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

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany e-mail: fcomeron@eso.org
² Observatoire de Paris-Meudon, GEPI, UMR 8111 du CNRS, 92195 Meudon Cedex, France e-mail: ana.gomez@obspm.fr
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain e-mail: jordi@am.ub.es

Corresponding author: F. Comerón, fcomeron@eso.org

Received: 13 November 2002
Accepted: 19 December 2002

Abstract

We present observations of J004804.8-251749, a blue object in the direction of the halo of NGC 253 with , and a stellar spectrum showing the Balmer lines redshifted to a velocity of  km s^-1. These data are consistent with a B5/B8 supergiant star with a mass   and a maximum age  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.