Ia Supernovae : colliding white dwarfs, not accreting white dwarfs!

My world has been rocked.

A paper by Gilfanov and Bogdan, and an accompanying press release yesterday from the Chandra Space Telescope, presents results that suggest that the progenitors of Type Ia supernovae are not what most of us have assumed all along. Before I go any further, I want to make it clear that this doesn’t call into question any of the results that have used Type Ia supernovae as standard candles, including the observation that the expansion of the Universe is accelerating. Those results don’t depend on what Ia supernovae are; they depend on the empirically observed fact that the luminosities of the explosions are remarkably consistent. Of course, if we really want to push those observations further, we need to understand the objects better, so that we can deal with outliers and exceptions, but the core results remain robust.

But, of course, as with everything in the Universe, we want to understand it.

I should also say that the model of two colliding white dwarfs— the “double degenerate” model in jargon terms— isn’t completely new. Back when I was with the Supernova Cosmology Project, we admitted that we didn’t really know how these supernovae started, and admitted the possibility of a double-degenerate source, even though I (and I think most other people) personally didn’t expect that to be a substantial contributor to Type Ia supernovae.

The old model for a Type Ia Supernova Progenitor

The traditional model we always used is show in the picture to the right. Indeed, when I’ve given talks about supernovae, or talks about Supernova Cosmology, I’ve shown this picture (or, in Second Life, the 3D object that I made) as our model for where these supernovae come from. Everybody showed a version of this picture. The idea was that you had a white dwarf star with a companion red giant (or other) star. The orbit of the two was close enough that its gravity could pull some of gas off of the outside of the companion star. That gas would then swirl around the white dwarf in an accretion disk, building up on the white dwarf. The white dwarf would eventually reach a critical mass of about 1.4 times the mass of the sun, and blow itself away in a runaway thermonuclear explosion that we would observe as a Type Ia supernova.

This seemed very reasonable. It’s not too hard to imagine how you’d get these systems. If you have a binary star system where the two stars aren’t the same mass— and we observe lots of those— one would “die” first, leaving behind a white dwarf. The second star would eventually reach its giant stage, puffing up and leaving its outer layers not terribly well gravitationally bound, and the white dwarf would have a relatively easy time pulling mass from it. This is still the model we have for regular novae. (With a regular nova, just the outer layers of the white dwarf explode in a nuclear fusion explosion, not the whole thing.)

But… modeling of these systems predict that they would emit a substantial fraction of X-rays. I don’t know exactly the details, but in brief, the center of the accretion disk gets extremely hot, and thus emits some X-rays. (Doubtless magnetic fields somehow get involved as well, but as I said, I don’t know the details.) What Chandra observed, however, was that there is not enough X-rays coming from galaxies for there to be enough of these accreting systems to explain the Type~Ia supernovae we see.

This leaves us with the conclusion that most Type~Ia supernovae must come from collisions of white dwarfs. (Or, possibly, from something else that we haven’t thought of yet.) And, to me, this is surprising, because I would have expected those collisions to be far rarer and harder to produce than the accreting systems I always thought that Type Ia supernovae came from.

I look forward to seeing more developments as the astrophysics community receives, thinks about, responds to, and does further investigation of this result.

Internet Astrophysics Celebs Carroll & Gay to Speak in Second Life

For those of you who haven’t been following my blog or watching my twitter or facebook updates, you may not realize that there are regular public-outreach astronomy talks in Second Life. These are designed for the general public, and are open to anybody. Indeed, because a Second Life account is free, they really are open to anybody. These talks are on Saturdays at 10 AM pacific time / 1 PM eastern time, are sponsored by the Meta-Institute of Computational Astrophysics, and are held in MICA Large Amphitheater of the StellaNova region in Second Life.

This series has historically been called “Dr. Knop Talks Astronomy”, because through the end of 2009 I have given the lion’s share of the talks. Indeed, I was thinking about it the other day. The last semester before I left Vanderbilt, I was contacted by the people who make the CD series called “The Great Courses”. These are CDs with lectures from university professors on their topics of expertise. Based on some podcasts of mine that were online from several years ago, they contacted me and asked me to come audition. The idea was that if I passed it, they might well produce a course from me. However, when I left Vanderbilt, and could no longer call myself a “Professor of Physics and Astronomy”, they were no longer interested in me.

It occurs to me that given the number of talks I’ve presented so far, those who have come to most or all of my talks have effective received the equivalent of one of these CD series on “hot topics in astronomy.” Indeed, it was more than that, for not only were there visuals (i.e. my slides), but it was interactive. You could ask questions, and also discuss the talk with the others present.

If you look at our schedule for this coming semester, you’ll see that we’re starting to mix things up some more. I’m still giving more of the talks than any other single person, but we’ve got a larger range of guest speakers. You can see who’s coming up soon by looking at the Upcoming Public Events page.

Last week, we had Nobel Prize winner John Mather speaking about the history of the whole Universe, and of hopes and plans for the upcoming Webb Space Telescope. In the next two weeks, we’ll have two “Internet Celebrities” talking. Sean Carroll, one of the authors of the popular physics blog Cosmic Variance, will be talking about his recently released book on the nature of time, From Eternity to Here. Then, Pamela Gay, host of 365 Days of Astronomy and a member of Astronomy Cast, will be speaking in two weeks.

Drop by and hear us. These talks can be very good… and, while you shouldn’t believe me if I tell you my own talks are good, others have told me that they are. The immersive environment of virtual worlds means that you really feel you are there— John Mather was commenting on this just last week after giving his talk.