This work was funded by ANID, Millennium Science Initiative, ICN12 009. LM acknowledges support from UNRN PI2022 40B1039 grant. MO acknowledges support from UNRN PI2022 40B1039 and grant PICT-2020-SERIEA-01141. EB and JD are supported in part by NASA grant 80NSSC20K0538. LG acknowledges financial support from the European Social Fund (ESF) ’Investing in your future’ under the 2019 Ramón y Cajal program RYC2019-027683-I. LG and TEMB acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033 under the PID2020-115253GA-I00 HOSTFLOWS project, and from Centro Superior de Investigaciones Científicas (CSIC) under the PIE project 20215AT016. LG, TEMB, and NER acknowledge partial support from the program Unidad de Excelencia María de Maeztu CEX2020-001058-M. BJS is supported by NSF grants AST-1920392, AST-1911074, and AST-1911074. MDS is funded by the Independent Research Fund Denmark (IRFD, grant number 10.46540/2032-00022B). The Aarhus supernova group is funded by the Independent Research Fund Denmark (IRFD, grant number 8021–00170B and 10.46540/2032-00022B), and by the VILLUM FONDEN (grant number 28021). NER acknowledges partial support from MIUR, PRIN 2017 (grant 20179ZF5KS), from the Spanish MICINN grant PID2019-108709GB-I00 and FEDER funds. HK was funded by the Academy of Finland projects 324504 and 328898. TEMB acknowledges financial support from Centro Superior de Investigaciones Científicas (CSIC) under the I-LINK 2021 LINKA20409. JTH is supported by NASA award 80NSSC22K0127. LK is supported by the Hungarian National Research, Development and Innovation Office grant PD-134784. TS, LK, and KV are supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. RKT, TS, and KV are supported by the ÚNKP 22–4, and Bolyai + grant 22–5 and ÚNKP-22-5-ELTE-1093 New National Excellence Programs of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund, respectively. MG is supported by the EU Horizon 2020 research and innovation programme under grant agreement No 101004719. MN is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 948381) and by a Fellowship from the Alan Turing Institute. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004719. This project has been supported by the GINOP-2-3-2-15-2016-00033 project of the National Research, Development and Innovation Office of Hungary (NKFIH) funded by the European Union, as well as by NKFIH grants OTKA FK-134432, 2019–2.1.11-TÉT-2019-00056, OTKA K131508 and the Élvonal grant KKP-143986. Authors acknowledge the financial support of the Austrian-Hungarian Action Foundation (101öu13).

We present extensive ultraviolet (UV) and optical photometric and optical spectroscopic follow-up of supernova (SN) 2021gno by the ‘Precision Observations of Infant Supernova Explosions’ (POISE) project, starting less than 2 d after the explosion. Given its intermediate luminosity, fast photometric evolution, and quick transition to the nebular phase with spectra dominated by [Ca II] lines, SN 2021gno belongs to the small family of Calcium-rich transients. Moreover, it shows double-peaked light curves, a phenomenon shared with only four other Calcium-rich events. The projected distance from the centre of the host galaxy is not as large as other objects in this family. The initial optical light-curve peaks coincide with a very quick decline of the UV flux, indicating a fast initial cooling phase. Through hydrodynamical modelling of the bolometric light curve and line velocity evolution, we found that the observations are compatible with the explosion of a highly stripped massive star with an ejecta mass of 0.8 M and a 56Ni mass of 0.024 M. The initial cooling phase (first light-curve peak) is explained by the presence of an extended circumstellar material comprising ∼10−2 M with an extension of 1100 R. We discuss if hydrogen features are present in both maximum-light and nebular spectra, and their implications in terms of the proposed progenitor scenarios for Calcium-rich transients.

With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2020-001058-M).

K. Ertini et al.

Peer reviewed