Detection of Helium in Sub-luminous Thermonuclear Supernovae
Connecting Explosion Composition Models with Progenitor Channels
Researchers at the University of California Santa Cruz have used the elemental composition of a peculiar supernova explosion to understand its complex origins. Using the NERSC supercomputers at Lawrence Berkeley Laboratories, the UCSC team modeled the material ejected in a new class of thermonuclear explosion called a Type Iax supernova (SN Iax). This recently discovered class of stellar explosion is characterized by lower observed luminosities and slower explosion velocities than other thermonuclear SNe currently known. However, like many classes of SN, the star system responsible for SNe Iax is still relatively unknown.
From their simulations, the researchers were able to study the presence of ionized helium produced in these under-luminous explosions as it relates to their stellar origins. The explosion models created by the UCSC team demonstrate how SNe Iax can arise from a binary star system wherein a white dwarf accretes mass from a helium star companion until it violently explodes. The team modeled hundreds of SNe Iax observations on the Edison supercomputer in order to explain how such a binary star system can produce these unique stellar explosions.
Thermonuclear supernovae such as SNe Iax are thought to arise from the explosion of at least one white dwarf star in a binary system. These SNe are responsible for the synthesis of most heavy elements such Iron, Nickel and Cobalt found in the universe. Consequently, thermonuclear SNe enrich the chemical composition of galaxies and are even responsible for heavy metals here on Earth. Furthermore, the physics responsible for these SNe is closely linked to binary stellar evolution and the deaths of stars within such star systems. Despite the crucial role thermonuclear SNe play in stellar evolution and galaxy enrichment, the exact binary companion to the white dwarf and the mechanisms responsible for these explosions remains uncertain.
In an attempt to understand their origins, the UCSC team has analyzed the elemental signatures within SNe Iax in order to test the proposed white dwarf + helium star progenitor system for these objects. Such a binary system is consistent with stellar environments where SNe Iax are found and is also the leading progenitor channel based on pre-explosion imaging of an individual SN Iax. Because the binary companion to the white dwarf is supposably comprised of helium, the UCSC group has searched for observable helium emission in the spectra of SNe Iax. Being first to examine the chemical composition of the entire SN class, the UCSC study used the detection of helium in multiple SNe Iax to constrain the current understanding of how white dwarf + helium star binaries explode. The presence of helium in any thermonuclear SNe is rare, but has direct implications for the binary star system responsible for these explosions. Overall, the study has identified the complexity of SNe Iax progenitor systems while also restricting the possible explosion scenarios for these peculiar events.
The primary objective of the research project was to search for helium in SN Iax spectra in order to constrain the proposed WD + He star progenitor channel for these objects. The project was led by undergraduate researcher Wynn Jacobson-Galán and was supervised by Professors Ryan Foley and Shawfeng Dong (full list of collaborators available in publication). With only ~50 objects comprising the SN class, the team created a sample of 110 spectral observations from 44 total SNe Iax. The spectra were then modeled using the spectral synthesis software SYNAPPS, which was run exclusively on the Edison supercomputer at NERSC. Access to the NERSC facility was essential for simulating the ejecta material of all 110 SN Iax observations because the project relied on both efficient parallel computing and Markov Chain fitting of hundreds of free parameters. Furthermore, the storage capabilities of NERSC were essential to the preservation of large amounts of data and spectral models generated for this research.
The SYNAPPS software allowed for the modeling of ions in SNe Iax and their individual dynamic properties i.e., ejecta velocities, temperatures and opacities. 11 individual ions (e.g. C I, O I, Fe II etc.) were included to model each SN Iax spectrum, resulting in a total of 59 free parameters to fit separately. Each simulation took ~10 wall clock hours on 6 computer nodes and were run for 6 months on Edison. The SYNAPPS code employs parallel computing through the Message Passing Interface (MPI) and OpenMP software dependencies. The team used SYNAPPS to model the elemental composition of each spectrum while at the same time masking out (i.e. not fitting) regions of the spectrum where ionized helium is known to emit. This allowed the team to subtract the synthetic SYNAPPS spectrum from the true data in order to detect helium emission that could have originated from interaction with the helium star companion in the original progenitor system. The ability to create highly accurate fits to SN Iax spectra was crucial for understanding the helium abundance within these objects and the luminosities at which this element could be detected.
After fitting the ionic composition of entire SN Iax sample, the UCSC team created a statistical method for determining true detections of helium in the residuals of each SYNAPPS model. Because the helium lines being searched for were considered to be pure emission (i.e. represented by Gaussian profiles), the team cross- correlated helium line Gaussian profiles with all model residuals in order to find true helium detections. This method yielded two significant detections of helium emission in SNe Iax 2004cs and 2007J; these being the first confirmed cases of early-time helium emission in a thermonuclear SN. However, the lack of helium in other SNe Iax demonstrates the complexity of these explosions. Following these detections, the team used the helium emission luminosity in SNe Iax to calculate the maximum SN Iax luminosities at which helium emission can still be detected. This analysis will be applied in future searches for SNe Iax because it has shown that helium cannot be observed in all SNe Iax due to their variable explosion luminosities and the defined luminosity limit for helium detection for these objects.
The presence of helium in only a few SN Iax spectra has demonstrated the diversity of these explosions, even within the SN class. A major component of this project was identifying possible mechanisms within the explosion that could allow for detectable helium to be present in only a fraction of objects. In their paper, the team showed how SNe Iax with helium could have been the result of nova eruptions that occurred in the binary system before explosion. Such an ejection occurs as the white dwarf is accreting helium from the companion helium star, but only occurs in a very specific white dwarf + helium star configuration. This is consistent with the low fraction of SNe Iax with detectable helium in their spectra. However, as discussed above, the study shows that the lack of detected helium could also be the result of an inability to observe ionized helium at a given luminosity. While all intrinsically sub-luminous, these SNe Iax have a spread of luminosities, many of which could be too high to detect helium at all. Nonetheless, the UCSC team has laid the foundation for future observations of helium emission in these SNe, which will further constrain the current model for how these particular explosions come to be.
About NERSC and Berkeley Lab
The National Energy Research Scientific Computing Center (NERSC) is a U.S. Department of Energy Office of Science User Facility that serves as the primary high-performance computing center for scientific research sponsored by the Office of Science. Located at Lawrence Berkeley National Laboratory, the NERSC Center serves more than 7,000 scientists at national laboratories and universities researching a wide range of problems in combustion, climate modeling, fusion energy, materials science, physics, chemistry, computational biology, and other disciplines. Berkeley Lab is a DOE national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California for the U.S. Department of Energy. »Learn more about computing sciences at Berkeley Lab.