I've posted before but I really do enjoy reading the summation of articles offered by the site Astrobites. They have had two articles this week that follow up on official papers published in Nature this week about SN 2011fe.
Here is my sketch of SN 2011fe (see the bright star next to the core, go directly across to 3 stars and the very bright one in the middle is SN 2011fe).
The Astrobites articles by Elizabeth Lovegrove from December 20th, 2011 (found here) and the follow up or Part II is by Michelle Kislak (found here) are well worth the entire read, especially if you viewed the supernova or followed it online.
The crux of the articles sum up a major paper in Nature by a well known team of astronomers and scientists. The first article includes some wonderful tidbets of information such as when the Palomar Transit Factory captured the initial supernova, which was at 0359 UTC on August 24th or 2059 MDT on August 23rd for those living in the Mountain Time Zone in the USA. After retooling the Swift Telescope that is in orbit and the robotic Liverpool Telescope in the Canary Islands, within 20 hours these two instruments confirmed this as a Type Ia supernova. The reason for this is no hydrogen lines were found in the light spectrum while strong silicon lines were detected. A Type II event would have strong hydrogen lines since the star would be relative young in terms of a star and from blowing up its shells.
So the first summation continues by showing how the team found the initial radius of the exploding object by "Measuring the initial radius of the progenitor requires knowing three things: the early luminosity, the early velocity, and the time since explosion." The paper then shows how this was done. The fact that I find fascinating here is that using a "simple power model" they could "fix the detonation time at 2011 August 23, 16:29 +/- 20 min UTC (modulo, of course, the 20.8 million year light travel time from M101), meaning that the initial observations caught the supernova just 11 hours after it exploded!" This allowed a very accurate measurement of the luminosity which allowed them to determine size of the progenitor. The size of the progenitor was smaller than that of our Sun which means it had to be a white dwarf. This combined with a remnant of carbon and oxygen means that this is most likely a carbon-oxygen white dwarf.
The first summation concludes by sharing that when the white-dwarf reached the Chandrasekhar limit and exploded (by drawing material off its companion star) the out-rushing ejecta from this supernova explosion would have struck the companion star and a shock-heat a portion of the companion, producing extra luminosity. If this white-dwarf had had a red giant companion, the shock wave would have caused a higher luminosity in the Red Giant's extended shell. This wasn't observed or measured so there is a possibility that the companion was a main sequence star "if viewed farther than 40 degrees off the symmetry axis of the system."
The second summation deals mainly with Figure 2 which in the link above shows the magnitude limit of SN 2011fe and compares it with the magnitude of other stars that could serve as a companion. From the measurements in the chart, it is easily to see the companion had to be 3.5 solar masses or smaller in order for the data to work. This rules out a Red Giant as the companion.
The article explains the difference between a double digit degenerate model, where two objects, held together by degenerate pressure ie two white dwarfs lose gravational radiation until their orbits come closer and closer and they slam into each other causing the supernova explosion. The paper cannot rule this out as a possible scenario based on the data. In a single-degenerate model "a white dwarf accretes mass from a donor such as a main sequence star, a subgiant, a red giant, or a helium star." The research was to try and either identify which it could be or which it couldn't be.
Based on the data gathered and analyze, it is confirmed that a Red Giant was not the companion. This means that the white-dwarf in this case had to be either a double digit degenerate model (two white-dwarfs that collided together) or the companion had to be a main sequence star, sub-giant or a helium star.
Anyway, feel free to skip my comments and read the well written Astrobites. I found the information fascinating and added to my overall experience to observing this object, while increasing my own knowledge and understanding.