Adam Fisher

Direct Analysis of Spectra of the Type Ic Supernova SN 1994I

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to observed photospheric-phase spectra of the Type Ic supernova SN 1994I. The observed optical spectra can be well matched by synthetic spectra that are based on the assumption of spherical symmetry. We consider the identification of the infrared absorption feature observed near 10250 ?, which previously has been attributed to He I ?10830 and regarded as strong evidence that SN 1994I ejected some helium. We have difficulty accounting for the infrared absorption with He I alone. It could be a blend of He I and C I lines. Alternatively, we find that it can be fitted by Si I lines without compromising the fit in the optical region. In synthetic spectra that match the observed spectra, from 4 days before to 26 days after the time of maximum brightness, the adopted velocity at the photosphere decreases from 17,500 to 7000 km s-1. Simple estimates of the kinetic energy carried by the ejected mass give values that are near the canonical supernova energy of 1051 ergs. The velocities and kinetic energies that we find for SN 1994I in this way are much lower than those that we find elsewhere for the peculiar Type Ic SNe 1997ef and 1998bw, which therefore appear to have been hyperenergetic.

On the High-Velocity Ejecta of the Type Ia Supernova SN 1994D

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared with spectra of the Type Ia SN 1994D that were obtained before the time of maximum brightness. Evidence is found for the presence of two-component Fe II and Ca II features, forming in high-velocity (≥20,000 km s-1) and lower velocity (≤16,000 km s-1) matter. Possible interpretations of these spectral splits, and implications for using early-time spectra of Type Ia supernovae to probe the metallicity of the progenitor white dwarf and the nature of the nuclear burning front in the outer layers of the explosion, are discussed.

Ion Signatures in Supernova Spectra

A systematic survey of ions that could be responsible for features in the optical spectra of supernovae is carried out. Six different compositions that could be encountered in supernovae are considered. For each composition, the LTE optical depth of one of the strongest optical lines of each ion is plotted against temperature. For each ion that can realistically be considered as a candidate for producing identifiable features in supernova spectra, a sample synthetic spectrum is displayed. The optical depth plots and the synthetic spectra can provide guidance for studies of line identifications in the optical spectra of all types of supernovae during their photospheric phases.

Evidence for a High-Velocity Carbon-rich Layer in the Type Ia SN 1990N

We use a parameterized spectrum-synthesis code to make a direct analysis of a high quality spectrum of the Type Ia SN 1990N that was obtained by Leibundgut et al. 14 days before the time of optical maximum. We suggest that the absorption feature observed near 6040 A, which has been attributed to blueshifted λ6355 of Si II, actually was produced by blueshifted λ6580 of C II, in a high-velocity (v > 26, 000 km s-1) carbon-rich region. Implications for SN Ia explosion models are briefly discussed.

On the spectrum and nature of the peculiar Type Ia supernova 1991T

A parametrized supernova synthetic-spectrum code is used to study line identifications in the photospheric-phase spectra of the peculiar Type Ia SN 1991T, and to extract some constraints on the composition structure of the ejected matter. The inferred composition structure is not like that of any hydrodynamical model for Type Ia supernovae. Evidence that SN 1991T was overluminous for a SN Ia is presented, and it is suggested that this peculiar event was probably a substantially super-Chandrasekhar explosion that resulted from the merger of two white dwarfs.

On the High--Velocity Ejecta of the Type Ia Supernova 1994D

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared with spectra of the Type Ia SN 1994D that were obtained before the time of maximum brightness. Evidence is found for the presence of two-component Fe II and Ca II features, forming in high-velocity (≥20,000 km s-1) and lower velocity (≤16,000 km s-1) matter. Possible interpretations of these spectral splits, and implications for using early-time spectra of Type Ia supernovae to probe the metallicity of the progenitor white dwarf and the nature of the nuclear burning front in the outer layers of the explosion, are discussed.

New insight into the spatial distribution of novae in M31

We use a Monte Carlo technique together with a simple model for the distribution of dust in M31 to investigate the observability and spatial distribution of classical novae in M31. By comparing our model positions of novae to the observed positions, we conclude that most M31 novae come from the disk population, rather than from the bulge population as has been thought. Our results indicate that the M31 bulge-to-disk nova ratio is as low as, or lower than, the M31 bulge-to-disk mass ratio.

On the spectrum of the peculiar type Ia supernova 1997br and the nature of SN 1991T-like events

Abstract Li et al. presented extensive observational data on the peculiar Type Ia SN 1997br and showed that it resembled the well-known peculiar SN 1991T . We present some results of a direct analysis of the spectra of SN 1997br . We raise the question of whether the Fe III and Ni III features in the early spectra are produced by 54 Fe and 58 Ni rather than by 56 Fe and 56 Ni as usually assumed, and discuss the issue of whether SN 1991T -like events are more powerful versions of normal SNe Ia rather than a physically distinct subgroup of events.

Type Ia Supernovae as Extragalactic Distance Indicators

Because Type Ia supernovae (SNe Ia) are not perfect standard candles, it is important to be able to use distance–independent observables (DIOs) to define subsets of SNe Ia that are “nearly standard candles” or to correct SN Ia absolute magnitudes to make them nearly homogeneous (“standardized candles”). This is not cruci al for the measurement of H o but it is for the measurement of q o and of parent–galaxy peculi ar velocities. We discuss the use of various photometric and spectroscopic SN Ia DIOs, and a parent–galaxy DIO, for this purpose. We also discuss the status of the absolute–magnitude calibration of SNe Ia. We find that SNe Ia, whether calibrated by means of (1) Cepheids in their parent g al axies, (2) fitting their optical –ultraviolet spectra with detailed non-LTE model atmosphere calculations, or (3) by considering that the light curve is powered by the decay of radioactive 56Ni, firmly indicate that the value of H o is low, less than or about 60 km s-1 Mpc-1. Some issues regarding the determination of q o by means of SNe Ia are discussed briefly. Finally, we conjecture that even if q o=0.5, there probably is no cosmic age problem.

On van den Bergh's Method for Measuring the Hubble Constant from Type Ia Supernovae

In a recent Letter, van den Bergh used the explosion-model light-curve calculations of Hoflich & Khokhlov to calibrate the Hamuy et al. sample of remote Type Ia supernovae (SNe Ia) and obtained low values of H0 in the range 55-60 km s-1 Mpc-1. Citing a higher value of H0 obtained from Cepheid-based distance determinations to a few galaxies in the Virgo Cluster complex, van den Bergh concluded that unless the Cepheid distance scale is wrong, the models of Hoflich & Khokhlov must be much too bright or much too red. Here we calibrate the same sample of remote SNe Ia in the same way as was suggested by van den Bergh, but now relying on Cepheid-based determinations of the distances to SN Ia parent galaxies instead of the explosion-model light-curve calculations. No corrections for extinction of the SNe Ia are required, and no SNe Ia need be excluded on grounds of peculiarity. We obtain H0 = 57 ± 5 km s-1 Mpc-1. Various subsamples of the data give H0 in the range 54-60 km s-1 Mpc-1. This shows that the fault lies not with the SN Ia models, nor with Cepheids, but with the hazardous route through the Virgo complex.