Normal or transitional? The evolution and properties of two type Ia supernovae in the Virgo cluster

¹ DARK, Niels Bohr Institute, University of Copenhagen Jagtvej 155A 2200 Copenhagen, Denmark
² INAF, Osservatorio Astronomico di Capodimonte Salita Moiariello 16 I-80121 Naples, Italy
³ School of Mathematics and Physics, University of Queensland Brisbane QLD 4072, Australia
⁴ Department of Astronomy, University of Illinois at Urbana-Champaign 1002 W. Green St. Champaign-Urbana IL 61801, USA
⁵ Institute for Astronomy, University of Hawaii 2680 Woodlawn Drive Honolulu HI 96822, USA
⁶ Kavli Institute for Theoretical Physics, University of California, Santa Barbara 552 University Road Goleta CA 93106-4030, USA
⁷ Las Cumbres Observatory, 6740 Cortona Drive Suite 102 Goleta CA 93117, USA
⁸ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto 50 St. George Street Toronto ON M5S 3H4, Canada
⁹ Institute of Astronomy and Kavli Institute for Cosmology Madingley Road Cambridge CB3 0HA, UK
¹⁰ Physics and Astronomy Department, Johns Hopkins University Baltimore MD 21218, USA
¹¹ Space Telescope Science Institute Baltimore MD 21218, USA
¹² Astrophysics Research Center of the Open University (ARCO), The Open University of Israel Ra’anana 4353701, Israel
¹³ Department of Natural Sciences, The Open University of Israel Ra’anana 4353701, Israel
¹⁴ Department of Astronomy and Astrophysics, University of California Santa Cruz CA 95064, USA
¹⁵ School of Physics, The University of Melbourne Melbourne VIC 3010, Australia
¹⁶ Graduate Institute of Astronomy, National Central University 300 Zhongda Road Zhongli Taoyuan 32001, Taiwan
¹⁷ INAF – Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 I-35122 Padova, Italy
¹⁸ Gran Sasso Science Institute (GSSI) I-67100 L’Aquila, Italy
¹⁹ Osservatorio Astronomico S. di Giacomo, AstroCampania Via S. di Giacomo I-80051 Agerola, Italy
²⁰ School of Physics and Astronomy, Birmingham University Birmingham B15 2TT, UK
²¹ Department of Physics, Lancaster University Lancaster LA1, UK
²² Department of Physics, University of Oxford Keble Road Oxford OX1 3RH, UK
²³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University Evanston IL 60208, USA
²⁴ Department of Astronomy, University of California Berkeley CA 94720-3411, USA
²⁵ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Padova Via F. Marzolo 8 I-35131 Padova, Italy
²⁶ INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 I-23807 Merate (LC), Italy
²⁷ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast Belfast BT7 1NN, UK
²⁸ The Thacher School 5025 Thacher Rd. Ojai CA 93023, USA
²⁹ Adler Planetarium 1300 South DuSable Lake Shore Drive Chicago IL 60605, USA

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Received: 15 July 2025
Accepted: 28 November 2025

Abstract

Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase in SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differentiate them from the population of normal SNe Ia such that their use as cosmological tools remains challenged. Here, we present a high-cadenced photometric and spectroscopic dataset of two SNe Ia, SNe 2020ue and 2020nlb, which were discovered in the nearby Virgo cluster of galaxies. Our study shows that SN 2020nlb is a normal SN Ia whose unusually red colour is intrinsic, arising from a lower photospheric temperature rather than interstellar reddening, providing clear evidence that colour diversity among normal SNe Ia can have a physical origin. In contrast, SN 2020ue has photometric properties, such as colour evolution and light curve decay rate, similar to those of transitional SNe. It is hence more spectroscopically aligned with normal SNe Ia. This is evident from spectroscopic indicators such as the pseudo-equivalent width of Si II lines. Thus, such SNe Ia, which lie photometrically at the edge of the standard normal SNe Ia range, may be missed in cosmological SNe Ia samples. Our results highlight that a spectroscopic analysis of SNe Ia around peak brightness is crucial for identifying intrinsic colour variations and constructing a more complete and physically homogeneous SN Ia sample for precision cosmology.