Massive star evolution: Luminous Blue Variables as unexpected Supernova progenitors

Abstract: Stars more massive than about 8 Msun end their lives as a Supernova (SN), an event of fundamental importance Universe-wide. Theoretically, these stars have been expected to be either at the red supergiant, blue supergiant, or Wolf-Rayet stage before the explosion. We performed coupled stellar evolution and atmospheric modeling of stars with initial masses between 20 Msun and 120 Msun. We found that the 20 Msun and 25 Msun rotating models, before exploding as SN, have spectra that do not resemble any of the aforementioned classes of massive stars. Rather, they have remarkable similarities with rare, unstable massive stars known as Luminous Blue Variables (LBV). While observations show that some SNe seem to have had LBVs as progenitors, no theoretical model had yet predicted that a star could explode at this stage. Our models provide theoretical support for relatively low-luminosity LBVs exploding as SN in the framework of single stellar evolution. This is a significant shift in paradigm, meaning that a fraction of LBVs could be the end stage of massive star evolution, rather than a transitory evolutionary phase. We suggest that type IIb SN could have LBV as progenitors, and a prime example could be SN 2008ax.

Massive Star Evolution: Luminous Blue Variables as Supernova Progenitors

The research paper titled "Massive star evolution: Luminous Blue Variables as unexpected Supernova progenitors" presents an intriguing theoretical exploration into the evolving understanding of massive star life cycles. Traditionally, the evolution of stars more massive than 8 solar masses before their end-of-life supernova event has been associated with either red supergiants (RSG), blue supergiants (BSG), or Wolf-Rayet (WR) stages. This paper departs from conventional wisdom by coupling stellar evolution and atmospheric modeling, proposing that Luminous Blue Variables (LBVs) could indeed directly precede supernovae.

The study utilizes the Geneva stellar evolution code combined with radiative transfer atmospheric modeling through CMFGEN to analyze stars with initial masses between 20 and 120 solar masses, focusing here on 20 and 25 solar mass rotating models. These models demonstrate spectra at the pre-supernova stage that closely resemble LBVs, a classification historically considered a transient phase rather than a terminal evolutionary stage. Remarkably, these findings provide theoretical support for the explosion of certain LBVs as supernovae, reinforcing previous observational evidence that linked some supernovae to LBV progenitors.

Key Findings

• Spectral Analysis: The paper's rotating stellar models exhibit spectra akin to LBVs, characterized by prominent H, He I, and N II lines with P-Cygni profiles, alongside mass-loss rates in line with LBV characteristics. This implies that stars could explode while in the LBV phase. Such results challenge entrenched paradigms by revealing LBVs as potential endpoints before supernova events rather than merely evolutionary transitions between other massive star stages.
• Evolutionary Implications: For stars of 20-25 solar masses, the paper posits the evolution sequence: Main Sequence O star to RSG, possibly re-entering the blue supergiant phase or transitioning to LBV just before undergoing a supernova explosion. Consequently, these stars might represent SN progenitors with a type IIb classification, as exemplified by SN 2008ax.

Theoretical and Practical Implications

The findings suggest that stellar mass loss at the RSG phase plays a crucial role in subsequent evolutionary paths, affecting final spectral signatures and pre-supernova phases. The models predict that low-luminosity LBVs exhibit distinct photometric properties with $B-V$ values between -0.14 and -0.11, distinguishing them from previously identified LBV-associated progenitors, thereby necessitating further observational validation.

Future Directions

Continued exploration into massive star evolution, particularly the dynamical processes governing LBV-stage supernova progenitors, remains crucial. Future studies could benefit from integrating more comprehensive datasets, including varied rotational velocities and metallicities, to refine model predictions. Observational campaigns to detect LBVs preceding supernova events may expand existing understanding of massive star life cycles and unveil the diversity of pathways leading to core-collapse supernovae.

In summary, this paper contributes significant theoretical insights into stellar evolution by proposing that LBVs can indeed culminate in supernova events for rotating massive stars, spurring revisions to current models and expectations of massive star lifecycles.