Secular Stellar Dynamics near a Massive Black Hole

The Astrophysical Journal (Impact Factor: 6.73). 10/2010; 738(1). DOI: 10.1088/0004-637X/738/1/99
Source: arXiv

ABSTRACT The angular momentum evolution of stars close to massive black holes (MBHs)
is driven by secular torques. In contrast to two-body relaxation, where
interactions between stars are incoherent, the resulting resonant relaxation
(RR) process is characterized by coherence times of hundreds of orbital
periods. In this paper, we show that all the statistical properties of RR can
be reproduced in an autoregressive moving average (ARMA) model. We use the ARMA
model, calibrated with extensive N-body simulations, to analyze the long-term
evolution of stellar systems around MBHs with Monte Carlo simulations.
We show that for a single-mass system in steady-state, a depression is carved
out near an MBH as a result of tidal disruptions. Using Galactic center
parameters, the extent of the depression is about 0.1 pc, of similar order to
but less than the size of the observed "hole" in the distribution of bright
late-type stars. We also find that the velocity vectors of stars around an MBH
are locally not isotropic. In a second application, we evolve the highly
eccentric orbits that result from the tidal disruption of binary stars, which
are considered to be plausible precursors of the "S-stars" in the Galactic
center. We find that RR predicts more highly eccentric (e > 0.9) S-star orbits
than have been observed to date.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we propose a method to study the nature of resonant relaxation in near-Keplerian systems. Our technique is based on measuring the fractal dimension of the angular momentum trails and we use it to analyze the outcome of N-body simulations. With our method, we can reliably determine the timescale for resonant relaxation, as well as the rate of change of angular momentum in this regime. We find that growth of angular momentum is more rapid than random walk, but slower than linear growth. We also determine the presence of long term correlations, arising from the bounds on angular momentum growth. We develop a toy model that reproduces all essential properties of angular momentum evolution.
    Monthly Notices of the Royal Astronomical Society 11/2010; 411(1). · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: When a star comes within a critical distance to a supermassive black hole (SMBH), immense tidal forces can remove a significant fraction of the star's mass, resulting in a stream of debris that falls back onto the black hole and powers a luminous flare. In this paper, we perform two hydrodynamical simulations of the disruption of a main-sequence star by a SMBH to characterize the evolution of the debris stream after a tidal disruption. We demonstrate that this debris stream is confined by self-gravity in the two directions perpendicular to the original direction of the star's travel, restricting its width and height to be only a factor of a few larger than its original size. As a consequence, the stream has a negligible surface area and makes almost no contribution to either the continuum or line emission. We propose that any observed emission lines are not the result of photoionization in the unbound debris, but are produced in the region above and below the forming elliptical accretion disk, the same region in which the broad-line region is expected to be produced in steadily-accreting active galactic nuclei. As each line within a broad-line region is observationally linked to a particular location in the accretion disk, we suggest that the absence of a line indicates that the accretion disk does not yet extend to the distance required to produce that line. This model can be used to understand the spectral properties of the tidal disruption event PS1-10jh, for which HeII lines are observed, but the Balmer series and HeI are not. Using a maximum likelihood analysis, we go on to show that a partial disruption of a main-sequence star of near-solar composition can reproduce this event. [abridged]
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We identify a gravitational-dynamical process in near-Keplerian potentials of galactic nuclei that occurs when an intermediate-mass black hole (IMBH) is migrating on an eccentric orbit through the stellar cluster towards the central supermassive black hole (SMBH). We find that, apart from conventional dynamical friction, the IMBH experiences an often much stronger systematic torque due to the secular (i.e., orbit-averaged) interactions with the cluster's stars. The force which results in this torque is applied, counterintuitively, in the same direction as the IMBH's precession and we refer to its action as "secular-dynamical anti-friction" (SDAF). We argue that SDAF, and not the gravitational ejection of stars, is responsible for the IMBH's eccentricity increase seen in the initial stages of previous N-body simulations. Our numerical experiments, supported by qualitative arguments, demonstrate that (1) when the IMBH's precession direction is artificially reversed, the torque changes sign as well, which decreases the orbital eccentricity, (2) the rate of eccentricity growth is sensitive to the IMBH migration rate, with zero systematic eccentricity growth for an IMBH whose orbit is artificially prevented from inward migration, and (3) SDAF is the strongest when the central star cluster is rapidly rotating. This leads to eccentricity growth/decrease for the clusters rotating in the opposite/same direction relative to the IMBH's orbital motion.
    The Astrophysical Journal 05/2012; 754(1). · 6.73 Impact Factor


Available from