Early type stars at high galactic latitudes - I. Ten young massive B-type stars

We present the results of quantitative spectral analyses of ten apparently normal B-type stars. These stars were found to be young massive B-type stars at distances of z = 2.6 to 7.6 kpc from the galactic plane based on their positions in the (T-eff, log g) diagram, normal abundance patterns and/or large projected rotational velocities. We discuss formation scenarios (runaway star scenarios versus a scenario for star formation in the halo) by comparing times-of-flight and evolutionary time scales. For all stars (except SB 357 and HS 1914+7139) both the scales are similar indicating that the stars could have formed in the galactic disk and been ejected from there soon after their birth. Derived ejection velocities range from 130 km s(-1) to 440 km s(-1) and may be used to constrain models for ejection mechanisms. Using new proper motion measurements we show that PHL 346, which was considered the most likely candidate for a young B-type star born in the halo, can be explained as a runaway star from the galactic plane.