305 research outputs found
The 44Ti-powered spectrum of SN 1987A
SN 1987A provides a unique opportunity to study the evolution of a supernova
from explosion into very late phases. Due to the rich chemical structure, the
multitude of physical process involved, and extensive radiative transfer
effects, detailed modeling is needed to interpret the emission from this and
other supernovae. In this paper, we analyze the late-time (~8 years) HST
spectrum of the SN 1987A ejecta, where 44Ti is the dominant power source. Based
on an explosion model for a 19 Msun progenitor, we compute a model spectrum by
calculating the degradation of positrons and gamma-rays from the radioactive
decays, solving the equations governing temperature, ionization balance and
NLTE level populations, and treating the radiative transfer with a Monte Carlo
technique. We obtain a UV/optical/NIR model spectrum which is found to
reproduce most of the lines in the observed spectrum to good accuracy. We find
non-local radiative transfer in atomic lines to be an important process also at
this late stage of the supernova, with ~30% of the emergent flux in the optical
and NIR coming from scattering/fluorescence. We investigate the question of
where the positrons deposit their energy, and favor the scenario where they are
locally trapped in the Fe/He clumps by a magnetic field. Energy deposition into
these largely neutral Fe/He clumps makes Fe I lines prominent in the emergent
spectrum. Using the best available estimates for the dust extinction, we
determine the amount of 44Ti produced in the explosion to 1.5\pm0.5 * 10^-4
Msun.Comment: 23 pages, 9 figures. 44Ti mass updated from 1.4E-4 to 1.5E-4 Msu
Spectral modeling of nebular-phase supernovae
Massive stars live fast and die young. They shine furiously for a few million
years, during which time they synthesize most of the heavy elements in the
universe in their cores. They end by blowing themselves up in a powerful
explosion known as a supernova. During this process, the core collapses to a
neutron star or a black hole, while the outer layers are expelled with
velocities of thousands of kilometers per second. The resulting fireworks often
outshine the entire host galaxy for many weeks.
The explosion energy is eventually radiated away, but powering of the newborn
nebula continues by radioactive isotopes synthesized in the explosion. The
ejecta are now quite transparent, and we can see the material produced in the
deep interiors of the star. To interpret the observations, detailed spectral
modeling is needed. This thesis aims to develop and apply state-of-the-art
computational tools for interpreting and modeling supernova observations in the
nebular phase. This requires calculation of the physical conditions throughout
the nebula, including non-thermal processes from the radioactivity, thermal and
statistical equilibrium, as well as radiative transport. The inclusion of
multi-line radiative transfer, which we compute with a Monte Carlo technique,
represents one of the major advancements presented in this thesis.Comment: PhD thesis. 84 page
Monte-Carlo methods for NLTE spectral synthesis of supernovae
We present JEKYLL, a new code for modelling of supernova (SN) spectra and
lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer.
The code assumes spherical symmetry, homologous expansion and steady state for
the matter, but is otherwise capable of solving the time-dependent radiative
transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used
was introduced in a series of papers by Lucy, but the full time-dependent NLTE
capabilities of it have never been tested. Here, we have extended the method to
include non-thermal excitation and ionization as well as charge-transfer and
two-photon processes. Based on earlier work, the non-thermal rates are
calculated by solving the Spencer-Fano equation. Using a method previously
developed for the SUMO code, macroscopic mixing of the material is taken into
account in a statistical sense. In addition, a statistical Markov-chain model
is used to sample the emission frequency, and we introduce a method to control
the sampling of the radiation field. Except for a description of JEKYLL, we
provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good
agreement in the calculated spectra as well as the state of the gas. In
particular, the comparison with CMFGEN, which is similar in terms of physics
but uses a different technique, shows that the Lucy method does indeed converge
in the time-dependent NLTE case. Finally, as an example of the time-dependent
NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a
set of models presented and discussed in detail in an accompanying paper. Based
on this model we investigate the effects of NLTE, in particular those arising
from non-thermal excitation and ionization, and find strong effects even on the
bolometric lightcurve. This highlights the need for full NLTE calculations when
simulating the spectra and lightcurves of SNe.Comment: Accepted for publication by Astronomy & Astrophysic
Emission line models for the lowest-mass core collapse supernovae. I: Case study of a 9 one-dimensional neutrino-driven explosion
A large fraction of core-collapse supernovae (CCSNe), 30-50%, are expected to
originate from the low-mass end of progenitors with . However, degeneracy effects make stellar evolution modelling of
such stars challenging, and few predictions for their supernova light curves
and spectra have been presented. Here we calculate synthetic nebular spectra of
a 9 Fe CCSN model exploded with the neutrino mechanism. The model
predicts emission lines with FWHM1000 km/s, including signatures from
each deep layer in the metal core. We compare this model to observations of the
three subluminous IIP SNe with published nebular spectra; SN 1997D, SN 2005cs,
and SN 2008bk. The prediction of both line profiles and luminosities are in
good agreement with SN 1997D and SN 2008bk. The close fit of a model with no
tuning parameters provides strong evidence for an association of these objects
with low-mass Fe CCSNe. For SN 2005cs, the interpretation is less clear, as the
observational coverage ended before key diagnostic lines from the core had
emerged. We perform a parameterised study of the amount of explosively made
stable nickel, and find that none of these three SNe show the high
Ni/Ni ratio predicted by current models of electron capture SNe
(ECSNe) and ECSN-like explosions. Combined with clear detection of lines from O
and He shell material, these SNe rather originate from Fe core progenitors. We
argue that the outcome of self-consistent explosion simulations of low-mass
stars, which gives fits to many key observables, strongly suggests that the
class of subluminous Type IIP SNe is the observational counterpart of the
lowest mass CCSNe.Comment: Resubmitted to MNRAS after referee comment
The nebular spectra of SN 2012aw and constraints on stellar nucleosynthesis from oxygen emission lines
We present nebular phase optical and near-infrared spectroscopy of the Type
IIP supernova SN 2012aw combined with NLTE radiative transfer calculations
applied to ejecta from stellar evolution/explosion models. Our spectral
synthesis models generally show good agreement with the ejecta from a MZAMS =
15 Msun progenitor star. The emission lines of oxygen, sodium, and magnesium
are all consistent with the nucleosynthesis in a progenitor in the 14 - 18 Msun
range. We also demonstrate how the evolution of the oxygen cooling lines of [O
I] 5577 A, [O I] 6300 A, and [O I] 6364 A can be used to constrain the mass of
oxygen in the non-molecularly cooled ashes to < 1 Msun, independent of the
mixing in the ejecta. This constraint implies that any progenitor model of
initial mass greater than 20 Msun would be difficult to reconcile with the
observed line strengths. A stellar progenitor of around MZAMS = 15 Msun can
consistently explain the directly measured luminosity of the progenitor star,
the observed nebular spectra, and the inferred pre-supernova mass-loss rate. We
conclude that there is still no convincing example of a Type IIP explosion
showing the nucleosynthesis expected from a MZAMS > 20 Msun progenitor.Comment: Accepted for publication in MNRA
Carbon monoxide formation and cooling in supernovae
The inclusion of molecular physics is an important piece that tends to be
missing from the puzzle when modeling the spectra of supernovae (SNe).
Molecules have both a direct impact on the spectra, particularly in the
infrared, and an indirect one as a result of their influence on certain
physical conditions, such as temperature. In this paper, we aim to investigate
molecular formation and non-local thermodynamic equilibrium (NLTE) cooling,
with a particular focus on CO, the most commonly detected molecule in
supernovae. We also aim to determine the dependency of supernova chemistry on
physical parameters and the relative sensitivity to rate uncertainties. We
implemented a chemical kinetic description of the destruction and formation of
molecules into the SN spectral synthesis code SUMO. In addition, selected
molecules were coupled into the full NLTE level population framework and, thus,
we incorporated molecular NLTE cooling into the temperature equation. We
produced a test model of the CO formation in SN 1987A between 150 and 600 days
and investigated the sensitivity of the resulting molecular masses to the input
parameters. We find that there is a close inter-dependency between the thermal
evolution and the amount of CO formed, mainly through an important
temperature-sensitive CO destruction process with O+. After a few hundred days,
CO completely dominates the cooling of the oxygen-carbon zone of the supernova
which, therefore, contributes little optical emission. The uncertainty of the
calculated CO mass scales approximately linearly with the typical uncertainty
factor for individual rates. We demonstrate how molecular masses can
potentially be used to constrain various physical parameters of the supernova
Towards Nebular Spectral Modeling of Magnetar-Powered Supernovae
Many energetic supernovae are thought to be powered by the rotational-energy
of a highly-magnetized, rapidly-rotating neutron star. The emission from the
associated luminous pulsar wind nebula (PWN) can photoionize the supernova
ejecta, leading to a nebular spectrum of the ejecta with signatures possibly
revealing the PWN. SN 2012au is hypothesized to be one such supernova. We
investigate the impact of different ejecta and PWN parameters on the supernova
nebular spectrum, and test if any photoionization models are consistent with SN
2012au. We study how constraints from the nebular phase can be linked into
modelling of the diffusion phase and the radio emission of the magnetar. We
present a suite of late-time (1-6y) spectral simulations of SN ejecta powered
by an inner PWN. Over a large grid of 1-zone models, we study the behaviour of
the SN physical state and line emission as PWN luminosity (),
injection SED temperature (), ejecta mass (), and
composition (pure O or realistic) vary. We discuss the resulting emission in
the context of the observed behaviour of SN 2012au, a strong candidate for a
PWN-powered SN. The supernova nebular spectrum varies as varies,
as the ejecta become less ionized as increases. Low ejecta mass
models at high PWN power obtain runaway ionization for O I and, in extreme
cases, also O II, causing a sharp decrease in their ion fraction over a small
change in the parameter space. Certain models can reproduce the oxygen lines
luminosities of SN 2012au reasonably well at individual epochs, but we find no
model that fits over the whole time evolution; this is likely due to the simple
model setup. Using our derived constraints from the nebular phase, we predict
that the magnetar powering SN 2012au had an initial rotation period 15
ms, and should be a strong radio source (F > 100 mJy) for decades.Comment: 26 pages, 22 figures, submitted to A&A. Comments welcom
Late-time spectral line formation in Type IIb supernovae, with application to SN 1993J, SN 2008ax, and SN 2011dh
We investigate line formation processes in Type IIb supernovae (SNe) from 100
to 500 days post-explosion using spectral synthesis calculations. The modeling
identifies the nuclear burning layers and physical mechanisms that produce the
major emission lines, and the diagnostic potential of these. We compare the
model calculations with data on the three best observed Type IIb SNe to-date -
SN 1993J, SN 2008ax, and SN 2011dh. Oxygen nucleosynthesis depends sensitively
on the main-sequence mass of the star and modeling of the [O I] 6300, 6364
lines constrains the progenitors of these three SNe to the M_ZAMS=12-16 M_sun
range (ejected oxygen masses 0.3-0.9 M_sun), with SN 2011dh towards the lower
end and SN 1993J towards the upper end of the range. The high ejecta masses
from M_ZAMS >= 17 M_sun progenitors give rise to brighter nebular phase
emission lines than observed. Nucleosynthesis analysis thus supports a scenario
of low/moderate mass progenitors for Type IIb SNe, and by implication an origin
in binary systems. We demonstrate how oxygen and magnesium recombination lines
may be combined to diagnose the magnesium mass in the SN ejecta. For SN 2011dh,
a magnesium mass of of 0.02-0.14 M_sun is derived, which gives a Mg/O
production ratio consistent with the solar value. Nitrogen left in the He
envelope from CNO-burning gives strong [N II] 6548, 6583 emission lines that
dominate over H-alpha emission in our models. The hydrogen envelopes of Type
IIb SNe are too small and dilute to produce any noticeable H-alpha emission or
absorption after ~150 days, and nebular phase emission seen around 6550 A is in
many cases likely caused by [N II] 6548, 6583. Finally, the influence of
radiative transport on the emergent line profiles is investigated...(abridged)Comment: Published versio
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