Making Black Holes in Supernovae

The possibility of making stellar mass black holes in supernovae that otherwise produce viable Type II and Ib supernova explosions is discussed and estimates given of their number in the Milky Way Galaxy. Observational diagnostics of stellar mass black hole formation are reviewed. While the equation of state sets the critical mass, fall back during the explosion is an equally important (and uncertain) element in determining if a black hole is formed. SN 1987A may or may not harbor a black hole, but if the critical mass for neutron stars is 1.5 - 1.6 M\sun, as Brown and Bethe suggest, it probably does. Observations alone do not yet resolve the issue. Reasons for this state of ambiguity are discussed and suggestions given as to how gamma-ray and x-ray observations in the future might help.Comment: 14 pages, uuencoded gzipped postscript, Accepted Nuclear Physics A, Gerry Brown Festschrift contributio

The Most Luminous Supernovae

Recent observations have revealed an amazing diversity of extremely luminous supernovae, seemingly increasing in radiant energy without bound. We consider here the physical limits of what existing models can provide for the peak luminosity and total radiated energy for non-relativistic, isotropic stellar explosions. The brightest possible supernova is a Type I explosion powered by a sub-millisecond magnetar. Such models can reach a peak luminosity of $\rm 2\times10^{46}\ erg\ s^{-1}$ and radiate a total energy of $\rm 4 \times10^{52}\ erg$. Other less luminous models are also explored, including prompt hyper-energetic explosions in red supergiants, pulsational-pair instability supernovae, and pair-instability supernovae. Approximate analytic expressions and limits are given for each case. Excluding magnetars, the peak luminosity is near $\rm 1\times10^{44}\ erg\ s^{-1}$ for the brightest models. The corresponding limits on total radiated power are $\rm3 \times 10^{51}\ erg$ (Type I) and $\rm1 \times 10^{51}\ erg$ (Type II). A magnetar-based model for the recent transient event, ASASSN-15lh is presented that strains, but does not exceed the limits of what the model can provide.Comment: 5 pages, 2 figures and 1 table. Submitted to The Astrophysical Journal Letter

On the Progenitors of Collapsars

We study the evolution of stars that may be the progenitors of common (long-soft) GRBs. Bare rotating helium stars, presumed to have lost their envelopes due to winds or companions, are followed from central helium ignition to iron core collapse. Including realistic estimates of angular momentum transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass loss, one is still able to create a collapsed object at the end with sufficient angular momentum to form a centrifugally supported disk, i.e., to drive a collapsar engine. However, inclusion of current estimates of magnetic torques (Spruit 2002) results in too little angular momentum for collapsars.Comment: 3 pages, 5 figures, in Proc. Woods Hole GRB meeting, ed. Roland Vanderspe

Low Mach Number Modeling of Type Ia Supernovae. IV. White Dwarf Convection

We present the first three-dimensional, full-star simulations of convection in a white dwarf preceding a Type Ia supernova, specifically the last few hours before ignition. For these long-time calculations we use our low Mach number hydrodynamics code, MAESTRO, which we have further developed to treat spherical stars centered in a three-dimensional Cartesian geometry. The main change required is a procedure to map the one-dimensional radial base state to and from the Cartesian grid. Our models recover the dipole structure of the flow seen in previous calculations, but our long-time integration shows that the orientation of the dipole changes with time. Furthermore, we show the development of gravity waves in the outer, stable portion of the star. Finally, we evolve several calculations to the point of ignition and discuss the range of ignition radii.Comment: 42 pages, some figures degraded to conserve space. Accepted to The Astrophysical Journal (http://journals.iop.org/

The Diversity of Type Ia Supernovae from Broken Symmetries

Type Ia supernovae result when carbon-oxygen white dwarfs in binary systems accrete mass from companion stars, reach a critical mass, and explode. The near uniformity of their light curves makes these supernovae good standard candles for measuring cosmic expansion, but a correction must be applied to account for the fact that the brighter supernovae have broader light curves. One-dimensional modelling, with a certain choice of parameters, can reproduce this general trend in the width-luminosity relation, but the processes of ignition and detonation have recently been shown to be intrinsically asymmetric. Here we report on multi-dimensional modelling of the explosion physics and radiative transfer that reveals that the breaking of spherical symmetry is a critical factor in determining both the width luminosity relation and the observed scatter about it. The deviation from sphericity can also explain the finite polarization detected in the light from some supernovae. The slope and normalization of the width-luminosity relation has a weak dependence on certain properties of the white dwarf progenitor, in particular the trace abundances of elements other than carbon and oxygen. Failing to correct for this effect could lead to systematic overestimates of up to 2% in the distance to remote supernovae.Comment: Accepted to Natur