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This paper presents late-time near-infrared and Spitzer mid-infrared photometric and spectroscopic observations of warm dust in the Type IIn SN 2005ip in NGC 2906. The spectra show evidence for two dust components with different temperatures. Spanning the peak of the thermal emission, these observations provide strong constraints on the dust mass, temperature, and luminosity, which serve as critical diagnostics for disentangling the origin and heating mechanism of each component. The results suggest that the warmer dust has a mass of [?]5 x 10-4 M , originates from newly formed dust in the ejecta, or possibly the cool, dense shell, and is continuously heated by the circumstellar interaction. By contrast, the cooler component likely originates from a circumstellar shock echo that forms from the heating of a large, pre-existing dust shell [?]0.01-0.05 M by the late-time circumstellar interaction. The progenitor wind velocity derived from the blue edge of the He I 1.083 mm P Cygni profile indicates a progenitor eruption likely formed this dust shell [?]100 years prior to the supernova explosion, which is consistent with a Luminous Blue Variable progenitor star.

Near-infrared photometric observations of the Type IIn supernova (SN) 2005ip in NGC 2906 reveal large fluxes (>1.3 mJy) in the Ks -band over more than 900 days. While warm dust can explain the late-time Ks -band emission of SN 2005ip, the nature of the dust heating source is ambiguous. Shock heating of pre-existing dust by post-shocked gas is unlikely because the forward shock is moving too slowly to have traversed the expected dust-free cavity by the time observations first reveal the Ks emission. While an infrared light echo model correctly predicts a near-infrared luminosity plateau, heating dust to the observed temperatures of [?] 1400-1600 K at a relatively large distance from the supernova ([?]1018 cm) requires an extraordinarily high early supernova luminosity ([?]1 x 1011 L ). The evidence instead favors condensing dust in the cool, dense shell between the forward and reverse shocks. Both the initial dust temperature and the evolutionary trend toward lower temperatures are consistent with this scenario. We infer that radiation from the circumstellar interaction heats the dust. While this paper includes no spectroscopic confirmation, the photometry is comparable to other supernovae that do show spectroscopic evidence for dust formation. Observations of dust formation in supernovae are sparse, so these results provide a rare opportunity to consider supernovae Type IIn as dust sources.

We present radio observations of the optically bright Type IIn supernova SN 1995N. We observed the SN at radio wavelengths with the Very Large Array for 11 years. We also observed it at low radio frequencies with the Giant Metrewave Radio Telescope at various epochs within 6.5 - 10 years since explosion. Although there are indications of an early optically thick phase, most of the data are in the optically thin regime so it is difficult to distinguish between synchrotron self absorption and free-free absorption (FFA) mechanisms. However, the information from other wavelengths indicates that FFA is the dominant absorption process. Model fits of radio emission with FFA give reasonable physical parameters. Making use of X-ray and optical observations, we derive the physical conditions of the shocked ejecta and the shocked circumstellar matter.

We present a comprehensive multiwavelength analysis of the bright, long-duration gamma-ray burst GRB 070125, comprised of observations in gamma-ray, X-ray, optical, millimeter, and centimeter wave bands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation and used to derive an upper limit of 2.4 x 1017 cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self-absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a windlike profile ( R[?]2). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that GRB 070125 is a burst with high radiative efficiency (>60%).

We present multicolor Hubble Space Telescope (HST) WFPC2 broadband observations of the Type Ic SN 1994I obtained [?]280 d after maximum light. We measure the brightness of the SN and, relying on the detailed spectroscopic database of SN 1994I, we transform the ground-based photometry obtained at early times to the HST photometric system, deriving light curves for the WFPC2 F439W, F555W, F675W, and F814W passbands that extend from 7 days before to 280 days after maximum. We use the multicolor photometry to build a quasi-bolometric light curve of SN 1994I, and compare it with similarly constructed light curves of other supernovae. In doing so, we propose and test a scaling in energy and time that allows for a more meaningful comparison of the exponential tails of different events. Through comparison with models, we find that the late-time light curve of SN 1994I is consistent with that of spherically symmetric ejecta in homologous expansion, for which the ability to trap the g-rays produced by the radioactive decay of 56Co diminishes roughly as the inverse of time squared. We also find that by the time of the HST photometry, the light curve was significantly energized by the annihilation of positrons.

We present multicolor Hubble Space Telescope(HST) WFPC2 broadband observations of the Type Ic SN 1994I obtained ?280 d after maximum light. We measure the brightness of the SN and, relying on the detailed spectroscopic database of SN 1994I, we transform the ground-based photometry obtained at early times to the HSTphotometric system, deriving light curves for the WFPC2 F439W, F555W, F675W, and F814W passbands that extend from 7 days before to 280 days after maximum. We use the multicolor photometry to build a quasi-bolometric light curve of SN 1994I, and compare it with similarly constructed light curves of other supernovae. In doing so, we propose and test a scaling in energy and time that allows for a more meaningful comparison of the exponential tails of different events. Through comparison with models, we find that the late-time light curve of SN 1994I is consistent with that of spherically symmetric ejecta in homologous expansion, for which the ability to trap the ?-rays produced by the radioactive decay of 56Co diminishes roughly as the inverse of time squared. We also find that by the time of the HSTphotometry, the light curve was significantly energized by the annihilation of positrons.

We set sensitive upper limits to the X-ray emission of four Type Ia supernovae (SNe Ia) using the Chandra X-Ray Observatory. SN 2002bo, a normal, although reddened, nearby SN Ia, was observed 9.3 days after explosion. For an absorbed, high-temperature bremsstrahlung model the flux limits are 3.2 x 10-16 ergs cm-2 s-1 (0.5-2 keV band) and 4.1 x 10-15 ergs cm-2 s-1 (2-10 keV band). Using conservative model assumptions and a 10 km s-1 wind speed, we derive a mass-loss rate of M ~ 2 x 10-5 M yr-1, which is comparable to limits set by the nondetection of Ha lines from other SNe Ia. Two other objects, SN 2002ic and SN 2005gj, observed 260 and 80 days after explosion, respectively, are the only SNe Ia showing evidence for circumstellar interaction. The SN 2002ic X-ray flux upper limits are ~4 times below predictions of the interaction model currently favored to explain the bright optical emission. To resolve this discrepancy, we invoke the mixing of cool dense ejecta fragments into the forward shock region, which produces increased X-ray absorption. A modest amount of mixing allows us to accommodate the Chandra upper limit. SN 2005gj is less well studied at this time. Assuming the same circumstellar environment as for SN 2002ic, the X-ray flux upper limits for SN 2005gj are ~4 times below the predictions, suggesting that mixing of cool ejecta into the forward shock has also occurred here. Our reanalysis of Swift and Chandra data on SN 2005ke does not confirm a previously reported X-ray detection. The host galaxies NGC 3190 (SN 2002bo) and NGC 1371 (SN 2005ke) each harbor a low-luminosity [LX ~ (3-4) x 1040 ergs s-1] active nucleus in addition to widespread diffuse soft X-ray emission.

Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these models, we find that single stars cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends that we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.

We propose new diagnostics for circumstellar interaction in Type IIP supernovae (SNe IIP) by the detection of high-velocity (HV) absorption features in Ha and He I 10830 A lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e., a depression in the blue absorption wing of Ha and a pronounced absorption of He I 10830 A at a radial velocity of about -104 km s-1. We identify HV absorption in Ha and He I 10830 A lines of SN 1999em and in Ha of SN 2004dj as being due to this effect. The derived mass-loss rate is close to 10-6 M yr-1 for both supernovae, assuming a wind velocity 10 km s-1. We argue that in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell (CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, [?]60 days. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.

The polarization of the supernova SN 2002ic interacting with a dense circumstellar envelope is calculated in the context of the asymmetric version of a previously proposed spherical interaction model. The circumstellar envelope is taken to be oblate. The observed polarization can be reproduced for an aspect ratio of 0.65-0.7, assuming inclination angles of >60deg. This model predicts a weak sensitivity of the line profiles to the orientation, which is in agreement with the absence of significant variations of the line profiles among SN 2002ic-like supernovae. We propose a test for distinguishing between the binary and single-star progenitor scenarios on the basis of the polarization distribution function for the growing sample of these events.

The presumed Wolf-Rayet star progenitors of Type Ib/c supernovae have fast, low-density winds, and the shock waves generated by the supernova interaction with the wind are not expected to be radiative at typical times of observation. The injected energy spectrum of radio-emitting electrons typically has an observed index p = 3, which is suggestive of acceleration in cosmic-ray-dominated shocks. The early, absorbed part of the radio light curves can be attributed to synchrotron self-absorption, which leads to constraints on the magnetic field in the emitting region and on the circumstellar density. The range of circumstellar densities inferred from the radio emission is somewhat broader than that for Galactic Wolf-Rayet stars, if similar efficiencies of synchrotron emission are assumed in the extragalactic supernovae. For the observed and expected ranges of circumstellar densities to roughly overlap, a high efficiency of magnetic field production in the shocked region is required ([?]B [?] 0.1). For the expected densities around a Wolf-Rayet star, a nonthermal mechanism is generally required to explain the observed X-ray luminosities of Type Ib/c supernovae. Inverse Compton emission is a candidate for the emission, if the observations are near optical maximum. In other cases we suggest that the mechanism is X-ray synchrotron emission in a situation in which the shock wave is cosmic-ray-dominated so that the electron energy spectrum flattens at high energy. More comprehensive X-ray observations of a Type Ib/c supernova are needed to determine whether this suggestion is correct.

HST images of the young supernova remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. ACS WFC images taken in 2004 March and December with Sloan F625W, F775W, and F850LP filters were used to identify 1825 high-velocity, outlying ejecta knots through measured proper motions of 0.''35-0.''90 yr-1, corresponding to Vtrans = 5500-14,500 km s-1 assuming d = 3.4 kpc. The distribution of derived transverse expansion velocities for these ejecta knots shows a striking bipolar asymmetry with the highest velocity knots (Vtrans [?] 10,500 km s-1) confined to nearly opposing northeast and southwest "jets" at P.A. = 45deg-70deg and 230deg-270deg, respectively. The jets have about the same maximum expansion velocity of [?]14,000 km s-1 and appear kinematically and chemically distinct in that they are the remnant's only S-rich ejecta with expansion velocities above the 10,000-11,000 km s-1 exhibited by outer nitrogen-rich ejecta, which otherwise represent the remnant's highest velocity debris. In addition, we find significant gaps in the spatial distribution of outlying ejecta in directions that are approximately perpendicular to the jets (P.A. = 145deg-200deg and 335deg-350deg). The remnant's central X-ray point source lies some 7'' to the southeast of the estimated expansion center (P.A. = 169deg +- 8.deg4) indicating a projected motion toward the middle of the broad southern ejecta knot gap. Extrapolations of measured 9 month proper motions for all 1825 outer ejecta knots and a selected subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 +- 27 and 1672 +- 18, respectively. We find some evidence for nonuniform deceleration in different directions around the remnant and find 126 knots located along the northwestern limb among the least decelerated ejecta, suggesting a convergence date of 1681 +- 19. A remnant age of around 325 yr would imply a [?]350 km s-1 transverse velocity for the central X-ray point source.

Type IIP (plateau) supernovae are thought to come from stars with initial mass ~8-25 M that end their lives as red supergiants. The expected stellar endpoints can be found from evolutionary calculations, and the corresponding mass-loss properties at these points can be estimated from typical values for Galactic stars. The mass-loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling could affect the radio-emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio-emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass-loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of supernovae in metal-poor regions could show how the mass loss depends on metallicity.

The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray, and Ha emission at an age of ~2.5 yr. Although the radio and X-ray emission have been attributed to an off-axis gamma-ray burst, we model the emission as the interaction of normal SN Ib/c ejecta with a dense, massive (~3 M) circumstellar shell at a distance of ~7 x 1016 cm. We investigate two models, in which the circumstellar shell has or has not been overtaken by the forward shock at the time of the X-ray observation. The circumstellar shell was presumably formed by vigorous mass loss with a rate of ~(2-10) x 10-3 M yr-1 at ~(1-2) x 103 yr prior to the supernova explosion. The hydrogen envelope was completely lost and subsequently was swept up and accelerated by the fast wind of the presupernova star up to a velocity of 30-50 km s-1. Although interaction with the shell can explain most of the late emission properties of SN 2001em, we need to invoke clumping of the gas to explain the low absorption at X-ray and radio wavelengths.

Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. In this paper, we concentrate on a [?]1.5 arcmin2 region located along the eastern limb of the remnant where we identify three main classes of outer ejecta: (1) knots dominated by [N II] ll6548, 6583 emission, (2) knots dominated by oxygen emission lines, especially [O II] ll7319, 7330, and (3) knots with emission-line strengths similar to the [S II]-strong fast-moving knot (FMK) ejecta commonly seen in the main emission shell. Mean transverse velocities derived from observed proper motion for N-rich, O-rich, and FMK-like knots identified in this region were found to be 8100, 7900, and 7600 km s-1, respectively. The discovery of a significant population of O-rich ejecta situated between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the progenitor's chemical layers were not completely disrupted by the supernova explosion outside of the remnant's northeast and southwest high-velocity "jet" regions. In addition, we find the majority of O-rich outer ejecta at projected locations out beyond the remnant's fastest moving Fe-rich X-ray emission material seen in Chandra and XMM-Newton data along the eastern limb, suggesting that penetration of Fe-rich material up through the S- and Si-rich mantle did not extend past the progenitor's N- or O-rich outer layers for this section of the remnant.

We observed supernova 1987A (SN 1987A) with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 September and again with the Advanced Camera for Surveys (ACS) on the HST in 2003 November. Our spectral observations cover ultraviolet (UV) and optical wavelengths from 1140 to 10266 A, and our imaging observations cover UV and optical wavelengths from 2900 to 9650 A. No point source is observed in the remnant. We obtain a limiting flux of Fopt [?] 1.6 x 10-14 ergs s-1 cm-2 in the wavelength range 2900-9650 A for any continuum emitter at the center of the supernova remnant (SNR). This corresponds to an intrinsic luminosity of Lopt [?] 5 x 1033 ergs s-1. It is likely that the SNR contains opaque dust that absorbs UV and optical emission, resulting in an attenuation of ~35% due to dust absorption in the SNR. Correcting for this level of dust absorption would increase our upper limit on the luminosity of a continuum source by a factor of 1.54. Taking into account dust absorption in the remnant, we find a limit of Lopt [?] 8 x 1033 ergs s-1. We compare this upper bound with empirical evidence from point sources in other supernova remnants and with theoretical models for possible compact sources. We show that any survivor of a possible binary system must be no more luminous than an F6 main-sequence star. Bright young pulsars such as Kes 75 or the Crab pulsar are excluded by optical and X-ray limits on SN 1987A. Other nonplerionic X-ray point sources have luminosities similar to the limits on a point source in SN 1987A; RCW 103 and Cas A are slightly brighter than the limits on SN 1987A, while Pup A is slightly fainter. Of the young pulsars known to be associated with SNRs, those with ages [?]5000 yr are all too bright in X-rays to be compatible with the limits on SN 1987A. Examining theoretical models for accretion onto a compact object, we find that spherical accretion onto a neutron star is firmly ruled out and that spherical accretion onto a black hole is possible only if there is a larger amount of dust absorption in the remnant than predicted. In the case of thin-disk accretion, our flux limit requires a small disk, no larger than 1010 cm, with an accretion rate no more than 0.3 times the Eddington accretion rate. Possible ways to hide a surviving compact object include the removal of all surrounding material at early times by a photon-driven wind, a small accretion disk, or very high levels of dust absorption in the remnant. It will not be easy to improve substantially on our optical-UV limit for a point source in SN 1987A, although we can hope that a better understanding of the thermal infrared emission will provide a more complete picture of the possible energy sources at the center of SN 1987A.

Ground-based and Hubble Space Telescope observations are presented for SN 1993J and SN 1998S. SN 1998S shows strong, relatively narrow circumstellar emission lines of N III-V and C III-IV, as well as broad lines from the ejecta. Both the broad ultraviolet and optical lines in SN 1998S indicate an expansion velocity of ~7000 km s-1. The broad emission components of Lya and Mg II are strongly asymmetrical after day 72 past the explosion and differ in shape from Ha. Different models based on dust extinction from dust in the ejecta or shock region, in combination with Ha from a circumstellar torus, are discussed. It is concluded, however, that the double-peaked line profiles are more likely to arise as a result of optical depth effects in the narrow, cool, dense shell behind the reverse shock than in a torus-like region. The ultraviolet lines of SN 1993J are broad, with a boxlike shape, coming from the ejecta and a cool, dense shell. The shapes of the lines are well fitted by a shell with inner velocity ~7000 km s-1 and outer velocity ~10,000 km s-1. For both SN 1993J and SN 1998S a strong nitrogen enrichment is found, with N/C [?] 12.4 in SN 1993J and N/C [?] 6.0 in SN 1998S. From a compilation of all supernovae with determined CNO ratios, we discuss the implications of these observations for the structure of the progenitors of Type II supernovae.

Massive star supernovae can be divided into four categories, depending on the amount of mass loss from the progenitor star and the star's radius: red supergiant stars with most of the H envelope intact (SN IIP), stars with some H but most lost (IIL and IIb), stars with all H lost (Ib and Ic), and blue supergiant stars with a massive H envelope (SN 1987A-like). Various aspects of the immediate aftermath of the supernova are expected to develop in different ways, depending on the supernova category: mixing in the supernova, fallback on the central compact object, expansion of any pulsar wind nebula, interaction with circumstellar matter, and photoionization by shock breakout radiation. The observed properties of young supernova remnants allow many of them to be placed in one of the supernova categories; all the categories are represented except for the SN 1987A-like type. Of the remnants with central pulsars, the pulsar properties do not appear to be related to the supernova category. There is no evidence that the supernova categories form a mass sequence, as would be expected in a single-star scenario for the evolution. Models for young pulsar wind nebulae expanding into supernova ejecta indicate initial pulsar periods of 10-100 ms and approximate equipartition between particle and magnetic energies. Ages are obtained for pulsar nebulae, including an age of 2400 +- 500 yr for 3C 58, which is not consistent with an origin in SN 1181. There is no evidence that mass fallback plays a role in neutron star properties.

We use two epochs of Hubble Space Telescope WFPC2 images separated by 2 yr to determine the location and propagation of the reverse shock (RS) in the young supernova remnant Cassiopeia A (Cas A). The images trace optical line emission from fast-moving knots and filaments of highly processed ejecta as they cross the RS, become heated and compressed, and radiatively cool. At numerous positions around the optical shell, new emission features are seen in the 2002 images that were not yet visible in the 2000 exposures. In a few instances emission features seen in the first epoch have completely disappeared in the second epoch. We concentrate on two regions along the rim of the main emission shell in Cas A for close inspection: one in the northwestern part of the shell and another along the southwestern part of it. In these regions the RS is viewed almost edge-on, and its precise position has been measured. The RS is coherent in these regions over arcminute (~1 pc) scales but is highly distorted perpendicular to the direction of expansion. We find the RS to be generally expanding at 50%-60% of the ~5500 km s-1 bulk velocity of the optical ejecta. We present shock models for the ejecta that are consistent with the high densities and short cooling times observed in the optical knots of the Cas A remnant.

The finding of a Type Ic supernova connected with GRB 030329 showed a massive star origin for this burst, supporting evidence for this association in previous bursts with light-curve bumps at the appropriate time for a supernova. Here we explore the possibility that all long bursts have massive star progenitors, interacting with either the freely expanding wind of the progenitor or the shocked wind. We present models for the afterglows of GRB 020405 and GRB 021211, which are a challenge to wind interaction models. Considering sources for which wind interaction models are acceptable, a range of wind densities is required, from values typical of Galactic Wolf-Rayet stars to values ~102 times smaller. The reason for the low densities is unclear, but it may involve low progenitor masses and/or low metallicities. If mass is a factor, a low-density event should be associated with a low-mass supernova. The interpretation of bursts apparently interacting with constant density media as interaction with a shocked wind requires both a range of mass-loss densities and a range of external pressures. The highest pressures, p/k [?] 108 cm-3 K, may be due to an extreme starburst environment, which would imply that the burst is superposed on an active star-forming region. Although the range of observed events can be accommodated by the shocked-wind theory, special circumstances are necessary to bring this about. Finally, we consider the high-velocity, high-ionization absorption features observed in some afterglow spectra. If the features are circumstellar, the presence of the burst in a starburst region may be important for the formation of clumps near the burst.

When a massive star produces a powerful stellar wind prior to its supernova event, theory predicts that the collision of the exploded stellar ejecta with the wind leads to a reverse shock, creating soft X-ray emission. To understand the frequency at which luminous young X-ray supernovae occur, we used ROSAT to observe a complete sample of nearby supernovae (vhelio < 1700 km s-1) that occurred in the period 1985.5 through 1994.3, which included eight Type Ia supernovae and 19 Type Ib and Type II events. Three supernovae are detected in this time frame, SN 1987A (LMC), SN 1993J (NGC 3031), and a previously unreported source, SN 1992ad, a Type II supernova in NGC 4411b. No supernova had 0.5-2 keV luminosities exceeding 2 x 1039 ergs s-1, so at the 95% confidence level, the probability of an individual supernova exceeding this luminosity limit is less than 12%. Two of these supernovae had luminosities brighter than 6 x 1038 ergs s-1 and at the 95% confidence level, the probability of a supernova being detected above this luminosity is in the range 8.7%-51%. It is unlikely for young supernovae to be a large component of the Intermediate Luminosity X-Ray Object (IXO or ULX) class, where the luminosity exceeds 2 x 1039 ergs s-1. The rate of successful detections appears to increase for sensitivities in the 1037 ergs s-1 range, especially when obtained close to the time of the event.

We present two-dimensional line profiles of high-velocity (~+-12,000 km s-1) Lya and Ha emission from supernova remnant 1987A obtained with the Space Telescope Imaging Spectrograph between 1997 September and 2001 September (days 3869-5327 after the explosion). This emission comes from hydrogen in the debris that is excited and ionized as it passes through the remnant's reverse shock. We use these profiles to measure the geometry and development of the reverse-shock surface. The observed emission is confined within ~+-30deg about the remnant's equatorial plane. At the equator, the reverse shock has a radius of ~75% of the distance to the equatorial ring. We detect marginal differences (6% +- 3%) between the location of the reverse-shock front in the northeast and southwest parts of the remnant. The radius of the reverse shock surface increases for latitudes above the equator, a geometry consistent with a model in which the supernova debris expands into a bipolar nebula. Assuming that the outer supernova debris has a power-law density distribution, we can infer from the reverse-shock emission light curve an expansion rate (in the northeast part of the remnant) of 3700 +- 900 km s-1, consistent with the expansion velocities determined from observations in radio (Manchester et al.) and X-ray (Park et al.; Michael et al.) wavelengths. However, our most recent observation (at day 5327) suggests that the rate of increase of mass flux across the northeast sector of the reverse shock has accelerated, perhaps because of deceleration of the reverse shock caused by the arrival of a reflected shock created when the blast wave struck the inner ring. Resonant scattering within the supernova debris causes Lya photons created at the reverse shock to be directed preferentially outward, resulting in a factor of ~5 difference in the observed brightness of the reverse shock in Lya between the near and far sides of the remnant. Accounting for this effect, we compare the observed reverse-shock Lya and Ha fluxes to infer the amount of interstellar extinction by dust as E(B - V) = 0.17 +- 0.01 mag. We also notice extinction by dust in the equatorial ring with E(B - V) [?] 0.02-0.08 mag, which implies dust-to-gas ratios similar to that of the LMC. Since Ha photons are optically thin to scattering, the observed asymmetry in brightness of Ha from the near and far sides of the remnant represents a real asymmetry in the mass flux through the reverse shock of ~30%. We discuss future observational strategies that will permit us to further investigate the reverse-shock dynamics and resonant scattering of the Lya line and to constrain better the extinction by dust within and in front of the remnant.

We use two-dimensional hydrodynamical simulations to investigate the properties of dense ejecta clumps (bullets) in a core-collapse supernova remnant, motivated by the observation of protrusions probably caused by clumps in the Vela supernova remnant. The ejecta, with an inner flat and an outer steep power-law density distribution, were assumed to freely expand into an ambient medium with a constant density, ~0.1 H atoms cm-3 for the case of Vela. At an age of 104 yr, the reverse shock front is expected to have moved back to the center of the remnant. Ejecta clumps with an initial density contrast kh ~ 100 relative to their surroundings are found to be rapidly fragmented and decelerated. In order to cause a pronounced protrusion on the blast wave, as observed in the Vela remnant, kh ~ 1000 may be required. In this case, the clump should be near the inflection point in the ejecta density profile, at an ejecta velocity ~3000 km s-1. These results apply to moderately large clumps; smaller clumps would require an even larger density contrast. Clumps can create ring structure in the shell of the Vela remnant, and we investigate the possibility that RX J0852-4622, an apparent supernova remnant superposed on Vela, is actually part of the Vela shell. Radio observations support this picture, but the possible presence of a compact object argues against it. The Ni bubble effect or compression in a pulsar wind nebula are possible mechanisms to produce the clumping.

Observations of the Type II-P (plateau) supernova SN 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio wavelengths. It is the least radio luminous and one of the least X-ray luminous SNe ever detected (except for the unusual and very close SN 1987A). The Chandra X-ray data indicate nonradiative interaction of SN ejecta with a power-law density profile (r [?] r-n, with n ~ 7) for a pre-SN wind with a low mass-loss rate of ~2 x 10-6 M yr-1 for a wind velocity of 10 km s-1, in agreement with radio mass-loss rate estimates. The Chandra data show an unexpected, temporary rise in the 0.4-2.0 keV X-ray flux at ~100 days after explosion. SN 1998S, at an age of more than 3 yr, is still bright in X-rays and is increasing in flux density at centimeter radio wavelengths. Spectral fits to the Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are overabundant with respect to solar values. We compare the observed elemental abundances and abundance ratios to theoretical calculations and find that our data are consistent with a progenitor mass of approximately 15-20 M if the heavy-element ejecta are radially mixed out to a high velocity. If the X-ray emission is from the reverse shock wave region, the supernova density profile must be moderately flat at a velocity ~104 km s-1, the shock front is nonradiative at the time of the observations, and the mass-loss rate is (1-2) x 10-4 M yr-1 for a presupernova wind velocity of 10 km s-1. This result is also supported by modeling of the radio emission, which implies that SN 1998S is surrounded by a clumpy or filamentary CSM established by a high mass-loss rate, ~2 x 10-4 M yr-1, from the presupernova star.

We report and interpret Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) long-slit observations of the optical and ultraviolet (1150-10270 A) emission line spectra of the rapidly brightening spot 1 on the equatorial ring of SN 1987A between 1997 September and 1999 October (days 3869-4606 after outburst). The emission is caused by radiative shocks created where the supernova blast wave strikes dense gas protruding inward from the equatorial ring. We measure and tabulate line identifications, fluxes, and, in some cases, line widths and shifts. We compute flux correction factors to account for substantial interstellar line absorption of several emission lines. Nebular analysis shows that optical emission lines come from a region of cool (Te [?] 104 K) and dense (ne [?] 106 cm-3) gas in the compressed photoionized layer behind the radiative shock. The observed line widths indicate that only shocks with shock velocities Vs < 250 km s-1 have become radiative, while line ratios indicate that much of the emission must have come from yet slower (Vs [?] 135 km s-1) shocks. Such slow shocks can be present only if the protrusion has atomic density n [?] 3 x 104 cm-3, somewhat higher than that of the circumstellar ring. We are able to fit the UV fluxes with an idealized radiative shock model consisting of two shocks (Vs = 135 and 250 km s-1). The observed UV flux increase with time can be explained by the increase in shock surface areas as the blast wave overtakes more of the protrusion. The observed flux ratios of optical to highly ionized UV lines are greater by a factor of ~2-3 than predictions from the radiative shock models, and we discuss the possible causes. We also present models for the observed Ha line widths and profiles, which suggest that a chaotic flow exists in the photoionized regions of these shocks. We discuss what can be learned with future observations of all the spots present on the equatorial ring.

Optical and ultraviolet observations of the Type IIn supernova SN 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor and is photoionized by X-rays from the shock. The intermediate component, which is dominated by newly processed oxygen, originates in a shell with velocity of 2500-5000 km s-1 and most likely comes from the ejecta. The hydrogen- and helium-dominated gas has a low ionization, a high density, and velocities that extend out to [?]10,000 km s-1. Strong signatures of Lya-pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~0.3 by number, and the nitrogen overabundance provide strong evidence for CNO-burning products. The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines. The Ha line profile shows strong evolution, with the red wing decreasing faster than the blue. Possible scenarios, involving either a clumpy circumstellar medium or an aspherical distribution of the surrounding gas, are discussed based on the line profiles and physical conditions. Finally, we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase.

For pulsars similar to the one in the Crab Nebula, most of the energy input to the surrounding wind nebula occurs on a timescale [?]103 yr; during this time, the nebula expands into freely expanding supernova ejecta. On a timescale ~104 yr, the interaction of the supernova with the surrounding medium drives a reverse shock front toward the center of the remnant, where it crushes the pulsar wind nebula (PWN). We have carried out one- and two-dimensional, two-fluid simulations of the crushing and reexpansion phases of a PWN. We show that (1) these phases are subject to Rayleigh-Taylor instabilities that result in the mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding interstellar medium give rise to asymmetries in the position of the PWN relative to the pulsar and explosion site. These effects are expected to be observable in the radio emission from evolved PWN because of the long lifetimes of radio-emitting electrons. The scenario can explain the chaotic and asymmetric appearance of the Vela X PWN relative to the Vela pulsar without recourse to a directed flow from the vicinity of the pulsar. The displacement of the radio nebulae in G327.1-1.1, MSH 15-56 (G326.3-1.8), G0.9+0.1, and W44 relative to the X-ray nebulae may be due to this mechanism. On timescales much greater than the nebular crushing time, the initial PWN may be mixed with thermal gas and become unobservable, so that even the radio emission is dominated by recently injected particles.

The simplest model of the afterglows of gamma-ray bursts (GRBs) envisions a spherical blast wave, with a power-law distribution of electron energy above some cutoff, running into a constant-density medium. A refinement involves a narrow jet, often invoked to explain the steep decline and/or steepening of light curves observed in some afterglows. The constant (ambient) density jet model has been applied to GRBs 991208 and 000301C, based to a large extent on radio observations. We show that, for these two sources, a spherical-wind model (with an r-2 density ambient medium) can fit the radio data as well as the jet model. The relatively steep decline and the fairly abrupt steepening of the R-band light curves of, respectively, GRB 991208 and GRB 000301C can be accounted for with a nonstandard, broken power-law distribution of electron energy. Our model predicts a slower late decline for the radio flux than does the jet model.

We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant-density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density profile. The interaction gives rise to a double-shocked structure susceptible to hydrodynamical instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. Provided the simulation is initiated early in the evolution, with a perturbation 1%, the instability reaches its fully developed nonlinear strength within 5 doubling times. The further nonlinear evolution does not depend on the initial conditions. Considering the small initial radii of Type Ia supernovae, they are likely to reach this fully developed phase. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the flatter part of the supernova density distribution. On the basis of observations of X-ray knots and the protrusion in the southeast outline of Tycho's supernova remnant, we include clumping in the ejecta because these features cannot be explained by instabilities growing from small perturbations. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging effect on the forward shock, the clump's initial density ratio to the surrounding ejecta must be at least 100 for the conditions in Tycho's remnant. The 56Ni bubble effect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a nonradioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-ray-emitting Si and S ejecta from the remnant center indicates that they were initially in clumps.

We present far-UV observations of the Crab Nebula and its pulsar made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Broad, blueshifted absorption arising in the nebula is seen in C IV?1550, reaching a blueward velocity of ~2500 km s-1. This can be interpreted as evidence for a fast outer shell surrounding the Crab Nebula, and we adopt a spherically symmetric model to constrain the properties of such a shell. From the line profile we find that the density appears to decrease outward in the shell. A likely lower limit to the shell mass is ~0.3 M?with an accompanying kinetic energy of ~1.5 × 1049ergs. A fast massive shell with 1051ergs cannot be excluded but is less likely if the density profile is much steeper than ?(R) R-4and the maximum velocity is 6000 km s-1. The observations cover the region 1140-1720 Å, which is further into the ultraviolet than has previously been obtained for the pulsar. With the time-tag mode of the spectrograph we obtain the pulse profile in this spectral regime. The profile is similar to that previously obtained by us in the near-UV, although the primary peak is marginally narrower. Together with the near-UV data, and new optical data from the Nordic Optical Telescope, our spectrum of the Crab pulsar covers the entire region from 1140 to 9250 Å. Dereddening the spectrum with a standard extinction curve we achieve a flat spectrum for the reddening parameters E(B-V) = 0.52, R= 3.1. This dereddened spectrum of the Crab pulsar can be fitted by a power law with spectral index ??= 0.11 ± 0.04. The main uncertainty in determining the spectral index is the amount and characteristics of the interstellar reddening, and we have investigated the dependence of ??on E(B-V) and R. In the extended emission covered by our 25'' × 05 slit in the far-UV, we detect C IV?1550 and He II?1640 emission lines from the Crab Nebula. Several interstellar absorption lines are detected along the line of sight to the pulsar. The Ly? absorption indicates a column density of (3.0 ± 0.5) × 1021cm-2of neutral hydrogen, which agrees well with our estimate of E(B-V) = 0.52 mag. Other lines show no evidence of severe depletion of metals in atomic gas.

Beginning with the ?-ray bursts GRB 970228 and GRB 970508, a standard model for the interpretation of gamma-ray burst (GRB) afterglows emerged involving synchrotron emission from a constant-energy blast wave expanding into a constant-density, "interstellar" medium. However, a massive star origin for GRBs implies a stellar wind environment, probably a Wolf-Rayet star, and we have previously suggested wind interaction models for the afterglows of GRBs 980326, 980519, and 980425/SN 1998bw. Here, we extend the theory of afterglows in winds, considering strong cooling phases, the transition to nonrelativistic motion, jets, and prompt reverse-shock emission. We find that, compared to the interstellar case, the optical prompt emission in the wind case could have a comparable magnitude but would die off faster. We examine the afterglows of other well-observed GRBs in the context of wind interaction models and find that GRBs 970228 and 970508 are likely wind interactors. The revision in the nonthermal afterglow emission from GRB 970228 caused by the recognition of late supernova emission favors wind interaction. The radio evolution of GRB 970508 provides especially strong evidence for wind interaction. For GRB 970508, the observations suggest a density that is compatible with that expected in a Wolf-Rayet star wind. Finally, observations of the afterglow evolution of GRBs 990123 and 990510 and the prompt optical emission from GRB 990123 favor interstellar interaction models, which would suggest compact star merger progenitors for these objects.

The locations of supernovae in the local stellar and gaseous environment in galaxies contain important clues to their progenitor stars. As part of a program to study the environments of supernovae using Hubble Space Telescope imaging data, we have examined the environment of the Type II-L supernova SN 1979C in NGC 4321 (M100). We place rigorous constraints on the mass of the SN progenitor, based on photometry of the stellar populations in its environment. The progenitor may have had an initial mass M [?] 17-18(+-3) M[?]. Moreover, 17 years after explosion we have recovered and measured the brightness of SN 1979C in several bands, e.g., m = 23.37 in F439W [[?]B; for comparison, mB(max) = 11.6].

The locations of supernovae in the local stellar and gaseous environment in galaxies contain important clues to their progenitor stars. As part of a program to study the environments of supernovae using Hubble Space Telescopeimaging data, we have examined the environment of the Type II-L supernova SN 1979C in NGC 4321 (M100). We place rigorous constraints on the mass of the SN progenitor, based on photometry of the stellar populations in its environment. The progenitor may have had an initial mass M ? 17-18(±3) M?. Moreover, 17 years after explosion we have recovered and measured the brightness of SN 1979C in several bands, e.g., m = 23.37 in F439W [?B; for comparison,mB(max) = 11.6].

Massive ([?]8 M) stars may end their lives in the molecular clouds in which they were born. O-type stars probably have sufficient photoionizing radiation and wind power to clear a region more than 15 pc in radius of molecular material. Early B stars (B1-B3 on the main sequence, or 8-12 M stars) are not capable of this and may interact directly with molecular gas when they explode. Molecular clouds are known to be clumpy, with dense molecular clumps occupying only a few percent of the volume. A supernova remnant then evolves primarily in the interclump medium, which has a density nH=5-25 H atoms cm-3. The remnant becomes radiative at a radius of ~6 pc, forming a shell that is magnetically supported. The structure of the shell can be described by a self-similar solution. When this shell interacts with the dense clumps, the molecular shock fronts are driven by a considerable overpressure compared to the pressure in the rest of the remnant. The expected range of clump sizes leads to a complex velocity distribution, with the possibility of molecular gas accelerated to a high velocity. Observations of the remnants W44 and IC 443 can be understood in this model. W44 has a shell expanding at ~150 km s-1 into a medium with density 4-5 cm-3. The shock emission expected in such a model is consistent with the observed Ha surface brightness and the [O I] 63 mm line luminosity. The clump interaction is seen in OH maser emission, which shows a magnetic field strength that is consistent with that expected in the model. IC 443 appears to be expanding at a lower velocity, 100 km s-1, into an interclump medium with a higher density, ~15 cm-3. The interaction of the radiative shell with molecular clumps can produce the molecular emission that is observed from IC 443. Both remnants are shell sources of radio synchrotron emission, which can be attributed to relativistic electrons in the cool radiative shell. If ambient cosmic-ray electrons are further accelerated by the shock front and by the postshock compression, the radio fluxes and the flat spectral indices of W44 and IC 443 can be explained. The energetic electrons are in a high-density shell and their bremsstrahlung emission can approximately produce the g-ray fluxes observed by EGRET. Molecular clouds have a significant uniform magnetic field component so that heat conduction is likely to be important in the hot interior and can explain the isothermal X-ray emission observed from the remnants.

SN 1998bw is an unusual Type Ic supernova that may be associated with the g-ray burst GRB 980425. We use a synchrotron self-absorption model for its radio emission to deduce that the synchrotron-emitting gas is expanding into a circumstellar medium of approximately r-2 density profile at a speed comparable to the speed of light. We assume that the efficiencies of production of relativistic electrons and magnetic field are constant through the evolution. The circumstellar density is consistent with that expected around the massive star core thought to be the progenitor of SN 1998bw. The explosion energy in material moving with velocity greater than 0.5c is ~1049 - (3 x 1050) ergs, with some preference for the high values. The rise in the radio light curves observed at days 20-40 is inferred to be the result of a rise in the energy of the blast wave by a factor of ~2.5. Interaction with a jump in the ambient density is not consistent with the observed evolution. We infer that the boost in energy is from a shell of matter from the explosion that catches up with the decelerating shock front. Both the high explosion energy and the nature of the energy input to the blast wave are difficult to reconcile with energy input from the shock-accelerated high-velocity ejecta from a supernova. The implication is that there is irregular energy input from a central engine, which is the type of model invoked for normal g-ray bursts. The link between SN 1998bw and GRB 980425 is thus strengthened.

The radio emission from supernovae has been modeled as synchrotron emission from the interaction between the supernova and a presupernova stellar wind. The emission shows a late power-law decline and early rise due to a low-frequency absorption process, which, in some cases (SN 1979C and SN 1980K), is well modeled as free-free absorption by the external wind. However, the flux rises in many radio supernovae (e.g., SN 1987A and Type Ib and Type Ic supernovae) have not been sufficiently well observed to define the absorption mechanism. The assumption of synchrotron self-absorption yields an approximate radius of the emission region at the time of peak flux. If another mechanism is dominant, the radius must be even larger. The large radii implied for SN 1987A and the observed Type Ib and Type Ic supernovae make external free-free absorption unlikely, given what else is known about these systems, so that synchrotron self-absorption is the probable absorption mechanism. Synchrotron self-absorption may play a role in the radio emission from SN 1981K (type unknown) and SN 1978K (Type IIn). Model supernova light curves with synchrotron self-absorption are presented. A combination of synchrotron self-absorption and external free-free absorption may be relevant to SN 1993J. The radii implied for the Type II supernovae SN 1979C and SN 1980K in the synchrotron self-absorption model are smaller than the radii expected for circumstellar interaction and are thus consistent with the free-free absorption mechanism.

AbstractSN 1987A has illuminated a great diversity of astrophysical processes – from neutrino emission during core collapse to the structure of the interstellar medium on a scale of hundreds of parsecs. Here I cover the evolution of SN 1987A from the outside in; the topics are interstellar light echoes, circumstellar light echoes, circumstellar shock waves, late emission and structure of the ejecta, and the effects of a central neutron star.

This paper reports on a Workshop on Supernova Remnants and the Physics of Strong Shock Waves hosted by North Carolina State University at Raleigh, North Carolina, September 16-18, 1993. The workshop brought together observers, shock theorists, cosmic-ray specialists, and simulators to address the role supernova remnants (SNRs) can play in furthering our understanding of the complex plasma physics associated with collisionless shocks and particle acceleration. Over fifty scientists presented papers on various aspects of supernova remnants. In lieu of a proceedings volume, we present here a synopsis of the workshop, in the form of brief summaries of each workshop session.

A newly formed neutron star in a supernova finds itself in a dense environment, in which the gravitational energy of accreting matter can be lost to neutrinos. For the conditions in SN 1987A, ≲0.1M⊙ may have fallen back onto the central neutron star on a timescale of hours after the explosion, after which the accretion rate is expected to drop sharply. Radiation is trapped in the flow until the mass accretion rate drops to 2×10−4M⊙ yr−1 at which point radiation can begin to escape from the shocked envelope at an Eddington limit luminosity. Between this neutrino limit and the Eddington limit, 3×10−8M⊙ yr−1, there are no steady, spherical solutions for neutron star accretion. SN 1987A should have reached the neutrino limit within a year of the explosion; the current lack of an Eddington luminosity can be attributed to black hole formation or to a clearing of the neutron star envelope. There is no evidence for newly formed neutron stars in supernovae. Radio supernovae, which were initially interpreted as pulsar activity, probably involve circumstellar interaction; SN 1993J shows especially good evidence for outer shock phenomena.

In the late stages of evolution, a star may make a transition from an extended, cool state where it has a slow, dense wind to a compact, hot state where it has a fast wind. The compact star can be a source of photoionizing radiation and may completely photoionize the dense wind. If the photoionized gas is isothermal, with sound speed a, and the fast wind power is steady, the subsequent expansion of the slow wind gas becomes self-similar. Starting from the center, the structure is characterized by a hot bubble of shocked fast wind, a region of shocked, photoionized slow wind, and a region of slow wind that is accelerated by the photoionization heating. In the limit of pure photoionization (previously studied by Meyer), a shock wave is generated with a minimum velocity of 2.56a. Even a weak fast wind tends to fill the central region of the flow. For conditions typical of planetary nebulae and a weak fast wind, the preshock flow is accelerated by roughly a above the initial slow wind velocity and the shock compression is roughly a factor of 2. As the inner wind power rises, the shock compression rises and the shocked slow wind region becomes thinner. Although the evolution in the actual stellar properties is likely to cause deviations from these solutions, they do give insight into the combined effects of stellar winds and photoionization.

We present ASCAand ROSATobservations of SN 1986J covering the period 1991 August to 1996 January. From observations with the ROSATHRI and PSPC, we find that the 0.5-2.5 keV flux decreased proportional to t-2during this period; the ASCAdata are consistent with this result and extend it to the 2-10 keV band. ASCAspectra from 1994 January and 1996 January are consistent with thermal emission from a solar metallicity plasma at an equilibrium temperature kT= 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae. These spectra also show a clear Fe K emission line at 6.7 keV with FWHM < 20,000 km s-1(90% confidence). This limit on the line width is consistent with the reverse shock model of Chevalier & Fransson but does not rule out the clumpy wind model of Chugai.

Type Ia supernovae (SNe) are presumed to arise from white dwarf progenitors, which may not appreciably modify their ambient medium. We study the interaction of the resulting supernova remnants with a constant density interstellar medium. Density profiles obtained from detailed explosion models of Type Ia SN explosions can be complex, but an exponential profile gives the best approximate representation of a set of profiles, and we emphasize this case. We describe the time evolution of dynamical quantities (such as radius, velocity, and expansion parameter) as a result of the interaction in terms of dimensionless variables and present the profiles of physical quantities. We compare our results to the power-law and constant ejecta density cases; a characteristic feature of the exponential case is that the shocked ejecta have a relatively constant temperature. The effect of a possible circumstellar wind region close to the supernova is to create a dense, cool shell near the contact discontinuity between the shocked ejecta and the surrounding medium. The complex density structure found in some supernova models persists in the shocked layer, giving rise to density and temperature variations. We apply our results to the two likely historical Type Ia SNe, SN 1006 and Tycho. The observed angular sizes and expansion rates are consistent with a distance of 1.95 +- 0.4 kpc and an ambient H density of 0.05-0.1 cm-3 for SN 1006. For Tycho's SNR, the results are not conclusive but indicate a distance around 2.3 kpc for an ambient density of 0.6-1.1 cm-3. In both cases, the low expansion rate limits the extent of a possible circumstellar wind region. The evidence for temperature variations in the ejecta of Tycho's remnant suggests that the supernova profile was more complex than an exponential profile and contained density inhomogeneities, or that there was early interaction with a circumstellar wind.

The Hubble Space Telescope 2000-8000 A spectrum of SN 1987A observed on 1995 January 7 (7.87 yr after the explosion) is dominated by Ha and UV lines, including Mg II-Mg I l2825 (equal to Ha in luminosity), Fe II UV 2 (two-thirds the intensity of Ha), Fe II UV 3 (one-half the intensity of Ha), and a 3730 A emission feature identified with a blend of [O II] l3727 and Fe I emission lines. [O I] l6300 and lines of [Ca II] and Na I, as well as some Fe II optical forbidden and permitted lines are present at visual wavelengths. Also present are a number of weak emission features, which are presumably metal lines produced by photon degradation as a result of reprocessing of UV radiation into metal lines. Modeling the Mg II-Mg I lines provides the velocity of the outer visible radius of the envelope, 9000 +- 500 km s-1 in the Mg II l2800 line, which is consistent with the earlier direct HST imaging at near-UV wavelengths. The UV/optical emission lines originate from the radioactive luminescence of the cool gas (T [?] 130-160 K). The metal lines reflect the instantaneous reprocessing of the energy deposited from 44Ti radioactive decays through collisions with fast electrons, while the Ha emission primarily comes from the recombination of previously ionized hydrogen. The overall luminosity of the Fe II emission lines, ~1035 ergs s-1, can be explained if the bulk of the positrons from a mass (1-2) x 10-4 M of 44Ti release their energy in the iron-rich material, which suggests the presence of a magnetic field B > 5 x 10-13 G prohibiting the escape of positrons into oxygen and hydrogen gas. The ionized fraction in the iron-rich material is small (0.2-0.3), and the total UV/optical emission from Fe I should be comparable to that from Fe II. Most of the 1036 ergs s-1 deposited by the 44Ti positrons should be emitted in the Fe II 26 mm line. The observed Ha luminosity decrease, by 5 orders of magnitude between the ages of 1 to 8 yr, is reproduced in a time-dependent model of ionization and cooling with the "standard" amount of radioactive nuclides. However, an additional source of energy at the present epoch with a deposition rate 30 ergs s-1 g-1 ([?] 1036 ergs s-1 in the whole envelope) is not ruled out. The present average temperature in the hydrogen envelope predicted by the time-dependent model is 130 K, which is lower than the value T [?] 350 K obtained from the observed Balmer continuum shape. However, the shape is affected by a possible contribution of metal lines to the Balmer continuum. The luminosity of the [O I] l6300 doublet is consistent with that expected for the deposited energy of g-rays from (1-2) x 10-4 M of 44Ti for an assumed 1.5-2 M of oxygen. If the oxygen mass does not exceed 2 M, 1 x 10-4 M is a lower limit for the amount of 44Ti in SN 1987A. The maximum fraction of the 44Ti positron energy deposited into oxygen-rich material does not exceed 5%, which is consistent with positron trapping in Fe-rich material. The [O I] l6300 line intensity rules out the presence of a central source of g-radiation (hn > 100 keV) with a luminosity Lg > 4 x 1036 ergs s-1.

IT is commonly assumed that a simple compression of the magnetic field occurs in interstellar shock waves. Recent space observations of the Earth's bow shock show that turbulent amplification of the magnetic field can occur in a collisionless shock. I show here that radio observations of Tycho's supernova remnant indicate the presence of a shock wave with such magnetic field amplification.

The supernova SN1986j was first discovered as a radio source in August 19861, although there were a number of pre-discovery observations. Here I show that the circumstellar interaction model for radio supernovae provides an adequate fit to the existing data, implies a time of explosion of about January 1983, and provides a guide for the future evolution of the radio source. A shell-like source structure is expected. The model together with very-long-baseline interferometry (VLBI) observations of the radio source, implies a shock velocity of ∼104km s−1and a pre-supernova mass loss rate of ∼10−4M⊙yr−1for a wind velocity of l0 kms−1. A search for an infrared dust echo is warranted and an X-ray luminosity of ∼1040erg s−1is expected. The time behaviour of the optical flux and its high luminosity suggest that a central pulsar nebula is the energy source. The required properties are comparable to those expected in the early phases of the Crab nebula. The radiated energy from the pulsar nebula is absorbed by expanding, helium-rich material extending out to a velocity of ∼700 km s−1or a radius of 8 x 1015cm on September 1986. The radius of the pulsar nebula at that time is ∼5x1015cm.