Type Iax Supernovae: A New Class of Stellar Explosion

Abstract: We describe observed properties of the Type Iax class of supernovae (SNe Iax), consisting of SNe observationally similar to its prototypical member, SN 2002cx. The class currently has 25 members, and we present optical photometry and/or optical spectroscopy for most of them. SNe Iax are spectroscopically similar to SNe Ia, but have lower maximum-light velocities (2000 < |v| < 8000 km/s), typically lower peak magnitudes (-14.2 > M_V,peak > -18.9 mag), and most have hot photospheres. Relative to SNe Ia, SNe Iax have low luminosities for their light-curve shape. There is a correlation between luminosity and light-curve shape, similar to that of SNe Ia, but offset from that of SNe Ia and with larger scatter. Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies, there are several indications that the progenitor stars are white dwarfs (WDs): evidence of C/O burning in their maximum-light spectra, low ejecta masses, strong Fe lines in their late-time spectra, a lack of X-ray detections, and deep limits on massive stars and star formation at the SN sites. However, two SNe Iax show strong He lines in their spectra. The progenitor system and explosion model that best fits all of the data is a binary system of a C/O WD that accretes matter from a He star and has a significant deflagration. At least some of the time, this explosion will not disrupt the WD. We estimate that in a given volume there are 31^+17_-13 SNe Iax for every 100 SNe Ia, and for every 1 M_sun of iron generated by SNe Ia at z = 0, SNe Iax generate 0.052^+0.017_-0.014 M_sun. Being the largest class of peculiar SNe, thousands of SNe Iax will be discovered by LSST. Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia.

• The paper reveals that Type Iax supernovae exhibit significantly lower velocities and luminosities compared to typical SNe Ia, indicating a distinct explosion mechanism.
• The paper employs comprehensive spectral and photometric analyses of 25 events to uncover a broader luminosity-light curve correlation than seen in SNe Ia.
• The paper supports a binary progenitor model involving a C/O white dwarf with a helium companion, offering new insights into incomplete deflagration processes.

Type Iax Supernovae: A Novel Class of Stellar Explosions

The study by Foley et al. presents an extensive investigation into a newly identified class of supernovae, designated as Type Iax (SNe Iax). This class is marked by its distinct observational and physical characteristics, distinguishing it from the well-known Type Ia supernovae (SNe Ia). The authors compile and analyze data on 25 SNe Iax, providing valuable insights into their properties and potential origins.

Key Observations and Characteristics

1. Spectral and Photometric Similarity with SNe Ia: While SNe Iax share some spectral similarities with SNe Ia, they exhibit notably lower maximum-light velocities (2000 km/s ≤ v ≤ 8000 km/s), lower peak magnitudes (-14.2 ≥ M_V,peak ≥ -18.9 mag), and typically exhibit hot photospheres.
2. Luminosity and Light Curve: SNe Iax demonstrate a correlation between luminosity and light-curve shape, akin to the well-known width-luminosity relation (WLR) of SNe Ia, though with greater scatter. Unlike SNe Ia, SNe Iax display relatively low luminosities for their light-curve shape.
3. Host-Galaxy Morphology: The host-galaxy morphology of SNe Iax is predominantly skewed towards late-type galaxies, with no detections in elliptical galaxies, suggesting younger stellar populations as the progenitor stars.
4. Progenitor Evidence: Several lines of evidence suggest that the progenitor stars of SNe Iax are C/O white dwarfs (WDs). This includes signs of C/O burning in their spectra, low ejecta masses (~0.5 M_☉), strong iron lines in late-time spectra, absence of X-ray detections, and lack of massive stars and active star formation at the SN sites. Additionally, helium lines are seen in two SNe Iax, indicating that helium may play a role in the progenitor system.
5. Explosion Model: The data support a model involving a binary system where a C/O WD accretes matter from a helium star and undergoes a deflagration. The explosion sometimes does not completely disrupt the WD, leading to different outcomes even within the same class.

Implications and Future Directions

The identification and characterization of SNe Iax have several implications for our understanding of supernovae and their progenitor systems. The variability in observed characteristics suggests a greater diversity in the end states of stellar evolution than previously realized, particularly among WDs in binary systems.

1. Understanding Progenitor Systems: The link between SNe Iax and C/O WDs in binary systems with helium-rich companions offers a unique opportunity to study thermonuclear processes in stellar explosions. This connection could refine models of WD interactions and mass-transfer dynamics.
2. Influence on Galactic Chemical Evolution: Although SNe Iax are less luminous and thus less influential than SNe Ia in terms of metal enrichment of the interstellar medium, they still contribute to the diversity of chemical elements, particularly iron group elements, which could have local effects on surrounding star-forming regions.
3. Theoretical Modeling: SNe Iax present an attractive testbed for theoretical models of incomplete deflagration and hybrid thermonuclear processes. Fine-tuning explosion simulations to replicate the observed diversity and peculiarities of SNe Iax can enhance our understanding of nuclear synthesis and explosion mechanisms.
4. Observational Strategies: Future observational campaigns, such as those by the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), are expected to significantly increase the detected sample of SNe Iax. Comprehensive multi-wavelength observations will be crucial to further delineate their properties and refine their classification.

In conclusion, SNe Iax expand the landscape of thermonuclear supernovae, prompting a reevaluation of stellar evolution pathways and explosion mechanisms. Continued research efforts are needed to unravel the complexities of these intriguing stellar phenomena, with significant contributions expected from both observations and theoretical advancements.