The Hills Mechanism and the Galactic Center S-stars

Abstract

The Galactic centre contains several young populations within its central parsec: a disk between ∼0.05 to 0.5 pc from the centre, and the isotropic S-star cluster extending an order of magnitude further inwards in radius. Recent observations (i.e. spectroscopy and hypervelocity stars) suggest that some S-stars originate in the disk. In particular, the S-stars may be remnants of tidally disrupted disk binaries. However, these is an apparent inconsistency in this scenario: the disk contains massive O and Wolf-Rayet stars while the S-stars are lower mass, B stars. We explore two different explanations for this apparent discrepancy: (i) a built-in bias in binary disruptions, where the primary star remains closer in energy to the centre-of-mass orbit than the secondary and (ii) selective tidal disruption of massive stars within the S-star cluster. The first explanation is plausible. On the other hand, tidal disruptions have not strongly affected the mass distribution of the S-stars over the last several Myr.

Constraints on the origins of hypervelocity stars: velocity distribution, mergers and star-formation history

“…However, this will only occur if the binaries undergo a slow diffusion in angular momentum, which implies a small disruption rate that is problematic for explaining the observed hypervelocity stars. Close-in binaries can be torqued to disruption more efficiently by a secular gravitational instability in a young stellar disc (Madigan et al 2009;Generozov & Madigan 2020). However, in this case binaries experience coherent changes in angular momentum over many orbits, rather than slow diffusion, and multiple encounters do not truncate the velocity distribution.…”

“…Binaries wider than ∼ 2 au will only produce hypervelocity stars when the loss cone is full, which is not the case here. Finally, as disruptions occur promptly after disc formation (Madigan et al 2009;Generozov & Madigan 2020), the stars are on the ZAMS at disruption in this case.…”

“…For each random walk, all pericentres within five times the (initial) nominal tidal radius are recorded. If the minimum pericentre is greater than three times the nominal tidal radius, then the binary has a small chance of being disrupted (Hills 1991;Bromley et al 2006;Sari et al 2010;Generozov & Madigan 2020), and we proceed to the next one. Otherwise, we simulate all close encounters with the FEWBODY code, stopping if the binary is disrupted.…”

“…Previous work has shown that a lopsided, eccentric disk in the Galactic Centre undergoes a secular gravitational instability that excites some of its orbits to high eccentricities. Binaries on such orbits can be tidally disrupted (Madigan et al 2009;Generozov & Madigan 2020).…”

“…This mechanism will only work if the progenitor binaries start very close to MBH, and experience many close encounters prior to disruption. Madigan et al (2009) and Generozov & Madigan (2020) describe how binaries in young disc(s) at small galactocentric radii (Levin & Beloborodov 2003;Paumard et al 2006) can be efficiently torqued to disruption by a secular gravitational instability. However, in this case, the binaries do not random walk to disruption (as assumed in Zhang et al 2010), but experience coherent changes in angular momentum over many orbits around the Galactic Centre, reducing the number of close encounters with the MBH.…”