Non-LTE effects in Na I spectral lines in stellar atmospheres

Mashonkina, L.; Shimanskii, V. V.; Сахибуллин, Н. А.

doi:10.1134/1.1327637

Cited by 89 publications

(96 citation statements)

References 14 publications

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“…NLTE corrections for O are well discussed in Takeda (1997), while the ones for Na are discussed in Mashonkina et al (2000). Both papers present a grid of corrections which fully covers the parameter space of our objects.…”

Section: Results Of the Spectroscopic Analysis And Determination Of Tmentioning

confidence: 99%

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The helium content of globular clusters: light element abundance correlations and HB morphology

Villanova¹,

Piotto²,

Gratton³

2009

A&A

109

102

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Context. In the context of the multiple stellar population scenario in globular clusters (GC), helium (He) has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology. However, up to now, He has never been directly measured in GC stars (8500 < T eff < 11 500 K) to verify this hypothesis. Aims. We studied hot blue horizontal branch (BHB) stars in the GC NGC 6752 to measure their He content. Our goal is to verify the feasibility of He measurement from high resolution spectra in stars cooler than 11 500 K, where chemical abundances are not altered by sedimentation or levitation. Methods. We observed 7 BHB stars using the UVES@VLT2 spectroscopic facility. Spectra of S/N ∼ 200 were obtained and the very weak He line at 5875 Å measured. We compared this feature with synthetic spectra to obtain He abundances. In addition, iron peak (Fe, Cr), α (Si, Ti), light (O, Na), and s-element (Ba) abundances were measured. Results. We could measure He abundance only for stars warmer than T eff = 8500 K. All our targets with measurable He are zero age HB (ZAHB) objects and turned out to have a homogeneous He content with a mean value of Y = 0.245 ± 0.012, compatible with the most recent measurements of the primordial He content of the Universe (Y ∼ 0.25). The whole sample of stars has a metallicity of [Fe/H] = −1.56 ± 0.03 and [α/Fe] = +0.21 ± 0.03, in agreement with other studies available in the literature. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars. Conclusions. This is the first direct measurement of the He abundance for a significative sample of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flasher HB stars.

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Section: Results Of the Spectroscopic Analysis And Determination Of Tmentioning

confidence: 99%

“…The O triplet at 6156-6158 Å and the Na doublet at 5889-5895 Å are known to be affected by NLTE, and we applied the corrections by Takeda (1997) and Mashonkina et al (2000), interpolated to the atmospheric parameters of our stars.…”

Section: Linelist Chemical Abundances and Rotationmentioning

confidence: 99%

The helium content of globular clusters: light element abundance correlations and HB morphology

Villanova¹,

Piotto²,

Gratton³

2009

A&A

109

102

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“…In other studies, the overabundances were detected just as an abundance ratio [Na-Al/Fe] greater than 0.1 dex in giants or clump stars. Randich et al (2006) argue that sodium enhancement in M 67 should be ascribed to non-LTE effects affecting sodium lines in giants more than dwarfs, as discussed in Mashonkina et al (2000). The extreme sensitivity of sodium to non-LTE effects was also noted by Sestito et al (2007).…”

Section: Introductionmentioning

confidence: 98%

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

Pace¹,

Danziger

Carraro³

et al. 2010

A&A

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Context. Classical chemical analyses may be affected by systematic errors that would cause observed abundance differences between dwarfs and giants. For some elements, however, the abundance difference could be real. Aims. We address the issue by observing 2 solar-type dwarfs in NGC 5822 and 3 in IC 4756, and comparing their composition with that of 3 giants in either of the aforementioned clusters. We determine iron abundance and stellar parameters of the dwarf stars, and the abundances of calcium, sodium, nickel, titanium, aluminium, chromium, silicon, and oxygen for both the giants and dwarfs. For the dwarfs, we also estimate the rotation velocities, and and lithium abundances. We improve the cluster parameter estimates (distance, age, and reddening) by comparing existing photometry with new isochrones. Methods. We acquired UVES high-resolution, of high signal-to-noise ratio (S /N) spectra. The width of the cross correlation profiles was used to measure rotation velocities. For abundance determinations, the standard equivalent width analysis was performed differentially with respect to the Sun. For lithium and oxygen, we derived abundances by comparing synthetic spectra with observed line features. Results. We find an iron abundance for dwarf stars equal to solar to within the margins of error for IC 4756, and slightly above for NGC 5822 ([Fe/H] = 0.01 and 0.05 dex respectively). The 3 stars in NGC 4756 have lithium abundances between Log N(Li) ≈ 2.6 and 2.8 dex, the two stars in NGC 5822 have Log N(Li) ≈ 2.8 and 2.5, respectively. Conclusions. For sodium, silicon, and titanium, we show that abundances of giants are significantly higher than those of the dwarfs of the same cluster (about 0.15, 0.15, and 0.35 dex). Other elements may also undergo enhancement, but all within 0.1 dex. Indications of much stronger enhancements can be found using literature data. But artifacts of the analysis may be partly responsible for this.

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“…Accordingly, it appears that a proper account of the non-LTE effect is mandatory for studying stellar sodium abundances, at least for the resonance Na Admittedly, several elaborate and important studies have recently been done concerning the non-LTE effect on Na abundance determinations for solar-type stars (e.g., Baumüller et al 1998;Gratton et al 1999;Korotin, Mishenina 1999;Mashonkina et al 2000). We would say, however, that their results are not presented in sufficient detail for the purpose of practical applications; i.e., they tend to be confined to only specific lines of interest or only representative atmospheric parameters, and it is not necessarily easy for the reader to assess how all those Na i lines of importance are affected by the non-LTE effect for stars showing a diversity of atmospheric parameters.…”

mentioning

confidence: 99%

Non-LTE Analysis of the Sodium Abundance of Metal-Poor Stars in the Galactic Disk and Halo

Takeda

Zhao

Takada-Hidai

et al. 2003

Chin. J. Astron. Astrophys.

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Non-LTE effects in Na I spectral lines in stellar atmospheres

Cited by 89 publications

References 14 publications

The helium content of globular clusters: light element abundance correlations and HB morphology

The helium content of globular clusters: light element abundance correlations and HB morphology

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

Non-LTE Analysis of the Sodium Abundance of Metal-Poor Stars in the Galactic Disk and Halo

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