Download The Swansong of Stars Orbiting Massive Black Holes

The Swansong of Stars Orbiting Massive Black Holes
Clovis Hopman
Weizmann Institute of Science
Ultraluminous X-ray Sources as Intermediate Mass
Black Holes Fed by Tidally Captured Stars
Orbital Inspiral into a Massive Black Hole
in a Galactic Center
Gravitational Waves: The Event Rates and
Orbital Statistics of LISA Sources
Hopman, Portegies Zwart & Alexander, ApJL 2004
Alexander & Hopman, ApJL 2003
Hopman & Alexander, in preparation
Stars may spiral into a MBH if they can dissipate their
orbital energy. Examples of dissipative processes are tidal
heating, or gravitational wave emission. These processes
are effective only when the star passes very close to the
MBH. Inspiral starts when the star is scattered into a highly
eccentric orbit, which brings it close to the MBH. Every
orbit it dissipates some orbital energy.
To dissipate all the energy and achieve tidal circularization
(left panel) or become a short-period LISA source (right
panel), the star needs to AVOID being scattered again at
all times.
Chandra image of the ULX in MGG-11
The nature of ULXs, with X-ray luminosities higher than the Eddington
luminosity of a stellar mass object, is not known. We argue that some
are powered by INTERMEDIATE MASS BLACK HOLES (IBHs) of
~1000 Msun, which tidally capture a star. We apply this scenario to the
X-ray source in cluster MGG-11 (see image).
INSPIRAL IS A RARE PHENOMENON.
-Only a few percent of all stars spiral into a MBH. The rest
fall directly in (or are tidally destroyed)
-Tidal capture has a negligible contribution to the growth of
a MBH by eating stars.
-Stars that are strongly perturbed by a close tidal
interaction (tidal scattering) are scattered back to the
ambient population. Many "weird stars" probably exist
around MBHs.
Our main results are:
From the LISA website
LISA will be able to detect compact objects that spiral into a MBH by GW emission
from up to a distance of a Gpc.
The signal is expected to be weak. To detect it, it is necessary to know in advance
the shape of the wave trains, and to do that, it is necessary to know the
eccentricity of the inspiral orbits. High eccentricity can change the nature of the
signal drastically. The eccentricity also determines how much time a star spends
within the LISA band before its final plunge.
We used dynamical Monte Carlo simulations to study how two-body scattering and
energy dissipation by GW emission, acting together, modify the inspiral orbits.
- Tidal capture can be followed by circularization of the star without
destroying it during in-spiral for IBHs. This is NOT possible for MBHs
of millions of solar masses, where tidal heating destroys the star!
- Roche Lobe overflow provides enough gas to feed the IBH and power
the ULX (see below)
- Life-time and capture rate are large enough so that there is ~10%
probability of presence of a ULX in a given IBH-cluster
- The capture rate is approximately independent of the relaxation time
- If the Roche Lobe overflow phase starts AFTER the cluster had
evaporated, the ULX will appear host-less. Such ULXs are indeed
observed! (Zezas et al. 2002)
We find that INSPIRALING STARS TEND TO HAVE HIGH ECCENTRICITIES.
We also find that the event rate is:
- Independent of the relaxation time.
- Lower than previously estimated: only a few detectable sources.
- Strongly depends on mass segregation.
Alexander & Hopman, ApJL 2003
Luminosity as a function of time for a
star filling its Roche Lobe on a
circular orbit around a 1000 solar
mass black hole. Hopman, Portegies
Zwart & Alexander, ApJL 2004
Eccentricity histograms for several compact objects for a MBH of a million
solar masses (left figure) and an IBH of a thousand solar masses (right
figure). Hopman & Alexander, in preparation.