The story of SN 2021aatd: A peculiar 1987A-like supernova with an early-phase luminosity excess

¹ Department of Experimental Physics, Institute of Physics, University of Szeged, Dóm tér 9, 6720 Szeged, Hungary
² HUN-REN–SZTE Stellar Astrophysics Research Group, Szegedi út, Kt. 766, 6500 Baja, Hungary
³ MTA-ELTE Lendület “Momentum” Milky Way Research Group, Szent Imre H. st. 112, 9700 Szombathely, Hungary
⁴ HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory, Konkoly Th. M. út 15-17., 1121 Budapest, Hungary
⁵ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁶ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9400 Szombathely, Hungary
⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
⁸ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, USA
⁹ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 6997801, Israel
¹⁰ CIFAR Azrieli Global Scholars Program, CIFAR, Toronto, Canada
¹¹ Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195-0002, USA
¹² Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
¹³ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹⁴ Center for Interdisciplinary Exploration and Research in Astrophysics and Department of Physics and Astronomy, Northwestern University, 1800 Sherman Ave., 8th Floor, Evanston, IL 60201, USA
¹⁵ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
¹⁶ Baja Astronomical Observatory of University of Szeged, Szegedi út, Kt. 766, 6500 Baja, Hungary
¹⁷ SETI Institute, 339 N. Bernardo Ave., Ste. 200, Mountain View, CA 94043, USA
¹⁸ Department of Physics and Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, South Korea
¹⁹ ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary

Received: 10 November 2023
Accepted: 1 June 2024

Abstract

Context. There is a growing number of peculiar events that cannot be assigned to any of the main classes. SN 1987A and a handful of similar objects, thought to be explosive outcomes of blue supergiant stars, is one of them: while their spectra closely resemble those of H-rich (IIP) SNe, their light curve (LC) evolution is very different.

Aims. Here we present the detailed photometric and spectroscopic analysis of SN 2021aatd, a peculiar Type II explosion. While its early-time evolution resembles that of the slowly evolving double-peaked SN 2020faa (although at a lower luminosity scale), after ∼40 days its LC shape becomes similar to that of SN 1987A-like explosions.

Methods. In addition to comparing LCs, color curves, and spectra of SN 2021aatd to those of SNe 2020faa, 1987A, and other objects, we compared the observed spectra with our own SYN++ models and with the outputs of published radiative transfer models. We also carried out a detailed modeling of the pseudo-bolometric LCs of SNe 2021aatd and 1987A with a self-developed semi-analytical code, assuming a two-component ejecta (core + shell), and involving the rotational energy of a newborn magnetar in addition to radioactive decay.

Results. We find that the photometric and the spectroscopic evolution of SN 2021aatd can be well described with the explosion of a ∼15 M_⊙ blue supergiant star. Nevertheless, SN 2021aatd shows higher temperatures and weaker Na I D and Ba II 6142 Å lines than SN 1987A, which is instead reminiscent of IIP-like atmospheres. With the applied two-component ejecta model (accounting for decay and magnetar energy), we can successfully describe the bolometric LC of SN 2021aatd, including the first ∼40-day phase showing an excess compared to 87A-like SNe, but being strikingly similar to that of the long-lived SN 2020faa. Nevertheless, finding a unified model that also explains the LCs of more luminous events (e.g., SN 2020faa) is still a matter of debate.