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 $2-4$ 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 $\gtrsim$ 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

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 ($M_{\rm Ch}$). 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$_{\odot}$ and 1.15 M$_{\odot}$, and a $^{56}\rm{Ni}$ yield between 0.08 M$_{\odot}$ and 0.84 M$_{\odot}$. Their lower ejecta density compared to $M_{\rm Ch}$ models results in a more rapid increase of the luminosity at early times and an enhanced $\gamma$-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 $^{56}\rm{Ni}$ 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 $B-V$ colour. For sub-$M_{\rm Ch}$ models fainter than $M_B\approx -18.5$ mag at peak, the greater bolometric decline and faster colour evolution lead to a larger $B$-band post-maximum decline rate, $\Delta M_{15}(B)$. In particular, all of our previously-published $M_{\rm Ch}$ models (standard and pulsational delayed detonations) are confined to $\Delta M_{15}(B) < 1.4$ mag, while the sub-$M_{\rm Ch}$ models with M_\rm{tot}\lesssim 1 M$_{\odot}$ extend beyond this limit to $\Delta M_{15}(B)\approx 1.65$ mag for a peak $M_B\approx -17$ 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

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

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 $\sim 23"$ 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