49 research outputs found
Multimessenger Universe with Gravitational Waves from Binaries
Future GW detector networks and EM observatories will provide a unique
opportunity to observe the most luminous events in the Universe involving
matter in extreme environs. They will address some of the key questions in
physics and astronomy: formation and evolution of compact binaries, sites of
formation of heavy elements and the physics of jets.Comment: 11 pages, two tables, White Paper submitted to the Astro-2020 (2020
Astronomy and Astrophysics Decadal Survey) by GWIC-3G Science Case Team
(GWIC: Gravitational-Wave International Committee
Normal Type Ia supernovae from violent mergers of white dwarf binaries
One of the most important questions regarding the progenitor systems of Type
Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to
explosions that reproduce observations of normal events. Here we present a
fully three-dimensional simulation of a violent merger of two carbon-oxygen
white dwarfs with masses of and
combining very high resolution and exact initial conditions. A well-tested
combination of codes is used to study the system. We start with the dynamical
inspiral phase and follow the subsequent thermonuclear explosion under the
plausible assumption that a detonation forms in the process of merging. We then
perform detailed nucleosynthesis calculations and radiative transfer
simulations to predict synthetic observables from the homologously expanding
supernova ejecta. We find that synthetic color lightcurves of our merger, which
produces about of , show good
agreement with those observed for normal SNe Ia in all wave bands from U to K.
Line velocities in synthetic spectra around maximum light also agree well with
observations. We conclude, that violent mergers of massive white dwarfs can
closely resemble normal SNe Ia. Therefore, depending on the number of such
massive systems available these mergers may contribute at least a small
fraction to the observed population of normal SNe Ia.Comment: 6 pages, 4 figures, accepted for publication in ApJ
A Study of Carbon Features in Type Ia Supernova Spectra
One of the major differences between various explosion scenarios of Type Ia
supernovae (SNe Ia) is the remaining amount of unburned (C+O) material and its
velocity distribution within the expanding ejecta. While oxygen absorption
features are not uncommon in the spectra of SNe Ia before maximum light, the
presence of strong carbon absorption has been reported only in a minority of
objects, typically during the pre-maximum phase. The reported low frequency of
carbon detections may be due to low signal-to-noise data, low abundance of
unburned material, line blending between C II 6580 and Si II 6355, ejecta
temperature differences, asymmetrical distribution effects, or a combination of
these. However, a survey of published pre-maximum spectra reveals that more SNe
Ia than previously thought may exhibit C II 6580 absorption features and relics
of line blending near 6300 Angstroms. Here we present new SN Ia observations
where spectroscopic signatures of C II 6580 are detected, and investigate the
presence of C II 6580 in the optical spectra of 19 SNe Ia using the
parameterized spectrum synthesis code, SYNOW. Most of the objects in our sample
that exhibit C II 6580 absorption features are of the low-velocity gradient
subtype. Our study indicates that the morphology of carbon-rich regions is
consistent with either a spherical distribution or a hemispheric asymmetry,
supporting the recent idea that SN Ia diversity may be a result of off-center
ignition coupled with observer line-of-sight effects.Comment: 10 papges, 9 figures, 3 table
Quark-Novae in Neutron Star-White-Dwarf Binaries: A model for luminous (spin-down powered) sub-Chandrasekhar-mass Type Ia Supernovae ?
We show that appealing to a Quark-Nova (QN) in a tight binary system
containing a massive neutron star and a CO white dwarf (WD), a Type Ia
explosion could occur. The QN ejecta collides with the WD driving a shock that
triggers Carbon burning under degenerate conditions (the QN-Ia). The conditions
in the compressed low-mass WD (M_WD < 0.9M_sun) in our model mimics those of a
Chandrasekhar mass WD. The spin-down luminosity from the QN compact remnant
(the quark star) provides additional power that makes the QN-Ia light-curve
brighter and broader than a standard SN-Ia with similar 56Ni yield. In QNe-Ia,
photometry and spectroscopy are not necessarily linked since the kinetic energy
of the ejecta has a contribution from spin-down power and nuclear decay.
Although QNe-Ia may not obey the Phillips relationship, their brightness and
their relatively "normal looking" light-curves means they could be included in
the cosmological sample. Light-curve fitters would be confused by the
discrepancy between spectroscopy at peak and photometry and would correct for
it by effectively brightening or dimming the QNe-Ia apparent magnitudes. Thus
over- or under-estimating the true magnitude of these spin-down powered SNe-Ia.
Contamination of QNe-Ia in samples of SNe-Ia used for cosmological analyses
could systematically bias measurements of cosmological parameters if QNe-Ia are
numerous enough at high-redshift. The strong mixing induced by spin-down wind
combined with the low 56Ni yields in QNe-Ia means that these would lack a
secondary maximum in the i-band despite their luminous nature. We discuss
possible QNe-Ia progenitors.Comment: Accepted for publication in RAA (Research in Astronomy and
Astrophysics). 24 journal pages (3 figures
The Long-Term Evolution of Double White Dwarf Mergers
In this paper, we present a model for the long-term evolution of the merger
of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the
merger, magnetic stresses rapidly redistribute angular momentum, leading to
nearly solid-body rotation on a viscous timescale of 1e-4 to 1 yr, long before
significant cooling can occur. Due to heating during the dynamical and viscous
phases, the less massive WD is transformed into a hot, slowly rotating, and
radially extended envelope supported by thermal pressure.
Following the viscous phase of evolution, the maximum temperature near the
envelope base may already be high enough to begin off-center convective
carbon-burning. If not, Kelvin-Helmholtz contraction of the inner region of the
envelope on a thermal timescale of 1e3-1e4 yr compresses the base of the
envelope, again yielding off-center burning. As a result, the long-term
evolution of the merger remnant is similar to that seen in previous
calculations: the burning shell diffuses inwards over ~1e4 yr, eventually
yielding a high-mass O/Ne WD or a collapse to a neutron star. During the
cooling and shell-burning phases, the merger remnant radiates near the
Eddington limit. Given the double WD merger rate of a few per 1000 yr, tens of
these ~1e38 erg/s sources should exist in a Milky Way-type galaxy.
While the end result is similar to that of previous studies, the physical
picture and the dynamical state of the matter in our model differ from previous
work. Furthermore, remaining uncertainties related to the convective structure
near the photosphere and mass loss during the thermal evolution may
significantly affect our conclusions. Thus, future work within the context of
the physical model presented here is required to better address the eventual
fate of double WD mergers, including those for which one or both of the
components is a He WD.Comment: Resubmitted to The Astrophysical Journal following the referee's
report; 11 pages, 8 figures. Changes include an updated thermal evolution
calculation, although our qualitative conclusions remain the sam
White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae
Limits on the companions of white dwarfs in the single degenerate scenario
for the origin of Type Ia supernovae (SNIa) have gotten increasingly tight. The
only type of non-degenerate stars that survive the limits on the companions of
SNIa in SNR 0509-67.5 and SN1572 are M dwarfs. M dwarfs have special properties
that have not been considered in most work on the progenitors of SNIa: they
have small but finite magnetic fields, and they flare frequently. These
properties are explored in the context of SNIa progenitors. White dwarf/M dwarf
pairs may be sufficiently plentiful to provide an adequate rate of explosions.
Even modest magnetic fields on the white dwarf and M dwarf will yield adequate
torques to lock the two stars together, resulting in a slowly rotating white
dwarf, with the magnetic poles pointing at one another in the orbital plane.
The mass loss will be channeled by a "magnetic bottle" connecting the two
stars, landing on a concentrated polar area on the white dwarf. This enhances
the effective rate of accretion compared to spherical accretion. Luminosity
from accretion and hydrogen burning on the surface of the white dwarf may
induce self-excited mass transfer. The combined effects of self-excited mass
loss, polar accretion, and magnetic inhibition of mixing of accretion layers
give possible means to beat the "nova limit" and grow the white dwarf to the
Chandrasekhar mass even at rather moderate mass accretion rates.Comment: 32 pages, 4 figures, accepted for publication in the Astrophysical
Journa
The delay-time distribution of type-Ia supernovae from Sloan II
We derive the delay-time distribution (DTD) of type-Ia supernovae (SNe Ia)
using a sample of 132 SNe Ia, discovered by the Sloan Digital Sky Survey II
(SDSS2) among 66,000 galaxies with spectral-based star-formation histories
(SFHs). To recover the best-fit DTD, the SFH of every individual galaxy is
compared, using Poisson statistics, to the number of SNe that it hosted (zero
or one), based on the method introduced in Maoz et al. (2011). This SN sample
differs from the SDSS2 SN Ia sample analyzed by Brandt et al. (2010), using a
related, but different, DTD recovery method. Furthermore, we use a
simulation-based SN detection-efficiency function, and we apply a number of
important corrections to the galaxy SFHs and SN Ia visibility times. The DTD
that we find has 4-sigma detections in all three of its time bins: prompt (t <
420 Myr), intermediate (0.4 2.4 Gyr),
indicating a continuous DTD, and it is among the most accurate and precise
among recent DTD reconstructions. The best-fit power-law form to the recovered
DTD is t^(-1.12+/-0.08), consistent with generic ~t^-1 predictions of SN Ia
progenitor models based on the gravitational-wave induced mergers of binary
white dwarfs. The time integrated number of SNe Ia per formed stellar mass is
N_SN/M = 0.00130 +/- 0.00015 Msun^-1, or about 4% of the stars formed with
initial masses in the 3-8 Msun range. This is lower than, but largely
consistent with, several recent DTD estimates based on SN rates in galaxy
clusters and in local-volume galaxies, and is higher than, but consistent with
N_SN/M estimated by comparing volumetric SN Ia rates to cosmic SFH.Comment: MNRAS, in pres