- ID:
- ivo://CDS.VizieR/J/ApJ/860/125
- Title:
- Six warm metal-poor stars iron abundances
- Short Name:
- J/ApJ/860/125
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high-resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H]~-2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate archival UV spectra for three other warm dwarfs with [Fe/H]~-3.3, -2.2, and -1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color-temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of FeI and FeII lines with wavelengths from 2290 to 6430{AA}. The [Fe/H] ratios derived separately from FeI and FeII lines agree in all six stars, with [FeII/H]-[FeI/H] ranging from +0.00+/-0.07 to -0.12+/-0.09dex, when strong lines and FeI lines with lower excitation potential <1.2eV are excluded. This constrains the extent of any deviations from LTE that may occur within this parameter range. While our result confirms non-LTE calculations for some warm, metal-poor dwarfs, it may not be generalizable to more metal-poor dwarfs, where deviations from LTE are predicted to be larger. We also investigate trends of systematically lower abundances derived from FeI lines in the Balmer continuum region (~3100-3700{AA}), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/807/171
- Title:
- SkyMapper Survey metal-poor star spectroscopy
- Short Name:
- J/ApJ/807/171
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The SkyMapper Southern Sky Survey is carrying out a search for the most metal-poor stars in the Galaxy. It identifies candidates by way of its unique filter set which allows for estimation of stellar atmospheric parameters. The set includes a narrow filter centered on the CaII K 3933{AA} line, enabling a robust estimate of stellar metallicity. Promising candidates are then confirmed with spectroscopy. We present the analysis of Magellan Inamori Kyocera Echelle high-resolution spectroscopy of 122 metal-poor stars found by SkyMapper in the first two years of commissioning observations. Forty-one stars have [Fe/H]<=-3.0. Nine have [Fe/H]<=-3.5, with three at [Fe/H]~-4. A 1D LTE abundance analysis of the elements Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, Ba, and Eu shows these stars have [X/Fe] ratios typical of other halo stars. One star with low [X/Fe] values appears to be "Fe-enhanced", while another star has an extremely large [Sr/Ba] ratio: >2 Only one other star is known to have a comparable value. Seven stars are "CEMP-no" stars ([C/Fe]>0.7, [Ba/Fe]<0). 21 stars exhibit mild r-process element enhancements (0.3<=[Eu/Fe]<1.0), while four stars have [Eu/Fe]>=1.0. These results demonstrate the ability to identify extremely metal-poor stars from SkyMapper photometry, pointing to increased sample sizes and a better characterization of the metal-poor tail of the halo metallicity distribution function in the future.
- ID:
- ivo://CDS.VizieR/J/MNRAS/451/2625
- Title:
- SLUGGS Globular Cluster CaT and [Z/H]
- Short Name:
- J/MNRAS/451/2625
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The outer halo regions of early-type galaxies carry key information about their past accretion history. However, spectroscopically probing the stellar component at such galactocentric radii is still challenging. Using the DEep Imaging Multi-Object Spectrograph on the Keck, we have been able to measure the metallicities of the stellar and globular cluster components in 12 early-type galaxies out to more than 10Re.
- ID:
- ivo://CDS.VizieR/J/A+A/423/683
- Title:
- Sodium abundances in nearby disk stars
- Short Name:
- J/A+A/423/683
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores.
- ID:
- ivo://CDS.VizieR/J/AZh/80/458
- Title:
- Sodium abundances in stellar atmospheres
- Short Name:
- J/AZh/80/458
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The non-LTE sodium abundances of 100 dwarf, giant and subgiant stars with metallicities -3<[Fe/H]<0.3 are determined using high-dispersion spectra with high signal-to-noise ratios.
- ID:
- ivo://CDS.VizieR/J/ApJS/208/7
- Title:
- Sodium excess objects. I. SDSS-DR7
- Short Name:
- J/ApJS/208/7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Several studies have reported the presence of sodium excess objects having neutral atomic absorption lines at 5895{AA} (Na.D) and 8190{AA} that are deeper than expected based on stellar population models that match the stellar continuum. The origin of these lines is therefore hotly debated. van Dokkum & Conroy (2010Natur.468..940V) proposed that low-mass stars (<~0.3M_{sun}_) are more prevalent in massive early-type galaxies, which may lead to a strong Na I 8190 line strength. It is necessary to test this prediction, however, against other prominent optical line indices such as Na.D, Mg b, and Fe 5270, which can be measured with a significantly higher signal-to-noise ratio than Na I 8190. We identified a new sample of roughly 1000 Na.D excess objects (NEOs; ~8% of galaxies in the sample) based on Na.D line strength in the redshift range 0.00<=z<=0.08 from the Sloan Digital Sky Survey (SDSS) DR7 through detailed analysis of galaxy spectra. We explore the properties of these new objects here. The novelty of this work is that the galaxies were carefully identified through direct visual inspection of SDSS images, and we systematically compared the properties of NEOs and those of a control sample of galaxies with normal Na.D line strengths. We note that the majority of galaxies with high velocity dispersions ({sigma}_e_>250km/s) show Na.D excesses.
- ID:
- ivo://CDS.VizieR/J/MNRAS/325/1365
- Title:
- Solar neighbourhood metallicity distribution
- Short Name:
- J/MNRAS/325/1365
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centered on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal-rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood have metallicities higher than [Fe/H]=0. The percentage of mid-metal-poor stars ([Fe/H}<-0.5) is approximately 4 per cent, in agreement with the present estimates of the thick disc.
- ID:
- ivo://CDS.VizieR/J/A+A/619/A130
- Title:
- Solar sibling candidates chemical abundances
- Short Name:
- J/A+A/619/A130
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Finding solar siblings, that is, stars that formed in the same cluster as the Sun, will yield information about the conditions at the Sun's birthplace. Finding possible solar siblings is difficult since they are spread widely throughout the Galaxy. We search for solar sibling candidates in AMBRE, the very large spectra database of solar vicinity stars. Since the ages and chemical abundances of solar siblings are very similar to those of the Sun, we carried out a chemistry- and age-based search for solar sibling candidates. We used high-resolution spectra to derive precise stellar parameters and chemical abundances of the stars. We used these spectroscopic parameters together with Gaia DR2 astrometric data to derive stellar isochronal ages. Gaia data were also used to study the kinematics of the sibling candidates. From the about 17000 stars that are characterized within the AMBRE project, we first selected 55 stars whose metallicities are closest to the solar value (-0.1<=[Fe/H]<=0.1dex). For these stars we derived precise chemical abundances of several iron-peak, {alpha}- and neutron-capture elements, based on which we selected 12 solar sibling candidates with average abundances and metallicities between -0.03 to 0.03dex. Our further selection left us with 4 candidates with stellar ages that are compatible with the solar age within observational uncertainties. For the 2 of the hottest candidates, we derived the carbon isotopic ratios, which are compatible with the solar value. HD186302 is the most precisely characterized and probably the most probable candidate of our 4 best candidates. Very precise chemical characterization and age estimation is necessary to identify solar siblings. We propose that in addition to typical chemical tagging, the study of isotopic ratios can give further important information about the relation of sibling candidates with the Sun. Ideally, asteroseismic age determinations of the candidates could solve the problem of imprecise isochronal ages.
- ID:
- ivo://CDS.VizieR/J/A+A/619/A73
- Title:
- Solar Twins age-chromospheric activity
- Short Name:
- J/A+A/619/A73
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- It is well known that the magnetic activity of solar-type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1-2Gyr that is followed by a relatively inactive constant phase. We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined. We measured the CaII H and K activity indices using 9000 HARPS spectra of 82 solar twins. In addition, the average solar activity was calculated through asteroids and Moon reflection spectra using the same instrumentation. Thus, we transformed our activity indices into the S Mount Wilson scale (S_MW_), recalibrated the Mount Wilson absolute flux and photospheric correction equations as a function of Te, and then computed an improved bolometric flux normalized activity index logR'_HK_(Teff) for the entire sample. New relations between activity and the age of solar twins were derived by assessing the chromospheric age-dating limits using logR'_HK_(Teff). We measured an average solar activity of S_MW_=0.1712+/-0.0017 during solar magnetic cycles 23-24 covered by HARPS observations, and we also inferred an average of S_MW_=0.1694+/-0.0025 for cycles 10-24, anchored on a sunspot number correlation of S index versus. We also found a simple relation between the average and the dispersion of the activity levels of solar twins. This enabled us to predict the stellar variability effects on the age-activity diagram, and consequently, to estimate the chromospheric age uncertainties that are due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6-7Gyr, in agreement with previous works using open clusters and field stars with precise ages. Our research confirms that CaII H& K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6-7Gyr. We found evidence that for the most homogenous set of old stars, the chromospheric activity indices seem to continue to decrease after the solar age toward the end of the main sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations eects. The Sun seems to have a normal activity level and variability for its age.
- ID:
- ivo://CDS.VizieR/J/MNRAS/417/1823
- Title:
- Sombrero galaxy globular clusters
- Short Name:
- J/MNRAS/417/1823
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a large sample of over 200 integrated-light spectra of confirmed globular clusters (GCs) associated with the Sombrero (M104) galaxy taken with the Deep Imaging Multi-Object Spectrograph (DEIMOS) instrument on the Keck telescope. A significant fraction of the spectra have signal-to-noise ratio levels high enough to allow measurements of GC metallicities using the method of Brodie & Huchra. We find a distribution of spectroscopic metallicities in the range -2.2<[Fe/H]<+0.1 that is bimodal, with peaks at [Fe/H]~-1.4 and -0.6. Thus, the GC system of the Sombrero galaxy, like a few other galaxies now studied in detail, reveals a bimodal spectroscopic metallicity distribution supporting the long-held belief that colour bimodality reflects two metallicity subpopulations. This further suggests that the transformation from optical colour to metallicity for old stellar populations, such as GCs, is not strongly non-linear. We also explore the radial and magnitude distribution with metallicity for GC subpopulations but small number statistics prevent any clear trends in these distributions.
