Supernovae
(C. Fransson, C. Kozma, P. Lundqvist, T. Nymark, J. Sollerman)

The supernova group at Stockholm Observatory is heavily involved in various observational projects using space observatories like HST, Chandra, FUSE and XMM. Data from IUE and ISO are also analyzed, and a substantial amount of ground-based data have been gathered using telescopes mainly at ESO, La Silla and ESO/VLT, Paranal. Much of the group's efforts have been to model these and other observations. The research has during 1999 focused mainly on SN 1987A, supernovae showing circumstellar emission, and on pulsars.

Radiation-hydrodynamical multi-group NLTE simulations of SN 1987A have been performed to model the shock breakout of the supernova, and the evolution of the emission from the ejecta during the subsequent $3-4$ months. The best models have energies in the range $E = (1.10 \pm 0.25) \times 10^{51}$ ergs. The maximum velocity of the ejecta ($\sim c/10$) in the models is similar to that indicated by early IUE spectra. Detailed NLTE radiation transfer has been added to make a detailed spectral analysis.

To prepare for HST/STIS and FUSE observations, the expected emission in narrow lines from the H II region inside the inner ring around the supernova 1987A was calculated. The strongest lines are N V $\lambda$1240 and O VI $\lambda$1034. A spin-off from the analysis of the ring itself is that one can derive an upper limit of $\sim 55$ kpc for the distance to the Large Magellanic Cloud.

HST images of a new unresolved source of emission near the inner circumstellar ring of SN 1987A have been obtained by the SINS team. The data show a brightening spot on the ring, which marks the onset of the collision between the SN 1987A ejecta and the dense gas in the ring.

Data obtained in 1998 with the ISO/SWS and ISO/LWS for SN 1987A have been reduced and analyzed. In particular, the upper limit on the flux of [Fe II] 25.99$\mu{\rm m}$ on day 3999 is $\sim 1.4$ Jy, which together with the group's theoretical models gives an upper limit of the mass of ejected $^{44}$Ti which is $1.5\times10^{-4}$ solar masses.

The temperature, ionization and line emission of the ejecta of SN 1987A has been modeled in its nebular phase, using a time-dependent model. It is found that freeze-out of the recombination is important in the hydrogen and helium zones. An IR-catastrophe is seen in the metal lines as a transition from thermal to non-thermal excitation, most clearly in the [O I]  $\lambda\lambda$6300, 6364 lines, and from the evolution of the [Fe II] lines. The distribution of hydrogen, helium, and oxygen is determined from the line profiles. Photometric UBVRI observations from day $\sim 3600$ have also been added to refine the model and to estimate the amount of ejected $^{44}$Ti.

Supernovae 1979C and 1980K have been studied with HST and from the ground, and these Type II linear supernovae (SNe II-L) spectra show general agreement with the lines expected in a circumstellar interaction model by Chevalier & Fransson. For SN 1979C WFPC2 imaging has been used to place constraints on the mass of the SN progenitor, based on photometry of the stellar populations in its environment. For the Type Ic SN 1994I synthetic spectra are compared to observed photospheric-phase spectra.

The members of the group have also studied the Crab pulsar. The pulsar's far-UV spectrum ($1140-1720$ Å) was studied with HST/STIS in time-tag mode which provides 125 $\mu$s time resolution. These data have been added to previously obtained near-UV data (see the Report from 1997-1998), and data obtained at NOT. The dereddened spectrum is flat with spectral index $\alpha_{\nu} = 0.10$. Absorption from the Crab nebula is seen in C IV $\lambda1550$, and modeling this absorption was initiated.

PSR 1706-44 was studied using the science verification observations with the test camera on VLT/UT1. (This made the Supernova group the first group in Sweden to analyze and publish VLT data.) The excellent seeing at Paranal made it possible to put a 3$\sigma$ limit on the optical flux from the pulsar at $V = 25.5$ if the pulsar is not close to a projected bright star.

In preparation for NGST and wide-field VLT instruments, explosion models coupled with radiation transfer of core-collapse supernovae have been made to estimate the possibility of detecting SNe II at different redshifts. Another study of this sort, using template supernova light curves, and including dust absorption is presented under Observational Cosmology.