151 research outputs found
Spectral modeling of type II supernovae. I. Dilution factors
We present substantial extensions to the Monte Carlo radiative transfer code
TARDIS to perform spectral synthesis for type II supernovae. By incorporating a
non-LTE ionization and excitation treatment for hydrogen, a full account of
free-free and bound-free processes, a self-consistent determination of the
thermal state and by improving the handling of relativistic effects, the
improved code version includes the necessary physics to perform spectral
synthesis for type II supernovae to high precision as required for the reliable
inference of supernova properties. We demonstrate the capabilities of the
extended version of TARDIS by calculating synthetic spectra for the
prototypical type II supernova SN1999em and by deriving a new and independent
set of dilution factors for the expanding photosphere method. We have
investigated in detail the dependence of the dilution factors on photospheric
properties and, for the first time, on changes in metallicity. We also compare
our results with two previously published sets of dilution factors by Eastman
et al. (1996) and by Dessart & Hillier (2005), and discuss the potential
sources of the discrepancies between studies.Comment: 16 pages, 12 figures, 2 tables, accepted for publication in A&
Spectral sequences of Type Ia supernovae. I. Connecting normal and sub-luminous SN Ia and the presence of unburned carbon
Type Ia supernovae are generally agreed to arise from thermonuclear
explosions of carbon-oxygen white dwarfs. The actual path to explosion,
however, remains elusive, with numerous plausible parent systems and explosion
mechanisms suggested. Observationally, type Ia supernovae have multiple
subclasses, distinguished by their lightcurves and spectra. This raises the
question whether these reflect that multiple mechanisms occur in nature, or
instead that explosions have a large but continuous range of physical
properties. We revisit the idea that normal and 91bg-like supernovae can be
understood as part of a spectral sequence, in which changes in temperature
dominate. Specifically, we find that a single ejecta structure is sufficient to
provide reasonable fits of both the normal type Ia supernova SN~2011fe and the
91bg-like SN~2005bl, provided that the luminosity and thus temperature of the
ejecta are adjusted appropriately. This suggests that the outer layers of the
ejecta are similar, thus providing some support of a common explosion
mechanism. Our spectral sequence also helps to shed light on the conditions
under which carbon can be detected in pre-maximum SN~Ia spectra -- we find that
emission from iron can "fill in" the carbon trough in cool SN~Ia. This may
indicate that the outer layers of the ejecta of events in which carbon is
detected are relatively metal poor compared to events where carbon is not
detected
Limits on stable iron in TypeIa supernovae from NIR spectroscopy
We obtained optical and near-infrared spectra of TypeIa supernovae
(SNeIa) at epochs ranging from 224 to 496 days after the explosion. The
spectra show emission lines from forbidden transitions of singly ionised iron
and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling
of the first and second ionisation stages of iron, nickel, and cobalt to fit
the spectra using a sampling algorithm allowing us to probe a broad parameter
space. We derive velocity shifts, line widths, and abundance ratios for iron
and cobalt. The measured line widths and velocity shifts of the singly ionised
ions suggest a shared emitting region. Our data are fully compatible with
radioactive Ni decay as the origin for cobalt and iron. We compare the
measured abundance ratios of iron and cobalt to theoretical predictions of
various SNIa explosion models. These models include, in addition to
Ni, different amounts of Ni and stable Fe. We can
exclude models that produced only Fe or only Ni in addition to
Ni. If we consider a model that has Ni, Ni, and
Fe then our data imply that these ratios are Fe / Ni
and Ni / Ni .Comment: 10 pages, 7 figures, Accepted for publication in A&
Nebular spectroscopy of SN 2014J: Detection of stable nickel in near infrared spectra
We present near infrared (NIR) spectroscopy of the nearby supernova 2014J
obtained 450 d after explosion. We detect the [Ni II] 1.939 m line
in the spectra indicating the presence of stable Ni in the ejecta. The
stable nickel is not centrally concentrated but rather distributed as the iron.
The spectra are dominated by forbidden [Fe II] and [Co II] lines. We use lines,
in the NIR spectra, arising from the same upper energy levels to place
constraints on the extinction from host galaxy dust. We find that that our data
are in agreement with the high and low found in earlier studies
from data near maximum light. Using a Ni mass prior from near maximum
light -ray observations, we find 0.05 M of stable nickel
to be present in the ejecta. We find that the iron group features are
redshifted from the host galaxy rest frame by 600 km s.Comment: 6 pages, 4 figures, submitted to A&
Modelling the early time behaviour of type Ia supernovae: effects of the 56Ni distribution
Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia)
at early times and highlighted a need for a better understanding of the
explosion physics as manifested by observations soon after explosion. To this
end, we present a Monte Carlo code designed to model the light curves of
radioactively driven, hydrogen-free transients from explosion to approximately
maximum light. In this initial study, we have used a parametrised description
of the ejecta in SNe Ia, and performed a parameter study of the effects of the
Ni distribution on the observed colours and light curves for a fixed
Ni mass of 0.6 . For a given density profile, we find that
models with Ni extending throughout the entirety of the ejecta are
typically brighter and bluer shortly after explosion. Additionally, the shape
of the density profile itself also plays an important role in determining the
shape, rise time, and colours of observed light curves. We find that the
multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent
with less extended Ni distributions, but show good agreement with models
that incorporate Ni throughout the entire ejecta. We further demonstrate
that comparisons with full colour light curves are powerful tools in
discriminating various Ni distributions, and hence explosion models.Comment: 14 pages, 8 figures, 2 tables. Minor changes in notation to match
published version in Astronomy & Astrophysic
KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
Knowing the metallicity distribution of stars in the Galactic Centre has
important implications for the formation history of the Milky Way nuclear star
cluster. However, this distribution is not well known, and is currently based
on a small sample of fewer than 100 stars. We obtained near-infrared K-band
spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way
nuclear star cluster with the integral-field spectrograph KMOS (VLT). We
analyse the medium-resolution spectra using a full-spectral fitting method
employing the G\"ottingen Spectral library of synthetic PHOENIX spectra. The
derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to
metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent
metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over
the entire observed field. The origin of metal-poor stars remains unclear. They
could originate from infalling globular clusters. For the metal-rich stellar
population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there
is only a very low fraction of metal-poor stars in the central 4 pc^2 of the
Galactic Centre, we believe that our data can discard a scenario in which the
Milky Way nuclear star cluster is purely formed from infalling globular
clusters.Comment: 18 pages, 9 Figures, accepted for publication in MNRA
Modelling the early time behaviour of type Ia supernovae: effects of the Ni-56 distribution
Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia) at early times and highlighted a need for a better understanding of the explosion physics as manifested by observations soon after explosion. To this end, we present a Monte Carlo code designed to model the light curves of radioactively driven, hydrogen-free transients from explosion to approximately maximum light. In this initial study, we have used a parametrised description of the ejecta in SNe Ia, and performed a parameter study of the effects of the Ni-56 distribution on the observed colours and light curves for a fixed Ni-56 mass of 0.6 M-circle dot. For a given density profile, we find that models with Ni-56 extending throughout the entirety of the ejecta are typically brighter and bluer shortly after explosion. Additionally, the shape of the density profile itself also plays an important role in determining the shape, rise time, and colours of observed light curves. We find that the multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent with less extended Ni-56 distributions, but show good agreement with models that incorporate Ni-56 throughout the entire ejecta. We further demonstrate that comparisons with full UVOIR colour light curves are powerful tools in discriminating various Ni-56 distributions, and hence explosion models
Searching for a Hypervelocity White Dwarf Companion: A Proper Motion Survey of SN 1006
Type Ia Supernovae (SNe Ia) are securely understood to come from the
thermonuclear explosion of a white dwarf as a result of binary interaction, but
the nature of that binary interaction and the secondary object is uncertain.
Recently, a double white dwarf model known as the dynamically driven
double-degenerate double-detonation (D6) model has become a promising
explanation for these events. One realization of this scenario predicts that
the companion may survive the explosion and reside within the remnant as a fast
moving ( km s), overluminous ()
white dwarf. Recently, three objects which appear to have these unusual
properties have been discovered in the Gaia survey. We obtained photometric
observations of the SN Ia remnant SN 1006 with the Dark Energy Camera over four
years to attempt to discover a similar star. We present a deep, high precision
astrometric proper motion survey of the interior stellar population of the
remnant. We rule out the existence of a high proper motion object consistent
with our tested realization of the D6 scenario ( km
s with ). We conclude that such a star does not exist within the
remnant, or is hidden from detection by either strong localized dust or the
unlikely possibility of ejection from the binary system near parallel to the
line of sight.Comment: 15 pages, 10 figure
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