Strong carbon-monoxide molecule emission in the low-luminosity and highly asymmetric type II SN 2020ocz

ATel #14653; Regis Cartier (CTIO), Jeonghee Rho (SETI), Peter Hoeflich (FSU), Morten Andersen (Gemini), Joseph P. Anderson (ESO), Priscila J. Pessi (ESO), Tom R. Geballe (Gemini) and Carlos Contreras (Las Campanas)
on 22 May 2021; 01:38 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Regis Cartier (rgcartier@gmail.com)

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We report the detection of very strong carbon monoxide first overtone emission in type II SN 2020ocz and the possible detection of the second CO overtone. The specific conditions under which the second CO overtone features at about 1.6 microns becomes strong are discussed in the appendix of Rho et al. (2021, ApJ, 908, 232). SN 2020ocz is a low luminosity type II supernova discovered on 2020 July 7.35 by the Gaia mission (Hodgkin et al. 2020, TNS Discovery Report, 2020-2061, 1). The most recent Gaia non-detection was on 2020 June 13.58. The explosion epoch is estimated at the halfway mark between the last non-detection and the discovery, that is on 2020 June 25.5, with an uncertainty of 12 days.

A high signal-to-noise ratio (S/N) optical classification spectrum was obtained by Cartier et al. (2020, TNS AstroNote, 190, 1) on the night of 2020 October 7.3, with the Goodman spectrograph mounted on the SOAR telescope at a phase of +104.8 days after the estimated epoch of the explosion. The high S/N spectra of SN 2020ocz, obtained over several nights, do not show evidence of Galactic or host-galaxy narrow Na I D absorption lines, consistent with the low Galactic reddening E(B-V) = 0.0106 (Schlafly & Finkbeiner 2011, ApJ, 737, 130) in the direction of the SN. To estimate the ⁵⁶Ni mass yield in SN 2020ocz, the procedure presented in Hamuy (2003a, ApJ, 582, 905) is followed. The V-band magnitudes are converted to bolometric luminosities using the bolometric corrections of Hamuy et al. (2001). Following this procedure a ⁵⁶Ni mass of 0.014 +/- 0.005 solar masses is obtained.

In the first near-infrared (NIR) spectrum obtained at +126.9 days after the explosion with TripleSpec4.1 mounted on the SOAR telescope, a very strong first overtone CO molecule emission is detected as well as the potential second CO overtone emission. The NIR spectral sequence of SN 2020ocz shows CO emission from +127 to 278 days, including a spectrum with S/N ~ 100 obtained with Flamingos-2 at Gemini on 2021 March 14 (GS-2021A-FT-201; PI J. Rho). The carbon monoxide emission in SN 2020ocz is 1-2 orders of magnitude stronger than those of the nearby type II SN 2017eaw; to our knowledge, SN 2020ocz shows the strongest first CO overtone emission detected in an SN to date.

The optical and the NIR spectra of SN 2020ocz show asymmetric emission line profiles similar in shape to SN 2004dj (see, e.g., Chugai et al. 2005, Astronomy Letters, 31, 792). The line profiles of SN 2020ocz exhibit a larger degree of asymmetry than in the case of SN 2004dj, showing three clear, distinct components with expansion velocities of about -2200 km/s, -200 km/s, and 2500 km/s. Follow-up observations of SN 2020ocz are encouraged.