7 Discussion

We have carried out quantitative spectral analyses of ten apparently normal B-type stars. Their positions in the ( ${T_{\rm eff}}$, $\log {g}$) diagram are consistent with models for main sequence stars. SB 357 shows emission in $\rm {H_{\beta}}$ and $\rm {H_{\gamma}}$, which confirms its classification as a Be star (Heber & Langhans 1986; Kilkenny 1989). Seven of the stars have rotational velocities >70 km $\rm {s^{-1}}$making detailed abundance analyses impossible. Mostly normal abundances with respect to $\iota$ Her were determined for BD-15$^\circ$115, PHL 159 and PHL 346. The Mg and O abundances of PHL 159 are significantly different from the comparison star and other normal B-type stars (Kilian 1994).

Calculated orbits based on measurements of radial velocity and proper motion allowed to determine times-of-flight from the galactic plane to their present position.

7.1 Runaway stars

Times-of-flight for PG 1511+367 and PG 1533+467 are smaller than the evolutionary times, indicating that these stars have been formed in the galactic plane and were then ejected (runaway stars). The times-of-flight are similar to the evolutionary times for PG 0122+214, PG 2219+094, PHL 159, BD-15$^\circ$115, and PG 1610+239, which implies that the stars could also have formed in the galactic disk and were then ejected very soon after their birth. Ejection velocities for all programme stars range from 130km s^-1 to 440km s^-1.

Three mechanisms for the production of runaway stars have been proposed in the literature:

i)

In the binary supernova scenario (Zwicky 1957; Blaauw 1961) the runaway star receives its velocity after a supernova explosion in a massive close binary. After the explosion the binary occasionally dissociates and the secondary is traveling with a velocity comparable to its pre-explosion orbital velocity. Calculations by Iben & Tutukov (1996) indicate that the runaway star can gain a velocity of 100 to 200km s^-1;

ii)

In the dynamical ejection scenario (Poveda et al. 1967; Gies & Bolton 1986) the runaway star gains its velocity through a dynamical interaction with one or more stars. The most efficient encounter is that between two close binaries in a stellar association or open cluster, which in most cases results in the ejection of two runaway stars and one eccentric binary (Hoffer 1983; Mikkola 1983). Calculations by Leonard (1991) show that even velocities in excess of 1000km $\rm {s^{-1}}$ can be gained in rare cases;

iii)

It has been conjectured that star formation can be triggered by the interaction of the gas of the galactic disk with an infalling satellite dwarf galaxy. The momentum transferred to the gas results in a significant velocity component of the newly born stars perpendicular to the galactic plane. This scenario has been supported by the discovery of the encounter with the Sagittarius dwarf galaxy (Ibata et al. 1994).

The availability of precise milliarcsecond astrometry for nearby stars through the Hipparcos satellite and pulsar astrometry and timing measurements have recently demonstrated that both the binary supernova scenario and the dynamical ejection scenario are viable. By calculating orbits for runaway stars, pulsars and open clusters it recently became possible to associate runaway stars and pulsars with their nascent clusters (Hoogerwerf et al. 2000, 2001).

Our programme stars are too far away and their space motions are therefore not known accurately enough to allow to identify their relation to a young cluster or association in the galactic plane. However, the ejection velocities determined for our programme stars may be important to identify the mechanism which led to their ejection from the galactic plane, once reliable theoretical predictions become available for the different scenarios discussed above. Six stars have escape velocities exceeding 300km s^-1 which seems too large to be achievable by the binary supernova scenario.

7.2 Stars born in the galactic halo?

Four stars in our sample have been proposed in the literature as candidates for B-type stars formed in the galactic halo because their times-of-flight were found to be considerably larer than the evolutionary time scales (Conlon et al. 1992; Keenan et al. 1986; Heber et al. 1995). As discussed above our new analysis of BD-15$^\circ$115 demonstrates that its time-of flight is consistent with the evolutionary time. Hence it could be runaway star, too.

For SB 357 and HS 1914+7139 the times of flight are more than twice as large as the evolutionary times, which would make formation in the disk unlikely. However, their times-of-flight are uncertain due to the lack of proper motion measurements. Such data are urgently needed before any firm conclusions can be drawn. Therefore we are reluctant to regard these stars as born in the halo.

PHL 346 has been proposed as a candidate massive B-type star born in the halo (Ryans et al. 1996; Hambly et al. 1996). Based on the new Tycho proper motion measurement, our analysis indicates that $T_{\rm flight}$ is marginally larger than ${T_{\rm evol}}$ and PHL 346 can be a runaway star, too.

Hence no conclusive candidate for a young massive B-type star formed in the halo remains in our sample. Proper motions for the four stars lacking any measurement should urgently be determined.

Acknowledgements

M.R. gratefully acknowledge financial support by the DFG (grant He1356/27-1). We thank Michael Odenkirchen who kindly provided us with his code ORBIT6 for the calculation of the kinematic orbits, Heinz Edelmann who carried out the DSAZ FOCES and ESO FEROS observations and Neil Reid, Ralf Napiwotzki and Klaus Werner who obtained the Keck HIRES spectra for us. S.M. was supported by a grant (50 OR 96029-ZA) from the Bundesministerium für Bildung und Forschung through the DLR.