There are two new routes of investigation into the causes of Type Ia supernovae (SN Ia) looking at objects in the nearby universe, according to a colloquium on February 14 led by Carlos Badenes, a postdoctoral fellow at Tel Aviv University and the Weizmann Institute of Science.

One of these routes is the investigation of supernova remnants (SNRs), created by the “interaction between ejecta and ambient medium,” which are visible in the sky for thousands of years after a supernova explosion. The other is the search for binary white dwarfs (BWDs) – binary systems containing two white dwarfs –  that may be identifiable through data from the Sloan Digital Sky Survey, which measured spectra for around 15,000 white dwarfs.

SN Ia are thermonuclear explosions of carbon- and oxygen-containing white dwarfs “prompted by accretion in a binary system,” according to Badenes. The spectra of such explosions contain silicon but do not contain any hydrogen because stars that have become white dwarfs have burned all of their hydrogen fuel.

The explosion mechanism and the types of binary systems that lead to SN Ia are unknown. There are two basic progenitor scenarios: single degeneracy, where the white dwarf has a normal companion star, and double degeneracy, where the binary system consists of two white dwarfs.

However, not everything is known about either case. “In fact,” Badenes said, “taking at face value what we know about theoretical stellar evolution, type Ia supernovae shouldn’t explode, but they do, so there is something wrong, and we might have to reappraise our ideas.”

Properties of SN Ia host galaxies could be used to place constraints on the progenitor systems, according to Badenes. He stated there is potential evidence for two progenitor populations: a “prompt” population of younger stars in star-forming galaxies (producing brighter explosions containing more nickel-56) and a “delayed” population of older stars in passive galaxies (producing dimmer explosions containing less nickel-56). However, this classification is imprecise because it involves averaging the properties of stars within SN Ia host galaxies.

One important factor to measure in the search for progenitors is the delay time distribution (DTD), the SN Ia rate as a function of time following a period of active star formation within the host galaxy.

To indirectly determine constraints on SN Ia progenitors observationally, one can investigate SNRs, which have bright X-ray spectra, provide large amounts of data, and do not limit researchers to single lines of sight.

Badenes established a “shopping list” of items that are necessary for applying SNR data to the search for SN Ia progenitors: being able to differentiate bright and dim SN Ia through X-ray spectra, estimating the age of the progenitors, and collecting enough data points to do meaningful statistical analysis.

SNRs can provide all of this information, Badenes explained. SNRs allow us to study resolved stellar populations, measuring metallicities and other properties of individual stars. This can give us an estimate for age. Age, combined with the X-ray spectra that depends on hydrodynamic models, allows differentiation of bright and dim SN Ia. Finally, there are 77 known SNRs in the nearby Magellanic Clouds, forming what is thought to be a fairly complete survey.

Combining these data points with an evolution model, astronomers can estimate the DTD for the Magellanic Clouds. Applying this technique to other galaxies, such as M33 where 144 SNRs are known, can refine the DTD estimate.

In the case of BWDs, the double degeneracy scenario neatly explains the lack of observed hydrogen. The Sloan Digital Sky Survey collected spectra of the white dwarfs it observed in multiple passes, allowing for “time-resolved spectroscopy,” according to Badenes. This allows for the measurement of radial velocity shifts, which would indicate binary systems and provide information about companion stars.

Badenes stated that the SWARM Survey is currently using Sloan data to categorize white dwarf binaries within the Milky Way, potentially identifying a double degeneracy population. Doing so will provide more data points for estimating values for the DTD, which may allow researchers to place further constraints on SN Ia progenitor scenarios.