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Astron. Astrophys. 356, 517-528 (2000)

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1. Introduction

The enigmatic phenomenon of the blue straggler stars has been widely discussed in the literature for almost half a century. Nowadays there is no doubt that they do not really follow the standard evolutionary scenario of the other cluster members. Despite the numerous efforts to explain this phenomenon, no general mechanism resulting in the remarkable apparent delay in their evolution has been proposed (see e.g. the detailed review by Stryker 1993 or the paper by Leonard & Linnell 1992). This discouraging result may be due to the absence of any systematic observational studies of these stars (i.e. spectroscopic analyses). Available fragmentary results do not allow a general picture to be drawn of the variations among the spectra of the blue stragglers themselves, or between the blue stragglers and normal stars of the same spectral type.

In this paper, which is to be regarded as part of a series, we continue a systematic observational study of blue stragglers in open clusters that was started by Andrievsky (1998), hereafter Paper I. We also follow up the detailed investigation of blue stragglers in several open clusters by Schönberner & Napiwotzki (1994). Those authors have shown that the most attractive scenario for the blue straggler phenomenon based on the evolution of single stars is probably not valid. With that conclusion, they clearly stress the urgent necessity of searching for other ways to account for the blue straggler phenomenon. It is quite clear that without any systematic observational work this can hardly be done.

We investigated objects in the open clusters NGC 3496, NGC 6475, NGC 6633 and IC 2602 (ages 0.23, 0.22, 0.66 and 0.036 Gyr respectively, according to Ahumada & Lapasset 1995).

  • [FORMULA] is a very poorly investigated open cluster. Ahumada & Lapasset (1995) mention four stars as its blue stragglers. They are, according to the Sher (1965) nomenclature, S 43, S 59, S 115 and S 117. No reliable information about their cluster membership is available.

  • NGC 6475 contains two blue stragglers. In the compilation of Ahumada & Lapasset (1995), HD 162374 is mentioned as a He-weak star with variable radial velocity (within 30 km s-1). Borra, Landstreet & Thompson (1983) tried to measure the surface magnetic field, but their result was not convincing: the detected field was of the same order of magnitude as the errors (100-400 G). The second blue straggler, HD 162586, is known to be a visual binary. For HD 162374, Gieseking (1985) gives a membership probability of 92%. Also, Snowden (1976) qualitatively confirms the membership of both blue stragglers.

  • Several objects have been investigated in the open cluster NGC 6633. Among them two stars are blue stragglers and three main sequence stars.

  • The very young open cluster IC 2602 contains only one very hot blue straggler, [FORMULA] Car, but this star was not the subject of present investigation. We observed several main sequence stars in this cluster.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000