Title: 1991T-Like Type Ia Supernovae as an Extension of the Normal Population
Authors: John T. O’Brien, Wolfgang E. Kerzendorf, Andrew Fullard, Reudiger Pakmor, Johannes Buchner, Christian Vogl, Nutan Chen, Patrick van der Smagt, Marc Williamson, Jaladh Singhal
First Author’s Institution: Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
Status: Published in the Astrophysical Journal [open access]
Famously, type Ia supernovae (SNe Ia) have been used to measure the local Hubble constant, or the rate at which our universe expands. These objects earned the nickname “standard candles” since their near-constant intrinsic luminosities allow us to measure distances in space. Slowly but surely, however, we’ve learned that some of our standard candles aren’t that “standard” after all…
Historically, SNe Ia were proposed to develop from the transfer of mass between two stars, where the star receiving the mass was a carbon-oxygen white dwarf–a star that’s reached the end of its life. After the white dwarf accretes a certain amount of mass, it explodes into a SN Ia. Spectroscopic studies of these supernovae over the decades have shown a wide range of absorption features, one major absorption line being silicon, a key element produced in the explosion. In fact, a subclassification scheme of SNe Ia–often referred to as the Branch classification– emerged based on the relative strengths of particular absorption features commonly identified in SNe Ia spectra (see Figure 1). One of these subclassifications is “shallow silicon,” which signifies a lack of silicon produced in the explosion. This subclassification (compared to other SN Ia subclassifications in Figure 1) shows how SNe Ia are like snowflakes: they have very similar structures yet vary in detail.
The supernova SN1991T was the first observed of its kind. What was so special about it? This event was considered over-luminous, or more luminous than the typical “near-intrinsic luminosity” of the average SNe Ia. Later, as observations improved, more events like SN1991T were detected, contributing to the growing class of aptly named “91T-like” events. The spectra of these events have shallow silicon lines compared to the normal range of SNe Ia. The peculiarity of these absorption lines hints at something unique about these events, and the answer lies in studying the ejecta, or the elements produced in the explosion. This paper is a step toward understanding what differentiates these events from the norm and what we can infer about their origins.
Outside of this work, recent hydrodynamical simulations of various progenitor models, or stellar origins, have successfully recreated some of the observable signatures of SNe Ia, including synthetic, or computed, optical spectra of theoretical events. Except, as previously mentioned, the observable signatures of SNe Ia can vary quite a bit amongst all these subtypes and classifications! Instead of hydrodynamical simulations, the authors of this paper chose to reconstruct SNe Ia ejecta using Bayesian inference and Active Learning conducted on early-time (within a few days after explosion) optical spectra of already-observed normal and 91T-like events. This is the time when 91T-like events show their features! After training on this data, they developed a model to link the optical spectra and the ejecta’s properties corresponding to normal and 91T-like events.
Their emulator successfully recreated both normal and 91T-like events, at least with 68% confidence (think one sigma!) Further, they discovered that the variety in the parameters used in their model illuminates some differences between these 91T events and normal SNe Ia. Remember those silicon features? They recreated those pesky absorption lines, particularly the major iron and silicon features experts look for. Their model successfully recreated suppressed, or not-as-deep, silicon absorption features. This indicates a low fraction of intermediate-mass elements, which range from lithium to iron, produced in the explosion compared to the total mass. They also matched the deep, major iron line seen in 91T-like events. Fewer intermediate-mass elements in 91T-like SNe Ia suggest that these elements exist at higher ionization states than in normal SN Ia (see Figure 2). This suggests that there isn’t just a single mechanism that produces a 91T-like SN Ia; it’s likely a combination of different physical processes.
The question now becomes: what can we learn about 91T-like origins from this? Can a single progenitor model lead to different pathways? Or do we need different progenitor models to explain these differences in spectroscopic features? The authors believe fewer intermediate-mass elements and higher ionization states hint at normal and 91T-like events sharing similar progenitor systems. In other words, 91T-like events might just be an extension, or extreme, of the normal population. Perhaps the candle just burned a bit too bright!
Aside from this work, in addition to these over-luminous 91T-like events, there also exists another interesting class of SNe Ia dubbed “super-luminous,” which are roughly one, maybe two, magnitudes brighter than normal SNe Ia. (Only in astronomy could the words over-luminous and super-luminous mean different things, right?) Because of this, researchers advocate for SNe Ia to be called “standardizable” candles instead because, as you now know, their intrinsic luminosities really aren’t that constant after all.
Astrobite edited by Ansh Gupta and Dee Dunne
Featured image credit: Figure 5 from O’Brien et al (2024)
