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We explore the possibility that ROSAT observations of thermal X‐ray emission from shell‐type supernova remnants (SNRs) can be used to reasonably constrain the distances to both Galactic and extragalactic remnants. The only major assumption is that the initial kinetic energy (eo) of a supernova explosion is eo=1051 ergs. Fortunately, the derived distance is only weakly dependent on this assumption (D∝eo0.4). We evaluate this technique by applying it to SNRs with relatively well known distances, and our initial results indicate that the distances so obtained are relatively accurate. It therefore appears that the ROSAT all sky survey can be used to establish good quality distances to a large number of extended, shell‐type SNRs. If confirmed by further tests, these data could greatly alleviate the long standing problem of unknown or very poorly established distance estimates to SNRs.

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at the wavelengths of 3.6 and 6 cm in the period 6 to 42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models., to appear in Astrophysical Journal Letters 10 pages of text, three figures (two of them in postscript format, one in gif format)

The radio emission from supernovae provides a direct probe of a supernova’s circumstellar environment, which presumably was established by mass-loss episodes in the late stages of the progenitor’s presupernova evolution. The observed synchrotron emission is generated by the SN shock interacting with the relatively high-density circumstellar medium which has been fully ionized and heated by the initial UV/X-ray flash. The study of radio supernovae therefore provides many clues to and constraints on stellar evolution. We will present the recent results on several cases, including SN 1980K, whose recent abrupt decline provides us with a stringent constraint on the progenitor’s initial mass; SN 1993J, for which the profile of the wind matter supports the picture of the progenitor’s evolution in an interacting binary system; and SN 1979C, where a clear change in presupernova mass-loss rate occurred about 104 years before explosion. Other examples, such as SNe 19941 and 1996cb, will also be discussed.

We have obtained 18 moderate-resolution (~2.5 Angstroms) spectra of the Galactic X-ray Nova Ophiuchi 1977 in quiescence with the first of the W. M. Keck 10 m telescopes. Cross-correlation with the spectra of late-type dwarfs (especially K7) yields excellent radial velocities for the companion star. The orbital period (P) is 0.5229 ± 0.0044 d, and the semiamplitude (Kc) is 447.6 ± 3.9 km s-1; these are consistent with the values previously derived by Remillard et al. (1996) from CTIO photometry (P = 0.5213 ± 0.0013 d) and spectroscopy (Kc = 420 ± 30 km s-1). Our derived mass function, f(Mx) = PKc3/2 pi G, is 4.86 ± 0.13 Modot (cf. 4.0 ± 0.8, Remillard et al.), the third highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (~3.2 Modot), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on an inclination i = 70circ (from ellipsoidal modulations) and Mc = 0.3-0.6 Modot are Mx = 6.4-6.9 Modot.