95 research outputs found
Type Ia supernovae: explosions and progenitors
Supernovae are the brightest explosions in the universe. Supernovae in our Galaxy, rare and happening only every few centuries, have probably been observed since the beginnings of mankind. At first they were interpreted as religious omens but in the last half millennium they have increasingly been used to study the cosmos and our place in it. Tycho Brahe deduced from his observations of the famous supernova in 1572, that the stars, in contrast to the widely believe Aristotelian doctrine, were not immutable.
More than 400 years after Tycho made his paradigm changing discovery using SN 1572, and some 60 years after supernovae had been identified as distant dying stars, two teams changed the view of the world again using supernovae. The found that the Universe was accelerating in its expansion, a conclusion that could most easily be explained if more than 70% of the Universe was some previously un-identified form of matter now often referred to as `Dark Energy'.
Beyond their prominent role as tools to gauge our place in the Universe, supernovae themselves have been studied well over the past 75 years. We now know that there are two main physical causes of these cataclysmic events. One of these channels is the collapse of the core of a massive star. The observationally motivated classes Type II, Type Ib and Type Ic have been attributed to these events. This thesis, however is dedicated to the second group of supernovae, the thermonuclear explosions of degenerate carbon and oxygen rich material and lacking hydrogen - called Type Ia supernovae (SNe Ia). White dwarf stars are formed at the end of a typical star's life when nuclear burning ceases in the core, the outer envelope is ejected, with the degenerate core typically cooling for eternity. Theory predicts that such stars will self ignite when close to 1.38 Msun (called the Chandrasekhar Mass). Most stars however leave white dwarfs with 0.6 Msun, and no star leaves a remnant as heavy as 1.38 Msun, which suggests that they somehow need to acquire mass if they are to explode as SN Ia. Currently there are two major scenarios for this mass acquisition. In the favoured single degenerate scenario the white dwarf accretes matter from a companion star which is much younger in its evolutionary state. The less favoured double degenerate scenario sees the merger of two white dwarfs (with a total combined mass of more than 1.38 Msun).
This thesis has tried to answer the question about the mass acquisition in two ways. First the single degenerate scenario predicts a surviving companion post-explosion. We undertook an observational campaign to find this companion in two ancient supernovae (SN 1572 and SN 1006). Secondly, we have extended an existing code to extract the elemental and energy yields of SNe Ia spectra by automating spectra fitting to specific SNe Ia. This type of analysis, in turn, help diagnose to which of the two major progenitor scenarios is right
Helium in Double-Detonation Models of Type Ia Supernovae
The double-detonation explosion model has been considered a candidate for
explaining astrophysical transients with a wide range of luminosities. In this
model, a carbon-oxygen white dwarf star explodes following detonation of a
surface layer of helium. One potential signature of this explosion mechanism is
the presence of unburned helium in the outer ejecta, left over from the surface
helium layer. In this paper we present simple approximations to estimate the
optical depths of important He I lines in the ejecta of double-detonation
models. We use these approximations to compute synthetic spectra, including the
He I lines, for double-detonation models obtained from hydrodynamical explosion
simulations. Specifically, we focus on photospheric-phase predictions for the
near-infrared 10830 \AA~and 2 m lines of He I. We first consider a double
detonation model with a luminosity corresponding roughly to normal SNe Ia. This
model has a post-explosion unburned He mass of 0.03 and our
calculations suggest that the 2 m feature is expected to be very weak but
that the 10830 \AA~feature may have modest opacity in the outer ejecta.
Consequently, we suggest that a moderate-to-weak He I 10830 \AA~feature may be
expected to form in double-detonation explosions at epochs around maximum
light. However, the high velocities of unburned helium predicted by the model
(~km~s) mean that the He I 10830 \AA~feature may be
confused or blended with the C I 10690~\AA~line forming at lower velocities. We
also present calculations for the He I 10830 \AA~and 2 m lines for a lower
mass (low luminosity) double detonation model, which has a post-explosion He
mass of 0.077 . In this case, both the He I features we consider are
strong and can provide a clear observational signature of the double-detonation
mechanism.Comment: 12 pages, 11 figures, accepted by A&
A reconnaissance of the possible donor stars to the Kepler supernova
The identity of Type Ia supernova progenitors remains a mystery, with various
lines of evidence pointing towards either accretion from a non-degenerate
companion, or the rapid merger of two degenerate stars leading to the
thermonuclear destruction of a white dwarf. In this paper we spectroscopically
scrutinize 24 of the brightest stars residing in the central 38" x 38" of the
SN 1604 (Kepler) supernova remnant to search for a possible surviving companion
star. We can rule out, with high certainty, a red giant companion star - a
progenitor indicated by some models of the supernova remnant. Furthermore, we
find no star that exhibits properties uniquely consistent with those expected
of a donor star down to L>10Lsun. While the distribution of star properties
towards the remnant are consistent with unrelated stars, we identify the most
promising candidates for further astrometric and spectroscopic follow-up. Such
a program would either discover the donor star, or place strong limits on
progenitor systems to luminosities with L<<Lsun.Comment: accepted by Ap
The rotation of surviving companion stars after type Ia supernova explosions in the WD+MS scenario
In the SD scenario of SNe Ia the companion survives the SN explosion and thus
should be visible near the center of the SN remnant and may show some unusual
features. A promising approach to test progenitor models of SNe Ia is to search
for the companion in SNRs. Here we present the results of 3D hydrodynamics
simulations of the interaction between the SN Ia blast wave and a MS companion
taking into consideration its orbital motion and spin. The primary goal of this
work is to investigate the rotation of surviving companions after SN Ia
explosions in the WD+MS scenario. We use Eggleton's code including the
optically thick accretion wind model to obtain realistic models of companions.
The impact of the SN blast wave on these companions is followed in 3D
hydrodynamic simulations employing the SPH code GADGET3. We find that the
rotation of the companion does not significantly affect the amount of stripped
mass and the kick velocity caused by the SN impact. However, in our
simulations, the rotational velocity of the companion is significantly reduced
to about 14% to 32% of its pre-explosion value due to the expansion of the
companion and the fact that 55%-89% of the initial angular momentum is carried
away by the stripped matter. Compared with the observed rotational velocity of
the presumed companion star of Tycho's SN, Tycho G, of 6 km/s the final
rotational velocity we obtain is still higher by at least a factor of two.
Whether this difference is significant, and may cast doubts on the suggestion
that Tycho G is the companion of SN 1572, has to be investigated in future
studies. Based on binary population synthesis results we present, for the first
time, the expected distribution of rotational velocities of companions after
the explosion which may provide useful information for the identification of
the surviving companion in observational searches in other historical SNRs.Comment: 13 pages, 15 figures, accepted for publication by Astronomy and
Astrophysic
Very late photometry of SN 2011fe
The Type Ia supernova SN 2011fe is one of the closest supernovae of the past decades. Due to its proximity and low dust extinction, this object provides a very rare opportunity to study the extremely late time evolution (>900 days) of thermonuclear su
Type Iax SNe as a few-parameter family
We present direct spectroscopic modeling of five Type Iax supernovae (SNe)
with the one dimensional Monte Carlo radiative transfer code TARDIS. The
abundance tomography technique is used to map the chemical structure and
physical properties of the SN atmosphere. Through via fitting of multiple
spectral epochs with self-consistent ejecta models, we can then constrain the
location of some elements within the ejecta. The synthetic spectra of the
best-fit models are able to reproduce the flux continuum and the main
absorption features in the whole sample. We find that the mass fractions of
IGEs and IMEs show a decreasing trend toward the outer regions of the
atmospheres using density profiles similar to those of deflagration models in
the literature. Oxygen is the only element, which could be dominant at higher
velocities. The stratified abundance structure contradicts the well-mixed
chemical profiles predicted by pure deflagration models. Based on the derived
densities and abundances, a template model atmosphere is created for the SN Iax
class and compared to the observed spectra. Free parameters are the scaling of
the density profile, the velocity shift of the abundance template, and the peak
luminosity. The results of this test support the idea that all SNe Iax can be
described by a similar internal structure, which argues for a common origin of
this class of explosions.Comment: 21 pages, 7 tables, 16 figures, accepted by MNRA
Discovery of low-metallicity stars in the central parsec of the Milky Way
We present a metallicity analysis of 83 late-type giants within the central 1
pc of the Milky Way. K-band spectroscopy of these stars were obtained with the
medium-spectral resolution integral-field spectrograph NIFS on Gemini North
using laser-guide star adaptive optics. Using spectral template fitting with
the MARCS synthetic spectral grid, we find that there is large variation in
metallicity, with stars ranging from [M/H] -1.0 to above solar metallicity.
About 6\% of the stars have [M/H] -0.5. This result is in contrast to
previous observations, with smaller samples, that show stars at the Galactic
center have approximately solar metallicity with only small variations. Our
current measurement uncertainties are dominated by systematics in the model,
especially at [M/H] 0, where there are stellar lines not represented in the
model. However, the conclusion that there are low metallicity stars, as well as
large variations in metallicity is robust. The metallicity may be an indicator
of the origin of these stars. The low-metallicity population is consistent with
that of globular clusters in the Milky Way, but their small fraction likely
means that globular cluster infall is not the dominant mechanism for forming
the Milky Way nuclear star cluster. The majority of stars are at or above solar
metallicity, which suggests they were formed closer to the Galactic center or
from the disk. In addition, our results indicate that it will be important for
star formation history analyses using red giants at the Galactic center to
consider the effect of varying metallicity.Comment: 11 pages, 10 figures, ApJ Accepte
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