Free Astronomy Magazine November-December 2014

SUPERNOVAE In acquiring a large amount of hydrogen, the companion star’s mass and temperature inevitably grow, with the result that the efficiency with which it transforms the sur- face hydrogen into helium increases, becom- ing a so-called “super-hot helium star”. An alternative mechanism that can make the more massive stars (over 30 solar masses) lose part of their hydrogen reserves with- out the intervention of a com- panion star is that involving powerful stellar winds, capable of propagating from the surfaces and drag along the gaseous out- er envelope, which then ends up lost in space. The models though indicate that only a small per- centage of these stars can lose in that way enough mass to be- come almost devoid of hydrogen (from 0.01 to 0.5 solar masses) at the time of the explosion. Con- sequently, the model foreseeing the existence of a companion star is clearly preferable. To verify how things really are, it would be necessary to observe one of those companion stars (or, at any rate, indirectly dem- onstrate its presence) at the point in which a Type II b supernova ex- ploded. But this is easier said than done, since that kind of research must necessarily be conducted in other galaxies −given that the last supernova visible in our galaxy blazed out in 1604 ─ and at dis- tances that even in the most fa- vourable cases are anyhow con- siderable. And that's not all. In fact, in order for the research to have sufficient probability of suc- cess it is useful to know the essen- tial characteristics of the progen- itors, which means having the pre- explosion measurements of their light (i.e. the spectrophotometric data of individual stars in other galaxies). To date, are only 4 the Type II b su- pernovae for which progenitors have been directly identified through im- ages acquired before their explosion, name- ly: SN 1993J, SN 2008ax, SN 2011dh and SN 2013df. Since several years must inevitably pass before the glow of the supernova and of its remnant become sufficiently faded to A tiny portion of the spiral arm of M81 in which the rem- nant of SN 1993J is expanding, visi- ble in the centre of the image. In- side that cocoon of light astrono- mers have recog- nized the pres- ence of the com- panion star of the progenitor. [NASA, ESA, and O. Fox (Univer- sity of California, Berkeley) et al.] R ight, an image of SN 1993J at the time of the ex- plosion, obtained in the X-rays by the Advanced Sa- tellite for Cosmo- logy and Astro- physics (ASCA). The high spatial resolution has al- lowed to distin- guish the super- nova contribution from that of other sources. [JAXA]