Dr Stephan Rosswog
Professor of Astrophysics
Jacobs University Bremen
Campus Ring 1
D-28759 Bremen, Germany
Tel: +49 421 200-3226
email: s.rosswog@jacobs-university.de
Tidal squeezing of White Dwarfs by moderately massive black holes:
a new type of supernova
Background
Intermediate mass black holes:
the existence of two classes of black holes is established beyond reasonable doubt:
"stellar-mass black holes" (a few solar masses) reveal themselves
as X-ray binaries and
"supermassive black holes" (beyond 106 solar masses) that lurk in the centers of galaxies
there are good reasons to believe that a third class of black holes (with about 1000 solar masses) exists, so-called
Intermediate-Mass Black Holes (IMBH), but their existence is to
date still controversial
Tidal disruption basics:
There is a certain distance from a black hole, the so-called
tidal radius, inside of which the tidal forces from the
black hole overcome the self-gravity of a star: it becomes disrupted
The tidal radius is given by
the tidal radius of a black hole only growth proportional to the
(black hole mass)1/3, but the Schwarzschild radius
of the black hole growth proportional to the black hole mass, this has as a
consequence that too massive black holes "swallow" the full star without
disrupting it
for example: a typical White Dwarf star (0.6 solar masses) is entirely swallowed
by black holes more massive than about 105 solar masses
Numerical Simulations
I have performed detailed simulations of tidal disruptions of white dwarfs by intermediate mass black holes. The simulations include (details in Rosswog et al. 2008a,b,c):
The stars are typically resolved with several million SPH-particles
Self-gravity of the star (via a parallel binary tree)
black hole gravity via a relativistic pseudo-potential (Paczynski-Wiita)
The best white dwarf equation of state available (HELMHOLTZ-EOS; electrons,
positrons, photons, a specifiable content of nuclei; no assumption is made
about the degree of degenracy or relativity of the leptons, the resulting integrals
are numerically integrated to machine machine precision; see Timmes & Swesty
2000)
A nuclear reaction network to follow the effects of nuclear burning (Hix et al.
1998)
Tidal compression and ignition of White Dwarfs
The tidal field has the property to compress gas flow in two directions while
stretching it in the other
This is illustrated by the following figure which shows a 3D hydrodynamics
simulation (smoothed particle hydrodynamics, about 5 000 000 particles) of
a 0.2 solar mass He-WD disrupted by a 1000 solar mass IMBH (source
Rosswog, Ramirez-Ruiz, Hix (2008))
For close enough encounters, temperatures can become very large
(close to nuclear-statistical equilibrium, around 4 109 Kelvin),
so that explosive nuclear burning can be triggered
The result is a thermonuclear supernova, but different from
"standard" type Ia supernovae (NOT triggered at Chandrasekhar mass, NOT
restricted to carbon-oxygen progenitors)
Accretion of white dwarf debris onto black hole: X-ray flares
about 30 % of the White Dwarf are accreted by the black hole
on returning to the black hole, the debris forms a "nozzle" and an
"accretion fan"
when matter that has passed the hole interacts with still infalling matter an
angular momentum re-distribution shock forms
tidal disruptions of white dwarfs can induce thermonuclear explosions via tidal compression
IF intermediate-mass black holes do exist, upcoming supernova surveys such as LSST should detect these peculiar thermonuclear supernovae
These explosions g along with a soft X-ray flare of several months duration
References
Tidal disruption and ignition of white dwarfs by moderately massive
black holes
S. Rosswog, E. Ramirez-Ruiz and R. Hix, 38 pages, submitted to ApJ (2008)
Atypical thermonuclear supernovae from tidally crushed white dwarfs
S. Rosswog, E. Ramirez-Ruiz and W.R. Hix, ApJ 679, 1385 (2008)
Simulating black hole white dwarf encounters
S. Rosswog, E. Ramirez-Ruiz and W.R. Hix, Computer Physics Communications
179, 184 (2008)
