Young stars at large distances from the galactic plane mechanisms of formation.pdf

a r X i v : a s t r o - p h / 0 2 1 2 2 7 2 v 1 1 1 D e c 2 0 0 2 YOUNG STARS AT LARGE DISTANCES FROM THE GALACTIC PLANE: MECHANISMS OF FORMATION Christine Allen ? T.D.Kinman ? Abstract We have collected from the literature data for a list of early-type stars, situated at large distnces from the galactic plane, for which evidence of youth seems convincing. We discuss two possible formation mechanisms for these stars: ejection from the plane by dynamical interactions within small clusters, and formation away from the plane , via induced shocks created by spiral density waves. We identify the stars that could be ex- plained by each mechanism. We conclude that the ejection mechanism could account for about two thirds of the stars, while a combination of star formation at z = 500-800 pc from the plane and ejection, can account for 90 percent of the stars. Neither mechanism, nor both together, can explain the most extreme examples. 1 Introduction There exists an anomalous group of early-type -hence mostly young- stars lo- cated far from the galactic plane, at z-distances ranging from 1 to more than 10 kpc. First studied were the A-type stars (Rodgers et al. 1981; Lance 1988). Such intermediate-age stars have most recently been studied by Preston and Sneden (2000) and are likely to be blue stragglers. More recently, a number of OB stars have been found, also situated at vertical distances of up to several kpc from the plane (Conlon et al. 1989; Conlon et al. 1990; Conlon 1992; Schmidt et al. 1997; Ringwald et al. 1998). The shorter lifetimes of these stars exacerbate the problems of their interpretation in terms of the standard picture of star formation and galactic evolution. The most extreme examples, if they originated in the plane, must have been ejected with velocities surpassing 1000 km/s, which clearly are unrealistically high values. ?Instituto de Astronomia, UNAM, Apartado, Postal 70-264 04510, Mexico, D.F. email: chris@astroscu.unam.mx ?KPNO/NOAO(Operated b