PREPRINTS AND PAPERS OF THE SUPERNOVA GROUP
at Stockholm Observatory

The LWS data display continuum emission as well as nebular lines of [O I], [C II] and [O III] from neighboring photoexcited regions in the LMC. The [O III] lines indicate an electron density of 120 +/- 75 cm^{-3}, and the continuum can be explained by dust with a temperature of roughly 37 K. A second dust component with approximately 10 K may also be present.

The distribution of the different zones, and therefore the gamma-ray deposition, is determined from the line profiles of the most important lines, where possible. We find that hydrogen extends into the core to < 700 km s^{-1}. The hydrogen envelope has a density profile close to proportional to V^{-2} from 2000 - 5000 km s^{-1}. The total mass of hydrogen-rich gas is roughly 7.7 solar masses, of which roughly 2.2 solar masses are mixed within 2000 km s^{-1}. The helium mass derived from the line fluxes is sensitive to assumptions about the degree of redistribution in the line. The mass of the helium dominated zone is consistent with roughly 1.9 solar masses, with a further roughly 3.9 solar masses of helium residing in the hydrogen component. Most of the oxygen-rich gas is confined to 400 - 2000 km s^{-1}, with a total mass of roughly 1.9 solar masses. Because of uncertainties in the modeling of the non-thermal excitation of the [O I] lines, the uncertainty in the oxygen mass is considerable. In addition, masses of nitrogen, neon, magnesium, iron and nickel are estimated.

The dominant contribution to the line luminosity often originates in a different zone from where most of the newly synthesized material resides. This applies to e.g., carbon, calcium and iron. The [C I] lines, mainly arising in the helium zone, indicate a substantially lower abundance of carbon mixed with helium than stellar evolution models give, and a more extended zone with CNO processed gas is indicated. The [Fe II] lines have in most phases a strong contribution from primordial iron, and at t > 600 - 800 days this component dominates the [Fe II] lines. The wings of the [Fe II] lines may therefore come from primordial iron, rather than synthesized iron mixed to high velocity. Lines from ions with low ionization potential indicate that the UV field below at least 1600 Å is severely quenched by dust absorption and resonance scattering.

The most important result in this paper refers to the evolution of the temperature and ionization of the various abundance zones. The metal-rich core undergoes a thermal instability, often referred to as the IR-catastrophe, at 600 - 1000 days. The hydrogen-rich zones evolve adiabatically after 500 - 800 days, while in the helium region both adiabatic cooling and line cooling are of equal importance after roughly 1000 days. Freeze-out of the recombination is important in the hydrogen and helium zones. Concomitant with the IR-catastrophe, the bulk of the emission shifts from optical and near-IR lines to the mid- and far-IR. After the IR-catastrophe, the cooling is mainly due to far-IR lines and adiabatic expansion.

Dust cooling is likely to be important in the zones where dust forms. We find that the dust condensation temperatures occur later than roughly 500 days in the oxygen-rich zones, and the most favorable zone for dust condensation is the iron core. The uncertainties introduced by the, in some cases, unknown charge transfer rates are discussed. Especially for ions of low abundances differences can be substantial.

Lundqvist & Cumming: SN 1994D. The presence of a circumstellar medium (CSM) in Type Ia supernovae (SNe Ia) is a direct observational way to discriminate between different types of progenitor systems. If embedded in a CSM, interaction of the ejecta with the CSM gives rise to X-ray, radio, UV and optical emission. Using refined modeling we find that X-ray observations in the range 5 - 10 keV, e.g., with AXAF, provide the most useful limits on the mass loss, while high-resolution optical spectroscopy offers the only direct way of identifying circumstellar hydrogen. We apply the models to early observations of SN 1994D, and find an upper limit on the mass loss rate 1.9EE{-5} solar masses per year, assuming a spherically symmetric wind of 10 km s^{-1}. If the supernova would have been observed as early as 3 days after explosion the limit would have been slightly lower 1.4EE{-5} solar masses per year. Major uncertainties of the analysis are the geometry of the CSM and the outer structure of the progenitor, which both affect the X-ray radiation from the supernova.

Lundqvist, Sollerman, & Cumming: SN 1994W. Spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W show evidence for circumstellar excitation and a low mass of ejected nickel-56. After the plateau phase (t > 120 days), the light curve dropped by roughly 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 cobalt-56, and by day 197 it was 3.6 magnitudes less luminous in R compared to SN 1987A. The low R-luminosity could indicate < 0.0026{+0.0017}{-0.0011} solar masses of nickel-56 ejected at the explosion, but the emission between 125 and 197 days must then have been dominated by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve was dominated by cobalt decay. In this case, the mass of the ejected nickale-56 was 0.015{+0.012}{-0.008} solar masses, 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. The implications of the type of progenitor is discussed. The supernova's narrow (of order 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 originated was most likely accelerated by the radiation from the supernova.

Only progenitors with M(ZAMS) = 8-10 solar masses and M(ZAMS) > 25 solar masses are expected to eject such low masses of Ni-56. If M(ZAMS) = 8-10 solar masses, the plateau phase indicates a low explosion energy, while for a progenitor with M(ZAMS) > 25 solar masses the energy can be the canonical 10^{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 solar mass 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 originated was most likely accelerated by the radiation from the supernova.

CIRCUMSTELLAR H FROM SN 1994D AND FUTURE TYPE Ia SUPERNOVAE: AN OBSERVATIONAL TEST OF PROGENITOR MODELS

Robert J. Cumming, Peter Lundqvist, Linda J. Smith, Max Pettini, & David L. King

AN OBSERVATIONAL LIMIT ON CIRCUMSTELLAR H FROM SUPERNOVA 1994D

Robert J. Cumming, Peter Lundqvist, Linda J. Smith, Max Pettini, & David L. King