Issue |
A&A
Volume 441, Number 2, October II 2005
|
|
---|---|---|
Page(s) | 675 - 684 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20053002 | |
Published online | 19 September 2005 |
On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918–549
1
SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: jeanz@sron.nl
2
Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
3
Physics Department, McGill University, 3600 Rue University, Montreal, QC, H3A 2T8, Canada
Received:
7
March
2005
Accepted:
19
June
2005
2S 0918–549 is a low-mass X-ray binary (LMXB) with a low optical
to X-ray flux ratio. Probably it is an ultracompact binary with an
orbital period shorter than 60 min. Such binaries cannot harbor
hydrogen rich donor stars. As with other (sometimes confirmed)
ultracompact LMXBs, 2S 0918–549 is observed to have a high
neon-to-oxygen abundance ratio (Juett et al. 2001, ApJ, 560, L59) which has been
used to argue that the companion star is a CO or ONe white
dwarf. However, type-I X-ray bursts have been observed from several
of these systems implying the presence of hydrogen or helium on the
neutron star surface. In this paper, we argue that the companion star
in 2S 0918–549 is a helium white dwarf. We first present a type-I X-ray
burst from 2S 0918–549 with a long duration of 40 min. We show
that this burst is naturally explained by accretion of pure helium at
the inferred accretion rate of ~ times the Eddington
accretion rate. At higher accretion rates of ~
Eddington,
hydrogen is required to explain long duration bursts. However, at low
rates the long duration is due to the large amount of helium that
accumulates prior to the burst. We show that it is possible to form a
helium white dwarf donor in an ultracompact binary if accretion
starts during the first ascent of the giant branch, when the core is
predominantly made of helium. Furthermore, this scenario naturally
explains the high neon-to-oxgen ratio, without requiring a CO or ONe
white dwarf companion. The only observational aspect of 2S 0918–549 that
we cannot explain is the absence of helium lines in the optical
spectrum. Model calculations of optical accretion disk spectra need
to be carried out in order to obtain limits on the helium abundance.
Key words: X-rays: binaries / X-rays: bursts / X-rays: individuals: 2S 0918–549=4U 0919-54 / stars: evolution
© ESO, 2005
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