Abstract
ABSTRACT We present optical spectroscopy together with ultraviolet, optical, and near-infrared photometry of SN 2019hcc, which resides in a host galaxy at redshift 0.044, displaying a sub-solar metallicity. The supernova spectrum near peak epoch shows a ‘w’ shape at around 4000 Å which is usually associated with O ii lines and is typical of Type I superluminous supernovae. SN 2019hcc post-peak spectra show a well-developed H α P-Cygni profile from 19 d past maximum and its light curve, in terms of its absolute peak luminosity and evolution, resembles that of a fast-declining Hydrogen-rich supernova (SN IIL). The object does not show any unambiguous sign of interaction as there is no evidence of narrow lines in the spectra or undulations in the light curve. Our tardis spectral modelling of the first spectrum shows that carbon, nitrogen, and oxygen (CNO) at 19 000 K reproduce the ‘w’ shape and suggests that a combination of non-thermally excited CNO and metal lines at 8000 K could reproduce the feature seen at 4000 Å. The Bolometric light-curve modelling reveals that SN 2019hcc could be fit with a magnetar model, showing a relatively strong magnetic field (B > 3 × 1014 G), which matches the peak luminosity and rise time without powering up the light curve to superluminous luminosities. The high-energy photons produced by the magnetar would then be responsible for the detected O ii lines. As a consequence, SN 2019hcc shows that a ‘w’ shape profile at around 4000 Å, usually attributed to O ii, is not only shown in superluminous supernovae and hence it should not be treated as the sole evidence of the belonging to such a supernova type.
Highlights
Supernovae (SNe) were initially classified according to specific observational characteristics, and a physically motivated classification scheme was built, providing insight into explosion physics and stellar evolution pathways
Such a ‘w’-shaped feature attributed to O II has never been identified and analysed in SNe II as such and only recognized in SN 2014G thanks to the analysis reported in this paper
Modelling of this ‘w’ feature using TARDIS (Kerzendorf & Sim 2014) suggested it could be produced by the excitation of CNO at a temperature of 19 000 K, which is more than twice that measured from the spectrum, suggesting these lines would be non-thermally excited
Summary
Supernovae (SNe) were initially classified according to specific observational characteristics, and a physically motivated classification scheme was built, providing insight into explosion physics and stellar evolution pathways. Wide-field surveys have revealed a large diversity of unusual transients that include extreme transitional objects (Modjaz, Gutierrez & Arcavi 2019) One such example is SN 2017ens (Chen et al 2018), a transition between a luminous broadline SN Ic and a SN IIn. SN 2017ivv is another, sharing properties with fast-declining SN II and SN IIb (Gutierrez et al 2020), or SN 2014C, which underwent a change from a SN Ib to SN IIn due to interaction with a hydrogen-rich CSM (Milisavljevic et al 2015). SN 2017ivv is another, sharing properties with fast-declining SN II and SN IIb (Gutierrez et al 2020), or SN 2014C, which underwent a change from a SN Ib to SN IIn due to interaction with a hydrogen-rich CSM (Milisavljevic et al 2015) Objects such as these can support physical continuity between progenitors and explosion mechanisms of different types (Filippenko 1988).
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