A bright early-type star in the halo of NGC 253:: Runaway or in situ formation?

We present observations of J004804.8-251749, a blue object in the direction of the halo of NGC 253 with (B - V) = -0.16 +/- 0.08, V = 22.09 +/- 0.06 and a stellar spectrum showing the Balmer lines redshifted to a velocity of 279 +/- 15 km s(-1). These data are consistent with a B5/B8 supergiant star with a mass M = 12 M. and a maximum age tau = similar or equal to20 Myr in the halo of NGC 253. Based on its position, radial velocity, and inferred maximum age, we consider the possibility that J004804.8-251749 may be a runaway star ejected from the disk of NGC 253. We derive a lower limit of 172 km s(-1) for the ejection velocity, or 148 km s(-1) if the lower limit of the distance fork to NGC 253 is adopted. While being within the range of velocities accessible to runaway stars through the dynamical ejection mechanism, such high velocities are rather unlikely, especially since the inferred value is only a lower limit. This leads us to consider in situ formation as a possible alternative scenario, in which the formation of J004804.8-251749 would have been triggered by the interaction between the starburst-driven superwind stemming from the center of NGC 253 and a cloud of cold hydrogen in the halo. Such a formation mechanism would be consistent with our previous finding of an excess of blue point sources in the direction of the halo of NGC 253, which we interpreted as indicating the existence of superwind-triggered star formation.