Type Ia single degenerate survivors must be overluminous

I noticed a paper on the arXiv today with exactly this title (well, except that I removed the superfluous capitalisation of words), that is due to be published in the Astrophysical Journal. The abstract of the paper says:

In the single-degenerate (SD) channel of a Type Ia supernovae (SN Ia) explosion, a main-sequence (MS) donor star survives the explosion but it is stripped of mass and shock heated. An essentially unavoidable consequence of mass loss during the explosion is that the companion must have an overextended envelope after the explosion. While this has been noted previously, it has not been strongly emphasized as an inevitable consequence. We calculate the future evolution of the companion by injecting $2$-$6\times10^{47}$ ergs into the stellar evolution model of a $1\,M_\odot$ donor star based on the post-explosion progenitors seen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the envelope, the companion must become significantly more luminous ($10$-$10^3\, L_\odot$) for a long period of time ($10^3$-$10^4$ years). The lack of such a luminous "leftover" star in the LMC supernova remnant SNR 0609-67.5 provides another piece of evidence against the SD scenario. We also show that none of the stars proposed as the survivors of the Tycho supernova, including Tycho G, could plausibly be the donor star. Additionally, luminous donors closer than $\sim10$ Mpc should be observable with the Hubble Space Telescope starting $\sim2$ years post-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe. Thus, the SD channel is already ruled out for at least two nearby SNe Ia and can be easily tested for a number of additional ones. We also discuss similar implications for the companions of core-collapse SNe.

Now, technical scientific papers are full of jargon and maybe the meaning of that paragraph isn't immediately clear to everyone (there's an accompanying Youtube video purporting to explain the content of the paper, but I didn't think it quite achieved that aim!). But I think this result is really quite interesting and probably important in a broader cosmological sense.

The Structure of a Scientific Revolution

A few days ago, Peter Coles posted an interesting comment about skepticism on his blog. In it, he showed the first graphs with data showing evidence for the acceleration of the expansion of the Universe from observations of distant supernovae. His point was that though the plot isn't much to look at, the statistical evidence for cosmic acceleration from supernovae data was quickly established and very soon became very widely believed.

Coles goes on to make some points about global warming skepticism, and most of the long discussion in the comments that follow is concerned with this. Which is unfortunate, as none of that discussion provides an exception to the general rule that arguments about global warming are a complete waste of time, because people have already decided what they want to believe before they start, and are subsequently impervious to any evidence, logic or persuasion. So although you may want to read the discussion there, let's not talk about the climate here.

Instead, I want to stick with the cosmology and pick up on a question that is more interesting. Let's take a quick look at the picture of the supernovae data from the High-Z Supernova Search Team and the Supernova Cosmology Project:

Evidence for dark energy from the Hubble diagram of supernovae. The top panel shows the distance modulus of the supernovae as a function of redshift, with three different theoretical model curves. The bottom panel shows the residual distance modulus relative to that in the model with the dotted curve (a Milne universe).

As Coles points out, most people who are not cosmologists (and some who are) look at these data and find them deeply unconvincing. The error bars are large. The points are widely scattered, and at least both the top two curves seem to give reasonably good fits. On top of that, there appear to be two distinct groups of supernovae: one group rather close to us, and the other much further away at higher redshifts (and therefore observed as they were a long time ago). Any inferences we draw from relative calibrations of these two groups rest on the assumption that deep down they are the same kind of objects, and that the Universe they existed in remained the same kind of Universe.

So why exactly did this cause such a revolution in the field? How did so many physicists become convinced, almost overnight, of the existence of this mysterious dark energy, that constitutes most of the energy of the Universe, but that we didn't understand then and don't understand now?

What makes a supernova?

At the beginning of the month we had a short cosmology meeting here in Bielefeld, known as Kosmologietag (though in fact it was spread over two days). One of the main talks was given by Fritz Röpke, of the Universität Würzburg. Fritz spoke about a very important sub-class of supernova explosions, known as Type Ia, and our current understanding of what causes them to explode in the first place.

Although I have worked in the past with supernovae data in a cosmological context, much of what he said during his talk was new to me, and also was very interesting. Supernovae are particularly amazing events, after all! The slides of his talk are available here though unfortunately some of the movies he showed will not work (incidentally, all the other talks from the meeting — including my own — can be found here, in case anyone is interested).

This post is somewhat experimental because I'm writing about a subject I don't have much expertise in, and also because I will try to convey some of the new things I learned from Fritz's research-level talk while assuming little or no prior knowledge about the subject among my readers. So please bear with me. I suppose I'd better start by briefly explaining what a supernova is, and what the 'Type Ia' business refers to.

Supernovae are explosions that occur at the end of the life cycle of some types of massive stars, that cause the exploding star to increase rapidly in brightness for a short period of time. The first thing everyone should know about supernovae is that they are truly catastrophic explosions, the most dramatic events that occur in the Universe today, which cause a single star to become as bright as an entire galaxy containing about a hundred billion stars. Indeed when the star whose remnants are now known as the Crab Nebula went supernova in 1054 AD, it was bright enough to be seen in the daytime, and was recorded by Chinese astronomers of the time as a 'guest star'.

Here's a picture from the Hubble Space Telescope of Supernova 1994D (this means it was the fourth supernova observed in 1994) in Galaxy NGC 4526:

The second thing you should know is that all the metal in the computer that you are reading this on was formed in such a gigantic supernova several billion years ago. We, and everything around us, are really composed of the debris of one of these stellar explosions. Now that should be enough to capture your interest!