Over the past several decades, we've built a large number of new observatories, developed improved adaptive optics for those on earth, and sent others into space. The net result is that we now have a growing collection of what could be considered time lapse observations of various parts of the sky. This paid off recently, when a history of observations helped us identify planets orbiting distant stars based on their movements over several years. In the past week, it has paid off again, as scientists have managed to produce before-and-after images of some of the stars that spawned supernovae, giving theorists a chance to test their models that describe how stars sometimes meet an explosive end.
The first of these reports appeared last week in Science Express, where Science magazine gives its readers an advanced peek at upcoming publications. In that case, a collaboration of researchers in Denmark and California identify two different precursors of Type II supernovae, with the results suggesting the theorists were on the money.
The first supernova they describe is 2003gd. In this case, they had pre-explosion images from both the Hubble Space Telescope and Hawaii's Gemini Observatory that predate the explosion; both show a red supergiant that is about eight times more massive than the sun. The authors decided that five years was sufficient time for the explosion to fade sufficiently, so they directed the Gemini to scan the same area of space; the red supergiant was no longer present.
Supernova 1993J was significantly more complex, following the pattern of a Type IIb explosion, which occurs in binary systems where the companion star has stolen significant amounts of hydrogen from the supernova progenitor. In this case, the progenitor of the supernova had been identified previously; in this case, the authors were able to spot the star that used to be its companion.
In both these cases, the observations line up well with theory. In the case of 2003gd, the type and mass of the star appears to match nicely with previous predictions; for 1993J, the fact that the progenitor was in a binary system lines up well with theory. Any confidence astronomers might be developing, however, probably took a hit from a publication that was released by Nature on Sunday. In that paper, the authors (another international collaboration, this one from authors in Israel and San Diego) identify the progenitor of SN2005gl as being too young to explode.
Researchers had previously proposed that an extremely luminous star called NGC266_LBV 1 had produced SN2005gl (LBV standing for luminous blue variable), but this didn't sit well with many people, as luminous blue variables should not be exploding at this stage in their stellar evolution. Whether they should or not, this one did—researchers pointed the Hubble at the site of the explosion, and NGC266_LBV 1 is no longer there. Their imaging of the remnants confirms that the progenitor star was quite heavy, and hadn't ejected much mass within the year or so before it exploded.
This is a bad match for theory on at least two counts. According to existing models, stars of this mass are expected to both end their luminous blue variable phase and eject most of their hydrogen before going boom. Neither of these seem to have happened here, and the authors clearly state that the models are going to have to undergo significant revision. They suggest keeping an eye on the remains of 2003gl, as further observations may provide a greater perspective on what sort of material was ejected by the star before its demise.
Given that this only brings our total of supernova progenitors up to four, who knows what other surprises might be awaiting the theorists.