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Observations of type Ia supernovae at high redshifts have become an important tool for studying the geometry of the universe. The relation between the duration of the peak phase of a type Ia supernova’s lightcurve and its luminosity (broader is brighter) forms the cornerstone of this measurement, yet it is a purely empirical relation. In this paper we show that the relation is a natural consequence of the radiation transport in type Ia supernovae and suggest constraints on the nature of the explosions which arise from our interpretation of the observed relation. The principle parameter underlying the relation is the mass of radioactive 56Ni produced in the explosion. The relation is shown to be relatively insensitive to most other parameters in Chandrasekhar-mass explosions. Cosmological results are thus unlikely to suffer from systematic effects stemming from evolution in the explosions’ progenitors.

We examine the nature of the opacity and radiation transport in Type Ia supernovae. The dominant opacity arises from line transitions. We discuss the nature of line opacities and diffusion in expanding media and the appropriateness of various mean and expansion opacities used in light-curve calculations. Fluorescence is shown to be the dominant physical process governing the rate at which energy escapes the supernova. We present a sample light curve that was obtained using a time-dependent solution of the radiative transport equation with a spectral resolution of 80 km s-1 and employing an LTE equation of state. The result compares favorably with light curves and spectra of typical supernovae and is used to illustrate the physics controlling the evolution of the light curve and especially the secondary maxima seen in infrared photometry.

We develop an analytic solution of the radiation transport problem for Type Ia supernovae (SNe Ia) and show that it reproduces bolometric light curves produced by more detailed calculations under the assumption of a constant-extinction coefficient. This model is used to derive the thermal conditions in the interior of SNe Ia and to study the sensitivity of light curves to various properties of the underlying supernova explosions. Although the model is limited by simplifying assumptions, it is adequate for demonstrating that the relationship between SNe Ia maximum-light luminosity and rate of decline is most easily explained if SNe Ia span a range in mass. The analytic model is also used to examine the size of various terms in the transport equation under conditions appropriate to maximum light. For instance, the Eulerian and advective time derivatives are each shown to be of the same order of magnitude as other order v/c terms in the transport equation. We conclude that a fully time-dependent solution to the transport problem is needed in order to compute SNe Ia light curves and spectra accurate enough to distinguish subtle differences of various explosion models.

The light curve of Supernova (SN) 1993J is calculated using two approaches to radiation transport as exemplified by the two computer codes, STELLA and EDDINGTON. Particular attention is paid to shock breakout and the photometry in the U, B, and V bands during the first 120 days. The hydrodynamical model, the explosion of a 13 Msun star which had lost most of its hydrogenic envelope to a companion, is the same in each calculation. The comparison elucidates differences between the approaches and also serves to validate the results of both. STELLA includes implicit hydrodynamics and is able to model supernova evolution at early times, before the expansion is homologous. STELLA also employs multi-group photonics and is able to follow the radiation as it decouples from the matter. EDDINGTON uses a different algorithm for integrating the transport equation, assumes homologous expansion, and uses a finer frequency resolution. Good agreement is achieved between the two codes only when compatible physical assumptions are made about the opacity. A new result for SN 1993J is a prediction of the continuum spectrum near the shock breakout (calculated by STELLA) which is superior to the results of other standard single energy group hydrocodes such as VISPHOT or TITAN. Based on the results of our independent codes, we discuss the uncertainties involved in the current time dependent models of supernova light curves., 43 pages with 22 eps figures, aaspp4.sty + epsf.sty, Accepted by ApJ, to appear in March 20, 1998 issue, Vol. 496

The infrared colour evolution of type Ia supernovae is very uniform, exhibiting a dip in the J-H light curve approximately 30 d after maximum light. Spectroscopically, this dip has been identified with what appears as a broad absorption feature (5×10 4 km s −1 ) at 1.2 μm. Although many candidate transitions have been investigated in the past, no plausible species of ion or molecule has been found that can account for an absorption at this wavelength. We suggest that this absorption is in fact a lack of emission caused by the absence of any significant opacity in this wavelength interval. At this epoch the supernova is merely a broad-line nebula with no lines near 1.2 μm

Supernova 1993J in the galaxy M81 is the second-brightest type II supernova observed this century, surpassed only by SN1987A in the Large Magellanic Cloud. Here we report the evolution of the photometric and spectral properties of SN1993J for the first 50 d following its discovery. The behavior of this supernova is unusual, showing features typical of type II supernovae near the initial maximum, but with the strong helium lines characteristic of type Ib supernovae at later times. This implies that the progenitor star had an unusually thin hydrogen envelope (compared to normal type II progenitors), suggesting that significant mass loss had taken place before the explosion. Application of an expanding photosphere model to our data provides an estimate of the distance to the supernova of 2.6 +/- 0.4 Mpc, broadly consistent with the distance to M81 determined using Cepheid variable stars. Supernova models that more closely match the atypical spectral features of SN1993J may change the inferred distance, and should provide better constraints on the structure of the progenitor.

We present 30 optical spectra and 49 photometric epochs sampling the first 517 days after discovery of supernova (SN) 1999em, and derive its distance through the expanding photosphere method (EPM). SN 1999em is shown to be a Type II-plateau (II-P) event, with a photometric plateau lasting until about 100 days after explosion. We identify the dominant ions responsible for most of the absorption features seen in the optical portion of the spectrum during the plateau phase. We find the distance to SN 1999em to be D = 8.2 +/- 0.6 Mpc, with an explosion date of 5.3 +/- 1.4 days before discovery. We examine potential sources of systematic error in EPM-derived distances, and find the most significant to result from uncertainty in the theoretical modeling of the flux distribution emitted by the SN photosphere (i.e., the ``flux dilution factor''). We compare previously derived EPM distances to 5 SNe II in galaxies for which a recently revised Cepheid distance exists from the HST Key Project and find D(Cepheids) / D(EPM) = 0.96 +/- 0.09. Finally, we investigate the possible use of SNe II-P as standard candles and find that for 8 photometrically confirmed SNe II-P with previously derived EPM distances and SN 1999em, the mean plateau absolute brightness is M_V(plateau) = -16.4 +/- 0.6 mag, implying that distances good to ~30% (1-sigma) may be possible without the need for a complete EPM analysis. At M_V(plateau) = -15.9 +/- 0.2 mag, SN 1999em is somewhat fainter than the average SN II-P. The general consistency of absolute SNe II-P brightness during the plateau suggests that the standard candle assumption may allow SNe II-P to be viable cosmological beacons at z > 2., 79 pages, 26 figures, accepted for publication in the Publications of the Astronomical Society of the Pacific

Currently popular models for Type Ia supernovae (SNe Ia) fall into two general classes. The first comprises explosions of nearly pure carbon/oxygen (C/O) white dwarfs at the Chandrasekhar limit which ignite near their centers. The second consists of lower-mass C/O cores which are ignited by the detonation of an accreted surface helium layer. Explosions of the latter type produce copious Fe, Co and Ni K-alpha emission from 56Ni and 56Co decay in the detonated surface layers, emission which is much weaker from Chandrasekhar-mass models. The presence of this emission provides a simple and unambiguous discriminant between these two models for SNe Ia. Both mechanisms may produce 0.1-0.6 solar masses of 56Ni, making them bright gamma-ray line emitters. The time to maximum brightness of 56Ni decay lines is distinctly shorter in the sub-Chandrasekhar mass class of model (approximately 15 days) than in the Chandrasekhar mass model (approximately 30 days), making gamma-ray line evolution another direct test of the explosion mechanism. It should just be possible to detect K-shell emission from a sub-Chandrasekhar explosion from SNe Ia as far away as the Virgo cluster with the XMM Observatory. A 1 to 2 square meter X-ray telescope such as the proposed Con-X Observatory could observe K-alpha emission from sub-Chandrasekhar mass SNe Ia in the Virgo cluster, providing not just a detection, but high-accuracy flux and kinematic information., Comment: 30 pages, 17 figures

We have developed an implicit, multi-group, time-dependent, spherical neutrino transport code based on the Feautrier variables, the tangent-ray method, and accelerated ${\bf \Lambda}$ iteration. The code achieves high angular resolution, is good to O($v/c$), is equivalent to a Boltzmann solver (without gravitational redshifts), and solves the transport equation at all optical depths with precision. In this paper, we present our formulation of the relevant numerics and microphysics and explore protoneutron star atmospheres for snapshot post-bounce models. Our major focus is on spectra, neutrino-matter heating rates, Eddington factors, angular distributions, and phase-space occupancies. In addition, we investigate the influence on neutrino spectra and heating of final-state electron blocking, stimulated absorption, velocity terms in the transport equation, neutrino-nucleon scattering asymmetry, and weak magnetism and recoil effects. Furthermore, we compare the emergent spectra and heating rates obtained using full transport with those obtained using representative flux-limited transport formulations to gauge their accuracy and viability. Finally, we derive useful formulae for the neutrino source strength due to nucleon-nucleon bremsstrahlung and determine bremsstrahlung's influence on the emergent $\nu_{\mu}$ and $\nu_{\tau}$ neutrino spectra., Comment: 58 pages, single-spaced LaTeX, 23 figures, revised title, also available at http://jupiter.as.arizona.edu/~burrows/papers, accepted for publication in the Ap.J

Recently a Type Ic supernova, SN 1998bw, was discovered coincident with a gamma-ray burst, GRB 980425. The supernova had unusual radio, optical, and spectroscopic properties. We explore here models based upon helium stars in the range 9 - 14 Msun and carbon-oxygen stars 6 - 11 Msun which experience unusually energetic explosions (kinetic energy 0.5 - 2.8 x 10**52 erg). Bolometric light curves and multi-band photometry are calculated and compared favorably with observations. Both LTE and non-LTE spectra are also calculated for the model that agrees best with the light curve, a carbon-oxygen core of 6 Msun exploded with a kinetic energy of 2.2 x 10**52 erg. We also examine potential mechanisms for producing the observed gamma-ray burst (GRB) - shock break-out and relativistic shock deceleration in circumstellar material. For spherically symmetric models, both fail to produce a GRB of even the low luminosity inferred for GRB 980425. The high explosion energies required to understand the supernova are in contrast to what is expected for such massive stars and may indicate that a new sort of explosion has been identified, possibly the consequence of a collapsar (Woosley 1993, 1996). Indeed a more likely explanation for what was seen is a highly asymmetric explosion in which the GRB was produced by a relativistic jet, perhaps viewed obliquely, and only a fraction of the total stellar mass was ejected, the remainder accreting into a black hole. The ejected mass (but not the 56Ni mass), explosion energy, and velocities may then be smaller. Other Type Ic supernova - GRB coincidences are suggested., 24 pages including 6 figures, submitted to The Astrophysical Journal. latex uses aaspp4.sty

It has been said more than once that the critical link between explosion models and observations is the ability to accurately simulate cooling and radiation transport in the expanding ejecta of Type Ia supernovae. It is perhaps frustrating to some of the theorists who study explosion mechanisms, and to some of the observers too, that more definitive conclusions have not been reached about the agreement, or lack thereof, between various Type Ia supernova models and the data. Although claims of superlative accuracy in transport simulations are sometimes made, I will argue here that there are outstanding issues of critical importance and in need of addressing before radiation transport calculations are accurate enough to discriminate between subtly different explosion models.

For the theorist, Type Ib and Ic supernovae are the explosion of massive stars that have lost their hydrogen envelopes. For the observer, Type Ib supernovae are those that show no evidence of hydrogen, a weak or absent Si II absorption feature near 6150 A at peak light, and strong oxygen emission at late times. The light curves of Type Ib are also about four times fainter and somewhat broader than Type Ia. Unlike Type la, Type Ib supernovae can be strong radio sources and show a strong preference for star forming regions in spiral galaxies. No Type Ib supernova has ever been discovered in an elliptical galaxy. Type Ic greatly resembles Ib, but additionally is characterized by a weak or absent He feature at 5876 A and, in at least one case, a narrower light curve.

A recent survey of massive stellar evolution & supernovae by Weaver & Woosley (1994) and Woosley & Weaver (1994a) has provided detailed abundance predictions for Type II supernovae of mass 11, 12, 13, 15, 18, 19, 20, 22, 25, 30, 35 and 40 M⊙. The computer code used was KEPLER, a 1-D implicit hydrodynamics code with appropriate physics to follow advanced stellar evolution and supernova explosions. (Weaver, Zimmermann, &. Woosley 1978). A total of 77 supernova models were explored. Variations on the initial metallicity (0.0, 10−4, 10−2, 0.1, and and 1.0 Z ⊙); explosion kinetic energy; the inclusion or omission of semi-convection; and the value of the 12C(α,γ) 16O reaction rate were explored for all masses. In the results presented here, the 17 models with initial solar metallicity, semi-convection, and a 12C(α,γ)16O rate increased by 1.7 above the value given by Caughlan and Fowler (1988) are most relevant.

We present early-time near-infrared spectra of the type Ia supernova 1991T and latetime spectra of SNe 1991T and 1987A obtained with IRIS on the AAT and CGS4 on UKIRT. A large mass of iron is present in the ejecta of type Ia supernovae. The early-time 1.2 μm depression in the Ia spectra is due to a lack of line blanketing opacity rather than a blend of absorbers. Late-time spectra of SN 1987A obtained at an unprecedented 5 years after explosion are shown.

We use the Expanding Photosphere Method to determine distances to 10 type II supernovae. The effects of asymmetries, extinction, and flux dilution are explored. Using empirical evidence and time-independent, spherical models which treat H and He in non-LTE, we show that blackbody corrections caused by flux dilution are small for type II supernovae in the infrared, and in the optical when their color temperatures are less than 6000~K. The extinction to a type II-P supernova can be estimated from its light curve: the uncertainty introduced into a distance measurement due to extinction is usually less than 10\%. Correcting for extinction and flux dilution we derive distances to 10 supernovae: SN 1968L, SN 1969L, SN 1970G, SN 1973R, SN 1979C, SN 1980K, SN 1987A, SN 1988A, SN 1990E, and SN 1990ae. The distance measurements span a wide range, 50 kpc to 120 Mpc, which is unique among the methods for establishing the extragalactic distance scale. The distances measured to SN 1970G in M101 and SN 1987A in the LMC are in good agreement with distances determined from Cepheid variable stars. Our distance to the Virgo Cluster, 22 +- 3 Mpc, is larger than recent distances estimates made using surface brightness fluctuations, planetary nebula luminosity functions, and the Tully-Fisher method. Using the distances determined from these type II supernovae we derive a value of $H_0 = 60 \pm 10$ km sec$^{-1}$Mpc$^{-1}$. This value is subject to errors caused by local deviations in the Hubble flow, but will soon be improved by applying the Expanding Photosphere Method to several distant type II supernovae., 21 pages

Hydrodynamic calculations of Type II supernova (SNII) explosions have been highly successful at reproducting the major observational properties of these events, particularly in the case of SN87A (e.g. [l], [2], [3]) Basic properties which can be determined from the data, such as the gas velocity and the bolometric luminosity are reasonably well fit by the hydrodynamic models. What makes these calculations feasible however is a compromise treatment of details concerning the radiation field. Most hydro codes treat the radiation transport by a simple flux limited radiative diffusion scheme and use a Lagrangian mass grid which is too course to yield up more than crude information on details of the emergent spectrum. Ultraviolet, optical and infrared spectrophotometry of supernovae are pregnant with information about their source, but in order to decipher most of this information it is first necessary to perform more refined calculations of the gas excitation/ionization and the radiation field structure in the photospheric and overlying layers.

The Expanding Photosphere Method (EPM) determines distances to Type II supernova (SNe II) by comparing the photospheric angular size with the expansion velocity measured from spectral lines. The photospheres of SNe II are low density and are dominated by electron scattering, and consequently the photospheric flux is dilute relative to a Planck function at the best-fitting continuum color temperature. The reliability of EPM distances depends on how the dilution is related to physical properties of the supernova atmosphere. We have calculated 63 different model atmospheres relevant to SNe II. The excitation, ionization, and thermal structure are described for the case of high effective temperature in which the atmosphere is completely ionized, and for the case of cooler effective temperatures in which the photosphere is formed in a region of recombining hydrogen. The general spectral features of both cases are discussed. We explore how the computed spectrum changes with density structure, helium abundance, metallicity, expansion rate, and luminosity or effective temperature. The most important variable in determining spectral appearance is the effective temperature. The amount by which the emergent flux is dilute relative to the best-fitting blackbody depends on a number of factors, most important of which are the temperature and,more » in short-wavelength bandpasses, density at the photosphere. For each model we derive distance correction factors for application in EPM, using the four filter combinations {l_brace}{ital BV}{r_brace}, {l_brace}{ital VI}{sub {ital c}}{r_brace}, {l_brace}{ital BVI}{sub {ital c}}{r_brace}, and {l_brace}{ital JHK}{r_brace}. The main differences may be expressed in terms of the observable color temperature and a slowly varying dependence on density. Functional fits to the distance correction factor are used to estimate the photospheric angular size from broadband photometry. The effect of uncertain dust extinction on angular size is small.« less

We have used observations gathered at Cerro Tololo Inter-American Observatory (CTIO) to measure distances by the expanding photosphere method (EPM) to five Type II supernovae. These supernovae lie at redshifts from cz = 1100 km/s to cz = 5500 km/s, and increase to 18 the number of distances measured using EPM. We compare distances derived to 11 Type II supernovae with distances to their galaxies measured using the Tully-Fisher method. We find that the Tully-Fisher distances average 11% +/- 7% smaller. The comparison shows no significant evidence of any large distance-dependent bias in the Tully-Fisher distances. We employ the sample of EPM distances from 4.5 Mpc to 180 Mpc to derive a value for the Hubble constant. We find H(sub 0) = 73 +/- 6 (statistical) +/- 7 (systematic) km/s/Mpc.

The light curve and other observable diagnostics are presented for what should be a typical Type II 'plateau' supernova, the explosion of a 15 solar mass red supergiant. The calculations are carried out using a multi-frequency radiation transport code that includes opacity from all important mechanisms-bound-bound, bound-free, free-free, and electron scattering. It is found that the inclusion of opacity sources besides electron scattering increases the computed lenght of the plateau in the 15 solar mass model by approximately 30 days. Especially important is the ultraviolet cutoff caused by a thick forest of lines. Deposition and escape of gamma-rays from radioactive decay are also accurately simulated. We find that the presence of 0.06 solar mass of ejected Ni-56 extends the length of the plateau from approximately 100 days to approximately 140 dys. Because of the large hydrogen envleope and radius in this model, mixing does not appreciably alter the bolometric light curve, although it does change the gamma-ray light curve significantly. With 0.06 solar mass of ejected Ni-56, the unmixed (mixed) model peaks in escaping gamma-rays at 576 (466) days with gamma-ray luminosity of 3 x 10(exp 38) ergs/s. Except for the local group, the gamma-rays from an extragalactic 15 solar mass SNe IIp will be too faint to detect with any exsiting detectors. However, a Galactic SNe IIp would be quite bright and easily detectable.

The evolution of the bright Type II supernova discovered last year in M81, SN 1993J, is consistent with that expected for the explosion of a star which on the main sequence had a mass of 13-16 Solar Mass but which, owing to mass exchange with a binary companion (a intially approximately 3-5 AU, depending upon the actual presupernova radius and the masses of the two stars) lost almost all of its hydrogen-rich envelope during late helium burning. At the time of explosion, the helium core mass was 4.0 +/- 0.5 Solar Mass and the hydrogen envelope, 0.20 +/- 0.05 Solar Mass. The envelope was helium and nitrogen-rich (carbon-deficient) and the radius of the star, 4 +/- 1 x 10(exp 13) cm. The luminosity of the presupernova star was 3 + 1 x 10(exp 38) ergs/s, with the companion star contributing an additional approximately 10(exp 38) ergs/s. The star may have been a pulsating variable at the time of the explosion. For an explosion energy near 10(exp 51) ergs (KE at infinity) and an assumed distance of 3.3 Mpc, a mass of Ni-56 in the range 0.07 +/- 0.01 Solar Mass was produced and ejected. This presciption gives a light curve which compares favorably with the bolomatric observations. Color photometry is more restrictive and requires a model in which the hydrogen-envelope mass is low and the mixing of hydrogen inward has been small, but in which appreciable Ni-56 has been mixed outward into the helium and heavy-element core. It is possible to obtain good agreement with B and V light curves during the first 50 days, but later photometry, especially in bands other than B and V, will require a non-local thermo-dynamic equilibrium (LTE) spectral calculation for comparison. Based upon our model, we predict a flux of approximately 10(exp -5)(3.3 Mpc/D)(exp 2) photons/sq cm/s in the 847 keV line of CO-56 at peak during 1993 August. It may be easier to detect the Computonized continuum which peaks at a few times 10(exp -4) photons /s/sq cm/MeV at 40 keV a few months after the explosion (though neither of these signals were, or should have been, detected by the Compton Gamma-Ray observatory (CGRO). The presupernova star was filing its Roche lobe at the time of the explosion and thus its envelope was highly deformed (about 3:2). The companion star is presently embedded in the supernova, but should becopme visable at age 3 yr (perhaps earlier in the ultraviolet) when the supernova has faded below 10(exp 38) ergs/s. Indeed, if 'kicks' have not played an important role, it is still bound to the neutron star.

The study combines several novel techniques for spectrum simulation in the Eddington computer program which solves the comoving frame equation of transfer coupled with the statistical and radiative equilibrium equations. One of these is a generalization of the accelerated lambda iteration (ALI) scheme to include an approximate frequency-derivative operator. This greatly enhances the convergence rate of ALI in optically thick, high-velocity shear flows. Another is a partial linearization technique which is capable of efficiently solving a very large number of rate equations on a moderately sized computer. An expansion opacity and emissivity approximation is derived which makes it possible to determine the effect on the transfer and statistical equilibrium of a very large number of lines not explicitly represented in the frequency grid and additionally to treat line-blanketing from species not explicitly included in the rate equations. The utility of these techniques is illustrated with models of two supernovae.

We present photometry and spectroscopy of Supernova (SN) 1992am for five months following its discovery by the Calan Cerro-Tololo Inter-American Observatory (CTIO) SN search. These data show SN 1992am to be type II-P, displaying hydrogen in its spectrum and the typical shoulder in its light curve. The photometric data and the distance from our own analysis are used to construct the supernova's bolometric light curve. Using the bolometric light curve, we estimate SN 1992am ejected approximately 0.30 solar mass of Ni-56, an amount four times larger than that of other well studied SNe II. SN 1992am's; host galaxy lies at a redshift of cz = 14 600 km s(exp -1), making it one of the most distant SNe II discovered, and an important application of the Expanding Photsphere Method. Since z = 0.05 is large enough for redshift-dependent effects to matter, we develop the technique to derive luminosity distances with the Expanding Photosphere Method at any redshift, and apply this method to SN 1992am. The derived distance, D = 180(sub -25) (sup +30) Mpc, is independent of all other rungs in the extragalactic distance ladder. The redshift of SN 1992am's host galaxy is sufficiently large that uncertainties due to perturbations in the smooth Hubble flow should be smaller than 10%. The Hubble ratio derived from the distance and redshift of this single object is H(sub 0) = 81(sub -15) (sup +17) km s(exp -1) Mpc(exp -1). In the future, with more of these distant objects, we hope to establish an independent and statistically robust estimate of H(sub 0) based solely on type II supernovae.

A code which simultaneously solves the non-LTE equations of statistical equilibrium and the time-independent equation of radiation transport in an expanding atmosphere is applied to analyze the spectrum of SN 1987A. The behavior of a model atmosphere in which the density structure is a power-law function of radius has been studied and compared closely with SN 1987A data. This model atmosphere is used to derive corrections to the expanding photosphere method for determining distances to supernovae. The distance to the Large Magellanic Cloud derived from SN 1987A observations by this 'custom yardstick' is 49 + or - 6 kpc in excellent accord with results for RR Lyrae stars and for Cepheids. This empirical test of the expanding photosphere method provides the basis for hope that it may make a significant contribution to the extragalactic distance scale at cosmologically interesting distances.