Main sequence B-type stars located far away from the galactic plane are a rare, albeit known phenomenon. In their pioneering paper, Greenstein & Sargent (1974) studied faint blue stars at high galactic latitudes and classified 25% of them as apparently normal OB-type stars at distances from the galactic plane of z= 1-3 kpc. More detailed studies have shown that many, but not all, of the apparently normal stars were in fact highly evolved low-mass stars. Surveys for UV excess objects (e.g., Palomar Green, Hamburg-Schmidt, Edinburgh Cape) have found many new candidates (e.g., Saffer et al. 1997; Rolleston et al. 1999; Magee et al. 1998) and some may be known even in other galaxies (M 31, Smoker et al. 2000). Their properties, possible evolutionary histories and formation mechanisms were reviewed by, e.g., Tobin (1987), Keenan (1992), and Heber et al. (1997).
Tobin (1987) also discusses the problem that some highly evolved stars spectroscopically mimic massive stars almost perfectly. The most striking example is PG 0832+676 which has been analysed several times. Its abundance pattern is close to normal. Only recently, Hambly et al. (1996) were able to firmly establish slight underabundances and a very low projected rotation velocity. Combining both results they concluded that PG 0832+676 in fact is a highly evolved star. Abundance analyses as well as determinations of rotational velocities are thus of essential importance for the verification of massive B-type star candidates. A high rotational velocity generally excludes a late evolutionary status of the star, as old, low-mass stars cannot rotate as fast as massive stars. This fact was used, e.g., by Heber et al. (1995, HS 1914+7139) and Schmidt et al. (1996, PG 0009+036) to identify massive B-type stars far from the Galactic plane from medium-resolution spectra.
The massive B-type stars in galactic halos can be separated kinematically into two
different categories: those stars with a main sequence
lifetime larger than the time they would need to travel from the plane to
their present position and those with a main sequence lifetime too small to
reach their current position assuming an acceptable velocity vertical to the
galactic disk. The former ones are assumed to be born in the disk and
thereafter ejected from it (runaway stars), while the latter are supposed to be born in the
halo (see Conlon et al. 1988, 1990; Hambly et al.
1993, for more details). Since many years runaway stars are known
to exist, whereas formation of massive stars in the halo has not yet
been confirmed convincingly. The best studied candidate is PHL 346 (Ryans
et al. 1996), a 
Cephei star in the halo (Dufton et al.
1998). A search for coeval stars around
PHL 346 (Hambly et al. 1996) met with limited success, since only one
out of 16 A- and B-type stars around
PHL 346 was found to have the appropriate spectral type and radial velocity.
Calculations of galactic orbits are thus very important to determine the true nature of the stars, since they also allow to determine ejection velocities from the galactic disk. However, accurate proper motions are a prerequisite for such an analysis. A big step forward has been achieved by Thejll et al. (1997) and the Hipparcos/Tycho mission (Perryman et al. 1997; Høg et al. 2000).
In this paper we present the analysis of new high-resolution spectra for 10 apparently normal B-type stars. Half of the sample are new discoveries whereas the other half has already been studied previously (Conlon et al. 1992; Ryans et al. 1996; Rolleston et al. 1999; Heber et al. 1995). For the latter the times-of-flight quoted in the literature appear to be larger than the evolutionary times indicating they might have formed in the halo. However, proper motions were not available rendering these estimates of the times-of-flight uncertain. Since proper motion measurements became available recently for three of these stars, it was deemed necessary to reanalyse them from new high resolution spectra.
Copyright ESO 2001