Supernova explosions of massive stars or dense white dwarfs send material hurtling into space at thousands of kilometers per second. The initial blast wave rapidly decelerates as it plows through the interstellar medium; the deceleration triggers yet another shock wave that moves inward and reheats the ejected debris. Researchers have long analyzed x-ray spectra of the reheated material to learn about supernova remnants (SNRs) and the explosions that produced them, but because of their low resolution those measurements provide only a fuzzy picture. Now Ivo Seitenzahl at the University of New South Wales in Australia and his colleagues have used optical spectroscopy to expose multiple elemental layers of SNRs. Applying the technique to probe type Ia supernovae triggered by white dwarfs could help reveal the mechanism by which those stars explode.
Seitenzahl and colleagues focused on three type Ia SNRs in the nearby Large Magellanic Cloud. Poring over spectral data from the 8.2 m Very Large Telescope in Chile, the researchers found visible-wavelength emission lines associated with several highly ionized states of iron and sulfur. The spectra for two of the SNRs revealed sharp, concentric shells of species including Fe13+ (green), Fe8+ (blue), and S11+ (red), as shown in the image of SNR 0509-67.5. The spectrograph’s superior resolution to those on x-ray telescopes enabled the researchers to take measurements of the Doppler shifts of the lines, which they used to calculate both the location and speed of the inward-moving shock wave. Using a simple model of a spherically symmetrical expansion, Seitenzahl and colleagues calculated various parameters for the two SNRs; most of their results matched well with both theory and earlier x-ray measurements.
In a final step, the researchers constrained the masses of the progenitor objects based on the observed SNR characteristics. For one remnant, the approximate progenitor mass was 1.4 solar masses, right around the Chandrasekhar limit that’s thought to trigger an explosion (see Physics Today, May 2010, page 11). But the progenitor mass associated with SNR 0509 was only about 1.0 solar mass. That finding demonstrates how much astrophysicists still have to learn about the mechanism behind type Ia supernovae, despite their widespread use as standard candles in cosmology (see the article by Saul Perlmutter, Physics Today, April 2003, page 53). Along with x-ray studies, supernova remnant tomography, as the researchers call their new technique, may help astrophysicists determine whether the explosions are triggered by individual white dwarfs, collisions between pairs of white dwarfs, or some combination of both. (I. R. Seitenzahl et al., Phys. Rev. Lett. 123, 041101, 2019.)
