1,653 research outputs found
The Gravitational Wave Signature of Core-Collapse Supernovae
We review the ensemble of anticipated gravitational-wave (GW) emission
processes in stellar core collapse and postbounce core-collapse supernova
evolution. We discuss recent progress in the modeling of these processes and
summarize most recent GW signal estimates. In addition, we present new results
on the GW emission from postbounce convective overturn and protoneutron star
g-mode pulsations based on axisymmetric radiation-hydrodynamic calculations.
Galactic core-collapse supernovae are very rare events, but within 3-5 Mpc from
Earth, the rate jumps to 1 in ~2 years. Using the set of currently available
theoretical gravitational waveforms, we compute upper-limit optimal
signal-to-noise ratios based on current and advanced LIGO/GEO600/VIRGO noise
curves for the recent SN 2008bk which exploded at ~3.9 Mpc. While initial LIGOs
cannot detect GWs emitted by core-collapse events at such a distance, we find
that advanced LIGO-class detectors could put significant upper limits on the GW
emission strength for such events. We study the potential occurrence of the
various GW emission processes in particular supernova explosion scenarios and
argue that the GW signatures of neutrino-driven, magneto-rotational, and
acoustically-driven core-collapse SNe may be mutually exclusive. We suggest
that even initial LIGOs could distinguish these explosion mechanisms based on
the detection (or non-detection) of GWs from a galactic core-collapse
supernova.Comment: Topical Review, accepted for publication in CQG. 51 pages, 13
figures, a version of the article with high-resolution figures is available
from http://stellarcollapse.org/papers/Ott_SN_GW_review2008.pdf. Update:
Added section on core collapse simulations and the treatment of general
relativit
The Progenitor Dependence of the Preexplosion Neutrino Emission in Core-Collapse Supernovae
We perform spherically-symmetric general-relativistic simulations of core
collapse and the postbounce preexplosion phase in 32 presupernova stellar
models of solar metallicity with zero-age-main-sequence masses of 12 M_{sun} to
120 M_{sun}. Using energy-dependent three-species neutrino transport in the
two-moment approximation with an analytic closure, we show that the emitted
neutrino luminosities and spectra follow very systematic trends that are
correlated with the compactness (~M/R) of the progenitor star's inner regions
via the accretion rate in the preexplosion phase. We find that these
qualitative trends depend only weakly on the nuclear equation of state, but
quantitative observational statements will require independent constraints on
the equation of state and the rotation rate of the core as well as a more
complete understanding of neutrino oscillations. We investigate the simulated
response of water Cherenkov detectors to the electron antineutrino fluxes from
our models and find that the large statistics of a galactic core collapse event
may allow robust conclusions on the inner structure of the progenitor star.Comment: 16 emulateapj pages, 10 figures, 1 table. matches published versio
The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions
The neutrino-heated "gain layer" immediately behind the stalled shock in a
core-collapse supernova is unstable to high-Reynolds-number turbulent
convection. We carry out and analyze a new set of 19 high-resolution
three-dimensional (3D) simulations with a three-species neutrino
leakage/heating scheme and compare with spherically-symmetric (1D) and
axisymmetric (2D) simulations carried out with the same methods. We study the
postbounce supernova evolution in a - progenitor star and vary the
local neutrino heating rate, the magnitude and spatial dependence of
asphericity from convective burning in the Si/O shell, and spatial resolution.
Our simulations suggest that there is a direct correlation between the strength
of turbulence in the gain layer and the susceptability to explosion. 2D and 3D
simulations explode at much lower neutrino heating rates than 1D simulations.
This is commonly explained by the fact that nonradial dynamics allows accreting
material to stay longer in the gain layer. We show that this explanation is
incomplete. Our results indicate that the effective turbulent ram pressure
exerted on the shock plays a crucial role by allowing multi-D models to explode
at a lower postshock thermal pressure and thus with less neutrino heating than
1D models. We connect the turbulent ram pressure with turbulent energy at large
scales and in this way explain why 2D simulations are erroneously exploding
more easily than 3D simulations.Comment: 13 pages, 8 figures, accepted by Ap
Supernova Fallback onto Magnetars and Propeller-powered Supernovae
We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields (~10^(15) G) and short spin periods (~1-10 ms) have an important influence on how the magnetar interacts with the infalling material. At long spin periods, weak magnetic fields, and high accretion rates, sufficient material is accreted to form a black hole, as is commonly found for massive progenitor stars. When B ≾ 5 × 10^(14) G, accretion causes the magnetar to spin sufficiently rapidly to deform triaxially and produces gravitational waves, but only for ≈50-200 s until it collapses to a black hole. Conversely, at short spin periods, strong magnetic fields, and low accretion rates, the magnetar is in the "propeller regime" and avoids becoming a black hole by expelling incoming material. This process spins down the magnetar, so that gravitational waves are only expected if the initial protoneutron star is spinning rapidly. Even when the magnetar survives, it accretes at least ≈0.3 M_☉, so we expect magnetars born within these types of environments to be more massive than the 1.4 M_☉ typically associated with neutron stars. The propeller mechanism converts the ~10^(52)erg of spin energy in the magnetar into the kinetic energy of an outflow, which shock heats the outgoing supernova ejecta during the first ~10-30 s. For a small ~5 M_☉ hydrogen-poor envelope, this energy creates a brighter, faster evolving supernova with high ejecta velocities ~(1-3) × 10^4 km s^(–1) and may appear as a broad-lined Type Ib/c supernova. For a large ≳ 10 M_☉ hydrogen-rich envelope, the result is a bright Type IIP supernova with a plateau luminosity of ≳ 10^(43)erg s^(–1) lasting for a timescale of ~60-80 days
Low-mass X-ray binaries from black-hole retaining globular clusters
Recent studies suggest that globular clusters (GCs) may retain a substantial
population of stellar-mass black holes (BHs), in contrast to the long-held
belief of a few to zero BHs. We model the population of BH low-mass X-ray
binaries (BH-LMXBs), an ideal observable proxy for elusive single BHs, produced
from a representative group of Milky Way GCs with variable BH populations. We
simulate the formation of BH-binaries in GCs through exchange interactions
between binary and single stars in the company of tens to hundreds of BHs.
Additionally, we consider the impact of the BH population on the rate of
compact binaries undergoing gravitational wave driven mergers. The
characteristics of the BH-LMXB population and binary properties are sensitive
to the GCs structural parameters as well as its unobservable BH population. We
find that GCs retaining BHs produce a galactic population of ejected BH-LMXBs whereas GCs retaining only BHs produce zero
ejected BH-LMXBs. Moreover, we explore the possibility that some of the
presently known BH-LMXBs might have originated in GCs and identify five
candidate systems.Comment: 27 pages, 18 figures, 7 tables, submitted to MNRA
Massive Computation for Understanding Core-Collapse Supernova Explosions
How do massive stars explode? Progress toward the answer is driven by increases in compute power. Petascale supercomputers are enabling detailed 3D simulations of core-collapse supernovae that are elucidating the role of fluid instabilities, turbulence, and magnetic field amplification in supernova engines
Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae
(Abridged) In the implicit large eddy simulation (ILES) paradigm, the
dissipative nature of high-resolution shock-capturing schemes is exploited to
provide an implicit model of turbulence. Recent 3D simulations suggest that
turbulence might play a crucial role in core-collapse supernova explosions,
however the fidelity with which turbulence is simulated in these studies is
unclear. Especially considering that the accuracy of ILES for the regime of
interest in CCSN, weakly compressible and strongly anisotropic, has not been
systematically assessed before. In this paper we assess the accuracy of ILES
using numerical methods most commonly employed in computational astrophysics by
means of a number of local simulations of driven, weakly compressible,
anisotropic turbulence. We report a detailed analysis of the way in which the
turbulent cascade is influenced by the numerics. Our results suggest that
anisotropy and compressibility in CCSN turbulence have little effect on the
turbulent kinetic energy spectrum and a Kolmogorov scaling is
obtained in the inertial range. We find that, on the one hand, the kinetic
energy dissipation rate at large scales is correctly captured even at
relatively low resolutions, suggesting that very high effective Reynolds number
can be achieved at the largest scales of the simulation. On the other hand, the
dynamics at intermediate scales appears to be completely dominated by the
so-called bottleneck effect, \ie the pile up of kinetic energy close to the
dissipation range due to the partial suppression of the energy cascade by
numerical viscosity. An inertial range is not recovered until the point where
relatively high resolution , which would be difficult to realize in
global simulations, is reached. We discuss the consequences for CCSN
simulations.Comment: 17 pages, 9 figures, matches published versio
Results From Core-Collapse Simulations with Multi-Dimensional, Multi-Angle Neutrino Transport
We present new results from the only 2D multi-group, multi-angle calculations
of core-collapse supernova evolution. The first set of results from these
calculations was published in Ott et al. (2008). We have followed a nonrotating
and a rapidly rotating 20 solar mass model for ~400 ms after bounce. We show
that the radiation fields vary much less with angle than the matter quantities
in the region of net neutrino heating. This obtains because most neutrinos are
emitted from inner radiative regions and because the specific intensity is an
integral over sources from many angles at depth. The latter effect can only be
captured by multi-angle transport. We then compute the phase relationship
between dipolar oscillations in the shock radius and in matter and radiation
quantities throughout the postshock region. We demonstrate a connection between
variations in neutrino flux and the hydrodynamical shock oscillations, and use
a variant of the Rayleigh test to estimate the detectability of these neutrino
fluctuations in IceCube and Super-K. Neglecting flavor oscillations,
fluctuations in our nonrotating model would be detectable to ~10 kpc in
IceCube, and a detailed power spectrum could be measured out to ~5 kpc. These
distances are considerably lower in our rapidly rotating model or with
significant flavor oscillations. Finally, we measure the impact of rapid
rotation on detectable neutrino signals. Our rapidly rotating model has strong,
species-dependent asymmetries in both its peak neutrino flux and its light
curves. The peak flux and decline rate show pole-equator ratios of up to ~3 and
~2, respectively.Comment: 13 pages, 9 figures, ApJ accepted. Replaced with accepted versio
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