528 research outputs found
Large Late-time Asphericities in Three Type IIP Supernovae
Type II-plateau supernovae (SNe IIP) are the results of the explosions of red
supergiants and are the most common subclass of core-collapse supernovae. Past
observations have shown that the outer layers of the ejecta of SNe IIP are
largely spherical, but the degree of asphericity increases toward the core. We
present evidence for high degrees of asphericity in the inner cores of three
recent SNe IIP (SNe 2006my, 2006ov, and 2007aa), as revealed by late-time
optical spectropolarimetry. The three objects were all selected to have very
low interstellar polarization (ISP), which minimizes the uncertainties in ISP
removal and allows us to use the continuum polarization as a tracer of
asphericity. The three objects have intrinsic continuum polarizations in the
range of 0.83-1.56% in observations taken after the end of the photometric
plateau, with the polarization dropping to almost zero at the wavelengths of
strong emission lines. Our observations of SN 2007aa at earlier times, taken on
the photometric plateau, show contrastingly smaller continuum polarizations
(~0.1%). The late-time H-alpha and [O I] line profiles of SN 2006ov provide
further evidence for asphericities in the inner ejecta. Such high core
polarizations in very ordinary core-collapse supernovae provide further
evidence that essentially all core-collapse supernova explosions are highly
aspherical, even if the outer parts of the ejecta show only small deviations
from spherical symmetry.Comment: 15 pages, 13 figures, minor revisions to match published version
(2010, ApJ, 713, 1363
Simulations of stellar/pulsar wind interaction along one full orbit
The winds from a non-accreting pulsar and a massive star in a binary system
collide forming a bow-shaped shock structure. The Coriolis force induced by
orbital motion deflects the shocked flows, strongly affecting their dynamics.
We study the evolution of the shocked stellar and pulsar winds on scales in
which the orbital motion is important. Potential sites of non-thermal activity
are investigated. Relativistic hydrodynamical simulations in two dimensions,
performed with the code PLUTO and using the adaptive mesh refinement technique,
are used to model interacting stellar and pulsar winds on scales ~80 times the
distance between the stars. The hydrodynamical results suggest the suitable
locations of sites for particle acceleration and non-thermal emission. In
addition to the shock formed towards the star, the shocked and unshocked
components of the pulsar wind flowing away from the star terminate by means of
additional strong shocks produced by the orbital motion. Strong instabilities
lead to the development of turbulence and an effective two-wind mixing in both
the leading and trailing sides of the interaction structure, which starts to
merge with itself after one orbit. The adopted moderate pulsar-wind Lorentz
factor already provides a good qualitative description of the phenomena
involved in high-mass binaries with pulsars, and can capture important physical
effects that would not appear in non-relativistic treatments. Simulations show
that shocks, instabilities, and mass-loading yield efficient mass, momentum,
and energy exchanges between the pulsar and the stellar winds. This renders a
rapid increase in the entropy of the shocked structure, which will likely be
disrupted on scales beyond the simulated ones. Several sites of particle
acceleration and low- and high-energy emission can be identified. Doppler
boosting will have significant and complex effects on radiation.Comment: 8 pages, 11 figures, Astronomy and Astrophysics, in press, minor
changes after acceptanc
Spectroscopy of High-Redshift Supernovae from the ESSENCE Project: The First Four Years
We present the results of spectroscopic observations from the ESSENCE
high-redshift supernova (SN) survey during its first four years of operation.
This sample includes spectra of all SNe Ia whose light curves were presented by
Miknaitis et al. (2007) and used in the cosmological analyses of Davis et al.
(2007) and Wood-Vasey et al. (2007). The sample represents 273 hours of
spectroscopic observations with 6.5 - 10-m-class telescopes of objects detected
and selected for spectroscopy by the ESSENCE team. We present 174 spectra of
156 objects. Combining this sample with that of Matheson et al. (2005), we have
a total sample of 329 spectra of 274 objects. From this, we are able to
spectroscopically classify 118 Type Ia SNe. As the survey has matured, the
efficiency of classifying SNe Ia has remained constant while we have observed
both higher-redshift SNe Ia and SNe Ia farther from maximum brightness.
Examining the subsample of SNe Ia with host-galaxy redshifts shows that
redshifts derived from only the SN Ia spectra are consistent with redshifts
found from host-galaxy spectra. Moreover, the phases derived from only the SN
Ia spectra are consistent with those derived from light-curve fits. By
comparing our spectra to local templates, we find that the rate of objects
similar to the overluminous SN 1991T and the underluminous SN 1991bg in our
sample are consistent with that of the local sample. We do note, however, that
we detect no object spectroscopically or photometrically similar to SN 1991bg.
Although systematic effects could reduce the high-redshift rate we expect based
on the low-redshift surveys, it is possible that SN 1991bg-like SNe Ia are less
prevalent at high redshift.Comment: 21 pages, 17 figures, accepted to A
Steady shocks around black holes produced by sub-keplerian flows with negative energy
We discuss a special case of formation of axisymmetric shocks in the
accretion flow of ideal gas onto a Schwarzschild black hole: when the total
energy of the flow is negative. The result of our analysis enlarges the
parameter space for which these steady shocks are exhibited in the accretion of
gas rotating around relativistic stellar objects. Since keplerian disks have
negative total energy, we guess that, in this energy range, the production of
the shock phenomenon might be easier than in the case of positive energy. So
our outcome reinforces the view that sub-keplerian flows of matter may
significantly affect the physics of the high energy radiation emission from
black hole candidates. We give a simple procedure to obtain analytically the
position of the shocks. The comparison of the analytical results with the data
of 1D and 2D axisymmetric numerical simulations confirms that the shocks form
and are stable.Comment: 5 pages, 5 figures, accepted by MNRAS on 10 November 200
Non-stationary Rayleigh-Taylor instability in supernovae ejecta
The Rayleigh-Taylor instability plays an important role in the dynamics of
several astronomical objects, in particular, in supernovae (SN) evolution. In
this paper we develop an analytical approach to study the stability analysis of
spherical expansion of the SN ejecta by using a special transformation in the
co-moving coordinate frame. We first study a non-stationary spherical expansion
of a gas shell under the pressure of a central source. Then we analyze its
stability with respect to a no radial, non spherically symmetric perturbation
of the of the shell. We consider the case where the polytropic constant of the
SN shell is and we examine the evolution of a arbitrary shell
perturbation. The dispersion relation is derived. The growth rate of the
perturbation is found and its temporal and spatial evolution is discussed. The
stability domain depends on the ejecta shell thickness, its acceleration, and
the perturbation wavelength.Comment: 16 page
Properties of the ultraviolet flux of type Ia supernovae: an analysis with synthetic spectra of SN 2001ep and SN 2001eh
The spectral properties of type Ia supernovae in the ultraviolet (UV) are
investigated using the early-time spectra of SN 2001ep and SN 2001eh obtained
using the Hubble Space Telescope (HST). A series of spectral models is computed
with a Monte Carlo spectral synthesis code, and the dependence of the UV flux
on the elemental abundances and the density gradient in the outer layers of the
ejecta is tested. A large fraction of the UV flux is formed by reverse
fluorescence scattering of photons from red to blue wavelengths. This process,
combined with ionization shifts due to enhanced line blocking, can lead to a
stronger UV flux as the iron-group abundance in the outer layers is increased,
contrary to previous claims.Comment: 14 pages, 13 figures. Replaced with revised version accepted for
publication in MNRA
Berkeley Supernova Ia Program II: Initial Analysis of Spectra Obtained Near Maximum Brightness
In this second paper in a series we present measurements of spectral features
of 432 low-redshift (z < 0.1) optical spectra of 261 Type Ia supernovae (SNe
Ia) within 20 d of maximum brightness. The data were obtained from 1989 through
the end of 2008 as part of the Berkeley SN Ia Program (BSNIP) and are presented
in BSNIP I (Silverman et al. 2012). We describe in detail our method of
automated, robust spectral feature definition and measurement which expands
upon similar previous studies. Using this procedure, we attempt to measure
expansion velocities, pseudo-equivalent widths (pEW), spectral feature depths,
and fluxes at the centre and endpoints of each of nine major spectral feature
complexes. We investigate how velocity and pEW evolve with time and how they
correlate with each other. Various spectral classification schemes are employed
and quantitative spectral differences among the subclasses are investigated.
Several ratios of pEW values are calculated and studied. The so-called Si II
ratio, often used as a luminosity indicator (Nugent et al. 1995), is found to
be well correlated with the so-called "SiFe" ratio and anticorrelated with the
analogous "SSi ratio," confirming the results of previous studies. Furthermore,
SNe Ia that show strong evidence for interaction with circumstellar material or
an aspherical explosion are found to have the largest near-maximum expansion
velocities and pEWs, possibly linking extreme values of spectral observables
with specific progenitor or explosion scenarios. [Abridged]Comment: 73 pages, 18 figures, 15 tables, revised version re-submitted to
MNRAS. Measured values of the spectral features (i.e., Appendix B) will be
publicly available when the paper is accepte
Starburst-Driven Galactic Winds: Filament Formation and Emission Processes
We have performed a series of three-dimensional simulations of the
interaction of a supersonic wind with a non-spherical radiative cloud. These
simulations are motivated by our recent three-dimensional model of a
starburst-driven galactic wind interacting with an inhomogeneous disk, which
show that an optically emitting filament can be formed by the break-up and
acceleration of a cloud into a supersonic wind. In this study we consider the
evolution of a cloud with two different geometries (fractal and spherical) and
investigate the importance of radiative cooling on the cloud's survival. We
have also undertaken a comprehensive resolution study in order to ascertain the
effect of the assumed numerical resolution on the results. We find that the
ability of the cloud to radiate heat is crucial for its survival. While an
adiabatic cloud is destroyed over a short period of time, a radiative cloud is
broken up via the Kelvin-Helmholtz instability into numerous small, dense
cloudlets, which are drawn into the flow to form a filamentary structure. The
degree of fragmentation is highly dependent on the resolution of the
simulation, with the number of cloudlets formed increasing as the
Kelvin-Helmholtz instability is better resolved. Nevertheless, there is a clear
qualitative trend, with the filamentary structure still persistent at high
resolution. We confirm the mechanism behind the formation of the H-alpha
emitting filaments found in our global simulations of a starburst-driven wind.
Based on our resolution study, we conclude that bow shocks around accelerated
gas clouds, and their interaction, are the main source of the soft X-ray
emission observed in these galactic-scale winds. [ABRIDGED]Comment: Accepted to ApJ, 39 pages, 21 figures, movie file can obtained at
http://www.mso.anu.edu.au/~jcooper/movie/halpha.mo
Supernova remnant S147 and its associated neutron star(s)
The supernova remnant S147 harbors the pulsar PSR J0538+2817 whose
characteristic age is more than an order of magnitude greater than the
kinematic age of the system (inferred from the angular offset of the pulsar
from the geometric center of the supernova remnant and the pulsar proper
motion). To reconcile this discrepancy we propose that PSR J0538+2817 could be
the stellar remnant of the first supernova explosion in a massive binary system
and therefore could be as old as its characteristic age. Our proposal implies
that S147 is the diffuse remnant of the second supernova explosion (that
disrupted the binary system) and that a much younger second neutron star (not
necessarily manifesting itself as a radio pulsar) should be associated with
S147. We use the existing observational data on the system to suggest that the
progenitor of the supernova that formed S147 was a Wolf-Rayet star (so that the
supernova explosion occurred within a wind bubble surrounded by a massive
shell) and to constrain the parameters of the binary system. We also restrict
the magnitude and direction of the kick velocity received by the young neutron
star at birth and find that the kick vector should not strongly deviate from
the orbital plane of the binary system.Comment: 9 pages, 5 figures, revised version accepted for publication in A&
Variable Sodium Absorption in a Low-Extinction Type Ia Supernova
Recent observations have revealed that some Type Ia supernovae exhibit
narrow, time-variable Na I D absorption features. The origin of the absorbing
material is controversial, but it may suggest the presence of circumstellar gas
in the progenitor system prior to the explosion, with significant implications
for the nature of the supernova progenitors. We present the third detection of
such variable absorption, based on six epochs of high-resolution spectroscopy
of the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3
months, from 5 days before maximum light to 90 days after maximum. We find that
one component of the Na D absorption lines strengthened significantly with
time, indicating a total column density increase of ~2.5 x 10^12 cm^-2. The
changes are most prominent after maximum light rather than at earlier times
when the UV flux from the SN peaks. As with SN 2006X, we detect no change in
the Ca II H&K lines over the same time period, rendering line-of-sight effects
improbable and suggesting a circumstellar origin for the absorbing material.
Unlike the previous two SNe exhibiting variable absorption, SN 2007le is not
highly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar rather
than interstellar absorption. Photoionization models show that the data are
consistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pc
from the explosion. These results broadly support the single-degenerate
scenario previously proposed to explain the variable absorption, with mass loss
from a nondegenerate companion star responsible for providing the circumstellar
gas. We also present tentative evidence for narrow Halpha emission associated
with the SN, which will require followup observations at late times to confirm.
[abridged]Comment: 16 pages, 10 figures (8 in color), 5 tables. Accepted for publication
in Ap
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