Stellar Iron Abundances: Non‐LTE Effects

Thévenin, F.; Idiart, T. P.

doi:10.1086/307578

Cited by 333 publications

(385 citation statements)

References 25 publications

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“…Although we do not have in hand specific computations for the Ca ii IR triplet to demonstrate this effect, we know it can be seen for other lines (see e.g. (Gratton et al 1999;Korn et al 2003;Thévenin & Idiart 1999). This value should also be appropriate for SDSS J1742+2531, which has similar atmospheric parameters.…”

Section: Effects Of Departures From Local Thermodynamic Equilibriummentioning

confidence: 78%

TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

170

232

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Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < −5.0 and measure the Ca abundance, with [Ca/H] = −5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < −4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas Article published by EDP Sciences A28, page 1 of 20 A&A 579, A28 (2015) cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

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Section: Effects Of Departures From Local Thermodynamic Equilibriummentioning

confidence: 78%

TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

170

232

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“…Since most of the stars in this study fall below the magnitude threshold of the Hipparcos mission, one must rely on an alternate method for estimating log g. Results from Gratton et al (1997) with Hipparcos-based gravities also suggest that there are few significant effects due to non-LTE (NLTE) processes that influence the behavior of neutral and ionized Fe lines. However, NLTE analyses of Fe line formation suggest that NLTE processes may affect neutral Fe lines in metal-poor stars (Fuhrmann 1998;Feltzing & Gustafsson 1998;Thévenin & Idiart 1999). Until further research corroborates these initial findings, we assume that the LTE ionization balance gravities are reasonable.…”

Section: Surface Gravitymentioning

confidence: 81%

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Stephens

Boesgaard

2002

The Astronomical Journal

145

182

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There are stars in the halo of the Galaxy whose orbital properties are distinctly different from the majority of the halo population. One suggestion to account for these kinematically peculiar stars is that they have been gravitationally subsumed by the Milky Way through the breakup of nearby dwarf galaxies. One test of this hypothesis lies in determining the chemical composition of the deviant halo stars relative to that of normal ('' native '') halo field stars. The possibly accreted stars are predicted to have different chemical enrichments due to a slower rate of star formation in nearby low-mass spheroidal or irregular galaxies. We have obtained echelle spectra of metal-poor halo dwarfs with unusual orbital properties: from the outer halo, from the '' high '' halo, and on retrograde orbits. Our spectra are at high spectral resolution (35,000-48,000) and high signal-to-noise ratios (median value 140), primarily from the Keck I 10 m telescope with HIRES (53 stars), but also from the echelle spectrometer of the KPNO Mayall 4 m Telescope (three stars). The spectra cover a large spectral range: $4000-6800 Å . The strengths of approximately 9000 lines have been measured to determine the stellar parameters (T eff , log g, [Fe/H], and ) and the abundances of 10 elements, including Na, -fusion products (Mg, Si, Ca, Ti), iron peak elements (Cr, Fe, Ni), and neutron capture elements (Y, Ba). A model atmosphere was computed for each star, which was used to determine the composition under the assumption of LTE. The comparison of the abundances with the kinematic properties revealed no special trends, except that the ratio of the mean of the -fusion elements to Fe is weakly correlated with stellar apogalactica in the sense that the outermost stars have lower ratios of [ /Fe] than those with smaller apogalactica. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the -elements (e.g., O, Mg, Si, Ca, Ti) with respect to Fe, a consequence of sporadic bursts of star formation within the dwarf galaxies. However [Fe/H]. The kinematically peculiar stars in our sample must have had their origins in localized star-forming regions far removed from the Galactic center. They do not carry the chemical signature of an accreted population. It seems clear that the chemical enrichment in our sample comes primarily from Type II supernovae with only a small component from Type Ia supernovae. Our stars would have formed at most $1 Gyr after the beginning of a star formation episode in the remote halo. The general conclusion extracted from these data is that the formation of the nascent Milky Way was not dominated by the late accretion of dwarf galaxies like the ones that currently orbit the Galaxy. However, the assimilation of fragments early in the evolution of the Galaxy is a natural by-product of hierarchical models of structure formation and can explain many properties of the halo population.

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“…Gratton et al (1999) found that non-LTE corrections for Fe lines are very small in dwarfs at any T eff , and only small corrections (<0.1 dex) are expected for stars on the red giant branch. Thévenin & Idiart (1999) found that non-LTE corrections become more important as [Fe/H] versus T eff is (+0.6 AE 1.0)Â 10 À4 dex K À1 , which is nearly flat. We conclude that non-LTE effects are negligible in our iron abundance determination, with a maximum change of 0.12 AE 0.2 dex in the Fe ionization equilibrium from the tip of the RGB to the main-sequence turnoff in M5.…”

Section: Iron Abundancementioning

confidence: 96%

“…The main non-LTE effect in late-type stars arises from overionization of electron donor metals by ultraviolet radiation (Auman & Woodrow 1975). Gratton et al (1999) and Thévenin & Idiart (1999) studied non-LTE effects in Fe abundances in metal-poor late-type stars. Gratton et al (1999) found that non-LTE corrections for Fe lines are very small in dwarfs at any T eff , and only small corrections (<0.1 dex) are expected for stars on the red giant branch.…”

Section: Iron Abundancementioning

confidence: 99%

Abundances in Stars from the Red Giant Branch Tip to near the Main-Sequence Turnoff in M5

Ramírez

Cohen

2003

The Astronomical Journal

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We present the iron abundance and abundance ratios for 18 elements with respect to Fe in a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff in the globular cluster M5. The analyzed spectra, obtained with the High Resolution Echelle Spectrograph at the Keck Observatory, are of high dispersion (R = /D = 35,000). We find that the neutron capture, the iron peak, and the -element abundance ratios show no trend with T eff and low scatter around the mean between the top of the red giant branch and near the main-sequence turnoff, suggesting that at this metallicity non-LTE effects are not important over the range of stellar parameters spanned by our sample. To within the precision of the measurements (about AE0.1 dex), gravitationally induced heavy-element diffusion does not appear to be present among the stars near the main-sequence turnoff studied here. Our work and other recent studies suggest that heavy-element diffusion is inhibited in the surface layers of metal-poor stars. Differences in the Na abundance from star to star that extend to the main-sequence turnoff are detected in our sample in M5. The anticorrelation between O and Na abundances, observed in other metal-poor globular clusters, is not detected in our sample, but it may be hidden among stars with only upper limits for their O abundances. As we found in M71, there is a hint of star-to-star variation in the Zr abundance. Overall the abundance ratios of M5 appear very similar to those of M71, with the possible exception of the neutron capture element Ba, for which we argue that the apparent difference may be due to difficulties in the analysis. As in M71, the -elements Mg, Ca, Si, and Ti are overabundant relative to Fe. The results of our abundance analysis of 25 stars in M5 provide further evidence of abundance variations among specific light elements at unexpectedly low luminosities, which cannot be explained by our current understanding of stellar evolution.

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Stellar Iron Abundances: Non‐LTE Effects

Cited by 333 publications

References 25 publications

TOPoS

TOPoS

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Abundances in Stars from the Red Giant Branch Tip to near the Main-Sequence Turnoff in M5

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