74 research outputs found
Implications of the metallicity dependence of Wolf-Rayet winds
Aims: Recent theoretical predictions for the winds of Wolf-Rayet stars
indicate that their mass-loss rates scale with the initial stellar metallicity
in the local Universe.We aim to investigate how this predicted dependence
affects the models of Wolf-Rayet stars and their progeny in different chemical
environments. Methods: We compute models of stellar structure and evolution for
Wolf-Rayet stars for different initial metallicities, and investigate how the
scaling of the Wolf-Rayet mass-loss rates affects the final masses, the
lifetimes of the WN and WC subtypes, and how the ratio of the two populations
vary with metallicity. Results: We find significant effects of metallicity
dependent mass-loss rates for Wolf-Rayet stars. For models that include the
scaling of the mass-loss rate with initial metallicity, all WR stars become
neutron stars rather than black holes at twice the solar metallicity; at lower
, black holes have larger masses. We also show that our models that include
the mass-loss metallicity scaling closely reproduce the observed decrease of
the relative population of WC over WN stars at low metallicities.Comment: 8 pages, 9 figures, accepted by Astronomy & Astrophysic
Quantitative spectroscopic analysis of and distance to SN1999em
This work presents a detailed quantitative spectroscopic analysis of, and the
determination of the distance to, the type II supernovae (SN) SN1999em with
CMFGEN (Dessart & Hillier 2005a), based on spectrophotometric observations at
eight dates up to 40 days after discovery. We use the same iron-group metal
content for the ejecta, the same power-law density distribution (with exponent
n~10), and a Hubble-velocity law at all times. We adopt a H/He/C/N/O abundance
pattern compatible with CNO-cycle equilibrium values for a RSG/BSG progenitor,
with C/O enhanced and N depleted at later times. Based on our synthetic fits to
spectrophotometric observations of SN1999em, we obtain a distance of 11.5Mpc,
similar to that of Baron et al. (2004) and the Cepheid distance to the galaxy
host of 11.7Mpc (Leonard et al. 2003). Similarly, based on such models, the
Expanding Photosphere Method (EPM) delivers a distance of 11.6Mpc, with
negligible scatter between photometric bandpass sets; there is thus nothing
wrong with the EPM as such. Previous determinations using the tabulated
correction factors of Eastman et al. (1996) all led to 30-50% underestimates:
we find that this is caused by 1) an underestimate of the correction factors
compared to the only other study of the kind by Dessart & Hillier (2005b), 2) a
neglect of the intrinsic >20% scatter of correction factors, and 3) the use of
the EPM at late times when severe line blanketing makes the method inaccurate.
The need of detailed model computations for reliable EPM distance estimates
thus defeats the appeal and simplicity of the method. However, detailed fits to
SN optical spectra, based on tailored models for individual SN observations,
offers a promising approach to obtaining distances with 10-20% accuracy, either
through the EPM or a la Baron et al. (2004).Comment: 20 pages, 13 figures, accepted for publication in A&
Transcriptional regulatory network discovery via multiple method integration: application to e. coli K12
Transcriptional regulatory network (TRN) discovery from one method (e.g. microarray analysis, gene ontology, phylogenic similarity) does not seem feasible due to lack of sufficient information, resulting in the construction of spurious or incomplete TRNs. We develop a methodology, TRND, that integrates a preliminary TRN, microarray data, gene ontology and phylogenic similarity to accurately discover TRNs and apply the method to E. coli K12. The approach can easily be extended to include other methodologies. Although gene ontology and phylogenic similarity have been used in the context of gene-gene networks, we show that more information can be extracted when gene-gene scores are transformed to gene-transcription factor (TF) scores using a preliminary TRN. This seems to be preferable over the construction of gene-gene interaction networks in light of the observed fact that gene expression and activity of a TF made of a component encoded by that gene is often out of phase. TRND multi-method integration is found to be facilitated by the use of a Bayesian framework for each method derived from its individual scoring measure and a training set of gene/TF regulatory interactions. The TRNs we construct are in better agreement with microarray data. The number of gene/TF interactions we discover is actually double that of existing networks
Fast and accurate frequency-dependent radiation transport for hydrodynamics simulations in massive star formation
Context: Radiative feedback plays a crucial role in the formation of massive
stars. The implementation of a fast and accurate description of the proceeding
thermodynamics in pre-stellar cores and evolving accretion disks is therefore a
main effort in current hydrodynamics simulations.
Aims: We introduce our newly implemented three-dimensional frequency
dependent radiation transport algorithm for hydrodynamics simulations of
spatial configurations with a dominant central source.
Methods: The module combines the advantage of the speed of an approximate
Flux Limited Diffusion (FLD) solver with the high accuracy of a frequency
dependent first order ray-tracing routine.
Results: We prove the viability of the scheme in a standard radiation
benchmark test compared to a full frequency dependent Monte-Carlo based
radiative transfer code. The setup includes a central star, a circumstellar
flared disk, as well as an envelope. The test is performed for different
optical depths. Considering the frequency dependence of the stellar
irradiation, the temperature distributions can be described precisely in the
optically thin, thick, and irradiated transition regions. Resulting radiative
forces onto dust grains are reproduced with high accuracy. The achievable
parallel speedup of the method imposes no restriction on further radiative
(magneto-) hydrodynamics simulations.
Conclusions: The proposed approximate radiation transport method enables
frequency dependent radiation hydrodynamics studies of the evolution of
pre-stellar cores and circumstellar accretion disks around an evolving massive
star in a highly efficient and accurate manner.Comment: 16 pages, 11 figure
Supernova Properties from Shock Breakout X-rays
We investigate the potential of the upcoming LOBSTER space observatory (due
circa 2009) to detect soft X-ray flashes from shock breakout in supernovae,
primarily from Type II events. LOBSTER should discover many SN breakout
flashes, although the number is sensitive to the uncertain distribution of
extragalactic gas columns. X-ray data will constrain the radii of their
progenitor stars far more tightly than can be accomplished with optical
observations of the SN light curve. We anticipate the appearance of blue
supergiant explosions (SN 1987A analogs), which will uncover a population of
these underluminous events. We consider also how the mass, explosion energy,
and absorbing column can be constrained from X-ray observables alone and with
the assistance of optically-determined distances. These conclusions are drawn
using known scaling relations to extrapolate, from previous numerical
calculations, the LOBSTER response to explosions with a broad range of
parameters. We comment on a small population of flashes with 0.2 < z < 0.8 that
should exist as transient background events in XMM, Chandra, and ROSAT
integrations.Comment: 14 pages, 9 figures, accepted by MNRAS, presented at AAS 203rd
meetin
A Hybrid Godunov Method for Radiation Hydrodynamics
From a mathematical perspective, radiation hydrodynamics can be thought of as
a system of hyperbolic balance laws with dual multiscale behavior (multiscale
behavior associated with the hyperbolic wave speeds as well as multiscale
behavior associated with source term relaxation). With this outlook in mind,
this paper presents a hybrid Godunov method for one-dimensional radiation
hydrodynamics that is uniformly well behaved from the photon free streaming
(hyperbolic) limit through the weak equilibrium diffusion (parabolic) limit and
to the strong equilibrium diffusion (hyperbolic) limit. Moreover, one finds
that the technique preserves certain asymptotic limits. The method incorporates
a backward Euler upwinding scheme for the radiation energy density and flux as
well as a modified Godunov scheme for the material density, momentum density,
and energy density. The backward Euler upwinding scheme is first-order accurate
and uses an implicit HLLE flux function to temporally advance the radiation
components according to the material flow scale. The modified Godunov scheme is
second-order accurate and directly couples stiff source term effects to the
hyperbolic structure of the system of balance laws. This Godunov technique is
composed of a predictor step that is based on Duhamel's principle and a
corrector step that is based on Picard iteration. The Godunov scheme is
explicit on the material flow scale but is unsplit and fully couples matter and
radiation without invoking a diffusion-type approximation for radiation
hydrodynamics. This technique derives from earlier work by Miniati & Colella
2007. Numerical tests demonstrate that the method is stable, robust, and
accurate across various parameter regimes.Comment: accepted for publication in Journal of Computational Physics; 61
pages, 15 figures, 11 table
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
SN Ib 1990I: Clumping and Dust in the Ejecta?
Photometry and spectra of the type Ib SN 1990I are presented and analysed,
covering about 400 days of evolution. The presence of optical helium lines is
shown. SN 1990I seems to show higher velocities compared to a sample of type Ib
events. The nebular emission lines display a high degree of asymmetry and the
presence of fine structures. Using the [O I] 6300,64A flux, we estimate a lower
limit on the oxygen mass to fall in the range (0.7-1.35) M_solar. The oxygen
mass requires a filling factor as small as 10^{-2} on day 254, indicating a
highly clumpy distribution of the oxygen material. A blueshift of the order 600
km/s is reported in the [O I] 6300,64A after day 254. The [Ca II] 7307.5A
emission profile appears blueshifted as well at late epochs . We recover the
quasi-bolometric "BVRI" light curve of SN 1990I. The constructed bolometric
light curve shows a change of slope at late phases, with an e-folding time of
60 +/-2 d in the [50:200] d time interval, considerably faster than the one of
{56}Co decay (i.e. 111.3 d), suggesting the gamma-rays escape with lower
deposition, owing to the low mass nature of the ejecta. After day 200, an
-folding time ~47 +/-2.8 d is measured. A simplified gamma-ray deposition
model is applied after adding a contribution of about 35% to the computed
pseudo-bolometric light curves to account for near-IR luminosities to estimate
the ejecta and {56}Ni masses (M({56}Ni)=0.11 M_solar and M_{ej}=3.7 M_solar).
The deficit in luminosity is estimated to be about 50% around day 308. The
observed spectral blueshift combined with the dramatic and sudden drop in the
pseudo-bolometric light curve and (B-V) colour is interpreted to be a
consequence of dust condensation in the ejecta of SN 1990I around day 250.Comment: 17 pages, 11 gigures and 5 tables. To appear in A & A (accepted
Modeling the Radio and X-ray Emission of SN 1993J and SN 2002ap
Modeling of radio and X-ray observations of supernovae interacting with their
circumstellar media are discussed, with special application to SN 1993J and SN
2002ap. We emphasize the importance of including all relevant physical
mechanisms, especially for the modeling of the radio light curves. The
different conclusions for the absorption mechanism (free-free or synchrotron
self-absorption), as well as departures from an CSM, as
inferred by some authors, are discussed in detail. We conclude that the
evidence for a variation in the mass loss rate with time is very weak. The
results regarding the efficiencies of magnetic field generation and
relativistic particle acceleration are summarized.Comment: 10 pages, 2 figures. Uses svmult.cls. To appear in proceedings of IAU
Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia,
Spain, eds. J. M. Marcaide and K. W. Weile
Optical studies of SN 2009jf: A type Ib supernova with an extremely slow decline and aspherical signature
Optical photometry and medium resolution spectroscopy of the type Ib
supernova SN 2009jf, during the period to +250days with respect to
the maximum are reported. The light curves are broad, with an extremely
slow decline. The early post-maximum decline rate in the band is similar to
SN 2008D, however, the late phase decline rate is slower than other studied
type Ib supernovae. With an absolute magnitude of
magnitude at peak, SN 2009jf is a normally bright supernova. The peak
bolometric luminosity and the energy deposition rate via Ni
Co chain indicate that
M of Ni was ejected during the explosion. He\,I 5876 \AA\ line
is clearly identified in the first spectrum of day , at a velocity of
km sec. The [O\,I] 6300-6364 \AA\ line seen in the nebular
spectrum has a multi-peaked and asymmetric emission profile, with the blue peak
being stronger. The estimated flux in this line implies \ga 1.34 M
oxygen was ejected. The slow evolution of the light curves of SN 2009jf
indicates the presence of a massive ejecta. The high expansion velocity in the
early phase and broader emission lines during the nebular phase suggest it to
be an explosion with a large kinetic energy. A simple qualitative estimate
leads to the ejecta mass of M M, and kinetic energy
E erg. The ejected mass estimate is indicative
of an initial main-sequence mass of \ga 20- 25 M.Comment: 14 pages, 13 figures; accepted for publication in MNRA
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