Massive stars in subparsec rings around galactic centres

Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 10/2006; 372(1):143 - 150. DOI: 10.1111/j.1365-2966.2006.10772.x
Source: arXiv


We consider the structure of self-gravitating marginally stable accretion discs in galactic centres in which a small fraction of the disc mass has been converted into protostars. We find that protostars accrete gaseous disc matter at prodigious rates. Mainly due to the stellar accretion luminosity, the disc heats up and thickens geometrically, shutting off further disc fragmentation. The existing protostars, however, continue to gain mass by gas accretion. As a result, the initial mass function for disc-born stars at distances R∼ 0.03–3 pc from the supermassive black hole should be top-heavy. The effect is most pronounced at around R∼ 0.1 pc. We suggest that this result explains observations of rings of young massive stars in our Galaxy and in M31, and we predict that more such rings will be discovered.

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    ABSTRACT: Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the stellar orbits are nearly Keplerian and so do not precess significantly. The resulting coherent torques efficiently change the magnitude of the angular momenta and rotate the orbital inclination in all directions. As a result, many of the tightly bound stars very near the MBH are rapidly destroyed by falling into the MBH on low-angular momentum orbits, while the orbits of the remaining stars are efficiently randomized. We solve numerically the Fokker-Planck equation in energy for the steady state distribution of a single mass population with a RR sink term. We find that the steady state current of stars, which sustains the accelerated drainage close to the MBH, can be up to ~10 times larger than that due to non-coherent 2-body relaxation alone. RR mostly affects tightly bound stars, and so it increases only moderately the total tidal disruption rate, which is dominated by stars originating from less bound orbits farther away. We show that the event rate of gravitational wave (GW) emission from inspiraling stars, originating much closer to the MBH, is dominated by RR dynamics. The GW event rate depends on the uncertain efficiency of RR. The efficiency indicated by the few available simulations implies rates ~10 times higher than those predicted by 2-body relaxation, which would improve the prospects of detecting such events by future GW detectors, such as LISA. However, a higher, but still plausible RR efficiency can lead to the drainage of all tightly bound stars and strong suppression of GW events from inspiraling stars. We apply our results to the Galactic MBH, and show that the observed dynamical properties of stars there are consistent with RR. Comment: Accepted to ApJ; Minor revisions
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    ABSTRACT: The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Centre. The resulting obscuration pattern is very patchy, with some lines of sight becoming optically thick to Thomson scattering. A fixed observer would see column depth changing by factors of many over time scales of order months to hundreds of years, depending on the physical size of the outflow region. Such winds may be relevant for obscuration of some AGN and especially "changing look AGN". However, averaged over the sky as seen from the central source, these winds are always optically thin unless wind outflow rates are super-Eddington. A simple scaling argument shows that this is true not only for stellar-driven winds but for any AGN winds. We therefore conclude that AGN winds are unable to account for the vast majority of optically thick obscured AGN (a significant fraction of all AGN). We suggest that the most likely source of optically thick obscuration in AGN is a warped parsec scale accretion disc.
    Astronomy and Astrophysics 03/2006; 465(1). DOI:10.1051/0004-6361:20065541 · 4.38 Impact Factor
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    ABSTRACT: The essential features of the stellar Initial Mass Function are, rather generally, (1) a peak at a mass of a few tenths of a solar mass, and (2) a power-law tail toward higher masses that is similar to the original Salpeter function. Recent work suggests that the IMF peak reflects a preferred scale of fragmentation associated with the transition from a cooling phase of collapse at low densities to a nearly isothermal phase at higher densities, where the gas becomes thermally coupled to the dust. The Salpeter power law is plausibly produced, at least in part, by scale-free accretion processes that build up massive stars in dense environments. The young stars at the Galactic Center appear to have unusually high masses, possibly because of a high minimum mass resulting from the high opacity of the dense star-forming gas.
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