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Astron. Astrophys. 317, 815-822 (1997)

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1. Introduction

HD 48798 is a single-lined spectroscopic binary with an orbital period of 1.55 d (Thackeray 1970, Stickland & Lloyd 1994). This hydrogen-deficient subdwarf of spectral type O6 is one of the brightest subdwarfs and has been studied extensively (Kudritzki & Simon 1978, Hamann et al. 1981). In the HR-diagram O subdwarfs generally are near the blue end of the horizontal branch. They are believed to be evolved objects on or just off the horizontal branch with cores nearly as massive as their total mass (Thejll et al. 1994, and references therein). Because of its close binarity HD 49798 has an unusual position among the subdwarfs and, as we will show, a different evolutionary history as well.

Recently, Israel et al. (1995) reported the detection of regular X-ray pulsations with a period of 13.18 s from this binary. The X-ray spectrum of the source is very soft, but has a high-energy excess. Clearly the X-ray pulsations must arise from a compact companion, either a neutron star or a white dwarf.

In this paper we discuss the possible nature of this companion and the evolutionary history of the system. The hydrogen-deficient nature of HD 49798 shows that this star is the stripped core of an initially much more massive star. The overabundance of N and underabundance of C shows that its present surface layers have been processed by the CNO-cycle, implying that they belonged to the (outer part of the) hydrogen burning core of a massive star, when this star was on the main-sequence. This, in combination with the short orbital period of the binary, implies that the star lost practically all of its hydrogen-rich envelope in a common-envelope phase which resulted in spiral-in.

In Sect. 2 we discuss the constraints on the masses of both components and the evolutionary history of the binary. The nature of the compact companion is discussed in Sect. 3.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998