294 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
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Gamma-Ray Light Curves and Spectra of Models for Type-Ia Supernovae
Based on detailed Monte Carlo calculations, we present gamma-ray energy deposition functions, gamma-ray light curves, and gamma-ray spectra for a large set of theoretical models of Type Ia supernovae including ''classical'' detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of Ni-56 synthesized in the event can be determined on the basis of the gamma-ray light curves and spectra if gamma-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times gamma-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the gamma-ray flux by CGRO experiment for our understanding of SN 1991 T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc), i.e., several models can be ruled out by the gamma-ray observations.Astronom
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
Increase in Forage Maize Production by Bacterial Fertilisers
Farmers in many countries value green material generated from maize as a high quality forage. Its inclusion in dairy cow diets can improve forage intake, increase animal performance and has the potential to reduce production costs (Phipps, 1994). Restrictions on the use of chemical fertilisers and a renewed interest in organic sustainable farming systems in general, has restored attention to crop rotations The increase of maize production using bacterial fertilisers also gives alternative ways to reduce chemical fertilisers in forage production. Increased uptake of nutrients such as N, P, and K and crop yield was reported with rhizobacteria (Lazarovits & Nowak, 1997). This paper presents studies carried out to evaluate the effects of bacterial fertilisers on maize production
On the gamma-ray emission of Type Ia Supernovae
A multi-dimension, time-dependent Monte Carlo code is used to compute sample
gamma-ray spectra to explore whether unambiguous constraints could be obtained
from gamma-ray observations of Type Ia supernovae. Both spherical and
aspherical geometries are considered and it is shown that moderate departures
from sphericity can produce viewing-angle effects that are at least as
significant as those caused by the variation of key parameters in
one-dimensional models. Thus gamma-ray data could in principle carry some
geometrical information, and caution should be applied when discussing the
value of gamma-ray data based only on one-dimensional explosion models. In
light of the limited sensitivity of current gamma-ray observatories, the
computed theoretical spectra are studied to revisit the issue of whether useful
constraints could be obtained for moderately nearby objects. The most useful
gamma-ray measurements are likely to be of the light curve and time-dependent
hardness ratios, but sensitivity higher than currently available, particularly
at relatively hard energies (~2-3 MeV), is desirable.Comment: 10 pages, 8 figures. Accepted by MNRAS. Minor changes to clarify
discussion in Section
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
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
X- and Gamma-Ray Flashes from Type Ia Supernovae?
We investigate two potential mechanisms that will produce X-ray and gamma-ray
flashes from Type Ia supernovae (SN-Ia). The mechanisms are the breakout of the
thermonuclear burning front as it reaches the surface of the white dwarf and
the interaction of the rapidly expanding envelope with an accretion disk. Based
on the delayed-detonation scenario and detailed radiation-hydro calculation
which include nuclear networks, we find that both mechanisms produce ~1 second
flashes of high energy radiation with peak luminosities of 10^48 to 10^50
erg/sec with fast rises and exponential declines. The X- and gamma-ray
visibility of a SN-Ia will depend strongly on self absorption within the
progenitor system, specifically on the properties of the accretion disk and its
orientation towards the observer. Such X-ray and gamma-ray flashes could be
detected as triggered events by Gamma-Ray Burst (GRB) detectors on satellites,
with events in current GRB catalogs. We have searched through the GRB catalogs
(for the BATSE, HETE, and Swift experiments) for GRBs that occur at the
extrapolated time of explosion and in the correct direction for known Type Ia
supernovae with radial velocity of less than 3,000 km/s. For BATSE about
12.9+-3.6 nearby SNe Ia should have been detected, but only 0.8+-0.7
non-coincidental matches have been found. With the HETE and Swift satellites,
we expect to see 5.6+-1.3 SN-Ia flashes from known nearby SNe Ia but, yet, no
SN-Ia flashes were detected. These place observational limits that the
bolometric peak luminosity of SN-Ia Flashes must be less ~10^46 erg/s. We
attribute the difference between theory and observational limits to the
absorption of the X- and gamma-rays by the accretion disk of large scale height
or common envelope that would be smothering the white dwarf.Comment: Astrophysical Journal, in pres
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