Vertical abundance stratification in the blue horizontal branch star HD 135485

Khalack, V.; Leblanc, F.; Bohlender, D.; Wade, G. A.; Behr, Bradford B.

doi:10.1051/0004-6361:20066647

Cited by 25 publications

(63 citation statements)

References 25 publications

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“…Observational evidence for such a stratification of Fe in the atmospheres of BHB stars has been found (Khalack et al 2007LeBlanc et al 2009LeBlanc et al , 2010. How is the shape of the spectral lines affected by vertical stratification?…”

Section: Peculiar Line Profiles and Stratificationmentioning

confidence: 99%

Hot subdwarf stars in close-up view

Geier

2013

A&A

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Context. Hot subdwarf B stars (sdBs) are considered to be core helium-burning stars with very thin hydrogen envelopes situated on or near the extreme horizontal branch. The formation of sdBs is still unclear as well as the chemical composition of their atmospheres. The observed helium depletion is attributed to atmospheric diffusion. Metal abundances have been determined for about a dozen sdBs only resulting in puzzling patterns with enrichment of heavy metals and depletion of lighter ones. Aims. We present a detailed metal abundance analysis of 106 sdBs. Methods. From high resolution spectra we measured elemental abundances of up to 24 different ions per star. A semi-automatic analysis pipeline was developed to calculate and fit LTE models to a standard set of spectral lines. Results. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen, and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from star to star, the general abundance patterns in most sdBs are similar. State-of-the-art diffusion models predict such patterns and are in qualitative agreement with our results. However, the highest enrichments measured cannot be explained with these models. Peculiar line shapes of the strongest metal lines in some stars indicate vertical stratification to be present in the atmospheres. Such effects are not accounted for in current diffusion models and may be responsible for some of the yet unexplained abundance anomalies.

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Section: Peculiar Line Profiles and Stratificationmentioning

confidence: 99%

Hot subdwarf stars in close-up view

Geier

2013

A&A

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“…These models do not treat the atmosphere in detail and assume that it is chemically homogeneous. However, recent observational evidence (Khalack et al 2007 and shows that the atmospheres of such stars exhibit vertical abundance stratification (see discussion below).…”

Section: Introductionmentioning

confidence: 99%

Stellar model atmospheres with abundance stratification

Leblanc

Monin

Hui-Bon-Hoa

et al. 2009

A&A

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Context. Atomic diffusion is believed to be an important physical process in the atmospheres of several types of stars. Stellar atmospheres, including the stratification of the elements due to diffusion, are then needed to properly compare theoretical results to observations for such stars. Aims. This paper aims to estimate the effect of vertical abundance stratification on the atmospheric structure of stars and its potential importance regarding observational anomalies for various types of stars. Methods. Simulations using a modified version of the PHOENIX atmosphere code will be described, while taking vertical abundance stratification into account. Results. Our results show that large abundance gradients can exist in the atmospheres of Ap and blue horizontal branch stars. Stratification can also lead to relatively large atmospheric structural changes. The effect of elemental stratification on the atmospheric structure might well be able to explain the well-known core-wing anomaly of the Balmer lines observed for cool Ap stars.

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“…The detection of vertical stratification of some elements, especially iron, in the atmospheres of blue HB stars lends additional support to the scenario of atomic diffusion being active in these regions (Khalack et al 2007(Khalack et al , 2008, but is at variance with the mixed zone introduced by Michaud et al (2008Michaud et al ( , 2011. Hui-Bon-Hoa et al (2000) and LeBlanc et al (2009) presented stellar atmosphere models of blue HB stars that include the effect of vertical abundance stratification on the atmospheric structure.…”

Section: Introductionmentioning

confidence: 70%

Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters

Moehler

Leblanc

Khalack

et al. 2014

A&A

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Context. NGC 288 is a globular cluster with a well-developed blue horizontal branch (HB) covering the u-jump that indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue HB stars. Aims. We compare observed abundances with predictions from stellar evolution models calculated with diffusion and from stratified atmospheric models. We verify the effect of using stratified model spectra to derive atmospheric parameters. In addition, we investigate the nature of the overluminous blue HB stars around the u-jump. Methods. We defined a new photometric index sz from uvby measurements that is gravity-sensitive between 8000 K and 12 000 K. Using medium-resolution spectra and Strömgren photometry, we determined atmospheric parameters (T eff , log g) and abundances for the blue HB stars. We used both homogeneous and stratified model spectra for our spectroscopic analyses. Results. The atmospheric parameters and masses of the hot HB stars in NGC 288 show a behaviour seen also in other clusters for temperatures between 9000 K and 14 000 K. Outside this temperature range, however, they instead follow the results found for such stars in ω Cen. The abundances derived from our observations are for most elements (except He and P) within the abundance range expected from evolutionary models that include the effects of atomic diffusion and assume a surface mixed mass of 10 −7 M . The abundances predicted by stratified model atmospheres are generally significantly more extreme than observed, except for Mg. When effective temperatures, surface gravities, and masses are determined with stratified model spectra, the hotter stars agree better with canonical evolutionary predictions. Conclusions. Our results show definite promise towards solving the long-standing problem of surface gravity and mass discrepancies for hot HB stars, but much work is still needed to arrive at a self-consistent solution.

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Vertical abundance stratification in the blue horizontal branch star HD 135485

Cited by 25 publications

References 25 publications

Hot subdwarf stars in close-up view

Hot subdwarf stars in close-up view

Stellar model atmospheres with abundance stratification

Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters

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