Hen 3-519 and AG Car, both at $\sim$2 kpc, are much closer than the 6--8 kpc distances previously assumed. For Hen 3-519, this moves the star far from the locus of LBVs on the HR Diagram. AG Car has been considered a defining example of a classical LBV, but its lower luminosity also moves it off the S~Dor instability strip. The lower luminosities allow AG Car and Hen 3-519 to have passed through a previous red supergiant phase, they lower the mass estimates for their shell nebulae, and imply that binary evolution is needed to account for their peculiar properties. These lower luminosities and initial masses for LBVs may also have important implications for understanding LBVs as potential supernova progenitors. Improved distances from next year's Gaia data release, which will include additional LBVs, may alter our traditional view of LBVs.

The Gaia-ESO Survey: calibration strategy

The Gaia-ESO survey (GES) is now in its fifth and last year of observations, and has already produced tens of thousands of high-quality spectra of stars in all Milky Way components. This paper presents the strategy behind the selection of astrophysical calibration targets, ensuring that all GES results on radial velocities, atmospheric parameters, and chemical abundance ratios will be both internally consistent and easily comparable with other literature results, especially from other large spectroscopic surveys and from Gaia. The calibration of GES is particularly delicate because of: (i) the large space of parameters covered by its targets, ranging from dwarfs to giants, from O to M stars, and with a large range of metallicities, as well as including fast rotators, emission line objects, stars affected by veiling and so on; (ii) the variety of observing setups, with different wavelength ranges and resolution; and (iii) the choice of analyzing the data with many different state-of-the art methods, each stronger in a different region of the parameter space, which ensures a better understanding of systematic uncertainties. An overview of the GES calibration and homogenization strategy is also given, along with some examples of the usage and results of calibrators in GES iDR4 - the fourth internal GES data release, that will form the basis of the next GES public data release. The agreement between GES iDR4 recommended values and reference values for the calibrating objects are very satisfactory. The average offsets and spreads are generally compatible with the GES measurement errors, which in iDR4 data already meet the requirements set by the main GES scientific goals.

On the detectability of solar-like oscillations with the NASA TESS mission

The upcoming NASA TESS mission will perform an all-sky survey for planets transiting bright nearby stars. In addition, its excellent photometric precision will enable asteroseismology of solar-type and red-giant stars. We apply a newly developed detection test along a sequence of stellar evolutionary tracks in order to predict the detectability of solar-like oscillations with TESS.

The rotation of brown dwarfs

One of the characteristic features of low-mass stars is their propensity to shed large amounts of angular momentum throughout their evolution. This distinguishs them from brown dwarfs which remain fast rotators over timescales of gigayears. Brown dwarfs with rotation periods longer than a couple of days have only been found in star forming regions and young clusters. This is a useful constraint on the mass dependency of mechanisms for angular momentum regular in stars. Rotational braking by disks and winds become highly inefficient in the substellar regime. In this short review I discuss the observational evidence for the fast rotation in brown dwarfs, the implications, and the link to the spin-mass relation in planets.

R-modes and neutron star recycling scenario [Cross-Listing]

To put new constraints on the r-mode instability window, we analyse formation of millisecond pulsars (MSPs) within the recycling scenario, making use of three sets of observations: (a) X-ray observations of neutron stars (NSs) in low-mass X-ray binaries; (b) timing of millisecond pulsars (MSPs); and (c) X-ray and UV observations of MSPs. As shown in previous works, r-mode dissipation by shear viscosity is not sufficient to explain observational set (a), and enhanced r-mode dissipation at internal temperatures $T^\infty\sim 10^8$ K is required to stabilize the observed NSs. Here we argue, that models with enhanced bulk viscosity can hardly lead to a self-consistent explanation of observational set (a) due to strong neutrino emission, which is typical for these models (unrealistically powerful energy source is required to keep NSs at the observed temperatures). We also demonstrate that the observational set (b) requires enhanced r-mode dissipation at low temperatures, $T^\infty\sim(1-2)\times 10^7$ K. Observational set (c) allows us to set an upper limit on the red-shifted internal temperatures of MSPs, $T^\infty<2\times 10^7$ K (assuming a canonical NS with the accreted crust). Recycling scenario can produce MSPs at these temperatures only if r-mode instability is suppressed in the whole MSP spin frequency range ($\nu\lesssim 750$ Hz) at temperatures $2\times 10^7\lesssim T^\infty\lesssim 3 \times 10^7$ K, providing thus a new constraint on the r-mode instability window. These observational constraints are analysed in more details in application to the resonance uplift scenario of Gusakov et al. [Phys. Rev. Lett., 112 (2014), 151101].

R-modes and neutron star recycling scenario

To put new constraints on the r-mode instability window, we analyse formation of millisecond pulsars (MSPs) within the recycling scenario, making use of three sets of observations: (a) X-ray observations of neutron stars (NSs) in low-mass X-ray binaries; (b) timing of millisecond pulsars (MSPs); and (c) X-ray and UV observations of MSPs. As shown in previous works, r-mode dissipation by shear viscosity is not sufficient to explain observational set (a), and enhanced r-mode dissipation at internal temperatures $T^\infty\sim 10^8$ K is required to stabilize the observed NSs. Here we argue, that models with enhanced bulk viscosity can hardly lead to a self-consistent explanation of observational set (a) due to strong neutrino emission, which is typical for these models (unrealistically powerful energy source is required to keep NSs at the observed temperatures). We also demonstrate that the observational set (b) requires enhanced r-mode dissipation at low temperatures, $T^\infty\sim(1-2)\times 10^7$ K. Observational set (c) allows us to set an upper limit on the red-shifted internal temperatures of MSPs, $T^\infty<2\times 10^7$ K (assuming a canonical NS with the accreted crust). Recycling scenario can produce MSPs at these temperatures only if r-mode instability is suppressed in the whole MSP spin frequency range ($\nu\lesssim 750$ Hz) at temperatures $2\times 10^7\lesssim T^\infty\lesssim 3 \times 10^7$ K, providing thus a new constraint on the r-mode instability window. These observational constraints are analysed in more details in application to the resonance uplift scenario of Gusakov et al. [Phys. Rev. Lett., 112 (2014), 151101].

The influence of dust grain porosity on the analysis of debris disc observations

Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution -- the so-called 'blowout size' -- is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of two. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, are barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend.

Strong correlations of neutron star radii with the slopes of nuclear matter incompressibility and symmetry energy at saturation

We examine the correlations of neutron star radii with the nuclear matter incompressibility, symmetry energy, and their slopes, which are the key parameters of the equation of state (EoS) of asymmetric nuclear matter. The neutron star radii and the EoS parameters are evaluated using a representative set of 24 Skyrme-type effective forces and 18 relativistic mean field models, and two microscopic calculations, all describing 2$M_\odot$ neutron stars. Unified EoSs for the inner-crust-core region have been built for all the phenomenological models, both relativistic and non-relativistic. Our investigation shows the existence of a strong correlation of the neutron star radii with the linear combination of the slopes of the nuclear matter incompressibility and the symmetry energy coefficients at the saturation density. Such correlations are found to be almost independent of the neutron star mass in the range $0.6\text{-}1.8M_{\odot}$. This correlation can be linked to the empirical relation existing between the star radius and the pressure at a nucleonic density between one and two times saturation density, and the dependence of the pressure on the nuclear matter incompressibility, its slope and the symmetry energy slope. The slopes of the nuclear matter incompressibility and the symmetry energy coefficients as estimated from the finite nuclei data yield the radius of a $1.4M_{\odot}$ neutron star in the range $11.09\text{-}12.86$ km.

Strong correlations of neutron star radii with the slopes of nuclear matter incompressibility and symmetry energy at saturation [Cross-Listing]

We examine the correlations of neutron star radii with the nuclear matter incompressibility, symmetry energy, and their slopes, which are the key parameters of the equation of state (EoS) of asymmetric nuclear matter. The neutron star radii and the EoS parameters are evaluated using a representative set of 24 Skyrme-type effective forces and 18 relativistic mean field models, and two microscopic calculations, all describing 2$M_\odot$ neutron stars. Unified EoSs for the inner-crust-core region have been built for all the phenomenological models, both relativistic and non-relativistic. Our investigation shows the existence of a strong correlation of the neutron star radii with the linear combination of the slopes of the nuclear matter incompressibility and the symmetry energy coefficients at the saturation density. Such correlations are found to be almost independent of the neutron star mass in the range $0.6\text{-}1.8M_{\odot}$. This correlation can be linked to the empirical relation existing between the star radius and the pressure at a nucleonic density between one and two times saturation density, and the dependence of the pressure on the nuclear matter incompressibility, its slope and the symmetry energy slope. The slopes of the nuclear matter incompressibility and the symmetry energy coefficients as estimated from the finite nuclei data yield the radius of a $1.4M_{\odot}$ neutron star in the range $11.09\text{-}12.86$ km.

Combined Nucleosynthetic Yields of Multiple First Stars

Modern numerical simulations of the formation of the first stars predict that the first stars formed in multiples. In those cases, the chemical yields of multiple supernova explosions may have contributed to the formation of a next generation star. We match the chemical abundances of the oldest observed stars in the universe to a database of theoretical supernova models, to show that it is likely that the first stars formed from the ashes of two or more progenitors.

Submillimeter polarization observation of the protoplanetary disk around HD 142527

We present the polarization observations toward the circumstellar disk around HD 142527 by using Atacama Large Millimeter/submillimeter Array (ALMA) at the frequency of 343 GHz. The beam size is $0.51 " \times 0.44 "$, which corresponds to the spatial resolution of $\sim$ 71 $\times$ 62 AU. The polarized intensity displays a ring-like structure with a peak located on the east side with a polarization fraction of $P= 3.26 \pm 0.02$ %, which is different from the peak of the continuum emission from the northeast region. The polarized intensity is significantly weaker at the peak of the continuum where $P= 0.220 \pm 0.010$ %. The polarization vectors are in the radial direction in the main ring of the polarized intensity, while there are two regions outside at the northwest and northeast areas where the vectors are in the azimuthal direction. If the polarization vectors represent the magnetic field morphology, the polarization vectors indicate the toroidal magnetic field configuration on the main ring and the poloidal fields outside. On the other hand, the flip of the polarization vectors is predicted by the self-scattering of thermal dust emission due to the change of the direction of thermal radiation flux. Therefore, we conclude that self-scattering of thermal dust emission plays a major role in producing polarization at millimeter wavelengths in this protoplanetary disk. Also, this puts a constraint on the maximum grain size to be approximately 150 ${\rm \mu m}$ if we assume compact spherical dust grains.

Top Argelander Stars: Pedagogy & Prize

Stellar astronomy, fueled by massive capital investments, advances in numerical modeling and theory, is resurgent and arguably is on the verge of a magnificent renaissance. Powerful time domain optical surveys, both on ground and in space, are producing data on variable stars on an unprecedented industrial scale. Those with deep knowledge of variable stars will stand to benefit from this resurgence. Notwithstanding these developments, in some astronomical communities, classical stellar astronomy has been in the doldrums. I offer a modest proposal to establish a basic level of familiarity with variable stellar phenomenology and an attractive scheme to make research in variable star astronomy visible, alluring and fashionable.

The radii and limb darkenings of Alpha Centauri A and B - Interferometric measurements with VLTI/PIONIER

The photospheric radius is one of the fundamental parameters governing the radiative equilibrium of a star. We report new observations of the nearest solar-type stars Alpha Centauri A (G2V) and B (K1V) with the VLTI/PIONIER optical interferometer. The combination of four configurations of the VLTI enable us to measure simultaneously the limb darkened angular diameter thetaLD and the limb darkening parameters of the two solar-type stars in the near-infrared H band (lambda = 1.65 microns). We obtain photospheric angular diameters of thetaLD(A) = 8.502 +/- 0.038 mas (0.43%) and thetaLD(B) = 5.999 +/- 0.025 mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of alpha(A) = 0.1404 +/- 0.0050 (3.6%) and alpha(B) = 0.1545 +/- 0.0044 (2.8%), which closely bracket the observed solar value (alpha_sun = 0.15027). Combined with the parallax pi = 747.17 +/- 0.61 mas recently determined, we derive linear radii of RA = 1.2234 +/- 0.0053 Rsun (0.43%) and RB = 0.8632 +/- 0.0037 Rsun (0.43%). The power law exponents that we derive for the two stars indicate a significantly weaker limb darkening than predicted by both 1D and 3D stellar atmosphere models. As this discrepancy is also observed on near-infrared limb darkening profile of the Sun, an improvement of the calibration of stellar atmosphere models is clearly needed. The reported PIONIER visibility measurements of Alpha Cen A and B provide a robust basis to validate the future evolutions of these models.

A new asteroseismic diagnostic for internal rotation in $\gamma$ Doradus stars

With four years of nearly-continuous photometry from Kepler, we are finally in a good position to apply asteroseismology to $\gamma$ Doradus stars. In particular several analyses have demonstrated the possibility to detect non-uniform period spacings, which have been predicted to be directly related to rotation. In the present work, we define a new seismic diagnostic for rotation in $\gamma$ Doradus stars that are too rapidly rotating to present rotational splittings. Based on the non uniformity of their period spacings, we define the observable $\Sigma$ as the slope of the period spacing when plotted as a function of period. We provide a one-to-one relation between this observable $\Sigma$ and the internal rotation, which applies widely in the instability strip of $\gamma$ Doradus stars. We apply the diagnostic to a handful of stars observed by Kepler. Thanks to g-modes in $\gamma$ Doradus stars, we are now able to determine the internal rotation of stars on the lower main sequence, which is still not possible for Sun-like stars.

Hubble Tarantula Treasury Project V. The star cluster Hodge 301: the old face of 30 Doradus

Based on color-magnitude diagrams (CMDs) from the Hubble Space Telescope Hubble Tarantula Treasury Project (HTTP) survey, we present the star formation history (SFH) of Hodge~301, the oldest star cluster in the Tarantula Nebula. The HTTP photometry extends faint enough to reach, for the first time, the cluster pre-main sequence (PMS) turn-on, where the PMS joins the main sequence. Using the location of this feature, along with synthetic CMDs generated with the latest PARSEC models, we find that Hodge~301 is older than previously thought, with an age between 26.5 and 31.5 Myr. From this age, we also estimate that between 38 and 61 supernovae Type-II exploded in the region. The same age is derived from the main sequence turn-off, whereas the age derived from the post-main sequence stars is younger and between 20 and 25 Myr. Other relevant parameters are a total stellar mass of $\approx 8800\,\pm 800$M$_{\odot}$ and average reddening E(B$-$V) $\approx 0.22-0.24$ mag, with a differential reddening $\delta$E(B$-$V)$\approx 0.04$ mag.

Inference of Heating Properties from "Hot" Non-flaring Plasmas in Active Region Cores. II. Nanoflare Trains

Despite its prediction over two decades ago, the detection of faint, high-temperature ("hot") emission due to nanoflare heating in non-flaring active region cores has proved challenging. Using an efficient two-fluid hydrodynamic model, this paper investigates the properties of the emission expected from repeating nanoflares (a nanoflare train) of varying frequency as well as the separate heating of electrons and ions. If the emission measure distribution ($\mathrm{EM}(T)$) peaks at $T = T_m$, we find that $\mathrm{EM}(T_m)$ is independent of details of the nanoflare train, and $\mathrm{EM}(T)$ above and below $T_m$ reflects different aspects of the heating. Below $T_m$ the main influence is the relationship of the waiting time between successive nanoflares to the nanoflare energy. Above $T_m$ power-law nanoflare distributions lead to an extensive plasma population not present in a monoenergetic train. Furthermore, in some cases characteristic features are present in $\mathrm{EM}(T)$. Such details may be detectable given adequate spectral resolution and a good knowledge of the relevant atomic physics. In the absence of such resolution we propose some metrics that can be used to infer the presence of "hot" plasma.

Close stellar conjunctions of alpha Centauri A and B until 2050 - An mK = 7.8 star may enter the Einstein ring of alpha Cen A in 2028

The rapid proper motion of the alpha Cen pair (3.7 arcsec/yr) and its location close to the galactic plane on a rich stellar background combine to make them excellent candidates for stellar conjunctions with distant stars. Adding new astrometry to archival data, we have refined the orbital parameters, barycentric proper motion and parallax of alpha Cen and compute its apparent trajectory on sky over the coming decades. We present a catalog of the expected close conjunctions until 2050. An exceptional event will take place in early May 2028, when alpha Cen A will come within 0.015 +/- 0.135 arcseconds of the mK = 7.8 star 2MASS 14392160-6049528 (hereafter S5). In terms of impact parameter and contrast, this is the most favorable stellar conjunction of alpha Cen within at least the next three decades. With an angular diameter of LD = 0.47 +/- 0.05 mas, it is likely that S5 is a red giant or supergiant located at several kiloparsecs. The approached stars will act as moving light probes in transmission through the environment of alpha Cen. The observation of these close conjunctions holds great promises to search for planets and other low mass objects in the alpha Cen system using photometry and astrometry. The relativistic deflection of the approached star images will be in the milliarcsecond range. The small impact parameter of the conjunction with S5 means that this star has a probability of 45% of entering the Einstein ring of alpha Cen A. The gravitational amplification of the flux of S5 could reach a factor five for the combination of the two lensed images. The proper motion, orbital parameters and parallax of alpha Cen will be measurable with an extreme accuracy from differential astrometry with the S stars. This will be valuable, for example to prepare the Breakthrough Starshot initiative to send interstellar nanocrafts to alpha Centauri.

Long-Term Trends In The Solar Wind Proton Measurements

We examine the long-term time evolution (1965-2015) of the relationships between solar wind proton temperature (Tp) and speed (Vp) and between the proton density (np) and speed using OMNI solar wind observations taken near Earth. We find a long-term decrease in the proton temperature-speed (Tp-Vp) slope that lasted from 1972 to 2010, but has been trending upward since 2010. Since the solar wind proton density-speed (np-Vp) relationship is not linear like the Tp-Vp relationship, we perform power law fits for np-Vp. The exponent (steepness in the np-Vp relationship) is correlated with the solar cycle. This exponent has a stronger correlation with current sheet tilt angle than with sunspot number because the sunspot number maxima vary considerably from cycle to cycle and the tilt angle maxima do not. To understand this finding, we examined the average np for different speed ranges, and found that for the slow wind np is highly correlated with the sunspot number with a lag of ~4 years. The fast wind np variation was less, but in phase with the cycle. This phase difference may contribute to the np-Vp exponent correlation with the solar cycle. These long-term trends are important since empirical formulas based on fits to Tp and Vp data are commonly used to identify ICMEs, but these formulas do not include any time dependence. Changes in the solar wind density over a solar cycle will create corresponding changes in the near Earth space environment and the overall extent of the heliosphere.

Toward High Precision Seismic Studies of White Dwarf Stars: Parametrization of the Core and Tests of Accuracy

We present a prescription for parametrizing the chemical profile in the core of white dwarfs in the light of the recent discovery that pulsation modes may sometimes be deeply confined in some cool pulsating white dwarfs. Such modes may be used as unique probes of the complicated chemical stratification that results from several processes that occurred in previous evolutionary phases of intermediate-mass stars. This effort is part of our ongoing quest for more credible and realistic seismic models of white dwarfs using static, parametrized equilibrium structures. Inspired from successful techniques developed in design optimization fields (such as aerodynamics), we exploit Akima splines for the tracing of the chemical profile of oxygen (carbon) in the core of a white dwarf model. A series of tests are then presented to better seize the precision and significance of the results that can be obtained in an asteroseismological context. We also show that the new parametrization passes an essential basic test, as it successfully reproduces the chemical stratification of a full evolutionary model.

Testing stellar evolution models with the retired A star HD 185351

The physical parameters of the retired A star HD 185351 were analysed in great detail by Johnson et al. (2014) using interferometry, spectroscopy and asteroseismology. Results from all independent methods are consistent with HD 185351 having a mass in excess of $1.5\mathrm{M}_{\odot}$. However, the study also showed that not all observational constraints could be reconciled in stellar evolutionary models, leading to mass estimates ranging from $\sim 1.6-1.9\mathrm{M}_{\odot}$ and casting doubts on the accuracy of stellar properties determined from asteroseismology. Here we solve this discrepancy and construct a theoretical model in agreement with all observational constraints on the physical parameters of HD 185351. The effects of varying input physics are examined as well as considering the additional constraint of the observed g-mode period spacing. This quantity is found to be sensitive to the inclusion of additional mixing from the convective core during the main sequence, and can be used to calibrate the overshooting efficiency using low-luminosity red giant stars. A theoretical model with metallicity $\left[\mathrm{Fe/H}\right]=0.16$dex, mixing-length parameter $\alpha_{\mathrm{MLT}}=2.00$, and convective overshooting efficiency parameter $f=0.030$ is found to be in complete agreement with all observational constraints for a stellar mass of $M\simeq1.60\mathrm{M}_{\odot}$.

Sparse Bayesian Inference and the Temperature Structure of the Solar Corona

Measuring the temperature structure of the solar atmosphere is critical to understanding how it is heated to high temperatures. Unfortunately, the temperature of the upper atmosphere cannot be observed directly, but must be inferred from spectrally resolved observations of individual emission lines that span a wide range of temperatures. Such observations are "inverted" to determine the distribution of plasma temperatures along the line of sight. This inversion is ill-posed and, in the absence of regularization, tends to produce wildly oscillatory solutions. We introduce the application of sparse Bayesian inference to the problem of inferring the temperature structure of the solar corona. Within a Bayesian framework a preference for solutions that utilize a minimum number of basis functions can be encoded into the prior and many ad hoc assumptions can be avoided. We demonstrate the efficacy of the Bayesian approach by considering a test library of 40 assumed temperature distributions.

Arcade Implosion Caused by a Filament Eruption in a Flare

Coronal implosions - the convergence motion of plasmas and entrained magnetic field in the corona due to a reduction in magnetic pressure - can help to locate and track sites of magnetic energy release or redistribution during solar flares and eruptions. We report here on the analysis of a well-observed implosion in the form of an arcade contraction associated with a filament eruption, during the C3.5 flare SOL2013-06-19T07:29. A sequence of events including magnetic flux-rope instability and distortion, followed by filament eruption and arcade implosion, lead us to conclude that the implosion arises from the transfer of magnetic energy from beneath the arcade as part of the global magnetic instability, rather than due to local magnetic energy dissipation in the flare. The observed net contraction of the imploding loops, which is found also in nonlinear force-free field extrapolations, reflects a permanent reduction of magnetic energy underneath the arcade. This event shows that, in addition to resulting in expansion or eruption of overlying field, flux-rope instability can also simultaneously implode unopened field due to magnetic energy transfer. It demonstrates the "partial opening of the field" scenario, which is one of the ways in 3D to produce a magnetic eruption without violating the Aly-Sturrock hypothesis. In the framework of this observation we also propose a unification of three main concepts for active region magnetic evolution, namely the metastable eruption model, the implosion conjecture, and the standard "CSHKP" flare model.

The HADES RV Programme with HARPS-N@TNG IV. Time resolved analysis of the Ca ii H&K and H{\alpha} chromospheric emission of low-activity early-type M dwarfs

M dwarfs are prime targets for planet search programs, particularly of those focused on the detection and characterization of rocky planets in the habitable zone. Understanding their magnetic activity is important because it affects our ability to detect small planets, and it plays a key role in the characterization of the stellar environment. We analyze observations of the Ca II H&K and H{\alpha} lines as diagnostics of chromospheric activity for low-activity early-type M dwarfs. We analyze the time series of spectra of 71 early-type M dwarfs collected for the HADES project for planet search purposes. The HARPS-N spectra provide simultaneously the H&K doublet and the H{\alpha} line. We develop a reduction scheme able to correct the HARPS-N spectra for instrumental and atmospheric effects, and to provide flux-calibrated spectra in units of flux at the stellar surface. The H&K and H{\alpha} fluxes are compared with each other, and their variability is analyzed. We find that the H and K flux excesses are strongly correlated with each other, while the H{\alpha} flux excess is generally less correlated with the H&K doublet. We also find that H{\alpha} emission does not increase monotonically with the H&K line flux, showing some absorption before being filled in by chromospheric emission when H&K activity increases. Analyzing the time variability of the emission fluxes, we derive a tentative estimate of the rotation period (of the order of a few tens of days) for some of the program stars, and the typical lifetime of chromospheric active regions (a few stellar rotations). Our results are in good agreement with previous studies. In particular, we find evidence that the chromospheres of early-type M dwarfs could be characterized by different filaments coverage, affecting the formation mechanism of the H{\alpha} line. We also show that chromospheric structure is likely related to spectral type.

The HADES RV Programme with HARPS-N@TNG. III. Flux-flux and activity-rotation relationships of early-M dwarfs

(Abridged) Understanding stellar activity in M dwarfs is crucial for the physics of stellar atmospheres as well as for ongoing radial velocity exoplanet programmes. Despite the increasing interest in M dwarfs, our knowledge of the chromospheres of these stars is far from being complete. We aim to test whether the relations between activity, rotation, and stellar parameters and flux-flux relationships also hold for early-M dwarfs on the main-sequence. We analyse in an homogeneous and coherent way a well defined sample of 71 late-K/early-M dwarfs that are currently being observed in the framework of the HArps-n red Dwarf Exoplanet Survey (HADES). Rotational velocities are derived using the cross-correlation technique while emission flux excesses in the Ca II H & K and Balmer lines from Halpha up to Hepsilon are obtained by using the spectral subtraction technique. The relationships between the emission excesses and the stellar parameters are studied. Relations between pairs of fluxes of different chromospheric lines are also studied. We find that the strength of the chromospheric emission in the Ca II H & K and Balmer lines is roughly constant for stars in the M0-M3 spectral range. Our data suggest that a moderate but significant correlation between activity and rotation might be present as well as a hint of kinematically selected young stars showing higher levels of emission. We find our sample of M dwarfs to be complementary in terms of chromospheric and X-ray fluxes with those of the literature, extending the analysis of the flux-flux relationships to the very low flux domain. Our results agree with previous works suggesting that the activity-rotation-age relationship known to hold for solar-type stars also applies to early-M dwarfs. We also confirm previous findings that the field stars which deviate from the bulk of the empirical flux-flux relationships show evidence of youth.

NLTE Analysis of High Resolution H-band Spectra. II. Neutral Magnesium

Aiming at testing the validity of our magnesium atomic model and investigating the effects of non-local thermodynamical equilibrium (NLTE) on the formation of the H-band neutral magnesium lines, we derive the differential Mg abundances from selected transitions for 13 stars either adopting or relaxing the assumption of local thermodynamical equilibrium (LTE). Our analysis is based on high-resolution and high signal-to-noise ratio H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and optical spectra from several instruments. The absolute differences between the Mg abundances derived from the two wavelength bands are always less than 0.1 dex in the NLTE analysis, while they are slightly larger for the LTE case. This suggests that our Mg atomic model is appropriate for investigating the NLTE formation of the H-band Mg lines. The NLTE corrections for the Mg I H-band lines are sensitive to the surface gravity, becoming larger for smaller log g values, and strong lines are more susceptible to departures from LTE. For cool giants, NLTE corrections tend to be negative, and for the strong line at 15765 \AA\ they reach -0.14 dex in our sample, and up to -0.22 dex for other APOGEE stars. Our results suggest that it is important to include NLTE corrections in determining Mg abundances from the H-band Mg I transitions, especially when strong lines are used.

NLTE Analysis of High Resolution H-band Spectra. I. Neutral Silicon

We investigated the reliability of our silicon atomic model and the influence of non-local thermodynamical equilibrium (NLTE) on the formation of neutral silicon (Si I) lines in the near-infrared (near-IR) H-band. We derived the differential Si abundances for 13 sample stars with high-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), as well as from optical spectra, both under local thermodynamical equilibrium (LTE) and NLTE conditions. We found that the differences between the Si abundances derived from the H-band and from optical lines for the same stars are less than 0.1 dex when the NLTE effects included, and that NLTE reduces the line-to-line scatter in the H-band spectra for most sample stars. These results suggest that our Si atomic model is appropriate for studying the formation of H-band Si lines. Our calculations show that the NLTE corrections of the Si I H-band lines are negative, i.e. the final Si abundances will be overestimated in LTE. The corrections for strong lines depend on surface gravity, and tend to be larger for giants, reaching ~ -0.2 dex in our sample, and up to ~ -0.4 dex in extreme cases of APOGEE targets. Thus, the NLTE effects should be included in deriving silicon abundances from H-band Si I lines, especially for the cases where only strong lines are available.

The 2012 July 23 Backside Eruption: An Extreme Energetic Particle Event?

The backside coronal mass ejection (CME) of 2012 July 23 had a short Sun to Earth shock transit time (18.5 hours). The associated solar energetic particle (SEP) event had a >10 MeV proton flux peaking at ~5000 pfu, and the energetic storm particle (ESP) event was an order of magnitude larger, making it the most intense event in the space era at these energies. By a detailed analysis of the CME, shock, and SEP characteristics, we find that the July 23 event is consistent with a high-energy SEP event (accelerating particles to GeV energies). The time of maximum and fluence spectra in the range 10-100 MeV were very hard, similar to those of ground level enhancement (GLE) events. We found a hierarchical relationship between the CME initial speeds and the fluence spectral indices: CMEs with low initial speeds had SEP events with the softest spectra, while those with highest initial speeds had SEP events with the hardest spectra. CMEs attaining intermediate speeds result in moderately hard spectra. The July 23 event was in the group of hard-spectrum events. During the July 23 event, the shock speed (>2000 km s-1), the initial acceleration (~1.70 km s-2), and the shock formation height (~1.5 solar radii) were all typical of GLE events. The associated type II burst had emission components from metric to kilometric wavelengths suggesting a strong shock. These observation confirm that the 2012 July 23 event is likely to be an extreme event in terms of the energetic particles it accelerated.

SCExAO and GPI $YJH$ Band Photometry and Integral Field Spectroscopy of the Young Brown Dwarf Companion to HD 1160

We present high signal-to-noise ratio, precise $YJH$ photometry and $Y$ band (\gpiwave~$\mu$m) spectroscopy of HD 1160 B, a young substellar companion discovered from the Gemini NICI Planet Finding Campaign, using the Subaru Coronagraphic Extreme Adaptive Optics instrument and the Gemini Planet Imager. HD 1160 B has typical mid-M dwarf-like infrared colors and a spectral type of M5.5$^{+1.0}_{-0.5}$, where the blue edge of our $Y$ band spectrum rules out earlier spectral types. Atmospheric modeling suggests HD 1160 B having an effective temperature of 3000--3100 $K$, a surface gravity of log $g$ = 4--4.5, a radius of~\bestfitradius~$R_{\rm J}$, and a luminosity of log $L$/$L_{\odot} = -2.76 \pm 0.05$. Neither the primary's Hertzspring-Russell diagram position nor atmospheric modeling of HD 1160 B show evidence for a sub-solar metallicity. The interpretation of the HD 1160 B depends on which stellar system components are used to estimate an age. Considering HD 1160 A, B and C jointly, we derive an age of 80--125 Myr, implying that HD 1160 B straddles the hydrogen-burning limit (70--90 $M_{\rm J}$). If we consider HD 1160 A alone, younger ages (20--125 Myr) and a brown dwarf-like mass (35--90 $M_{\rm J}$) are possible. Interferometric measurements of the primary, a precise GAIA parallax, and moderate resolution spectroscopy can better constrain the system's age and how HD 1160 B fits within the context of (sub)stellar evolution.

The Habitability of Planets Orbiting M-dwarf Stars

The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sun-like stars. Over the past decade, significant progress has been made by both space- and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky planets, with roughly a third of these rocky M-dwarf planets orbiting within the habitable zone, where they have the potential to support liquid water on their surfaces. Theoretical studies have also quantified the effect on climate and habitability of the interaction between the spectral energy distribution of M-dwarf stars and the atmospheres and surfaces of their planets. These and other recent results fill in knowledge gaps that existed at the time of the previous overview papers published nearly a decade ago by Tarter et al. (2007) and Scalo et al. (2007). In this review we provide a comprehensive picture of the current knowledge of M-dwarf planet occurrence and habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field.

The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity

We present the results of our survey of 1612 MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud. We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from IR stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust driven winds: $v_{exp} \propto Z L^{0.4}$. We compare the sub-solar metallicity LMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic Bulge. For 8 of the Bulge stars we derive pulsation periods for the first time, using near-IR photometry from the VVV survey. We have modeled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass loss rates by scaling the models to the results from maser profiles. We have done this also for samples in the Galactic Centre and Bulge and derived a new mass loss prescription that includes luminosity, pulsation period, and gas-to-dust ratio $\dot{M} = 1.06^{+3.5}_{-0.8} \rm{ \cdot }10^{-5}\,(L/10^4\,\rm{L}_\odot)^{0.9\pm0.1}(P/500\,\rm{d})^{0.75\pm0.3} (r_{gd}/200)^{-0.03\pm0.07}\,\rm{M_{\odot}}\, yr^{-1}$. The tightest correlation is found between mass loss rate and luminosity. We find that the gas-to-dust ratio has little effect on the mass loss of oxygen-rich AGB stars and RSGs within the Galaxy and the LMC. This suggests that mass loss of oxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a half and twice solar.

Assessing magnetic torques and energy fluxes in close-in star-planet systems

Planets in close-in orbit interact with the magnetized wind of their hosting star. This magnetic interaction was proposed to be a source for enhanced emissions in the chromosphere of the star, and to participate in setting the migration time-scale of the close-in planet. The efficiency of the magnetic interaction is know to depend on the magnetic properties of the host star, of the planet, and on the magnetic topology of the interaction. We use a global, three-dimensional numerical model of close-in star planet systems, based on the magnetohydrodynamics approximation, to compute a grid of simulations for varying properties of the orbiting planet. We propose a simple parametrization of the magnetic torque that applies to the planet, and of the energy flux generated by the interaction. The dependancy upon the planet properties and the wind properties are clearly identified in the derived scaling laws, which can be used in secular evolution codes to take into account the effect of magnetic interactions in planet migration. They can also be used to estimate a potential magnetic source of enhanced emissions in observed close-in star-planet systems, in order to constrain observationally possible exoplanetary magnetic fields.

The OmegaWhite survey for Short-Period Variable Stars III: Follow-up Photometric and Spectroscopic Observations

We present photometric and spectroscopic follow-up observations of short-period variables discovered in the OmegaWhite survey: a wide-field high-cadence g-band synoptic survey targeting the Galactic Plane. We have used fast photometry on the SAAO 1.0-m and 1.9-m telescopes to obtain light curves of 27 variables, and use these results to validate the period and amplitude estimates from the OmegaWhite processing pipeline. Furthermore, 57 sources (44 unique, 13 also with new light curves) were selected for spectroscopic follow-up using either the SAAO 1.9-m telescope or the Southern African Large Telescope. We find many of these variables have spectra which are consistent with being delta Scuti type pulsating stars. At higher amplitudes, we detect four possible pulsating white dwarf/subdwarf sources and an eclipsing cataclysmic variable. Due to their rarity, these targets are ideal candidates for detailed follow-up studies. From spectroscopy, we confirm the symbiotic binary star nature of two variables identified as such in the SIMBAD database. We also report what could possibly be the first detection of the `Bump Cepheid' phenomena in a delta Scuti star, with OW J175848.21-271653.7 showing a pronounced 22% amplitude dip lasting 3 minutes during each pulsational cycle peak. However, the precise nature of this target is still uncertain as it exhibits the spectral features of a B-type star.

Production and Distribution of 44Ti and 56Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A [Cross-Listing]

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae (SNe) can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive 44Ti with high-energy satellite telescopes (NuSTAR, INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in Supernova 1987A, and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here, we discuss a three-dimensional (3D) simulation of a neutrino-driven explosion, using a parametrized neutrino engine, whose 44Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron-star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither progenitor nor explosion were fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high 44Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions genericly eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass-ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tug-boat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly "one-sided" Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.

Production and Distribution of 44Ti and 56Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae (SNe) can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive 44Ti with high-energy satellite telescopes (NuSTAR, INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in Supernova 1987A, and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here, we discuss a three-dimensional (3D) simulation of a neutrino-driven explosion, using a parametrized neutrino engine, whose 44Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron-star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither progenitor nor explosion were fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high 44Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions genericly eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass-ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tug-boat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly "one-sided" Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.

Production and Distribution of 44Ti and 56Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A [Cross-Listing]

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae (SNe) can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive 44Ti with high-energy satellite telescopes (NuSTAR, INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in Supernova 1987A, and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here, we discuss a three-dimensional (3D) simulation of a neutrino-driven explosion, using a parametrized neutrino engine, whose 44Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron-star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither progenitor nor explosion were fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high 44Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions genericly eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass-ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tug-boat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly "one-sided" Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.

A revised moving cluster distance to the Pleiades open cluster

Aims: We provide a new distance estimate for the Pleiades based on the moving cluster method, which will be useful to further discuss the so-called Pleiades distance controversy and compare it with the very precise parallaxes from the Gaia space mission. Methods: We apply a refurbished implementation of the convergent point search method to an updated census of Pleiades stars to calculate the convergent point position of the cluster from stellar proper motions. Then, we derive individual parallaxes for 64 cluster members using radial velocities compiled from the literature, and approximate parallaxes for another 1146 stars based on the spatial velocity of the cluster. This represents the largest sample of Pleiades stars with individual distances to date. Results: The parallaxes derived in this work are in good agreement with previous results obtained in different studies (excluding Hipparcos) for individual stars in the cluster. We report a mean parallax of $7.44\pm 0.08$~mas and distance of $134.4^{+2.9}_{-2.8}$pc that is consistent with the weighted mean of $135.0\pm 0.6$pc obtained from the non-Hipparcos results in the literature. Conclusions: Our result for the distance to the Pleiades open cluster is not consistent with the Hipparcos catalog, but favors the recent and more precise distance determination of $136.2\pm 1.2$pc obtained from Very Long Baseline Interferometry observations. It is also in good agreement with the mean distance of $133\pm 5$pc obtained from the first trigonometric parallaxes delivered by the Gaia satellite for the brightest cluster members in common with our sample.

Explosion Dynamics of Parametrized Spherically Symmetric Core-Collapse Supernova Simulations

We report on a method, PUSH, for triggering core-collapse supernova (CCSN) explosions of massive stars in spherical symmetry. This method provides a framework to study many important aspects of core collapse supernovae: the effects of the shock passage through the star, explosive supernova nucleosynthesis and the progenitor-remnant connection. Here we give an overview of the method, compare the results to multi-dimensional simulations and investigate the effects of the progenitor and the equation of state on black hole formation.

Understanding the photometric variability of {\zeta} Ori Aa

We studied the variability of the magnetic O-type supergiant $\zeta$ Ori Aa using multi-colour BRITE photometry. We confirmed the known rotation frequency $f_{\rm rot} = 0.15 \pm 0.02$ c/d, and detected some of its higher harmonics, of which $4f_{\rm rot}$ is compatible with the known DAC recurrence timescale. Thanks to simultaneous high-resolution CHIRON spectroscopy, we could identify another frequency $f_{\rm env} = 0.10 \pm 0.02$ c/d, caused by the circumstellar environment. Variations in the circumstellar environment are believed to cause the observed difference between the BRITE lightcurves.

Preparing and correcting extracted BRITE observations

Extracted BRITE lightcurves must be carefully prepared and corrected for instrumental effects before a scientific analysis can be performed. Therefore, we have created a suite of Python routines to prepare and correct the lightcurves, which is publicly available. In this paper we describe the method and successive steps performed by these routines.

Clumpy molecular structures revolving the B[e] supergiant MWC 137

The peculiar emission-line star MWC 137 with its extended optical nebula was recently classified as B[e] supergiant. To study the spatial distribution of its circumstellar molecular gas on small and large scales, we obtained near-infrared and radio observations using SINFONI and APEX, respectively. We find that the hot CO gas is arranged in moving clumpy ring and shell structures close to the star, while a cold CO envelope is encircling the borders of the optical nebula from the south to the west.

A possible mechanism for over luminous type Ia supernovae explosions inspired by dark matter [Cross-Listing]

Dark matter is believed to be a major component of our universe. In this paper we propose a new mechanism based on dark matter inspired super-Chandrasekhar mass white dwarf to explain the recent observation of super luminous type Ia supernovae explosions. Our calculation shows when a white dwarf accretes enough dark matter, due to the Pauli exclusive principle between fermionic dark matter particles, the mass of corresponding dark white dwarf (which means the white dwarf mixed with dark matters) can significantly exceeds the Chandrasekhar limit. Moreover, we investigate some physical observable quantities, such as the redshift and moment of inertia of the dark white dwarf and found that these quantities are sensitive to the dark matter particle's distributions and thus can be potentially used to probe the relevant information of dark matter particles in the future.

A possible mechanism for over luminous type Ia supernovae explosions inspired by dark matter

Dark matter is believed to be a major component of our universe. In this paper we propose a new mechanism based on dark matter inspired super-Chandrasekhar mass white dwarf to explain the recent observation of super luminous type Ia supernovae explosions. Our calculation shows when a white dwarf accretes enough dark matter, due to the Pauli exclusive principle between fermionic dark matter particles, the mass of corresponding dark white dwarf (which means the white dwarf mixed with dark matters) can significantly exceeds the Chandrasekhar limit. Moreover, we investigate some physical observable quantities, such as the redshift and moment of inertia of the dark white dwarf and found that these quantities are sensitive to the dark matter particle's distributions and thus can be potentially used to probe the relevant information of dark matter particles in the future.

The pre-penumbral magnetic canopy in the solar atmosphere

Penumbrae are the manifestation of magnetoconvection in highly inclined (to the vertical direction) magnetic field. The penumbra of a sunspot tends to form, initially, along the arc of the umbra antipodal to the main region of flux emergence. The question of how highly inclined magnetic field can concentrate along the antipodal curves of umbrae, at least initially, remains to be answered. Previous observational studies have suggested the existence of some form of overlying magnetic canopy which acts as the progenitor for penumbrae. We propose that such overlying magnetic canopies are a consequence of how the magnetic field emerges into the atmosphere and are, therefore, part of the emerging region. We show, through simulations of twisted flux tube emergence, that canopies of highly inclined magnetic field form preferentially at the required locations above the photosphere.

What's the nature of sdA stars?

White dwarfs with log g lower than 7.0 are called Extremely Low Mass white dwarfs (ELMs). They were first found as companions to pulsars, then to other white dwarfs and main sequence stars (The ELM Survey: 2010 to 2016), and can only be formed in interacting binaries in the age of the Universe. In our SDSS DR12 white dwarf catalog (Kepler et al. 2016), we found a few thousand stars in the effective temperature and surface gravity ranges attributed to ELMs. We have called these objects sdAs, alluding to their narrow hydrogen line spectra showing sub-main sequence log g. One possible explanation for the sdAs is that they are ELMs. Increasing the ELMs sample would help constrain the number of close binaries in the Galaxy. Interestingly, if they turn out to be A stars with an overestimated log g, the distance modulus would put these young stars in the Galaxy's halo.

Kinetic temperature of massive star forming molecular clumps measured with formaldehyde

For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are strongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature.We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps.Three 218 GHz transitions (JKAKC=303-202, 322-221, and 321-220) of para-H2CO were observed with the 15m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H2CO 322-221/303-202and 321-220/303-202 ratios. The gas kinetic temperatures derived from the para-H2CO (321-220/303-202) line ratios range from 30 to 61 K with an average of 46 K. A comparison of kinetic temperature derived from para-H2CO, NH3, and the dust emission indicates that in many cases para-H2CO traces a similar kinetic temperature to the NH3 (2,2)/(1,1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H2CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H2CO trace turbulence to a higher degree than NH3 (2,2)/(1,1) in the massive clumps. The non-thermal velocity dispersions of para-H2CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.

Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune

In an atmosphere, a cloud condensation region is characterized by a strong vertical gradient in the abundance of the related condensing species. On Earth, the ensuing gradient of mean molecular weight has relatively few dynamical consequences because N$_2$ is heavier than water vapor, so that only the release of latent heat significantly impacts convection. On the contrary, in an hydrogen dominated atmosphere (e.g. giant planets), all condensing species are significantly heavier than the background gas. This can stabilize the atmosphere against convection near a cloud deck if the enrichment in the given species exceeds a critical threshold. This raises two questions. What is transporting energy in such a stabilized layer, and how affected can the thermal profile of giant planets be? To answer these questions, we first carry out a linear analysis of the convective and double-diffusive instabilities in a condensable medium showing that an efficient condensation can suppress double-diffusive convection. This suggests that a stable radiative layer can form near a cloud condensation level, leading to an increase in the temperature of the deep adiabat. Then, we investigate the impact of the condensation of the most abundant species---water---with a steady-state atmosphere model. Compared to standard models, the temperature increase can reach several hundred degrees at the quenching depth of key chemical tracers. Overall, this effect could have many implications for our understanding of the dynamical and chemical state of the atmosphere of giant planets, for their future observations (with Juno for example), and for their internal evolution.

On the East-West Longitudinally Asymmetric Distribution of Solar Proton Events [Cross-Listing]

A large data set of 78 solar proton events observed near the Earth's orbit during 1996-2011 is investigated. An East-West longitudinal (azimuthal) asymmetry is found to exist in the distribution of flare sources of solar proton events. With the same longitudinal separation between the flare sources and the magnetic field line footpoint of observer, the number of the solar proton events originating from solar sources located on the eastern side of the nominal magnetic footpoint of observer is larger than the number of the solar proton events from solar sources located on the western side. We emphasize the importance of this statistical investigation in two aspects. On the one hand, this statistical finding confirms our previous simulation results obtained by numerically solving five-dimensional Fokker-Planck equation of solar energetic particle (SEP) transport. On the other hand, the East-West longitudinally (azimuthally) asymmetric distribution of solar proton events accumulated over a long time period provides an observational evidence for the effects of perpendicular diffusion on the SEP propagation in the heliosphere. We further point out that, in the sense of perpendicular diffusion, our numerical simulations and statistical results of SEP events confirm each other. We discuss in detail the important effects of perpendicular diffusion on the formation of the East-West azimuthal (longitudinal) asymmetry of SEP distribution in two physical scenarios, i.e., 'multiple SEP events with one spacecraft' and 'one SEP event with multiple spacecraft'. A functional relation I_{max}(r)=kr^{-1.7} quantifying the radial dependence of SEP peak intensities is obtained and utilized in the analysis of physical mechanism. The relationship between our results and those of Dresing et al. is also discussed.

First observations of W Virginis stars with K2: detection of period doubling

We present the first analysis of W Vir stars observed by the Kepler space telescope in the K2 mission. Clear cycle-to-cycle variation were detected in the light curves of KT Sco and the globular cluster member M80-V1. While the variations in the former star seems to be irregular on the short time scale of the K2 data, the latter appears to experience period doubling in its pulsation. Ground-based colour data confirmed that both stars are W Vir-type pulsators, while a comparison with historical photometric time-series data revealed drastic period changes in both stars. For comparison we reexamine ground-based observations of W Vir, the prototype of the class, and conclude that it shows period doubling instead of mode beating. These results support the notion that nonlinear dynamics plays an important role in the pulsation of W Virginis-type stars.

Constraints of the physics of low-mass AGB stars from CH and CEMP stars

We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis usingmodel atmospheres, and thus, they represent the largest homogeneous abundance data available for CH stars up to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass AGB stars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enriched surfaces. We discuss the effects induced on the surface AGB s-process distributions by different prescriptions for convection and rotation. Our reference theoretical FRUITY set fits only part of the observations. Moreover, the s-process observational spread for a fixed metallicity cannot be reproduced. At Fe/H]>-1, a good fit is found when rotation and a different treatment of the inner border of the convective envelope are simultaneously taken into account. In order to increase the statistics at low metallicities, we include in our analysis a selected number of CEMP stars and, therefore, we compute additional AGB models down to [Fe/H]=-2.85. Our theoretical models are unable to attain the large [hs/ls] ratios characterizing the surfaces of those objects. We speculate on the reasons for such a discrepancy, discussing the possibility that the observed distribution is a result of a proton mixing episode leading to a very high neutron density (the so-called i-process)

Photospheric carbon and oxygen abundances of F-G type stars in the Pleiades cluster [Replacement]

In order to investigate the carbon-to-oxygen ratio of the young open cluster M45 (Pleiades), the C and O abundances of selected 32 F-G type dwarfs (in the effective temperature range of Teff~5800-7600K and projected rotational velocity range of vesini~10-110km/s) belonging to this cluster were determined by applying the synthetic spectrum-fitting technique to C I 5380 and O I 6156-8 lines. The non-LTE corrections for these C I and O I lines were found to be practically negligible (less than a few hundredths dex). The resulting C and O abundances (along with the Fe abundance) turned out nearly uniform without any systematic dependence upon Teff or vesini. We found, however, in spite of almost solar Fe abundance ([Fe/H]~0), carbon turned out to be slightly subsolar ([C/H]~-0.1) while oxygen slightly supersolar ([O/H]~+0.1). This lead to a conclusion that [C/O] ratio was moderately subsolar (~-0.2) in the primordial gas from which these Pleiades stars were formed ~120--130 Myr ago. Interestingly, similarly young B-type stars are reported to show just the same result ([C/O]~-0.2), while rather aged (~1-10 Gyr) field F-G stars of near-solar metallicity yield almost the solar value ([C/O]~0) on the average. Such a difference in the C/O ratio between two star groups of distinctly different ages may be explained as a consequence of orbit migration mechanism which Galactic stars may undergo over a long time.