Kinetic equilibrium of iron in the atmospheres of cool stars

Korn, A. J.; Shi, Jianrong; Gehren, T.

doi:10.1051/0004-6361:20030907

Cited by 117 publications

(163 citation statements)

References 37 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: 77%

TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

174

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: 77%

TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

174

232

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“…We treated the overall effects of rotation and macroturbulence for the remaining stars as radialtangential macroturbulence. The macroturbulence value V mac was determined for each star either by Fuhrmann (1998Fuhrmann ( , 2004 or in our previous studies (Korn et al 2003;Mashonkina et al 2008) from the analysis of an extended list of lines of various chemical species. For a given star, V mac was allowed to vary by ±0.3 km s −1 (1σ).…”

Section: Methodsmentioning

confidence: 99%

“…There is no need anymore for the enforced upper level thermalization procedure that was applied by Gehren et al (2001a); Korn et al (2003), or Collet et al (2005).…”

Section: Effect Of the Model Atom Completeness On The Statistical Equmentioning

confidence: 99%

“…For example, Korn et al (2003) found a negligible discrepancy between the non-LTE spectroscopic and Hipparcos astrometric distances of the halo star HD 84937, while the non-LTE calculations of Thévenin & Idiart (1999) resulted in a 34% smaller spectroscopic distance of that same star.…”

Section: Introductionmentioning

confidence: 99%

“…The highest levels presented in the model atom did not couple thermally to the ground state of Fe ii, which indicates substantial term incompleteness. To force the levels near the continuum into LTE, an upper level thermalization procedure was applied (more details in Gehren et al 2001a;Korn et al 2003;Collet et al 2005).…”

Section: Introductionmentioning

confidence: 99%

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A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars

Mashonkina

Gehren

Shi

et al. 2011

A&A

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233

266

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Aims. We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for the two ions of iron and check the ionization equilibrium between Fe i and Fe ii in model atmospheres of the cool reference stars based on the best available complete model atom for neutral and singly-ionized iron.Methods. We present a comprehensive model atom for Fe with more than 3000 measured and predicted energy levels. As a test and first application of the improved model atom, iron abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. The efficiency of inelastic collisions with hydrogen atoms in the statistical equilibrium of iron is empirically estimated from inspection of their different influence on the Fe i and Fe ii lines in the selected stars. Results. Non-LTE leads to systematically depleted total absorption in the Fe i lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of such corrections is smaller compared to the earlier results. These non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe i/Fe ii ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe ii lines non-LTE corrections do not exceed 0.01 dex in absolute value over the whole range of stellar parameters that are considered. This study reveals two problems. The first one is that g f -values available for the Fe i and Fe ii lines are not accurate enough to pursue high-accuracy absolute stellar abundance determinations. For the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56 ± 0.09 and the mean abundance from 18 Fe ii lines varies between 7.41 ± 0.11 and 7.56 ± 0.05 depending on the source of the g f -values. The second problem is that lines of Fe i give, on average, a 0.1 dex lower abundance compared with those of Fe ii lines for HD 61421 and HD 102870, even when applying a differential line-by-line analysis with regard to the Sun. A disparity between neutral atoms and first ions points to problems of stellar atmosphere modelling or/and effective temperature determination.

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Detailed investigation of two high‐speed evolved Galactic stars

et al. 2021

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The study of metal poor stars provides clarification and knowledge about the primordial Universe. Specially, halo stars provide explanations of the nature of the first generations of stars and the nucleosynthesis in the metal‐poor regime. We present a detailed chemical analysis and determination of the kinematic and orbital properties of two stars characterized by high speed with respect to the Sun. We analyzed two high‐resolution Subaru spectra employing the MyGIsFOS code, which allows to derive the detailed chemical abundances for 28 elements (C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu), and abundance from two ionization states in the case of four elements (Ti, Cr, Fe, and Zr). TYC 622–742–1 and TYC 1193–1918–1 are metal‐poor stars ([Fe/H] of −2.37 and −1.60), they are similar in the chemical pattern with respect to Fe, they are α enhanced and show a slight excess in Eu abundance. Both giant stars are poor in C and rich in N, as expected for evolved stars, and this fact is supported by the low 12Cfalse/13C isotopic ratio in TYC 1193–1918–1. Nevertheless, the C abundance of TYC 622–742–1 is particularly low. TYC 622–742–1 and TYC 1193–1918–1 have a similar chemical composition to the other Galactic halo stars of comparable metallicity. According to their kinematics, both stars belong to the Galactic halo, but they are not a part of the Gaia‐Sausage‐Enceladus structure.

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Kinetic equilibrium of iron in the atmospheres of cool stars

Cited by 117 publications

References 37 publications

TOPoS

TOPoS

A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars

Detailed investigation of two high‐speed evolved Galactic stars

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