The Massive Runaway Binaries in NGC 3603 Cluster postprint

Abstract: Massive stars tend to form in clustered environments. The origin of the massive stars and binaries outside clusters is one key to understand the formation and evolution of massive stars. The authors selected two colliding wind candidates, MTT68 and MTT71, and probed their origin by analyzing the \emph{Chandra} X--ray observation and the proper motions queried from the \emph{Gaia} Data Release 2 (\emph{Gaia}~DR2) catalog. Those two targets are located on the margin of NGC 3603, the galactic massive cluster 7.6\;kpc away. MTT68 has been resolved as a visual binary of MTT68A/B, and MTT68A itself was proposed as a close massive binary. The authors compared the centroid of the X--ray emission and the coordinate queried from \emph{Gaia}~DR2 catalog, to those of MTT68A/B on the \emph{HST} image, and found that both of them are close to MTT68A but far away from MTT68B, which suggests the X--ray characteristic and the astrometry measurement both mainly describe MTT68A instead of MTT68B. They also carried out the X-ray spectral analysis of MTT68A and MTT71, and found (1) the existence of the Fe XXV line in their spectra, (2) a second component of temperature $kT\gtrsim2.0$\;keV is required when fitting their spectra with hot plasma models, and (3) their ratio of X-ray to bolometric luminosity, lg($L_{\rm X}/L_{\rm bol})\gtrsim-5.8$, is at least one order of magnitude higher than the value of isolated O-type stars. All above features further prove their colliding wind scenario. Besides that, they analyzed the astrometry measurements of MTT68A and MTT71 queried from the \emph{Gaia}~DR2 database. MTT71 has an absolute proper motion similar to the overall value of NGC 3603, which suggests it formed associated with NGC 3603 without ejection process. In contrast, MTT68A has a significant proper motion relative to NGC 3603: the relative proper motion in the right ascension direction $\Delta\mu_\alpha=(-2.5\pm1.1)$\;mas\,$\cdot$\,yr$^{-1}$ and declination direction $\Delta\mu_\delta=(3.3\pm0.9)$\;mas\,$\cdot$\,yr$^{-1}$, resulting a direction that opposites to the core region of NGC 3603. Hence the authors propose that MTT68A is a massive binary ejected 20\;kyr ago, and single-binary and binary--binary interactions are possible mechanisms. A search for the massive star with reverse relative proper motion at the opposite side of the cluster will be helpful to verify this hypothesis.