260 research outputs found
Hydrogen-Poor Circumstellar Shells from Pulsational Pair-Instability Supernovae with Rapidly Rotating Progenitors
In certain mass ranges, massive stars can undergo a violent pulsation
triggered by the electron/positron pair instability that ejects matter, but
does not totally disrupt the star. After one or more of these pulsations, such
stars are expected to undergo core-collapse to trigger a supernova explosion.
The mass range susceptible to this pulsational phenomena may be as low as 50-70
Msun if the progenitor is of very low metallicity and rotating sufficiently
rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and
composition of the matter ejected in the pulsation are important aspects to
determine the subsequent observational characteristics of the explosion. We
examine the dynamics of a sample of stellar models and rotation rates and
discuss the implications for the first stars, for LBV-like phenomena, and for
superluminous supernovae. We find that the shells ejected by pulsational
pair-instability events with rapidly rotating progenitors (>30% the critical
value) are hydrogen-poor and helium and oxygen-rich.Comment: 14 pages, 2 figure
Spitzer Mid-infrared Study of Compact HII Regions in the Magellanic Clouds
We present a study of the mid-infrared properties and dust content of a
sample of 27 HII ``blobs'', a rare class of compact HII regions in the
Magellanic Clouds. A unique feature of this sample is that even though these
HII regions are of high and low excitation they have nearly the same physical
sizes ~1.5-3 pc. We base our analysis on archival 3-8 microns infrared imagery
obtained with the Infrared Array Camera (IRAC) on board the Spitzer Space
Telescope. We find that despite their youth, sub-solar metallicity and varied
degrees of excitation, the mid-infrared colors of these regions are similar to
those of typical HII regions. Higher excitation ``blobs'' (HEBs) display
stronger 8 micron emission and redder colors than their low-excitation
counterparts (LEBs).Comment: 8 pages, 4 figures, Accepted for publication in Astronomy &
Astrophysics (higher resolution version is available in
http://www.physics.uoc.gr/~vassilis/papers/blobs_spitzer.pdf
Multidimensional Simulations of Rotating Pair Instability Supernovae
We study the effects of rotation on the dynamics, energetics and Ni-56
production of Pair Instability Supernova explosions by performing rotating
two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution
of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN
progenitors with initial surface rotational velocities 50% that of the critical
Keplerian value using the stellar evolution code MESA. We allow for both the
inclusion and the omission of the effects of magnetic fields in the angular
momentum transport and in chemical mixing, resulting in slowly-rotating and
rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation
for carbon-oxygen cores of the same mass and chemical stratification leads to
less energetic PISN explosions that produce smaller amounts of Ni-56 due to the
effect of the angular momentum barrier that develops and slows the dynamical
collapse. We find a non-monotonic dependence of Ni-56 production on rotational
velocity in situations when smoother composition gradients form at the outer
edge of the rotating cores. In these cases, the PISN energetics are determined
by the competition of two factors: the extent of chemical mixing in the outer
layers of the core due to the effects of rotation in the progenitor evolution
and the development of angular momentum support against collapse. Our 2.5-D
PISN simulations with rotation are the first presented in the literature. They
reveal hydrodynamic instabilities in several regions of the exploding star and
increased explosion asymmetries with higher core rotational velocity.Comment: 31 pages, 23 figures, accepted for publication in the Ap
Is Betelgeuse the Outcome of a Past Merger?
We explore the possibility that the star alpha Orionis (Betelgeuse) is the
outcome of a merger that occurred in a low mass ratio (q = M2/M1 = 0.07 - 0.25)
binary system some time in the past hundreds of thousands of years. To that
goal, we present a simple analytical model to approximate the perturbed
internal structure of a post-merger object following the coalescence of a
secondary in the mass range 1-4 Msun into the envelope of a 15-17 Msun primary.
We then compute the long-term evolution of post-merger objects for a grid of
initial conditions and make predictions about their surface properties for
evolutionary stages that are consistent with the observed location of
Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger
occurred after the end of the primary's main-sequence phase, while it was
expanding toward becoming a red supergiant star and typically with radius ~200
- 300 Rsun, then it's envelope is spun-up to values which remain in a range
consistent with the Betelgeuse observations for thousands of years of
evolution. We argue that the best scenario that can explain both the fast
rotation of Betelgeuse and its observed large space velocity is one where a
binary was dynamically ejected by its parent cluster a few million years ago
and then subsequently merged. An alternative scenario in which the progenitor
of Betelgeuse was spun up by accretion in a binary and released by the
supernova explosion of the companion requires a finely tuned set of conditions
but cannot be ruled out.Comment: 20 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Generalized Semi-Analytical Models of Supernova Light Curves
We present generalized supernova (SN) light curve (LC) models for a variety
of power inputs. We provide an expression for the power input that is produced
by self-similar forward and reverse shocks in SN ejecta - circumstellar matter
(CSM) interaction. We find that this ejecta-CSM interaction luminosity is in
agreement with results from multi-dimensional radiation hydrodynamics
simulations in the optically-thin case. We develop a model for the case of an
optically-thick CSM by invoking an approximation for the effects of radiative
diffusion. In the context of this model, we provide predictions for the time of
forward shock break-out from the optically-thick part of the CSM envelope. We
also introduce a hybrid LC model that incorporates ejecta-CSM interaction plus
Ni-56 and Co-56 radioactive decay input. We fit this hybrid model to the LC of
the Super-Luminous Supernova (SLSN) 2006gy. We find that this model provides a
better fit to the LC of this event than previously presented models. We also
address the relation between Type IIL and Type IIn SN with ejecta-CSM
interaction models. Forward and reverse shock power input due to CSM
interaction can produce the LCs of Type IIn SNe in terms of duration, shape and
decline rate. This model can also produce LCs that are symmetric in shape
around peak luminosity. We conclude that the observed LC variety of SNe Type
IIn and of the SLSNe is likely to be a byproduct of the large range of
conditions relevant to significant ejecta-CSM interaction as a power source.Comment: 48 pages, 13 figure
Effects of Rotation on the Minimum Mass of Primordial Progenitors of Pair Instability Supernovae
The issue of which stars may reach the conditions of electron/positron pair
formation instability is of importance to understand the final evolution both
of the first stars and of contemporary stars. The criterion to enter the pair
instability regime in density and temperature is basically controlled by the
mass of the oxygen core. The main sequence masses that produce a given oxygen
core mass are, in turn, dependent on metallicity, mass loss, and convective and
rotationally-induced mixing. We examine the evolution of massive stars to
determine the minimum main sequence mass that can encounter pair-instability
effects, either a pulsational pair instability (PPISN) or a full-fledged
pair-instability supernova (PISN). We concentrate on zero-metallicity stars
with no mass loss subject to the Schwarzschild criterion for convective
instability, but also explore solar metallicity and mass loss and the Ledoux
criterion. As expected, for sufficiently strong rotationally-induced mixing,
the minimum main sequence mass is encountered for conditions that induce
effectively homogeneous evolution such that the original mass is converted
almost entirely to helium and then to oxygen. For this case, we find that the
minimum main sequence mass is ~40 Msun to encounter PPISN and ~65 Msun to
encounter a PISN. When mass-loss is taken into account those mass limits become
~50 Msun for PPISN and ~80 Msun for PISN progenitors. The implications of these
results for the first stars and for contemporary supernovae is discussed.Comment: 23 pages, 8 figure
Extreme Supernova Models for the Superluminous Transient ASASSN-15lh
The recent discovery of the unprecedentedly superluminous transient
ASASSN-15lh (or SN 2015L) with its UV-bright secondary peak challenges all the
power-input models that have been proposed for superluminous supernovae. Here
we examine some of the few viable interpretations of ASASSN-15lh in the context
of a stellar explosion, involving combinations of one or more power inputs. We
model the lightcurve of ASASSN-15lh with a hybrid model that includes
contributions from magnetar spin-down energy and hydrogen-poor circumstellar
interaction. We also investigate models of pure circumstellar interaction with
a massive hydrogen-deficient shell and discuss the lack of interaction features
in the observed spectra. We find that, as a supernova ASASSN-15lh can be best
modeled by the energetic core-collapse of a ~40 Msun star interacting with a
hydrogen-poor shell of ~20 Msun. The circumstellar shell and progenitor mass
are consistent with a rapidly rotating pulsational pair-instability supernova
progenitor as required for strong interaction following the final supernova
explosion. Additional energy injection by a magnetar with initial period of 1-2
ms and magnetic field of 0.1-1 x 10^14 G may supply the excess luminosity
required to overcome the deficit in single-component models, but this requires
more fine-tuning and extreme parameters for the magnetar, as well as the
assumption of efficient conversion of magnetar energy into radiation. We thus
favor a single-input model where the reverse shock formed in a strong SN
ejecta-CSM interaction following a very powerful core-collapse SN explosion can
supply the luminosity needed to reproduce the late-time UV-bright plateau.Comment: 8 pages, 3 figure
Direct Wolf summation of a polarizable force field for silica
We extend the Wolf direct, pairwise r^(-1) summation method with spherical
truncation to dipolar interactions in silica. The Tangney-Scandolo interatomic
force field for silica takes regard of polarizable oxygen atoms whose dipole
moments are determined by iteration to a self-consistent solution. With Wolf
summation, the computational effort scales linearly in the system size and can
easily be distributed among many processors, thus making large-scale
simulations of dipoles possible. The details of the implementation are
explained. The approach is validated by estimations of the error term and
simulations of microstructural and thermodynamic properties of silica.Comment: See http://link.aip.org/link/?JCP/132/194109 - 8 pages, 6 figures.
Changes in v3: Copyright notice added, minor typographical changes. Changes
in v2: 1. Inserted Paragraph in Sec. IV B describing the limitations of the
TS potential. 2. We corrected transcription errors in Tab. II, and adjusted
the deviation percentages mentioned in Sec. IV B, first paragraph,
accordingl
- …