Master Thesis Defense by Kristina Johansen
Title: The nature of the 2021bug: An outburst of a luminous blue variable star
Abstract: At the final stage of most massive stars’ life, they explode as supernovae thus ending their lives. However, some supernovae turn out not to be true terminal explosions. One such case is 2021bug, which underwent an outburst of peak brightness M_r = −13.91±0.063 mag in February, 2021.
In this thesis, I present the spectroscopic and photometric analysis of 2021bug, and compare it to other SN imposter, or LBV-like objects in literature. Modeling 2021bug as a blackbody, I found peak bolometric luminosity of L_bol = 1.51 · 10^41 erg/s ±1.07 · 10^40 erg/s, with corresponding peak temperature of T = 10942.45 K ± 162.10 K. The radius of the photosphere reaches a maximum of R ∼ 1.2 · 10^14 cm a few days after peak brightness. The ejecta velocity around two and four days before peak brightness of the H-alpha emission lines was found to be 2008.91 km/s ± 40.58 km/s and 1552.76 km/s ± 59.46 km/s, respectively, with a P-cygni absorption line showing 1560.44 km/s ± 409.24 km/s at four days before peak brightness. Through analytic SN lightcurve fitting, 2021bug was found to mostly not fit the characteristics of a normal type IIP SN. 2021bug was placed on the evolutionary track of extremely massive stars (∼120 solar masses), though this result is associated with large uncertainties. The combined photometric and spectroscopic results indicate that the progenitor star of 2021bug is a massive, LBV-like SN imposter, which in February 2021 underwent a spontaneous, giant eruption of over 3 visual magnitudes - akin to that of nCar and the pre-SN outburst of SN 2009ip and SN 2015bh.
Supervisor:
- Christa Gall, University of Copenhagen, Niels Bohr Institute
Censor:
- Hans Kjeldsen, Aarhus University
Post Defense Reception to be held at: Aud. M at NBI Blegdamsvej 21