113 research outputs found
Hoyle-Lyttleton Accretion in Three Dimensions
We investigate the stability of gravitational accretion of an ideal gas onto
a compact object moving through a uniform medium at Mach 3. Previous
three-dimensional simulations have shown that such accretion is not stable, and
that strong rotational 'disk-like' flows are generated and accreted on short
time scales. We re-address this problem using overset spherical grids that
provide a factor of seven improvement in spatial resolution over previous
simulations. With our higher spatial resolution we found these 3D accretion
flows remained remarkably axisymmetric. We examined two cases of accretion with
different sized accretors. The larger accretor produced very steady flow, with
the mass accretion rate varying by less than 0.02% over 30 flow times. The
smaller accretor exhibited an axisymmetric breathing mode that modulated the
mass accretion rate by a constant 20%. Nonetheless, the flow remained highly
axisymmetric with only negligible accretion of angular momentum in both cases.Comment: 6 pages, 6 figures. Submitted to Ap
Axisymmetric circumstellar interaction in supernovae
Multiwavelength observations of Type II supernovae have shown evidence for
the interaction of supernovae with the dense slow winds from the red supergiant
progenitor stars. Observations of planetary nebulae and the nebula around SN
1987A show that the slow winds from extended stars frequently have an axisymme-
tric structure with a high density in the equatorial plane. We have carried out
numerical calculations of the interaction of a supernova with such an axisymme-
tric density distribution. For small values of the angular density gradient at
the pole, the asymmetry in the interaction shell is greater than, but close to,
that expected from purely radial motion. If the angular density gradient is
above a moderate value, the flow qualitatively changes and a protrusion emerges
along the axis. For a power-law supernova density profile, the flow approaches
a self-similar state in which the protrusion length is times the radius
of the main shell. The critical density gradient is larger for steeper density
profiles of the ejecta. Most of our calculations are axisymmetric, but we have
carried out a 3-dimensional calculation to show that the protrusion is not a
numerical artifact along the symmetry axis. For typical supernova parameters,
the protrusions take several years to develop. The appearance of the
shell with protrusions is similar to that observed in VLBI radio images of the
remnant 41.9 +58 in M82 and, possibly, of SN 1986J. We also considered the
possibility of asymmetric ejecta and found that it had a relatively small
effect on the asymmetry of the interaction region.Comment: 22 page postscript file (gzipped and uuencoded), 10 gzipped
postscript figures may be retrieved from
ftp://www.astro.su.se/pub/supernova/blc96_asym/ Submitted to Ap
Linear growth of spiral SASI modes in core-collapse supernovae
Two-dimensional axisymmetric simulations have shown that the post-bounce
accretion shock in core collapse supernovae is subject to the Spherical
Accretion Shock Instability, or SASI. Recent three-dimensional simulations have
revealed the existence of a non-axisymmetric mode of the SASI as well, where
the postshock flow displays a spiral pattern. Here we investigate the growth of
these spiral modes using two-dimensional simulations of the post-bounce
accretion flow in the equatorial plane of a core-collapse supernova. By
perturbing a steady-state model we are able to excite both one, two and
three-armed spiral modes that grow exponentially with time, demonstrating that
these are linearly unstable modes closely related to the original axisymmetric
sloshing modes. By tracking the distribution of angular momentum, we show that
these modes are able to efficiently separate the angular momentum of the
accretion flow (which maintains a net angular momentum of zero), leading to a
significant spin-up of the underlying accreting proto-neutron star.Comment: To be published in The Astrophysical Journa
Stability of Standing Accretion Shocks, With an Eye Toward Core Collapse Supernovae
We examine the stability of standing, spherical accretion shocks. Accretion
shocks arise in core collapse supernovae (the focus of this paper), star
formation, and accreting white dwarfs and neutron stars. We present a simple
analytic model and use time-dependent hydrodynamics simulations to show that
this solution is stable to radial perturbations. In two dimensions we show that
small perturbations to a spherical shock front can lead to rapid growth of
turbulence behind the shock, driven by the injection of vorticity from the now
non-spherical shock. We discuss the ramifications this instability may have for
the supernova mechanism.Comment: 21 pages, 13 figures; to be published in The Astrophysical Journa
Evidence for sub-Chandrasekhar-mass progenitors of Type Ia supernovae at the faint end of the width-luminosity relation
The faster light-curve evolution of low-luminosity Type Ia supernovae (SNe
Ia) suggests that they could result from the explosion of white dwarf (WD)
progenitors below the Chandrasekhar mass (). Here we present 1D
non-LTE time-dependent radiative transfer simulations of pure central
detonations of carbon-oxygen WDs with a mass (M_\rm{tot}) between 0.88
M and 1.15 M, and a yield between 0.08
M and 0.84 M. Their lower ejecta density compared to
models results in a more rapid increase of the luminosity at early
times and an enhanced -ray escape fraction past maximum light.
Consequently, their bolometric light curves display shorter rise times and
larger post-maximum decline rates. Moreover, the higher
M(^{56}\rm{Ni})/M_\rm{tot} ratio at a given mass enhances the
temperature and ionization level in the spectrum-formation region for the less
luminous models, giving rise to bluer colours at maximum light and a faster
post-maximum evolution of the colour. For sub- models fainter
than mag at peak, the greater bolometric decline and faster
colour evolution lead to a larger -band post-maximum decline rate, . In particular, all of our previously-published models
(standard and pulsational delayed detonations) are confined to mag, while the sub- models with
M_\rm{tot}\lesssim 1 M extend beyond this limit to mag for a peak mag, in better agreement
with the observed width-luminosity relation (WLR). Regardless of the precise
ignition mechanism, these simulations suggest that fast-declining SNe Ia at the
faint end of the WLR could result from the explosion of WDs whose mass is
significantly below the Chandrasekhar limit.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
An X-ray and Radio Study of the Varying Expansion Velocities in Tycho's Supernova Remnant
We present newly obtained X-ray and radio observations of Tycho's supernova
remnant using {\it Chandra} and the Karl G. Jansky Very Large Array in 2015 and
2013/14, respectively. When combined with earlier epoch observations by these
instruments, we now have time baselines for expansion measurements of the
remnant of 12-15 year in the X-rays and 30 year in the radio. The remnant's
large angular size allows for proper motion measurements at many locations
around the periphery of the blast wave. We find, consistent with earlier
measurements, a clear gradient in the expansion velocity of the remnant,
despite its round shape. The proper motions on the western and southwestern
sides of the remnant are about a factor of two higher than those in the east
and northeast. We showed in an earlier work that this is related to an offset
of the explosion site from the geometric center of the remnant due to a density
gradient in the ISM, and using our refined measurements reported here, we find
that this offset is towards the northeast. An explosion center
offset in such a circular remnant has implications for searches for progenitor
companions in other remnants.Comment: Accepted for publication in ApJ Letter
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