Scientific Method / Science & Exploration

Remnants of a stellar suicide pact

A companion star remnant helps us understand the blast that destroyed it.

Artist's impression of a red giant companion star dumping gas onto a white dwarf. Such a binary system may be the progenitor of some type Ia supernovae.

Type Ia supernovae (SN Ia) are some of the most important objects in observational cosmology. They explode in very similar ways, and their extreme brightness allows them to be used to determine the distance to far galaxies. The overwhelming consensus is that they arise from the explosion of white dwarfs, which are the remains of stars like our Sun. However, the particular mechanism that leads them to explode is still the subject of argument. SN Ia explosions might reach an explosive mass by stealing matter from a companion star, or by merging with another white dwarf—or both possibilities might occur.

Why is it so hard to figure out what's going on? Observations have failed to identify SN Ia progenitor systems. However, astronomers studying the supernova PTF 11kx have observed clear signs of a stellar companion: a star in mutual orbit with the white dwarf that provided mass that ultimately triggered the explosion. Based on these data, the researchers concluded the progenitor system was a binary system consisting of a red giant star and a white dwarf

White dwarfs are the cores of stars similar to the Sun, which shed their outer layers at the end of their lives. Without the heat from fusion, these stars would normally suffer gravitational collapse. But they are held from collapse by quantum degeneracy pressure; normally, they'd collapse until their components occupied the same quantum state, but the Pauli exclusion principle keeps that from happening. The end result is referred to as a degenerate stellar remnants.

If a white dwarf's mass exceeds a certain maximum—the Chandrasekhar limit—it explodes as a type Ia supernova. According to the two best models, extra mass can be added either when the white dwarf strips gas off a companion star, or when two white dwarfs merge. The first scenario is known as the single-degenerate (SD) system, since it consists of a single white dwarf and an ordinary star; the second type is a double-degenerate (DD) system.

Previous observations have failed to identify companion stars, leading some astronomers to argue that SN Ia explosions may all be DD. Similarly, a paper we covered previously hinted that the number of binary white dwarf systems is consistent with the total SN Ia rate in spiral galaxies. However, binaries containing white dwarfs and ordinary stars do exist, and we can detect the stellar companion transferring mass to the white dwarf because it creates flares known as recurrent novas. RS Ophiuchi is one well-known example of a recurrent nova.

As described in a new Science paper, the supernova PTF 11kx provides clear evidence for an ordinary star companion. It was identified by the Palomar Transient Factory (PTF, hence its name) in January of 2011; follow-up observations using the High Resolution Echelle Spectrometer (HIRES) at Keck provided a complete spectrum of the explosion. The observers found a number of interesting spectral features, marking absorption and emission by gas moving rapidly (65 kilometers per second) away from the progenitor system. Details within the spectrum indicated the presence of several distinct expanding gas layers.

These gas clouds appeared to be consistent with layers of a star that had interacted with the white dwarf before the explosion—the products of a recurrent nova. The system doesn't resemble explosions that have been identified as likely to be the product of merging dwarfs, further hinting that PTF 11kx could have been a SD system.

The absorption and emission features in the spectrum of PTF 11kx had not been identified in earlier SN Ia explosions. However, when the spectrum from PTF 11kx was compared to similar supernovae SN1999aa and SN2002ic, they broadly matched each other, meaning the explosions were probably driven by the same mechanism. In other words, if PTF 11kx was a SD system, then SN1999aa and other similar SN Ia explosions likely were SD as well, according to the researchers. This subclass of SN Ia comprises between 1 and 30 percent of all type Ia systems.

While that's a large range of possibilities, if PTF 11kx truly was a SD system, then it shows definitively that not all SN Ia explosions are DD—itself a significant result. Further observations, especially of the subclass of type Ia supernovae including PTF 11kx and SN1999aa, should turn up signs of companion stars. With more statistics, astronomers will be able to determine how many explosions fall into which category, bringing us closer to an understanding of these important supernovae.

Science, 2012. DOI: 10.1126/science.1219164  (About DOIs).

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