Massive runaway stars in the Large Magellanic Cloud

Abstract

The origin of massive field stars in the Large Magellanic Cloud (LMC) has long been an enigma. The recent measurements of large offsets (∼100 km s −1 ) between the heliocentric radial velocities of some very massive (O2-type) field stars and the systemic LMC velocity provides a possible explanation of this enigma and suggests that the field stars are runaway stars ejected from their birthplaces at the very beginning of their parent cluster's dynamical evolution. A straightforward way to prove this explanation is to measure the proper motions of the field stars and to show that they are moving away from one of the nearby star clusters or OB associations. This approach is, however, complicated by the long distance to the LMC, which makes accurate proper motion measurements difficult. We used an alternative approach for solving the problem (first applied for Galactic field stars), based on the search for bow shocks produced by runaway stars. The geometry of detected bow shocks would allow us to infer the direction of stellar motion, thereby determining their possible parent clusters. In this paper we present the results of a search for bow shocks around six massive field stars that have been proposed as candidate runaway stars. Using archival Spitzer Space Telescope data, we found a bow shock associated with one of our programme stars, the O2 V((f*)) star BI 237, which is the first-ever detection of bow shocks in the LMC. Orientation of the bow shock suggests that BI 237 was ejected from the OB association LH 82 (located at 120 pc in projection from the star). A by-product of our search is the detection of bow shocks generated by four OB stars in the field of the LMC and an arc-like structure attached to the candidate luminous blue variable R81 (HD 269128). The geometry of two of these bow shocks is consistent with the possibility that their associated stars were ejected from the 30 Doradus star-forming complex. We discuss implications of our findings for the problem of the origin of runaway stars and the early dynamical evolution of star clusters.

The VLT-FLAMES Tarantula Survey

“…Whilst there is currently no evidence for a bow shock, one of the possible explanations for the MIR excess is the presence of a bow-shock (e.g. Gvaramadze et al 2010). Alternative explanations for the MIR excess may involve its (line-of-sight) association with an active star forming region, or a nebula formed by vigorous mass loss since the object started to burn hydrogen in its core.…”

Massive runaway stars in the Small Magellanic Cloud

“…The high velocities of runaway OB stars can be revealed either directly, via measurement of their proper motions and/or radial velocities (e.g. Moffat et al 1998;Mdzinarishvili & Chargeishvili 2005;Massey et al 2005;Evans et al 2010), or indirectly, through the detection of bow shocks generated ahead of supersonically moving stars (Gvaramadze & Bomans 2008b;Gvaramadze et al 2010a, hereafter Paper I). The proper motions can be used to trace the trajectories of the field stars back to their parent clusters, hence to prove their runaway nature (e.g.…”

4U 1907+09: an HMXB running away from the Galactic plane

“…Like many other bow shocks discovered with Spitzer (e.g. Gvaramadze et al 2010aGvaramadze et al ,b, 2011Gvaramadze & Gualandris 2011), the bow shock associated with 4U 1907+09 is only visible at 24 μm (although the IRAC 5.8 and 8 μm images show the gleam of emission possibly associated with the bow shock). The bow shock has a clear arcuate shape with the apex at 0.…”

“…For runaway stars located far from star-forming regions and their associated gas outflows (caused by stellar winds and supernovae), one can assume that the peculiar velocity of the ambient medium is negligible compared with the space velocity of the star, meaning that the symmetry axis of the bow shock coincides well with the direction of stellar motion. In this case, the orientation of the bow shock can be used to back-trace the trajectory of the star to the parent cluster even for those stars whose proper motions cannot be measured with a high confidence (Gvaramadze & Bomans 2008;Gvaramadze et al 2010aGvaramadze et al ,b, 2011. The symmetry axis of the bow shock around 4U 1907+09 (see Fig.…”

“…van Buren & McCray 1988;van Buren et al 1995;Gvaramadze & Bomans 2008;Gvaramadze et al 2010a;). The first-ever detection of a bow shock associated with a HMXB was reported by Kaper et al (1997), who discovered a very symmetric Hα arc around the HMXB Vela X-1 (a system similar to 4U 1907+09).…”