324 research outputs found
The Proto-neutron Star Phase of the Collapsar Model and the Route to Long-soft Gamma-ray Bursts and Hypernovae
Recent stellar evolutionary calculations of low-metallicity massive
fast-rotating main-sequence stars yield iron cores at collapse endowed with
high angular momentum. It is thought that high angular momentum and black hole
formation are critical ingredients of the collapsar model of long-soft
gamma-ray bursts (GRBs). Here, we present 2D multi-group,
flux-limited-diffusion MHD simulations of the collapse, bounce, and immediate
post-bounce phases of a 35-Msun collapsar-candidate model of Woosley & Heger.
We find that, provided the magneto-rotational instability (MRI) operates in the
differentially-rotating surface layers of the millisecond-period neutron star,
a magnetically-driven explosion ensues during the proto-neutron star phase, in
the form of a baryon-loaded non-relativistic jet, and that a black hole,
central to the collapsar model, does not form. Paradoxically, and although much
uncertainty surrounds stellar mass loss, angular momentum transport, magnetic
fields, and the MRI, current models of chemically homogeneous evolution at low
metallicity yield massive stars with iron cores that may have too much angular
momentum to avoid a magnetically-driven, hypernova-like, explosion in the
immediate post-bounce phase. We surmise that fast rotation in the iron core may
inhibit, rather than enable, collapsar formation, which requires a large
angular momentum not in the core but above it. Variations in the angular
momentum distribution of massive stars at core collapse might explain both the
diversity of Type Ic supernovae/hypernovae and their possible association with
a GRB. A corollary might be that, rather than the progenitor mass, the angular
momentum distribution, through its effect on magnetic field amplification,
distinguishes these outcomes.Comment: 5 pages, 1 table, 2 figures, accepted to ApJ
A Two-Dimensional MagnetoHydrodynamics Scheme for General Unstructured Grids
We report a new finite-difference scheme for two-dimensional
magnetohydrodynamics (MHD) simulations, with and without rotation, in
unstructured grids with quadrilateral cells. The new scheme is implemented
within the code VULCAN/2D, which already includes radiation-hydrodynamics in
various approximations and can be used with arbitrarily moving meshes (ALE).
The MHD scheme, which consists of cell-centered magnetic field variables,
preserves the nodal finite difference representation of div(\bB) by
construction, and therefore any initially divergence-free field remains
divergence-free through the simulation. In this paper, we describe the new
scheme in detail and present comparisons of VULCAN/2D results with those of the
code ZEUS/2D for several one-dimensional and two-dimensional test problems. The
code now enables two-dimensional simulations of the collapse and explosion of
the rotating, magnetic cores of massive stars. Moreover, it can be used to
simulate the very wide variety of astrophysical problems for which multi-D
radiation-magnetohydrodynamics (RMHD) is relevant.Comment: 22 pages, including 11 figures; Accepted to the Astrophysical
Journal. Higher resolution figures available at
http://zenith.as.arizona.edu/~burrows/mhd-code
Features of the Acoustic Mechanism of Core-Collapse Supernova Explosions
In the context of 2D, axisymmetric, multi-group, radiation/hydrodynamic
simulations of core-collapse supernovae over the full 180 domain, we
present an exploration of the progenitor dependence of the acoustic mechanism
of explosion. All progenitor models we have tested with our Newtonian code
explode. We investigate the roles of the Standing-Accretion-Shock-Instability
(SASI), the excitation of core g-modes, the generation of core acoustic power,
the ejection of matter with r-process potential, the wind-like character of the
explosion, and the fundamental anisotropy of the blasts. We find that the
breaking of spherical symmetry is central to the supernova phenomenon and the
blasts, when top-bottom asymmetric, are self-collimating. We see indications
that the initial explosion energies are larger for the more massive
progenitors, and smaller for the less massive progenitors, and that the
neutrino contribution to the explosion energy may be an increasing function of
progenitor mass. The degree of explosion asymmetry we obtain is completely
consistent with that inferred from the polarization measurements of Type Ic
supernovae. Furthermore, we calculate for the first time the magnitude and sign
of the net impulse on the core due to anisotropic neutrino emission and suggest
that hydrodynamic and neutrino recoils in the context of our asymmetric
explosions afford a natural mechanism for observed pulsar proper motions.
[abridged]Comment: Accepted to the Astrophysical Journal, 23 pages in emulateapj format,
including 12 figure
Improved distance determination to M51 from supernovae 2011dh and 2005cs
The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M51,
provides an opportunity to derive an improved distance to their host galaxy by
combining the observations of both SNe. We apply the Expanding Photosphere
Method to get the distance to M51 by fitting the data of these two SNe
simultaneously. In order to correct for the effect of flux dilution, we use
correction factors (zeta) appropriate for standard type II-P SNe atmospheres
for 2005cs, but find zeta ~ 1 for the type IIb SN 2011dh, which may be due to
the reduced H-content of its ejecta. The EPM analysis resulted in D_M51 = 8.4
+/- 0.7 Mpc. Based on this improved distance, we also re-analyze the HST
observations of the proposed progenitor of SN 2011dh. We confirm that the
object detected on the pre-explosion HST-images is unlikely to be a compact
stellar cluster. In addition, its derived radius (~ 277$ R_sun) is too large
for being the real (exploded) progenitor of SN 2011dh. The supernova-based
distance, D = 8.4 Mpc, is in good agreement with other recent distance
estimates to M51.Comment: 6 pages, 5 figures, accepted for publication in A&
Remote Sensing analysis of the meanders migration in the Mamorecillo River between 1985 and 2012, Bolivia
The morphology of a channel, in a space-time resolution, suffers with sedimentological processes of erosion, transport and deposition. Processes that are more accentuated in meandering channels. In the present work the objective is to analyze, identify and discuss the changes occurred in spatial and temporal sequence in the form of the meanders and river bed of a section of the Mamorecillo River between the provinces of Cochabamba and Santa Cruz - Bolivia. Characterized by being an extensively meandering river - characteristic of the river basin in which it is inserted, the Amazon Basin, the present study, through remote sensing resources, seeks to discuss and correlate the changes occurred in the channel in a period of twenty-seven years (1985-2012). As well as understanding the processes of migration of the meanders that this section of the Mamorecillo River suffered through the sedimentological processes of erosion and deposition and consequently understand the processes in the Amazon basin. For the accomplishment of the work, the studied area was delimited and allied with geoprocessing tools such as software and aerial images, the main geomorphological features were identified and their changes discussed during the studied period. In this work, we intend to correlate and verify the inter-dependence (referring to the sedimentological and / or hydrological contribution) that exists between the Mamorecillo River and the Chimoreacute; and Ichilo rivers
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Far-UV FUSE spectroscopy of the OVI resonance doublet in Sand2 (WO)
We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectroscopy of Sand
2, a LMC WO-type Wolf-Rayet star, revealing the OVI resonance P Cygni doublet
at 1032-38A. These data are combined with HST/FOS ultraviolet and Mt Stromlo
2.3m optical spectroscopy, and analysed using a spherical, non-LTE,
line-blanketed code. Our study reveals exceptional stellar parameters:
T*=150,000K, v_inf=4100 km/s, log (L/Lo)=5.3, and Mdot=10^-5 Mo/yr if we adopt
a volume filling factor of 10%. Elemental abundances of C/He=0.7+-0.2 and
O/He=0.15(-0.05+0.10) by number qualitatively support previous recombination
line studies. We confirm that Sand 2 is more chemically enriched in carbon than
LMC WC stars, and is expected to undergo a supernova explosion within the next
50,000 yr.Comment: 17 pages, 4 figures, AASTeX preprint format. This paper will appear
in a special issue of ApJ Letters devoted to the first scientific results
from the FUSE missio
Type II supernovae as probes of environment metallicity: observations of host HII regions
Spectral modelling of SNII atmospheres indicates a clear dependence of metal
line strengths on progenitor metallicity. This motivates further work to
evaluate the accuracy with which these SNe can be used as metallicity
indicators. To assess this accuracy we present a sample of SNII HII-region
spectroscopy, from which environment abundances are derived. These environment
abundances are compared to the observed strength of metal lines in SN spectra.
Combining our sample with measurements from the literature, we present oxygen
abundances of 119 host HII regions, by extracting emission line fluxes and
using abundance diagnostics. Then, following Dessart et al., these abundances
are compared to equivalent widths of Fe 5018 A at various time and colour
epochs. Our distribution of inferred SNII host HII-region abundances has a
range of ~0.6 dex. We confirm the dearth of SNeII exploding at metallicities
lower than those found (on average) in the Large Magellanic Cloud. The
equivalent width of Fe 5018 A at 50 days post explosion shows a statistically
significant correlation with host HII-region oxygen abundance. The strength of
this correlation increases if one excludes abundance measurements derived far
from SN explosion sites. The correlation significance also increases if we only
analyse a 'gold' IIP sample, and if a colour epoch is used in place of time. In
addition, no evidence is found of correlation between progenitor metallicity
and SN light-curve or spectral properties - except for that stated above with
respect to Fe 5018 A equivalent width - suggesting progenitor metallicity is
not a driving factor in producing the diversity observed in our sample. This
study provides observational evidence of the usefulness of SNII as metallicity
indicators. We finish with a discussion of the methodology needed to use SN
spectra as independent metallicity diagnostics throughout the Universe.Comment: Accepted for publication in Astronomy and Astrophyci
SN 2005cs in M51 I. The first month of evolution of a subluminous SN II plateau
Early time optical observations of supernova (SN) 2005cs in the Whirlpool
Galaxy (M51), are reported. Photometric data suggest that SN 2005cs is a
moderately under-luminous Type II plateau supernova (SN IIP). The SN was
unusually blue at early epochs (U-B ~ -0.9 about three days after explosion)
which indicates very high continuum temperatures. The spectra show relatively
narrow P-Cygni features, suggesting ejecta velocities lower than observed in
more typical SNe IIP. The earliest spectra show weak absorption features in the
blue wing of the He I 5876A absorption component and, less clearly, of H
and H. Based on spectral modelling, two different interpretations can
be proposed: these features may either be due to high-velocity H and He I
components, or (more likely) be produced by different ions (N II, Si II).
Analogies with the low-luminosity, Ni-poor, low-velocity SNe IIP are
also discussed.
While a more extended spectral coverage is necessary in order to determine
accurately the properties of the progenitor star, published estimates of the
progenitor mass seem not to be consistent with stellar evolution models.Comment: 12 pages, 11 Figures. Accepted for publication in MNRA
The first year of SN 2004dj in NGC 2403
New BVRI photometry and optical spectroscopy of the Type IIp supernova 2004dj
in NGC 2403, obtained during the first year since discovery, are presented. The
progenitor cluster, Sandage 96, is also detected on pre-explosion frames. The
light curve indicates that the explosion occured about 30 days before
discovery, and the plateau phase lasted about +110 \pm 20 days after that. The
plateau-phase spectra have been modelled with the SYNOW spectral synthesis code
using H, NaI, TiII, ScII, FeII and BaII lines. The SN distance is inferred from
the Expanding Photosphere Method and the Standard Candle Method applicable for
SNe IIp. They resulted in distances that are consistent with each other as well
as earlier Cepheid- and Tully-Fisher distances. The average distance, D = 3.47
\pm 0.29 Mpc is proposed for SN 2004dj and NGC 2403. The nickel mass produced
by the explosion is estimated as 0.02 \pm 0.01 M_o. The SED of the progenitor
cluster is reanalysed by fitting population synthesis models to our observed
BVRI data supplemented by U and JKH magnitudes from the literature. The
chi^2-minimization revealed a possible "young" solution with cluster age T_{cl}
= 8 Myr, and an "old" solution with T_{cl} = 20 - 30 Myr. The "young" solution
would imply a progenitor mass M > 20 M_o, which is higher than the previously
detected progenitor masses for Type II SNe.Comment: 19 pages, accepted in MNRA
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