Discovery of a spectroscopic binary comprising two hot helium-rich subdwarfs

Ahmad, Amir; Jeffery, C. S.; Fullerton, A. W.

doi:10.1051/0004-6361:20035917

Cited by 27 publications

(30 citation statements)

References 14 publications

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“…Interestingly, we find that other He-dominated objects (e.g. He-rich subdwarfs; Müller-Ringat 2013; Németh et al 2012;Naslim et al 2010;Hirsch 2009;Ahmad et al 2004;Rauch et al 1998Rauch et al , 1991Husfeld et al 1989;Stroeer et al 2007, DO WDs Werner et al 2014;Mahsereci 2011;Hügelmeyer et al 2006;Dreizler & Werner 1996) also show these differences in the CNO abundances 13 . Some objects are enriched in N but not in C, some C-enriched objects do not show N, but there also are objects which are enriched in both.…”

Section: Resultsmentioning

confidence: 48%

“…Zhang & Jeffery (2012a) compared their result with He-rich sdO stars from the sample of Hirsch (2009) and He-rich sdB stars from the sample Naslim et al (2010). We extended this comparison to all He-rich sdB and sdO stars, that are enriched in C and/or N (Müller-Ringat 2013; Németh et al 2012;Naslim et al 2010;Hirsch 2009;Ahmad et al 2004;Rauch et al 1998Rauch et al , 1991Husfeld et al 1989;Stroeer et al 2007), the O(He) stars of our work, and those of Werner et al (2014); Wassermann et al (2010), and the known DO WDs (Werner et al 2014;Mahsereci 2011;Hügelmeyer et al 2006;Dreizler & Werner 1996). We found that all of them fit this scenario.…”

Section: Merger Scenariosmentioning

confidence: 89%

See 1 more Smart Citation

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

Reindl¹,

Rauch²,

Werner³

et al. 2014

A&A

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Context. About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are not yet understood. Aims. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. A precise analysis of their photospheric abundances will establish constraints to their evolution. Methods. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. Results. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf (WD) merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. Conclusions. A helium-dominated stellar evolutionary sequence exists that may be fed by different types of mergers or CE scenarios. It appears likely that all these pass through the O(He) phase just before they become WDs.

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Section: Resultsmentioning

confidence: 48%

Section: Merger Scenariosmentioning

confidence: 89%

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

Reindl¹,

Rauch²,

Werner³

et al. 2014

A&A

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“…Until recently, only one double-lined He-sdB+He-sdB binary could be analysed (Ahmad & Jeffery 2004).…”

Section: Nature Of the Companionsmentioning

confidence: 99%

Hot subdwarf stars in close-up view

Geier¹,

Heber²,

Podsiadlowski

et al. 2010

A&A

162

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The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Since most of the known sdB systems are single-lined spectroscopic binaries, it is difficult to derive masses and unravel the companions' nature, which is the aim of this paper. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. We found orbital synchronisation most likely to be established in binaries with orbital periods shorter than 1.2 d. Only in five cases it was impossible to decide whether the sdB's companion is a white dwarf or an M dwarf. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases (including the well known system KPD1930+2752) the companion mass exceeds 0.9 M , four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion. The sample also contains a candidate post-RGB star, which rotates fast despite its long orbital period. The post-RGB stars are expected to spin-up caused by their ongoing contraction. The age of the sdB is another important factor. If the EHB star is too young, the synchronisation process might not be finished yet. Estimating the ages of the target stars from their positions on the EHB band, we found PG 2345+318, which is known not to be synchronised, to lie near the zero-age extreme horizontal branch as are the massive candidates PG 1232−136, PG 1432+159 and PG 1101+249. These star may possibly be too young to have reached synchronisation. The derived large fraction of putative massive sdB binary systems ...

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“…Considerable effort has been spent on discovering and exploring these stars, with spectacular results. They turn out to fall into diverse groups, including the double subdwarf PG1544+488 (Schulz et al 1991;Ahmad et al 2004;Şener & Jeffery 2014), the binary CPD-20…”

Section: Introductionmentioning

confidence: 99%

GALEX J184559.8−413827: a new extreme helium star identified using SALT★

Jeffery

2017

Monthly Notices of the Royal Astronomical Society

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A high-resolution spectrum of the helium-rich 'hot subdwarf' GALEX J184559.8−413827 ( J1845−4138) obtained with SALT HRS demonstrates it to be the first extreme helium (EHe) star to be discovered in nearly 40 years. A quantitative analysis demonstrates it to have an atmosphere described by T eff = 26 170 ± 750 K, log g/cm s −2 = 4.22 ± 0.10, and a surface chemistry characterised by CNO-processed helium, a 1% contamination of hydrogen (by number), and a metallicity 0.4 dex subsolar. Its distance and position are consistent with membership of the Galactic bulge. Its sharp absorption lines place strong constraints on both the rotation and microturbulent velocities. Spectroscopically, J1845−4138 closely resembles the pulsating EHe star V652 Her, generally considered to be the product of a double helium white dwarf merger evolving to become a helium-rich sdO star.

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Discovery of a spectroscopic binary comprising two hot helium-rich subdwarfs

Cited by 27 publications

References 14 publications

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

Hot subdwarf stars in close-up view

GALEX J184559.8−413827: a new extreme helium star identified using SALT★

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