143 research outputs found
Aspherical Explosion Models for SN 1998bw/GRB 980425
The recent discovery of the unusual supernova SN1998bw and its apparent
correlation with the gamma-ray burst GRB 980425 has raised new issues
concerning both the GRB and supernovae. Although the spectra resemble those of
TypeIc supernovae, there are distinct differences at early times and SN1998bw
appeared to be unusually bright and red at maximum light. The apparent
expansion velocities inferred by the Doppler shift of (unidentified) absorption
features appeared to be high, making SN1998bw a possible candidate for a
"hypernova" with explosion energies between 20 and 50E51 erg and ejecta masses
in excess of 6 - 15 M_o. Based on light curve calculations for aspherical
explosions and guided by the polarization observations of "normal" SNIc and
related events, we present an alternative picture that allows SN1998bw to have
an explosion energy and ejecta mass consistent with core collapse supernovae
(although at the 'bright' end). We show that the LC of SN1998bw can be
understood as result of an aspherical explosion along the rotational axis of a
basically spherical, non-degenerate C/O core of massive star with an explosion
energy of 2foe and a total ejecta mass of 2 M_o if it is seen from high
inclinations with respect to the plane of symmetry. In this model, the high
expansion velocities are a direct consequence of an aspherical explosion which,
in turn, produces oblate iso-density contours. It suggests that the fundamental
core-collapse explosion process itself is strongly asymmetric.Comment: 12 pages, 8 figures, latex, aas2pp4.sty, submitted to Ap
Properties of Deflagration Fronts and Models for Type Ia Supernovae
Detailed models of the explosion of a white dwarf, which include
self-consistent calculations of the light curve and spectra, provide a link
between observational quantities and the underlying explosion.These
calculations assume spherical geometry and are based on parameterized
descriptions of the burning front during the deflagration phase. Recently,
first multi-dimensional calculations for nuclear burning fronts have been
performed. Although a fully consistent treatment of the burning fronts is
beyond the current state of the art, these calculations provided a new and
better understanding of the physics, and new descriptions for the flame
propagation have been proposed. Here, we have studied the influence on the
results of previous analyses of Type Ia Supernovae, namely, the nucleosynthesis
and structure of the expanding envelope. Our calculations are based on a set of
delayed detonation models with parameters that give a good account of the
optical and infrared light curves, and of the spectral evolution. In this
scenario, the burning front propagates first in a deflagration mode and,
subsequently, turns into a detonation. The explosions and light curves are
calculated using a one-dimensional Lagrangian radiation-hydro code, including a
detailed nuclear network.Comment: 9 pages, 4 figures, macros 'crckapb.sty'. The Astrophysical Journal
(accepted
SN 2005hj: Evidence for Two Classes of Normal-Bright SNe Ia and Implications for Cosmology
HET Optical spectra covering the evolution from about 6 days before to about
5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the
"Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxy
shows HII region lines at redshift of z=0.0574, which puts the peak unfiltered
absolute magnitude at a somewhat over-luminous -19.6. The spectra show weak and
narrow SiII lines, and for a period of at least 10 days beginning around
maximum light these profiles do not change in width or depth and they indicate
a constant expansion velocity of ~10,600 km/s. We analyzed the observations
based on detailed radiation dynamical models in the literature. Whereas delayed
detonation and deflagration models have been used to explain the majority of
SNe Ia, they do not predict a long velocity plateau in the SiII minimum with an
unvarying line profile. Pulsating delayed detonations and merger scenarios form
shell-like density structures with properties mostly related to the mass of the
shell, M_shell, and we discuss how these models may explain the observed SiII
line evolution; however, these models are based on spherical calculations and
other possibilities may exist. SN 2005hj is consistent with respect to the
onset, duration, and velocity of the plateau, the peak luminosity and, within
the uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun.
Our analysis suggests a distinct class of events hidden within the
Branch-normal SNe Ia. If the predicted relations between observables are
confirmed, they may provide a way to separate these two groups. We discuss the
implications of two distinct progenitor classes on cosmological studies
employing SNe Ia, including possible differences in the peak luminosity to
light curve width relation.Comment: ApJ accepted, 31 page
Low Carbon Abundance in Type Ia Supernovae
We investigate the quantity and composition of unburned material in the outer
layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr and 20 04bw.
Pristine matter from a white dwarf progenitor is expected to be a mixture of
oxygen and carbon in approximately equal abundance. Using near-infrared (NIR,
0.7-2.5 microns) spectra, we find that oxygen is abundant while carbon is
severely depleted with low upper limits in the outer third of the ejected mass.
Strong features from the OI line at rest wavelength = 0.7773 microns are
observed through a wide range of expansion velocities approx. 9,000 - 18,000
km/s. This large velocity domain corresponds to a physical region of the
supernova with a large radial depth. We show that the ionization of C and O
will be substantially the same in this region. CI lines in the NIR are expected
to be 7-50 times stronger than those from OI but there is only marginal
evidence of CI in the spectra and none of CII. We deduce that for these three
normal SNe Ia, oxygen is more abundant than carbon by factors of 100 - 1,000.
MgII is also detected in a velocity range similar to that of OI. The presence
of O and Mg combined with the absence of C indicates that for these SNe Ia,
nuclear burning has reached all but the extreme outer layers; any unburned
material must have expansion velocities greater than 18,000 km/s. This result
favors deflagration to detonation transition (DD) models over pure deflagration
models for SNe Ia.Comment: accepted for publication in Ap
Hard X- and Gamma-Rays from Type Ia Supernovae
The gamma-ray light curves and spectra are presented for a set of theoretical
Type Ia supernova models including deflagration, detonation, delayed
detonation, and pulsating delayed detonations of Chandrasekhar mass white
dwarfs as well as merger scenarios that may involve more than the Chandrasekhar
mass and helium detonations of sub-Chandrasekhar mass white dwarfs. The results
have been obtained with a Monte Carlo radiation transport scheme which takes
into account all relevant gamma-transitions and interaction processes. The
result is a set of accurate line profiles which are characteristic of the
initial Ni-mass distribution of the supernova models. The gamma-rays probe the
isotopic rather than just the elemental distribution of the radioactive
elements in the ejecta. Details of the line profiles including the line width,
shift with respect to the rest frame, and line ratios are discussed. With
sufficient energy and temporal resolution, different model scenarios can
clearly be distinguished. Observational strategies are discussed for current
and immediately upcoming generations of satellites (CGRO and INTEGRAL) as well
as projected future missions including concepts such as Laue telescopes. With
CGRO, it is currently possible with sufficiently early observations (near
optical maximum) to distinguish helium detonations from explosions of
Chandrasekhar mass progenitors and of those involving mergers up to a distance
of about 15 Mpc. This translates into one target of opportunity every eight
years. SNe Ia up to about 10 Mpc would allow detailed CGRO studies of line
ratios of Co lines.Comment: 32 pages, Tex, ApJ, in pres
The Becklin-Neugebauer Object as a Runaway B Star, Ejected 4000 years ago from the theta^1C system
We attempt to explain the properties of the Becklin-Neugebauer (BN) object as
a runaway B star, as originally proposed by Plambeck et al. (1995). This is one
of the best-studied bright infrared sources, located in the Orion Nebula
Cluster -- an important testing ground for massive star formation theories.
From radio observations of BN's proper motion, we trace its trajectory back
to Trapezium star theta^1C, the most massive (45 Msun) in the cluster and a
relatively tight (17 AU) visual binary with a B star secondary. This origin
would be the most recent known runaway B star ejection event, occurring only
\~4000 yr ago and providing a unique test of models of ejection from multiple
systems of massive stars. Although highly obscured, we can constrain BN's mass
(~7 Msun) from both its bolometric luminosity and the recoil of theta^1C.
Interaction of a runaway B star with dense ambient gas should produce a compact
wind bow shock. We suggest that X-ray emission from this shocked gas may have
been seen by Chandra: the offset from the radio position is ~300 AU in the
direction of BN's motion. Given this model, we constrain the ambient density,
wind mass-loss rate and wind velocity. BN made closest approach to the massive
protostar, source ``I'', 500 yr ago. This may have triggered enhanced accretion
and thus outflow, consistent with previous interpretations of the outflow being
a recent (~10^3 yr) "explosive" event.Comment: 6 pages, accepted to ApJ Letter
Three Dimensional Simulation of Gamma Ray Emission from Asymmetric Supernovae and Hypernovae
Hard X- and -ray spectra and light curves resulting from radioactive
decays are computed for aspherical (jet-like) and energetic supernova models
(representing a prototypical hypernova SN 1998bw), using a 3D energy- and
time-dependent Monte Carlo scheme. The emission is characterized by (1) early
emergence of high energy emission, (2) large line-to-continuum ratio, and (3)
large cut-off energy by photoelectric absorptions in hard X-ray energies. These
three properties are not sensitively dependent on the observer's direction. On
the other hand, fluxes and line profiles depend sensitively on the observer's
direction, showing larger luminosity and larger degree of blueshift for an
observer closer to the polar () direction. Strategies to derive the degree
of asphericity and the observer's direction from (future) observations are
suggested on the basis of these features, and an estimate on detectability of
the high energy emission by the {\it INTEGRAL} and future observatories is
presented. Also presented is examination on applicability of a gray effective
-ray opacity for computing the energy deposition rate in the aspherical
SN ejecta. The 3D detailed computations show that the effective -ray
opacity cm g reproduces the
detailed energy-dependent transport for both spherical and aspherical
(jet-like) geometry.Comment: 24 pages, 13 figures. Figure 7 added in the accepted version. ApJ,
644 (01 June 2006 issue), in press. Resolution of figures lower than the
published versio
Asymmetric Supernovae, Pulsars, Magnetars, and Gamma-Ray Bursts
We outline the possible physical processes, associated timescales, and
energetics that could lead to the production of pulsars, jets, asymmetric
supernovae, and weak gamma-ray bursts in routine circumstances and to a
magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in
the collapse of the bare core of a massive star. The production of a
LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a
weak gamma-ray burst when the jet accelerates down the stellar density gradient
of a hydrogen-poor photosphere. The matter-dominated jet would be formed
promptly, but requires 5 to 10 s to reach the surface of the progenitor of a
Type Ib/c supernova. During this time, the newly-born neutron star could
contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In
addition, the light cylinder will contract from a radius large compared to the
Alfven radius to a size comparable to that of the neutron star. This will
disrupt the structure of any organized dipole field and promote the generation
of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude
Electromagnetic Waves (LAEMW) at low density. The generation of the these waves
would be delayed by the cooling time of the neutron star about 5 to 10 seconds,
but the propagation time is short so the UMHDW could arrive at the surface at
about the same time as the matter jet. In the density gradient of the star and
the matter jet, the intense flux of UMHDW and LAEMW could drive shocks,
generate pions by proton-proton collision, or create electron/positron pairs
depending on the circumstances. The UMHDW and LAEMW could influence the
dynamics of the explosion and might also tend to flow out the rotation axis to
produce a collimated gamma-ray burst.Comment: 31 pages, LaTeX, revised for referee comments, accepted for ApJ, July
10 issu
The Axially Symmetric Ejecta of Supernova 1987A
Extensive early observations proved that the ejecta of supernova 1987A (SN
1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble
Space Telescope has resolved the rapidly expanding ejecta. The late-time images
and spectroscopy provide a geometrical picture that is consistent with early
observations and suggests a highly structured, axially symmetric geometry. We
present here a new synthesis of the old and new data. We show that the Bochum
event, presumably a clump of Ni, and the late-time image, the locus of
excitation by Ti, are most naturally accounted for by sharing a common
position angle of about 14\degree, the same as the mystery spot and early
speckle data on the ejecta, and that they are both oriented along the axis of
the inner circumstellar ring at 45\degree to the plane of the sky. We also
demonstrate that the polarization represents a prolate geometry with the same
position angle and axis as the early speckle data and the late-time image and
hence that the geometry has been fixed in time and throughout the ejecta. The
Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in
spatially resolved HST spectra of the ejecta can be consistently integrated
into this geometry. The radioactive clump is deduced to fall approximately
along the axis of the inner circumstellar ring and therefore to be redshifted
in the North whereas the [Ca II]/[O II] 7300 \AA emission is redshifted in the
South. We present a jet-induced model for the explosion and argue that such a
model can account for many of the observed asymmetries. In the jet models, the
oxygen and calcium are not expected to be distributed along the jet, but
primarily in an expanding torus that shares the plane and northern blue shift
of the inner circumstellar ring.Comment: To Appear in Ap
Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-velocity Ejecta
SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic
polarization signal. In addition, during the epochs prior to maximum light, the
CaII IR triplet absorption is seen distinctly and separately at both normal
photospheric velocities and at very high velocities. The high-velocity triplet
absorption is highly polarized, with a different polarization angle than the
rest of the spectrum. The unique observation allows us to construct a
relatively detailed picture of the layered geometrical structure of the
supernova ejecta: in our interpretation, the ejecta layers near the photosphere
(v \approx 10,000 km/s) obey a near axial symmetry, while a detached,
high-velocity structure (v \approx 18,000-25,000 km/s) with high CaII line
opacity deviates from the photospheric axisymmetry. By partially obscuring the
underlying photosphere, the high-velocity structure causes a more incomplete
cancellation of the polarization of the photospheric light, and so gives rise
to the polarization peak and rotated polarization angle of the high-velocity IR
triplet feature. In an effort to constrain the ejecta geometry, we develop a
technique for calculating 3-D synthetic polarization spectra and use it to
generate polarization profiles for several parameterized configurations. In
particular, we examine the case where the inner ejecta layers are ellipsoidal
and the outer, high-velocity structure is one of four possibilities: a
spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The
synthetic spectra rule out the spherical shell model, disfavor a toroid, and
find a best fit with the clumped shell. We show further that different
geometries can be more clearly discriminated if observations are obtained from
several different lines of sight.Comment: 14 pages (emulateapj5) plus 18 figures, accepted by The Astrophysical
Journa
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