We present optical and infrared spectroscopy of the Ðrst 2 months of evolution of the Type II supernova SN 1999em. We combine these data with high-quality optical/infrared photometry beginning only 3 days after shock breakout, in order to study the performance of the "" expanding photosphere method ÏÏ (EPM) in the determination of distances. With this purpose, we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra. The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11% in velocity from an individual spectrum. Our analysis shows that EPM is quite robust to the e †ects of dust. In particular, the distances derived from the V I Ðlters change by only 7% when the adopted visual extinction in the host galaxy is varied by 0.45 mag. The superb time sampling of the BV IZJHK light curves of SN 1999em permits us to study the internal consistency of EPM and test the dilution factors computed from atmosphere models for Type II plateau supernovae. We Ðnd that, in the Ðrst week since explosion, the EPM distances are up to 50% lower than the average, possibly because of the presence of circumstellar material. Over the following 65 days, on the other hand, our tests lend strong credence to the atmosphere models, and conÐrm previous claims that EPM can produce consistent distances without having to craft speciÐc models to each supernova. This is particularly true for the V I Ðlters, which yield dis- tances with an internal consistency of 4%. From the whole set of BV IZJHK photometry, we obtain an average distance of 7.5^0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of the internal precision of the method obtained from the analysis of the Ðrst 70 days of the supernova evolution.

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The light curve of supernova (SN) 1993J is calculated using two approaches to radiation transport as exempliÐed 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 Ðrst 120 days. The hydrodynamical model, the explosion of a 13 star that has lost most of its hydrogenic envelope to a com-M _ panion, is the same in each calculation. The comparison elucidates di †erences 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 multigroup photonics and is able to follow the radiation as it decouples from the matter. EDDINGTON uses a di †erent algorithm for integrating the transport equation, assumes homologous expansion, and uses a Ðner frequency resolution. Good agreement is achieved between the two codes only when compatible physical assumptions are made about the opacity. In particular, the line opacity near the principal (second) peak of the light curve must be treated primarily as absorptive, even though the electron density is too small for collisional deexcitation to be a dominant photon destruction mechanism. JustiÐcation is given for this assumption and involves the degradation of photon energy by "" line splitting,ÏÏ i.e., Ñuores-cence. The fact that absorption versus scattering matters to the light curve is indicative of the fact that departures from equilibrium radiative di †usion are important. 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.

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. Among other things, it was especially bright for a Type Ic and rose quickly to maximum. When modeled in the usual way as a spherically symmetric explosion, this requires a large mass of 56 Ni, 0.45 -0.60 M ⊙ , a quite massive star, and a very large explosion energy. We explore here models based upon helium stars in the range 9 -14 M ⊙ and carbon-oxygen stars 6 -11 M ⊙ which experience unusually energetic explosions (kinetic energy 0.5 − 2.8 × 10 52 erg). Bolometric light curves and multi-band photometry are calculated and compared favorably with observations. No spectroscopic data are available at this time, but both LTE and non-LTE spectra are calculated for the model that agrees best with the light curve, a carbon-oxygen core of 6 M ⊙ exploded with a kinetic energy of 2.2 × 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(Woosley , 1996 whose main sequence mass was ∼ 35 M ⊙ (helium core mass 14 M ⊙ ). 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 56 Ni mass), explosion energy, and velocities may then be