skip to main content

Title: Non-LTE analysis of K I in late-type stars
Context. Older models of Galactic chemical evolution (GCE) predict [K/Fe] ratios as much as 1 dex lower than those inferred from stellar observations. Abundances of potassium are mainly based on analyses of the 7698 Å resonance line, and the discrepancy between GCE models and observations is in part caused by the assumption of local thermodynamic equilibrium (LTE) in spectroscopic analyses. Aims. We study the statistical equilibrium of K  I , focusing on the non-LTE effects on the 7698 Å line. We aim to determine how non-LTE abundances of potassium can improve the analysis of its chemical evolution, and help to constrain the yields of GCE models. Methods. We construct a new model K  I atom that employs the most up-to-date atomic data. In particular, we calculate and present inelastic e+K collisional excitation cross-sections from the convergent close-coupling (CCC) and the B -Spline R -matrix (BSR) methods, and H+K collisions from the two-electron model (LCAO). We constructed a fine, extended grid of non-LTE abundance corrections based on 1D MARCS models that span 4000 < T eff ∕K < 8000, 0.50 < log g < 5.00, − 5.00 < [Fe/H] < + 0.50, and applied the corrections to potassium abundances extracted from the literature. more » Results. In concordance with previous studies, we find severe non-LTE effects in the 7698 Å line. The line is stronger in non-LTE and the abundance corrections can reach approximately − 0.7 dex for solar-metallicity stars such as Procyon. We determine potassium abundances in six benchmark stars, and obtain consistent results from different optical lines. We explore the effects of atmospheric inhomogeneity by computing for the first time a full 3D non-LTE stellar spectrum of K  I lines for a test star. We find that 3D modeling is necessary to predict a correct shape of the resonance 7698 Å line, but the line strength is similar to that found in 1D non-LTE. Conclusions. Our non-LTE abundance corrections reduce the scatter and change the cosmic trends of literature potassium abundances. In the regime [Fe/H] ≲−1.0 the non-LTE abundances show a good agreement with the GCE model with yields from rotating massive stars. The reduced scatter of the non-LTE corrected abundances of a sample of solar twins shows that line-by-line differential analysis techniques cannot fully compensate for systematic LTE modelling errors; the scatter introduced by such errors introduces a spurious dispersion to K evolution. « less
Authors:
; ; ; ; ; ; ; ; ;
Award ID(s):
1834740 1803844
Publication Date:
NSF-PAR ID:
10127674
Journal Name:
Astronomy & Astrophysics
Volume:
627
Page Range or eLocation-ID:
A177
ISSN:
0004-6361
Sponsoring Org:
National Science Foundation
More Like this
  1. Context. Rubidium is one of the few elements produced by the neutron capture s - and r -processes in almost equal proportions. Recently, a Rb deficiency ([Rb/Fe] < 0.0), amounting to a factor of about two with respect to the Sun, has been found in M dwarfs of near-solar metallicity. This stands in contrast to the close-to-solar [Sr, Zr/Fe] ratios derived in the same stars. This deficiency is difficult to understand from the point of view of observations and of nucleosynthesis. Aims. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars across a similar metallicity range, extracted from the AMBRE Project. Methods. We used high-resolution and high signal-to-noise spectra to derive Rb and Zr abundances in a sample of 54 bright giant stars with metallicities in the range of −0.6 ≲ [Fe/H] ≲ +0.4 dex, via spectral synthesis in both local and non-local thermodynamic equilibrium (LTE and NLTE, respectively). We also studied the impact of the Zeeman broadening in the profile of the Rb  I at λ 7800 Å line. Results. The LTE analysis also results in a Rb deficiency in giant stars, however, it is considerablymore »lower than that obtained in M dwarfs. However, once NLTE corrections are performed, the [Rb/Fe] ratios are very close to solar (average −0.01 ± 0.09 dex) in the full metallicity range studied here. This stands in contrast to the value found for M dwarfs. The [Zr/Fe] ratios derived are in excellent agreement with those obtained in previous studies in FGK dwarf stars with a similar metallicity. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. Although the former phenomenon has a negligible impact on the surface Rb abundance, the presence of an average magnetic field with an intensity that is typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. This may explain the Rb deficiency in M dwarfs, but not fully. On the other hand, the new [Rb/Fe] and [Rb/Zr] versus [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-to-intermediate mass stars (these latter also producing Zr) are considered, without the need to deviate from the standard s -process nucleosynthesis in asymptotic giant branch stars, as suggested previously.« less
  2. Abstract Individual chemical abundances for 14 elements (C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Ni) are derived for a sample of M dwarfs using high-resolution, near-infrared H -band spectra from the Sloan Digital Sky Survey-IV/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The quantitative analysis included synthetic spectra computed with 1D LTE plane-parallel MARCS models using the APOGEE Data Release 17 line list to determine chemical abundances. The sample consists of 11 M dwarfs in binary systems with warmer FGK dwarf primaries and 10 measured interferometric angular diameters. To minimize atomic diffusion effects, [X/Fe] ratios are used to compare M dwarfs in binary systems and literature results for their warmer primary stars, indicating good agreement (<0.08 dex) for all studied elements. The mean abundance difference in primaries minus this work’s M dwarfs is −0.05 ± 0.03 dex. It indicates that M dwarfs in binary systems are a reliable way to calibrate empirical relationships. A comparison with abundance, effective temperature, and surface gravity results from the APOGEE Stellar Parameter and Chemical Abundances Pipeline (ASPCAP) Data Release 16 finds a systematic offset of [M/H], T eff , log g = +0.21 dex, −50 K, andmore »0.30 dex, respectively, although ASPCAP [X/Fe] ratios are generally consistent with this study. The metallicities of the M dwarfs cover the range of [Fe/H] = −0.9 to +0.4 and are used to investigate Galactic chemical evolution via trends of [X/Fe] as a function of [Fe/H]. The behavior of the various elemental abundances [X/Fe] versus [Fe/H] agrees well with the corresponding trends derived from warmer FGK dwarfs, demonstrating that the APOGEE spectra can be used to examine Galactic chemical evolution using large samples of selected M dwarfs.« less
  3. Context. Stellar models applied to large stellar surveys of the Milky Way need to be properly tested against a sample of stars with highly reliable fundamental stellar parameters. We have established a programme aiming to deliver such a sample of stars. Aims. Here we present new fundamental stellar parameters of nine dwarf stars that will be used as benchmark stars for large stellar surveys. One of these stars is the solar-twin 18 Sco, which is also one of the Gaia -ESO benchmarks. The goal is to reach a precision of 1% in effective temperature ( T eff ). This precision is important for accurate determinations of the full set of fundamental parameters and abundances of stars observed by the surveys. Methods. We observed HD 131156 ( ξ Boo), HD 146233 (18 Sco), HD 152391, HD 173701, HD 185395 ( θ Cyg), HD 186408 (16 Cyg A), HD 186427 (16 Cyg B), HD 190360, and HD 207978 (15 Peg) using the high angular resolution optical interferometric instrument PAVO at the CHARA Array. We derived limb-darkening corrections from 3D model atmospheres and determined T eff directly from the Stefan–Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surfacemore »gravities were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE spectrograph and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron. Results. For eight of the nine stars we measure the T eff ⪅ 1%, and for one star better than 2%. We determined the median uncertainties in log  g and [Fe/H] as 0.015 dex and 0.05 dex, respectively. Conclusions. This study presents updated fundamental stellar parameters of nine dwarf stars that can be used as a new set of benchmarks. All the fundamental stellar parameters were based on consistently combining interferometric observations, 3D limb-darkening modelling, and spectroscopic analysis. The next paper in this series will extend our sample to giants in the metal-rich range.« less
  4. Context. Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures ( T eff ) in this regime. Aims. We aim to construct a new set of metal-poor benchmark stars based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% in T eff , as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources. Methods. We observed ten late-type metal-poor dwarfs and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170, and HD 224930. Only three of them (HD 103095, HD 122563, and HD 140283) have previously been used as benchmark stars. For the observations, we used the high-angular-resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb-darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log( g ))more »were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron. Results. We inferred T eff to better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283, and HD 224930). The effective temperatures of the other five stars are reliable to between 2 and 3%; the higher uncertainty on the T eff for those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log( g ) and [Fe/H] with median uncertainties of 0.03 dex and 0.09 dex, respectively. Conclusions. This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb-darkening-modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.« less
  5. Context. Large spectroscopic surveys of the Milky Way must be calibrated against a sample of benchmark stars to ensure the reliable determination of atmospheric parameters. Aims. Here, we present new fundamental stellar parameters of seven giant and subgiant stars that will serve as benchmark stars for large surveys. The aim is to reach a precision of 1% in the effective temperature. This precision is essential for accurate determinations of the full set of fundamental parameters and abundances for stars observed by the stellar surveys. Methods. We observed HD 121370 ( η Boo), HD 161797 ( μ Her), HD 175955, HD 182736, HD 185351, HD 188512 ( β Aql), and HD 189349, using the high angular resolution optical interferometric instrument PAVO at the CHARA Array. The limb-darkening corrections were determined from 3D model atmospheres based on the STAGGER grid. The T eff were determined directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. We estimated surface gravities from comparisons to Dartmouth stellar evolution model tracks. The spectroscopic observations were collected from the ELODIE and FIES spectrographs. We estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutralmore »and singly ionised iron. Results. For six of the seven stars, we measured the value of T eff to better than 1% accuracy. For one star, HD 189349, the uncertainty on T eff is 2%, due to an uncertain bolometric flux. We do not recommend this star as a benchmark until this measurement can be improved. Median uncertainties for all stars in log  g and [Fe/H] are 0.034 dex and 0.07 dex, respectively. Conclusions. This study presents updated fundamental stellar parameters of seven giant and subgiant stars that can be used as a new set of benchmarks. All the fundamental stellar parameters were established on the basis of consistent combinations of interferometric observations, 3D limb-darkening modelling, and spectroscopic analysis. This paper in this series follows our previous papers featuring dwarf stars and stars in the metal-poor range.« less