56 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
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
NLTE Spectra of Kilonovae
The electromagnetic transient following a binary neutron star merger is known
as a kilonova (KN). Owing to rapid expansion velocities and small ejecta
masses, KNe rapidly transition into the Non-Local Thermodynamic Equilibrium
(NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5
to 20 days after merger using the \texttt{SUMO} spectral synthesis code. We
study three homogeneous composition, 1D multi-zone models with characteristic
electron fractions of and . We find that emission
features in the spectra tend to emerge in windows of reduced line blocking, as
the ejecta are still only partially transparent even at 20 days. For the (lanthanide-free) ejecta, we find that the neutral and singly
ionised species of Rb, Sr, Y and Zr dominate the spectra, all with good
potential for identification. We directly test and confirm an impact of Sr on
the 10000 angstrom spectral region in lanthanide-free ejecta, but also see that
its signatures may be complex. We suggest the Rb I -
7900 angstrom transition as a candidate for the 7500--7900
angstrom P-Cygni feature in AT2017gfo. For the and
compositions, lanthanides are dominant in the spectral formation, in particular
Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably
that scattering and fluorescence play important roles even up to 20 days after
merger, implying that the KN ejecta are not yet optically thin at this time.Comment: 20 pages (29 with appendices), 17 figures, resubmitted to MNRAS after
referee repor
Modelling supernova nebular lines in 3D with
We present (EXplosive TRAnsient Spectral Simulator), a
newly developed code aimed at generating 3D spectra for supernovae in the
nebular phase by using modern multi-dimensional explosion models as input. It
is well established that supernovae are asymmetric by nature, and that the
morphology is encoded in the line profiles during the nebular phase, months
after the explosion. In this work, we use to study one such
simulation of a He-core explosion
(, erg)
modelled with the code and evolved to the homologous
phase. Our code calculates the energy deposition from the radioactive decay of
Ni Co Fe and uses this to
determine the Non-Local-Thermodynamic-Equilibrium temperature, excitation and
ionization structure across the nebula. From the physical condition solutions
we generate the emissivities to construct spectra depending on viewing angles.
Our results show large variations in the line profiles with viewing angles, as
diagnosed by the first three moments of the line profiles; shifts, widths, and
skewness. We compare line profiles from different elements, and study the
morphology of line-of-sight slices that determine the flux at each part of a
line profile. We find that excitation conditions can sometimes make the
momentum vector of the ejecta emitting in the excited states significantly
different from that of the bulk of the ejecta of the respective element, thus
giving blueshifted lines for bulk receding material, and vice versa. We compare
the 3.3 He-core model to observations of the Type Ib supernova SN
2007Y.Comment: 20 pages, 15 Figures 2 Tables. Accepted for publication in MNRA
The progenitor mass of the Type IIP supernova SN 2004et from late-time spectral modeling
SN 2004et is one of the nearest and best-observed Type IIP supernovae, with a
progenitor detection as well as good photometric and spectroscopic
observational coverage well into the nebular phase. Based on nucleosynthesis
from stellar evolution/explosion models we apply spectral modeling to analyze
its 140-700 day evolution from ultraviolet to mid-infrared. We find a M_ZAMS=
15 Msun progenitor star (with an oxygen mass of 0.8 Msun) to satisfactorily
reproduce [O I] 6300, 6364 {\AA} and other emission lines of carbon, sodium,
magnesium, and silicon, while 12 Msun and 19 Msun models under- and overproduce
most of these lines, respectively. This result is in fair agreement with the
mass derived from the progenitor detection, but in disagreement with
hydrodynamical modeling of the early-time light curve. From modeling of the
mid-infrared iron-group emission lines, we determine the density of the
"Ni-bubble" to rho(t) = 7E-14*(t/100d)^-3 g cm^-3, corresponding to a filling
factor of f = 0.15 in the metal core region (V = 1800 km/s). We also confirm
that silicate dust, CO, and SiO emission are all present in the spectra.Comment: 21 pages, 15 figures. Accepted for publication in A&
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