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Relativistic Hydrodynamics

Astrophysics Sector

SISSA International School for Advanced Studies

Theoretical Astrophysics and Relativity

California Institute of Technology

Max Planck Institute for Gravitational Physics – Albert Einstein Institute

Laboratoire d'Astrophysique

Section of Astrophysics, Astronomy & Mechanics

Aristotle University of Thessaloniki

The final evolutionary state of a star with mass below about 10 solar masses is a white dwarf, which is composed mostly carbon and oxygen (and possibly neon and magnesium) atoms. Unlike in a regular star, this very dense matter is supported against gravitational collapse by electron degeneracy pressure. This leads to a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit which amounts to approximately 1.4 solar masses. A carbon-oxygen white dwarf approaching this mass limit, typically by mass transfer from a companion star, is expected to explode as a Type Ia supernova.

However, in an oxygen-neon-magnesium white dwarf, accretion of matter from a companion star may trigger a gravitational collapse instead, leading a rebound of the collapsing core (the core bounce), the subsequent formation of a proto-neutron star composed of even denser matter, and a collapse-driven supernova explosion. In the accretion-induced collapse scenario, the supernova explosion energy is expected to be small and the resulting transient shortlived, making it hard to detect by electromagnetic means alone. The observation of the associated gravitational wave signature may provide crucial information necessary to reveal a potential accretion-induced collapse in space.

Motivated by the need for systematic predictions of the gravitational wave signature of accretion-induced collapse, we have performed a general relativistic simulation study with an extensive set of accretion-induced collapse models using a microphysical non-zero temperature equation of state and an approximate treatment of deleptonization during collapse [Abdikamalov, et al., 2009], very similar to the modeling used for our two recent sophisticated studies of rotating gravitational core collapse of massive stars (with variation of rotation alone [Dimmelmeier, et al., 2007] and with variation of progenitor mass and equation of state as well [Dimmelmeier, et al., 2008]).

By investigating a set of 114 progenitor models in axisymmetric rotational equilibrium, with a wide range of rotational configurations, temperatures and central densities, and resulting white dwarf masses, we have extended previous Newtonian studies and find that the gravitational wave signal has a generic shape already known as a Type III signal in the literature (see figure below for an exemplary time–frequency diagram of such a waveform). Despite this reduction to a single type of waveform, we show that the emitted gravitational waves still carry information that can be used to constrain the progenitor and the postbounce rotation.

We have assessed the detectability of the emitted gravitational waves, showing that the signal-to-noise ratio for current or next-generation interferometer detectors could be high enough to detect such events in our Galaxy. Furthermore, by contrasting the gravitational wave signals of accretion-induced collapse and rotating massive star iron core collapse we have found that they can be distinguished, but only if the distance to the source is known and a detailed reconstruction of the gravitational wave time series from detector data is possible.

Our simulations also predict that some accretion-induced collapse models form massive quasi-Keplerian accretion disks after bounce. The disk mass is very sensitive to progenitor mass and angular momentum distribution. In rapidly differentially rotating models whose precollapse masses are significantly larger than the Chandrasekhar mass, the resulting disk mass can be as large as 0.8 solar masses. Slowly and/or uniformly rotating models that are limited to masses near the Chandrasekhar mass produce much smaller disks or no disk at all.

Finally, we have found that the postbounce cores of rapidly spinning white dwarfs can reach sufficiently rapid rotation to develop a gravito-rotational bar-mode instability. Moreover, many of our models exhibit sufficiently rapid and differential rotation to become subject to recently discovered low-rotation-rate-type dynamical instabilities, which is as well a source of gravitational radiation with a very distinct quasi-periodic signal waveform. This additional signal contribution could break up the near-degeneracy of the burst signal from the collapse and bounce phase.

We provide a waveform catalog of all simulated models on an external website.

• Abdikamalov, E.B., Ott, C.D., Rezzolla, L., Dessart, L., Dimmelmeier, H., Marek, A., Janka, H.-T.,
  "Axisymmetric general relativistic simulations of the accretion-induced collapse of white dwarfs",
  Phys. Rev. D, submitted, (2009),
  [Article in astro-ph].


• Dimmelmeier, H., Ott, C.D., Janka, H.-T., Marek, A., and Müller, E.,
  "Generic gravitational wave signals from the collapse of rotating stellar cores",
  Phys. Rev. Lett., 98, 251101, (2007),
  [Article in astro-ph].


• Dimmelmeier, H., Ott, C.D., Marek, A., and Janka, H.-T.,
  "The gravitational wave burst signal from core collapse of rotating stars",
  Phys. Rev. D, 78, 064056, (2008),
  [Article in astro-ph].