1 Introduction

The field of gravitational wave (GW) astronomy will soon become a reality. The first generation of ground-based interferometric detectors (LIGO [45 ], VIRGO [126 ], GEO 600 [104 ], TAMA 300 [182 ]) are beginning their search for GWs. Toward the end of this decade, two of these detectors (LIGO, VIRGO) will begin upgrades that should allow them to reach the sensitivities necessary to regularly detect emissions from astrophysical sources. A space-based interferometric detector, LISA [181 ], could be launched in the early part of the next decade. One important class of sources for these observatories is stellar gravitational collapse. This class covers an entire spectrum of stellar masses, from the accretion induced collapse (AIC) of a white dwarf through the collapse of massive stars ( $M > 8 Mo .$ ) including the “collapsar” engine believed to power long-duration gamma-ray bursts [261 ], very massive Population III stars ( $M = 100 -500 Mo .$ ), and supermassive stars (SMSs, $6 M > 10 Mo .$ ). Some of these collapses result in explosions (Type II, Ib/c supernovae and hypernovae) and all leave behind neutron star or black hole remnants.

Strong GWs can be emitted during a gravitational collapse/explosion and, following the collapse, by the resulting compact remnant [244 , 172 , 173 , 73 , 214 , 91 , 88 , 120 ]. GW emission during the collapse itself may result if the collapse or explosion involves aspherical bulk mass motion or convection. Rotational or fragmentation instabilities encountered by the collapsing star will also produce GWs. Asymmetric neutrino emission can also produce a strong gravitational wave signature. Neutron star remnants of collapse may emit GWs due to the growth of rotational or $r$ -mode instabilities. Black hole remnants will also be sources of GWs if they experience accretion induced ringing or if the disks around the black hole develop instabilities. All of these phenomena have the potential of being detected by gravitational wave observatories because they involve the rapid change of dense matter distributions.

Observation of gravitational collapse by gravitational wave detectors will provide unique information, complementary to that derived from electromagnetic and neutrino detectors. Gravitational radiation arises from the coherent superposition of mass motion, whereas electromagnetic emission is produced by the incoherent superposition of radiation from electrons, atoms, and molecules. Thus, GWs carry different kinds of information than other types of radiation. Furthermore, electromagnetic radiation interacts strongly with matter and thus gives a view of the collapse only from lower density regions near the surface of the star, and it is weakened by absorption as it travels to the detector. In contrast, gravitational waves can propagate from the innermost parts of the stellar core to detectors without attenuation by intervening matter. With their weak interaction cross-sections, neutrinos can probe the same region probed by GWs. But whereas neutrinos are extremely sensitive to details in the microphysics (equation of state and cross-sections), GWs are most sensitive to physics driving the mass motions (e.g., rotation). Combined, the neutrino and the GW signals can teach us much about the conditions in the collapsing core and ultimately the physics that governs stellar collapse (e.g., [7 , 87 ]). Update

The characteristics of the GW emission from gravitational collapse have been the subject of much study. Core collapse supernovae, in particular, have been investigated as sources of gravitational radiation for more than three decades (see, e.g., [203 , 245 , 204 , 57 , 179 , 171 , 233 , 74 , 170 , 271 , 198 , 86 , 88 ]). However, during this time research has produced estimates of GW strength that vary over orders of magnitude. This is due to the complex nature of core collapse. Important theoretical and numerical issues include

construction of accurate progenitor models, including realistic angular momentum distributions,
proper treatment of microphysics, including the use of realistic equations of state and neutrino transport,
simulation in three-dimensions to study non-axisymmetric effects,
inclusion of general relativistic effects,
inclusion of magnetic field effects, and
study of the effect of an envelope on core behavior.

To date, collapse simulations generally include state-of-the-art treatments of only one or two of the above physics issues (often because of numerical constraints). For example, those studies that include advanced microphysics have often been run with Newtonian gravity (and approximate evaluation of the GW emission; see Section 2.4). A 3D, general relativistic collapse simulation that includes all significant physics effects is not feasible at present. However, good progress has been made on the majority of the issues listed above; the more recent work will be reviewed in some detail here.

The remainder of this article is structured as follows. Each category of gravitational collapse will be discussed in a separate section (AIC in Section 2, collapse of massive stars in Section 3, collapsar models in Section 4, collapse of Population III stars in Section 5, and collapse of SMSs in Section 6). Each of these sections (2, 3, 4, 5, 6) is divided into subsection topics: collapse scenario, formation rate, GW emission mechanisms, and numerical predictions of GW emission. In the subsections on numerical predictions, the detectability of the GW emission from various phenomena associated with collapse is examined. In particular, the predicted characteristics of GW emission are compared to the sensitivities of LIGO (for sources with frequencies of $1$ to $104 Hz$ ) and LISA (for sources with lower frequencies in the range of $10- 4$ to $1 Hz$ ).