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Дата изменения: Fri Jun 21 20:14:31 1996
Дата индексирования: Sat Dec 22 04:47:45 2007
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Поисковые слова: galactic collision

Introduction

Since IRS 7 at the Galactic center was identified as a luminous red supergiant (Lebofsky et al. 1982), a lasting debate has been initiated whether the nucleus of the Milky Way galaxy had experienced a recent star formation episode. For more than a decade, many arguments both pro and contra have been accumulated. One of the strongest arguments for the presence of a young stellar population was the finding of luminous HeI/HI emission line stars both in IRS 16 (Hall et al. 1982) and in the central stellar cluster (Krabbe et al. 1991). The spectroscopic study of the AF star, one of the most remarkable representatives of these stars, came up with the conclusion that they are probably related to luminous, blue supergiants in a short-lived phase of intense mass loss (Najarro et al. 1993). However Morris (1993) proposed that the interpretation of the HeI/HI emission line stars in terms of young, massive stars is not the only possibility. The finding of a very high stellar density in the central star cluster (Eckart et al. 1993) is seemingly consistent with the other interpretations: the HeI/HI stars could be either 10 black holes (BHs) that have collided with giants (Morris 1993) or could result from collisions and mergers of less massive stars in the cluster (for discussion of these possibilities, see Genzel et al. 1994).

Until recently, this conflicting arguments have remained insufficient to resolve the issue. Ozernoy et al. (1993) suggested a new approach by showing that the 10 KeV gas in the central 200 pc of the Galaxy (Koyama et al. 1989) could be produced by multiple supernova explosions. Such supernovae, if sequential, might be associated with a starburst whose parameters are a scaled-down version of those of starburst galaxies (Ozernoy 1994). Related evidence was suggested by Sofue (1994) using the North Polar Spur data; it remains to be seen, however, whether this spur is associated with the Galactic center indeed rather than being a local feature. Tamblyn & Rieke (1993) and Schaerer (1994) proposed starburst models which would be able to account for a young stellar population at the Galactic center, although the origin of some peculiar hot stars still remains to be explained (Tamblyn et al. 1996). A recent finding of a possible WR star at a projected distance of 0.5 pc (Blum et al. 1995) seems to be one of yet missing links to massive stars produced during the starburst.

The purpose of this paper is to explore some consequences of the putative starburst at the Galactic center by elaborating the population synthesis model for the late evolutionary stages of massive binary stars created in the starburst, with emphasis on X-ray source statistics. As is known, X-ray observations of the Galactic center have revealed a number of energetic X-ray sources located in the innermost regions of the Galaxy (Pavlinsky et al. 1994, Churasov et al. 1994), with a part of them being attributed, by their spectral characteristics, to BH candidates. These observations demonstrate a substantially enhanced, compared to the average galactic value, spatial density of X-ray binary systems in the central region of the Galaxy. In this paper, we show that such a situation can be natural consequence of binary stellar evolution if a starburst occurred a few million years ago at the Galactic center. We apply Monte Carlo simulations to the evolution of a large ensemble of binary systems, with proper accounting for the spin evolution of magnetized compact stars (Lipunov 1992, Lipunov et al. 1994). This method has been shown to be a powerful tool for studying different products of stellar evolution both in spiral and elliptical galaxies under a wide range of initial conditions and star formation histories (see e.g. Kornilov & Lipunov 1984, Lipunov & Postnov 1987, Lipunov et al. 1994 and references therein).

In this paper, we will focus on the most prominent, from the observational point of view, representatives of the late stages of massive binary evolution, such as X-ray transients (NS in a highly eccentric orbit around a main sequence star, like A0535+26), super-accreting BHs (observationally seen as SS 433 if in pair with a Roche lobe filling secondary component), and BH + supergiant binaries (like Cyg X-1, with an evolved supergiant underfilling its Roche lobe). Different kinds of binary pulsars which appear after a star formation burst were considered previously (Lipunov et al. 1995a). Here we calculate the numbers of the corresponding X-ray sources and diccuss their spatial distribution. We include in our study only massive X-ray systems, because during the first 10 Myr only massive stars () can leave the main sequence.