17 research outputs found
A very low mass of Ni-56 in the ejecta of SN 1994W
We present spectroscopic and photometric observations of the luminous narrow-
line Type IIP (plateau) supernova 1994W. After the plateau phase (t >120 days),
the light curve dropped by 3.5 mag in V in only 12 days. Between 125 and 197
days after explosion the supernova faded substantially faster than the decay
rate of Co-56, and by day 197 it was 3.6 magnitudes less luminous in R compared
to SN 1987A. The low R-luminosity could indicate less than 0.0026 {+0.0017}/
{-0.0011} Msun of Ni-56 ejected at the explosion, but the emission between 125
and 197 days must then have been dominated by an additional power source, pre-
sumably circumstellar interaction. Alternatively, the late light curve was
dominated by Co-56 decay. In this case, the mass of the ejected Ni-56 was 0.015
{+0.012}/{-0.008} Msun, and the rapid fading between 125 and 197 days was most
likely due to dust formation. Though this value of the mass is higher than in
the case with the additional power source, it is still lower than estimated for
any previous Type II supernova. Only progenitors with M(ZAMS) = 8-10 Msun and
M(ZAMS) > 25 Msun are expected to eject such low masses of Ni-56. If M(ZAMS) =
8-10 Msun, the plateau phase indicates a low explosion energy, while for a
progenitor with M(ZAMS) > 25 Msun the energy can be the canonical 1.0E{51}
ergs. As SN 1994W was unusually luminous, the low-mass explosion may require an
uncomfortably high efficiency in converting explosion energy into radiation.
This favors a M(ZAMS) > 25 Msun progenitor. The supernova's narrow (roughly
1000 km s^{-1}) emission lines were excited by the hot supernova spectrum,
rather than a circumstellar shock. The thin shell from which the lines origi-
nated was most likely accelerated by the radiation from the supernova.Comment: 19 pages AASTeX v.4.0, including 5 Postscript figures; ApJ, in pres
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
Circumstellar Na I and Ca II lines in type IIP supernovae and SN 1998S
We study a possibility of detection of circumstellar absorption lines of Na I
D and Ca II H,K in spectra of type IIP supernovae at the photospheric
epoch. The modelling shows that the circumstellar lines of Na I doublet will
not be seen in type IIP supernovae for moderate wind density, e.g.,
characteristic of SN 1999em, whereas rather pronounced Ca II lines with P Cygni
profile should be detectable. A similar model is used to describe Na I and Ca
II circumstellar lines seen in SN 1998S, type IIL with a dense wind. We show
that line intensities in this supernova are reproduced, if one assumes an
ultraviolet excess, which is caused primarily by the comptonization of
supernova radiation in the shock wave.Comment: To be published in Astronomy Letter
Radiation hydrodynamics of SN 1987A: I. Global analysis of the light curve for the first 4 months
The optical/UV light curves of SN 1987A are analyzed with the multi-energy
group radiation hydrodynamics code STELLA. The calculated monochromatic and
bolometric light curves are compared with observations shortly after shock
breakout, during the early plateau, through the broad second maximum, and
during the earliest phase of the radioactive tail. We have concentrated on a
progenitor model calculated by Nomoto & Hashimoto and Saio, Nomoto, & Kato,
which assumes that 14 solar masses of the stellar mass is ejected. Using this
model, we have updated constraints on the explosion energy and the extent of
mixing in the ejecta. In particular, we determine the most likely range of E/M
(explosion energy over ejecta mass) and R_0 (radius of the progenitor). In
general, our best models have energies in the range E = (1.1 +/- 0.3) x 10^{51}
ergs, and the agreement is better than in earlier, flux-limited diffusion
calculations for the same explosion energy. Our modeled B and V fluxes compare
well with observations, while the flux in U undershoots after about 10 days by
a factor of a few, presumably due to NLTE and line transfer effects. We also
compare our results with IUE observations, and a very good quantitative
agreement is found for the first days, and for one IUE band (2500-3000 A) as
long as for 3 months. We point out that the V flux estimated by McNaught &
Zoltowski should probably be revised to a lower value.Comment: 27 pages AASTeX v.4.0 + 35 postscript figures. ApJ, accepte