- ID:
- ivo://CDS.VizieR/VI/156
- Title:
- M-dwarf Lum-Temp-Radius relationships
- Short Name:
- VI/156
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- There is growing evidence that M-dwarf stars suffer radius inflation when compared to theoretical models, suggesting that models are missing some key physics required to completely describe stars at effective temperatures (TSED) less than about 4000K. The advent of Gaia DR2 distances finally makes available large datasets to determine the nature and extent of this effect.We employ an all-sky sample, comprising of >15000 stars, to determine empirical relation-ships between luminosity, temperature and radius.This is accomplished using only geometric distances and multiwave-band photometry, by utilising a modified spectral energy distribution fitting method. The radii we measure show an inflation of 3-7% compared to models, but nomore than a 1-2% intrinsic spread in the inflated sequence. We show that we are currently able to determine M-dwarf radii to an accuracy of 2.4% using our method. However, we determine that this is limited by the precision of metallicity measurements, which contribute 1.7% to the measured radius scatter. We also present evidence that stellar magnetism is currently unable to explain radius inflation in M-dwarfs.
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- ID:
- ivo://CDS.VizieR/J/MNRAS/463/1844
- Title:
- M dwarfs rotation-activity relation
- Short Name:
- J/MNRAS/463/1844
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We study the relation between stellar rotation and magnetic activity for a sample of 134 bright, nearby M dwarfs observed in the Kepler Two-Wheel (K2) mission during campaigns C0 to C4. The K2 lightcurves yield photometrically derived rotation periods for 97 stars (79 of which without previous period measurement), as well as various measures for activity related to cool spots and flares. We find a clear difference between fast and slow rotators with a dividing line at a period of ~10d at which the activity level changes abruptly. All photometric diagnostics of activity (spot cycle amplitude, flare peak amplitude and residual variability after subtraction of spot and flare variations) display the same dichotomy, pointing to a quick transition between a high-activity mode for fast rotators and a low-activity mode for slow rotators. This unexplained behavior is reminiscent of a dynamo mode-change seen in numerical simulations that separates a dipolar from a multipolar regime. A substantial number of the fast rotators are visual binaries. A tentative explanation is accelerated disk evolution in binaries leading to higher initial rotation rates on the main-sequence and associated longer spin-down and activity lifetimes. We combine the K2 rotation periods with archival X-ray and UV data. X-ray, FUV and NUV detections are found for 26, 41, and 11 stars from our sample, respectively. Separating the fast from the slow rotators, we determine for the first time the X-ray saturation level separately for early- and for mid-M stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/794/146
- Title:
- M dwarfs with IR excess
- Short Name:
- J/ApJ/794/146
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) spectroscopic catalog, we searched the WISE AllWISE catalog to investigate the occurrence of warm dust, as inferred from IR excesses, around field M dwarfs (dMs). We developed SDSS/WISE color selection criteria to identify 175 dMs (from 70841) that show IR flux greater than the typical dM photosphere levels at 12 and/or 22{mu}m, including seven new stars within the Orion OB1 footprint. We characterize the dust populations inferred from each IR excess and investigate the possibility that these excesses could arise from ultracool binary companions by modeling combined spectral energy distributions. Our observed IR fluxes are greater than levels expected from ultracool companions (>3{sigma}). We also estimate that the probability the observed IR excesses are due to chance alignments with extragalactic sources is <0.1%. Using SDSS spectra we measure surface gravity-dependent features (K, Na, and CaH 3) and find <15% of our sample indicates low surface gravities. Examining tracers of youth (H{alpha}, UV fluxes, and Li absorption), we find <3% of our sample appear young, indicating we are observing a population of field stars >~1Gyr, likely harboring circumstellar material. We investigate age-dependent properties probed by this sample, studying the disk fraction as a function of Galactic height. The fraction remains small and constant to |Z|~700pc and then drops, indicating little to no trend with age. Possible explanations for disks around field dMs include (1) collisions of planetary bodies, (2) tidal disruption of planetary bodies, or (3) failed planet formation.
- ID:
- ivo://CDS.VizieR/J/A+A/635/A161
- Title:
- Measured atmospheric parameters of NGC6397 stars
- Short Name:
- J/A+A/635/A161
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recent work has used spectra of ~5000 stars in NGC 6397 that were extracted from a MUSE mosaic to determine the atmospheric parameters for these stars by fitting the spectra against the Goettingen Spectral Library. A significant change in metallicity between the turn off and the red giant branch was found and was discussed as a possible manifestation of predicted effects of atomic diffusion. However, the small amplitude of the effect and inconsistency with earlier measurements call for more attention before this result is interpreted. Systematic effects due to the interpolation or to the synthetic spectra cannot be ruled out at this level of precision. We reanalyze the data with : the ELODIE and MILES reference libraries in order to assess the robustness of the result. These empirical libraries have a finer metallicity coverage down to approximately the cluster metallicity turn-off. Methods. We used the ULySS full-spectrum fitting package, together with the library interpolators to remeasure the three atmospheric parameters effective temperature, surface gravity, and [Fe/H] metallicity. We find a very low [Fe/H] dispersion along the isochrone (0.07dex), consistent with our error estimate (0.05dex). However, the [Fe/H] trend is not reproducible. This shows that the data have the potential to reveal patterns of the magnitude of the expected physical effects, but the analysis methods need to be refined to cull systematic effects that currently dominate the patterns.
- ID:
- ivo://CDS.VizieR/J/A+A/637/A43
- Title:
- Membership in the Cepheus association
- Short Name:
- J/A+A/637/A43
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Young field stars are hardly distinguishable from older ones because their space motion rapidly mixes them with the stellar population of the Galactic plane.Nevertheless, a careful target selection allows for young stars to be spotted throughout the sky. We aim to identify additional sources associated with the four young comoving stars that we discovered towards the CO Cepheus void and to provide a comprehensive view of the Cepheus association. Based on multivariate analysis methods, we have built an extended sample of 193 young star candidates, which are the optical and infrared counterparts of ROSAT All-Sky Survey and XMM-Newton X-ray sources. From optical spectroscopic observations, we measured their radial velocity with the cross-correlation technique. We derived their atmospheric parameters and projected rotational velocity with the code ROTFIT. We applied the subtraction of inactive templates to measure the lithium equivalent width, from which we infer their lithium abundance and age. Finally, we studied their kinematics using the second Gaia data release. Our sample is mainly composed of young or active stars and multiple systems. We identify two distinct populations of young stars that are spatially and kinematically separated. Those with an age between 100 and 300Myr are mostly projected towards the Galactic plane. In contrast, 23 of the 37 sources younger than 30Myr are located in the CO Cepheus void, and 21 of them belong to the stellar kinematic group that we previously reported in this sky area. We report a total of 32 bona fide members and nine candidates for this nearby (distance=157+/-10pc) young (age=10-20Myr) stellar association. According to the spatial distribution of its members, the original cluster is already dispersed and partially mixed with the local population of the Galactic plane.
- ID:
- ivo://CDS.VizieR/J/A+A/573/A13
- Title:
- Metal abundances in 6 globular cluster
- Short Name:
- J/A+A/573/A13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have observed almost one third of the globular clusters in the Milky Way, targeting distant and/or highly reddened objects, as well as a few reference clusters. A large sample of red giant stars was observed with FORS2 at VLT/ESO at R~2000. The method for deriving stellar parameters is presented with application to six reference clusters. We aim at deriving the stellar parameters effective temperature, gravity, metallicity, and {alpha}-element enhancement as well as the radial velocity to confirm the membership of individual stars in each cluster. We analyse the spectra collected for the reference globular clusters NGC 6528 ([Fe/H]~-0.1), NGC 6553 ([Fe/H]~-0.2), M 71 ([Fe/H]~-0.8), NGC 6558 ([Fe/H]~-1.0), NGC 6426 ([Fe/H]~-2.1), and Terzan 8 ([Fe/H]~-2.2). They cover the full range of globular cluster metallicities, and are located in the bulge, disc, and halo. Full spectrum-fitting techniques were applied by comparing each target spectrum with a stellar library in the optical region at 4560-5860{AA}. We employed the library of observed spectra MILES, and the Coelho synthetic library. We validated the method by recovering the known atmospheric parameters for 49 well-studied stars that cover a wide range in the parameter space. We adopted as final stellar parameters (effective temperatures, gravities, metallicities) the average of results using the observed and synthetic spectral libraries.
- ID:
- ivo://CDS.VizieR/J/A+A/653/L8
- Title:
- Metallicities on multiple MSs of Omega Centauri
- Short Name:
- J/A+A/653/L8
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Omega Cen is a rare example of a globular cluster where the iron abundance of the stars spans more than one order of magnitude. Many spectroscopic investigations of its red-giant- and sub-giant- branches have revealed multiple peaks in the iron abundance distribution. The metallicity distribution of main-sequence (MS) stars is not well characterized yet, due to the faintness of the stars and lack of data. So far, almost all studies of MS stars are based on photometric measurements. Our goal is to investigate the metallicity distribution of a statistically significant sample of MS stars in Omega Cen. In particular, we aim at revisiting the metallicity difference between the red and blue MS of the cluster. We use MUSE spectra obtained for the central region of omega Cen to derive metallicities for ~4200 MS stars. We find that blue MS stars are on average ~0.1dex more metal-rich than their red counterparts. On the basis of this new estimate, we find that the two sequences can be fit on the Hubble Space Telescope color-magnitude diagram with two isochrones having the same global metallicity and age but a higher helium abundance for the blue MS, i.e. {DELTA}Y~<0.1. Furthermore, we determine the average metallicity of the five main populations along Omega Cen MS and these estimates are consistent with expectations from previous photometric studies.
- ID:
- ivo://CDS.VizieR/J/A+A/551/A112
- Title:
- Metallicity-giant planet correlation
- Short Name:
- J/A+A/551/A112
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- It is generally accepted that the presence of a giant planet is strongly dependent on the stellar metallicity. A stellar mass dependence has also been investigated, but this dependence does not seem as strong as the metallicity dependence. Even for metallicity however, the exact form of the correlation has not been established. In this paper, we test several scenarios to describe the frequency of giant planets as a function of its host parameters. We perform this test on two volume-limited samples (from CORALIE and HARPS). By using a Bayesian analysis, we can quantitatively compare the different scenarios. We confirm that giant planet frequency is indeed a function of metallicity. However, there is no statistical difference between a constant or an exponential function for stars with subsolar metallicities contrarily to what has been previously stated in the literature. The dependence on stellar mass could not be confirmed, neither discarded.
- ID:
- ivo://CDS.VizieR/J/A+A/567/A55
- Title:
- Metallicity of the {gamma} Vel cluster
- Short Name:
- J/A+A/567/A55
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Knowledge of the abundance distribution of star forming regions and young clusters is critical to investigate a variety of issues, from triggered star formation and chemical enrichment by nearby supernova explosions to the ability to form planetary systems. In spite of this, detailed abundance studies are currently available for relatively few regions. In this context, we present the analysis of the metallicity of the Gamma Velorum cluster, based on the products distributed in the first internal release of the Gaia-ESO Survey. The Gamma Velorum candidate members have been observed with FLAMES, using both UVES and Giraffe, depending on the target brightness and spectral type. In order to derive a solid metallicity determination for the cluster, membership of the observed stars must be first assessed. To this aim, we use several membership criteria including radial velocities, surface gravity estimates, and the detection of the photospheric lithium line. Out of the 80 targets observed with UVES, we identify 14 high-probability members. We find that the metallicity of the cluster is slightly sub-solar, with a mean [Fe/H]=-0.057+/-0.018dex. Although J08095427-4721419 is one of the high-probability members, its metallicity is significantly larger than the cluster average. We speculate about its origin as the result of recent accretion episodes of rocky bodies of about 60 Msun hydrogen-depleted material from the circumstellar disc.
- ID:
- ivo://CDS.VizieR/J/AJ/145/13
- Title:
- Metal-poor stars from SDSS/SEGUE. I. Abundances
- Short Name:
- J/AJ/145/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Chemical compositions are determined based on high-resolution spectroscopy for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). High-resolution spectra with moderate signal-to-noise (S/N) ratios were obtained with the High Dispersion Spectrograph of the Subaru Telescope. Most of the sample (approximately 80%) are main-sequence turnoff stars, including dwarfs and subgiants. Four cool main-sequence stars, the most metal-deficient such stars known, are included in the remaining sample. Good agreement is found between effective temperatures estimated by the SEGUE stellar parameter pipeline, based on the SDSS/SEGUE medium-resolution spectra, and those estimated from the broadband (V - K)_0_ and (g - r)_0_colors. Our abundance measurements reveal that 70 stars in our sample have [Fe/H] < -3, adding a significant number of EMP stars to the currently known sample. Our analyses determine the abundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe] > +0.7) among the 25 giants in our sample is as high as 36%, while only a lower limit on the fraction (9%) is estimated for turnoff stars. This paper is the first of a series of papers based on these observational results. The following papers in this series will discuss the higher-resolution and higher-S/N observations of a subset of this sample, the metallicity distribution function, binarity, and correlations between the chemical composition and kinematics of extremely metal-poor stars.
