119 research outputs found
Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae
We have developed a quantitative, empirical method for estimating the age of
Type Ia supernovae (SNe Ia) from a single spectral epoch. The technique
examines the goodness of fit of spectral features as a function of the temporal
evolution of a large database of SNe Ia spectral features. When a SN Ia
spectrum with good signal-to-noise ratio over the rest frame range 3800 to 6800
A is available, the precision of a spectral feature age (SFA) is (1-sigma) ~
1.4 days. SFA estimates are made for two spectral epochs of SN 1996bj (z=0.574)
to measure the rate of aging at high redshift. In the 10.05 days which elapsed
between spectral observations, SN 1996bj aged 3.35 3.2 days, consistent
with the 6.38 days of aging expected in an expanding Universe and inconsistent
with no time dilation at the 96.4 % confidence level. The precision to which
individual features constrain the supernova age has implications for the source
of inhomogeneities among SNe Ia.Comment: 14 pages (LaTex), 7 postscript figures to Appear in the Astronomical
Journa
Analytic Inversion of Emission Lines of Arbitrary Optical Depth for the Structure of Supernova Ejecta
We derive a method for inverting emission line profiles formed in supernova
ejecta. The derivation assumes spherical symmetry and homologous expansion
(i.e., ), is analytic, and even takes account of occultation by
a pseudo-photosphere. Previous inversion methods have been developed which are
restricted to optically thin lines, but the particular case of homologous
expansion permits an analytic result for lines of {\it arbitrary} optical
depth. In fact, we show that the quantity that is generically retrieved is the
run of line intensity with radius in the ejecta. This result is
quite general, and so could be applied to resonance lines, recombination lines,
etc. As a specific example, we show how to derive the run of (Sobolev) optical
depth with radius in the case of a pure resonance scattering
emission line.Comment: 6 pages, no figures, to appear in Astrophysical Journal Letters,
requires aaspp4.sty to late
The Axially Symmetric Ejecta of Supernova 1987A
Extensive early observations proved that the ejecta of supernova 1987A (SN
1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble
Space Telescope has resolved the rapidly expanding ejecta. The late-time images
and spectroscopy provide a geometrical picture that is consistent with early
observations and suggests a highly structured, axially symmetric geometry. We
present here a new synthesis of the old and new data. We show that the Bochum
event, presumably a clump of Ni, and the late-time image, the locus of
excitation by Ti, are most naturally accounted for by sharing a common
position angle of about 14\degree, the same as the mystery spot and early
speckle data on the ejecta, and that they are both oriented along the axis of
the inner circumstellar ring at 45\degree to the plane of the sky. We also
demonstrate that the polarization represents a prolate geometry with the same
position angle and axis as the early speckle data and the late-time image and
hence that the geometry has been fixed in time and throughout the ejecta. The
Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in
spatially resolved HST spectra of the ejecta can be consistently integrated
into this geometry. The radioactive clump is deduced to fall approximately
along the axis of the inner circumstellar ring and therefore to be redshifted
in the North whereas the [Ca II]/[O II] 7300 \AA emission is redshifted in the
South. We present a jet-induced model for the explosion and argue that such a
model can account for many of the observed asymmetries. In the jet models, the
oxygen and calcium are not expected to be distributed along the jet, but
primarily in an expanding torus that shares the plane and northern blue shift
of the inner circumstellar ring.Comment: To Appear in Ap
Evidence for Asphericity in the Type IIn Supernova 1998S
We present optical spectropolarimetry obtained at the Keck-II 10-m telescope
on 1998 March 7 UT along with total flux spectra spanning the first 494 days
after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN)
1998S. The SN is found to exhibit a high degree of linear polarization,
implying significant asphericity for its continuum-scattering environment.
Prior to removal of the interstellar polarization, the polarization spectrum is
characterized by a flat continuum (at p ~ 2%) with distinct changes in
polarization associated with both the broad (FWZI >= 20,000 km/s) and narrow
(unresolved, FWHM < 300 km/s) line emission seen in the total flux spectrum.
When analyzed in terms of a polarized continuum with unpolarized broad-line
recombination emission, an intrinsic continuum polarization of p ~ 3% results
(the highest yet found for a SN), suggesting a global asphericity of >= 45%
from the oblate, electron-scattering dominated models of Hoflich (1991). The
smooth, blue continuum evident at early times is shown to be inconsistent with
a reddened, single-temperature blackbody, instead having a color temperature
that increases with decreasing wavelength. Broad emission-line profiles with
distinct blue and red peaks are seen in the total flux spectra at later times,
perhaps suggesting a disk-like or ring-like morphology for the dense (n_e ~
10^7 cm^{-3}) circumstellar medium. Implications of the circumstellar
scattering environment for the spectropolarimetry are discussed, as are the
effects of uncertain removal of interstellar polarization.Comment: 25 pages + 2 tables + 14 figures, Submitted to The Astrophysical
Journa
The Earliest Near-infrared Time-series Spectroscopy of a Type Ia Supernova
We present ten medium-resolution, high signal-to-noise ratio near-infrared
(NIR) spectra of SN 2011fe from SpeX on the NASA Infrared Telescope Facility
(IRTF) and Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North, obtained
as part of the Carnegie Supernova Project. This data set constitutes the
earliest time-series NIR spectroscopy of a Type Ia supernova (SN Ia), with the
first spectrum obtained at 2.58 days past the explosion and covering -14.6 to
+17.3 days relative to B-band maximum. C I {\lambda}1.0693 {\mu}m is detected
in SN 2011fe with increasing strength up to maximum light. The delay in the
onset of the NIR C I line demonstrates its potential to be an effective tracer
of unprocessed material. For the first time in a SN Ia, the early rapid decline
of the Mg II {\lambda}1.0927 {\mu}m velocity was observed, and the subsequent
velocity is remarkably constant. The Mg II velocity during this constant phase
locates the inner edge of carbon burning and probes the conditions under which
the transition from deflagration to detonation occurs. We show that the Mg II
velocity does not correlate with the optical light-curve decline rate
{\Delta}m15. The prominent break at ~1.5 {\mu}m is the main source of concern
for NIR k-correction calculations. We demonstrate here that the feature has a
uniform time evolution among SNe Ia, with the flux ratio across the break
strongly correlated with {\Delta}m15. The predictability of the strength and
the onset of this feature suggests that the associated k-correction
uncertainties can be minimized with improved spectral templates.Comment: 14 pages, 13 figures, accepted for publication in Ap
Direct Confirmation of the Asymmetry of the Cas A Supernova with Light Echoes
We report the first detection of asymmetry in a supernova (SN) photosphere
based on SN light echo (LE) spectra of Cas A from the different perspectives of
dust concentrations on its LE ellipsoid. New LEs are reported based on
difference images, and optical spectra of these LEs are analyzed and compared.
After properly accounting for the effects of finite dust-filament extent and
inclination, we find one field where the He I and H alpha features are
blueshifted by an additional ~4000 km/s relative to other spectra and to the
spectra of the Type IIb SN 1993J. That same direction does not show any shift
relative to other Cas A LE spectra in the Ca II near-infrared triplet feature.
We compare the perspectives of the Cas A LE dust concentrations with recent
three-dimensional modeling of the SN remnant (SNR) and note that the location
having the blueshifted He I and H alpha features is roughly in the direction of
an Fe-rich outflow and in the opposite direction of the motion of the compact
object at the center of the SNR. We conclude that Cas A was an intrinsically
asymmetric SN. Future LE spectroscopy of this object, and of other historical
SNe, will provide additional insight into the connection of explosion mechanism
to SN to SNR, as well as give crucial observational evidence regarding how
stars explode.Comment: 13 pages, 7 figures, accepted for publication in Ap
Preliminary Spectral Analysis of the Type II Supernova 1999em
We have calculated fast direct spectral model fits to two early-time spectra
of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The
first is an extremely early blue optical spectrum and the second a combined HST
and optical spectrum obtained one week later. Spectroscopically this supernova
appears to be a normal Type II and these fits are in excellent agreement with
the observed spectra. Our direct analysis suggests the presence of enhanced
nitrogen. We have further studied these spectra with the full NLTE general
model atmosphere code PHOENIX. While we do not find confirmation for enhanced
nitrogen (nor do we rule it out), we do require enhanced helium. An even more
intriguing possible line identification is complicated Balmer and He I lines,
which we show falls naturally out of the detailed calculations with a shallow
density gradient. We also show that very early spectra such as those presented
here combined with sophisticated spectral modeling allows an independent
estimate of the total reddening to the supernova, since when the spectrum is
very blue, dereddening leads to changes in the blue flux that cannot be
reproduced by altering the ``temperature'' of the emitted radiation. These
results are extremely encouraging since they imply that detailed modeling of
early spectra can shed light on both the abundances and total extinction of SNe
II, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54
Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae
We have coordinated Hubble Space Telescope photometry with ground-based
discovery for three supernovae: two SN Ia near z~0.5 (SN 1997ce, SN 1997cj) and
a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST
separates each supernova from its host galaxy and leads to good precision in
the light curves. The HST data combined with ground-based photometry provide
good temporal coverage. We use these light curves and relations between
luminosity, light curve shape, and color calibrated from low-z samples to
derive relative luminosity distances which are accurate to 10% at z~0.5 and 20%
at z=1. The redshift-distance relation is used to place constraints on the
global mean matter density, Omega_matter, and the normalized cosmological
constant, Omega_Lambda. When the HST sample is combined with the distance to SN
1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is
insufficient to produce a flat Universe. Specifically, for
Omega_matter+Omega_Lambda=1, Omega_matter is less than 1 with >95% confidence,
and our best estimate of Omega_matter is -0.1 +/- 0.5 if Omega_Lambda=0.
Although the present result is based on a very small sample whose systematics
remain to be explored, it demonstrates the power of HST measurements for high
redshift supernovae.Comment: Submitted to ApJ Letters, 3 figures, 1 plate, additional tabl
The Carnegie Supernova Project I: photometry data release of low-redshift stripped-envelope supernovae
The first phase of the Carnegie Supernova Project (CSP-I) was a dedicated
supernova follow-up program based at the Las Campanas Observatory that
collected science data of young, low-redshift supernovae between 2004 and 2009.
Presented in this paper is the CSP-I photometric data release of low-redshift
stripped-envelope core-collapse supernovae. The data consist of optical
(uBgVri) photometry of 34 objects, with a subset of 26 having near-infrared
(YJH) photometry. Twenty objects have optical pre-maximum coverage with a
subset of 12 beginning at least five days prior to the epoch of B-band maximum
brightness. In the near-infrared, 17 objects have pre-maximum observations with
a subset of 14 beginning at least five days prior to the epoch of J-band
maximum brightness. Analysis of this photometric data release is presented in
companion papers focusing on techniques to estimate host-galaxy extinction
(Stritzinger et al., submitted) and the light-curve and progenitor star
properties of the sample (Taddia et al., submitted). The analysis of an
accompanying visual-wavelength spectroscopy sample of ~150 spectra will be the
subject of a future paper.Comment: Updated a couple of small error
- âŠ