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Register a new userRunaway and hypervelocity stars. The supernova connection
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We present an investigation of the known sample of runaway stars. The orbits of these stars are traced back to their origin in the Galactic disc. The velocity distribution of these stars is compared to theoretical predictions. We conclude that the majority of stars is well explained by the standard binary ejection mechanism (BEM) and the dynamical ejection mechanism (DEM). However, we find a sample of ten stars which has ejection velocities in excess of those predicted by standard scenarios. We discuss how these can be explained by a variant of the BEM. This mechanism can create runaway stars exceeding the Galactic escape velocity (known as hypervelocity stars). The number of runaway stars in our Galaxy is estimated and compared to the known sample of high mass X-ray binaries, whose formation is linked to the BEM channel.
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We measure proper motions with the Hubble Space Telescope for 16 extreme radial velocity stars, mostly unbound B stars in the Milky Way halo. Twelve of these stars have proper motions statistically consistent with zero, and thus have radial trajectories statistically consistent with a Galactic center hypervelocity star origin. The trajectory of HE 0437-5439 is consistent with both Milky Way and Large Magellanic Cloud origins. A Galactic center origin is excluded at 3-sigma confidence for two of the lowest radial velocity stars in our sample, however. These two stars are probable disk runaways and provide evidence for ~500 km/s ejections from the disk. We also measure a significant proper motion for the unbound sdO star US 708. Its 1,000 km/s motion is in some tension with proposed supernova ejection models, but can be explained if US 708 was ejected from the stellar halo. In the future, we expect Gaia will better constrain the origin of these remarkable unbound stars.
The dwarf carbon (dC) star SDSS J112801.67+004034.6 has an unusually high radial velocity, 531$pm 4$ km s$^{-1}$. We present proper motion and new spectroscopic observations which imply a large Galactic rest frame velocity, 425$pm 9$ km s$^{-1}$. Several other SDSS dC stars are also inferred to have very high galactocentric velocities, again each based on both high heliocentric radial velocity and also confidently detected proper motions. Extreme velocities and the presence of $C_2$ bands in the spectra of dwarf stars are both rare. Passage near the Galactic center can accelerate stars to such extreme velocities, but the large orbital angular momentum of SDSS J1128 precludes this explanation. Ejection from a supernova in a binary system or disruption of a binary by other stars are possibilities, particularly as dC stars are thought to obtain their photospheric $C_2$ via mass transfer from an evolved companion.
Runaway OB stars are ejected from their parent clusters via two mechanisms, both involving multiple stars: the dynamical ejection scenario (DES) and the binary supernova scenario (BSS). We constrain the relative contributions from these two ejection mechanisms in the Small Magellanic Cloud (SMC) using data for 304 field OB stars from the spatially complete, Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4). We obtain stellar masses and projected rotational velocities $v_rsin i $ for the sample using RIOTS4 spectra, and use transverse velocities $v_{rm loc}$ from $it{Gaia}$ DR2 proper motions. Kinematic analyses of the masses, $v_rsin i $, non-compact binaries, high-mass X-ray binaries, and Oe/Be stars largely support predictions for the statistical properties of the DES and BSS populations. We find that dynamical ejections dominate over supernova ejections by a factor of $sim 2-3$ in the SMC, and our results suggest a high frequency of DES runaways and binary ejections. Objects seen as BSS runaways also include two-step ejections of binaries that are reaccelerated by SN kicks. We find that two-step runaways likely dominate the BSS runaway population. Our results further imply that any contribution from $it{in-situ}$ field OB star formation is small. Finally, our data strongly support the post-mass-transfer model for the origin of classical Oe/Be stars, providing a simple explanation for the bimodality in the $v_rsin i $ distribution and high, near-critical, Oe/Be rotation velocities. The close correspondence of Oe/Be stars with BSS predictions implies that the emission-line disks are long-lived.
We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelocity Star Survey. The measurements reveal a clear pattern in the B-type stars. Halo stars dominate the sample at speeds about 100 km/s below Galactic escape velocity. Disk runaway stars have speeds up to 100 km/s above Galactic escape velocity, but most disk runaways are bound. Stars with speeds about 100 km/s above Galactic escape velocity originate from the Galactic center. Two bound stars may also originate from the Galactic center. Future Gaia measurements will enable a large, clean sample of Galactic center ejections for measuring the massive black hole ejection rate of hypervelocity stars, and for constraining the mass distribution of the Milky Way dark matter halo.
Wolf-Rayet stars are advanced evolutionary stages of massive stars. Despite their large mass-loss rates and high wind velocities, none of them display a bow shock, although a fraction of them are classified as runaway. Our 2.5-D numerical simulations of circumstellar matter around a 60Mo runaway star show that the fast Wolf-Rayet stellar wind is released into a wind-blown cavity filled with various shocks and discontinuities generated throughout the precedent evolutionary phases. The resulting fast-wind slow-wind interaction leads to the formation of spherical shells of swept-up dusty material similar to those observed in near infrared 24 micron with Spitzer, and which appear to be co-moving with the runaway massive stars, regardless of their proper motion and/or the properties of the local ambient medium. We interpret bright infrared rings around runaway Wolf-Rayet stars in the Galactic plane, like WR138a, as indication of their very high initial masses and a complex evolutionary history. Stellar-wind bow shocks become faint as stars run in diluted media, therefore, our results explain the absence of detected bow shocks around Galactic Wolf-Rayet stars such as the high-latitude, very fast-moving objects WR71, WR124 and WR148. Our results show that the absence of a bow shock is consistent with a runaway nature of some Wolf-Rayet stars. This questions the in-situ star formation scenario of high-latitude Wolf-Rayet stars in favor of dynamical ejection from birth sites in the Galactic plane.
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