Free Astronomy Magazine November-December 2014

SUPERNOVAE the Hubble Space Telescope (HST) and the Keck-1 telescope in Hawaii, but in no circum- stance researchers were able to attribute it with certainty to the companion star. This also because the star field in which SN 1993J appeared is rather populated and hard to resolve into individual stars, with the result that the spectroscopically analyzed light is in reality the sum of multiple sources. One of the early studies, led by Justyn Maund in 2004, had determined an upper limit to the contribution of the field stars closer to the supernova’s position, and concluded that there was still an ultraviolet residue possibly attributable to a not directly visible source, compatible with the flux emitted by a blue giant star of spectral type B (surface tempe- rature: 10,000 to 30,000 kelvin). The conclusions drawn by Maund and col- leagues remained unconfirmed for a decade when, just this summer, a new study based on observations made since 2012 with the Cosmic Origins Spectrograph (COS) and the Wide-Field Camera 3 of the HST, supplement- ed by observations made from the ground, added new details to the scenario, confirm- ing the validity of the leading model for Type II b supernovae. The authors of the new study are a dozen researchers from the USA, Sweden and India, including the coor- dinators Ori D. Fox and Alexei V. Filippenko (both of the University of California, Berke- ley) −the latter is one of the most assidu- ous students of SN 1993J. The team was able to measure the properties of the ultraviolet spectrum attributable to the companion star and found that it is actually consistent with the contribution of a blue giant of Type B2, and that its brightness, tempera- ture and gravity are indicative of a mass equivalent to about 20 times that of the Sun, hence comparable to that of the progenitor prior to the transfer of hydrogen and to the explosion. Everything, as already mentioned, in good agreement with the theoret- ical predictions. For the first time astronomers were able to determine with sufficient accuracy the proper- ties of the companion star of the progenitor of a Type II b supernova and reconstruct the conditions of the system prior to the explosion. n B ottom left, a sequence ob- tained in radio waves by the Very Long Base- line Array, show- ing the first 7 years of evolution of the remnant of SN 1993J. [NRAO/ AUI and N. Bartel, M. Bietenholz, M. Rupen, et al.] Left, an image that demon- strates how the field studied by the team led by Fox and Filippen- ko ─ outlined by the dashed circle− was contaminated by the signal of numerous sources different than those from the SN, making diffi- cult the task of extracting the sig- nal coming from the progenitor’s companion. [Ori D. Fox et al.]