Caltech Fireworks day 1: Mostly Thermonuclear

The first day of the Fireworks Meeting was spent reviewing and discussing what is known, and what we hope to find out, about thermonuclear supernovae and related events. By thermonuclear we mean events in which the dominant energy source for the explosion is provided by the nuclear fusion of atoms within some astrophysical object, which we call the progenitor. A supernova is an event in which enough energy is liberated to completely unbind the star, ejecting all of its mass out into the surrounding space. There can also be thermonuclear events which don’t quite unbind the star, such as classical novae, which are thought to result (for example) from the explosive nuclear burning of a shell of helium on the surface of a white dwarf — a very small (size of the Earth), dense (mass of the Sun) remnant left over from the gradual evolution of a normal star much like our own Sun.

Progenitors and explosion mechanisms of type Ia supernovae

My own primary interest, personally, is of course in type Ia supernovae (SNe Ia). The generally accepted theory explaining these supernovae is that they result from the complete thermonuclear incineration of a white dwarf, converting a star made mainly of carbon and oxygen to fast-moving (10,000 km/s) ejecta made mostly of elements ranging from silicon to nickel on the periodic table. These events, as I’ve mentioned before, are interesting to particle physicists because they can be used to accurately measure the rate of acceleration of the universe’s expansion and thereby probe the nature of gravity on large scales. However, it’s important to remember that “SN Ia” is an observational designation, since the events are identified by looking for evidence of the presence or absence of particular elements. SNe Ia are simply bright events which show no evidence for hydrogen or helium, but plenty of silicon.

Many of the day’s talks reminded us that while there are several competing ideas about how to blow up white dwarfs to produce something that looks like a SN Ia, none of them are completely satisfactory in explaining the data. It seems clear that a white dwarf sitting around by itself won’t blow up on its own; it’ll just radiate its energy away gradually, slowly cooling down over billions of years. The influence of some other star is needed to trigger the explosion.

  • If that star is a normal, “non-degenerate” star, the explosion might be triggered when the white dwarf sucks material off the surface of this star, eventually collecting enough to exceed a critical mass necessary to trigger the explosion. You might expect to see evidence of part of its hydrogen or helium atmosphere being swept up in the explosion; people have looked for it and haven’t found it. Moreover, the number of binary systems like this so far identified in the Milky Way, and the number of such systems predicted to exist in other galaxies, are far too few to explain the number of SNe Ia we see. Finally, the burning proceeds by propagation of a flame through the star, running into fuel and leaving ash behind it. To reproduce the distribution of elements we see — a layered composition with silicon on top and iron underneath — there needs to be a transition where the flame starts out traveling slower than the sound speed in the white dwarf material (deflagration), and ends up traveling much faster (detonation). Theorists don’t yet quite understand how the transition occurs, or even how the flame is ignited to begin with.

  • Alternately, the white dwarf might suck matter off the surface of a star made mostly out of helium, which builds up on the surface until it explodes. (Yes, helium is chemically inert, but remember we’re talking about nuclear burning here.) That explosion could then be the spark that triggers the explosion of the rest of the white dwarf without its having to reach a particular critical mass. Since you don’t need a lot of fuel, this scenario works for fairly limited, small explosions as well. The problem is that you make too many relatively heavy elements, like chromium and titanium, on the surface, which isn’t what we see in SNe Ia. Various fixes have been tried for this situation, including lowering the mass of helium on the white dwarf’s surface and mixing carbon in with the helium — but the amount of post facto fudging involved is a bit excessive and there’s no natural understanding of how such configurations might arise in nature. It’s also not entirely clear how the burning proceeds in the mostly-helium layer, and how successful it is in igniting the carbon and oxygen inside the white dwarf itself.

  • In a third scenario, you could try to have two white dwarfs merge or collide. A head-on collision of two white dwarfs produces really bright explosions with a lot of nickel and iron, but that should happen extremely rarely, even in very dense systems where you would expect to have stars bumping into each other all the time. (It may be our best guess right now to explain the most violent of SN Ia events.) Trying to get merging white dwarfs to explode is very tricky, and theorists have seen their simulations produce frustrating results often — the merger produces a single, rapidly rotating white dwarf which may or may not explode; the merger triggers a weak explosion which doesn’t produce enough nickel to match observations of normal SNe Ia; the merger produces a collapse to a neutron star; the merger produces an explosion enshrouded in a deep cloud of gas, which is then heated and glows in ultraviolet light far more brightly than the initial explosion itself. All of these events may be observable as different kinds of explosions which our new surveys can pick up, but guesses about their appearance are varied except that they aren’t expected to produce events that look like SNe Ia.

This is really just a sampling of what was discussed. It’s likely that several of these pathways to the explosions we observe are realized in nature, but that we haven’t got our descriptions of any of the processes straight yet. The limits of order-of-magnitude estimates have long since been reached, and the only way to get even a qualitative theoretical understanding of what’s really going on is with the use of 3-D simulations run on some of the most powerful computers available to scientists today. These simulations use a wide variety of algorithms to track intricate plumes of fluid, kilometers long and centimeters thick, circulating through a space the size of the Earth, with associated nuclear reaction networks and the exchange of parcels of light at millions of individual, distinct wavelengths dictated by the quantum-mechanical structure of the atoms involved.

In short: This is hard, folks.

Other weird transients possibly of thermonuclear origin

There was also quite a lot of discussion about explosive transients which have appearances similar to SNe Ia, but might be different kinds of eruptions, although with a thermonuclear origin. Various “telephone numbers” were thrown around: SN 2002bj, SN 2002cx, SN 2005E, PTF10bhp, PTF10iuv. These are usually fainter than normal SNe Ia, suggesting they might be “failed” explosions which fail to blow up the star, making it “burp” instead as the material settles back down after the outburst, or which only just barely overcome the star’s own gravity to produce an explosion. There’s also a subclass of events with different phone numbers like SN 2003fg, SN 2007if, and SN 2009dc which are much brighter than normal SNe Ia but show the same basic identifying patterns of elements. They might have far more than the average amount of carbon/oxygen fuel for nuclear burning, which raises the question of how you can gather so much fuel together in one place without blowing it up much earlier.

The one I really want to know more about was PTF09dav, which, judging by the lack of a paper to be found online, is still being written up. Usually thermonuclear burning proceeds in such a way that important elements seen in the expanding atmospheres overwhelmingly have even numbers of protons in the nucleus — magnesium (12), silicon (14), sulfur (16), calcium (20), chromium (22), titanium (24), iron (26), nickel (28); sometimes cobalt (27), but this is formed after the explosion, from the radioactive decay of certain isotopes of nickel. This event had strong evidence for scandium (21), as well as an extremely cool atmosphere, and even a hint of strontium (38) — much heavier than nickel, usually the heaviest element we observe in these explosions. It’s a weird one, and I haven’t the foggiest idea what it all means.

What’s a “thermonuclear” event?

With such a motley crew of weird-looking mostly-similar events, the question arose, perhaps understandably, of what we meant by “thermonuclear” transients. In a theoretical context it’s clear what we mean, but as research scientists we don’t have access to the answer in the back of the book — we have to make our best guess based on the information we have available.

And in general we have very limited information. We can measure the amount of light emitted from the events, which may vary with wavelength and with time. Just as a friendly reminder: The result of recording the brightness of an event in various color filters as a function of time is called a light curve, which is usually easier and less time-consuming to construct than a spectrum (the result of spreading the light out with a prism or diffraction grating). If properly calibrated, spectra contain more information than colored filter measurements at a given time, and in particular contain the patterns of light that allow us to infer which chemical elements are contained in the gas from which the light was emitted. But because we don’t get much light from distant supernovae, it’s easy to literally spread it too thin onto our detectors, where it’s overwhelmed by the much brighter glow from the Earth’s atmosphere. Thus to take a spectrum we usually need telescopes at least twice as big as the one with which we found the supernova, and often much bigger even than that.

In the past, supernovae have typically been classified using what are called photospheric spectra, taken when the supernova is still so dense and hot that it’s opaque to visible light, so that we can only see the elements that are on the surface. Some of my colleagues suggested that we start sub-classifying explosive events by what are called nebular spectra, which are taken after the supernova has expanded enough to become transparent so that we can see all the way through to its center.

That’s all well and good, and potentially quite interesting. But the problem is that you have to wait a long time for the supernova to go nebular — almost a year after the explosion, by which time it has usually faded to a tiny fraction of its original brightness. We can only take nebular spectra for things that are very nearby in cosmic terms (closer than about 100 million light years), and this reminds us what rare events supernovae are. We’ve only been able to study supernovae by the thousands because our telescopes survey such a large volume of space in which these rare events are able to occur. We certainly couldn’t take nebular spectra for most supernovae far enough away to use for measurements of the dark energy. So the cosmologists had better hope that, if nebular spectra reveal any pieces of physics which have a bearing on how bright a supernova is (i.e., how precisely we can know its distance), the same information can somehow be inferred from photospheric spectra.

This is all pretty intense stuff, and thirsty work, leaving us scientists no choice but to retire to the pub in the basement of Caltech’s Athenaeum soon after ending the discussion. More booze was had in the form of margaritas with Mexican food after that, and the crowd I was with was regaled by tales of debauchery past at other conferences I missed. Perfectly good times were had here, of course.

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About Richard

I'm an American scientist who is building a new life in Australia. This space will contain words about science and math, but also philosophy, policy, literature, my travels, occasional rants, all sorts of things I find strange and awesome. The views expressed in this blog do not necessarily reflect the opinions of my employer at the time (currently University of Sydney), though personally, I think they should.
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