A low-energy explosion yields the underluminous Type IIP SN 2020cxd

Abstract

Context. We present observations and analysis of SN 2020cxd, a low-luminosity (LL), long-lived Type IIP supernova (SN). This object was a clear outlier in the magnitude-limited SN sample recently presented by the Zwicky Transient Facility (ZTF) Bright Transient Survey. Aims. We demonstrate that SN 2020cxd is an additional member of the group of LL SNe, and discuss the rarity of LL SNe in the context of the ZTF survey, and how further studies of these faintest members of the core-collapse (CC) SN family might help understand the underlying initial mass function for stars that explode. Methods. We present optical light curves (LCs) from the ZTF in the gri bands and several epochs of ultra-violet data from the Neil Gehrels Swift Observatory as well as a sequence of optical spectra. We construct colour curves, a bolometric LC, compare ejectavelocity and black-body temperature evolutions for LL SNe, as well as for typical Type II SNe. Furthermore, we adopt a Monte Carlo code that fits semi-analytic models to the LC of SN 2020cxd, which allows the estimation of physical parameters. Using our late-time nebular spectra, we also compare against SN II spectral synthesis models from the literature to constrain the progenitor properties of SN 2020cxd.Results. The LCs of SN 2020cxd show great similarity with those of LL SNe IIP, in luminosity, timescale and colours. Also the spectral evolution of SN 2020cxd is that of a Type IIP SN. The spectra show prominent and narrow P-Cygni lines, indicating low expansion velocities. This is one of the faintest LL SNe observed, with an absolute plateau magnitude of M r = −14.5 mag, and also one with the longest plateau lengths, with a duration of 118 days. Finally, the velocities measured from the nebular emission lines are among the lowest ever seen in a SN, with intrinsic Full Width at Half Maximum of 478 km s −1 . The underluminous late-time exponential LC tail indicates that the mass of 56 Ni ejected during the explosion is much smaller than the average of normal SNe IIP, we estimate M56 Ni = 0.003 M . The Monte Carlo fitting of the bolometric LC suggests that the progenitor of SN 2020cxd had a radius of R 0 = 1.3 × 10 13 cm, kinetic energy of E kin = 4.3 × 10 50 erg, and ejecta mass M ej = 9.5 M . From the bolometric LC, we estimate the total radiated energy E rad = 1.52 × 10 48 erg. Using our late-time nebular spectra, we compare against SN II spectral synthesis models to constrain the progenitor Zero-age Main-sequence mass and found it likely to be 15 M . Conclusions. SN 2020cxd is a LL Type IIP SN. The inferred progenitor parameters and the features observed in the nebular spectrum favour a low-energy, Ni-poor, iron CC SN from a low mass (∼ 12 M ) red supergiant.

Low luminosity Type II supernovae -- IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

“…For SN 2020cxd in particular, the velocity measurements performed on the H𝛼 line showed that the line forming region moves in velocity space monotonically from 5900 km s −1 at 2 d, to 2560 km s −1 immediately after the drop from the plateau (134 d), and finally to 1020 km s −1 at 245 d. The H𝛼 expansion velocity after 90 days is measured from the Full Width at Half Maximum (FWHM) of the line, since the rise of the Ba II 𝜆 6497 makes it impossible to identify clearly the position of the minimum of the P Cygni profile. The results are reported in Table 2 and plotted in Figure 8, along with other values from LL SNe IIP taken from Pastorello et al (2004Pastorello et al ( , 2009 For SN 2021aai, the velocities measured from the H𝛼 P Cygni profiles range from 7000 km s −1 at 8 d to 4200 km s −1 at 35 d. Subsequently, the rise of the Ba II 𝜆 6497 line in the absorption part of the P Cygni profile forces us to estimate the expansion velocities from the FWHM of the emission component of the H𝛼 line, as previously done by Yang et al (2021) for SN 2020cxd. As already mentioned, metal lines are characterised by a lower optical depth, leading to their formation closer to the photosphere compared to H lines, which form in the outer layers of the ejecta and therefore yield higher velocity measurements.…”

“…SN 2020cxd is a LL SN IIP discovered on 2020 Feb 19 (Nordin et al 2020) at the coordinates RA = 17 ℎ 26 𝑚 29 𝑠 .26 Dec = +71°05' 38".58 in the spiral galaxy NGC 6395, classified as Scd (de Vaucouleurs et al 1991) and with a redshift z = 0.003883 ± 0.000002 (Springob et al 2005). As noticed by Yang et al (2021), the distance measurements for the host galaxy vary between 19 and 23 Mpc, depending on the methodology used. In this paper we adopt a distance modulus of 𝜇 = 31.60 ± 0.20 mag (or 20.9 ± 1.4 Mpc), obtained by averaging the six different estimates obtained through the Tully-Fisher method and reported on the NED database (Bottinelli et al 1985;Tully et al 2013Tully et al , 2016Sorce et al 2014;Willick et al 1997;Tully & Fisher 1988).…”

“…SN 2020cxd has been the studied byYang et al (2021). Here we provide additional photometric and spectroscopic coverage of this target.…”