skip to main content

Title: Forming young and hypervelocity stars in the Galactic Centre via tidal disruption of a molecular cloud

The Milky Way Galaxy hosts a four million solar mass black hole, Sgr A*, that underwent a major accretion episode approximately 3–6 Myr ago. During the episode, hundreds of young massive stars formed in a disc orbiting Sgr A* in the central half parsec. The recent discovery of a hypervelocity star (HVS) S5-HVS1, ejected by Sgr A* five Myr ago with a velocity vector consistent with the disc, suggests that this event also produced binary star disruptions. The initial stellar disc has to be rather eccentric for this to occur. Such eccentric discs can form from the tidal disruptions of molecular clouds. Here, we perform simulations of such disruptions, focusing on gas clouds on rather radial initial orbits. As a result, stars formed in our simulations are on very eccentric orbits ($\bar{e}\sim 0.6$) with a lopsided configuration. For some clouds, counterrotating stars are formed. As in previous work, we find that such discs undergo a secular gravitational instability that leads to a moderate number of particles obtaining eccentricities of 0.99 or greater, sufficient for stellar binary disruption. We also reproduce the mean eccentricity of the young disc in the Galactic Centre, though not the observed surface density profile. We discuss more » missing physics and observational biases that may explain this discrepancy. We conclude that observed S-stars, HVSs, and disc stars tightly constrain the initial cloud parameters, indicating a cloud mass between a few × 104 and $10^5\, {\rm M}_{\odot }$, and a velocity between ∼40 and 80 km s−1 at 10 pc.

« less
 ;  ;  
Publication Date:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range or eLocation-ID:
p. 4100-4115
Oxford University Press
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT The hyper-velocity star S5-HVS1, ejected 5 Myr ago from the Galactic Centre at 1800 km s−1, was most likely produced by tidal break-up of a tight binary by the supermassive black hole SgrA*. Taking a Monte Carlo approach, we show that the former companion of S5-HVS1 was likely a main-sequence star between 1.2 and 6 M⊙ and was captured into a highly eccentric orbit with pericentre distance in the range of 1–10 au and semimajor axis about 103 au. We then explore the fate of the captured star. We find that the heat deposited by tidally excited stellar oscillation modes leads to runaway disruption if the pericentre distance is smaller than about $3\rm \, au$. Over the past 5 Myr, its angular momentum has been significantly modified by orbital relaxation, which may stochastically drive the pericentre inwards below $3\rm \, au$ and cause tidal disruption. We find an overall survival probability in the range 5 per cent to 50 per cent, depending on the local relaxation time in the close environment of the captured star, and the initial pericentre at capture. The pericentre distance of the surviving star has migrated to 10–100 au, making it potentially the most extreme member of the S-star cluster. From the ejection rate ofmore »S5-HVS1-like stars, we estimate that there may currently be a few stars in such highly eccentric orbits. They should be detectable (typically $K_{\rm s}\lesssim 18.5\,$ mag) by the GRAVITY instrument and by future Extremely Large Telescopes and hence provide an extraordinary probe of the spin of SgrA*.« less
  2. ABSTRACT Active galactic nuclei (AGN) are powered by the accretion of discs of gas on to supermassive black holes (SMBHs). Stars and stellar remnants orbiting the SMBH in the nuclear star cluster (NSC) will interact with the AGN disc. Orbiters plunging through the disc experience a drag force and, through repeated passage, can have their orbits captured by the disc. A population of embedded objects in AGN discs may be a significant source of binary black hole mergers, supernovae, tidal disruption events, and embedded gamma-ray bursts. For two representative AGN disc models, we use geometric drag and Bondi–Hoyle–Littleton drag to determine the time to capture for stars and stellar remnants. We assume a range of initial inclination angles and semimajor axes for circular Keplerian prograde orbiters. Capture time strongly depends on the density and aspect ratio of the chosen disc model, the relative velocity of the stellar object with respect to the disc, and the AGN lifetime. We expect that for an AGN disc density $\rho \gtrsim 10^{-11}{\rm g\, cm^{-3}}$ and disc lifetime ≥1 Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.
  3. ABSTRACT We report the formation of bound star clusters in a sample of high-resolution cosmological zoom-in simulations of z ≥ 5 galaxies from the Feedback In Realistic Environments project. We find that bound clusters preferentially form in high-pressure clouds with gas surface densities over $10^4\, \mathrm{ M}_{\odot }\, {\rm pc}^{-2}$, where the cloud-scale star formation efficiency is near unity and young stars born in these regions are gravitationally bound at birth. These high-pressure clouds are compressed by feedback-driven winds and/or collisions of smaller clouds/gas streams in highly gas-rich, turbulent environments. The newly formed clusters follow a power-law mass function of dN/dM ∼ M−2. The cluster formation efficiency is similar across galaxies with stellar masses of ∼107–$10^{10}\, \mathrm{ M}_{\odot }$ at z ≥ 5. The age spread of cluster stars is typically a few Myr and increases with cluster mass. The metallicity dispersion of cluster members is ∼0.08 dex in $\rm [Z/H]$ and does not depend on cluster mass significantly. Our findings support the scenario that present-day old globular clusters (GCs) were formed during relatively normal star formation in high-redshift galaxies. Simulations with a stricter/looser star formation model form a factor of a few more/fewer bound clusters per stellar mass formed, whilemore »the shape of the mass function is unchanged. Simulations with a lower local star formation efficiency form more stars in bound clusters. The simulated clusters are larger than observed GCs due to finite resolution. Our simulations are among the first cosmological simulations that form bound clusters self-consistently in a wide range of high-redshift galaxies.« less
  4. ABSTRACT We study the growth of stellar discs of Milky Way-sized galaxies using a suite of cosmological simulations. We calculate the half-mass axis lengths and axis ratios of stellar populations split by age in galaxies with stellar mass $M_{*}=10^7\!-\!10^{10}\, \mathrm{M}_{\odot }$ at redshifts z > 1.5. We find that in our simulations stars always form in relatively thin discs, and at ages below 100 Myr are contained within half-mass height z1/2 ∼ 0.1 kpc and short-to-long axial ratio z1/2/x1/2 ∼ 0.15. Disc thickness increases with the age of stellar population, reaching median z1/2 ∼ 0.8 kpc and z1/2/x1/2 ∼ 0.6 for stars older than 500 Myr. We trace the same group of stars over the simulation snapshots and show explicitly that their intrinsic shape grows more spheroidal over time. We identify a new mechanism that contributes to the observed disc thickness: rapid changes in the orientation of the galactic plane mix the configuration of young stars. The frequently mentioned ‘upside-down’ formation scenario of galactic discs, which posits that young stars form in already thick discs at high redshift, may be missing this additional mechanism of quick disc inflation. The actual formation of stars within a fairly thin plane is consistent with the correspondingly flatmore »configuration of dense molecular gas that fuels star formation.« less

    The majority of binary star systems that host exoplanets will spend the first portion of their lives within a star-forming cluster that may drive dynamical evolution of the binary-planet system. We perform numerical simulations of S-type planets, with masses and orbital architecture analogous to the Solar system’s four gas giants, orbiting within the influence of a $0.5\, \mathrm{M}_{\odot }$ binary companion. The binary-planet system is integrated simultaneously with an embedded stellar cluster environment. ∼10 per cent of our planetary systems are destabilized when perturbations from our cluster environment drive the binary periastron towards the planets. This destabilization occurs despite all of our systems being initialized with binary orbits that would allow stable planets in the absence of the cluster. The planet–planet scattering triggered in our systems typically results in the loss of lower mass planets and the excitement of the eccentricities of surviving higher mass planets. Many of our planetary systems that go unstable also lose their binary companions prior to cluster dispersal and can therefore masquerade as hosts of eccentric exoplanets that have spent their entire histories as isolated stars. The cluster-driven binary orbital evolution in our simulations can also generate planetary systems with misaligned spin–orbit angles. This is typicallymore »done as the planetary system precesses as a rigid disc under the influence of an inclined binary, and those systems with the highest spin–orbit angles should often retain their binary companion and possess multiple surviving planets.

    « less