Astronomers Probe the Environment of Exploding Stars with HET, Computer Models

DENVER, Colo.— University of Texas at Austin astronomer Chris Gerardy and a host of colleagues are reporting today that they have probed the structure of the environment surrounding type Ia supernovae, exploding stars scientists use as "standard candles" to study the universe’s past, present, and future. Their study provides strong support for the idea that type Ia supernovae originate in close binary star systems, a notion which has long been believed on purely theoretical grounds with little direct observational evidence. Their work combined Gerardy’s observations with the 9.2-meter Hobby-Eberly Telescope (HET) at McDonald Observatory in West Texas with complex computer models of supernova explosions.

Gerardy is reporting their research today at the 204th meeting of the American Astronomical Society in Denver. The work was published in the May 20 edition of The Astrophysical Journal.

The upshot, according to Gerardy, is that "if you can detect certain spectral lines from a type Ia, you can constrain the properties of the progenitor system." It’s important for astronomers to completely understand the workings of type Ia supernovae, because their role as "standard candles" allows the calculation of distances to galaxies outside our own Milky Way — information critical to calculations about the universe’s age, size, and fate.

The most widely accepted view of the "progenitor star" of a type Ia supernova is that it is a close binary star system, with one of the pair being a white dwarf. The white dwarf is in end-stages of life. It has already ballooned into a giant star, then released its outer layers of gas into space, with only a dense core about the size of Earth remaining. The companion star is in an orbit so close to the white dwarf that its gas is being sucked away, falling into a disk orbiting the white dwarf. When the white dwarf takes on too much mass, it explodes.

This widely held view hasn’t been backed up by much direct observational evidence, though, and indeed even the type of the companion star is not known, Gerardy said. To prove that this model is correct, astronomers have been looking in spectra of type Ia supernovae for evidence of material either from the disk that surrounded the white dwarf before it exploded, or blown off of the surface of the close companion star by the supernova explosion. However, Gerardy and colleagues’ work shows that previous studies may have been looking for the wrong evidence, and in the wrong place. In most searches for evidence of gas from the donor star in supernova debris, astronomers have for looked for spectral lines of hydrogen or helium, the most abundant elements in the material being transferred to the white dwarf. So far, they’ve come up empty.

But Gerardy’s HET spectrum of supernova 2003du showed a different interesting feature — an extra set of spectral lines of calcium (specifically called the "calcium II infrared triplet") that has only been seen in few other type Ia supernovae. This particular feature shows up at a wavelength around 8,000 angstroms, Gerardy said, which is at a shorter wavelength than the calcium lines that are usually seen in supernovae. This shift is caused by the Doppler Effect where light that is emitted or absorbed from gas moving a large fraction of the speed of light is shifted to shorter or longer wavelengths depending on whether the material is moving toward or away from the observer. In this case, the shift means that the calcium is moving faster than most of the ejected supernova debris.

Successive spectra of SN 2003du made over several days with HET show that the high-velocity calcium lines change rapidly, compared to other features on its spectra. That means the calcium source is likely a thin fast-moving shell which dissipates quickly. Such a shell could be created from the disk surrounding the white dwarf. When the white dwarf explodes as a supernova, gas in the disk surrounding it is pushed outward and swept up into a shell. As the white dwarf’s expanding gas remnant expands rapidly, it quickly overtakes this shell.

The team’s computer models of supernova explosions indicate that this could indeed be the case. They modified the model to include a shell of material around the exploding star, and ran the model. The model produces predictions of what the spectrum of a supernova will look like. In this case, it produced a spectrum remarkably similar to Gerardy’s spectrum of SN 2003du, including the unusual calcium feature. And it did not produce any spectral lines for hydrogen or helium that could come from the circumstellar matter. While most of the gas in the shell is hydrogen and helium, a tiny amount of calcium actually makes a stronger spectral line. "It’s like food coloring," said Gerardy "it doesn’t affect the taste much but it turns your food blue."

Gerardy and collaborators plan to use HET to observe more type Ia supernovae. "The HET is good for this work for a couple of reasons," Gerardy said. "It’s queue-scheduled, which means we can study a supernova right after it’s been discovered elsewhere, meaning we don’t have to wait months to get telescope time. Also, the HET will allow us to see fainter supernovae," he said. "We need to go to fainter objects so that we can get dozens of them. Having access to this is a great resource."

Studying more candidates will help them to figure out if the disk idea for type Ia supernova progenitor is right, Gerardy said. "Statistics on how often you see this calcium feature in type Ia supernovae will tell you about the geometry," he said. "If this material really is coming from a disk, you’ll only see this calcium feature if that disk is oriented edge-on to our line of sight." How often you see the feature then tells you about how much of the star is covered by the surrounding material.

The Hobby-Eberly Telescope is a joint project of The University of Texas at Austin, The Pennsylvania State University (Penn State), Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Unversität Göttingen.

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Note to Editors: Poster session 63.08, "SN 2003du: Signatures of the Circumstellar Evironment in a Normal Type-Ia Supernova?," by C. L. Gerardy, et al, will occur at 10:00 a.m. MDT on June 2, 2004, at the American Astronomical Society meeting in Denver.