Runaway OB Stars in the Small Magellanic Cloud: The Case for Dynamical Ejection Dominance over Supernova Ejections
Abstract
For decades, the existence of massive stars located in sparsely-populated regions outside of star clusters has both challenged and advanced our understanding of stellar kinematics. Most of these stars were ejected from their parent clusters via: (1) the dynamical ejection scenario (DES) involving close interactions with a multiple star system, or (2) the binary supernova scenario (BSS) involving the explosion of a binary companion star. We use GAIA DR2 proper motions (PMs) for 311 field OB stars from the spatially complete Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC to constrain the relative contributions from the two ejection mechanisms. We obtain local residual transverse velocities (vloc) by correcting for the systemic PM of each star's neighborhood. Although both ejection mechanisms produce stars with runaway velocities (vloc >30 km/s), the DES mechanism results in higher velocity runaways than the BSS. Thus, assuming that all of our stars with vloc >75 km/s are dynamically ejected, we can extrapolate the predicted v-3/2 DES velocity distribution to obtain the total number of DES runaways. Furthermore, since the BSS produces stars with the highest rotational velocities, we use the number of Oe/Be runaways as a crude estimate for the number of our BSS runaways. Both of these methods suggest that the DES accounts for about ¾ of our runaways and that the BSS accounts for ¼. Further, the number of runaway high-mass X-ray binaries (HMXBs) and Oe/Be stars implies that the BSS produces surprisingly fast runaways, which may be due to the low SMC metallicity and greater black hole natal kick velocities. Finally, the well-known HMXB SMC X-1 apparently has vloc = 90 km/s; given its high mass, this suggests that SMC X-1 may have been produced via a two-step ejection scenario, combining both DES and BSS.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23511018D