1,303 research outputs found
Non-LTE time-dependent spectroscopic modelling of type II-plateau supernovae from the photospheric to the nebular phase: case study for 15 and 25Msun progenitor stars
We present the first non-LTE time-dependent radiative-transfer simulations of
supernovae (SNe) II-Plateau (II-P) covering both the photospheric and nebular
phases, from ~10 to >~1000d after the explosion, and based on 1.2B
piston-driven ejecta produced from a 15Msun and a 25Msun non-rotating
solar-metallicity star. The radial expansion of the gradually cooling
photosphere gives rise to a near-constant luminosity up to >~100d after
explosion. The photosphere remains in the outer 0.5Msun of the ejecta for up to
~50d after explosion. As the photosphere reaches the edge of the helium core,
the SN luminosity drops by an amount mitigated by the progenitor radius and the
56Ni mass. Synthetic light-curves exhibit a bell-shape morphology, evolving
faster for more compact progenitors, and with an earlier peak and narrower
width in bluer filters. UV and U-band fluxes are very sensitive to
line-blanketing, the metallicity, and the adopted model atoms. During the
recombination epoch synthetic spectra are dominated by HI and metal lines, and
are largely insensitive to the differing H/He/C/N/O composition of our two
progenitor stars. In contrast, synthetic nebular-phase spectra reveal a
broader/stronger OI doublet line in the higher-mass progenitor model,
reflecting the larger masses of oxygen and nickel that are ejected. Our
simulations overestimate the typical luminosity and the visual rise time of
standard SNe II-P, likely a consequence of our progenitor stars being too big
and/or too hydrogen rich. Comparison of our simulations with photospheric-phase
observations of SN1999em of the same color are satisfactory. Our neglect of
non-thermal processes leads to a fast disappearance of continuum radiation and
Balmer-line emission at the end of the plateau phase. With the exception of HI
lines, our nebular spectra show a striking similarity to contemporaneous
observations of SN1999em.Comment: Accepted to MNRA
Non-thermal excitation and ionization in supernovae
We incorporate non-thermal excitation and ionization processes arising from
non-thermal electrons that result from \gamma-ray energy deposition, into our
radiative transfer code CMFGEN. The non-thermal electron distribution is
obtained by solving the Spencer-Fano equation using the procedure of Kozma &
Fransson (1992). We applied the non-thermal calculations to the blue supergiant
explosion model whose early evolution was studied in Dessart & Hillier (2010).
Non-thermal processes generally increase excitation and ionization and decrease
the temperature of the ejecta. We confirm that non-thermal processes are
crucial for modeling the nebular spectra. Both optical HI and HeI lines are
significantly strengthened. While optical HeI lines are not easily discerned in
observational spectra due to severe blending with other lines, HeI 2.058 \mu m
provides an excellent opportunity to infer the influence of non-thermal
processes. We also discuss the processes controlling the formation of the HeI
lines during the nebular epoch. Most lines of other species are only slightly
affected. We also show that the inclusion of FeI has substantial
line-blanketing effects on the optical spectra. Our model spectra and synthetic
light curves are compared to the observations of SN 1987A. The spectral
evolution shows broad agreement with the observations, especially H\alpha. The
uncertainties of the non-thermal solver are studied, and are expected to be
small. With this new addition of non-thermal effects in CMFGEN, we now treat
all known important processes controlling the radiative transfer of a supernova
ejecta, whatever the type and the epoch.Comment: 18 pages, 22 figures, accepted to MNRA
Determining the main-sequence mass of Type II supernova progenitors
We present radiation-hydrodynamics simulations of core-collapse supernova
(SN) explosions, artificially generated by driving a piston at the base of the
envelope of a rotating or non-rotating red-supergiant progenitor star. We
search for trends in ejecta kinematics in the resulting Type II-Plateau (II-P)
SN, exploring dependencies with explosion energy and pre-SN stellar-evolution
model. We recover the trivial result that larger explosion energies yield
larger ejecta velocities in a given progenitor. However, we emphasise that for
a given explosion energy, the increasing helium-core mass with main-sequence
mass of such Type II-P SN progenitors leads to ejection of core-embedded
oxygen-rich material at larger velocities. We find that the photospheric
velocity at 15d after shock breakout is a good and simple indicator of the
explosion energy in our selected set of pre-SN models. This measurement,
combined with the width of the nebular-phase OI6303-6363A line, can be used to
place an upper-limit on the progenitor main-sequence mass. Using the results
from our simulations, we find that the current, but remarkably scant, late-time
spectra of Type II-P SNe support progenitor main-sequence masses inferior to
~20Msun and thus, corroborate the inferences based on the direct, but
difficult, progenitor identification in pre-explosion images. The narrow width
of OI6303-6363A in Type II-P SNe with nebular spectra does not support
high-mass progenitors in the range 25-30Msun. Combined with quantitative
spectroscopic modelling, such diagnostics offer a means to constrain the
main-sequence mass of the progenitor, the mass fraction of the core ejected,
and thus, the mass of the compact remnant formed.Comment: accepted to MNRA
Numerical simulations of super-luminous supernovae of type IIn
We present numerical simulations that include 1-D Eulerian multi-group
radiation-hydrodynamics, 1-D non-LTE radiative transfer, and 2-D polarised
radiative transfer for super-luminous interacting supernovae (SNe). Our
reference model is a ~10Msun inner shell with 10^51erg ramming into a ~3Msun
cold outer shell (the circumstellar-medium, or CSM) that extends from 10^15cm
to 2x10^16cm and moves at 100km/s. We discuss the light curve evolution, which
cannot be captured adequately with a grey approach. In these interactions, the
shock-crossing time through the optically-thick CSM is much longer than the
photon diffusion time. Radiation is thus continuously leaking from the shock
through the CSM, in disagreement with the shell-shocked model that is often
invoked. Our spectra redden with time, with a peak distribution in the near-UV
during the first month gradually shifting to the optical range over the
following year. Initially Balmer lines exhibit a narrow line core and the broad
line wings that are characteristic of electron scattering in the SNe IIn
atmospheres (CSM). At later times they also exhibit a broad blue shifted
component which arises from the cold dense shell. Our model results are broadly
consistent with the bolometric light curve and spectral evolution observed for
SN2010jl. Invoking a prolate pole-to-equator density ratio in the CSM, we can
also reproduce the ~2% continuum polarisation, and line depolarisation,
observed in SN2010jl. By varying the inner shell kinetic energy and the mass
and extent of the outer shell, a large range of peak luminosities and
durations, broadly compatible with super-luminous SNe IIn like 2010jl or
2006gy, can be produced.Comment: paper accepted to MNRA
Synthetic line and continuum linear-polarisation signatures of axisymmetric type II supernova ejecta
We present synthetic single-line and continuum linear-polarisation signatures
due to electron scattering in axially-symmetric Type II supernovae (SNe) which
we calculate using a Monte Carlo and a long-characteristic radiative-transfer
code. Aspherical ejecta are produced by prescribing a latitudinal scaling or
stretching of SN ejecta inputs obtained from 1-D non-LTE time-dependent
calculations. We study polarisation signatures as a function of inclination,
shape factor, wavelength, line identity, post-explosion time. At early times,
cancellation and optical-depth effects make the polarisation intrinsically low,
causing complicated sign reversals with inclination or continuum wavelength,
and across line profiles. While the line polarisation is positive (negative)
for an oblate (prolate) morphology at the peak and in the red wing, the
continuum polarisation may be of any sign. These complex polarisation
variations are produced not just by the asymmetric distribution of scatterers
but also of the flux. Our early-time signatures are in contradiction with
predictions for a centrally illuminated aspherical nebula, although this
becomes a better approximation at nebular times. For a fixed asymmetry, our
synthetic continuum polarisation is generally low, may evolve non-monotonically
during the plateau phase, but it systematically rises as the ejecta become
optically thin. Changes in polarization over time do not necessarily imply a
change in the asymmetry of the ejecta. The SN structure (e.g.,
density/ionization) critically influences the level of polarisation.
Importantly, a low polarisation (<0.5%) at early times does not necessarily
imply a low degree of asymmetry as usually assumed. Asphericity influences
line-profile morphology and the luminosity, which may compromise the accuracy
of SN characteristics inferred from these.Comment: 25 pages, 23 figures, accepted to MNRA
2D Simulations of the Line-Driven Instability in Hot-Star Winds: II. Approximations for the 2D Radiation Force
We present initial attempts to include the multi-dimensional nature of
radiation transport in hydrodynamical simulations of the small-scale structure
that arises from the line-driven instability in hot-star winds. Compared to
previous 1D or 2D models that assume a purely radial radiation force, we seek
additionally to treat the lateral momentum and transport of diffuse
line-radiation, initially here within a 2D context. A key incentive is to study
the damping effect of the associated diffuse line-drag on the dynamical
properties of the flow, focusing particularly on whether this might prevent
lateral break-up of shell structures at scales near the lateral Sobolev angle
of ca. . We first explore nonlinear simulations that cast the
lateral diffuse force in the simple, local form of a parallel viscosity.
Second, to account for the lateral mixing of radiation associated with the
radial driving, we next explore models in which the radial force is azimuthally
smoothed over a chosen scale. Third, to account for both the lateral line-drag
and the lateral mixing in a more self-consistent way, we explore further a
method first proposed by Owocki (1999), which uses a restricted 3-ray approach
that combines a radial ray with two oblique rays set to have an impact
parameter within the stellar core. From numerical simulations,
we find that, compared to equivalent 1-ray simulations, the high-resolution
3-ray models show systematically a much higher lateral coherence.... (Full
abstract in paper)Comment: Accepted by A&A, 12 pages, 7 figures, 3 only shown in version
available at http://www.mpa-garching.mpg.de/~luc/2778.ps.g
Time Dependent Radiative Transfer Calculations for Supernovae
In previous papers we discussed results from fully time-dependent radiative
transfer models for core-collapse supernova (SN) ejecta, including the Type
II-peculiar SN 1987A, the more "generic" SN II-Plateau, and more recently Type
IIb/Ib/Ic SNe. Here we describe the modifications to our radiative modeling
code, CMFGEN, which allowed those studies to be undertaken. The changes allow
for time-dependent radiative transfer of SN ejecta in homologous expansion. In
the modeling we treat the entire SN ejecta, from the innermost layer that does
not fall back on the compact remnant out to the progenitor surface layers. From
our non-LTE time-dependent line-blanketed synthetic spectra, we compute the
bolometric and multi-band light curves: light curves and spectra are thus
calculated simultaneously using the same physical processes and numerics. These
upgrades, in conjunction with our previous modifications which allow the
solution of the time dependent rate equations, will improve the modeling of SN
spectra and light curves, and hence facilitate new insights into SN ejecta
properties, the SN progenitors and the explosion mechanism(s). CMFGEN can now
be applied to the modeling of all SN typesComment: 20 pages, 10 figures, to appear in MNRA
Time-dependent Effects in Photospheric-Phase Type II Supernova Spectra
Spectroscopic modeling of Type II supernovae (SNe) generally assumes
steady-state. Following the recent suggestion of Utrobin & Chugai, but using
the 1D non-LTE line-blanketed model atmosphere code CMFGEN, we investigate the
effects of including time-dependent terms that appear in the statistical and
radiative equilibrium equations. We base our discussion on the ejecta
properties and the spectroscopic signatures obtained from time-dependent
simulations, investigating different ejecta configurations, and covering their
evolution from one day to six weeks after shock breakout. Compared to
equivalent steady-state models, our time-dependent models produce SN ejecta
that are systematically over-ionized, affecting helium at one week after
explosion, but ultimately affecting all ions after a few weeks. While the
continuum remains essentially unchanged, time-dependence effects on observed
spectral lines are large. At the recombination epoch, HI lines and NaID are
considerably stronger and broader than in equivalent steady-state models, while
CaII8500A is weakened. If time dependence is allowed for, the HeI lines at
5875A and 10830A appear about 3 times stronger at one week, and HeI10830A
persists as a blue-shifted absorption feature even at 6 weeks after explosion.
Time dependence operates through the energy gain from changes in ionization and
excitation, and, perhaps more universally across SN types, from the competition
between recombination and expansion, which in-turn, can be affected by
optical-depth effects. Our time-dependent models compare well with observations
of the low-luminosity low-velocity SN 1999br and the more standard SN 1999em,
reproducing the Halpha line strength at the recombination epoch, and without
the need for setting unphysical requirements on the magnitude of nickel mixing.Comment: 19 pages, 18 figures, accepted for publication in MNRAS,
high-resolution of the paper at
http://hermes.as.arizona.edu/~luc/pap_ddt/pap_ddt.ps.g
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