- 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.
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- 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.
- ID:
- ivo://CDS.VizieR/J/AJ/142/188
- Title:
- Metal-poor stars from the HES using CH G-band
- Short Name:
- J/AJ/142/188
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We describe a new method to search for metal-poor candidates from the Hamburg/ESO objective-prism survey (HES) based on identifying stars with apparently strong CH G-band strengths for their colors. The hypothesis we exploit is that large overabundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. The selection was made by considering two line indices in the 4300{AA} region, applied directly to the low-resolution prism spectra. This work also extends a previously published method by adding bright sources to the sample. The spectra of these stars suffer from saturation effects, compromising the index calculations and leading to an undersampling of the brighter candidates. A simple numerical procedure, based on available photometry, was developed to correct the line indices and overcome this limitation. Visual inspection and classification of the spectra from the HES plates yielded a list of 5288 new metal-poor (and by selection, carbon-rich) candidates, which are presently being used as targets for medium-resolution spectroscopic follow-up. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for 117 of the first candidates, based on follow-up medium-resolution spectra obtained with the SOAR 4.1m and Gemini 8m telescopes. There are eight newly discovered stars with [Fe/H] < -3.0 in our sample, including two with [Fe/H] < -3.5.
- ID:
- ivo://CDS.VizieR/J/AJ/158/239
- Title:
- Metal-rich host stars abundances & equivalent widths
- Short Name:
- J/AJ/158/239
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The relationship between the compositions of giant planets and their host stars is of fundamental interest in understanding planet formation. The solar system giant planets are enhanced above solar composition in metals, both in their visible atmospheres and bulk compositions. A key question is whether the metal enrichment of giant exoplanets is correlated with that of their host stars. Thorngren et al. (2016, J/ApJ/831/64) showed that in cool (T_eq_<1000 K) giant exoplanets, the total heavy-element mass increases with total M_p_ and the heavy-element enrichment relative to the parent star decreases with total M_p_. In their work, the host star metallicity was derived from literature [Fe/H] measurements. Here we conduct a more detailed and uniform study to determine whether different host star metals (C, O, Mg, Si, Fe, and Ni) correlate with the bulk metallicity of their planets, using correlation tests and Bayesian linear fits. We present new host star abundances of 19 cool giant planet systems, and combine these with existing host star data for a total of 22 cool giant planet systems (24 planets). Surprisingly, we find no clear correlation between stellar metallicity and planetary residual metallicity (the relative amount of metal versus that expected from the planet mass alone), which is in conflict with common predictions from formation models. We also find a potential correlation between residual planet metals and stellar volatile-to-refractory element ratios. These results provide intriguing new relationships between giant planet and host star compositions for future modeling studies of planet formation.
- ID:
- ivo://CDS.VizieR/J/AJ/160/18
- Title:
- M giant stars asteroseismology with Kepler and APOGEE
- Short Name:
- J/AJ/160/18
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Evolved stars near the tip of the red giant branch show solar-like oscillations with periods spanning hours to months and amplitudes ranging from ~1mmag to ~100mmag. The systematic detection of the resulting photometric variations with ground-based telescopes would enable the application of asteroseismology to a much larger and more distant sample of stars than is currently accessible with space-based telescopes such as Kepler or the ongoing Transiting Exoplanet Survey Satellite mission. We present an asteroseismic analysis of 493 M giants using data from two ground-based surveys: the Asteroid Terrestrial-impact Last Alert System (ATLAS) and the All-Sky Automated Survey for Supernovae (ASAS-SN). By comparing the extracted frequencies with constraints from Kepler, the Sloan Digital Sky Survey Apache Point Observatory Galaxy Evolution Experiment, and Gaia we demonstrate that ground-based transient surveys allow accurate distance measurements to oscillating M giants with a precision of ~15%. Using stellar population synthesis models we predict that ATLAS and ASAS-SN can provide asteroseismic distances to ~2x106 galactic M giants out to typical distances of 20-50kpc, vastly improving the reach of Gaia and providing critical constraints for Galactic archeology and galactic dynamics.