Luminous, rapidly declining supernovae as stripped transitional objects in low-metallicity environments: The case of SN 2022lxg

¹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
³ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
⁷ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁸ Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland
⁹ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
¹⁰ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
¹¹ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
¹² Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁴ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁵ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹⁶ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
¹⁷ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I23807 Merate, (LC), Italy
¹⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹⁹ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, 2200 Copenhagen, Denmark
²⁰ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
²¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
²² INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, I-64100 Teramo, Italy
²³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands

^⋆ Corresponding author: pachar@utu.fi

Received: 19 March 2025
Accepted: 12 June 2025

Abstract

We present an analysis of the optical and near-infrared properties of SN 2022lxg, a bright (M_g peak = −19.41 mag) and rapidly evolving supernova (SN). It was discovered within a day of explosion, and rose to peak brightness in ∼10 d. Two distinct phases of circumstellar interaction are evident in the data. The first is marked by a steep blue continuum (T > 15 000 K) with flash-ionisation features due to hydrogen and He II. The second, weaker phase is marked by a change in the colour evolution accompanied by changes in the shapes and velocities of the spectral line profiles. Narrow P-Cygni profiles (∼150 km s⁻¹) of He I further indicate the presence of slow-moving, unshocked material and suggest partial stripping of the progenitor. The fast decline of the light-curve from the peak (3.48 ± 0.26 mag (50 d)⁻¹ in g band) implies that the ejecta mass must be low. Spectroscopically, until +35 d there are similarities with some Type IIb SNe but then there is a transition to spectra that are more reminiscent of an interacting SN II. However, metal lines are largely absent in the spectra, even at epochs of ∼80 d. Its remote location (∼4.6 kpc projected offset) from the presumed host galaxy, a dwarf with M_B ∼ −14.4 mag, is consistent with our metallicity estimate – close to the values of the Small Magellanic Cloud – obtained from scaling relations. Furthermore, several lines of evidence (including intrinsic polarisation of p ∼ (0.5 − 1.0)%) point to deviations from spherical symmetry. We suggest that a plausible way of uniting the observational clues is to consider a binary system that underwent case C mass transfer. This failed to remove the entire H envelope of the progenitor before it underwent core collapse. In this scenario, the progenitor itself would be more compact and perhaps straddle the boundary between blue and yellow supergiants, which ties in with the early spectroscopic similarity to Type IIb SNe.