Expanding the parameter space of 2002es-like type Ia supernovae: On the underluminous ASASSN-20jq/SN 2020qxp

¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
² Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus, OH, 43210 USA
³ Center for Cosmology and AstroParticle Physics (CCAPP), The Ohio State University, 191 W. Woodruff Avenue, Columbus, OH, 43210 USA
⁴ Institute for Astronomy, University of Hawaiì at Manoa, 2680 Woodlawn Drive, Honolulu, HI, 96822 USA
⁵ Planetary Science Institute, 1700 E Fort Lowell Rd., Ste 106, Tucson, AZ, 85719 USA
⁶ Hamburger Sternwarte, Gojensbergweg 112, 21029 Hamburg, Germany
⁷ Department of Physics, Florida State University, 77 Chieftain Way, Tallahassee, FL, 32306 USA
⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
¹⁰ Post Astronomy, Lexington, MA, USA
¹¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
¹² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹³ Department of Astronomy and The Oskar Klein Centre, AlbaNova University Center, Stockholm University, 106 91 Stockholm, Sweden
¹⁴ School of Physics, The University of Melbourne, VIC, 3010 Australia
¹⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, 95064 USA
¹⁶ Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860 Macul, Santiago, Chile
¹⁷ Institut für Theoretische Physik, Goethe Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
¹⁸ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels, (Barcelona), Spain
¹⁹ Sorbonne Université, CNRS/IN2P3, LPNHE, F-75005 Paris, France
²⁰ Carnegie Observatories, Las Campanas Observatory, Casilla 601, La Serena, Chile

^⋆ Corresponding authors: email@subhashbose.com, max@phys.au.dk

Received: 6 January 2025
Accepted: 22 April 2025

Abstract

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernovae (SNe Ia) ASASSN-20jq/SN 2020qxp. It is a low-luminosity object, with a peak absolute magnitude of M_B = −17.1 ± 0.5 mag, while its post-peak light-curve decline rate of Δm₁₅(B) = 1.35 ± 0.09 mag and color-stretch parameter of s_BV ⪆ 0.82 is similar to that of normal luminosity SNe Ia. That makes it a prevalent outlier in both the SN Ia luminosity-width and the luminosity-color-stretch relations. The analysis of the early light curves indicates a possible “bump” during the first ≈1.4 days of explosion. ASASSN-20jq synthesized a low radioactive ⁵⁶Ni mass of 0.09 ± 0.01 M_⊙. The near-maximum light spectra of the supernova show strong Si II absorption lines, indicating a cooler photosphere than normal SNe Ia; however, it lacks Ti II absorption lines. Additionally, it shows unusually strong absorption features of O Iλ7773 and the Ca II near-infrared triplet. The nebular spectra of ASASSN-20jq show a remarkably strong but narrow forbidden [Ca II] λλ7291, 7324 doublet emission that has not been seen in SNe Ia except for a handful of Type Iax events. There is also a marginal detection of the [O I] λλ6300, 6364 doublet emission in nebular spectra, which is extremely rare. Both the [Ca II] and [O I] lines are redshifted by roughly 2000 km s⁻¹. ASASSN-20jq also exhibits a strong [Fe II] λ7155 emission line with a tilted-top line profile, which is identical to the [Fe II] λ16433 line profile. The asymmetric [Fe II] line profiles, along with the redshifted [Ca II] and emission lines, suggest a high central density white dwarf progenitor that underwent an off-center delayed-detonation explosion mechanism, synthesizing roughly equal amounts of ⁵⁶Ni during the deflagration and detonation burning phases. The equal production of ⁵⁶Ni in both burning phases distinguishes ASASSN-20jq from normal bright and subluminous SNe Ia. Assuming this scenario, we simultaneously modeled the optical and near-infrared nebular spectra, achieving a good agreement with the observations. The light curve and spectroscopic features of ASASSN-20jq do not align with any single sub-class of SNe Ia. However, the significant deviation from the luminosity versus light-curve shape relations (along with several light-curve and spectroscopic features) exhibits similarities to some 2002es-like objects. Therefore, we have identified ASASSN-20jq as an extreme candidate within the broad and heterogeneous parameter space of 2002es-like SNe Ia.