Hot subdwarf stars in close-up view

Abstract

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.

Hot subdwarf stars in close-up view

“…Their models, which assume extra mixing but no stellar-wind mass loss, are in general agreement with the observed metal abundances of sdB stars (Geier 2013) as well as the helium abundances for T eff > 25 000 K. Furthermore, they show that diffusion effects reach far deeper than the stellar atmospheres themselves and should also be dominant in the He + /He 2+ convection zone (Moehler, Michaud, priv. comm.…”

A subsynchronously rotating pulsating subdwarf B star in a short-period binary with a white dwarf companion

“…After making these symmetric we found that KIC 7664467 has an NLTE temperature of T eff = 27 440 ± 120 K, surface gravity of log g = 5.382 ± 0.019 dex, and He abundance of log(nHe/nH) = −2.57 ± 0.05 dex, which places it among most of the slow pulsating sdB stars. The metallicity of KIC 7664467 fits in the general abundance profile of sdB stars (Geier 2013): helium and light metals are depleted, while Fe is near or somewhat above the solar abundance. The nuclear history of sdB stars is not reflected by their surface parameters, because diffusion processes change their surface abundances (Michaud et al 2011).…”

The subdwarf B star SB 290 – A fast rotator on the extreme horizontal branch

“…The C ii lines at 4267 Å, as well as the Si iii lines at 4552, 4567, 4574, and 5739 Å, are strong enough to measure both the abundances and the rotational broadening in the way described in Geier (2013). The v rot sin i = 48 ± 2 km s −1 measured from those lines is somewhat smaller than the one measured from the Balmer and helium lines.…”

“…In the course of our studies of sdB stars at high spectral resolution (Geier et al 2010(Geier et al , 2011aGeier 2013), the sdB star SB 290 caught our attention because of its unusual line shapes. SB 290 (CD−38 222) was discovered as a Btype star in the objective prism survey of the south Galactic pole region by Slettebak & Brundage (1971).…”