194 research outputs found
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
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
A one-dimensional Chandrasekhar-mass delayed-detonation model for the broad-lined Type Ia supernova 2002bo
We present 1D non-local thermodynamic equilibrium (non-LTE) time-dependent
radiative-transfer simulations of a Chandrasekhar-mass delayed-detonation model
which synthesizes 0.51 Msun of 56Ni, and confront our results to the Type Ia
supernova (SN Ia) 2002bo over the first 100 days of its evolution. Assuming
only homologous expansion, this same model reproduces the bolometric and
multi-band light curves, the secondary near-infrared (NIR) maxima, and the
optical and NIR spectra. The chemical stratification of our model qualitatively
agrees with previous inferences by Stehle et al., but reveals significant
quantitative differences for both iron-group and intermediate-mass elements. We
show that +/-0.1 Msun (i.e., +/-20 per cent) variations in 56Ni mass have a
modest impact on the bolometric and colour evolution of our model. One notable
exception is the U-band, where a larger abundance of iron-group elements
results in less opaque ejecta through ionization effects, our model with more
56Ni displaying a higher near-UV flux level. In the NIR range, such variations
in 56Ni mass affect the timing of the secondary maxima but not their magnitude,
in agreement with observational results. Moreover, the variation in the I, J,
and K_s magnitudes is less than 0.1 mag within ~10 days from bolometric
maximum, confirming the potential of NIR photometry of SNe Ia for cosmology.
Overall, the delayed-detonation mechanism in single Chandrasekhar-mass white
dwarf progenitors seems well suited for SN 2002bo and similar SNe Ia displaying
a broad Si II 6355 A line. Whatever multidimensional processes are at play
during the explosion leading to these events, they must conspire to produce an
ejecta comparable to our spherically-symmetric model.Comment: Accepted for publication in MNRAS. The hydrodynamical input and
synthetic spectra are available at https://www-n.oca.eu/supernova/home.html .
Minor changes from v1: corrected several typos and updated acknowledgement
A Spectropolarimetric Comparison of the Type II-Plateau Supernovae SN 2008bk and SN 2004dj
The Type II-Plateau supernova (SN II-P) SN 2004dj was the first SN II-P for which
spectropolarimetry
data were obtained with fine temporal sampling before, during, and after its fall off of the
photometric plateau – the point that marks the transition from the photospheric to the nebular phase in
SNe II-P. Unpolarized during the plateau, SN 2004dj showed a dramatic spike in polarization during the
descent off of the plateau, and then exhibited a smooth polarization decline over the next two
hundred days. This behavior was interpreted by Leonard et al. (2006) as evidence for a strongly
non-spherical explosion mechanism that had imprinted asphericity only in the innermost ejecta. In this brief
report, we compare nine similarly well-sampled epochs of spectropolarimetry of the Type II-P SN
2008bk to those of SN 2004dj. In contrast to SN 2004dj, SN 2008bk became polarized well before the end
of the plateau and also retained a nearly constant level of polarization through the early nebular
phase. Curiously, although the onset and persistence of polarization differ between the
two objects, the detailed spectropolarimetric characteristics at the epochs of recorded maximum
polarization for the two objects are extremely similar, feature by feature. We briefly interpret the
data in light of non-Local-Thermodynamic Equilibrium, time-dependent radiative-transfer simulations
specifically crafted for SN II-P ejecta
Critical ingredients of supernova Ia radiative-transfer modeling
We explore the physics of SN Ia light curves and spectra using the 1-D
non-LTE time-dependent radiative-transfer code CMFGEN. Rather than adjusting
ejecta properties to match observations, we select as input one "standard" 1-D
Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore
the sensitivity of radiation and gas properties on radiative-transfer modeling
assumptions. The correct computation of SN Ia radiation is not exclusively a
solution to an "opacity problem", characterized by the treatment of a large
number of lines. It is also key to treat important atomic processes
consistently. Besides handling line blanketing in non-LTE, we show that
including forbidden line transitions of metals is increasingly important for
the temperature and ionization of the gas beyond maximum light. Non-thermal
ionization and excitation are also critical since they affect the color
evolution and the Delta-M15 of our model. While impacting little the bolometric
luminosity, a more complete treatment of decay routes leads to enhanced line
blanketing, e.g., associated with 48Ti in the U and B bands. Overall, we find
that SN Ia radiation properties are influenced in a complicated way by the
atomic data we employ, so that obtaining converged results is a challenge. We
nonetheless obtain a good match to the golden standard type Ia SN 2005cf in the
optical and near-IR, from 5 to 60d after explosion, suggesting that assuming
spherical symmetry is not detrimental to SN Ia radiative-transfer modeling at
these times. Multi-D effects no doubt matter, but they are perhaps less
important than accurately treating non-LTE processes [abridged].Comment: Accepted to MNRA
Time-dependence Effects in Photospheric-Phase Type II Supernova Spectra
We have incorporated time-dependent terms into the statistical and radiative
equilibrium calculations of the non-LTE line-blanketed radiative transfer code
CMFGEN. To illustrate the significant improvements in spectral fitting achieved
for photospheric phase Type II SN, and to document the effects associated with
time dependence, we model the outer 6.1Msun of ejecta of a BSG/RSG progenitor
star. Hopping by 3-day increments, we compute the UV to near-IR spectral
evolution for both continuum and lines, from the fully ionized conditions at
one week to the partially recombined conditions at 6 weeks after the explosion.
We confirm the importance of allowing for time-dependence in the modeling of
Type-II SN, as recently discussed by Utrobin & Chugai for SN1987A. However, we
allow in our approach for the full interaction between the radiation field and
level populations, and study the effects on the full spectrum. At the
recombination epoch, HI lines and NaD are stronger and broader than in
equivalent steady-state models, while CaII is weakened. Former successes of
steady-state CMFGEN models are unaffected, while former discrepancies are
cured. Time dependence affects all lines, while the continuum, from the UV to
the optical, changes only moderately. We identify two key effects: First, time
dependence together with the energy gain through changes in ionization and
excitation lead to an over-ionization in the vicinity of the photosphere,
dramatically affecting line optical depths and profiles. Second, the ionization
is frozen-in at large radii/velocities. This stems solely from the time-scale
contrast between recombination and expansion and will occur, modulo non-thermal
excitation effects, in all SN types. [abridged]Comment: 9 pages, 3 figures, to appear in conf. proc. of "The Multicoloured
Landscape of Compact Objects and their Explosive Progenitors: Theory vs
Observations", a conference held in Cefalu, Sicily, June 11-24, 200
[CoIII] versus NaID in type Ia supernova spectra
The high metal content and fast expansion of supernova (SN) Ia ejecta lead to
considerable line overlap in their optical spectra. Uncertainties in
composition and ionization further complicate the process of line
identification. In this paper, we focus on the 5900A emission feature seen in
SN Ia spectra after bolometric maximum, a line which in the last two decades
has been associated with [CoIII]5888A or NaID. Using non-LTE time-dependent
radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation
models, we find that NaID line emission is extremely weak at all post-maximum
epochs. Instead, we predict the presence of [CoIII]5888A after maximum in all
our SN Ia models, which cover a range from 0.12 to 0.87Msun of 56Ni. We also
find that the [CoIII]5888A forbidden line is present within days of bolometric
maximum, and strengthens steadily for weeks thereafter. Both predictions are
confirmed by observations. Rather than trivial taxonomy, these findings confirm
that it is necessary to include forbidden-line transitions in
radiative-transfer simulations of SNe Ia, both to obtain the correct ejecta
cooling rate and to match observed optical spectra.Comment: Accepted to MNRA
- …