# SN 2020jfo: A short plateau Type II supernova from a low mass progenitor

Rishabh Singh Teja Indian Institute of Astrophysics, II Block, Koramangala,
Bengaluru-560034, Karnataka, India  Pondicherry University, R.V. Nagar,
Kalapet, Pondicherry-605014, UT of Puducherry, India Rishabh Singh Teja
<EMAIL_ADDRESS><EMAIL_ADDRESS>Avinash Singh Hiroshima
Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima,
Hiroshima 739-8526, Japan  D.K. Sahu Indian Institute of Astrophysics, II
Block, Koramangala, Bengaluru-560034, Karnataka, India  G.C. Anupama Indian
Institute of Astrophysics, II Block, Koramangala, Bengaluru-560034, Karnataka,
India  Brajesh Kumar Aryabhatta Research Institute of Observational Sciences,
Manora Peak, Nainital-263001, Uttarakhand, India  Nayana A.J. Indian Institute
of Astrophysics, II Block, Koramangala, Bengaluru-560034, Karnataka, India

(Received January 12, 2022; Revised March 22, 2022; Accepted March 25, 2022)

###### Abstract

We present spectroscopic and photometric observations of the Type IIP
supernova, SN 2020jfo, in ultraviolet and optical wavelengths. SN 2020jfo
occurred in the spiral galaxy M61 (NGC 4303), with eight observed supernovae
in the past 100 years. SN 2020jfo exhibited a short plateau lasting $<\,65$ d,
and achieved a maximum brightness in _V_ -band of $\rm M_{V}=-17.4\pm 0.4$ mag
at about $\rm 8.0\pm 0.5$ d since explosion. From the bolometric light curve,
we have estimated the mass of $\rm{}^{56}Ni$ synthesised in the explosion to
be $\rm 0.033\pm 0.006\,M_{\odot}$. The observed spectral features are typical
for a type IIP supernova except for shallow H$\alpha$ absorption throughout
the evolution and the presence of stable 58Ni feature at 7378 Å, in the
nebular phase. Using hydrodynamical modelling in the MESA + STELLA framework,
an ejecta mass of $\rm\sim 5\,M_{\odot}$ is estimated. Models also indicate SN
2020jfo could be the result of a Red Super Giant progenitor with $\rm
M_{ZAMS}\,\sim\,12\,M_{\odot}$. Bolometric light curve modelling revealed the
presence of a secondary radiation source for initial $\rm\sim 20$ d, which has
been attributed to interaction with a circumstellar material of mass $\rm\sim
0.2\,M_{\odot}$, which most likely was ejected due to enhanced mass loss about
20 years prior to the supernova explosion.

Observational astronomy(1145) — Type II supernovae(1731) — Red supergiant
stars(1375) — Hydrodynamical simulations(767)

††facilities: HCT: 2 m, Swift (UVOT), Palomer-ZTF, DOT: 3.6 m††software: MESA
(release 15140), STELLA, PyRAF (v2.1.14), Astropy (Astropy Collaboration et
al., 2018), emcee (Foreman-Mackey et al., 2013), matplotlib (Hunter, 2007),
pandas (Wes McKinney, 2010), SciPy (Virtanen et al., 2020), seaborn (Waskom,
2021)

##  1 Introduction

Core-Collapse Supernovae (CCSNe) are a diverse and heterogeneous class, mainly
due to a vast variety and complexity of their possible progenitors,
surroundings, and the physics associated with these. Stars exceeding the
threshold of $\rm\sim 8\,M_{\odot}$, terminate their lives in these violent
explosions injecting recently synthesised elements, which in turn enrich the
interstellar medium. During the progenitor’s evolution, they develop a
degenerate core with an outer envelope surrounding it, and when this core
reaches its “Chandrasekhar mass” ($\rm\sim 1.5\ M_{\odot}$), they implode due
to gravitational instability (Burrows & Vartanyan, 2021). They leave neutron
stars or black holes as a remnant post collapse. Various routes have been
proposed for such events, viz. Electron-Capture SNe, Fe-Core SNe,
$\rm\gamma$-Ray Burst SNe, and Pair-Instability SNe (Janka, 2012 and
references therein).

The primary classification of supernovae is based on the presence of spectral
features around optical maximum (Filippenko, 1997). SNe with an absence of
hydrogen features in their spectra are termed as Type I events, whereas Type
II events show prominent hydrogen Balmer features in their spectra (Minkowski,
1941). Type II events are further classified based on their light curves, as a
Type IIP if a plateau phase of constant luminosity is observed and a Type IIL
when the decline from the peak is linear before falling to the tail powered by
the radioactive decay. Type IIP SNe are more commonly observed with a
definitive plateau of around 100 days, but with varied luminosity, before
falling to the radioactive-decay powered tail. In certain events, narrow and
strong emission features in the spectra arise due to the strong interaction of
ejecta with the circumstellar material (CSM), hence terming such events as
Type IIn. In addition to these, some events, labelled type IIb, show hydrogen
lines initially in their spectra, which weaken in the later phases when strong
helium features are visible.

Direct detection of a progenitor (Van Dyk, 2017 and references therein) have
tightly constrained the mass range of Red Super Giant (RSG) progenitors of
Type IIP supernovae. Type IIP SNe, such as the SN 2012aw (Van Dyk et al.,
2012; Fraser et al., 2012) and SN 2012ec (Maund et al., 2013) have a directly
detected progenitor in archival images that puts a mass limit of $\rm
8\lesssim M/M_{\odot}\lesssim 17$ (Branch & Wheeler, 2017; Arcavi, 2017) for
the Type IIP progenitors. However, theoretical models predict a much higher
upper limit for the progenitor mass.

Many statistical studies, including both observational (Anderson et al., 2014;
Sanders et al., 2015; Valenti et al., 2016; Gutiérrez et al., 2017a) and
synthetic/hydrodynamical modelling (Eldridge et al., 2018; Hiramatsu et al.,
2021a) have shown that Type IIP, IIL, and IIb form a continuous sequence of
objects with the plateau or decline depending on the outer hydrogen envelope
mass. It has been seen statistically in both types of attempts that the
probability of events occurring with a characteristic small plateau of tens of
days is relatively low.

Apart from the above-mentioned properties, detecting the presence of CSM
around these events has become quite usual. Conspicuous signatures of CSM have
been observed in many CCSNe events both in spectra and light curves (Bullivant
et al., 2018a; Rui et al., 2019; Singh et al., 2019; Zhang et al., 2022;
Jacobson-Galán et al., 2022). RSGs have a significant mass-loss history, and
it is intuitive for the CSM to be present (Mauron & Josselin, 2011). However,
in many cases, the CSM evades from being detected directly (Andrews et al.,
2019; Dong et al., 2021). Nonetheless, the heating effects of such hidden CSM
are at times observed in the form of enhanced luminosity (Nagao et al., 2020;
Haynie & Piro, 2021).

Even with a robust and ever-increasing classification scheme for these
phenomena, some do not fit very well in either one of the classified
designations. With the mushrooming supernova numbers discovered by the various
existing regular night-sky surveys and those expected from forthcoming survey
projects, there would be more such discrete events that would not be
considered standard. Hence, studying individual events in detail with an
endeavour to learn about their origins would help significantly in
understanding of the classes as a whole.

SN 2020jfo (also known as ZTF20aaynrrh) was discovered by Zwicky Transient
Facility (ZTF, Bellm et al., 2019), using the Palomar 1.2m Oschin telescope on
2020 May 06 (JD = 2458975.70) in the galaxy M61 (NGC 4303) at
$\alpha=12^{h}21^{m}50^{s}.479,\ \delta=+04\arcdeg 28\arcmin 54.14\arcsec\
(J2000)$. It was discovered at an AB magnitude of 16.0 mag in ZTF $r$-band.
Merely within a day after the discovery, spectroscopic classification of SN
2020jfo was performed by the ZTF group (Perley et al., 2020) using spectra
obtained with LT/SPRAT, NOT/ALFOSC, and P60/SEDM. Cross-correlation of the
observed spectra with the SNID (Blondin & Tonry, 2007) library showed a good
match to Type IIP supernova SN 1999gi, about 7 days before maximum light. SN
2020jfo was suggested to be a young Type II supernova. Based on nebular
spectra, Sollerman et al. (2021) suggest the SN to have a low mass progenitor.

0.9

Figure 1: The $I$ band image of SN 2020jfo in M61 obtained on 2020 May 08. The
positions of the three regions of the archival SDSS-spectra [dark orange
squares] along with SN 2020jfo [white circle] have been marked.

In this paper, we present a comprehensive photometric and spectroscopic
analysis along with hydrodynamical modelling of SN 2020jfo. Subsequent
sections are divided in the following manner. Section 2 describes optical
observations of SN 2020jfo along with a brief outline of the data reduction
procedure. Host galaxy properties, light curve analysis and various physical
parameters of SN 2020jfo are presented in Section 3. Section 4 deals with the
spectroscopic properties, while possible progenitor and its properties
estimated using various methods viz. semi-analytical modelling, nebular phase
spectrum, and hydrodynamical modelling are discussed in Section 5. Section 6
provides a brief summary of the results presented.

##  2 Observations and Data Reduction

Table 1: Optical Photometry of SN 2020jfo from HCT JD | Phase | _U_ | _B_ | _V_ | _R_ | _I_
---|---|---|---|---|---|---
_(2458900+)_ | _(d)_ | _(mag)_ | _(mag)_ | _(mag)_ | _(mag)_ | _(mag)_
77.2 | 3.2 | 13.89 $\pm$ 0.17 | 14.63 $\pm$ 0.05 | 14.85 $\pm$ 0.05 | - | 14.84 $\pm$ 0.01
78.3 | 4.3 | - | 14.57 $\pm$ 0.02 | 14.80 $\pm$ 0.03 | 14.74 $\pm$ 0.07 | 14.68 $\pm$ 0.02
79.2 | 5.2 | - | 14.51 $\pm$ 0.03 | 14.61 $\pm$ 0.04 | 14.57 $\pm$ 0.04 | 14.48 $\pm$ 0.05
80.2 | 6.2 | 13.84 $\pm$ 0.08 | 14.55 $\pm$ 0.03 | 14.61 $\pm$ 0.03 | - | 14.34 $\pm$ 0.05
81.1 | 7.1 | 13.85 $\pm$ 0.02 | 14.57 $\pm$ 0.02 | - | 14.32 $\pm$ 0.05 | -
83.1 | 9.1 | - | 14.57 $\pm$ 0.01 | 14.59 $\pm$ 0.01 | 14.38 $\pm$ 0.02 | 14.36 $\pm$ 0.03
85.1 | 11.1 | 14.09 $\pm$ 0.05 | 14.63 $\pm$ 0.01 | 14.57 $\pm$ 0.02 | - | 14.39 $\pm$ 0.02
86.1 | 12.1 | - | 14.55 $\pm$ 0.03 | - | - | 14.30 $\pm$ 0.03
87.1 | 13.1 | - | 14.72 $\pm$ 0.02 | - | 14.46 $\pm$ 0.04 | -
89.3 | 15.3 | 14.41 $\pm$ 0.07 | 14.80 $\pm$ 0.02 | 14.70 $\pm$ 0.03 | 14.42 $\pm$ 0.02 | 14.36 $\pm$ 0.03
90.2 | 16.2 | 14.54 $\pm$ 0.06 | 14.84 $\pm$ 0.02 | - | 14.47 $\pm$ 0.04 | -
100.2 | 26.2 | 15.64 $\pm$ 0.11 | 15.35 $\pm$ 0.03 | 14.80 $\pm$ 0.03 | 14.47 $\pm$ 0.03 | -
102.3 | 28.3 | 15.66 $\pm$ 0.13 | 15.47 $\pm$ 0.05 | 14.80 $\pm$ 0.05 | 14.50 $\pm$ 0.02 | 14.35 $\pm$ 0.03
104.3 | 30.3 | - | 15.57 $\pm$ 0.04 | 14.82 $\pm$ 0.02 | 14.51 $\pm$ 0.02 | 14.33 $\pm$ 0.14
109.2 | 35.2 | 16.28 $\pm$ 0.11 | 15.69 $\pm$ 0.02 | 14.86 $\pm$ 0.02 | 14.54 $\pm$ 0.02 | 14.35 $\pm$ 0.02
111.1 | 37.1 | 16.46 $\pm$ 0.06 | 15.70 $\pm$ 0.01 | 14.91 $\pm$ 0.02 | 14.53 $\pm$ 0.03 | 14.36 $\pm$ 0.04
119.2 | 45.2 | 16.77 $\pm$ 0.13 | 15.96 $\pm$ 0.03 | 15.01 $\pm$ 0.02 | 14.62 $\pm$ 0.05 | 14.40 $\pm$ 0.03
130.2 | 56.2 | - | 16.36 $\pm$ 0.04 | 15.20 $\pm$ 0.03 | 14.79 $\pm$ 0.03 | 14.51 $\pm$ 0.02
144.2 | 70.2 | 19.04 $\pm$ 0.14 | 18.15 $\pm$ 0.03 | 16.90 $\pm$ 0.01 | 16.14 $\pm$ 0.02 | 15.89 $\pm$ 0.04
267.5 | 193.5 | - | 19.50 $\pm$ 0.07 | 18.67 $\pm$ 0.03 | 17.71 $\pm$ 0.06 | 17.54 $\pm$ 0.04
280.5 | 206.5 | - | 19.51 $\pm$ 0.02 | 18.71 $\pm$ 0.04 | 17.92 $\pm$ 0.02 | 17.85 $\pm$ 0.05
309.5 | 235.5 | - | 19.75 $\pm$ 0.04 | 19.21 $\pm$ 0.03 | 18.40 $\pm$ 0.04 | 18.36 $\pm$ 0.01
316.4 | 242.4 | - | 19.50 $\pm$ 0.49 | 19.48 $\pm$ 0.29 | 18.46 $\pm$ 0.17 | 18.55 $\pm$ 0.26
341.4 | 267.4 | - | - | 19.71 $\pm$ 0.05 | 18.93 $\pm$ 0.04 | -

###  2.1 Optical Photometry with 2m HCT

A quick follow up of SN 2020jfo began on 2020 May 07 (JD 2458977.2), i.e.,
$\sim$ 2 days after discovery, with the Himalayan Faint Object Spectrograph
Camera (HFOSC, Prabhu, 2014) mounted on the 2-m Himalayan Chandra Telescope
(HCT), situated at the Indian Astronomical Observatory (IAO), Hanle, India. It
was monitored in two phases. In the first phase, it was observed until 2020
July 14 (JD 2459044.1), after which it went into Solar conjunction. When it
reappeared in the night sky, the second phase of observations were carried out
from 2020 November 14 (JD 2459167.5) to 2021 January 26 (JD 2459241.5). Broad-
band photometric observations in Bessell $UBVRI$ filters were carried out for
a total of 23 epochs. The HCT optical data presented here are supplemented
with data from the ZTF in $g$ and $r$ bands, obtained through ALeRCE (Sánchez-
Sáez et al., 2021).

HFOSC is equipped with an E2V CCD chip having a dimension of $2048\times 4096$
pixels. The readout noise and gain of the camera are $\rm 5.75\,e^{-}$ and
$\rm 0.28\,e^{-}\,ADU^{-1}$, respectively. The central $\rm 2K\times 2K$
pixels used for imaging, covers a field of view (FOV) of $\rm 10\arcmin\times
10\arcmin$ at an image scale of $\rm 0.296\arcsec\,pixel^{-1}$. Object frames
were obtained in multiple filters. In addition to the object frames, several
bias and sky flat frames were observed at each epoch. The data were pre-
processed by performing the standard tasks of bias subtraction, flat fielding,
and cosmic ray removal through packages available in IRAF implemented through
pyRAF as given in Singh (2021). At certain epochs, especially during the late
phase, multiple frames in the same band were observed, which were later
aligned and combined to improve the signal-to-noise ratio (SNR) in the
resultant object frame. The zero points used to calibrate the secondary
standards in the SN field were determined using the average colour terms for
the telescope detector system and field stars calibrated from SN 2014dt field
(Singh et al., 2018b).

Figure 2: Panchromatic light curves for SN 2020jfo with photometry from HCT,
Swift/$UVOT$ and ZTF. The time period for which SN 2020jfo went behind the Sun
has been obliterated from the plot and is marked by the discontinuity in the
abscissa. Offsets in the apparent magnitudes are for visual clarity.

Since SN 2020jfo is situated in an outer spiral arm of M61, the host
brightness could significantly affect the supernova luminosity, especially
during the late phase. Hence, we used template images of the host, obtained as
a part of our monitoring programme of SN 2008in, to remove the contribution
from the host. The template images were aligned with the field of SN 2020jfo,
background subtracted, PSF-matched, and scaled. The scaled templates were then
subtracted, leaving only the SN in the resultant images. Aperture photometry
of the SN was then performed, and the SN magnitudes were calibrated using the
nightly zero-points obtained from the original images. The template
subtraction procedure adopted is given in Singh et al. (2019). The estimated
magnitudes are listed in Table 1 and plotted in Figure 2.

###  2.2 UV-optical photometry with _S_ wift/UVOT

Table 2: UV-Optical Photometry of SN 2020jfo from _Swift_ /UVOT JD | Phase | _UVW2_ | _UVM2_ | _UVW1_ | _UVU_ | _UVB_ | _UVV_
---|---|---|---|---|---|---|---
_(2458900+)_ | _(d)_ | _(mag)_ | _(mag)_ | _(mag)_ | _(mag)_ | _(mag)_ | _(mag)_
76.6 | 2.6 | 12.78 $\pm$ 0.02 | 12.84 $\pm$ 0.03 | 12.92 $\pm$ 0.03 | 13.54 $\pm$ 0.03 | 14.87 $\pm$ 0.03 | 14.93 $\pm$ 0.05
79.9 | 5.9 | 13.39 $\pm$ 0.03 | 13.17 $\pm$ 0.03 | 13.11 $\pm$ 0.03 | 13.38 $\pm$ 0.03 | 14.61 $\pm$ 0.03 | 14.53 $\pm$ 0.05
80.7 | 6.7 | 13.76 $\pm$ 0.03 | 13.44 $\pm$ 0.03 | 13.22 $\pm$ 0.03 | 13.39 $\pm$ 0.03 | 14.62 $\pm$ 0.03 | 14.43 $\pm$ 0.04
82.1 | 8.1 | 14.34 $\pm$ 0.04 | 13.98 $\pm$ 0.04 | 13.56 $\pm$ 0.04 | 13.49 $\pm$ 0.04 | 14.75 $\pm$ 0.04 | 14.54 $\pm$ 0.07
86.2 | 12.2 | 15.35 $\pm$ 0.07 | 15.20 $\pm$ 0.06 | 14.52 $\pm$ 0.05 | 13.84 $\pm$ 0.04 | 14.82 $\pm$ 0.05 | 14.69 $\pm$ 0.07
94.4 | 20.4 | 17.60 $\pm$ 0.13 | 17.63 $\pm$ 0.12 | 16.34 $\pm$ 0.09 | 15.16 $\pm$ 0.06 | 15.06 $\pm$ 0.04 | 14.59 $\pm$ 0.05
96.0 | 22.0 | 17.55 $\pm$ 0.13 | 17.90 $\pm$ 0.13 | 16.64 $\pm$ 0.11 | 15.57 $\pm$ 0.07 | 15.14 $\pm$ 0.04 | 14.63 $\pm$ 0.06
103.0 | 29.0 | 18.59 $\pm$ 0.17 | 17.43 $\pm$ 0.14 | 17.81 $\pm$ 0.18 | 16.29 $\pm$ 0.09 | 15.54 $\pm$ 0.05 | 14.73 $\pm$ 0.06
107.5 | 33.5 | 18.05 $\pm$ 0.17 | - | 17.89 $\pm$ 0.19 | 16.48 $\pm$ 0.10 | 15.72 $\pm$ 0.06 | 14.71 $\pm$ 0.06
112.7 | 38.7 | 18.72 $\pm$ 0.20 | 18.16 $\pm$ 0.14 | 18.04 $\pm$ 0.21 | 17.11 $\pm$ 0.15 | 15.79 $\pm$ 0.06 | 14.92 $\pm$ 0.06
118.5 | 44.5 | - | 18.50 $\pm$ 0.20 | 17.94 $\pm$ 0.23 | 17.70 $\pm$ 0.22 | 16.04 $\pm$ 0.07 | 14.90 $\pm$ 0.07
148.5 | 74.5 | - | - | - | - | 20.31 $\pm$ 0.21 | 17.27 $\pm$ 0.26
159.5 | 85.5 | - | - | - | - | - | 17.08 $\pm$ 0.22
164.5 | 90.5 | - | - | - | - | - | 17.18 $\pm$ 0.23
169.6 | 95.6 | - | - | - | - | - | 17.23 $\pm$ 0.50

The _Neil Gehrels Swift Observatory_ (Gehrels et al., 2004) database indicated
that SN 2020jfo was observed with the Ultra Violet Optical Telescope (UVOT,
Roming et al., 2005) onboard _Neil Gehrels Swift Observatory_ (Gehrels et al.,
2004) in the _UVW2_ , _UVM2_ , _UVW1_ , _UVU_ , _UVB_ and _UVV_ bands starting
from 2020 May 07 (JD 2458976.6) and continued till 2020 August 08 (JD
2459069.6). The openly accessible archival images111https://www.swift.ac.uk/
were reduced using packages available via High Energy Astrophysics Software
(HEASOFT, v6.27) and with the latest calibration database for the UVOT
instrument, following the methods as described in Poole et al. (2008), and
Brown et al. (2009). The SN magnitude was extracted using UVOTSOURCE task with
an aperture size of 5 arcsec for the source and a similar aperture size to
extract the background counts. The final UVOT magnitudes (see Figure 2) were
obtained in the Vega system and are tabulated in Table 2. Template subtraction
for UVOT images was performed using the mean background flux estimated at the
SN 2020jfo location from the archival images of M61 obtained during follow-up
of SN 2014dt. A similar flux was also obtained at the SN location as the light
curve in the UV filters ($UVW2$, $UVM2$ and $UVW1$) flattened out during the
post-plateau phase ($\rm\gtrsim 60\,d$).

###  2.3 Optical spectroscopy

Table 3: Log of Spectroscopic observations of SN 2020jfo JD | Phase | Wavelength
---|---|---
_(2458900+)_ | _(d)_ | _(Å)_
77.1 | 3 | 4000–8000; 5200–9000
78.3 | 4 | 4000–8000; 5200–9000
79.2 | 5 | 4000–8000; 5200–9000
81.2 | 7 | 4000–8000; 5200–9000
85.1 | 11 | 4000–8000; 5200–9000
86.2 | 12 | 4000–8000; 5200–9000
89.3 | 15 | 4000–8000; 5200–9000
102.2 | 28 | 4000–8000; 5200–9000
110.1 | 36 | 4000–8000; 5200–9000
112.1 | 38 | 4000–8000; 5200–9000
119.2 | 45 | 4000–8000; 5200–9000
129.2 | 55 | 4000–8000; 5200–9000
144.1 | 70 | 4000–8000
270.5 | 196 | 4000–8000
276.5 | 202 | 4000–8000; 5200–9000
309.4 | 235 | 4000–8000; 5200–9000
341.4 | 267 | 4000–8000; 5200–9000
366.4 | 292 | 4000-9000 [DOT]

Spectroscopic observations of SN 2020jfo were primarily carried out with the
HCT starting from 2020 May 07 (JD 2458977.1) to 2021 January 26 (JD
2459241.4), using HFOSC with grisms Gr7 and Gr8. One spectrum at nebular phase
was obtained on 2021 February 21 (JD 2459266.5) with the ADFOSC instrument
mounted at the 3.6 m Devasthal Optical Telescope (DOT, Omar et al., 2019;
Sagar et al., 2019). Standard IRAF packages were used to extract, reduce and
calibrate the spectra obtained with both the instruments. Details of the
reduction procedure are mentioned in Singh et al. (2018a). The spectra of SN
2020jfo were corrected for the host redshift using $z=0.00502$ (Perley et al.,
2020). Log of spectroscopic observations is provided in Table 3.

##  3 Analysis

###  3.1 Reddening, Distance and Metallicity

The Milky Way line-of-sight reddening for M61 is $E(B-V)_{\rm{MW}}=0.0194\pm
0.0001$ mag which is obtained from IRSA222 NASA/IPAC Infrared Space Archive
Galactic Dust Reddening and Extinction map (Schlafly & Finkbeiner, 2011). We
also noted a prominent host Na I D absorption with an equivalent width (EW) of
$1.14\pm 0.04$ Å, in the co-added spectrum obtained from three early phase
spectra, spanning 11 to 15 d from the date of explosion (see Section 3.2 for
explosion epoch). Here, we have adopted two independent methods to estimate
the host galaxy reddening ($E(B-V)_{\rm{host}}$). The empirical relations
between $E(B-V)$ and equivalent width of Na I D absorption lines provided by
Barbon et al. (1990) and Poznanski et al. (2012) were used to infer
$E(B-V)_{\rm{host}}$ of $0.29\pm 0.01$ mag and $0.30\pm 0.08$ mag,
respectively. Secondly, reddening was also estimated using Balmer decrements,
with host galaxy spectra from three regions, marked in Figure 1, obtained from
the SDSS archive (Ahumada et al., 2020). The ratio of $\rm H\alpha$ and $\rm
H\beta$ line fluxes were measured and the colour excess was estimated using
the relation given by Domínguez et al. (2013), which resulted in an
$E(B-V)_{\rm{host}}$ = $0.25\pm 0.02$ mag. The reddening estimated using the
two independent methods agree within errors. A weighted mean from the above
estimates results in $E(B-V)_{\rm{host}}=0.27\pm 0.08$ mag. A total reddening
of $E(B-V)=0.29$ mag is adopted throughout this work.

A plethora of distance estimates to the host galaxy M61 are available on the
NASA/IPAC Extragalactic Database (NED)333http://ned.ipac.caltech.edu, ranging
from 7.59 Mpc (Bottinelli et al., 1984) to 35.50 Mpc (Sparks, 1994) including
both redshift-dependent and redshift-independent measurements. Recent
redshift-independent measurements based on SN 2008in constrain the distance
from 12-20 Mpc (see Rodríguez et al., 2014; Bose & Kumar, 2014). A simple mean
of all these estimates could not be adopted as the values are not continuous
but at extremes. Steer (2020) has defined a robust method to get enhanced mean
estimate distances (MED) using weighted mean for the distances from various
primary and secondary sources. From the various means, we have estimated MED
7, which is a combination of the unweighted (MED 2), error-weighted (MED 3),
and date-weighted (MED 4) means with weights of 1:2:4, respectively. The
distance obtained is, $\rm D_{L}\,=\,16.45\pm 2.69$ Mpc ($\rm\mu\,=\,31.08\pm
0.36$ mag).

Table 4: O3N2-index and E(B-V) estimated from the SDSS spectra of 3 regions in M 61. The regions have been marked in the Figure 1. Regions | O3N2-index | 12+log[O/H] | _E(B-V)_
---|---|---|---
SDSS-Reg 1 | 1.16 | 8.36 | 0.28
SDSS-Reg 2 | -0.32 | 8.83 | 0.22
SDSS-Reg 3 | -0.48 | 8.88 | 0.26

To estimate the host environment properties, we used archival SDSS spectra of
the three regions in M61, as indicated earlier (refer Figure 1). Fluxes of the
strong emission lines viz. H$\alpha$, H$\beta$, [N II] 6584 Å and [O III] 5007
Å were measured and the O3N2 index as prescribed by Pettini & Pagel (2004) was
estimated. The gas-phase oxygen abundance (12+log[O/H]) was computed using the
relation given in Pettini & Pagel (2004). The metallicity estimates for all
the three regions are listed in Table 4. We find that towards the outer edge
of the galaxy, the metallicity is sub-solar with an oxygen abundance of $\sim$
8.36 dex($\rm\sim 0.5\,Z_{\odot}$)444Solar value for 12+log[O/H] is taken from
Asplund et al. (2006) which is $\rm 8.66\pm 0.05$ dex, whereas in the regions
on the spiral arms (Regions 2 and 3), the metallicity is 8.83 dex and 8.88 dex
($\rm\sim 1.6\,Z_{\odot}$), respectively.

Table 5: Peak magnitudes from UV/Optical light curves  Band | t(mmax) | mmax
---|---|---
| $(MJD)$ | $(mag)$
$U$ | 58978.71$\pm$0.70 | 13.83$\pm$0.08
$B$ | 58979.19$\pm$0.28 | 14.53$\pm$0.03
$V$ | 58981.54$\pm$0.35 | 14.55$\pm$0.03
$R$ | 58982.58$\pm$0.25 | 14.38$\pm$0.02
$I$ | 58981.31$\pm$0.56 | 14.30$\pm$0.05
ZTF-$g$ | 58980.63$\pm$0.21 | 14.46$\pm$0.02
ZTF-$r$ | 58982.35$\pm$0.37 | 14.47$\pm$0.03
$UVU$ | 58979.23$\pm$0.28 | 13.37$\pm$0.03
$UVB$ | 58980.23$\pm$0.31 | 14.61$\pm$0.03
$UVV$ | 58981.54$\pm$0.58 | 14.45$\pm$0.05

###  3.2 Light and colour curves evolution

The last non-detection of SN 2020jfo was on UT 2020 May 02.27 (JD 2458971.8)
with a limiting AB magnitude of 19.7 mag in the $g$-band ZTF filter (Nordin et
al., 2020). The supernova was discovered on UT 2020 May 06.26 (JD 2458975.7).
The mid epoch between the last non-detection and the first detection is JD
2458973.75. Hence, JD 2458974$\pm$2 is taken as date of explosion and has been
used for defining phase throughout this work.

The light curve evolution of SN 2020jfo in Bessell $U,B,V,R,I$, the ZTF $g,r$
bands and in _Swift_ $UVOT$-bands is shown in Figure 2. Optical light curves
show a relatively fast rise to the maximum in all bands. To estimate peak
magnitudes and rise times to the peak in various bands, we fitted a cubic
spline to the observed photometric data. The estimated peak magnitude and date
of maximum in different bands are given in Table 5. The rise time ranges from
5.2 d in $U$ to 9.1 d in $R$ band, with a similar trend seen in _Swift_ -UVOT
bands from 5.7 d in $UVU$ to 8.0 d in $UVV$, and 7.1 d and 8.9 d in ZTF $g$
and $r$-bands, respectively. Early phase light curves show a bump around the
maximum which is prominent in the redder bands ($R$ and ZTF-$r$). Post peak,
the light curves vary very slowly in the redder bands and settle onto a
plateau, that appears to be short.

To have a better estimate of the plateau length, observations during
transition from plateau to late declining phase are required. Unfortunately,
only one observation could be made during this phase due to observational
constraints, however, we notice a steep decline in ZTF-$g$ band at +60 d.
Moreover, observations around +70 d and beyond (in $UVV$) indicate that the SN
has already entered into the radioactive decay tail. This puts an upper limit
on the length of the plateau to be 70 d. Also, we do not see any change in
slope in the $V$ band light curve until +56 d. With this the lower limit of
the plateau length is constrained as 56 d. With these limits the plateau
(OPTd, Anderson et al., 2014) length is estimated as $\simeq$63$\pm$7 d.

Figure 3: Estimated Steepness of SN 2020jfo using the functional form from
Elmhamdi et al. (2003b).

Another way to estimate the upper limit of the plateau length is by estimating
the date of inflection during the transition phase, which is defined as the
point of maximum steepness/slope. We use the formulation from Elmhamdi et al.
(2003b) to fit the late-plateau and radioactive decay phase of the $V$ band
light curve (Figure 3). We could include some more points in $V$ band during
transition from plateau to nebular phase using ZTF-$g$ and $UVV$ magnitudes.
The $g$-band magnitudes were transformed to $V$ magnitudes using the
transformation relations given by Jester et al. (2005). Fit to the better
sampled $V$-band light curve yields a steepness parameter of 0.143$\pm$0.002
$\rm mag\,d^{-1}$ and day of inflection as 65.2$\pm$0.5 d. This is in
concurrence with our plateau length estimate.

Figure 4: Colour evolution of SN 2020jfo from the early rise up to the plateau
phase is plotted along with some other Type II SNe.

The mean plateau length for a large sample of type IIP SNe was found to be
$\sim$ 100 d (Anderson et al., 2014), while the estimated plateau length is
much shorter, $\sim$ 63 d for SN 2020jfo. Only a handful of such objects have
been discovered till now, namely, SN 2006Y, SN 2006ai, SN 2008bp, SN 2008bu
(Anderson et al., 2014), SN 2014G (Terreran et al., 2016), and SN 2016egz
(Hiramatsu et al., 2021a). In a recent study, Hiramatsu et al. (2021a)
estimated that only a small fraction ($\rm\sim 4\,\%$, (3/78)) of short
plateau objects were there in a large sample of Type II SNe studied by
Gutiérrez et al. (2018). The rarity of short plateau objects could also be
seen in the supernova lightCURVE POPulation Synthesis by Eldridge et al.
(2018, CURVEPOPS) for Type II SN, as they obtained a mere $\rm 4.7\%$ short-
plateau Type IIP events out of their 637 models.

Temporal evolution of $UVW2-UVM2$, $g-r$, $U-B$ and $B-V$ colors for SN
2020jfo, during early phase is shown in Figure 4. The colours have been
corrected for reddening estimated in Section 3.1. The colour evolution of some
other well studied objects is also plotted in the same figure for comparison.
The colour evolution of SN 2020jfo follows the blue to red trend, indicating
cooling of the ejecta as the supernova evolves. SN 2020jfo shows overall bluer
colour, the $UVM2-UVW1$, $B-V$, and $g-r$ colour of SN 2020jfo is bluer than
all other supernovae used for comparison, with the exception of SN 2009au.

###  3.3 Absolute V-band light curve

Figure 5: Absolute V-band light curve of SN 2020jfo is plotted with some other
Type II SNe. Distance and extinction correction for individual objects are
obtained from their references as provided in Section 3.3. The decline rates
during early plateau (s1), late-plateau (s2) and the nebular (s3) phases
determined using linear fit are also mentioned.

Absolute $V$ band light curve of SN 2020jfo is obtained after correcting the
observed $V$ band magnitude for extinction and distance estimated in Section
3.1 and $\rm R_{V}=3.1$ (Cardelli et al., 1989). The $V$ band light curve
peaked on $\sim\rm 8.0$ d (JD 2458982.04$\pm$0.35) after explosion with an
absolute magnitude, $M_{V}=-17.40\pm 0.37$ mag. This puts it under the
category of luminous Type IIP events. We estimated the light curve slopes
during different phases $s1$, $s2$ and $s3$, (Anderson et al., 2014) for SN
2020jfo as $1.4^{+0.5}_{-0.6}$, $1.5^{+0.4}_{-0.3}$ and $1.6^{+0.2}_{-0.2}$
mag per 100 days, respectively. Based on a large sample of Type II SNe light
curves, Anderson et al. (2014) estimate mean values of 2.65 ($s1$), 1.23
($s2$) and 1.47 ($s3$) mag per 100 days, indicating a clear transition from
the early decline to the plateau phase. The estimated values of $s1$ and $s2$
in the case of SN 2020jfo indicate the absence of such a clear transition,
although the rise to maximum is similar to other Type II SNe. It thus appears
that either the $s1$ phase lasted for a very short period, or is missing
entirely.

The comparison of $V$-band absolute magnitude light curve of SN 2020jfo with
other Type II SNe including short plateau events is shown in Figure 5. As the
number of short plateau objects studied in detail so far is small, we compared
the light curve of SN 2020jfo with a sample of objects including archetypal
Type IIP SNe, SN 1999em (Elmhamdi et al., 2003a) and SN 2004et (Sahu et al.,
2006), Type II SNe with CSM-signatures, SN 2009au (Rodríguez et al., 2020), SN
2013fs (Bullivant et al., 2018a), and SN 2014G (Terreran et al., 2016), and
faster declining or short plateau Type II SNe, SN 2013by (Valenti et al.,
2015), SN 2014dw (Valenti et al., 2016) and SN 2016X (Huang et al., 2018).
Although SN 2014G and SN 2013by are brighter than SN 2020jfo during the pre-
maximum, early decline and plateau phase, whereas in the nebular phase their
light curve merges with that of SN 2020jfo. In case of SN 2013fs, the early
post-maximum decline is faster in comparison to SN 2020jfo but the plateau
brightness is similar.

Figure 6: Quasi-bolometric light curve (Q-bol) of SN 2020jfo along with other
Type II SNe. Q-bol with contribution from UV fluxes and from SuperBol (without
BB-corrections) are also plotted. Inset shows Optical and UV+Optical Q-bol
during early phase.

###  3.4 Quasi-Bolometric LC

The quasi-bolometric light curve (Q-bol) of SN 2020jfo is estimated using the
observed magnitudes in $UVW2,UVM2,UVW1,U,B,$ ZTF$-g,V,R$ and $I$ filters,
corrected for reddening due to the Milky Way and the host galaxy. Extinction
corrections in individual photometric bands is applied using the relations by
Cardelli et al. (1989, $R_{V}=3.1$). The extinction-corrected apparent
magnitudes were converted to monochromatic fluxes at the effective filter
wavelength, using the magnitude-to-flux conversion zero points listed in
(Bessell et al., 1998). Zero-point for bands other than Bessell
$U,\,B,\,V,\,R,\,I$ filters were taken from the SVO Filter Profile
Service555http://svo2.cab.inta-csic.es/theory/fps/. The spectral energy
distribution (SED) curve for each epoch was estimated by interpolating the
estimated flux in different bands using a Cubic spline. Finally, the quasi-
bolometric flux was estimated by integrating the SED through the initial
wavelength of the first band to the upper cut-off wavelength of the last band.
On the nights when magnitudes were not available for some bands, we used
linear interpolation to estimate them.

The quasi-bolometric luminosity for initial epochs, i.e., up to $+$28 d,
includes UV fluxes obtained from Swift-UVOT and, beyond that, the contribution
is computed only using $UBgVRI$ filters. Figure 6 shows the quasi-bolometric
light curve with and without UV contribution. It is evident from Figure 6 (and
its inset) that during the first $\sim$ 15 days, contribution from UV bands to
the bolometric flux is significant, and beyond this, it becomes very small in
comparison to the optical flux. During the late nebular phase, where only ZTF
data are available, bolometric correction ($BC_{g}$) was derived using the
last few points in the LC for which the $BgVRI$ bolometric luminosity could be
obtained. The estimated $BC_{g}$ was applied to ZTF-$g$ band magnitudes to
obtain the bolometric luminosity till the very late phase.

For comparing our bolometric estimates we also use SuperBol (Nicholl, 2018),
with ZTF-$g$ as the reference band. SuperBol fits a polynomial to bands with
missing data and integrates those at epochs of the reference band. It seems to
slightly underestimate the luminosity at the earlier epochs, where we see some
signs of enhanced flux in individual optical light curves. It might be due to
the smoothing of the data with a polynomial approximation. At other phases,
quasi-bolometric light flux estimated in two different ways, match quite well.
The contribution from optical flux to the UV+Optical bolometric flux is
$\approx 20\%$ at +3 d, which increases to $\approx 80\%$ at $\sim$
$\rm+15\,d$ and almost in entirety at $\sim$ $\rm+28\,d$.

Clearly, discrete decline trends are visible in the Q-bol light curve where
the initial decline from $\rm+6\,d$ to $\rm+15\,d$ is significantly steeper
than other supernovae. Each decline phase is linearly fitted using Python’s
emcee routine. For comparison, the slopes for other objects during similar
phases are computed and tabulated in Table 6.

Table 6: Best fit slopes for various phases where decline is conspicuous in the Q-bol light curve of SN 2020jfo. Slopes for other Type II SNe have also been estimated for comparison wherever possible. SNe$\Rightarrow$ | 2020jfo | 1999em | 2004et | 2009au | 2013by | 2013fs | 2014G | 2016X
---|---|---|---|---|---|---|---|---
Phases $\Downarrow$ | Slopes$\rm(dex\\{log[L(erg\ s^{-1}]\\}\ 100\ d^{-1})$
Early | $-4.00^{+1.02}_{-1.09}$ | $-1.27^{+0.06}_{-0.05}$ | $-1.33^{+0.02}_{-0.02}$ | $-1.84^{+0.01}_{-0.01}$ | $-1.60^{+0.01}_{-0.01}$ | $-1.33^{+0.01}_{-0.01}$ | $-1.32^{+0.02}_{-0.02}$ | $-1.56^{+0.02}_{-0.02}$
Plateau | $-0.91^{+0.19}_{-0.20}$ | $-0.08^{+0.01}_{-0.01}$ | $-0.22^{+0.01}_{-0.01}$ | - | $-0.89^{+0.01}_{-0.01}$ | - | $-0.98^{+0.03}_{-0.04}$ | $-0.52^{+0.01}_{-0.01}$
Nebular | $-0.32^{+0.03}_{-0.04}$ | $-0.30^{+0.01}_{-0.01}$ | $-0.44^{+0.01}_{-0.01}$ | - | $-0.45^{+0.05}_{-0.04}$ | - | $-0.68^{+0.04}_{-0.03}$ | $-0.77^{+0.02}_{-0.02}$
Early UV+Optical | $-5.33^{+0.42}_{-0.41}$ | - | - | - | - | - | - | -

Q-bol light curve of SN 2020jfo peaks at $\sim\,4.3\pm 1.4\times 10^{42}$ $\rm
erg\ s^{-1}$ in optical bands around $\rm+6\,d$, whereas we missed the peak in
the UV+Optical data. During the very early phase, Q-bol declines at a rate of
$\rm 4.00^{+1.02}_{-1.09}\,dex\,100\,d^{-1}$ and $\rm
5.33^{+0.42}_{-0.41}\,dex\,100\,d^{-1}$ in Optical and UV+Optical
respectively, whereas for the other SNe this early phase decline is less
steeper. For the Type IIP events SN 1999em and SN 2004et, and SN 2013fs we
estimated an early phase decline of 1.27 $\rm dex\,100\,d^{-1}$, 1.33 $\rm
dex\,100\,d^{-1}$, and 1.33 $\rm dex\,100\,d^{-1}$, respectively. For SN
2009au (1.84 $\rm dex\,100\,d^{-1}$), SN 2013by (1.60 $\rm dex\,100\,d^{-1}$)
and SN 2016X (1.56 $\rm dex\,100\,d^{-1}$), we find the decline to be steeper
than normal Type II SNe, but significantly lower than SN 2020jfo (see Table
6). During the plateau phase and nebular phase, we find decline rates for SN
2020jfo to be $\rm 0.91^{+0.19}_{-0.20}\,dex\,100\,d^{-1}$ and $\rm
0.32^{+0.03}_{-0.04}\,dex\,100\,d^{-1}$, respectively. Plateau phase decline
is found to be similar to SN 2013by (0.89 $\rm dex\,100\,d^{-1}$) and SN 2014G
(0.98 $\rm dex\,100\,d^{-1}$). In terms of magnitude, the slope in the nebular
phase is found to be $0.80^{+0.08}_{-0.10}\rm\,mag\,100\,d^{-1}$.

###  3.5 56Ni Mass

In addition to ionising and heating the outer envelope, the shock assists in
the synthesis of heavy radioactive nuclei that decay and radiate (Arnett,
1980). Out of all these, $\rm{}^{56}Ni$ is the most significant contributor
whose decay results in the daughter nuclei $\rm{}^{56}Co$ with a half-life of
$\sim$ 6.1 d. These daughter nuclei then decay to $\rm{}^{56}Fe$ with a half-
life of $\sim$ 77.3 d.

To calculate the synthesised mass of $\rm{}^{56}Ni$, we employed two
independent methods. Firstly, we used the following relations by Hamuy (2003):

$\frac{M(^{56}Ni)}{M_{\odot}}=\frac{L_{t}}{L_{*}}\exp\left[\frac{(t_{t}-t_{exp})/(1+z)-t_{1/2}(^{56}Ni)}{t_{e-folding}(^{56}Co)}\right]$
(1)

where $\rm L_{*}=1.271\times 10^{43}\,erg\,s^{-1}$, $\rm t_{1/2}(^{56}Ni)$ is
6.1 d and the $e$-folding time of $\rm{}^{56}Co$ decay used is 111.26 d. Using
the quasi-bolometric luminosity from $\sim$ 192 d onward as tail luminosity,
$L_{t}$, the mass of synthesised $\rm{}^{56}Ni$ is estimated as $\rm 0.019\pm
0.005\,M_{\odot}$. It is to be noted that IR contribution to the bolometric
luminosity is not included, and hence, it could be considered as a lower limit
on $\rm{}^{56}Ni$ mass.

Secondly, we compared the late phase quasi-bolometric luminosity of SN 2020jfo
with that of SN 1987A. For SN 1987A the bolometric luminosity and the mass of
$\rm{}^{56}Ni$ synthesised in the explosion is estimated with significant
accuracy (0.075 $\rm M_{\odot}$, Turatto et al., 1998). Assuming that the
$\gamma$-ray deposition in SN 2020jfo is similar to SN 1987A, mass of
$\rm{}^{56}Ni$ in SN 2020jfo was estimated using, $\rm
M_{Ni}=M_{Ni}(1987A)\times L_{bol}(2020jfo)/L_{bol}(1987A)\,M_{\odot}$. If a
constant fraction of about $35\%$ (as estimated by Patat et al., 2001 and
Elmhamdi et al., 2003b) is added to the quasi-bolometric flux to account for
missing NIR flux, the mass of $\rm{}^{56}Ni$ synthesised in SN 2020jfo becomes
$\rm 0.033\pm 0.004\,M_{\odot}$, which is consistent with our earlier estimate
if a similar IR correction is used. This value is also typical of Type II SNe
as it is similar to the mean value of $\rm{}^{56}Ni$ mass
($\rm=0.033\,M_{\odot}$) obtained by Anderson (2019) for a sample consisting
of more than 40 Type II supernovae.

It was empirically shown by Elmhamdi et al. (2003b) that the $\rm{}^{56}Ni$
mass anti-correlates with the maximum of the steepness parameter ($\rm
S=dM_{V}/dt$) during the transition from the plateau phase to the nebular
phase. This relation was further refined by Singh et al. (2018a) by
incorporating a larger sample of Type IIP SNe including low-luminosity events.
Mass of $\rm{}^{56}Ni$ estimated using steepness parameters of 0.143 $\rm
mag\,d^{-1}$ (refer Section 3.2) is 0.030$\pm$0.002 $\rm M_{\odot}$, which is
similar to earlier estimates.

Maguire et al. (2012) showed that the mass of $\rm{}^{56}Ni$ is correlated
with full-width at half-maximum (FWHM) of H$\alpha$ feature during the late
nebular phase. The FWHM of H$\alpha$ line was measured in the spectrum
obtained at +292 d by fitting a Gaussian profile. The observed FWHM was
corrected for instrumental broadening using the width of the night sky
emission lines present in the spectrum. Mass of $\rm{}^{56}Ni$ estimated using
this method is found to be $\rm 0.047^{+0.005}_{-0.004}\,M_{\odot}$, which is
higher than our earlier estimates. It clearly signifies a broadened line
emission profile in SN 2020jfo, implying a larger velocity dispersion in the
line forming region, whereas in a typical Type IIP SN, the dispersion would
have been lower due to a massive hydrogen envelope.

##  4 Spectroscopic Evolution

The optical spectral sequence of SN 2020jfo spanning from +3 d to around +292
d is shown in Figure 7. The spectral evolution at various phases together with
a comparison with other type II supernovae is discussed in this Section.

Figure 7: Spectral evolution of SN 2020jfo from 3 d until 292 d post
explosion. Lines have been identified following Gutiérrez et al., 2017b, some
of the prominent lines are marked. All spectra are flux calibrated and
corrected for reddening and redshift.

.

###  4.1 Pre-Maximum Spectral Evolution

In the first spectrum obtained on +3 d, we detect a broad absorption trough at
6266 Å, which is likely due to $\rm H\alpha$ and yields a line velocity of
around 13,500 $\rm km\,s^{-1}$ (see Figure 7, 8). If we look for an $\rm
H\beta$ counterpart at a similar velocity, we should detect an absorption dip
at 4650 Å. Instead, we observe a broad P-Cygni feature with emission at around
4686 Å and its absorption counterpart at roughly 4466 Å. The feature is likely
a broad feature of He II 4686 Å at roughly 14,000 $\rm km\,s^{-1}$, consistent
with the line velocity of $\rm H\alpha$ feature. This feature faded after $+$4
d, and a feature redward of this started appearing, which was identified as
$\rm H\beta$ owing to a similar velocity with the $\rm H\alpha$ feature. Broad
He II 4686 Å was also seen in SN 2013fs (Bullivant et al., 2018b; Chugai,
2020) and is indicative of the presence of a cold dense shell (CDS) above the
photosphere. The He II feature has a blue-skewed boxy profile which suggests a
geometrically thin and unfragmented CDS (Chugai, 2020). The presence of He II
in the early spectrum typically arises from the rapid recombination resulting
from the interaction of the SN ejecta with extended supergiant atmosphere
(Bruch et al., 2021), however, these would lead to the existence of narrow
emission lines in the spectrum. The presence of a broad P-Cygni feature
indicates that the line originated in the SN ejecta. This would require that
the ejecta and the nearby CSM is highly ionised by the passage of the shock,
which was also seen in SN 2006bp (Quimby et al., 2007).

Figure 8: Early phase (+4.0 d) spectrum of the SN 2020jfo is compared with
synthetic spectra generated using tardis, indicating the presence of ionised
Helium. The relative contributions due to different elemental compositions are
plotted. In the square brackets ’e’ implies the enhanced abundance for He in
the composition.

To ascertain the identification of the He II feature in the early spectra, we
used rapid spectral modelling code tardis (Kerzendorf & Sim, 2014).
Incorporating modifications from Vogl et al. (2019), tardis is now capable of
synthesising spectra for Type II events as well. For our initial setup, we
used uniform density configuration with a density profile in the form of power
law (Vogl et al., 2019). Hydrogen was treated in the non-local thermodynamic
equilibrium (NLTE) approximation. We used different compositions for the outer
layers, including CNO, H only, He only, H+He only and H+He+CNO. We fixed
luminosity parameters for +4 d calculations and used temperature as a free
parameter. The observed velocities ($\rm\sim 14000-16000\,km\,s^{-1}$) in the
spectrum are used as velocities of the envelope layers. The resulting spectral
luminosity was scaled with distance to obtain the observed flux values. The
synthesised spectra along with the SN 2020jfo spectrum at +4 d are shown in
Figure 8. It was noticed that, reproducing the ionised Helium feature required
a temperature range of $\sim$9000 K to $\sim$18000 K along with a higher
Helium abundance than Solar values. On the other hand, for this feature to be
a blend of Nitrogen and Carbon, the modelling required much higher CNO
abundances which are almost order of magnitudes higher than the Solar values
and is rather non-physical. However, some amount of blending along with the
Helium could not be ruled out altogether. This strengthens the case that the
observed broad absorption feature is likely a He II feature.

Figure 9: The pre-maximum (+2.8 d) spectrum of SN 2020jfo compared with the
spectra of other Type II SNe at similar early phase.

The spectrum obtained during $+$4 d to $+$7 d shows gradual development of
Balmer spectral features. Absorption trough around 5600Å is seen in the
spectrum obtained on $+$4 d which is likely due to He I 5876 Å which evolved
into a fully developed P-Cygni profile on $+$7 d. As the SN evolves, the
continuum becomes redder. The spectrum of SN 2020jfo obtained on $+$3 d is
compared with the spectrum of some other objects at comparable epochs and
shown in Figure 9. The early phase spectrum of SN 1999em and SN 2004et shows a
blue continuum with broad absorption due to hydrogen Balmer lines, while the
early spectrum of SN 2009au, SN 2013by and SN 2014G show narrow flash-ionised
lines. The spectrum of SN 2020jfo appears different than the other objects
with shallow absorption due to H$\alpha$ and the presence of broad absorption
due to He II.

###  4.2 Plateau Phase Spectral Evolution

Figure 10: Spectrum of SN 2020jfo during the plateau phase (+44.9 d) compared
with other Type II SNe at a similar epoch.

As the SN enters the plateau phase, the photosphere cools to the recombination
temperature and stays in the hydrogen envelope leading to the development of
various metallic lines of Iron, Scandium, Oxygen and Calcium in the spectra.
He I feature in the early phase slowly vanishes by +15 d and the Na I D
feature from the SN appears at its place. The shallow absorption feature seen
at 5000 Å in the spectrum obtained on $+$12 d is due to the Fe II (multiplet
42) features at 4924 Å, 5018 Å, and 5169 Å). This feature strengthens as the
photosphere moves deep inside the hydrogen envelope. Hydrogen Balmer lines
become stronger, and other metallic lines such as, Sc II (5663 Å), Sc II/Fe II
(5531 Å), He I/Na ID, Ba I and Calcium NIR triplet develop in the spectra. We
also detect O I 7774 Å absorption feature in the spectrum of +28 d.

The spectrum of SN 2020jfo obtained around +45 d is compared with the spectra
of some other well studied objects in Figure 10. Most spectral features are
identical in all the Type II SNe used for comparison. The H$\alpha$ absorption
feature in SN 2020jfo is shallower compared to the normal Type II SNe like SN
1999em and SN 2004et, whereas it is similar to fast-declining Type II SNe such
as SN 2009au, SN 2013by and SN 2014G. Further, in SN 2020jfo, the H$\alpha$
absorption trough is broader than other objects.

Figure 11: Line velocity evolution of Balmer, Fe II and Sc II features
obtained using their absorption minima are shown here. A comparison with mean
Type II SNe velocities from Gutiérrez et al. (2014) is also shown. The solid
line represents mean value while the shaded region displays the 1-$\rm\sigma$
scatter from the mean.

The velocities inferred from the metallic lines are similar and they fall in
the range $\rm\sim 5000\,km\,s^{-1}$ at $+$28 d to $\rm\sim 2000\,km\,s^{-1}$
at $+$70 d. The expansion velocities obtained using various species is
compared with velocity estimates for a larger sample of Type II supernovae
(Gutiérrez et al., 2014) and is shown in Figure 11. Except for the early phase
($<+25\,\rm d$), where a steep decline in the H$\alpha$ velocity is observed,
the velocities measured using H$\alpha$, H$\beta$ and H$\gamma$ lines are
similar to the average velocity for Type II sample and closely follow the
observed trend in Type II supernovae throughout the photospheric phase. The
velocities calculated using Fe and Sc lines are found to be marginally lower
than the mean velocities of the Type II sample. This might probably indicate
that SN 2020jfo was an explosion with lower energy, albeit the higher
luminosity indicates otherwise. The steep decline observed during the early
phase in the H$\alpha$ velocity could possibly hint towards slowing down of
outer layers while encountering circumstellar matter around the progenitor.
Nevertheless, if we look at the Figure 6, we could see that the higher
luminosity, in comparison to other Type II events, is only visible initially
and reaches a moderate value later achieved through a faster decline, again
indicating a short-lived source of secondary radiation, likely CSM.

From +36 d onward, the H$\alpha$ absorption feature starts to broaden up and a
deep and narrow absorption feature (Cachito) starts to develop blue-wards of
H$\alpha$. This feature is prominently visible on the spectrum of +55 d at a
wavelength of 6365 Å. The possibility of this feature arising due to Si II
6355 Å (Valenti et al., 2014) or Ba II 6497 Å was explored. The Cachito
absorption lies redwards of the rest-wavelength of Si II and is hence unlikely
to be related to it. Assuming the feature originated due to the Ba II line,
the line velocity inferred is $\rm\sim 6000\,km\,s^{-1}$ which is almost twice
the velocity obtained from other metal lines ($\rm\sim 3000\,km\,s^{-1}$ for
Fe II) at the same epoch.

Chugai et al. (2007) proposed the emergence of high velocity absorption
features of hydrogen as a result of the interaction between the RSG wind and
SN ejecta. We also explored the possibility of this feature being a high
velocity feature of hydrogen. The measured velocity of this HV-H$\alpha$
feature is $\rm\sim 9000\,km\,s^{-1}$ and this velocity is similar to the
post-maximum expansion velocity of H$\alpha$. However, we did not observe a
clear H$\beta$ counterpart, likely due to the blending of several metallic
lines in that region. This HV feature is similar to the “narrow and deeper”
Cachito, seen in Gutiérrez et al. (2017b, e.g. SN 2003hl) Type II SN sample
study, and is similar to the case of low-velocity/low-luminosity SNe, where no
H$\beta$ counterpart is seen. The likely presence of the HV feature of
H$\alpha$ in the photospheric spectra favours the case of circumstellar
interaction (Gutiérrez et al., 2017b).

###  4.3 Nebular Phase Spectral Evolution

Nebular spectra of SN 2020jfo during $+$196 d to $+$292 d is plotted at the
lower panel of Figure 7. As the recombination phase ends, the photosphere
recedes into the innermost part of the ejecta. The luminosity during this
phase varies in direct proportion to the $\rm{}^{56}Ni$, which was synthesised
during explosion (Srinivasaragavan et al., 2021). The light curve in this
phase is mainly powered by the radioactive decay of $\rm{}^{56}Co$ to
$\rm{}^{56}Fe$. The nebular phase spectrum of SN 2020jfo is dominated by
prominent emission lines of Na ID, [O I], H$\alpha$ and [Ca II].

Narrow emission lines from metals are also seen, which become progressively
more prominent as the supernova evolves into the late nebular phase. The bluer
part of the spectrum is dominated by lines due to Fe, Ba, Sc, Mg etc. Hydrogen
Balmer lines are seen with decreased absorption strength. As the medium
becomes more rarefied forbidden lines of Fe, Ca, and O appear in the spectrum.
The prominent lines seen in the nebular phase spectrum are identified and
marked in Figure 13. The spectra taken during this phase could be used to
estimate the Zero-Age-Main-Sequence (ZAMS) mass of the progenitor when
compared with line strengths of model spectra at similar phases. This method
has been deployed in many cases in the literature to constrain progenitor mass
of Type II SNe (e.g., Van Dyk et al. (2019); Szalai et al. (2019); Hiramatsu
et al. (2021b).

Figure 12: Spectrum of SN 2020jfo during the nebular phase (+292.2 d) is
compared with other Type II SNe at similar epochs.

Figure 13: Identification of lines in the nebular spectrum (+292 d) of SN
2020jfo.

Nebular spectrum of SN 2020jfo at $+$292 d is compared with nebular phase
spectra of other Type II SNe at similar epochs (see Figure 12). We find that
the prominent emission lines of [O I], H$\alpha$, [Ca II] and Ca II NIR
triplet is similar to that of other normal and fast-declining Type II SNe.
However, if we look closely, the spectrum of SN 2020jfo shows a clear blue
excess and a forest of features due to lines of [Fe II] and Fe II. The Na ID
feature in SN 2020jfo is similar to that of normal type II SN 1999em but is
more pronounced in comparison to fast-declining events SN 2013by and SN 2014G.
The red-wing of [Ca II] shows a clear secondary peak due to [Ni II] which is
not seen in normal Type II SNe like SN 1999em and SN 2004et, but seen in the
fast-declining Type II SN 2013by.

##  5 Characteristics of the possible progenitor

In this section, we perform observational analysis along with the semi-
analytical and hydrodynamical modelling to discuss possible progenitor
scenarios for SN 2020jfo.

###  5.1 Semi-Analytical Modelling

Figure 14: Best fitting model curves to the bolometric light curve using two-
component model from Nagy & Vinkó (2016). Individual contributions from the
shell and core component are shown with the combined bolometric luminosity.

To obtain estimates on progenitor properties, we used a semi-analytical model
described in Nagy & Vinkó (2016), which was initially described by Arnett & Fu
(1989). It models the supernova as a two-component system consisting of a core
region that is dense and a shell region with a low mass extended envelope. It
assumes that the SN ejecta is spherically symmetric and expanding
homologously. For the density profile of the ejecta, the core region is
assumed with a flat or constant density profile with a constant Thompson-
scattering opacity of $\rm\kappa=0.4\,cm^{2}\,g^{-1}$ whereas the shell region
has density profile which decreases as a power-law function ($\rm n=2$) or as
an exponential ($\rm a=0$) with an opacity of $\rm\kappa=0.2\,cm^{2}\,g^{-1}$
(Nagy & Vinkó, 2016). We obtained an ejecta mass of $\sim$ 7.5 $\rm M_{\odot}$
(core+shell), an RSG radius ranging from 310-340 $\rm R_{\odot}$, and a total
energy (thermal and kinetic) of $\sim$ 3 foe based on the best fitting model
shown in Figure 14. The mass of $\rm{}^{56}Ni$ obtained from this semi-
analytical model, with the amount of gamma-leakage added to the model to
follow the nebular light curve evolution is $\rm 0.03\pm 0.01\,M_{\odot}$,
which is in corroboration with our earlier estimates.

###  5.2 Estimate from nebular spectrum

Figure 15: Nebular spectrum (+292 d) of SN 2020jfo is compared with the model
spectra for 12, 15, 19 and 25 $\rm M_{\odot}$ models, at 250 d since
explosion. The model spectra obtained from Jerkstrand et al. (2014) are scaled
for distance and nickel mass, and corrected for phase mismatch using the
characteristic decay time corresponding to SN 2020jfo.

To constrain the progenitor mass, we compared the nebular phase spectrum of
$+$292 d with model spectra from Jerkstrand et al. (2014) (Figure 15). The
model spectra for different progenitor masses viz. 12, 15, 19, and 25 $\rm
M_{\odot}$ have been scaled with respect to $\rm{}^{56}Ni$ mass and the
distance of SN 2020jfo (in contrast to 5.5 Mpc for model spectra). In order to
account for the difference in phase between the model spectra and the observed
spectrum, the observed spectrum was scaled by the amount determined from the
characteristic time scale of $\rm{}^{56}Ni$-decay chain and the dissimilarity
in phases. The comparison of [O I] 6300 Å, 6364 Å line fluxes of the observed
spectra with the spectral models suggest a lower mass progenitor of $\rm\sim$
12 $\rm M_{\odot}$. However, the flux of $\rm H\alpha$ is quite weak compared
to the 12 $\rm M_{\odot}$ progenitor, indicating a stripped hydrogen envelope
in SN 2020jfo. In a core-collapse SN, the mass of calcium synthesised is
insensitive to the ZAMS mass of the progenitor, whereas the mass of oxygen
synthesised depends on it, and the [Ca II] / [O I] flux ratio is an indicator
of the progenitor mass (Fransson & Chevalier, 1989). The observed [Ca II] / [O
I] flux ratio of $\sim$ 1.5 in the spectrum of $\sim$ 292 d is also suggestive
of a low-mass progenitor for SN 2020jfo.

SN 2020jfo is also one of the few hydrogen-rich SNe where a clear, distinct
spectral feature of [Ni II] 7378 Å is seen adjacent to the [Ca II] feature in
nebular phase spectral evolution. The feature has its origins from stable
$\rm{}^{58}Ni$, synthesised during explosive nucleosynthesis (Jerkstrand et
al., 2015a). Following the methodology described in Jerkstrand et al. (2015b),
we computed a Ni/Fe luminosity ratio for SN 2020jfo (see Figure 16) as
$2.10\pm 0.43$ (similar to the value obtained in Sollerman et al., 2021). This
translates to a Ni/Fe ratio by mass as $0.18\pm 0.04$, which is roughly
$3.0\pm 0.6$ times the Solar value. This could be achieved either by a neutron
excess usually found in the Silicon layer or due to a very high progenitor
metallicity ($\rm>5\,Z_{\odot}$) that increases the neutron excess in Oxygen
shell (Jerkstrand et al., 2015a). The estimated metallicity close to the SN
site is $\rm\sim 1.5\,Z_{\odot}$ which is not high enough to produce such a
Ni/Fe ratio in the ejecta. The only plausible scenario for such excess is seen
in spherically symmetric models of $\rm M_{ZAMS}\lesssim 13\,M_{\odot}$
(Jerkstrand et al., 2015a) that house a thick Si layer with a neutron excess.
This is concurrent with our estimates of a lower mass progenitor.

Figure 16: Multi-component Gaussian fit to the nebular spectrum of SN 2020jfo
(+292 d). The fit was performed by having different values of line broadening
for [Ca II] and [Fe II]/[Ni II] owing to their differing origins in the
ejecta. The FWHM values obtained are mentioned in the figure.

###  5.3 Hydrodynamical Modelling

We resort to detailed hydrodynamical modelling for better constraints about
the progenitor, its evolution, mass loss history and its immediate
environment. We used the publicly available 1-D stellar evolution code MESA
r-15140 (Paxton et al., 2011, 2013, 2015, 2018, 2019) and a simplified version
of STELLA (Blinnikov & Sorokina, 2004; Baklanov et al., 2005; Blinnikov et
al., 2006) included with MESA to simulate light curves and photospheric
velocities of SN 2020jfo. MESA + STELLA has been successfully used in many
studies to investigate properties of Type IIP SNe progenitors (Moriya et al.,
2011; Goldberg et al., 2019; Hiramatsu et al., 2021a). We also try this
framework to get more insights about the progenitor of SN 2020jfo. Some of the
aspects regarding various hydrodynamical parameters are as follows:

  1. 1.

The built-in nuclear reactions rates were taken from
‘approx21_cr60_plus_co56.net’. Nuclear reaction rates are mostly from the
Nuclear Astrophysics Compilation of Reaction rates, (NACRE, Angulo, 1999) and
the Joint Institute for Nuclear Astrophysics, JINA reaction rates (REACLIB,
Cyburt et al., 2010).

  2. 2.

Cool and hot wind schemes for the Red Giant Branch or Asymptotic Giant Branch
phase are taken as ‘Dutch’, as described in MESA IV. This wind scheme for
massive stars is a combination of results from work by various Dutch authors.
The particular combination chosen is based on the work by Glebbeek et al.
(2009). Typically, if the surface hydrogen has a mass fraction less than 0.4
and an effective temperature greater than $\rm 10^{4}\,K$, the prescription
used is from Vink et al. (2001), otherwise it is taken from Nugis & Lamers
(2000).

  3. 3.

The mixing length parameter (MLT_option) is set to $Henyey$, which is based on
the work by Henyey et al. (1965), with $\rm\alpha_{MLT}=1.5$, where,
$\rm\alpha_{MLT}$ is the ratio of mixing length to the pressure scale height
($\rm=P/g\rho$).

  4. 4.

To determine the position of the convective boundaries, the default $Ledoux$
criterion is used.

From the evolution of pre-main-sequence star to finally retrieving the optical
light curve post-explosion, the modelling process was completed in three steps
inside the MESA framework. First, a pre–MS star was evolved till the Fe-core
developed, and there was an onset of rapid infall of the iron core. It was
accomplished using ‘make_pre_ccsnIIp’ test suite provided in MESA. Default
values of the controls in inlists were used with slight variations for
convergence with the help from Farmer et al. (2016). We fixed our metallicity
to $\rm Z=0.024$ as estimated in Section 3.1 for all the simulations.

Figure 17: Pre-supernova mass fractions for an evolved 12 $\rm M_{\odot}$
model for different species present in the ‘approx21_cr60_plus_co56.net’
(approx21) network used in MESA modelling.

Since the explosion could not be achieved directly by MESA, we proceeded to
the second step, which closely followed the ‘ccsn_IIp’ test suite. In this
step, a section of the core was removed, which would have eventually collapsed
onto a proto–NS. This centre section was removed from the model at the
location where $\rm entropy/baryon=4k_{B}$. Later, the explosion was induced
by the synthetic injection of energy into a thin layer of $\rm\approx
0.01\,M_{\odot}$ at the inner boundary (IB) for 5 ms, and the rate was scaled
such that the $\rm E_{exp}$ reached the desired input value. Shock then
proceeded through the various steps until it reached just below the surface
where the hand-off was performed from MESA to STELLA (Paxton et al., 2018).
STELLA then dealt with the shock-breakout and post-explosion evolution. For
the second step, we did not vary $\rm{}^{56}Ni$ mass estimate of 0.033 M⊙
obtained in Section 3.5 in order to reduce the parameter space.

A recent study by Hiramatsu et al. (2021a) discusses the possibility of
obtaining shorter plateaus in Type II SN light curves from progenitors with
ZAMS mass of 18-25 $\rm M_{\odot}$ with an enhanced mass loss
($\dot{M}\rm\simeq 10^{-2}\,M_{\odot}\,yr^{-1}$) in the decades prior to
collapse. The objects of interest in their study were more luminous events
($\rm Peak\,M_{V}<-18\,mag$), with a higher $\rm{}^{56}Ni$ yield and higher
expansion velocities in contrast to SN 2020jfo, which has an average
$\rm{}^{56}Ni$ mass and lower expansion velocities in comparison to a typical
Type II SNe. In addition, a number of other numerical modelling works (Dessart
et al., 2010; Sukhbold et al., 2016) showed that low mass progenitors of $\rm
M\leq 18\,M_{\odot}$ were not able to produce a shorter plateau duration of
around 60 days. In all, none of the simulations for masses $\rm M\leq
15\,M_{\odot}$ under standard conditions were able to produce light curves
with short plateaus (Curtis et al., 2020; Hiramatsu et al., 2021a; Dessart et
al., 2010; Sukhbold et al., 2016). Instead of exploring the parameter space
favoured by other works for short plateau Type II SNe, we took a different
approach, where the inputs were driven from the results of semi-analytical
modelling and nebular phase spectra. Hence, we went ahead with the evolution
of a ZAMS model of $\rm 12\,M_{\odot}$ and tried variations in the evolution
schemes to achieve a shorter plateau.

A 12 $\rm M_{\odot}$ progenitor was evolved with an initial metallicity
slightly higher than Solar and with a finite amount of rotation
($\rm\Omega=0.1\,\Omega_{critical}$). Figure 17 shows the pre-Supernova mass
fractions of an evolved model for ‘approx21’ network. It was found that, for a
typical mass loss rate due to winds, a short plateau was not possible as there
was not enough stripping of the hydrogen envelope of the progenitor’s ejecta.
Hence, an enhanced mass loss due to winds was applied during the evolution,
which is highly possible in a higher metallicity environment with a rotating
progenitor. Mass loss was controlled by the wind scaling factor (wsf) in MESA.
We tried varying this parameter from a default value of 1.0 onwards. At a
value of $\rm wsf=5.0$, we could get enough material stripped off from its
surface in order to achieve a short plateau with a similar period as SN
2020jfo. We also note that the sharp transition could not be produced using
physical mass loss schemes. However, a sharp transition is achieved if the
mass is removed by hand to leave the final mass as 5.0 M⊙. Exploring amount of
nickel mixing in layers, density structure of the progenitor, etc. is beyond
the scope of this work. Some of the models did not converge as the central
density was not sufficient enough for the ignition of higher masses during the
course of evolution. The simulated light curves obtained for various wind
scaling factors along with Q-bol for SN 2020jfo are shown in the Figure 18 and
corresponding pre-SN values for various models are presented in Table 7.

Table 7: Pre-SN parameters for different models $\rm M_{i}\ =\ 12.0\
M_{\odot},\ \Omega\ =\ 0.1\Omega_{c},\ Z\ =\ 0.024$
---
$\rm M_{f}$ | Age | $\rm\alpha_{Dutch}$ | Radius | $\rm E_{tot}$
$\rm(M_{\odot})$ | (Myr) |  | $\rm(R_{\odot})$ | (foe)
10.9 | 18.7 | 1.0 | 470 | –0.94
8.9 | 19.2 | 3.0 | 780 | –1.00
8.7 | 19.3 | 3.2 | 723 | –0.93
6.6 | 19.7 | 5.0 | 679 | –0.91

Figure 18: Quasi-bolometric light curves obtained from MESA + STELLA modelling
with different values of wsf (1.0, 3.0, 3.2 and 5.0) for the $\rm
12\,M_{\odot}$ ZAMS model. Q-bol of SN 2020jfo is over-plotted for comparison.

Figure 19: Photospheric velocity evolution as obtained from MESA + STELLA
modelling for two different values of optical depth ($\rm\tau_{Sobolev}$=1.0
and 2.0) compared with the observed photospheric velocities.

It was demonstrated by comprehensive modelling that the model of ZAMS with 12
$\rm M_{\odot}$ and the final mass of the progenitor as 6.6 $\rm M_{\odot}$,
fitted closely the decline to nebular phase and late phase evolution of the
observed light curve (Figure 18). It had an ejected mass of about 5 $\rm
M_{\odot}$ and an excised core of about 1.6 $\rm M_{\odot}$. The explosion
energy for best-fitting models was $\rm E_{exp}=0.2-0.4\,foe$. The
photospheric velocity evolution for this model is in agreement with the
estimates from the observed spectral sequence (see Figure 19). Slow velocity
evolution could also be attributed to the low energy of the explosion as
obtained from hydrodynamic modelling ($\rm 0.2-0.4\,foe$) along with the low
ejecta mass as most of the mass was blown away by winds during evolution. A
nickel mass of 0.033 $\rm M_{\odot}$ used in models substantiate our earlier
Ni mass estimates. While the modelled light curve matched with the observed
plateau duration, decline to the nebular phase, and late phase light curve
evolution, it failed to reproduce the early steep rise to a high luminosity
observed in the quasi-bolometric light curve. As the calculations are based on
normal type IIP SNe, only primary radiation sources viz. shock breakout and
cooling, hydrogen recombination, and radioactive decay are considered. The
inadequacy of the current model to fit the early part of the light curve
indicates the need to introduce a secondary source of radiation for early
times, and the best possible source could be the presence of CSM close to the
progenitor.

Due to the lack of direct signatures of CSM interaction, we could not
calculate the extent and density of the CSM. In order to estimate the same, we
used STELLA, where it was possible to place the CSM around the progenitor with
its configuration defined by the parameters wind velocity, mass loss rate, and
its duration. The density profile in STELLA is dependent on the radius, $r$,
away from the progenitor’s centre as:

$\rho(r)_{w}=\frac{\dot{M_{w}}}{4\pi r^{2}v_{w}},$ (2)

where $\dot{M_{w}}$ is mass loss rate in $\rm M_{\odot}\,yr^{-1}$ due to winds
and vw is the wind velocity. We allocated 40 zones out of 400 for CSM
configuration and the bolometric flux was obtained at four extents
($\rm~{}2,10,20,40\ AU$) with various mass loss rates
($\rm~{}0.001,0.005,0.01,0.05\,M_{\odot}\ yr^{-1}$). A typical wind velocity
of $\rm~{}10^{6}\,cm\,s^{-1}$ was affixed for all configurations. The modelled
light curve with $\dot{M}\rm=0.01\,M_{\odot}\,yr^{-1},\ t=20\,yr$ (which
corresponds to a CSM extent of roughly 40 AU) is found to fit the observed
quasi-bolometric light curve well (see Figure 20). The He II feature
considered as proxy for interaction signature is not observed beyond +10 d.
This could be due to formation and increasing strength of other lines in the
spectra. Furthermore, it could also be due to decrease in CSM-ejecta
interaction, giving rise to a steep decline in the quasi-bolometric light
curve. Light curve modelling is suggestive of interaction up to +15 d, which
is considered as an upper bound. Further, we compare $U-B$ colour evolution
(Figure 21) of models with observed $U-B$ colour. We find the colour to be
flat for initial 8 d and later it evolves towards red. This initial trend is
only seen in the models with added CSM.

Figure 20: Quasi-bolometric light curves as obtained from MESA + STELLA
modelling for a $\rm 12\,M_{\odot}$ ZAMS progenitor ($\rm
M_{f}=6.6\,M_{\odot}$) with different CSM configurations. Q-bol light curve of
SN 2020jfo is over-plotted for comparison.

Figure 21: Colour evolution as obtained from MESA + STELLA modelling for a
$\rm 12\,M_{\odot}$ ZAMS progenitor ($\rm M_{f}=6.6\,M_{\odot}$) with
different CSM configurations.

###  5.4 CSM Interaction

There have been numerous instances where studies have provided enough evidence
for CSM surrounding Type II progenitors both in spectra and light curves.
Förster et al. (2018) attributed the steeper light curve rise and delayed
shock emergence to the dense CSM from their sample of 26 Type II SNe. Another
study, combining light curve modelling and observations (Morozova et al.,
2018) summarised that $\sim 70\%$ SNe have CSM, and the estimated CSM masses
ranged between $\rm 0.18-0.83\,M_{\odot}$. Bruch et al. (2021) emphasised the
appearance of narrow flash emission features, especially He II 4686 Å, in the
very early spectra, ideally taken less than 48 hrs of explosion. We do not see
such narrow signatures of CSM interaction in our earlier spectra, although we
do see broad ionised lines of Helium, which were likely formed at the CDS
arising due to the shock ionisation of the outer layers or the CSM close to
the ejecta. Along with this, the presence of HV $\rm H\alpha$ feature in the
mid to late plateau phase spectra is an indication of CSM’s presence.

The higher peak luminosity and steeper early phase decline seen in SN 2020jfo
also strengthen the case of CSM close to its progenitor. High decline rates in
the early phase of Type II SNe have been attributed to the interaction with
CSM. The diagnostic in SN 2013by was the presence of asymmetric line profiles
with photospheric signatures of high velocity features of hydrogen (Valenti et
al., 2015). In SN 2014G, the presence of highly ionised spectroscopic features
was attributed to a metal-rich CSM accumulated from the mass-loss events prior
to the explosion (Terreran et al., 2016). It is likely that the higher
luminosity of SN 2020jfo during the early epoch is due to interaction with the
nearby CSM, and its density profile is such that this is not sustained for
prolonged periods. We ascertained this possibility with hydro-dynamical
modelling using MESA + STELLA.

Furthermore, the colour evolution of SN 2020jfo in the early phase is bluer as
compared to other SNe, while in the late phase, it flattens out and merges
with the normal Type II SN colour evolution. The bluer early phase colour
evolution is similar to the CSM-interacting events. The Q-bol of SN 2020jfo
during the early phase is comparable to SN 2009au and SN 2014G (see Figure 6),
which showed clear signs of interaction in their spectra. Though the
luminosity is higher during the early phase, the steeper decline in the
plateau phase leads to a luminosity comparable to normal Type IIP events such
as SN 2016X towards the end of the plateau. The additional source giving rise
to the higher luminosity in the early phase is likely due to CSM interaction,
however, the CSM remains hidden. Nagao et al. (2020) showed that a CSM
distributed in the form of a disc, when viewed from a polar angle, would only
cause enhancement in flux due to heating effects and would not leave any
signatures of interaction in the spectra. Although, as clearly shown in Nagao
et al. (2020), overluminous Type IIP SNe might not be powered by such disc
interaction, but a slight enhancement is a likely proposition during the early
phase.

To have a complete picture, we also looked at the field of SN 2020jfo for any
radio detection. The field was observed on October 17, 2021 (JD 2459504.5) in
the VLA sky survey (VLASS) (image cut-outs can be found here
666http://cutouts.cirada.ca/). No significant radio emission was detected at
the SN position, and a flux density limit of 309 $\rm\mu$Jy ($\rm 3\sigma$
upper limit) at 3 GHz was obtained. Unsurprisingly, the SN being $>$ 500 days
old at the time of VLASS observations, the radio flux density declined below
the sensitivity limits of current radio telescopes around this period, even
for radio bright Type IIP SNe. Using the expression for mass-loss rate given
in Weiler et al. (1986), we obtained an upper limit of $\dot{M}<2.5\times
10^{-5}\,\rm M_{\odot}\ yr^{-1}$. This value is somewhat consistent with
typical Type IIP SNe but smaller for our case where we estimated a higher
mass-loss rate (see Section 5.3) from the MESA+STELLA modelling of the early
phase bolometric light curve. This is likely due to the difference in the
epochs of observations of the radio and the modelled light curve with CSM.
Sollerman et al. (2021) also looked for X-ray emission post-explosion for SN
2020jfo and could only cite an upper limit based on their estimates. It might
be the case that the X-ray emission from dense CSM was earlier on and could
have been missed as it was absorbed by nearby CSM (similar scenario as pointed
out in Jacobson-Galán et al., 2021).

Figure 22: Temporal evolution of pEW of Fe II 5018 Å in comparison with models
from Dessart et al. (2013) having different metallicities (0.4, 1 and 2 $\rm
Z_{\odot}$). The black solid line represents the mean value of the pEW of Fe
II 5018 Å and shaded region shows its dispersion from the extensive sample of
Gutiérrez et al. (2017a).

###  5.5 Case for a stripped low mass progenitor

The metallicity estimated, was slightly higher than the Solar values and might
have helped in escalating the mass-loss rate of the progenitor star. It has
been argued in Dessart et al. (2013) that lower envelope mass at higher
metallicity ought to produce a Type II SN with a shorter duration plateau. A
direct result of this is seen in the temporal evolution of pseudo-equivalent
widths (pEW) of Fe II 5018 Å of SN 2020jfo in comparison to the estimates from
spectral models from Dessart et al. (2013) (see Figure 22). We find that with
increasing metallicities, the pEW increases. The higher pEW evolution of SN
2020jfo during the photospheric phase corroborates with the enhanced
metallicity environment. However, the implied metallicity is much higher than
that estimated for its host (i.e. $\sim$ 1.5 $\rm Z_{\odot}$ in Section 3.1).
Dessart et al. (2013) have shown that the compactness (i.e. radius) of the
progenitor affects the pEW of the metal features during the plateau phase.
This is depicted in Figure 22, which shows 3 variants of 1 $\rm Z_{\odot}$
metallicity progenitor, with different final progenitor radii.
Observationally, this indicates the presence of a stripped hydrogen envelope,
which would make the progenitor more compact, and probably explain the lack of
a clear early decline ($s1$) phase prior to the onset of the plateau ($s2$)
phase.

The semi-analytical modelling of the bolometric light curve of SN 2020jfo in
Section 5.1 infers a progenitor of low mass with an ejected mass, $\rm M_{ej}$
of $\rm\sim 7.3\,M_{\odot}$ and an RSG radius of $\rm\sim 350\ R_{\odot}$.
Considering a typical Neutron Star (NS) remnant core of $\rm\sim
1.5\,M_{\odot}$, we infer a pre-SN mass of $\rm\sim\,9\,M_{\odot}$.

From MESA+STELLA modelling, we obtained a stripped progenitor with a pre-SN
mass of $\sim 6.6\rm\,M_{\odot}$ where the initial ZAMS mass was
$12\rm,M_{\odot}$. A similar ZAMS value for the progenitor was also obtained
using late nebular spectra (see Section 5.2). However, we did not implement
any synthetic mass loss scheme (deliberate removal of the H envelope mass) to
achieve stripping of progenitor (see Section 5.3). Nevertheless, there could
be numerous short time windows where enhanced mass loss is possible (Decin et
al., 2006), which are hard to predict and hence challenging to include in
modelling. Observationally, mass-loss rates have not been constrained very
well and vary by over two orders of magnitudes ($10^{-4}$ to $10^{-6}\,\rm
M_{\odot}\,yr^{-1}$; see van Loon et al., 2005; Mauron & Josselin, 2011). The
mass loss rate adopted for the progenitor of SN 2020jfo in MESA is five times
the typical mass-loss rates for an RSG progenitor, but is well within the
observed limits. It is difficult to predict what could have caused such a high
mass-loss rate, whether it was solely due to the rotation and high metallicity
environment, or due to other factors such as interaction with a binary
companion or multiple episodes of enhanced mass loss.

It was shown by Eldridge et al. (2018) that the initial progenitor masses
around 8-15 $\rm M_{\sun}$ in the binary scenario possibly give light curves
with shorter plateaus of the order of tens of days. However, their physical
parameter space was limited, and not much could be said quantitatively about
the progenitor properties. Another attempt by Hiramatsu et al. (2021a) showed
that the RSG progenitors with initial masses of 18-25 $\rm M_{\sun}$ with
enhanced mass-loss rates could reproduce shorter duration plateaus. However,
the observed properties viz. nebular spectra, the mass of synthesised
radioactive nickel, and evolution velocities of the events (SN 2006Y, SN
2006ai and SN 2016egz) were also supportive of higher mass progenitors. Both
these studies had shown a continuous population of Type IIP-IIL-IIb events,
wherein a higher progenitor mass leads to an increased amount of stripping of
the hydrogen envelope. However, SN 2020jfo poses a question to the standard
progenitor scenario. The arguments presented in our analysis and discussion
weigh in on a low mass progenitor with enhanced mass loss that gave birth to
the short plateau supernova SN 2020jfo.

##  6 Summary

In this work, we presented an extensive multi-band photometric and optical
spectroscopic study of a short-plateau Type II event, namely SN 2020jfo.

  1. 1.

We estimated a plateau duration of $<65$ d for SN 2020jfo, putting it under
the category of the rare, short plateau Type IIP SNe.

  2. 2.

The observational properties associated with SN 2020jfo are: peak V-band
absolute magnitude of $\rm M_{V}=-17.4\pm 0.4$ mag, peak optical luminosity of
$\rm 4.3\pm 1.4\times 10^{42}\,erg\,s^{-1}$, and a synthesised $\rm{}^{56}Ni$
mass of $\rm 0.033\pm 0.006\,M_{\odot}$.

  3. 3.

Using nebular phase spectrum, we estimated a progenitor of mass $\sim 12$ $\rm
M_{\odot}$.

  4. 4.

We estimated the progenitor properties for SN 2020jfo from hydrodynamical
modelling and concluded that the most plausible progenitor is a Red Super
Giant with an initial mass around 12 $\rm M_{\odot}$, radius $\rm\sim
679\,R_{\odot}$ and a final pre-supernova mass $\rm<6.6\,M_{\odot}$. It
evolved in a relatively high metallicity environment and shredded a
significant amount of mass during its course of evolution.

  5. 5.

A high Ni/Fe ratio of 0.18$\pm$0.04 by mass was estimated for SN 2020jfo that
is consistent with a low mass progenitor ($\rm M_{ZAMS}\leq 13\,M_{\odot}$).

  6. 6.

The evolution of pEW values of Fe II 5018 Å are much higher than the other
Type II SNe, confirming the presence of a high metallicity environment and a
compact progenitor.

  7. 7.

The presence of ionised He II line, HV $\rm H\alpha$ feature, higher
luminosity in contrast to slower velocity evolution, and the steeper decline
in luminosity indicated the presence of CSM, which was confirmed by
MESA+STELLA hydrodynamical modelling. It was deduced that a $\rm
0.2\,M_{\odot}$ CSM, extended to a region of $\rm\sim 40\,AU$ around
progenitor was required to explain the early higher luminosity and a faster
decline of the bolometric light curve.

## Acknowledgements

We thank the referee for a thorough evaluation of the manuscript that helped
in improving it. We thank the staff of IAO, Hanle and CREST, Hosakote, that
made these observations possible. The facilities at IAO and CREST are operated
by the Indian Institute of Astrophysics, Bangalore. RST acknowledges Anirban
Dutta for helpful discussions during this work. NAJ would like to acknowledge
DST-INSPIRE Faculty Fellowship (IFA20- PH-259) for supporting this research.
This research has made use of the High Performance Computing (HPC)
resources777https://www.iiap.res.in/?q=facilities/computing/nova made
available by the Computer Center of the Indian Institute of Astrophysics,
Bangalore. This research made use of
RedPipe888https://github.com/sPaMFouR/RedPipe (Singh, 2021), an assemblage of
data reduction and analysis scripts written by AS. This research is also based
on observations obtained at the 3.6m Devasthal Optical Telescope (DOT), which
is a National Facility run and managed by Aryabhatta Research Institute of
Observational Sciences (ARIES), an autonomous Institute under the Department
of Science and Technology, Government of India. This research has also made
use of the NASA/IPAC Extragalactic Database
(NED999https://ned.ipac.caltech.edu), which is funded by the National
Aeronautics and Space Administration and operated by the California Institute
of Technology. We used the Open Supernova Catalog101010https://sne.space (OSC,
Guillochon et al., 2017) to retrieve all the light curve and spectral data for
comparison. We also acknowledge Wiezmann Interactive Supernova data
REPository111111https://wiserep.weizmann.ac.il (Yaron & Gal-Yam, 2012,
(WISeREP)) for spectral data downloads. This research also made use of tardis,
a community-developed software package for spectral synthesis in supernovae
(Kerzendorf & Sim, 2014). The development of tardis received support from
GitHub, the Google Summer of Code initiative, and from ESA’s Summer of Code in
Space program. tardis is a fiscally sponsored project of NumFOCUS. tardis
makes extensive use of Astropy and Pyne.

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