Title: Single-Degenerate Type Ia Supernovae Are Preferentially Overluminous
Authors: Robert Fisher & Kevin Jumper
First Author’s institution: University of Massachusetts Dartmouth
Status: Accepted to The Astrophysical Journal
Type Ia supernovae (SNe) are often the archetype of an astronomical standardizable candle — something that has a known luminosity which we can use to measure its distance. Scientists famously used type Ia SNe to discover that our universe is accelerating and won the Nobel prize in 2011. However, one of astronomy’s dirtiest secrets is that we don’t know exactly how type Ia SNe materialize or why they might even be standard candles.
Two Mechanisms for Type Ia Supernovae
If you recall, supernovae are the explosive deaths of stars. They have a range of spectral types and energies that depend on the nature of the explosion and the progenitor stars. Type Ia SNe detonate in one of two ways: via the single degenerate or double degenerate model. In the single degenerate model, a white dwarf orbits a massive main-sequence star and eats aways at its partner’s outer layers. The white dwarf gains mass and eventually tips over the Chandrasekhar limit and collapses on itself and explodes. In the double-degenerate model, a binary system of two white dwarfs loses energy due to gravitational waves and the white dwarfs eventually collide. These two mechanisms are schematically shown in Figure 1.
There is no reason to think that only one model can be the true type Ia mechanism; most astronomers agree that there is likely a mixture of the two means. The problem is that these two scenarios may not be standardizable when put together. In particular, the single degenerate scenario implies that the progenitor of type Ia SNe is always around the Chandrasekhar mass (1.4 solar masses), while the double degenerate model leads to a range of plausible progenitor masses. Because the mass is correlated with the luminosity, it is not clear why the double degenerate case should be standardizable, and it is definitely not obvious that both populations are standardizable when mixed. Unfortunately, we have yet to see a clear distinction between the two populations observationally. Today’s paper suggests a few observational differences between these two scenarios and how the single degenerate model may account for very bright (superluminous) type Ia SNe.
Igniting with a Bubble
Fisher and Jumper start with a simple model in which they ignite a bubble somewhere with a white dwarf in order to jump-start the supernova. It turns out that where this “flame bubble” ignited has a significant effect on the SN explosion. Bubbles which are close to the core of the white dwarf lead to slow burning (known as deflagration), causing the white dwarf to expand. Eventually, this drastically transitions into a detonation of the remaining material which causes a supernova. This process is known as deflagration-to-detonation (DDT), and leads to type Ia SNe which are less luminous; you can see both deflagration and detonation in action in the video below. If the ignition bubble is offset from the center, the deflagration phase is minimal, and the detonation leads to much brighter SNe. The question is: how far away from the center of the white dwarf does the ignition bubble need to be for deflagration to significantly affect the luminosity of the supernova?
The authors approach this problem analytically. They take into account the speed of the growth of the flame bubble, the density profile of the bubble and its surroundings, and the gravitational acceleration around the bubble. They find that a characteristic offset of ~19 km is where deflagration becomes less important (meaning that Ia SNe explode without slow burning). This is an incredibly small offset! A typical white dwarf has a radius of ~7000 km; according to the authors a bubble greater than just 3% away from the core is enough to clobber the possibility of prolonged deflagration. To reiterate, this means that white dwarfs in the single degenerate model are likely to be superluminous compared to typical type Ia SNe.
This is a useful hypothesis for a number of reasons. For one, the predicted rate of type Ia SNe is much higher than the single degenerate model predicts. The rate of superluminous SNe that we see, however, is in line with the single degenerate model. Additionally, it would agree with the observational fact that most type Ia SNe seem to have double degenerate progenitors and make a cleaner separation between the two classes.
Finally, the authors provide some clear tests to confirm the nature of superluminous type Ia SNe. One of the most obvious tests is to look at the post-explosion site for a possible companion star. If a main sequence star exists, than the supernova was from the single degenerate variety. We can also try to find traces of hydrogen in the SNe spectra; hydrogen indicates that a main sequence companion star was in the process of feeding the white dwarf during the time of collapse. Due to the low numbers of superluminous type Ia SNe (and perhaps low numbers of single degenerate SNe), the observational evidence is hindered by looking for the single degenerate needle in the haystack of all type Ia SNe. Perhaps with deeper and larger surveys such as LSST, we can begin to untangle this mystery.