We present optical photometry and spectroscopy of the Type IIP supernova SN 2004et that occurred in the nearby galaxy NGC 6946. The observations span a time range of 8-541 d after explosion. The late time bolometric luminosity and the Hα luminosity in the nebular phase indicate that 0.06 ± 0.02 M of 56 Ni was synthesized during the explosion. The plateau luminosity, its duration and the expansion velocity of the supernova at the middle of the plateau indicate an explosion energy of E exp = 1.20 +0.38 −0.30 × 10 51 erg. The late time light curve and the evolution of the [O I] and Hα emission-line profiles indicate the possibility of an early dust formation in the supernova ejecta. The luminosity of [O I] 6300, 6364 Å doublet, before the dust formation phase, is found to be comparable to that of SN 1987A at similar epochs, implying an oxygen mass in the range 1.5-2 M , and a main-sequence mass of 20 M for the progenitor.

We present optical spectra of 45 intermediate-mass Herbig Ae/ Be stars. Together with the multiepoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using preYmain-sequence evolutionary tracks, we have studied the evolution of emission-line activity in them. We find that, on average, the H emission line strength decreases with increasing stellar age in Herbig Ae/ Be stars, indicating that the accretion activity gradually declines during the preYmain-sequence phase. This would hint at a relatively longlived (a few Myr) process being responsible for the cessation of accretion in Herbig Ae/ Be stars. We also find that the accretion activity in these stars drops substantially by $3 Myr. This is comparable to the timescale in which most intermediate-mass stars are thought to lose their inner disks, suggesting that inner disks in intermediate-mass stars are dissipated rapidly after the accretion activity has fallen below a certain level. We further find a relatively tight correlation between strength of the emission line and near-infrared excess due to inner disks in Herbig Ae/ Be stars, indicating that the disks around Herbig Ae/ Be stars cannot be entirely passive. We suggest that this correlation can be understood within the framework of the puffed-up inner rim disk models if the radiation from the accretion shock is also responsible for the disk heating.

On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.