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Astron. Astrophys. 356, 665-675 (2000)

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1. Introduction

LB 3459 (HD 269696) is a very blue ([FORMULA], [FORMULA], Wesselink 1962) peculiar early-type foreground object ([FORMULA]) of the Large Magellanic Cloud region (Feast et al. 1960) with a variable spectrum.

Early studies of LB 3459 by Kilkenny et al. (1978, 1979, 1981) and Paczynski (1980) established that the system is composed of an sdO primary star and a low-mass degenerate dwarf star of low temperature.

The first spectral analysis of the sdO primary star of LB 3459 by means of NLTE model atmosphere techniques was performed by Kudritzki et al. (1982). They determined [FORMULA], [FORMULA] (cgs), [FORMULA] [FORMULA] (by number) which is extraordinary small, [FORMULA], and [FORMULA]. This analysis was hampered by the lack of adequate numerical methods and computational capacity at that time, and thus only rudimental model atoms of hydrogen and helium could be considered (Kudritzki 1976).

Kudritzki et al. (1982) proposed diffusion to cause the He depletion in the photosphere of LB 3459. This was supported by an analysis of the carbon and silicon abundances by Lynas-Gray et al. (1984) using high-resolution IUE spectra. They found that these elements are depleted by factors of 100 and 10, respectively.

Based on the newly developed Accelerated Lambda Iteration (ALI) method (Werner & Husfeld 1985, Werner 1986) Rauch (1987) calculated new hydrogen NLTE models (lowest 15 H I levels treated in NLTE, all respective line transitions) in order to verify that the low [FORMULA] ratio was not an artifact due to an error in the determination of [FORMULA] (Heber et al. 1988). Rauch (1987) reproduced well the observations at [FORMULA] and excluded definitely [FORMULA]. He concluded that the [FORMULA] ratio is even lower (at least a tenth) than found by Kudritzki et al. (1982, see Table 4).

LB 3459 has been classified to be a post-common envelope (CE) sdOB + MS binary (de Kool & Ritter 1993). Binaries containing an sdOB star are rosetta stones for the understanding of CE evolution since the lifetime of this component is much shorter than the orbital evolution time scale of the binary (de Kool & Ritter 1993). The CE evolution is discussed in detail e.g. by Iben & Livio (1993).

Hilditch et al. (1996, hereafter HHH ) have recently analyzed the light and radial-velocity curves of LB 3459 and determined its orbital parameters precisely. Since a wide range for the primary mass ([FORMULA]) is possible within the error limits of [FORMULA] given by Kudritzki et al. (1982), and hence, even a very low secondary mass of [FORMULA] appears likely, HHH illustratively assumed that the primary has a mass of [FORMULA] and arrived at a mass of [FORMULA] for the secondary. Since they have presented a well defined orbital solution for LB 3459, their [FORMULA] is as good as their [FORMULA] assumption. Thus, the secondary might be a brown dwarf with a lower mass as well. HHH concluded that a new determination of [FORMULA] is urgently required in order to solve this problem.

To make progress, we have collected new spectra with ESO telescopes at La Silla (Table 1) and have now a complete coverage of the optical wavelength range. Together with IUE spectra which are available in the Final Archive, we determine the photospheric properties of LB 3459 within small error ranges (Sect. 3.2). The metal abundances (C, N, O, Mg, Si, Fe, and Ni) are determined by means of state-of-the-art NLTE model atmosphere techniques (Sect. 3.4). The masses of both, primary and secondary, are determined from comparison with evolutionary tracks and from the mass function of LB 3459 (Sect. 3.4.8).


Table 1. IUE spectra of LB 3459 retrieved from the Final Archive

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000