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Astron. Astrophys. 347, 69-76 (1999)

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1. Introduction

In a previous paper (Saffer et al. 1997), we have presented low-resolution optical spectroscopy for a complete sample of blue stellar objects that have been taken from the Palomar-Green ultraviolet excess survey (Green et al. 1986). This sample represents the largest that has been obtained to date, in an attempt to resolve a number of key questions concerning the nature and origin of the faint, blue stars that are observed at intermediate and high Galactic latitudes (see Keenan 1992, for a review). Most are at a post main-sequence evolution stage, viz. white dwarfs (Bergeron et al. 1994), hot subdwarfs or extended horizontal-branch stars (Saffer et al. 1994; Hambly et al. 1997, hereafter Paper I) or post-asymptotic giant branch stars (McCausland et al. 1992). In addition, there exists a small subset of objects that are spectroscopically indistinguishable from normal, young Population I B-type stars found in the Galactic disk (Conlon et al. 1992). The majority of this subset are plausible `runaway' stars, recently formed in the Galactic disk and subsequently ejected by some mechanism (see Leonard & Duncan 1988). However, in a few cases such as PHL 346 (Keenan et al. 1986), these apparently normal stars are found at large distances from the Galactic disk, and their evolutionary ages are too short for the objects to have attained their current Galactic locations, even given the estimated uncertainty in deriving those quantities (Ryans et al. 1996).

There is no doubt that star formation at large distances from the Galactic plane is a controversial idea. Many objects that were originally identified as distant, young halo formation candidates on the basis of low- or intermediate-resolution optical spectra, have subsequently been shown to belong to one or other of the classes of evolved, low-mass stars (see, for example, PG 0832+676, Brown et al. 1989; Hambly et al. 1996b). On the other hand, there is a certain amount of observational evidence for the existence of young stars and associations in lower gas density environments. For example, young associations of massive stars have been discovered in the `bridge' of material between the Magellanic Clouds (Irwin et al. 1985) and model-atmosphere analyses of individual objects have confirmed their evolutionary status (Hambly et al. 1994; Rolleston et al. 1998). Also, the high Galactic latitude, young, open cluster, Blanco 1, around Zeta Sculptoris (Blanco 1949) was probably formed as a result of shocks produced during the collision of an infalling halo high-velocity cloud (HVC) with the disk interstellar medium (Edvardsson et al. 1995). Shock induced star formation between halo HVCs has been postulated as the origin of the apparently young, distant B-type stars (Dyson & Hartquist 1983) where formation in situ seems to be the only possible explanation for their existence.

Previously, intermediate dispersion spectroscopy of a magnitude limited sample in an equatorial survey of limited solid angle (325 square degrees), suggested that there may be as many as several hundreds of these anomalous young stars present in the Galactic halo (Hambly et al. 1993; Little et al. 1995). More recently, Rolleston et al. (1997) reported results from an on-going Southern hemisphere survey for 25 high-latitude stars identified as having B-type spectra. Model-atmosphere analyses of the high-resolution optical spectroscopy confirm a Population I status for 17 objects. However, to date, no object in this sample has been identified that cannot be explained as a Galactic disk runaway. In this paper, we present high-resolution optical spectroscopy of a new sample of distant, apparently young B-type stars identified from the Palomar-Green Survey. A detailed model-atmosphere/ kinematical analysis has been undertaken in an attempt to elucidate the evolutionary history of each object.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999