SN 2020cpg: an energetic link between Type IIb and Ib supernovae

MNRAS 506, 1832–1849 (2021) https://doi.org/10.1093/mnras/stab1761 Advance Access publication 2021 June 22 SN 2020cpg: an energetic link between Type IIb and Ib supernovae K. Medler ,1‹P. A. Mazzali,1,2J. Teffs ,1S. J. Prentice ,3C. Ashall ,4M. Amenouche,5J. P. Anderson,6J. Burke,7,8T. W. Chen,9L. Galbany ,10 M. Gromadzki ,11 C. P. Guti´ errez ,12,13 D. Hiramatsu,7,8D. A. Howell,7,8C. Inserra ,14 E. Kankare,13 C. McCully,7T. E. M¨ uller-Bravo ,15 M. Nicholl,16 C. Pellegrino7,8and J. Sollerman9 1Astrophysical Research Institute Liverpool John Moores University, Liverpool L3 5RF, UK 2Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany 3Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK 4Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Dr., Hawai’i, HI 96822, USA 5Universit´ e Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6European Southern Observatory, Alonso de C´ ordova 3107, Casilla 19, Santiago, Chile 7Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA 8Department of Physics, University of California, Santa Barbara 93106, USA 9The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden 10Departamento de F´ ısica Te´ orica y del Cosmos, Universidad de Granada, E-18071 Granada, Spain 11Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, PL-00-478 Warszawa, Poland 12Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland 13Tuorla Observatory, Department of Physics and Astronomy, FI-20014 University of Turku, Finland 14School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24 3AA, UK 15School of Physics and Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ, UK 16Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK Accepted 2021 June 16. Received 2021 May 24; in original form 2021 March 17 ABSTRACT Stripped-envelope supernovae (SE-SNe) show a wide variety of photometric and spectroscopic properties. This is due to the different potential formation channels and the stripping mechanism that allows for a large diversity within the progenitors outer envelope compositions. Here, the photometric and spectroscopic observations of SN 2020cpg covering ∼130 d from the explosion date are presented. SN2020cpg (z=0.037) is a bright SE-SNe with the B-band peaking at MB=−17.75 ±0.39 mag and a maximum pseudo-bolometric luminosity of Lmax =6.03 ±0.01 ×1042 erg s−1. Spectroscopically, SN 2020cpg displays a weak highand low-velocity H αfeature during the photospheric phase of its evolution, suggesting that it contained a detached hydrogen envelope prior to explosion. From comparisons with spectral models, the mass of hydrogen within the outer envelope was constrained to be ∼0.1 M. From the pseudo-bolometric light curve of SN2020cpg a 56Ni mass of MNi ∼ 0.27 ±0.08 Mwas determined using an Arnett-like model. The ejecta mass and kinetic energy of SN 2020cpg were determined using an alternative method that compares the light curve of SN2020cpg and several modelled SE-SNe, resulting in an ejecta mass of Mejc ∼5.5 ±2.0 Mand a kinetic energy of EK∼9.0 ±3.0 ×1051 erg. The ejected mass indicates a progenitor mass of 18−25 M. The use of the comparative light curve method provides an alternative process to the commonly used Arnett-like model to determine the physical properties of SE-SNe. Key words: supernovae: general – supernovae: individual (SN 2020cpg). 1 INTRODUCTION Core-collapse supernovae (CC-SNe) result from the death of stars with a Zero-Age Main Sequence (ZAMS) mass of MZAMS >8M  (Woosley, Langer & Weaver 1995;Smartt2009). These CC-SNe, separated into multiple categories based on their photometric and spectroscopic properties, are known as the H-rich Type II SNe (SNeII) and the H-poor stripped envelope SNe (SE-SNe). SNeII undergo little to no stripping of their outer hydrogen envelope prior E-mail: [email protected] to explosion and as such display strong hydrogen features throughout their spectral evolution. SE-SNe, however, lack the strong hydrogen features and display a variety of different spectroscopic properties depending on their elemental composition prior to core-collapse. The type of SE-SN can be determined by the presence and strength of both hydrogen and helium features within their spectra. These SNe include the H/He-rich Type IIb SNe (SNe IIb), the H-poor/He-rich Type Ib SNe (SNe Ib), and the H/He-poor Type Ic SNe (SNeIc). SNe Ib(c) lack any prominent hydrogen (and helium) spectral lines (Filippenko 1997), as their progenitor stars are thought to have been fully stripped of their outer hydrogen and H/He envelopes prior to the C 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1833 core-collapse event. The stripping of the outer envelopes for these progenitor stars is expected to occur over the last stages of the stars life cycle prior to core-collapse. The process required to strip the outer envelope from these massive stars is still under investigation. The predominant methods include binary interaction where mass is transferred to the companion star via Roche lobe overflow (e.g. Podsiadlowski, Joss & Hsu 1992; Stancliffe & Eldridge 2009;Soker 2017), and a single star formation channel where the outer envelope is stripped prior to collapse, during the Wolf–Rayet phase, by either stellar winds (e.g. Georgy et al. 2012;Gr ¨ afener & Vink 2015)or via rotational stripping (Groh, Meynet & Ekstr¨ om 2013). Despite the existence of multiple potential formation channels for SE-SNe, the binary star model seems to be favoured in recent years as the dominant source of SE-SNe progenitors. This is because the singlestar model is unable to produce the number of progenitors required to account for all the SE-SNe observed (Smith et al. 2011). However, if the degree of stripping is not high enough to fully remove all of the hydrogen from the progenitor, a hydrogen envelope is present during the explosion resulting in a SNeIIb (see Woosley et al. 1994 for SN 1993J one of the best followed examples of SN IIb). SNeIIb are different to the other SE-SNe by the clear hydrogen features within their spectra that can persist for several months before slowly fading as the SN evolves into the nebular phase (see Filippenko 1997,2000). Photometrically, SNe IIb are very similar to other SE-SNe displaying a main peak within the first two to three weeks from the explosion. Several SNeIIb also display a bright initial peak within a few days of the explosion prior to the main peak seen in all SE-SNe (see Bersten et al. 2012; Piro 2015). The initial luminous peak is thought to result from the shock cooling near the stellar surface (Waxman & Katz 2017), while the second main peak is a result of the radioactive decay of 56Ni and 56Co synthesized during the explosion. The dual peak light curve has been seen in several of the well observed SNe IIb, such as SN 1993J (Wheeler et al. 1993) and SN 2016gkg (Arcavi et al. 2017; Bersten et al. 2018), and also in some SNeIb, such as SN 1999ex (Mazzali et al. 2002) and SN 2008D (Malesani et al. 2009). Although this feature is seen in both SNeIIb and Ib, it is not observed in the majority of SNe, because the progenitor compactness causes the shock to cool more quickly than surveys can observe the shock breakout. However, thanks to the improving cadence of present surveys, the probability of covering and detecting this feature will increase. Spectroscopically SNe IIb differentiate themselves from SNe Ib by the presence of the hydrogen features which fade over time as the spectra of SNeIIb become more Ib-like, with the helium features becoming dominant. From detailed modelling of H-rich and H-poor SNe the mass of hydrogen within the outer envelope, MH, required to form a SNe IIb has been found to be within the range of 0.01−1.0 M(Sravan, Marchant & Kalogera 2019). Hachinger et al. (2012) constructed a detailed set of spectral models to determine the amount of hydrogen and helium that can be hidden within the outer envelope of SNeIb/c, respectively. From their synthetic spectra, Hachinger et al. (2012) concluded that as little as 0.025– 0.033 Mis required to form a strong H αabsorption feature, suggesting that some Type Ib’s may display H αfeatures further blending the distinction between IIb and Ib SE-SNe. More recently, Prentice & Mazzali (2017) showed that the distinction between the He-rich SE-SNe can be further blurred based on the strength of Hαemission within the spectra. Prentice & Mazzali (2017) created two further SE-SNe subcategories; the Type IIb(I), which display moderate H-rich spectra where the H αP-Cygni profile is dominated by the absorption component relative to the emission profile, and the Ib(II), whose spectra only show some weak H αwith no obvious Balmar lines more energetic than H α. The classification scheme of Prentice & Mazzali (2017) along with the findings of Hachinger et al. (2012) demonstrates that SNe IIb and Ib are likely more related than previously thought. Here, we present the photometric and spectroscopic evolution for SN2020cpg, a Type Ib SN with a thin hydrogen layer, during the first ∼130 d. SN 2020cpg was initially classified with the Supernova Identification code SNID (Blondin & Tonry 2007)as a Type Ib SN, from the spectrum obtained on 19/02/2020 with the Liverpool Telescope (LT; Steele et al. 2004). However, followup spectral observations suggest that SN2020cpg displayed H α features as seen in Type IIb SNe. In Section 2.2, we present the BgVri-band photometry for SN2020cpg from the first 130 d after the explosionobtainedthroughvariousLasCumbresObservatoryGlobal Telescopenetworktelescopes(LCO;Brownetal. 2013), aspartofthe Global Supernova Project (GSP; Howell & Global Supernova Project 2017). The spectroscopic observation of SN 2020cpg are presented in Section 2.3. In Section 3, we discuss the construction of the pseudobolometric light curve and the Arnett-like model used to obtain the physical parameters. In Section 4, we present the light curves for the BgVri-band photometry and the constructed pseudo-bolometric light curve, along with physical properties obtained by an Arnett-like model. In Section 4.3, we obtain the line velocity evolution, along with a comparison of SN 2020cpg spectra with other well followed Type Ib and IIb SNe. In Section 5, we discuss the potential presence of a hydrogen envelope and the spectral modelling done to determine its presence. We also discuss the use of hydrodynamical models to obtain more realistic explosion parameters and compare the results with those produced by the Arnett-like model. Finally, in Section 6, we summarize the finding on SN 2020cpg, giving final estimates for the physical parameters and a value of the progenitors initial mass. 2 OBSERVATIONS AND DATA REDUCTION 2.1 Explosion date and host galaxy SN2020cpg was first detected on 15/02/2020 (MJD =58894.54) by Nordin et al. (2020) on behalf of the Zwicky Transient Facility (ZTF; Bellm et al. 2019). The last non-detection of SN 2020cpg, on 06/02/2020 (MJD =58885.52), predates the ZTF discovery by 9 d. To place a better constraint on the explosion date of SN 2020cpg, we modified the pseudo-bolometric light curve model to include the explosion date as a parameter (see Section 3.2). From this fit, we obtain an explosion date of 08/02/20, MJD =58887.7 ±2.1 d, which we adopt throughout the rest of the paper. SN 2020cpg was associated with the galaxy SDSS J135219.64 +133432.9 and was located 1.14 arcsec south and 24.07 arcsec west from the galaxy centre, just off the outer end of the host galaxy’s western spiral arm, as seen in Fig. 1. Using the cosmological parameters of H0= 73.0±5.0kms−1Mpc−1, matter =0.27 and vacuum =0.73 gives aredshift distance of 158.6±11.1 Mpc, with the distance calculation based on the local velocity field model from Mould et al. (2000). The host redshift of z=0.037 implies a distance modulus of m−M=36.05 ±0.15mag. 2.2 Photometry The initial g-andr-band photometry was obtained by ZTF, using the ZTF-cam mounted on the Palomar 1.2m Samuel Oschin telescope several days (t[MJD] ≈58894.5) before continuous follow-up occurred. This photometry was run through the automated ZTF pipeline (Masci et al. 2019) and is presented on Lasair transient MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 1834 K. Medler et al. Figure 1. Image of SN 2020cpg and the host galaxy, obtained by combining LCO observations in BgVri filters on 20/02/2020, stacked and aligned using AstroImageJ (Collins et al. 2017). The field of view is 2.6×2.4arcmin2. broker(Smithetal.2019).1Afterthediscovery,theBgVri-bandswere followed by the Las Cumbres Observatory Global Telescope network (LCO; Brown et al. 2013) and reduced using the BANZAI pipeline (McCully et al. 2018). Full BgVri-band photometry was obtained until 23/03/2020 from which point only Vri-band photometry could be obtained. Observation was obtained from a combination of 1m telescopes from the Siding Spring Observatory (code: COJ), the South African Astronomical Observatory (code: CPT), the McDonald Observatory (code: ELP), and the Cerro Tololo Interamerican Observatory (code: LSC). Both cand o-band photometry were also obtained by the Asteroid Terrestrial-impact Last Alert System (ATLAS;Smithetal.2020)andreducedthroughthestandard ATLAS pipeline (Tonry et al. 2018). The BgVri +co-band absolute light curve from the follow-up campaigns are shown in Fig. 2.The photometry has been corrected for reddening using a Milky Way (MW) extinction of E(B−V)MW =0.025 ±0.001 mag, obtained using the Galactic dust map calibration of Schlafly & Finkbeiner (2011) and extinction factor RV=3.1. The host galaxy extinction was taken to be negligible relative to MW extinction, as there was no noticeable Na IDλλ 5890,5896 lines at the SN rest frame (e.g. Poznanski, Prochaska & Bloom 2012). Also, it should be noted that, as seen in Fig. 1, SN 2020cpg was located far from the galactic centre where the effect of dust is likely reduced. All uncorrected LCO photometry and ATLAS photometry are given in Tables A1 and A2, respectively. 2.3 Spectroscopy Spectra from multiple telescopes were obtained over an 80-d period post-explosion and reduced through standard means available within each observatory pipeline. The classification spectrum of SN2020cpg (Poidevin et al. 2020) was obtained with the LT, on 19/02/2020 using the Spectrograph for the Rapid Acquisition of Transients (SPRAT; Piascik et al. 2014) and was reduced by the 1https://lasair.roe.ac.uk/object/ZTF20aanvmdt/ LT automatic pipeline2(see Barnsley, Smith & Steele 2012,for details on the pipeline). Several later spectra were also obtained using the LT. Additional spectra for SN 2020cpg were obtained by the advanced Public ESO Spectroscopic Survey for Transient Objects (ePESSTO +)3(Smartt et al. 2015), using the ESO Faint Object Spectrograph and Camera mounted on the New Technology Telescope (NTT; EFOSC2; Buzzoni et al. 1984). ePESSTO +data were reduced as described in Smartt et al. (2015). The Alhambra Faint Object Spectrograph and Camera (ALFOSC) mounted on the Nordic Optical Telescope (NOT; Djupvik & Andersen 2010) provided several spectra of SN2020cpg, which were reduced by the Foscgui pipeline.4Multiple spectra were taken by the LCO 2m Faulkes Telescope South (FTS) at COJ and Faulkes Telescope North (FTN) at the Haleakala Observatory (code: OGG). We attempted to obtain further spectra after two and a half months post-explosion; however, SN 2020cpg was too dim at this point for the available telescopes to obtain good-quality spectra. All spectra have been binned to improve the S/N ratio, and de-reddened, assuming a standard RV=3.1 and the E(B−V) given in Section 2.1. All spectra can be seen in Fig. 3. The details on the phase from B-band max, observatory and instrument alongside the observed range are given in Table 1. 3 METHOD 3.1 Pseudo-bolometric light curve From the BgVri-band photometry obtained for SN2020cpg, we constructed a pseudo-bolometric light curve, shown in Fig. 4,using the pseudo-bolometric light curve code of Nicholl (2018). As we lack any ultraviolet (UV) or near-infrared (NIR) data, we approximate the missing luminosity in these bands by extrapolating the blackbody spectral energy distributions that were fit to the BgVri-bands into the UV and NIR regions. The UV and NIR contributions to the pseudobolometric light curve are relatively small at peak time, contributing ∼10–20 per cent and ∼15–25 per cent, respectively, compared to the optical contribution, which accounts for ∼50–60 per cent of total flux near bolometric peak (Lyman, Bersier & James 2013). We conclude that our extrapolation to UV and NIR bands does not introduce a significant error to the bolometric light curve. Along with the pesudo-bolometric light curve of SN 2020cpg, we construct pesudo-bolometric light curves for SN1993J (Richmond et al. 1994; Barbon et al. 1995; Richmond et al. 1996), SN 2003bg (Hamuy et al. 2009), SN 2008ax (Pastorello et al. 2008;Tsvetkov et al. 2009), SN 2009jf (Sahu et al. 2011; Bianco et al. 2014), SN2011dh (Tsvetkov et al. 2012; Sahu, Anupama & Chakradhari 2013; Brown et al. 2014), iPTF13bvn (Brown et al. 2014; Folatelli et al. 2016; Fremling et al. 2016), SN 2013ge (Drout et al. 2016), 2015ap (Prentice et al. 2019), and SN 2016gkg (Brown et al. 2014; Arcavi et al. 2017; Bersten et al. 2018). The comparison between theseSE-SNeisshowninFig.5. These SE-SNe were chosen, as they all have comprehensive coverage over the first ∼100 d post-explosion, they all have well-defined explosion dates and photospheric velocities both of which are required for the Arnettlike model used to obtain physical parameters. For these SE-SNe, we excluded any UV and NIR data available when constructing the pseudo-bolometriclight curveensuring the effectsof theUV and NIR 2http://telescope.livjm.ac.uk/TelInst/Inst/SPRAT/ 3www.pessto.org 4http://graspa.oapd.inaf.it/foscgui.html MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1835 Figure 2. The absolute magnitude photometry of SN 2020cpg in the BgVri-bands along with the ATLAS cand o-band, covering ∼130 d from the explosion date. The individual band light curves have been corrected for extinction, shifted by a constant magnitude and are shown in rest frame. The red dashed lines denote the epochs at which spectra were taken. extrapolation did not greatly influence the comparison between the SE-SNe. Where SNe lacked Sloan Digital Sky Survey (SDSS) filters, we used the corresponding Johnson–Cousins (J–C) filters to cover a similar wavelength range allowing for a more accurate comparison between the pseudo-bolometric light curves. 3.2 Physical parameters The bolometric luminosity of an SN is intrinsically linked to several physical parameters, those being the mass of nickel synthesized during the explosion, the amount of material ejected from the outer layers of the progenitor and the kinetic energy of the ejected mass. This relation was first formulated for Type Ia SNe by Arnett (1982) who assumed that all the energy that powers the bolometric light curve originated from the decay of 56Ni →56Co and the decay of 56Co →56Fe. While the model was initially formulated for SNe that do not undergo a hydrogen recombination phase, such as those seen in SE-SNeIb/c and SNe IIb, it has been used regularly for multiple types of SNe. This is done by ignoring the recombination phase and restricting the fitting to the rise and fall of the peak of the bolometric light curve that is powered by radioactive activity, as done in Lyman et al. (2016). The Arnett-like model also assumes that all 56Ni is located in a point at the centre of the ejecta, that the optical depth of the ejecta is constant throughout the evolution of the light curve, the initial radius prior to explosion is very small and that the diffusion approximation used for the model is that of photons. While these assumptions are acceptable, the approximation of constant opacity has a severe effect on diffusion time-scale which is dependent on the estimated ejecta mass and kinetic energy of the SN. The effect of neglecting the time-dependent diffusion on the 56Ni mass was discussed by Khatami & Kasen (2019), who concluded that this results in an over estimation of the 56Ni mass. Through alternative modelling methods it was seen that the 56Ni mass was overestimated by the Arnett-like model by ∼30–40 per cent (see e.g. Dessart et al. 2016; Woosley, Sukhbold & Kasen 2020). We initially used the Arnett-like model to determine the physical parameters of SN2020cpg and compare the results to several other SE-SNe. In the Arnett-like model the kinetic energy and ejecta mass have a strong dependence on the diffusion time-scale, τm,ofthe bolometric light curve, which is given as τm=κopt βc  1 26M3 ejc 5Ek 1 4.(1) WhereMejc is the mass of ejected material and Ekis the kinetic energy of the supernovae. Also cis the speed of light, βis the constant of integration derived by Arnett (1982) that takes the value of β≈13.8 and κopt is the optical opacity of the material ejected by the SN. For the Arnett-like model, a constant value of κopt =0.06 ±0.01cm2g−1 wasused. The degeneracybetween the ejecta mass andkinetic energy was broken using the photospheric velocity the event obtained from the velocity of theFe II 5169 Å linemeasured atmaximumbolometric luminosity. This is the epoch when the outer ejecta has the largest MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 1836 K. Medler et al. Figure 3. Spectroscopic evolution of SN2020cpg with details of the observations given in Table 1. The epochs on the right side are relative to Bmax in rest frame. The H α,HeI,FeII,CaII,andOIfeatures have been noted along with the main telluric feature at 7600, ⊕. The spectra have been binned to reduce the noise. contribution to the luminosity under the assumption of homogeneous density.Themodel wasalsoadjustedtoincludethe SNexplosiondate to allow for an improved fit and to place a constraint on the rise time of the SNe. For SNe with well observed pre-maximum and welldefined explosion dates we use the dates provided. The explosion date of SN2020cpg was obtained by constraining the fitting to limit the potential explosion date to after the date of last non-detection and prior to the initial observation. Due to the known problems with the Arnett-like model, in Section 5.3 we discuss an alternative method for determining the ejecta mass and kinetic energy of SN 2020cpg by comparing the light curve properties and physical properties determined by hydrodynamical MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1837 Table 1. Details of the spectroscopic observations of SN 2020cpg. Phase from both the predicted explosion date (Phaseexp) and the date of Bmax (PhaseBmax ) are given in rest frame. Date Phaseexp PhaseBmax Telescope+Range (d) (d) Instrument (Å) 17/02 +9−6 FTS en12 3500–10000 19/02 +11 −4 LT SPRAT 4000–8000 20/02 +12 −3 NTT EFOSC2 3685–9315 24/02 +16 +1 FTN FLOYDS 3500–9000 25/02 +17 +2 NTT EFOSC2 3380–10320 28/02 +20 +5 FTS en12 3500–10000 29/02 +21 +6 LT SPRAT 4000–8000 02/03 +23 +8 LT SPRAT 4000–8000 09/03 +30 +15 NOT ALFOSC 3200–9600 09/03 +30 +15 FTN FLOYDS 3500–10000 17/03 +38 +23 NTT EFOSC2 3380–10320 23/03 +44 +30 NTT EFOSC2 3380–10320 30/04 +82 +67 NOT ALFOSC 3200–9600 Figure 4. The pseudo-bolometric light curve of SN2020cpg constructed from the BgVri photometry. Luminosity is shown relative to days from the peak of the pseudo-bolometric light curve in rest frame and follows approximately 120 d from explosion. The red dashed lines indicate the epochs where spectra were taken and the black dashed line is the yielded explosion date. modelling of other SE-SNe, as done for SN2010ah in Mazzali et al. (2013, here after PM13). This method re-scales the physical parameters of other SE-SNe using equation (1) under the assumption that the optical opacity of the two SNe are equivalent. This is physically a more robust assumption than a fixed opacity for all SE-SNe as adopted by the Arnett-like model. A comparison between the results obtained from the Arnett-like model and the PM13 model is presented later in Section 5.3. 4 RESULTS 4.1 Multicolour light curves The early time rise of both the B-andV-bands were missed in the follow-up campaign; however, the peaks in both bands were observed, shown in Fig. 2. The bluer bands peaked several days beforethered bands, trise blue ≈15 d post-explosion and trise red ≈19 d,see Table 2. Both the Band gbands were followed for ∼30dbyLCO before the photometry bands dropped below the brightness threshold Figure 5. Pseudo-bolometric light curves of SN1993J, SN2003bg, SN2008ax, SN2009jf, SN2011dh, iPTF13bvn, SN 2013ge, SN2015ap, SN2016gkg,andSN2020cpg,coveringa periodof 100 d from their estimated explosion date. The Arnett-like model fit to the pseudo-bolometric light curves, detailed in Section 3.2, are shown as lines and were fitted out to ∼40 d before they started to strongly diverge from the pseudo-bolometric light curves. The velocities used to break the degeneracy for each SN, along with the predicted physical parameters, are given in Table 4. Table 2. Epoch of light-curve maximum, rise time in rest frame, and peak absolute magnitude for the BgVri photometry bands for SN 2020cpg. Band MJDmax Rise time (d) mmax Mmax B58902.1 14.7 ±2.5 18.38 ±0.02 −17.75 ±0.39 g58903.1 16.0 ±2.1 18.05 ±0.02 −18.04 ±0.40 V58904.7 17.0 ±2.1 18.16 ±0.02 −17.91 ±0.38 c58906.0 18.8 ±2.3 18.10 ±0.05 −17.97 ±0.38 r58906.2 18.6 ±2.1 18.06 ±0.02 −18.00 ±0.38 o58908.3 21.1 ±2.4 18.05 ±0.05 −18.01 ±0.39 i58909.2 22.0 ±2.1 18.05 ±0.02 −18.00 ±0.38 required for follow-up. The brightness for the Band gbands fell by ∼2 mag in the 30 d from the photometric peak, as a result of the SN rapidly cooling. The remaining bands fell at a slower rate, dropping by roughly 1 mag in the same time period, before their decline slowed down as the light curve transitioned to the exponential tail produced by the radioactive decay 56Co synthesized in the explosion. The ATLAS c-band was followed for approximately 100 d from the expected explosion date with the oband being followed for a further 30 d. The peaks in both bands were not well observed, especially in the c-band. As with the other bands the redder o-band declines at a slower rate just after maximum light when compared to the cband. The ATLAS bands have a greater error associated with them compared to the BgVri-bands, and as the ATLAS bands cover a similar wavelength range as the BgVri they were not used when constructing the pseudo-bolometric light curve. The light curves for He-rich CC-SNe display a variation within the evolution of their light curves due to the range of progenitor properties. As such the BgVri-band photometry for SN 2020cpg was compared with those of SN1993J, SN2003bg, SN 2009jf, SN2011dh, iPTF13bvn, SN2013ge, SN 2015ap, and SN 2016gkg. The absolute magnitude photometry for these SNe relative to SN2020cpg is shown in Fig. 6, with the details on each SN given in Table 3. SN 2020cpg is brighter than the majority of the other SNe that we compare to, with only SN2009jf and SN 2015ap being MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 1838 K. Medler et al. Figure 6. Comparison of the absolute magnitude light curves of several SNeIb and IIb with SN 2020cpg. All photometry is relative to Bmax light that was either taken from the literature or by fitting a Gaussian to the B-band peak. The light curves have been corrected for time dilation as well as corrected for both Milky Way and host galaxy reddening when possible. Primed bands are SDSS photometry bands and unprimed are the J–C photometry bands. Error on absolute magnitudes not included. of similar brightness. The Bandgbands evolve in a similar way to that of SN2015ap, while the other bands evolve more similar to SN2009jf. Due to the lack of pre-maximum light observations, it is not possible to determine if SN2020cpg had a shock breakout cooling peak similar to that seen in several other SE-SNe, such as SN1993J and SN 2016gkg. 4.2 Pseudo-bolometric light curves The pseudo-bolometric rise time for SN 2020cpg is tbol rise ≈16.0±2.5 d. Once peak luminosity had been reached the light curve rapidly declines for the next ≈34 d before settling on the exponential tail. Due to lack of much pre-peak photometry, the rise of the pseudobolometric light curve is not as well constrained as the post-peak light curve. SN2020cpg reaches a peak luminosity of log(Lmax)= 42.78 ±0.08[erg s−1], which is higher than the average luminosity of Type IIb +Ib(II), which has a value of log(Lmax)=42.2+0.4 −0.1[erg s−1], and the average maximum luminosity of Type IIb +IIb(I), log(Lmax)=42.09 ±0.17 [erg s−1], as given in Prentice et al. (2019), showing that SN2020cpg lies at the brighter end of the SE-SNe regime. We fit the pseudo-bolometric light curve of SN2020cpg with the Arnett-like model, using a photospheric velocity of vph ≈12500 ±1500 km s−1to break the degeneracy between the kinetic MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1839 Table 3. Details for several historical Type Ib and IIb SNe which have been compared to SN2020cpg. SN Explosion date Bmax date Redshift Distance E(B−V)MW E(B−V)Host Reference (MJD) (MJD) (Mpc) (mag) (mag) 1993J 49072.0 49093.48 −0.000113 2.9 0.069 0.11 1, 2, 3 2003bg 52695.0 52718.35 0.00456 20.25 0.018 - 5 2008ax 54528.8 54546.86 0.001931 5.1 0.0188 0.28 4, 6 2009jf 55101.33 55120.91 0.0079 31 0.097 0.03 7, 10 2011dh 55712.5 55730.82 0.001638 7.25 0.0309 0.05 8, 9, 11 iPTF13bvn 56458.17 56474.95 0.00449 19.94 0.0436 0.17 11, 12, 13 2013ge 56602.5 56618.93 0.004356 19.342 0.0198 0.047 14 2015ap 57270.0 57283.0 0.01138 50.082 0.037 – 17 2016gkg 57651.15 57669.67 0.0049 21.8 0.0166 0.09 11, 15, 16 2020cpg 58887.6 58902.07 0.037 158.6 0.0246 – – References. 1: Richmond et al. (1994), 2: Barbon et al. (1995), 3: Richmond et al. (1996), 4: Pastorello et al. (2008), 5: Hamuy et al. (2009), 6: Tsvetkov et al. (2009), 7: Sahu et al. (2011), 8: Tsvetkov et al. (2012), 9: Sahu et al. (2013), 10: Bianco et al. (2014), 11: Brown et al. (2014), 12: Fremling et al. (2016), 13: Folatelli et al. (2016), 14: Drout et al. (2016), 15: Arcavi et al. (2017), 16: Bersten et al. (2018), and 17: Prentice et al. (2019). Table 4. Physical properties of several SE-SNe derived from the fitting of the Arnett-like model described in Section 3 and shown in Fig. 5.The photospheric velocity used for each SN was taken from their discovery paper. SN vph MNi Mejc Ek (km s−1)(M )(M )(10 51erg) 1993J 8000 ±1000 0.11 ±0.03 2.0 ±0.8 0.8 ±0.3 2003bg 10000 ±500 0.13 ±0.04 2.8 ±0.9 1.7 ±0.5 2008ax 7500 ±500 0.14 ±0.04 2.6 ±1.0 0.9 ±0.3 2009jf 11000 ±500 0.27 ±0.09 4.0 ±1.4 2.9 ±1.0 2011dh 7000 ±1000 0.09 ±0.02 1.8 ±0.5 0.5 ±0.1 iPTF13bvn 8000 ±1000 0.07 ±0.02 1.7 ±0.4 0.6 ±0.2 2013ge 10500 ±500 0.12 ±0.03 2.7 ±0.8 1.8 ±0.5 2015ap 16000 ±1000 0.22 ±0.05 1.4 ±0.4 2.2 ±0.6 2016gkg 8000 ±1000 0.10 ±0.02 1.9 ±0.4 0.7 ±0.2 2020cpg 12500 ±1200 0.27 ±0.08 3.4 ±1.0 2.9 ±0.9 energy and ejecta mass. The value of vph was obtained from the averageFe II linevelocitiesatpeaklight.Theaveragevalueofthe Fe II triplet was used instead of the commonly employed FeII λ5169 line due to the low signal-to-noise ratio within the FeII region of the spectrum taken around peak luminosity. From the Arnett-like model fit to SN2020cpg’spseudo-bolometric light curve,we deriveanickelmass of MNi =0.27 ±0.08 M. The ejecta mass and kinetic energy given bythe fithad avalueof Mejc =3.4 ±1.0 Mand Ek=2.9 ±0.9×1051 erg, respectively. This process was then repeated for the bolometric light curves of the other SE-SNe shown in Fig. 5and the derived physical parameters are given in Table 4. As expected the Arnett-like model deviates from the pseudo-bolometric light curves at later times (t40 d) when the SNe start to transition into the nebular phase. Relative to the other SNe, SN2020cpg has a high nickel mass similar to both SN2009jf and SN 2015ap, shown in Table 4. The similar MNi between SN 2020cpg and both SN 2009jf and SN2015ap is expected from their comparable peak luminosities. The ejecta mass and kinetic energy of SN2020cpg is also higher than the majority of the SE-SNe, we have looked atsuggesting thatthe progenitorof SN 2020cpg wasa high-mass star prior to the stripping of the outer envelope. However, due to the problems associated with the Arnett-like approach, we discuss an alternative approach to obtain the values for Mejc and Ek in Section 5.3, we then use the values for Mejc and Ekderived using the PM13 method to estimate the progenitor mass. 4.3 Spectral evolution and comparison At early times, the spectra of SN2020cpg (Fig. 3) shows a large blue excess. The spectra rapidly cool until around +15 d from Bmax. Prominent He Ilines are present throughout the spectral evolution with the HeI5876 Å line being the most prominent and the 6678 Å line becoming stronger at around +23 d. Around +1 d post Bmax,the spectrumdevelopsan absorptionfeaturelocatedinthe Hαregionthat persists for ∼30 d. At earlier times during the spectral evolution, the Hαfeature is split into a high-velocity and low-velocity components that merge into a single H αfeature at later times. The presence of the H αline provides strong evidence that SN 2020cpg is not a standard Type Ib SN and may be an intermediate SN between the H-rich and H-poor SE-SNe. While the feature around 6300Å may be interpreted as the presence of silicon, this is not likely, because it would imply that absorption from other silicon transitions, around 4100 and 5900 Å should be detected in this and later spectra which is not observed. Moreover, when identified as silicon, the line shift would indicate a velocity of 3000 km s−1which is far too slow for this epoch. These pieces of evidence alongside the lack of silicon in the spectra of other well-observed Type Ib/c SNe give strong evidence that the feature is the result of the presence of hydrogen within the outer envelope. Later, the spectral evolution shows the development of FeII λλ4924, 5018, 5169 lines, although it should be noted that the FeII lines are located close to He Ilines making the separation of these lines difficult, especially given the high noise in this region of the spectra. The evolution of the line velocities for H α,HeIλλ5876, 6678, 7065 and Fe II λλ4924, 5018, 5169 were determined by the fitting of a Gaussian to each feature to locate the minima. The line evolution of each elemental feature is shown in Fig. 7. The line velocities derived from the Gaussian fits are given in Fig. 8. The main source of error for these elemental line velocities comes from the low S/N of the spectra, especially on the fringes where the Fe II line is located, which makes the fitting of the Gaussian more difficult. This results in an error derived from the Gaussian fitting of approximately 15 percent, with a negligible error associated with the redshift. For the H αfeature, we separate the minimum into two distinct highand low-velocity components. The high-velocity feature is visible from the second spectrum, −4 d, until approximately +15 d post Bmax,asshown by the solid red line in Fig. 7. At this point, the high-velocity and low-velocity components blend together in the later spectra to form asingleHαfeature. There is a clear separation between the highand low-velocity H αcomponents, with the low velocity remaining MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 1840 K. Medler et al. Figure 7. Evolution of SN2020cpg spectra. The spectra have been plotted between 4000 and 8000 Å to highlight the regions where prominent H α,HeI,and FeII features are visible. The different elements are shown by the different lines, with H α=red, He I=blue, and Fe II =green, and different element lines given by different styles. Lines are only shown when line features are clearly visible within the spectra. MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1847 with the University of Copenhagen and NOTSA, with observation having been made with the Nordic Optical Telescope, operated at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grant nos NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; by-products of the NEO search include images and catalogues from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, and the Space Telescope Science Institute. 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Apparent BgVri LCO Photomety of SN 2020cpg, no k-correction or extinction correction applied. MJDBB(err)MJD g’g’(err)MJD VV(err) MJDr’r’(err) MJDi’i’(err) (mag) (mag) (mag) (mag) (mag) 58900.362 18.39(0.02) 58894.544 18.55(0.09) 58900.367 18.25(0.02) 58894.501 18.49(0.08) 58900.380 18.41(0.02) 58900.364 18.37(0.02) 58900.371 18.05(0.01) 58900.369 18.28(0.02) 58900.376 18.35(0.02) 58900.382 18.40(0.02) 58902.316 18.35(0.02) 58900.374 18.20(0.01) 58902.322 18.20(0.02) 58900.378 18.24(0.02) 58902.335 18.24(0.02) 58902.319 18.36(0.02) 58902.326 18.08(0.01) 58902.324 18.18(0.02) 58902.331 18.16(0.02) 58902.336 18.22(0.02) 58903.337 18.49(0.02) 58902.328 18.06(0.01) 58903.343 18.10(0.02) 58902.333 18.14(0.02) 58903.355 18.20(0.02) 58903.340 18.45(0.02) 58903.346 18.06(0.01) 58903.345 18.08(0.02) 58903.352 18.16(0.02) 58903.357 18.17(0.02) 58905.101 18.35(0.02) 58903.349 18.07(0.01) 58905.238 18.10(0.02) 58903.354 18.13(0.02) 58905.251 18.17(0.02) 58905.233 18.39(0.02) 58905.242 18.05(0.01) 58905.240 18.11(0.02) 58905.247 18.07(0.02) 58905.253 18.18(0.02) 58905.236 18.41(0.02) 58905.245 18.06(0.01) 58906.256 18.24(0.02) 58905.249 18.09(0.02) 58906.269 18.06(0.02) 58906.251 18.47(0.02) 58906.260 18.10(0.01) 58906.258 18.25(0.02) 58906.265 18.11(0.02) 58906.271 18.04(0.02) 58906.254 18.56(0.02) 58906.263 18.09(0.01) 58907.277 18.21(0.02) 58906.267 18.08(0.02) 58907.290 18.12(0.02) 58907.272 18.59(0.02) 58907.281 18.14(0.01) 58907.279 18.24(0.02) 58907.286 18.02(0.02) 58907.291 18.06(0.02) 58907.274 18.58(0.02) 58907.283 18.16(0.01) 58909.253 18.35(0.02) 58907.288 18.05(0.02) 58909.265 18.08(0.02) 58909.248 18.63(0.02) 58909.256 18.16(0.02) 58909.255 18.37(0.02) 58909.262 18.01(0.02) 58909.267 18.03(0.02) 58909.250 18.73(0.02) 58909.259 18.17(0.02) 58910.336 18.09(0.02) 58909.264 18.04(0.02) 58910.349 18.01(0.02) 58910.331 18.48(0.02) 58910.340 18.29(0.02) 58910.338 18.06(0.02) 58910.345 18.07(0.02) 58910.350 18.06(0.02) 58910.333 18.52(0.02) 58910.342 18.23(0.02) 58912.105 18.18(0.02) 58910.347 18.10(0.02) 58912.118 18.05(0.04) 58912.100 18.77(0.02) 58912.109 18.37(0.02) 58912.107 18.34(0.02) 58912.114 18.11(0.02) 58912.119 18.06(0.04) 58912.102 18.78(0.02) 58912.111 18.39(0.02) 58914.389 18.45(0.02) 58912.116 18.09(0.02) 58914.401 18.09(0.03) 58914.383 19.01(0.02) 58914.392 18.63(0.02) 58914.390 18.44(0.02) 58914.397 18.17(0.02) 58914.403 18.14(0.03) 58914.386 19.10(0.02) 58914.395 18.55(0.02) 58916.355 18.76(0.02) 58914.399 18.18(0.02) 58916.368 18.19(0.02) 58916.350 19.38(0.04) 58916.359 18.80(0.02) 58916.357 18.70(0.02) 58916.364 18.20(0.02) 58916.369 18.22(0.02) 58916.352 19.37(0.04) 58916.361 18.80(0.02) 58917.323 18.48(0.02) 58916.366 18.28(0.02) 58917.347 18.37(0.05) 58917.313 19.31(0.05) 58917.331 18.89(0.02) 58917.327 18.56(0.02) 58917.341 18.28(0.02) 58917.349 18.49(0.05) 58917.318 19.24(0.05) 58917.336 18.98(0.02) 58920.620 18.62(0.06) 58917.344 18.34(0.02) 58920.643 18.46(0.05) 58920.610 19.47(0.09) 58920.627 19.24(0.02) 58920.623 18.71(0.06) 58920.638 18.54(0.02) 58920.646 18.40(0.05) 58920.615 19.47(0.09) 58920.632 19.14(0.02) 58924.184 18.87(0.04) 58920.640 18.50(0.02) 58924.208 18.40(0.03) 58924.174 19.94(0.06) 58924.192 19.45(0.02) 58924.188 18.85(0.04) 58924.202 18.59(0.02) 58924.210 18.40(0.03) 58924.179 19.90(0.06) 58924.197 19.57(0.02) 58927.094 19.09(0.04) 58924.205 18.60(0.02) 58927.118 18.59(0.02) 58927.084 20.25(0.06) 58927.102 19.75(0.02) 58927.098 19.00(0.04) 58927.112 18.81(0.02) 58927.120 18.62(0.02) 58927.089 20.19(0.06) 58927.107 19.67(0.02) 58931.054 19.37(0.04) 58927.115 18.85(0.02) 58931.080 18.81(0.01) 58931.044 20.65(0.05) 58931.062 20.11(0.02) 58931.058 19.52(0.04) 58931.072 19.09(0.02) 58931.084 18.78(0.01) 58931.049 20.42(0.05) 58931.067 20.05(0.02) 58949.708 19.92(0.12) 58931.076 19.15(0.02) 58939.657 19.12(0.01) – – – – 58951.685 19.99(0.08) 58939.653 19.34(0.02) 58951.692 19.53(0.03) – – – – 58959.235 20.11(0.05) 58951.689 19.66(0.04) 58959.243 19.54(0.04) – – – – 58974.207 20.20(0.15) 58959.239 19.77(0.03) 58974.215 19.76(0.11) – – – – 58982.144 20.48(0.15) 58974.211 20.01(0.10) 58982.151 19.88(0.05) – – – – 58985.538 20.53(0.07) 58982.147 20.03(0.04) 58985.546 19.91(0.05) – – – – 58993.431 20.36(0.09) 58985.542 20.07(0.03) 58993.439 20.14(0.09) – – – – 59000.823 20.67(0.14) 58993.435 20.17(0.11) 59000.831 20.43(0.15) – – – – 59008.891 20.54(0.15) 59000.827 20.21(0.10) 59008.898 20.17(0.06) – – – – – – 59008.894 20.40(0.12) – – MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021 SN 2020cpg: an energetic SE-SNe 1849 Table A2. Apparent c+oband ATLAS photometry for SN 2020cpg. Photometry has not been corrected for either extinction or k-correction. MJDcc(err) MJDoo(err) MJDoo(err) (mag) (mag) (mag) 58903.464 18.10(0.06) 58901.489 18.33(0.08) 58953.488 19.32(0.17) 58903.499 18.10(0.05) 58901.493 18.23(0.07) 58957.400 19.80(0.30) 58903.503 18.19(0.06) 58901.500 18.24(0.06) 58957.412 19.59(0.20) 58903.512 18.18(0.05) 58901.511 18.22(0.06) 58957.415 19.87(0.27) 58911.503 18.13(0.05) 58905.562 18.07(0.05) 58961.441 19.38(0.16) 58931.502 19.27(0.13) 58905.565 18.09(0.05) 58961.444 19.78(0.22) 58931.522 19.59(0.17) 58905.573 18.06(0.05) 58961.465 19.67(0.21) 58931.530 19.45(0.15) 58905.583 18.10(0.05) 58965.446 19.26(0.20) 58931.539 19.46(0.16) 58913.454 18.20(0.07) 58965.457 19.73(0.29) 58935.583 19.78(0.25) 58913.458 18.10(0.06) 58965.491 19.55(0.21) 58935.586 19.55(0.21) 58913.463 18.24(0.07) 58969.404 20.12(0.30) 58959.426 20.14(0.30) 58913.477 18.19(0.07) 58969.417 19.83(0.23) 58959.430 19.66(0.19) 58917.449 18.25(0.18) 58969.421 20.02(0.27) 58959.438 20.18(0.28) 58917.457 18.32(0.19) 58969.428 19.75(0.22) 58959.455 20.05(0.27) 58917.467 18.78(0.27) 58971.423 19.68(0.26) 58967.415 19.96(0.23) 58925.533 18.70(0.21) 58971.439 19.54(0.25) 58967.425 20.10(0.26) 58925.538 18.65(0.17) 58977.446 19.21(0.28) 58967.460 20.10(0.28) 58933.537 19.36(0.18) 58981.387 19.63(0.20) 58982.391 20.06(0.25) 58933.543 19.02(0.13) 58981.404 20.07(0.30) 58987.399 20.12(0.27) 58933.547 19.14(0.15) 58981.415 19.76(0.26) – – 58933.559 18.86(0.12) 58985.366 19.94(0.23) – – 58937.496 19.27(0.13) 58985.380 20.12(0.28) – – 58937.498 19.32(0.12) 58989.354 19.76(0.18) – – 58937.508 18.99(0.29) 58989.358 20.08(0.25) – – 58937.522 19.47(0.17) 58989.393 20.06(0.24) – – 58941.433 18.80(0.13) 58997.324 20.07(0.28) – – 58941.445 18.91(0.15) 58997.331 20.13(0.28) – – 58941.448 19.50(0.24) 58997.335 20.06(0.30) – – 58941.463 19.18(0.27) 58999.352 19.83(0.28) – – 58943.479 19.44(0.25) 58999.355 19.77(0.29) – – 58943.484 19.47(0.30) 59006.346 19.44(0.30) – – 58949.477 19.20(0.26) 59013.351 20.26(0.30) – – 58949.482 19.30(0.30) 59013.357 20.25(0.29) – – 58951.461 18.89(0.28) 59013.367 20.23(0.28) – – 58951.469 19.57(0.29) 59021.328 20.08(0.27) – – 58951.482 19.17(0.29) 59021.346 20.04(0.27) – – 58953.467 19.28(0.19) 59025.321 20.26(0.30) – – 58953.474 19.92(0.28) 59029.318 19.44(0.30) – – 58953.478 19.46(0.19) 59037.297 19.95(0.29) This paper has been typeset from a TEX/L A TEX file prepared by the author. MNRAS 506, 1832–1849 (2021) Downloaded from https://academic.oup.com/mnras/article/506/2/1832/6307516 by Universidad de Granada - Biblioteca user on 21 October 2021