Thermonuclear Explosions Cause Most Heated of Discussions

NASA and H. Richer (U. of British Columbia)

As an organizer of the program on degenerate stars, KITP permanent member Lars Bildsten represents the participants who are particularly interested in the explosions of carbon-oxygen white dwarfs.

As Bildsten said, “Some participants don’t care at all about the objects themselves, they just want to use them. We want to understand them. We would like to understand these astrophysical events as well as any others that are occurring that are indicators of accretion and explosion on white dwarfs."

“Historically,” said Bildsten, “there are two types of supernovae — the ones [type I] with no hydrogen in their spectra and the ones [type II ] with hydrogen in their spectra. Type Ia supernovae exhibit spectral lines of silicon and calcium — very heavy elements; you don’t see hydrogen or helium. We think their origin is the complete incineration of a white dwarf from what was probably a solar mass of carbon and oxygen to mostly the most stable nucleus you can make out of incinerating that material — nickel 56.”

The energy for supernovae II comes from the gravitational energy released in the collapse of a stellar core to a neutron star that sends out a shock that disrupts the envelope and that engenders the supernova, said Bildsten. By contrast, the origin of the type Ia supernova energy is completely thermonuclear, coming from the fusion of carbon to nickel 56.

With supernovae Ia there exists an empirical relationship between the peak brightness and the decline rate, he explains. The brighter the supernova, the slower will be its decline; and conversely the dimmer the supernova, the more rapid will be its decline. This key relationship, called “the Phillips relationship” after its discoverer (program participant Mark Phillips of Las Campanas Observatory, Chile), enabled supernovae Ia to be used as tools for cosmology. In other words, the shape of the light curve tells observers whether they are looking at a bright or dim supernova so that their assessment of its distance is independent of the apparent brightness of the object; otherwise, dimmer ones could look further away (relative to bright ones) than they really are.

“This Phillips relation is still not understood theoretically,” said Bildsten. “We don’t really know what the controlling parameter is. We think most of the brighter ones may be more successful at incinerating most of the white dwarf. So the simplest scenario is that they all have roughly the same mass, but differ in terms of how much nickel they make. If they have a lot of nickel, they are bright; if they have less radioactive heating, they are faint. What nobody can do is explain the rapid decay of the ones that fade fast.”

Nickel is crucial to brightness because we do not see the actual explosion, but days later the cloud of radioactively decaying nickel; so, the reasoning goes, the more nickel, the brighter the supernova.

To recapitulate, some white dwarfs detonate (though unclear how) and set off a chain reaction in which carbon and oxygen isotopes fuse to form radioactive nickel 56. The fusion energy then overcomes the gravitational energy and unbinds the white dwarf, and all the stuff just leaves the scene at the rate of 10,000 kilometers per second. Days later, the brilliant glow we see from earth is the radioactive nickel decaying to radioactive cobalt 56, which in turn visibly decays to stable iron 56.

In addition to tools for cosmology, supernovae Ia provide another service — of making about two-thirds of the iron in the universe.

The carbon-oxygen white dwarf is the endpoint for most stars, which have converted hydrogen to helium and helium to carbon and oxygen. Incidentally, the carbon so key to life molecules comes not from supernovae Ia, but from supernovae II (the core collapsers). The thermonuclear explosion of type Ia’s converts all the carbon to magnesium and then to nickel, so there is no carbon left to seed the intergalactic medium for the formation of more stars and solar systems and life forms.

“We would like to know,” said Bildsten, “whether the origin of supernovae Ia depends on the kind of galaxy they are in.” Elliptical galaxies harbor only old stars; and because all the stars are old, they are also comparatively low mass (since the more massive a star, the shorter its life). Observers see no type II supernovae in elliptical galaxies though they do see supernovae Ia. In spiral galaxies, where star formation is vigorous and young stars abound, both types of supernovae are observed. The supernovae Ia seen in elliptical galaxies tend to be less bright than the supernovae Ia seen in spirals.

“What has become clear in only the last two or three years,” said Bildsten, “is that all evidence points to the rate of those supernovae popping off in a young stellar population knowing about the young stars.” In other words, where there is a vigorous star formation rate, there is a vigorous type Ia rate.

“Those supernovae might well have a different path to explosion than the ones in ellipticals,” said Bildsten. “But if there are two different populations,” he asked, “why should they all lie on the same Phillips relationship? There are data now that seem to point to two paths depending on what kinds of stars are around to make these events. At high redshifts [i.e. the greater the redshift, the further away and the longer ago the object], vigorous star formation occurs, and there are very few aged stars; so we would think that at high redshifts, we are mostly seeing the bright ones. If they obey the same relationship that is seen nearby, then everything is okay,” said Bildsten, meaning the use of supernovae Ia as tools for cosmology. “But that’s not my problem."

“What we are trying to do in the program is to have a critical discussion of this path to ignition. How do these things explode?”

Though the trigger is unclear, what seems to be clear is that additional mass is required and the only source is from another star.

Bildsten emphasizes, “The carbon fuses as a function of rising density, not rising temperature. This density effect is the real cold fusion!” he exclaims.

Astrophysicists call the effect “pycnonuclear” burning from the Greek “pyknos,” meaning “dense.”

Fifteen years ago at a program on exploding stars at the KITP, two of the current participants, Ed van den Heuvel and Ken Nomoto, wrote a paper together, which matched then fresh, soft x-ray satellite-based observations with a model they proposed of a white dwarf in a very particular kind of binary. That paper “Accreting White Dwarf Models for CAL 83, CAL 87 and Other Ultrasoft X-ray Sources in the LMC” [i.e., “Large Magellanic Clouds”] has become a classic in the field, according to Bildsten, and still provides the basis for the preferred ignition scenario today because it describes the mechanism for white dwarfs that become massive enough through accretion to overcome the degenerate electron pressure counteracting gravity, to contract, and to ignite.

“Many of us,” said Bildsten, “have been working on alternative ignition scenarios because this one is so weird.”

All the program/conference organizers independently observed how “very contentious” were the discussions on how the explosion happens. Bildsten counted, “Five scenarios in terms of binaries that give you different kinds of triggers and probably would have different kinds of explosions.”

Apparently, the more models, the hotter the discussion gets?

“At a very fundamental level, there are huge holes in our understanding,” comments Bildsten.


KITP Newsletter, Spring 2007