# Photometric and spectroscopic analysis of the Type II SN 2020jfo with a
short plateau

B. Ailawadhi,1,2 R. Dastidar,3,4 K. Misra,1 R. Roy,5,6 D. Hiramatsu,7,8 D. A.
Howell,7,8 T. G. Brink,9 W. Zheng,9 L. Galbany,10,11 M. Shahbandeh,12 I.
Arcavi,13,14 C. Ashall,15 K. A. Bostroem,16 J. Burke,7,8 T. Chapman,9
Dimple,1,2 A. V. Filippenko,9,17 A. Gangopadhyay,18 A. Ghosh,1,19 A. M.
Hoffman,20,9,21 G. Hosseinzadeh,22 C. Jennings,9 V. K. Jha,1,2 A.
Kumar,1,19,23 E. Karamehmetoglu,24 C. McCully,7,8 E. McGinness,9 T. E. Müller-
Bravo,10 Y. S. Murakami,21,9,25 S. B. Pandey,1 C. Pellegrino,7,8 L.
Piscarreta,10 J. Rho,26 M. Stritzinger,22 J. Sunseri,9,21 S. D. Van Dyk,27 L.
Yadav2
1 Aryabhatta Research Institute of Observational Sciences, Manora Peak,
Nainital 263 001, India
2 Department of Physics, Deen Dayal Upadhyaya Gorakhpur University,
Gorakhpur-273009, India
3 Millennium Institute of Astrophysics, Nuncio Monsenor Sótero Sanz 100,
Providencia, Santiago, 8320000 Chile
4 Instituto de Astrofisica, Universidad Andres Bello, Fernandez Concha 700,
Las Condes, Santiago, Chile
5 Manipal Centre for Natural Sciences, Centre of Excellence, Manipal Academy
of Higher Education, Manipal 576104, Karnataka, India
6 Inter-University Center for Astronomy and Astrophysics, Ganeshkhind, Pune,
Maharashtra-411007, India
7 Department of Physics, University of California, Santa Barbara, CA
93106-9530, USA
8 Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA
93117-5575, USA
9 Department of Astronomy, University of California, Berkeley, CA 94720-3411,
USA
10 Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans,
s/n, E-08193 Barcelona, Spain
11 Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
12 Department of Physics, Florida State University, 77 Chieftan Way,
Tallahassee, FL 32306, USA
13 The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978,
Israel
14 CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada
15 Department of Physics, Florida State University, Tallahassee, FL 32306, USA
16 DiRAC Institute, Department of Astronomy, University of Washington, Box
351580, U.W., Seattle, WA 98195, USA
17 Miller Institute for Basic Research in Science, University of California,
Berkeley, CA 94720, USA
18 Hiroshima Astrophysical Science Centre, Hiroshima University, 1-3-1
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
19 School of Studies in Physics and Astrophysics, Pandit Ravishankar Shukla
University, Chattisgarh 492010, India
20 Institute for Astronomy, University of Hawai‘i at Manoa, 2680 Woodlawn Dr.,
Honolulu, HI 96822, USA
21 Department of Physics, University of California, Berkeley, CA 94720-7300,
USA
22 Steward Observatory, University of Arizona, 933 North Cherry Avenue,
Tucson, AZ 85721-0065, USA
23 Department of Physics, University of Warwick, Gibbet Hill Road, Coventry
CV4 7AL, UK
24 Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120,
DK-8000 Aarhus C, Denmark
25 Department of Physics and Astronomy, Johns Hopkins University, Baltimore,
MD 21218, USA
26 SETI Institute, 189 North Bernardo Avenue, Suite 200, Mountain View, CA
94043, USA
27 Caltech/IPAC, Mailcode 100-22, Pasadena, CA 91125, USA E-mail:
<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present high-cadence photometric and spectroscopic observations of SN
2020jfo in ultraviolet and optical/near-infrared bands starting from $\sim 3$
to $\sim 434$ days after the explosion, including the earliest data with the
10.4 m GTC. SN 2020jfo is a hydrogen-rich Type II SN with a relatively short
plateau duration ($67.0\pm 0.6$ days). When compared to other Type II
supernovae (SNe) of similar or shorter plateau lengths, SN 2020jfo exhibits a
fainter peak absolute $V$-band magnitude ($M_{V}=-16.90\pm 0.34$ mag). SN
2020jfo shows significant H$\alpha$ absorption in the plateau phase similar to
that of typical SNe II. The emission line of stable [Ni II] $\lambda$7378,
mostly seen in low-luminosity SNe II, is very prominent in the nebular-phase
spectra of SN 2020jfo. Using the relative strengths of [Ni II] $\lambda$7378
and [Fe II] $\lambda$7155, we derive the Ni/Fe production (abundance) ratio of
0.08–0.10, which is $\sim 1.5$ times the solar value. The progenitor mass of
SN 2020jfo from nebular-phase spectral modelling and semi-analytical modelling
falls in the range of 12–15 $M_{\odot}$. Furthermore, semi-analytical
modelling suggests a massive H envelope in the progenitor of SN 2020jfo, which
is unlikely for SNe II having short plateaus.

###### keywords:

techniques: photometric – techniques: spectroscopic – supernovae: general –
supernovae: individual: SN 2020jfo – galaxies: individual: M61

††pubyear: 2021††pagerange: Photometric and spectroscopic analysis of the Type
II SN 2020jfo with a short plateau–A

##  1 Introduction

Core-collapse supernovae (CCSNe) represent the end stages of stars having
zero-age main-sequence (ZAMS) masses $\gtrsim 8\,M_{\odot}$. These SNe emerge
from the gravitational collapse of the degenerate core of massive stars. The
presence of hydrogen (H) lines in the SN spectra around maximum brightness
distinguishes Type II SNe from other subtypes of CCSNe (Filippenko, 1997). The
plateau (P) feature (which lasts for $\sim 80$–120 days after maximum
brightness; Barbon et al. 1979) and the linear (L) decline feature in the
light curve characterises the events as Type IIP and Type IIL SNe,
respectively. However, as more SNe were discovered through systematic surveys,
it became evident that typical SN II light-curve morphologies formed a
continuous distribution (Anderson et al., 2014). Another subclass of SNe II
are the Type IIb, which initially show features of hydrogen and later
transition to helium, suggesting small amounts of H in the ejecta (Filippenko,
1988). Hereafter, we will refer to the hydrogen-rich class as Type II SNe. The
H recombination phase will be referred to as the “plateau." The Type II SNe
with a plateau decline rate greater than 1 mag (100 day)-1 will be referred to
as the fast-declining SNe II and those with a decline rate less than 1 mag
(100 day)-1 will be referred to as the slow-declining SNe II (Faran et al.,
2014b).

The formation of the plateau in SNe II occurs due to energy deposition in the
hydrogen-rich envelope of the star by the shock wave, which is gradually
radiated away as the recombination wave moves inward in mass coordinates. The
transition from plateau to the exponential decay/tail phase marks the end of
the recombination phase. There is a drop of 1.0–2.6 mag from the plateau to
the tail phase in the case of normal-luminosity SNe II (Valenti et al.,
2016b). On the other hand, a 3–5 mag drop has been observed in the low-
luminosity Type II SNe (e.g. $\sim 3.83$ mag in SN 2005cs; Tsvetkov et al.
2006; Spiro et al. 2014). However, recent studies have reported the presence
of outliers — for example, a large drop of 3.8 mag was reported in SN 2018zd
(Hiramatsu et al., 2021a), a normal-luminosity SN II, and a drop of $\sim 2$
mag was observed in the low-luminosity Type II SN 2016afq (Müller-Bravo et
al., 2020). After the drop, the SN enters the exponentially declining tail
phase, which is powered by the decay of 56Co to 56Fe.

In pre-explosion imaging studies of the galaxies hosting these SNe, red
supergiant (RSG) stars have been shown to be the progenitors of the slowly
declining SNe II, which preserve the hydrogen envelope at the time of
explosion. The mass range of the slowly declining Type II SN progenitors
estimated from pre-explosion imaging is 8–17 $M_{\odot}$, while the apparent
lack of higher-mass progenitors (17–30 $M_{\odot}$) gives rise to the “RSG
problem" (Smartt et al., 2009; Smartt, 2015).

In the recent study by Anderson et al. (2014) and Valenti et al. (2016b), the
estimated parameters in a sample of SNe II display a wide range of peak
absolute magnitudes ($-14>M_{V,{\rm peak}}>-19.5$ mag) and plateau
luminosities ($-14>M_{V,{\rm 50\,d}}>-18$ mag). The 56Ni mass is estimated to
lie in the range 0.005–0.280 $M_{\odot}$ (Müller et al., 2017; Rodríguez et
al., 2021) and the ejecta velocities lie between $9600>v_{\rm ph}>1500$ km s-1
(Gutiérrez et al. 2017). This shows the remarkable diversity in the features
of Type II SN population, encompassing faint, low-velocity, nickel-poor events
such as SN 1997D (Benetti et al., 2001; Chugai & Utrobin, 2000) to bright,
high-velocity, nickel-rich events such as SN 1992am (Nadyozhin, 2003).
Moreover, some SNe II are found to have a shorter plateau than normal (e.g.,
SNe 1995ad, 2006Y, 2006ai, 2007od, and 2016egz; Inserra et al. 2011, 2013;
Hiramatsu et al. 2021b), while for some other SNe II, longer plateaus have
been reported (e.g., SNe 2009ib; Takáts et al. 2015, 2015ba; Dastidar et al.
2018). The plateau length largely depends on the recombination time of all the
ionized hydrogen, which is proportional to the mass of the hydrogen envelope
(Popov, 1993; Kasen & Woosley, 2009). The analytical and numerical modelling
of the light curves of Type II SNe suggests that the length of the plateau
scales with the explosion and progenitor properties of SNe such as the initial
stellar mass, metallicity, and explosion energy (Popov, 1993; Sukhbold et al.,
2016; Goldberg et al., 2019).

Hiramatsu et al. (2021b) studied the properties of three SNe II (SNe 2006Y,
2006ai, 2016egz) with shorter plateau lengths ranging from 50 to 70 days. They
reported some properties of these three SNe which make them stand out from
normal Type IIP SNe. In their work, the three Type II SNe were identified as a
transitional class between Type IIL and IIb SNe with a narrow H-rich envelope
mass window ($M_{\rm H-env}\approx 1.7\,M_{\odot}$). The plateau phase of
these three SNe II has been claimed to be powered by 56Ni decay, unlike other
Type II SNe. The photospheric luminosity and the effective temperature of
these three SNe were found to lie in the range
$5.23<\log_{10}(L_{\mathrm{ph}}/L_{\odot})<5.53$ and
$3.65<\log_{10}(T_{\mathrm{eff}}/\mathrm{K})<3.72$ (respectively) from
modelling, and these values correspond to luminous RSGs.

In this paper, we present the photometric and spectroscopic analysis of the
Type II SN 2020jfo. The light curve and spectra of SN 2020jfo show similarity
with normal SNe II, but with a comparatively shorter plateau length in
comparison to the normal events. The paper is structured as follows. The
discovery of SN 2020jfo is presented in Section 2. Section 3 provides the
details of photometric and spectroscopic observations and data-reduction
techniques. The light curve and spectroscopic evolution are presented in
Sections 4 and 5, respectively. The properties of the progenitor and explosion
are discussed in Section 6. A comparison of SN 2020jfo with other Type II SNe
is given in Section 7, and our conclusions are presented in Section 8.

##  2 SN 2020jfo Discovery

SN 2020jfo (also known as ZTF20aaynrrh) was discovered on 2020-05-06 (UTC
dates are used throughout this paper; 58975.2 MJD) by the Zwicky Transient
Facility (ZTF) survey using the Palomar Schmidt 48-inch (P48) Samuel Oschin
telescope (Bellm et al., 2019; Graham et al., 2019). It was reported on the
Transient Name Server (TNS) (Nordin et al., 2019) within 2 hr of discovery.
The first clear detection reported by the ZTF was in the $r$ band at a
magnitude of $16.01\pm 0.04$. The last nondetection with ZTF was 4 days (on
2020-05-02) before the discovery, with a reported upper limit of $g>19.7$ mag.
SN 2020jfo was classified as a Type II SN (Perley et al. 2020) based on
spectra taken with the Liverpool Telescope (LT) and the Nordic Optical
Telescope (NOT). These spectra were taken $\sim 17$ hr after the first ZTF
detection. Our first spectrum with the Gran Telescopio Canarias (GTC) was
taken nearly half an hour earlier than the LT and NOT spectra. The independent
discoveries of this transient by ATLAS, PS2, Gaia, and MASTER were also
reported to the TNS. We note here that SN 2020jfo was earlier studied by
Sollerman et al. (2021) and Singh Teja et al. (2022).

SN 2020jfo exploded in the outskirts of the face-on spiral galaxy M61 at a
redshift $z=0.00522$ (from NED). M61 is a prolific SN-producing galaxy which
has hosted seven other SNe to date. In the work by Bose & Kumar (2014), the
expanding photosphere method (EPM) was employed using the optical dataset from
SN 2008in to estimate the distance ($14.51\pm 1.38$ Mpc) calibrated with the
atmosphere model provided by Dessart & Hillier (2005). We adopt this value of
the distance for our work ($\mu=30.81\pm 0.20$ mag). Sollerman et al. (2021)
used this same distance for their work, whereas Singh Teja et al. (2022) used
a distance of $16.45\pm 2.69$ Mpc for their analysis. These distance values
are in agreement with ours within the uncertainties.

The Galactic reddening along the line of sight to SN 2020jfo, obtained from
the infrared (IR) dust maps given by Schlafly & Finkbeiner (2011), is
$E(B-V)_{\rm Gal}=0.0194\pm 0.0001$ mag. To estimate the host-galaxy
extinction, empirical relations correlating the equivalent width (EW) of the
Na I D doublet ($\lambda\lambda 5890$, 5896) and the colour excess are often
used. The rest-frame spectra of SN 2020jfo do exhibit a conspicuous narrow
absorption feature around 5893 Å. The EW of Na I D is estimated to be $0.88\pm
0.21$ Å using the composite spectrum, constructed from the two highest signal-
to-noise-ratio (SNR) spectra (21.2 and 25.2 d) of SN 2020jfo (see Figure 1).
Using the Poznanski et al. (2012) empirical relations between EW and $E(B-V)$,
we obtained $E(B-V)_{\rm host}=0.15\pm 0.09$ mag. The host-galaxy extinction
can also be estimated from the colour (Olivares E. et al., 2010), but this
method is not very reliable (de Jaeger et al., 2018). Using this method, we
obtained negative values of $A_{V}$, which are unphysical. Since Na I D is a
well-known tracer of gas in galaxies, and Na I D absorption is prominent in
the spectra, we use the host-galaxy extinction estimated from the EW (although
the reliability of this method has also been questioned; e.g., see Phillips et
al. 2013). We adopt a total $E(B-V)=0.17\pm 0.09$ mag throughout this paper.
Sollerman et al. (2021) correct the photometric magnitudes only for the
Galactic extinction, and Singh Teja et al. (2022) use a total $E(B-V)=0.29\pm
0.08$ mag. In Table 1, we list the basic information on SN 2020jfo and its
host galaxy.

Table 1: Basic information on SN 2020jfo and the host galaxy M61.a

Host galaxy | NGC 4303
---|---
Galaxy type | SAB(rs) bc
Redshift | $0.00522\pm 0.00001$
Major axis diameter | $6.5^{\prime}$
Minor axis diameter | $5.8^{\prime}$
Heliocentric velocity |  $1566.12\pm 2.10$ km s-1
Distance |  $14.51\pm 1.38$ Mpcb
Total extinction $E(B-V)$ |  $0.17\pm 0.09$ magc
SN type | IIP
Date of discovery (MJD) | 58975.2
Estimated date of explosion (MJD) | 58973.1c

aThe host-galaxy parameters are taken from NED.
bBose & Kumar (2014).
cThis paper.

Figure 1: The narrow Na I D absorption dip due to the host galaxy, M61, in the
two highest-SNR spectra (21.2 and 25.2 d) and the composite spectrum of SN
2020jfo.

##  3 Observations and Data Reduction

###  3.1 Photometric observations

####  3.1.1 Swift/UVOT photometry

Photometry of SN 2020jfo was obtained with the Neil Gehrels Swift Observatory
(hereafter Swift) between 2020-05-07 and 2021-01-30 at 26 epochs. The
transient was bright in all near-UV (NUV) and optical filters of the
Swift/UVOT instrument. The UVOT data were reduced using the standard pipeline
available in the HEAsoft software
package111https://heasarc.nasa.gov/lheasoft/. Observations at each epoch were
conducted using one or several orbits. To improve the SNR in a given band at a
particular epoch, we coadded all orbit data for that corresponding epoch using
the HEAsoft routine uvotimsum. We used the routine uvotdetect to determine the
correct position of the transient (which is consistent with the ground-based
optical observations) and the routine uvotsource to measure the apparent
magnitude of the transient by performing aperture photometry. For source
extraction we used a small aperture of radius 3$\aas@@fstack{\prime\prime}$5,
while an aperture of radius $100^{\prime\prime}$ was used to determine the
background. There is a possibility that when multiple photons are coincident
on the same area of the UVOT detector, it does not count all the photons, an
effect known as “coincidence loss." The uvotsource routine makes the necessary
correction to compensate for the coincidence loss. All of the UVOT photometry
presented here is corrected for coincidence loss.

The field was also visited by Swift in the year 2008, to follow the Type II SN
2008in (Roy et al., 2011b), which exploded at the outskirt of another arm of
the host. We have used those previous UVOT frames to calculate the host
contribution at the location of SN 2020jfo. The host magnitudes at the
transient location are $19.38\pm 0.07$, $19.34\pm 0.08$, $19.02\pm 0.05$,
$18.49\pm 0.04$, $17.61\pm 0.07$, and $17.34\pm 0.09$ mag in the UVOT $uw2$,
$um2$, $uw1$, $uvu$, $uvb$, and $uvv$ bands, respectively. To obtain the host-
subtracted magnitudes of SN 2020jfo, we first subtracted the coincidence-loss-
corrected count rate of the galaxy computed from the pre-SN observations, from
the coincidence-loss-corrected count rate of SN 2020jfo calculated from the
images where the SN exists. Then the host-subtracted count rates of the SN
were converted to AB magnitudes (Oke & Gunn, 1983). The uncertainties in the
final photometry were obtained by propagating the error associated with the
host magnitude and the error in the “unsubtracted magnitudes" in quadrature.
Our photometry in AB mag is listed in Table 6.

####  3.1.2 Optical photometry

Photometric follow-up observations of SN 2020jfo were carried out with
telescopes in the Las Cumbres Observatory (LCO) under the Global Supernova
Project (GSP) in $BVg^{\prime}r^{\prime}i^{\prime}$ filters between 2020-05-07
to 2021-07-13 at 53 epochs. Data between 2020-05-07 to 2021-02-22 at 44 epochs
in the clear and $BVRI$ filters were also taken with the 1.04 m Sampurnanand
Telescope (ST), the 1.3 m Devasthal Fast Optical Telescope (DFOT), and the 3.6
m Devasthal Optical Telescope (DOT) located in ARIES, Nainital, India, as well
as with the 0.76 m Katzman Automatic Imaging Telescope (KAIT; Filippenko et
al. 2001) and the 1.0 m Nickel telescope at Lick Observatory. The earliest
$r$-band photometric point was taken with the 10.4 m GTC. The instrument
details of the respective telescopes are given in Table 7. Preprocessing of
the data, including bias and flat correction, was done using Python modules.
The Cosmicray task in IRAF222IRAF stands for Image Reduction and Analysis
Facility distributed by the National Optical Astronomy Observatories which is
operated by the Association of Universities for research in Astronomy, Inc.,
under cooperative agreement with the U.S. National Science Foundation. was
used for the removal of cosmic rays. Multiple frames observed on the same
night were median-combined in order to increase the SNR. We performed image
subtraction on the cleaned observed frames to compute the true flux of the
transient. For the ST and DFOT observations, we used an IRAF and DAOPHOT II
(Stetson, 1987) based routine developed by Roy et al. (2011a), with the
template taken on 2011-01-04 (see Roy et al. 2011b). The instrumental
magnitudes of the SN were estimated using DAOPHOT II.

The LCO GSP data were reduced with the
lcogtsnpipe333https://github.com/svalenti/lcogtsnpipe pipeline (see Valenti et
al. 2016a). For the LCO data, image subtraction was carried out using High
Order Transform of PSF ANd Template Subtraction
(HOTPANTS444https://github.com/acbecker/hotpants). We used the images obtained
on 2021-07-17, when the SN faded below the detection threshold of the 1 m LCO
telescopes, as the template. The instrumental SN magnitudes were calibrated
using the magnitudes of a number of local standard stars in the SN field
obtained from the AAVSO Photometric All-Sky Survey
(APASS555https://www.aavso.org/apass) catalogue, whereas the ST and DFOT
photometric data were calibrated using the magnitudes of the field standards
marked by Roy et al. (2011b). The data from KAIT and the 1.0 m Nickel
telescope were reduced using a custom
pipeline666https://github.com/benstahl92/LOSSPhotPypeline detailed by Stahl et
al. (2019). An image-subtraction procedure was also applied in order to remove
the host-galaxy light. The Pan-
STARRS1777http://archive.stsci.edu/panstarrs/search.php catalog was used for
calibration. Apparent magnitudes were all measured in the KAIT4/Nickel2
natural system, then transformed back to the standard system using local
calibrators and colour terms for KAIT4 and Nickel2 (Stahl et al., 2019).
Figure 20 shows the template-subtracted image of SN 2020jfo. The calibrated SN
magnitudes are listed in Tables 8, 9, and 10.

###  3.2 Optical and near-infrared spectroscopy

Spectroscopic observations of SN 2020jfo were conducted at 26 epochs between
2020-05-06 and 2021-05-04 using the 2.0 m Faulkes Telescope North (FTN) and
Faulkes Telescope South (FTS) of the LCO Network (again via the Global
Supernova Project), the 3.0 m Shane telescope at Lick Observatory, the 3.6 m
DOT (ADFOSC; Omar et al. 2019), and the 10.4 m GTC. The details of the
instruments used are given in Table 7. The one-dimensional (1D) wavelength-
and flux-calibrated spectra were extracted using the floydsspec
pipeline888https://github.com/svalenti/FLOYDSpipeline (Valenti et al., 2014)
for the LCO data. Spectroscopic reduction of the Shane, DOT, and GTC data were
done using standard tasks in IRAF which include preprocessing, extraction of
the 1D spectrum, wavelength calibration, and flux calibration. The slit-loss
corrections were accounted for by scaling the spectra to the photometry at the
same or nearby epoch. We also corrected the spectra for the heliocentric
redshift of the host galaxy. The log of spectroscopic observations is
presented in Table 11.

Near-infrared (NIR) spectra of SN 2020jfo were obtained with the SpeX (Rayner
et al., 2003) spectrograph installed on the 3.0 m NASA Infrared Telescope
Facility (IRTF) at three epochs (2020-05-13, 2020-05-24, and 2020-06-15). The
spectra were taken in both the PRISM and SXD mode with a slit size of
$0\aas@@fstack{\prime\prime}5\times 15^{\prime\prime}$. The spectra were taken
using the classic ABBA technique, and were reduced utilising the Spextool
software package (Cushing et al., 2004). The telluric absorption corrections
were done using the XTELLCOR software. The log of NIR spectroscopic
observations is given in Table 11.

##  4 Light curves of SN 2020jfo

###  4.1 Light-curve rise and explosion epoch

Figure 2: The ZTF-public $g$-band and $r$-band data of the rising phase of SN
2020jfo. The downward arrows show the upper limits. The black dashed vertical
line marks the epoch (MJD 58975.5) of the first spectrum reported on the TNS.
The blue dot-dashed vertical line marks the date of discovery (MJD 58975.203),
whereas the date of explosion ($t_{0}$) obtained through power-law fitting is
MJD 58975.18.

We used the ZTF-public data to compute the rise time of the transient. The
object was first detected before its peak in the ZTF $r$ band on MJD
58975.203999https://alerce.online/object/ZTF20aaynrrh, considered the epoch of
discovery in this paper. A third-order polynomial fit to the early part of the
ZTF $r$ and $g$ light curves shows that the transient attained its peak at MJD
$58981.698\pm 1.514$ in the observer’s frame. Figure 2 represents the early-
time ZTF photometry along with the pre-SN nondetection ($3\sigma$) limits. The
limit in nondetection (difference between last nondetection and first
detection) is roughly 3.9 days, which also indicates the early discovery of
the SN. The epoch of spectral classification (MJD 58975.5) is marked by the
black dashed vertical line, whereas the epoch of discovery is marked by the
blue dot-dashed line. We have assumed that during the early rising phase, the
transient’s flux ($f$) evolved with time ($t\geq t_{0}$) according to a power
law, $f\propto(t-t_{0})^{\beta}$ ($f=0$ for $t\leq t_{0}$). Here $t_{0}$ is
the epoch of explosion and $\beta$ is the temporal index. The typical upper
limits of the ZTF photometry during early nondetections are respectively
$20.49\pm 0.10$ mag and $20.47\pm 0.10$ mag in $g$ and $r$. We have further
assumed that for $t<t_{0}$, the fluxes at the SN location correspond to these
upper limits. The power-law fit to the rising part of the light curves gives
$t_{0}\approx 58975.18$, consistent with the date of explosion computed by
Sollerman et al. (2021) and Singh Teja et al. (2022).

A comparison of the classification spectrum taken with the Liverpool
Telescope/SPRAT on 2020-05-06 (MJD 58975.92)101010https://www.wis-
tns.org/object/2020jfo with early-time spectra of Type II events using
GELATO111111https://gelato.tng.iac.es/ shows that SN 2020jfo was roughly 3.8
days old on 2020-05-06. This implies that the epoch of explosion according to
the spectral classification is MJD 58972.12. Furthermore, we run the SED
matching code SNID (Blondin & Tonry, 2007) to constrain the explosion epoch of
SN 2020jfo. We perform the spectral fitting of the same classification
spectrum obtained with the Liverpool Telescope/SPRAT. The “rlap" parameter
quantifies the quality of the fit, with a higher value implying a better
correlation. The best match, with rlap parameter 4.9, is found with the
spectrum of SN 2013ej obtained 3.7 days after explosion. Based on this, we
estimate the age of SN 2020jfo on 2020-05-06 to be 3.7 days since explosion,
which translates to MJD 58972.20 as the date of explosion. Anderson et al.
(2014) and Gutiérrez et al. (2017) have indicated that this method is
consistent with the last-nondetection method with an uncertainty of $\pm 5$
days.

Figure 3: The fit to the Sapir & Waxman (2017) shock-cooling model using the
method described by Hosseinzadeh et al. (2018). Posterior probability
distributions and correlation between parameters of SN 2020jfo are shown in
the corner plot. The parameters corresponding to the temperature and
luminosity of the SN 1 day after the explosion ($T_{1}$, $L_{1}$), the time of
explosion ($t_{0}$), and the time at which the envelope becomes transparent
($t_{\rm tr}$) are shown. The top-right panel displays 100 fits randomly drawn
from the MCMC routine (Hosseinzadeh & Gomez, 2020) to fit the early-time
photometry. The $\sigma$ parameter depicts the intrinsic scatter of the model.
The fit deviates largely from the observed light curves, and hence the
estimated parameters from the fit are not used in the paper.

Additionally, we adopt early light-curve modeling, following the prescription
of Sapir & Waxman (2017), to estimate the explosion epoch. The early-time
emission from SNe is powered by shock cooling and provides useful insights on
the radius and pre-explosion evolution of the progenitor. The Markov Chain
Monte Carlo (MCMC) code developed by Hosseinzadeh & Gomez (2020) is used to
obtain posterior probability distributions of temperature and luminosity of
the SN 1 day after explosion ($T_{1}$, $L_{1}$), the time of explosion
($t_{0}$), and the time at which the envelope becomes transparent ($t_{\rm
tr}$) simultaneously. We model the early-time light curve of SN 2020jfo using
the multiband data up to MJD 58983.0 with $n=1.5$ for an RSG and uniform
priors for all parameters. The model outputs the blackbody temperature and
total luminosity as a function of time for each combination of the parameters,
which are then translated to measured fluxes for each photometric point and
all the light curves are simultaneously fitted. The parameter $\sigma$ is a
measure of the intrinsic scatter of the model for which a log uniform prior is
used. The posterior probability distributions and correlation between
parameters of SN 2020jfo are shown in Figure 3. The model fits the V, r, and i
bands reasonably well at least up to 58979 MJD. However, in the B, g, and UV
bands, the model does not fit well; specifically, the B and g data are much
redder and the UVW1 band data are bluer than the models allow. Also, the model
has a large intrinsic scatter ($\sigma=5.7$). Thus, although the fit converges
to an explosion epoch MJD $58973.6^{+0.4}_{-0.5}$ d, and the temperature and
luminosity 1 day after explosion respectively converge to
$T_{1}=24.8^{+1.0}_{-0.9}\times 10^{3}$ K and $L_{1}=4.7^{+0.4}_{-0.3}\times
10^{42}$ erg s-1, we cannot accept these numbers as reliable estimates owing
to the poor quality of the fits. The poor fitting of the model suggests that
there is likely some circumstellar material (CSM) interaction which is
powering the early-time light curve.

The explosion epoch from the different methods described above are 58975.0
(from the light curve rise), 58972.12 (from GELATO), and 58972.20 (from SNID).
Here we use the mean MJD 58973.1 as the explosion epoch and the standard
deviation of these estimates (1.6 day) as the uncertainty for further
analysis, consistent with the explosion epochs MJD $58974.70^{+0.5}_{-3.5}$
and $58973.8^{+0.49}_{-0.46}$ used by Sollerman et al. (2021) and Singh Teja
et al. (2022), respectively.

###  4.2 Light-curve evolution of SN 2020jfo

Figure 4: The Swift UVOT and optical light curves of SN 2020jfo since the date
of explosion. The inverted triangles represent upper limits in the UVOT light
curves. The light curves are offset by arbitrary numbers for display clarity.
The dashed line represents the best fit to the $V$ band using Valenti et al.
(2016b).

The Swift UVOT and multiband optical light curves of SN 2020jfo are shown in
Figure 4. The optical light curves of SN 2020jfo initially rise to a peak in
the BVg’r’i’ bands. We estimate the peak magnitudes and the corresponding
epoch at maximum brightness in the BVg’r’i’ bands by fitting a parabolic
function to the early-time data points ($t\leq 16$ day). The uncertainty in
the maximum epoch is obtained by adding in quadrature the fitting error and
the uncertainty in the explosion epoch. The values are listed in Table 2.
During the plateau phase, the SN declines with a rate of $2.12\pm 0.06$,
$0.70\pm 0.07$, and $0.30\pm 0.08$ mag (50 day)-1 in B, V, and r,
respectively. As the SN evolves, the light curve falls from maximum and
settles onto a plateau phase. The end of the plateau phase is marked by a
sharp drop in magnitude, followed by an exponentially decaying phase powered
by the radioactive decay of 56Co to 56Fe. The slope of the radioactive tail in
$V$ is $1.60\pm 0.07$ mag (100 d)-1, which is steeper than the 56Co to 56Fe
decay rate (0.98 mag (100 d)-1), indicating partial trapping of $\gamma$-rays
in the ejecta.

Table 2: Peak magnitudes and phase of peak time in BVg’r’i’ bands

Band | Epoch at maximuma | Peak magnitude
---|---|---
| ( days) | (mag)
B | 7.8$\pm$1.6 | 14.634$\pm$0.004
g’ | 8.1$\pm$1.6 | 14.484$\pm$0.001
V | 10.5$\pm$1.8 | 14.512$\pm$0.014
r’ | 10.2$\pm$1.6 | 14.460$\pm$0.008
i’ | 10.8$\pm$1.6 | 14.514$\pm$0.008

aPhase with respect to the explosion epoch (MJD 58973.1).

We fit the V-band light curve using the expression given by Valenti et al.
(2016b),

$y(t)=\frac{-a_{0}}{1\,+\,{\rm exp}{(t-t_{\rm pt})/w_{0}}}+p_{0}\,t+m_{0}$ .

This is used to estimate the drop in magnitude from plateau to the tail phase,
$a_{0}$, the duration of the transition phase ($\sim 6\times w_{0}$), $t_{\rm
pt}$, and a transition point between the end of the plateau (and the onset of
the radioactive tail) and the decline rate of the radioactive tail phase,
$p_{0}$. The estimated value of $t_{\rm pt}$ of SN 2020jfo is $67.0\pm 0.6$
days, which is similar to that of SN 2006Y ($66.8\pm 4$ days) but shorter than
that of the other SNe II in the comparison sample. Singh Teja et al. (2022)
estimated a plateau length of $58\pm 2$ days, which corresponds to the
parameter OPTd from Anderson et al. (2014), whereas we estimated $t_{\rm pt}$,
which is roughly equivalent to OPTd + 0.5 times the duration of the transition
phase. Considering this, our estimated $t_{pt}$ value is consistent with Singh
Teja et al. (2022) within the uncertainties. The best-fitting value of $a_{0}$
is $1.68\pm 0.09$ mag, which lies in the range of Type II SNe (1–2.6 mag) as
suggested by Valenti et al. (2016b). SN II 2006Y has also been reported to
have a similar drop in magnitude ($1.75^{+0.23}_{-0.15}$) during the
transition phase. However, the duration of the transition phase ($6\times
w_{0}$) of SN 2020jfo (13.3 days) is shorter than that of SN II 2006Y (25.7
days). This could be due to different amounts of Ni mixing (Goldberg et al.,
2019). The values of best-fit parameters are listed in Table 3 and the best
fit in the $V$ band is shown in Figure 4 by the dashed line.

Table 3: The best-fitted s50V slope of V-band lightcurve of SN 2020jfo between start and end phase. The measurement of slope is given in magnitudes per 50 days. The best fitted V-band light curve parameters using Valenti et al. (2016b) model are also given here.  Slope | Decline Rate | Start Phase | End Phase
---|---|---|---
s50V | 0.63$\pm$0.05 | 8.8 | 60.7
tpt (d) | a0 (mag) | w0 (d) | p0 (mag/d)
67.0 $\pm$ 0.6 | 1.68 $\pm$ 0.09 | 2.22 $\pm$ 0.68 | 0.0134 $\pm$ 0.0003
Table 4: Properties of the SNe in the comparison sample

Object | Host Galaxy | Distance | Total | Ejected | M${}_{V}^{peak}$ | M${}_{V}^{50d}$ | tpt | References
---|---|---|---|---|---|---|---|---
|  |  | Extinction |  56Ni Mass |  |  |  |
|  | (Mpc) |  E(B-V) (mag) | ($M_{\odot}$) | (mag) | (mag) | (days) |
SN 1995ad | NGC2139 | 22.91$\pm$1.64 | 0.035 | 0.028 | -17.64$\pm$0.29 | -16.27$\pm$0.11 | 50† | 6
SN 1999em | NGC 1637 | 11.7$\pm$0.1 | 0.10 | 0.042 | -16.86$\pm$0.07 | -16.71$\pm$0.06 | 118.1$\pm$1.0 | 1, 2, 3, 7
SN 2005cs | M 51 | 8.40 | 0.12 | <0.003 | -14.81$\pm$0.37 | -14.55$\pm$0.32 | 126.0$\pm$0.5 | 4, 5, 7
SN 2006Y | anonymous | 146.0 | 0.11 | >0.050 | -18.09$\pm$0.01 | -16.75$\pm$0.02 | 66.8$\pm$4 | 7, 8
SN 2006ai | ESO 5-G9 | 67.5 | 0.11 | >0.034 | -18.23$\pm$0.02 | -17.00$\pm$0.01 | 72.4$\pm$5 | 7, 8
SN 2007od | UGC12846 | 25.7$\pm$1.84 | 0.038 | 0.02 | -18.00$\pm$0.16 | -17.31$\pm$0.13 | 45† | 9
SN 2015ba | IC 1029 | 34.8$\pm$0.7 | 0.46$\pm$0.05 | 0.032$\pm$0.006 | -17.68$\pm$0.17 | -17.05$\pm$0.15 | 140.7$\pm$0.2 | 10
SN 2016egz | anonymous | 100.0 | 0.01 | – | -18.46$\pm$0.03 | -17.55$\pm$0.05 | 71.2‡ $\pm$ 0.5 | 8
SN 2020jfo | M61 | 14.51$\pm$1.38 | 0.17$\pm$0.09 | 0.03$\pm$0.01 | -16.90$\pm$0.34 | -16.69$\pm$0.44 | 67.0$\pm$0.6 | This work

  * •

† OPTd

  * •

‡ tpt estimated in this work

  * •

(1) Hamuy et al. (2001b), (2) Leonard et al. (2002), (3) Leonard et al.
(2003), (4) Pastorello et al. (2006), (5) Pastorello et al. (2009), (6)
Inserra et al. (2013), (7) Valenti et al. (2016b), (8) Hiramatsu et al.
(2021b), (9) Inserra et al. (2011), (10) Dastidar et al. (2018)

SN 2020jfo has good UVOT data coverage until late times, unlike most other SNe
II, which have low apparent luminosity owing to host-galaxy extinction and
large distances. The UVOT light curves decay rapidly until $\sim 23$ days
since explosion with a rate of $0.28\pm 0.01$, $0.30\pm 0.01$, and $0.21\pm
0.01$ mag day-1 in the uw2, um2, and uw1 bands respectively. We compare the UV
absolute magnitudes in the UVOT filters of SN 2020jfo with those of some other
well-observed Type II SNe in Figure 5. The sample is chosen based on the
availability of good cadence light curves in UV bands. This sample comprises
the following SNe: SN 2005cs (Pastorello et al. 2009; Brown et al. 2009), SN
2006bp (Dessart et al. 2008), SN 2012aw (Bose et al. 2013), SN 2013ab (Bose et
al. 2015a), SN 2013by (Valenti et al. 2015), SN 2013ej (Bose et al. 2015b), SN
2014G (Bose et al. 2016), and SN 2016at (Bose et al. 2019). All Type II SNe
show a rapid decline in the first 20 days of evolution in the UV filters. SN
2020jfo falls on the fainter end of the normal-luminosity SNe II and is
brighter than the low-luminosity SN 2005cs. Some of the comparison SNe with
good data coverage, such as SNe 2013by and 2013ab, show a constant-luminosity
phase in the UV bands corresponding to the plateau phase in optical bands.
However, the UV light curves of SN 2020jfo do not exhibit any plateau phase.

Figure 5: The comparison of Swift UVOT absolute magnitude light curves of SN
2020jfo with those of some well-observed Type IIP SNe observed with Swift.

To examine the behaviour of SN 2020jfo in the optical bands, we assemble a
comparison sample of SNe with varying plateau lengths and a range of explosion
properties: SNe 1995ad (short plateau; Inserra et al. 2012), 1999em (normal
Type IIP; Elmhamdi et al. 2003), 2005cs (low-luminosity Type IIP; Pastorello
et al. 2006, Pastorello et al. 2009), 2006Y (short plateau; Hiramatsu et al.
2021b), 2006ai (short plateau; Hiramatsu et al. 2021b), 2007od (short plateau;
Inserra et al. 2011), 2015ba (long plateau Type IIP; Dastidar et al. 2018),
and 2016egz (short plateau; Hiramatsu et al. 2021b). The parameters of the
comparison sample are listed in Table 4.

Figure 6: $(B-V)_{0}$ colour evolution of SN 2020jfo compared with the SN II
sample used in this paper.

The colour evolution provides information about the dynamics of the SN ejecta.
The temporal evolution of the reddening-corrected $(B-V)_{0}$ colour of SN
2020jfo is depicted in Figure 6 along with that of the comparison sample of
SNe and the sample of Type II SNe from de Jaeger et al. (2018) (shown in grey)
and a subset of this sample with negligible host extinction (shown with grey
stars). All SNe exhibit roughly the same trend in $(B-V)_{0}$ colour — a
gradual transition to redder colours in the first 50 days, followed by a
flattening until the end of the plateau phase. Owing to expansion of the
ejecta, the $(B-V)_{0}$ colour of SN 2020jfo becomes redder by 1.2 mag until
the end of the plateau phase. Thereafter, SN 2020jfo starts transitioning from
the plateau to the tail phase at $\sim 70$ days, which is marked by a steeper
rise to redder colours. As the SN settles in the exponentially decaying tail
phase, the $(B-V)_{0}$ colour gradually becomes bluer by 0.7 mag. SN 2020jfo
and the other SNe II with shorter plateau lengths (SNe 1995ad, 2006Y, 2006ai,
2007od, and 2016egz) do not show the constant-colour phase corresponding to
the plateau phase as seen in other SNe II such as SNe 1999em, 2005cs, and
2015ba; almost immediately after 50 days, the light curve transitions from the
plateau to the tail phase in Type II SNe with shorter plateau. Like SN
2020jfo, a sudden transition to redder colours at the end of the plateau phase
is also evident in SN 2005cs; however, the latter does not display a
transition to bluer colours in the nebular phase. SN 2006ai also exhibits this
transition to bluer colours in the nebular phase.

Figure 7: Absolute $V$-band light curve of SN 2020jfo compared with the Type
II SNe in the comparison sample. The grey lines represent the light curves of
116 SNe II having different plateau lengths from Anderson et al. (2014). The
inset shows the comparison of SN 2020jfo with other Type II SNe having similar
plateau lengths in the comparison sample used in the paper and the sample of
Anderson et al. (2014).

The absolute magnitude $V$-band light curve of SN 2020jfo is shown along with
the comparison sample in Figure 7. In terms of brightness, SN 2020jfo has
similar absolute magnitudes in the early and plateau phase as the prototypical
Type IIP SN 1999em, with a remarkably early drop from the plateau as compared
to SN 1999em. The plateau length of SN 2020jfo is rather similar to that of
the Type II SNe 1995ad, 2006Y, and 2006ai with plateau duration in the range
50–70 days. The peak of SN 2020jfo is $-16.90\pm 0.34$ mag, consistent within
uncertainties with that obtained by Singh Teja et al. (2022). The Type II SNe
having shorter plateau duration (SNe 1995ad, 2006Y, 2006ai, 2007od, and
2016egz) are brighter than SN 2020jfo by at least 0.7 mag at peak and have a
steeper post-peak decline rate (up to 20 days) than SN 2020jfo (2.5 mag (50
day)-1, SN 1995ad; 3.0 mag (50 day)-1, SN 2006Y; 1.7 mag (50 day)-1, SN
2006ai; 1.2 mag (50 day)-1, SN 2007od; 2.2 mag (50 day)-1, SN 2016egz). SN
2020jfo is brighter than the low-luminosity Type II SN 2005cs by $\sim 2$ mag
in the plateau phase and fainter than the long-plateau SN 2015ba by 0.5 mag.
The grey lines represent the light curves of 116 Type II SNe having different
plateau lengths from Anderson et al. (2014). The sample of Anderson et al.
(2014) also consists of a few SNe II with relatively shorter plateaus (SNe
1992af, 2004dy, 2008bp, and 2008bu), which we have shown separately in the
inset plot along with SNe 2020jfo, 1995ad, 2006Y, 2006ai, and 2016egz. Except
for SNe 1992af, the other SNe II (SNe 1995ad, 2004dy, 2008bp, and 2008bu) have
absolute magnitudes during the plateau phase similar to or lower than that of
SN 2020jfo. As noted by Hiramatsu et al. (2021b), SN 1992af has an uncertain
explosion epoch and hence might have a longer plateau duration. The tail-phase
luminosity of SN 2020jfo is similar to that of SN 1999em and lower than that
of the rest of the SNe in the comparison sample except for SN 2005cs. SN
2016egz has the highest tail luminosity, whereas SNe 2006Y and 2006ai have
tail luminosities similar to that of SN 1999em. The inset plot shows that the
Type II SNe with similar plateau magnitudes also have comparaable tail
luminosities.

The 56Ni mass is estimated from the tail bolometric luminosity following Hamuy
(2003) using the tail $V$-band magnitudes. The tail luminosity ($L_{t}$)
obtained from the $V$ magnitudes using the bolometric correction factor given
by Hamuy (2003) at two epochs (72.2 and 80.2 days) are $2.19\pm 0.71\times
10^{41}$ erg s-1 and $2.04\pm 0.66\times 10^{41}$ erg s-1, respectively, which
corresponds to a mean 56Ni mass of $0.03\pm 0.01$ M⊙. This is consistent with
the 56Ni mass obtained for SN 2020jfo by Sollerman et al. (2021) and Singh
Teja et al. (2022).

##  5 Spectral Analysis

###  5.1 Spectroscopic evolution

Figure 8: The spectral evolution of SN 2020jfo from 2.8 days to 365.2 days
since explosion with the prominent spectral features marked. Each spectrum is
normalised with the median flux.

The spectral evolution of SN 2020jfo from 2.8 to 365.2 days since explosion is
shown in Figure 8. At the early phase of the evolution (2.8–4.5 days), broad P
Cygni profiles of H Balmer lines can be seen superimposed on a blue continuum.
The spectra from 8.3 days reflect the transition from a hot to a cool SN
envelope, as the photosphere begins to penetrate deeper into the metal-rich
ejecta. These spectra mark the emergence of other lines from heavier atomic
species such as Ca, Fe, Sc, Ba, Ti, and Na. Among these, the Fe II
$\lambda\lambda$4950, 5169 lines appear from 18.9 days. Sc II lines, O I, and
the Ca II NIR triplet gain prominence from day 40.2 and get stronger over
time. By the end of the plateau phase, all weak and blended lines have evolved
and become conspicuous. A high-velocity absorption dip blueward of the
H$\alpha$ absorption minimum becomes apparent in the day 63–85 spectra.

For robust line identifications, we used SYNAPPS (Thomas, 2013) to reproduce
the early-time (3.5 d) and photospheric-phase (32.4 d) spectra of SN 2020jfo
(Figure 9). For modelling the early spectrum, we used H I and Ca II species.
The photospheric velocity and blackbody temperature at 3.5 d as constrained
from the modelling are 13,360 km s-1 and 11,650 K, respectively. The
photospheric spectrum is reproduced using H I, Ca II, Fe II, Na I, Ba II, Sc
II, and Ti II lines. The Ti II, Sc II, and Ba II species mainly contribute in
reproducing the 5200–6000 Å region. The photospheric velocity and blackbody
temperature at 32.4 d as constrained from the modelling are 5476 km s-1 and
8380 K, respectively. In both cases, the “detach" parameter has been
deactivated, which means that the minimum ejecta velocity of all the ions will
be the same as the photospheric velocity.

The day 63.3 spectrum marks the onset of the transition from the plateau to
the nebular phase. All subsequent spectra up to day 85.1 are representative of
the early nebular phase, when the outer ejecta have become optically thin. In
the late nebular phase spectra (253.9–365.2 day), in addition to the H$\alpha$
emission, a number of forbidden emission lines such as [O I]
$\lambda\lambda$6300, 6364 and [Ca II] can be seen. Also, the nebular spectra
of SN 2020jfo show strong [Ni II] and [Fe II] forbidden emission which can be
used as a diagnostic of the Ni-to-Fe mass ratio in the entire ejecta
(Jerkstrand et al., 2015).

Figure 9: SYNAPPS modelling of the early (3.5 day) and plateau phase (32.4
day) spectra of SN 2020jfo (shown in black) marked with the lines identified
from the spectral modelling.

The three NIR spectra of SN 2020jfo obtained on days 8.3, 19.3, and 41.2 are
shown in Figure 10. The day 8.3 spectrum exhibits weak lines of Pa$\alpha$,
Pa$\beta$, Pa$\gamma$, and He I, and a relatively strong Ca II NIR triplet.
The feature around $\lambda$10,800 in the day 8.3, 19.3, and 41.2 spectra is
probably a He I and Pa$\gamma$ blend, although there is a possibility of C I
formation at 10,691 Å. We used SYNAPPS to model the wavelength range
8000–13,500 Å in the day 19.3 spectrum using H I, He I, C I, and Ca II ions,
and can reproduce this region well as shown in Figure 10. The presence of the
C I line has been indicated in a few SNe II, such as SNe1999em (Hamuy et al.,
2001a), 2005cs (Pastorello et al., 2009), and 2012aw (Davis et al., 2019). We
compare the NIR spectra of SN 2020jfo with those of other SNe II like SNe
1999em (Hamuy et al., 2001a), 2005cs (Pastorello et al., 2009), 2012aw
(Dall’Ora et al., 2014), and 2012ec (Barbarino et al., 2015) at nearby epochs
in Figure 10. Overall, the NIR spectra of SN 2020jfo show similarity with
other SNe II. The day 8.3 spectrum of SN 2020jfo is similar to the day 5.7
spectrum of SN 1999em and the day 7.0 spectrum of SN 2012ec, all showing the
classical feature at 10,830 Å, while the day 8.0 spectrum of SN 2012aw does
not exhibit any conspicuous line at this wavelength. The 10,549 Å absorption
feature in the day 21.7 spectrum of SN 1999em was suggested to be possibly a
blend of C I $\lambda$10,691 and He I $\lambda$10,830 (Hamuy et al., 2001a).
However, in the day 62.0 spectrum of SN 2005cs, this feature was suggested to
be mostly due to P$\gamma$, Sr II $\lambda$10,915, and C I $\lambda$10,691,
with less dominance of He I $\lambda$ 10,830 than in other CCSNe (Pastorello
et al., 2009). In SN 2020jfo, this feature is mostly dominated by Pa$\gamma$,
He I, and C I. There is possibly no formation of Sr II at least until day 19.3
in SN 2020jfo; introducing the ion deteriorated the SYNAPPS fit.

Figure 10: The NIR spectral evolution of SN 2020jfo compared with that of
other Type II SNe. The SYNAPPS model of the day 19.3 spectrum in the
wavelength range 8000–13,500 Å using H I, He I, C I, and Ca II species is
shown in red.

###  5.2 Comparison with other SNe

We compare the spectra of SN 2020jfo at early times ($\sim 8.3$ day), the
plateau ($\sim 40.2$ day), and the nebular phase ($\sim 365.2$ day) with the
spectra of the comparison sample at similar epochs. The spectroscopic
comparison sample is composed of the same SNe II that are defined in Section
4.2 and listed in Table 4.

At the early phase ($\sim 8$ day), Figure 11 indicates that the spectrum of SN
2020jfo displays broad H Balmer emission on a blue continuum similar to SNe
1995ad, 1999em, and 2007od. The comparable-epoch spectra of Type II SNe 2006Y
(12.7 day), 2016egz (6.8 day), and SN 2015ba (7.8 day) are mostly blue and
featureless corresponding to the shock-cooling phase. In contrast, the day 8
spectrum of SN 2005cs shows well-developed P Cygni profiles of H Balmer lines,
Ca II H&K, and feebly visible Fe II, suggesting cooler photospheric
temperatures for SN 2005cs.

Figure 11: The spectrum of SN 2020jfo at $\sim 8$ day is compared with early-
time spectra of other SNe II in the comparison sample.

In Figure 12, we compare the plateau-phase spectrum ($\sim 40.2$ day) of SN
2020jfo with those of the comparison sample. The plateau-phase spectrum of SN
2020jfo is likewise similar to that of SNe 1995ad, 1999em, 2007od, and 2015ba.
Unlike the other Type II SNe 2006Y and 2016egz, the spectrum of SN 2020jfo
exhibits a substantial absorption component of the P Cygni profile of
H$\alpha$, which is shown by zooming in on the wavelength region 6000–6800 Å.
The plateau spectrum of SN 2020jfo also shows well-developed Fe II and feebly
visible Sc II and Ba II lines. The Na I D absorption is prominent in SN
2020jfo, similar to some Type II SNe with short plateau duration such as SNe
1995ad and 2006ai, but unlike the other short-plateau Type II SNe 2006Y,
2007od, and 2016egz. The Ca II NIR triplet is also prominent in SN 2020jfo,
similar to SNe 1995ad, 1999em, 2005cs, and 2015ba, and can be seen developing
in the spectra of SNe 2006Y, 2006ai, and 2016egz.

Figure 12: The plateau-phase spectrum of SN 2020jfo at $\sim 40$ day is
compared with those of other SNe in the comparison sample. The wavelength
region 6000–6800 Å is zoomed in and plotted in velocity space with respect to
H$\alpha$, which highlights the similarity of the absorption component of
H$\alpha$ in SN 2020jfo with the prototypical Type II SN 1999em.

In Figure 13, we compare the nebular ($\sim 365.2$ day) spectrum of SN 2020jfo
with that of other SNe in the comparison sample at similar phases. Only one
short-plateau SN 2016egz has a nebular spectrum after 300 days, which is
mostly featureless. This is most likely due to host-galaxy contamination given
its proximity to the host, as is also suggested by the the prominent narrow [O
III] $\lambda\lambda$4959, 5007 emission lines. The H$\alpha$ feature is
pronounced in emission in all of the comparison spectra. The [O I]
$\lambda\lambda$6300, 6364 doublet and [Ca II] $\lambda\lambda$7292, 7324 are
very prominent in SN 2020jfo, similar to the other Type II SNe 1999em, 2005cs,
and 2015ba. The nebular spectrum of SN 2020jfo exhibits strong [Ni II]
$\lambda$7378 emission which is indiscernible in the comparison sample.
Overall, the features in the SN 2020jfo nebular spectrum are more similar to
the spectra of SNe 1999em, 2005cs, and 2015ba, except the inconspicuous Ca II
NIR triplet in the spectrum of SN 2020jfo (unlike these other SNe).

Figure 13: The nebular-phase spectrum of SN 2020jfo at $\sim 365$ day is
compared with other SNe in the comparison sample. The nebular features in SN
2020jfo are similar to those seen in SN 1999em.

Thus, despite a short plateau duration, the spectral features of SN 2020jfo
are more similar to those of normal Type II SNe. The length of the plateau
phase is determined by the mass of the H envelope as well as the time it takes
for the H to recombine. The progenitor of SN 2020jfo is likely to have an H
envelope mass comparable to that of a typical SN II, but with a shorter
recombination time. However, rapidly expanding ejecta could explain the
shorter recombination time; except for the first two epochs, the ejecta
velocity of SN 2020jfo follows the mean velocity curve of normal Type II SNe
(see Figure 14). The plateau duration, however, is also affected by the extent
of 56Ni mixing in the ejecta, with a higher degree of mixing resulting in
shorter plateaus (Pumo & Zampieri, 2013; Goldberg et al., 2019).

###  5.3 Velocity evolution

The expansion velocity of the ejecta can be estimated from the minima of the
absorption component of the P Cygni profile of various lines. We estimate the
line velocities of a few prominent features in SN 2020jfo and show their
evolution with time in Figure 14 (top panel). The H$\alpha$ and H$\beta$
velocities are comparable at early phases (10,200 km s-1 and 9900 km s-1 at
day 8) and greater than the velocity of other metal lines. As time proceeds,
the H$\alpha$ velocity is higher than all other line velocities, showing it to
be part of the outer expanding layer of ejecta. The velocity of the Fe II line
gives a fair estimate of the expansion velocity of the photosphere, and it is
estimated to be around 8400 km s-1 at $\sim 15$ days; it decays down to 1600
km s-1 at the onset of the nebular phase. The decreasing trend of velocity
shows that as time proceeds we are looking at the inner, lower-velocity layers
of the ejecta owing to a decrease in the opacity resulting from the expansion
of the ejecta.

The line velocities of SN 2020jfo are compared with those of other SNe in the
sample. Figure 14 (bottom three panels) show the evolution of H$\alpha$,
H$\beta$, and Fe II line velocities. The mean velocity evolution and the
standard deviation for a sample of 122 Type II SNe from Gutiérrez et al.
(2017) is shown with a blue solid line and grey shaded region, respectively.
Overall, the velocities of SN 2020jfo are lower than the velocities of the SNe
in the comparison sample except SN 2005cs, and lie toward the edge of the grey
band. The velocities of other SNe II having shorter plateaus are typically
higher than that of SN 2020jfo. The Fe II velocity of SN 2020jfo at day 15
after explosion is $\sim 1900$ km s-1 higher than the mean velocity.

Figure 14: The evolution of line velocities of a few prominent lines in SN
2020jfo is shown in the top panel. The bottom three panels display a
comparison of the velocity evolution of H$\alpha$, H$\beta$, and Fe II lines
of SN 2020jfo and the comparison sample. The mean velocities and the standard
deviations of the velocities in the SN II sample from Gutiérrez et al. (2017)
are shown with a blue solid line and a grey band, respectively.

##  6 Progenitor and explosion properties

In this section, we adopt different techniques to estimate the progenitor
properties, and we provide a cohesive picture of this analysis in Section 8.

###  6.1 Estimations from the nebular spectrum

The observed strengths of nebular lines, especially the [O I]
$\lambda\lambda$6300, 6364 doublet, can serve as a diagnostic of the
progenitor mass, as the nucleosynthetic yield depends strongly on the mass of
the progenitor (Woosley & Weaver, 1995). Jerkstrand et al. (2014) generated
models of nebular spectra for 12, 15, 19, and 25 M⊙ progenitors for 0.062 M⊙
of 56Ni, assuming a distance of 5.5 Mpc. We compare the day 365.2 spectrum of
SN 2020jfo to these models to get a handle on the mass of the progenitor as
shown in Figure 15. As the SN is powered by the radioactive decay energy in
the nebular phase, we scaled the day 400 model spectrum flux with the ratio of
the 56Ni mass of SN 2020jfo (0.030 M⊙) to the 56Ni mass of the model spectrum
(0.062 M⊙). Then the day 400 model flux was rescaled to the distance and phase
(365.2 day) of the nebular spectrum of SN 2020jfo. We also used a scaling
factor of 0.5, similar to Sollerman et al. (2021), owing to the partial
trapping of $\gamma$-rays in SN 2020jfo. Before the comparison, we
deredshifted the SN spectrum, scaled the spectrum to match the photometric
flux so that the slit losses are taken care of, and applied reddening
corrections. The SN spectrum best matches with the 12 M⊙ model. However, the
H$\alpha$ flux in the spectrum of SN 2020jfo is a factor of 3 lower than the
model flux. The model also shows a prominent Ca II NIR triplet feature which
is absent in the SN 2020jfo spectrum. The [O I] flux in the SN 2020jfo
spectrum is comparable to the model spectrum, suggesting a progenitor ZAMS
mass of $\sim 12$ M⊙ for SN 2020jfo. We note here that Sollerman et al. (2021)
and Singh Teja et al. (2022) also carried out this comparison with the nebular
spectra of SN 2020jfo and our result is consistent with theirs.

Figure 15: Comparison of the day 365.2 spectrum of SN 2020jfo with that of the
scaled model spectra from (Jerkstrand et al., 2014).

The relative abundances of the different elements have been suggested to vary
substantially with the helium core mass (Fransson & Chevalier, 1989);
specifically, the [O I]/[Ca II] ratio has been used to estimate the
progenitor’s ZAMS mass by several authors (e.g., Fransson & Chevalier 1989;
Elmhamdi et al. 2004; Kuncarayakti et al. 2015). We estimated the [O I] and
[Ca II] luminosity in the nebular spectrum (365.2 day) of SN 2020jfo to be
$(1.13\pm 0.19)\times 10^{38}$ erg s-1 and $(1.96\pm 0.06)\times 10^{38}$ erg
s-1, respectively, and the [O I]/[Ca II] ratio is $0.58\pm 0.10$. This ratio
has been found to fall below 0.7 for Type II SNe, while for other CCSNe, the
ratio is higher than 0.7 (Kuncarayakti et al., 2015). Thus, for SN 2020jfo,
the [O I]/[Ca II] ratio coincides with the upper limit for SNe II. The fact
that the ratio increases with increasing main-sequence mass suggests that the
progenitor of SN 2020jfo falls on the upper end of the mass distribution for
Type IIP SN progenitors.

###  6.2 Ni/Fe production ratio

Ni and Fe are essentially produced in the same region of the ejecta in Fe
CCSNe, and the [Fe II] emission is stronger compared with [Ni II] in these
events (Jerkstrand et al., 2012). Jerkstrand et al. (2015) presented an
analytic method to determine the Ni/Fe ratio in nebular spectra of SNe in the
7100–7400 Å region, which we have applied here to SN 2020jfo. There are eight
prominent emission lines in the 7100–7500 Å region: [Ca II]
$\lambda\lambda$7291, 7323, [Fe II] $\lambda$7155, [Fe II] $\lambda$7172, [Fe
II] $\lambda$7388, [Fe II] $\lambda$7453, [Ni II] $\lambda$7378, and [Ni II]
$\lambda$7412\. We fit this spectral region in the day 253.9, 255.9, and 365.2
spectra of SN 2020jfo using Gaussian components for these lines. Following the
Jerkstrand model, we fixed the line ratios from the same species. Thus, the
iron lines are constrained by $L_{7453}=0.31\,L_{7155}$,
$L_{7172}=0.24\,L_{7155}$, and $L_{7388}=0.19\,L_{7155}$, and the nickel lines
are constrained by $L_{7412}=0.31\,L_{7378}$. Furthermore, in this method, the
full width at half-maximum (FWHM) velocity $v$ of all the emission lines are
forced to be the same for a given epoch, so that there are four free
parameters: $L_{7291,7323}$, $L_{7155}$, $L_{7378}$, and $v$.

In Figure 16, we show that a good fit in the day 365.2 spectrum is obtained
for $L_{7291,7323}=(1.96\pm 0.06)\times 10^{38}$ erg s-1, $L_{7155}=(0.57\pm
0.02)\times 10^{38}$ erg s-1, $L_{7378}=(0.84\pm 0.03)\times 10^{38}$ erg s-1,
and $v=2158$ km s-1. From this, we determine a ratio
$L_{7378}/L_{7155}=1.61\pm 0.11$ on day 365.2. We followed the same approach
and estimated the luminosity ratio of [Ni II] to [Fe II] to be $1.65\pm 0.18$
and $1.98\pm 0.21$ in the day 253.9 and 255.9 spectra of SN 2020jfo,
respectively. The luminosity ratio measured by Sollerman et al. (2021) with a
day 306 spectrum is 1.7 and by Singh Teja et al. (2022) with a day 292
spectrum is $2.10\pm 0.43$. Using $T=2700$ K, we obtain a Ni-to-Fe mass ratio
in the range 0.08–0.10, which is $\sim 1.5$ times the solar value. We see that
[Ni II] $\lambda$7378 emission is stronger than [Fe II] $\lambda$7155 in SN
2020jfo, which is usually seen in low-luminosity SNe II (Müller-Bravo et al.,
2020). While we derived a mean ratio using three nebular spectra, both
Sollerman et al. (2021) and Singh Teja et al. (2022) used a single nebular
spectrum and obtained a higher value of this mass ratio. The ratio estimated
using the day 306 spectrum by Sollerman et al. (2021) is 2 times the solar
value and that by Singh Teja et al. (2022) using the day 292 spectrum is 3
times the solar value; however, in the latter work, the temperature used to
estimate this quantity is not mentioned.

Figure 16: Section of the 365.2 day spectrum showing the Gaussian fits to
determine [Ni II] $\lambda$7378/[Fe II] $\lambda$7155 in SN 2020jfo.

###  6.3 Bolometric light curve modelling

We constructed the bolometric light curve of SN 2020jfo using superbol
(Nicholl, 2018) as shown in Figure 17. The dereddened magnitudes supplied to
the routine are interpolated to a common set of epochs and converted to
fluxes. The fluxes are used to construct the spectral energy distribution
(SED) at all the epochs. The routine then fits a blackbody function to the
SED, extrapolating to the UV and IR regimes to estimate the true bolometric
luminosity. For SN 2020jfo, we used the Swift $uvw2$, $uvm2$, and $uvw1$
bands, optical $BgVri$ bands, and IR $J$-band data from Sollerman et al.
(2021) to construct the first 50 days bolometric light curve using superbol.
For the rest of the light curve, we used only $BgVri$ data.

We employed the semi-analytical modelling of Nagy & Vinkó (2016) to which an
MCMC method was added by Jäger et al. (2020) for numerically fitting the
output model to the observed light curves. Nagy & Vinkó (2016) used a
core+shell combination to fit the early declining part with the shell
configuration. The plateau phase, drop, and tail are modelled with the core
configuration. However, the MCMC method added by Jäger et al. (2020) only
takes into account the core part, and hence the early declining part (the
first 10 days in the light curve) is not fitted using this method. For the
modelling, we have sampled five parameters: radius ($R_{0}$), ejecta mass
($M_{\mathrm{ej}}$), kinetic energy ($E_{\mathrm{kin}}$), thermal energy
($E_{\mathrm{th}}$), and opacity. A mean and sigma prior expansion velocity of
8140 km s-1 and 500 km s-1 (respectively) were used, along with a
recombination temperature of 5500 K. From the model, we estimated an initial
radius ($R_{0}$) of $\sim 1000$ R⊙, ejecta mass ($M_{\mathrm{ej}}$) of 13.6
M⊙, and total energy ($E_{\mathrm{kin}}+E_{\mathrm{th}}$) of 3.0 foe. The
bolometric magnitude evolution and the 50 best-fit light curves are shown in
Figure 17, and the corresponding parameter estimates and their 1$\sigma$
uncertainties are provided in Table 5.

There are two known parameter correlations (Arnett & Fu, 1989; Nagy et al.,
2014) between $M_{\mathrm{ej}}$ and $E_{\mathrm{kin}}$, and between $R_{0}$
and $E_{\mathrm{th}}$. Since the two parameter pairs
$M_{\mathrm{ej}}-E_{\mathrm{kin}}$ and $E_{\mathrm{th}}-R_{0}$ are
significantly correlated, separate determination of the quantities in the pair
is not possible. Rather, the product of the pairs should be used as an
independent parameter. The 56Ni mass estimated from the modelling using the
tail light curve is 0.022 M⊙. Assuming a remnant neutron star mass of 1.5–2
M⊙, the lower limit of the progenitor mass would be $\sim 15$ M⊙, which lies
within the expected mass range of RSGs exploding as Type IIP SNe. We note here
that we could not reproduce the early luminosity excess by solely using the
core component in the semi-analytical model. Thus, we cannot rule out the
possibility of CSM interaction from pre-explosion mass loss.

In Inserra et al. (2013), hydrodynamic modelling of the bolometric light curve
of SN 1995ad was performed, which estimated 5 M⊙ of ejecta mass and a
progenitor radius of $4\times 10^{13}$ cm. They proposed both super asymptotic
giant branch (SAGB) and Fe CC progenitors, and noted that the former is less
likely owing to the relatively high amount of 56Ni ejected. The range of
ejecta mass (5–7.5 M⊙) and progenitor radius ((4–7) $\times 10^{13}$ cm)
estimated for SN 2007od are also similar to SN 1995ad. In the work of
Hiramatsu et al. (2021b), three short-plateau SNe 2006ai, 2006Y, and 2016egz
were studied and the ZAMS mass ($M_{\rm ZAMS}$) of the progenitor was
suggested to be in the range $\sim 18$–22 M⊙, much higher than that of SNe
1995ad and 2007od. The study indicated that the progenitors of these SNe II
had experienced massive stripping prior to explosion (Hiramatsu et al.,
2021b), leading to a very low H-envelope mass ($\sim 1.7$ M⊙), much lower than
that of SNe 1995ad and 2005od. The ejecta mass estimated from semi-analytical
modelling, which mainly constitutes the H-envelope mass, for SN 2020jfo is
much higher than that of the SNe II studied by Inserra et al. (2013), Inserra
et al. (2011), and Hiramatsu et al. (2021b). Although the early luminosity
excess in SN 2020jfo and the high-velocity feature in H$\alpha$ indicate pre-
explosion mass loss, the envelope stripping in the progenitor of SN 2020jfo is
expectedly much less violent than in the Type II SNe 2006Y, 2006ai, and
2016egz.

Figure 17: The bolometric magnitude evolution of SN 2020jfo is shown with the 50 best-fit light curves overplotted, following Nagy & Vinkó (2016) and Jäger et al. (2020). Table 5: The best-fit core parameters and 1$\sigma$ confidence intervals for the bolometric light curve of SN 2020jfo using Nagy & Vinkó (2016).  $R_{0}$ ($10^{13}$ cm) | $6.89^{+0.45}_{-2.78}$ | Initial radius of ejecta
---|---|---
$M_{\mathrm{ej}}$ (M⊙) | $13.6^{+0.2}_{-2.5}$ | Ejecta mass
$E_{\mathrm{kin}}$ (foe) | $2.9^{+0.4}_{-0.5}$ | Initial kinetic energy
$E_{\mathrm{th}}$ (foe) | $0.066^{+0.038}_{-0.004}$ | Initial thermal energy

###  6.4 Metallicity

Metallicity is one of the key parameters that constrains the nature of the
stellar evolution and the mechanism of the SN explosion, particularly in the
case of high-mass stars (e.g., Heger et al. 2003). Pilyugin et al. (2004)
derived an empirical relation between absolute $B$-magnitude ($M_{B}$) and
metallicity (12 + log($N_{O}/N_{H}$)) for the nearby spiral galaxies (along
with M61, the host of SN 2020jfo) and computed the radial profile of their
metallicity distribution. The nuclear metallicity of M61 is 8.84 dex, and the
angular separation between the SN and the host center is $\sim 1.2^{\prime}$.
This implies that the metallicity at the SN location is roughly 8.57 dex (8.65
dex is the solar-neighbourhood metallicity; Asplund et al. 2009). Thus, the
estimated metallicity of the region of SN 2020jfo using this method appears to
be nearly solar, although it is higher than that of SN 2008in (8.44 dex; Roy
et al. 2011b) which was located in the same host galaxy.

Metallicity also plays a crucial role in the evolution of metal lines in the
photospheric phase as the SNe with low progenitor metallicity exhibit weaker
metal features (Dessart et al., 2014). Dessart et al. (2013) simulated model
spectra by evolving a 15 M⊙ ZAMS star up to the pre-SN stage at various
metallicities (0.1, 0.4, 1.0, 2.0 Z⊙). We compared the day 80.2 spectrum of SN
2020jfo with these models and found a reasonable match with both 1 and 2 Z⊙
model spectra considering the Fe II $\lambda$5018 and $\lambda$5169 features,
that are overplotted on the spectrum of SN 2020jfo in the top panel of Figure
18. However, the higher metallicity model matches the H$\alpha$ absorption
better. Furthermore, Anderson et al. (2016) estimated the oxygen abundances
using a sample of 119 host H II regions and found a positive correlation
between the host H II region oxygen abundance and the pseudo-equivalent width
(pEW) of Fe II $\lambda$5018\. In the bottom panel of Figure 18, the time
evolution of the mean pEW of Fe II $\lambda$5018 of the sample of Type II SNe
from Anderson et al. (2016) is shown along with its standard deviation (the
black line and the shaded region), over which the thick solid lines are the
time evolution of the pEW of Fe II $\lambda$5018 in the different metallicity
models of Dessart et al. (2013). The pEW measurements of Fe II $\lambda$5018
in the spectra of SN 2020jfo is shown with a dashed line which falls slightly
above the evolution of the 2 Z⊙ model and is higher than the mean pEW of the
sample. This estimate is higher than that obtained from radially decreasing
metallicity gradients in spiral galaxies. Generally, higher metallicity would
imply a higher extent of envelope stripping of the progenitor prior to
explosion Heger et al. 2003. Despite the limitations of both methods of
metallicity estimation, the higher metallicity scenario is more consistent
with SNe II having shorter plateaus, where the progenitors are expected to
have undergone comparatively extensive envelope stripping than Type II SNe
with long plateaus (Hiramatsu et al., 2021b). In the context of SN 2020jfo,
however, our analysis does not indicate significant envelope stripping. Thus,
we prefer the low-luminosity scenario for SN 2020jfo.

Figure 18: Top panel: Comparison of the day 80.2 spectrum of SN 2020jfo with
the best-matched model spectrum of Dessart et al. (2013). Bottom panel: The
evolution of the pEW of Fe II $\lambda$5018 of SN 2020jfo is shown with dashed
lines. The black line is the mean pEW of a sample of 119 Type II SNe from
(Anderson et al., 2016) and the shaded region shows the standard deviation
about the mean. The time evolution of the pEWs of Fe II $\lambda$5018 of the
four distinct metallicity models of (Dessart et al., 2013) is shown with thick
lines.

##  7 Collocation of SN 2020jfo in the SN II parameter space

We explored the collocation of SN 2020jfo in the parameter space of other SNe
II for which we have used the sample of Type II SNe from Anderson et al.
(2014), Faran et al. (2014a), and Valenti et al. (2016b).

Figure 19: The correlation between $s50V$ and $t_{\rm pt}$ is shown. The
objects are colour-coded with respect to the absolute $V$-band magnitude at
day 50.

In Figure 19, the plateau decline rate in the $V$ band and $t_{\rm pt}$ of SN
2020jfo are shown colour-coded with the $V$ absolute magnitude at day 50,
along with that of other SNe. There is a correlation among these parameters as
noted by Valenti et al. (2016b). The correlation implies that SNe with longer
plateaus tend to decline more slowly. The SNe with $t_{\rm pt}$ less than 80
days are SNe 1979C, 2006Y, and 2006ai. Although the plateau lengths of SNe
2006ai and 2006Y are consistent with SN 2020jfo within the uncertainties, SN
2006Y is brighter than SNe 2020jfo and 2006ai. We performed a linear fit and
have shown the 3$\sigma$ confidence interval with the grey shaded region. SN
2006ai falls within the 3$\sigma$ confidence interval whereas SN 2020jfo falls
outside it. Among the SNe with a short plateau duration, SN 2020jfo has the
shallowest decline rate. In general, the SNe with shorter plateaus are
brighter, with a higher decline rate than those with longer plateau. SNe with
absolute plateau magnitudes in the range $-18$ to $-19$ mag are spread over
all decline rates, while for the rest of the sample, SNe with lower luminosity
preferably have shallower decline rates. SN 1979C is considered to be a
prototypical Type IIL SN that does not show a clear light-curve drop. As
suggested by Anderson et al. (2014), SN 1979C may have a light-curve drop
around day 50 after explosion, though if there, the drop would have occurred
quite early and less pronounced than in all the other cases.

We plot the absolute magnitude in $V$ at day 50 against the logarithm of the
56Ni mass for a sample of Type II SNe. We find that the SNe II with higher
luminosity (and hence higher explosion energy) have a larger 56Ni yield. SN
2020jfo follows the correlation between these two parameters. Also, we plot
peak magnitude in $V$ against $a_{0}$ of SN 2020jfo along with those of other
Type II SNe. In general, SNe with higher peak magnitudes have shorter plateau
lengths. Hence, the fall in magnitude from the plateau to the radioactive tail
phase is expected to be lower. SN 2020jfo follows the trend, but the plateau
length of SN 2020jfo is shorter and slowly declining compared with other
short-plateau SNe.

The correlations suggest that although SN 2020jfo has a short plateau length,
its parameters are more similar to those of normal SNe IIP.

##  8 Conclusions

In this paper, we present a photometric and spectroscopic analysis of the Type
II SN 2020jfo. Using the light-curve equation from Valenti et al. (2016b), we
estimated a short-plateau length of $67.0\pm 0.6$ days for SN 2020jfo, which
is consistent with the result given by Sollerman et al. (2021) and Singh Teja
et al. (2022).

However, unlike other SNe II with shorter plateau duration, SN 2020jfo is
fainter, with a peak absolute $V$-band magnitude of $-16.90\pm 0.34$ mag, in
comparison with the luminous optical peaks of those SNe ($\lesssim-18.4$ mag).
While the plateau decline rates in $V$ of SN 2020jfo and other SNe II having
shorter plateau duration are similar, the short-plateau Type II SNe 1995ad,
2006Y, 2006ai, 2007od, and 2016egz display a steeply declining phase in $V$ up
to $\sim 20$ days, which is not observed in SN 2020jfo. The 56Ni mass yield of
SN 2020jfo estimated from the tail-phase luminosity is $0.03\pm 0.01$
$M_{\odot}$, which falls in the range of normal SNe II.

Despite the shorter plateau duration in SN 2020jfo, the H I absorption
features in its plateau-phase spectra are remarkably strong, indicating a
relatively high H-envelope mass. SN 2020jfo also shows strong metal lines in
comparison with other SNe II at similar epochs and having comparable plateau
lengths. Moreover, a discernible line of stable [Ni II] $\lambda$7378, which
has only been seen in a few H-rich SNe, could be identified in the nebular
spectra of SN 2020jfo. Analysis of the line ratio of [Ni II] $\lambda$7378 to
[Fe II] $\lambda$7155 suggests a supersolar Ni-to-Fe production ratio by mass.

From nebular-phase spectral comparison (Jerkstrand et al., 2014), we estimated
the ZAMS mass of the progenitor of SN 2020jfo to be around 12 M⊙. Moreover,
semi-analytical modelling using the prescription of Nagy & Vinkó (2016)
indicated an ejecta mass of $13.6^{+0.2}_{-2.5}$ M⊙. Adding a remnant neutron
star mass, the progenitor mass turns out to be $15^{+0.2}_{-2.5}$ M⊙. This can
be considered as a lower limit of the progenitor ZAMS mass, since the
analytical method does not estimate the mass lost by winds. The SN 2020jfo
ejecta mass estimated from semi-analytical modelling, which mainly constitutes
the H-envelope mass, is much higher than that of the Type II SNe studied by
Inserra et al. (2013), Inserra et al. (2011), and Hiramatsu et al. (2021b)
with similar plateau lengths. Thus, spectroscopic comparison and modelling
suggests that SN 2020jfo differs from SNe II with similar plateau lengths and
is rather spectroscopically similar to SN 1999em.

## Acknowledgements

The authors thank the referee for critically reviewing the manuscript which
has improved the presentation of the paper. This research made use of the
NASA/IPAC Extragalactic Database (NED) that is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract with the
National Aeronautics and Space Administration (NASA). This work makes use of
data obtained by the LCO network. Based in part on observations obtained at
the 3.6 m Devasthal Optical Telescope (DOT), which is a National Facility run
and managed by Aryabhatta Research Institute of Observational Sciences
(ARIES), an autonomous Institute under Department of Science and Technology,
Government of India. We thank (mostly U.C. Berkeley undergraduate students)
Raphael Baer-Way, Matthew Chu, Asia deGraw, Nachiket Girish, Evelyn Liu, Emily
Ma, Shaunak Modak, Derek Perera, and Keto D. Zhang for their effort in taking
Lick/Nickel data. The Kast red CCD detector upgrade, led by B. Holden, was
made possible by the Heising-Simons Foundation, William and Marina Kast, and
the University of California Observatories. KAIT and its ongoing operation
were made possible by donations from Sun Microsystems, Inc., the Hewlett-
Packard Company, AutoScope Corporation, the Lick Observatory, the US National
Science Foundation, the University of California, the Sylvia & Jim Katzman
Foundation, and the TABASGO Foundation. Research at Lick Observatory is
partially supported by a generous gift from Google.

B.A. acknowledges CSIR fellowship award for this work. R.D. acknowledges funds
by ANID grant FONDECYT Postdoctorado #3220449. K.M. and S.B.P. acknowledge
BRICS grant DST/IMRCD/BRICS/Pilotcall/ProFCheap/2017(G) for this work. L.G. is
grateful for financial support from the Spanish Ministerio de Ciencia e
Innovación (MCIN), the Agencia Estatal de Investigación (AEI)
10.13039/501100011033, and the European Social Fund (ESF) “Investing in your
future” under the 2019 Ramón y Cajal program RYC2019-027683-I and the
PID2020-115253GA-I00 HOSTFLOWS project, from Centro Superior de
Investigaciones Científicas (CSIC) under the PIE project 20215AT016, and the
program Unidad de Excelencia María de Maeztu CEX2020-001058-M. L.P.
acknowledges financial support from the CSIC project JAEICU-21-ICE-09. E.K.
and M.S. are funded by a Project 1 research grant from the Independent
Research Fund Denmark (IRFD, grant number 8021-00170B) and by a VILLUM FONDEN
Experiment grant (number 28021). A.V.F.’s supernova group at U.C. Berkeley is
grateful for support from the TABASGO Foundation, the Christopher R. Redlich
Fund, the Miller Institute for Basic Research in Science (in which he was a
Miller Senior Fellow), and many individual donors.

## Data Availability

The photometric and spectroscopic data underlying this article will be made
available upon request to the corresponding author.

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##  Appendix A Data tables

Table 6: Swift/UVOT photometry of SN 2020jfoa

Date | Phaseb | $uw2$ | $um2$ | $uw1$ | $uvu$ | $uvb$ | $uvv$
---|---|---|---|---|---|---|---
(UT) | (days) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag)
2020-05-07.13 | 3.52 | 14.75$\pm$ 0.09 | 14.75$\pm$ 0.06 | 14.64$\pm$ 0.08 | 14.55$\pm$ 0.06 | 14.74$\pm$ 0.05 | 14.99$\pm$0.06
2020-05-08.73 | 5.13 | 14.81$\pm$ 0.09 | 14.66$\pm$ 0.07 | 14.60$\pm$ 0.08 | 14.40$\pm$ 0.06 | 14.53$\pm$ 0.05 | 14.62$\pm$0.06
2020-05-09.23 | 5.63 | 15.18$\pm$ 0.09 | 14.86$\pm$ 0.06 | 14.79$\pm$ 0.08 | 14.42$\pm$ 0.06 | 14.58$\pm$ 0.05 | 14.62$\pm$0.06
2020-05-10.35 | 6.75 | 15.39$\pm$ 0.09 | 15.08$\pm$ 0.07 | 14.87$\pm$ 0.08 | 14.40$\pm$ 0.06 | 14.48$\pm$ 0.05 | 14.54$\pm$0.06
2020-05-10.42 | 6.82 | 15.47$\pm$ 0.09 | 15.12$\pm$ 0.07 | 14.97$\pm$ 0.08 | 14.48$\pm$ 0.06 | 14.53$\pm$ 0.06 | 14.58$\pm$0.07
2020-05-11.22 | 7.62 | 15.76$\pm$ 0.09 | 15.34$\pm$ 0.06 | 14.96$\pm$ 0.08 | 14.41$\pm$ 0.06 | 14.49$\pm$ 0.05 | 14.51$\pm$0.06
2020-05-12.76 | 9.15 | 16.31$\pm$ 0.09 | 15.91$\pm$ 0.07 | 15.36$\pm$ 0.08 | 14.54$\pm$ 0.06 | 14.59$\pm$ 0.05 | 14.58$\pm$0.06
2020-05-13.42 | 9.81 | 16.33$\pm$ 0.09 | 16.05$\pm$ 0.07 | 15.48$\pm$ 0.08 | 14.57$\pm$ 0.06 | 14.52$\pm$ 0.05 | 14.58$\pm$0.06
2020-05-15.67 | 12.07 | 16.83$\pm$ 0.10 | 16.73$\pm$ 0.07 | 15.95$\pm$ 0.08 | 14.78$\pm$ 0.06 | 14.58$\pm$ 0.06 | 14.75$\pm$0.07
2020-05-16.66 | 13.06 | 17.17$\pm$ 0.10 | 17.05$\pm$ 0.08 | 16.18$\pm$ 0.09 | 14.84$\pm$ 0.06 | 14.66$\pm$ 0.06 | 14.73$\pm$0.07
2020-05-24.91 | 21.31 | 19.93$\pm$ 0.12 | 19.90$\pm$ 0.10 | 18.20$\pm$ 0.10 | 16.23$\pm$ 0.07 | 14.96$\pm$ 0.06 | 14.66$\pm$0.06
2020-05-26.56 | 22.95 | 19.78$\pm$ 0.12 | 20.20$\pm$ 0.10 | 18.53$\pm$ 0.11 | 16.62$\pm$ 0.07 | 15.04$\pm$ 0.06 | 14.71$\pm$0.06
2020-06-02.76 | 30.16 | 20.89$\pm$ 0.11 | 21.34$\pm$ 0.09 | 19.30$\pm$ 0.10 | 17.33$\pm$ 0.07 | 15.45$\pm$ 0.05 | 14.79$\pm$0.06
2020-06-07.11 | 34.5 | 20.60$\pm$ 0.10 | 21.24$\pm$ 0.08 | 19.51$\pm$ 0.09 | 17.71$\pm$ 0.07 | 15.57$\pm$ 0.05 | 14.86$\pm$0.05
2020-06-13.09 | 40.48 | 21.19$\pm$ 0.11 | 21.79$\pm$ 0.09 | 19.96$\pm$ 0.10 | 18.12$\pm$ 0.08 | 15.72$\pm$ 0.05 | 14.93$\pm$0.06
2020-06-18.00 | 45.39 | 22.29$\pm$ 0.15 | 20.90$\pm$ 0.12 | 19.66$\pm$ 0.14 | 18.65$\pm$ 0.13 | 15.90$\pm$ 0.07 | 14.91$\pm$0.07
2020-07-18.41 | 75.81 | 21.50$\pm$ 0.10 | 22.63$\pm$ 0.08 | 21.83c | 20.79 | 18.58$\pm$ 0.07 | 17.51$\pm$0.08
2020-07-29.33 | 86.72 | 21.74$\pm$ 0.10 | 21.74$\pm$ 0.08 | 21.82$\pm$ 0.10 | 20.52 | 18.71$\pm$ 0.07 | 17.28$\pm$0.07
2020-08-03.41 | 91.81 | 21.72$\pm$ 0.10 | 21.48$\pm$ 0.08 | 21.63$\pm$ 0.10 | 20.60 | 19.00$\pm$ 0.07 | 17.53$\pm$0.08
2020-08-08.45 | 96.9 | 21.73$\pm$ 0.10 | 22.14 | 21.61 | 21.49$\pm$ 0.09 | 18.86$\pm$ 0.07 | 17.47$\pm$0.10
2020-11-14.65 | 195.0 | 21.86 | 22.69$\pm$ 0.10 | 21.87$\pm$ 0.10 | 20.28 | 20.28$\pm$ 0.09 | 18.58$\pm$0.15
2020-12-12.03 | 222.43 | 21.64$\pm$ 0.11 | 22.05$\pm$ 0.08 | 21.67 | 21.24$\pm$ 0.09 | 21.11$\pm$ 0.08 | 19.25$\pm$0.11
2020-12-24.41 | 234.81 | 22.94 | 22.14$\pm$ 0.07 | 22.17 | 21.12 | 21.41$\pm$ 0.07 | 19.55$\pm$0.08
2021-01-13.53 | 254.92 | 22.98 | 22.99 | 22.24 | 21.18 | 20.37 | 20.08$\pm$0.08
2021-01-28.99 | 270.39 | 22.31$\pm$ 0.14 | 21.63 | 20.99 | 20.03 | 19.29 | 20.95$\pm$0.18
2021-01-30.53 | 271.92 | 22.80 | 22.83 | 22.13 | 21.10 | 20.31 | 20.47$\pm$0.09

aThe magnitudes of the transient have been determined after subtracting the
host flux at the SN location from recent photometry. The host flux at the SN
location in different UVOT bands has been determined from previous Swift
observations (in the year 2008) of the field (see the text for details).
bPhase with respect to the explosion epoch (MJD = 58973.1).
cMagnitudes without uncertainties correspond to upper limits.

Table 7: Details of the instruments used for the observational campaign of SN 2020jfo. Telescope | Location | Instrument | Pixel Scale | Imaging | Dispersers/
---|---|---|---|---|---
|  |  | ("/pixel) | bands | Grisms
0.76m KAIT | Lick Observatory | - | 0.80 | $BVRI$ |
1.04m ST | ARIES Observatory, India | Tek 1k $\times$ 1k | 0.53 | $BVRI$ | -
1.0m Nickel | Lick Observatory | - | 0.184 | BVRI |
1.0m LCO | Siding Spring Observatory | Sinistro | 0.389 | $BVg^{\prime}r^{\prime}i^{\prime}$ | -
1.0m LCO | South African | Sinistro | 0.389 | $BVg^{\prime}r^{\prime}i^{\prime}$ | -
| Astronomical Observatory |  |  |  |
1.0m LCO | McDonald Observatory | Sinistro | 0.389 | $BVg^{\prime}r^{\prime}i^{\prime}$ | -
1.0m LCO | Cerro Tololo | Sinistro | 0.389 | $BVg^{\prime}r^{\prime}i^{\prime}$ | -
| Interamerican Observatory |  |  |  |
1.3m DFOT | ARIES Observatory, India | Andor $512\times 512$ | 0.64 | $UBVRI$ | -
2.0m FTN | Haleakala Observatory, USA | Spectral, FLOYDS | 0.304, 0.34 | - | Cross disperser
2.0m FTS | Siding Spring Observatory, | FLOYDS | 0.34 | - | Cross disperser
| Australia |  |  |  |
3.0m NASA IRTF | Mauna Kea | SpeX | 0.15 | - | Cross disperser
3.0m Shane | Lick Observatory | Kast | 0.43 | - | -
3.6m DOT | ARIES Observatory, India | ADFOSC | - | - | -
10.4m GTC | La Palma, Spain | OSIRIS | 0.254 | - | R1000B, R1000R
Figure 20: Measurement procedure of the true SN flux. The left and middle
cutouts respectively present the template and transient frames, marking the SN
locations observed from ST in the $V$ band. The template (left panel) frame
was acquired on 2011-01-04 (see Roy et al. 2011b), whereas the middle panel
represents the observation done on 2021-01-07. Both cutouts cover the same
region of the sky, each of dimension $\sim 2.5\arcmin\times 2.5\arcmin$, and
the location of the SN has been marked. In both cutouts north is up and east
is to the left. The right panel shows the subtracted frame, where the SN has
been marked. We performed the template subtraction on the ST and DFOT frames
using the routine outlined by Roy et al. (2011a). The point-spread-function
(PSF) photometry was done on each of the subtracted frames. The results
obtained from this routine are consistent with the photometry obtained from
HOTPANTS. Table 8: Optical photometry of SN 2020jfo with LCO network of
telescopes

Date | Phasea | $B$ | $V$ | $g$ | $r$ | $i$
---|---|---|---|---|---|---
(UT) | (days) | (mag) | (mag) | (mag) | (mag) | (mag)
2020-05-06.90 | 2.80 |  |  |  | 15.16$\pm$0.06b |
2020-05-07.51 | 3.41 | 14.78$\pm$0.08 | 14.77$\pm$0.09 | 14.61$\pm$0.11 | 15.02$\pm$0.05 | 15.07$\pm$0.12
2020-05-07.76 | 3.66 | 14.76$\pm$0.06 | 14.76$\pm$0.02 | 14.60$\pm$0.06 | 14.91$\pm$0.08 | 14.96$\pm$0.08
2020-05-08.44 | 4.34 | 14.67$\pm$0.02 | 14.63$\pm$0.03 | 14.66$\pm$0.50 | 14.67$\pm$0.10 | 14.86$\pm$0.01
2020-05-09.54 | 5.44 | 15.01$\pm$0.08 | - | 14.57$\pm$0.01 | 14.61$\pm$0.01 | 14.55$\pm$0.01
2020-05-10.36 | 6.26 | 14.54$\pm$0.02 | - | 14.38$\pm$0.01 | 14.42$\pm$0.04 | 14.55$\pm$0.04
2020-05-10.87 | 6.77 | 14.63$\pm$0.02 | 14.44$\pm$0.14 | 14.48$\pm$0.02 | 14.43$\pm$0.06 | 14.45$\pm$0.06
2020-05-11.56 | 7.46 | 14.61$\pm$0.05 | 14.47$\pm$0.06 | 14.23$\pm$0.02 | 14.37$\pm$0.04 | 14.51$\pm$0.04
2020-05-12.37 | 8.27 | 14.57$\pm$0.01 | 14.44$\pm$0.02 | 14.45$\pm$0.05 | 14.45$\pm$0.03 | 14.56$\pm$0.08
2020-05-12.85 | 8.75 | 14.67$\pm$0.03 | 14.53$\pm$0.02 | 14.49$\pm$0.04 | 14.56$\pm$0.06 | 14.49$\pm$0.30
2020-05-13.81 | 9.71 | 14.72$\pm$0.03 | 14.60$\pm$0.02 | 14.57$\pm$0.06 | 14.53$\pm$0.01 | 14.57$\pm$0.08
2020-05-14.83 | 10.73 | 14.82$\pm$0.04 | 14.53$\pm$0.02 | 14.55$\pm$0.01 | 14.60$\pm$0.01 | 14.69$\pm$0.02
2020-05-17.81 | 13.71 | 14.90$\pm$0.02 | 14.43$\pm$0.02 | 14.50$\pm$0.09 | 14.55$\pm$0.01 | 14.58$\pm$0.10
2020-05-19.35 | 15.25 | 14.98$\pm$0.12 | 15.29$\pm$0.04 | 13.32$\pm$0.11 | 14.81$\pm$0.02 | 14.85$\pm$0.38
2020-05-20.74 | 16.64 | 15.01$\pm$0.04 | 14.56$\pm$0.02 | 14.68$\pm$0.01 | 14.54$\pm$0.01 | 14.72$\pm$0.01
2020-05-24.84 | 20.74 | 15.25$\pm$0.02 | 14.59$\pm$0.02 | 14.80$\pm$0.01 | 14.62$\pm$0.01 | 14.67$\pm$0.01
2020-05-27.70 | 23.60 | 15.31$\pm$0.05 | 14.68$\pm$0.02 | 14.82$\pm$0.09 | 14.59$\pm$0.01 | 14.60$\pm$0.06
2020-05-29.70 | 25.60 | 15.45$\pm$0.05 | 14.63$\pm$0.02 | 14.96$\pm$0.02 | 14.58$\pm$0.01 | 14.61$\pm$0.01
2020-06-04.85 | 31.75 | 15.67$\pm$0.02 | 14.72$\pm$0.02 | 15.12$\pm$0.01 | 14.65$\pm$0.01 | 14.63$\pm$0.01
2020-06-09.75 | 36.65 | 15.83$\pm$0.02 | 14.78$\pm$0.02 | 15.23$\pm$0.01 | 14.80$\pm$0.01 | 14.74$\pm$0.01
2020-06-15.41 | 42.31 | 16.02$\pm$0.02 | 14.92$\pm$0.02 | 15.39$\pm$0.01 | 14.74$\pm$0.01 | 14.69$\pm$0.01
2020-06-19.45 | 46.35 | 15.99$\pm$0.02 | 14.80$\pm$0.02 | 15.43$\pm$0.01 | 14.68$\pm$0.01 | 14.57$\pm$0.01
2020-06-30.40 | 57.30 | 16.62$\pm$0.02 | 15.26$\pm$0.02 | 15.76$\pm$0.01 | - | 14.93$\pm$0.02
2020-07-03.80 | 60.70 | 16.75$\pm$0.02 | 15.29$\pm$0.02 | 15.93$\pm$0.01 | 14.99$\pm$0.01 | 14.91$\pm$0.01
2020-07-08.76 | 65.67 | 17.46$\pm$0.02 | 15.79$\pm$0.02 | 16.64$\pm$0.01 | 15.42$\pm$0.01 | 15.35$\pm$0.01
2020-07-15.73 | 72.63 | 18.81$\pm$0.03 | 16.99$\pm$0.02 | 17.89$\pm$0.01 | 16.49$\pm$0.01 | 16.49$\pm$0.01
2020-07-18.76 | 75.66 | 18.81$\pm$0.03 | - | - | - | -
2020-07-20.71 | 77.61 | 19.00$\pm$0.02 | 17.28$\pm$0.02 | 17.97$\pm$0.01 | 16.61$\pm$0.01 | 16.58$\pm$0.01
2020-07-23.74 | 80.64 | 18.68$\pm$0.02 | 17.11$\pm$0.03 | 17.77$\pm$0.02 | 16.51$\pm$0.01 | 16.49$\pm$0.02
2020-07-27.76 | 84.66 | 18.84$\pm$0.14 | 17.37$\pm$0.06 | 17.72$\pm$0.01 | 16.47$\pm$0.02 | 16.71$\pm$0.03
2020-07-31.38 | 88.28 | - | 17.78$\pm$0.39 | - | 17.17$\pm$0.18 | 17.13$\pm$0.35
2020-08-05.74 | 93.64 | 18.70$\pm$0.06 | 17.30$\pm$0.04 | 18.03$\pm$0.03 | 16.58$\pm$0.02 | 16.90$\pm$0.03
2020-08-10.37 | 98.27 | 18.51$\pm$0.09 | 17.41$\pm$0.09 | 17.83$\pm$0.03 | 16.59$\pm$0.04 | 16.91$\pm$0.03
2020-12-19.48 | 229.38 | 20.04$\pm$0.06 | 19.03$\pm$0.04 | 19.35$\pm$0.03 | 18.45$\pm$0.02 | 18.53$\pm$0.04
2020-12-23.45 | 233.35 | 20.16$\pm$0.07 | 19.12$\pm$0.08 | 19.44$\pm$0.03 | 18.61$\pm$0.02 | 18.91$\pm$0.05
2021-01-03.50 | 244.40 | 20.57$\pm$0.10 | 19.57$\pm$0.10 | 19.85$\pm$0.08 | 18.97$\pm$0.06 | 19.09$\pm$0.13
2021-01-15.07 | 255.97 | 20.44$\pm$0.08 | 19.41$\pm$0.06 | 19.54$\pm$0.03 | 19.12$\pm$0.04 | 18.85$\pm$0.06
2021-01-26.08 | 266.98 | 20.57$\pm$0.08 | 19.54$\pm$0.07 | 19.84$\pm$0.03 | 19.62$\pm$0.04 | 19.46$\pm$0.03
2021-02-06.08 | 277.98 | 20.90$\pm$0.11 | 19.68$\pm$0.06 | - | 19.66$\pm$0.05 | -
2021-02-09.34 | 281.24 | 21.20$\pm$0.16 | 19.83$\pm$0.06 | 19.92$\pm$0.04 | 19.72$\pm$0.06 | 19.39$\pm$0.07
2021-02-27.66 | 299.56 | 21.08$\pm$0.14 | 20.28$\pm$0.08 | 19.92$\pm$0.05 | 20.33$\pm$0.08 | 19.80$\pm$0.07
2021-03-11.23 | 311.13 | 21.69$\pm$0.28 | 20.32$\pm$0.09 | 20.29$\pm$0.05 | 20.46$\pm$0.13 | 19.85$\pm$0.08
2021-03-22.24 | 322.14 | 21.63$\pm$0.30 | 20.51$\pm$0.09 | 20.46$\pm$0.06 | 20.79$\pm$0.15 | 20.09$\pm$0.07
2021-04-03.95 | 334.85 | 22.24$\pm$0.34 | 20.66$\pm$0.14 | 20.58$\pm$0.06 | 21.44$\pm$0.18 | 20.56$\pm$0.13
2021-04-14.89 | 345.79 | 22.50$\pm$0.45 | 20.66$\pm$0.13 | 20.61$\pm$0.07 | 21.49$\pm$0.19 | 20.47$\pm$0.19
2021-04-27.18 | 358.08 | 21.53$\pm$0.13 | 21.60$\pm$0.39 | - | - | 20.88$\pm$0.38
2021-05-08.18 | 369.08 | 22.61$\pm$0.38 | 21.07$\pm$0.18 | 21.43$\pm$0.39 | - | -
2021-05-13.36 | 374.26 | - | 21.03$\pm$0.15 | - | - | -
2021-05-24.50 | 385.40 | 22.00$\pm$0.38 | 21.51$\pm$0.32 | 22.05$\pm$0.22 | 21.38$\pm$0.40 | 20.28$\pm$0.22
2021-06-04.71 | 396.61 | - | 21.18$\pm$0.21 | 21.64$\pm$0.16 | - | -
2021-06-06.41 | 398.31 | - | 21.34$\pm$0.24 | 21.30$\pm$0.12 | 21.92$\pm$0.28 | 21.48$\pm$0.44
2021-06-21.06 | 412.96 | 22.68$\pm$0.51 | 22.50$\pm$0.43 | - | 21.83$\pm$0.23 | 21.15$\pm$0.23
2021-07-02.04 | 423.94 | - | 21.53$\pm$0.31 | 21.91$\pm$0.26 | 22.01$\pm$0.34 | -
2021-07-13.00 | 434.90 | - | - | 21.52$\pm$0.50 | - | -

aPhase with respect to the explosion epoch (MJD = 58973.1).
bThe earliest photometric data obtained with the 10.4 m GTC.

Table 9: Optical photometry of SN 2020jfo with ARIES telescopes

Date | Phasea | $B$ | $V$ | $R$ | $I$
---|---|---|---|---|---
(UT) | (days) | (mag) | (mag) | (mag) | (mag)
2021-01-06.98 | 248.37 | $-$ | 19.26$\pm$0.05 | 19.09$\pm$0.09 | 19.89$\pm$0.17
2021-01-07.90 | 249.30 | 19.78$\pm$0.09 | 19.20$\pm$0.06 | 19.31$\pm$0.09 | 19.55$\pm$0.20
2021-01-07.91 | 249.31 | $-$ | 19.19$\pm$0.05 | 19.50$\pm$0.06 | 19.49$\pm$0.16
2021-01-09.90 | 251.26 | 20.14$\pm$0.21 | 19.21$\pm$0.05 | 19.20$\pm$0.08 | 19.66$\pm$0.16
2021-01-21.96 | 263.34 | 20.07$\pm$0.19 | 19.11$\pm$0.06 | 18.84$\pm$0.08 | 19.97$\pm$0.28
2021-02-02.98 | 275.38 | 20.4 | 19.77$\pm$0.14 | 18.81$\pm$0.10 | 19.86$\pm$0.27
2021-02-14.79 | 287.19 | 20.59$\pm$0.23 | 20.16$\pm$0.09 | 18.90$\pm$0.08 | 19.98$\pm$0.17
2021-02-22.81 | 295.37 | 21.21$\pm$0.32 | 20.17$\pm$0.10 | 19.67$\pm$0.13 | 21.12$\pm$0.32

aPhase with respect to the explosion epoch (MJD = 58973.1).
bMagnitude without error corresponds to the upper limit.

Table 10: Optical photometry of SN 2020jfo with Lick Observatory telescopes

Date | Phasea | $B$ | $V$ | $R$ | $I$ | Clear
---|---|---|---|---|---|---
(UT) | (days) | (mag) | (mag) | (mag) | (mag) | (mag)
2020-05-07.32 | 3.22 | 14.66$\pm$0.06 | 15.01$\pm$0.03 | 14.94$\pm$0.06 | 14.96$\pm$0.06 | 14.78$\pm$0.04
2020-05-08.35 | 4.25 | 14.58$\pm$0.04 | 14.77$\pm$0.02 | 14.71$\pm$0.02 | 14.64$\pm$0.03 | 14.55$\pm$0.03
2020-05-09.25 | 5.15 | 14.52$\pm$0.04 | 14.66$\pm$0.02 | 14.58$\pm$0.02 | 14.47$\pm$0.02 | 14.44$\pm$0.03
2020-05-10.34 | 6.24 | 14.54$\pm$0.05 | 14.63$\pm$0.02 | 14.48$\pm$0.02 | 14.37$\pm$0.03 | 14.38$\pm$0.02
2020-05-11.30 | 7.20 | 14.48$\pm$0.05 | 14.59$\pm$0.03 | 14.45$\pm$0.02 | 14.31$\pm$0.03 | 14.37$\pm$0.02
2020-05-16.27 | 12.17 | 14.66$\pm$0.03 | 14.71$\pm$0.01 | 14.51$\pm$0.01 | 14.45$\pm$0.02 | 14.44$\pm$0.02
2020-05-17.27 | 13.17 | 14.68$\pm$0.05 | 14.76$\pm$0.04 | 14.50$\pm$0.04 | 14.42$\pm$0.10 | 14.46$\pm$0.02
2020-05-21.24 | 17.14 | 14.79$\pm$0.08 | 14.66$\pm$0.02 | 14.48$\pm$0.02 | 14.37$\pm$0.03 | -
2020-05-21.31 | 17.21 | 14.87$\pm$0.04 | 14.73$\pm$0.02 | 14.51$\pm$0.02 | 14.43$\pm$0.03 | 14.46$\pm$0.03
2020-05-23.25 | 19.15 | 14.94$\pm$0.04 | 14.71$\pm$0.02 | 14.50$\pm$0.02 | 14.38$\pm$0.02 | 14.48$\pm$0.03
2020-05-24.27 | 20.17 | 15.01$\pm$0.04 | 14.73$\pm$0.02 | 14.50$\pm$0.02 | 14.39$\pm$0.03 | 14.49$\pm$0.03
2020-05-25.28 | 21.18 | 15.00$\pm$0.08 | 14.77$\pm$0.02 | 14.52$\pm$0.02 | 14.41$\pm$0.02 | 14.49$\pm$0.03
2020-05-26.25 | 22.15 | 15.10$\pm$0.04 | 14.75$\pm$0.02 | 14.51$\pm$0.02 | 14.38$\pm$0.03 | 14.51$\pm$0.02
2020-05-27.25 | 23.15 | 15.14$\pm$0.04 | 14.77$\pm$0.02 | 14.53$\pm$0.02 | 14.41$\pm$0.03 | 14.52$\pm$0.03
2020-05-28.26 | 24.16 | 15.20$\pm$0.04 | 14.78$\pm$0.02 | 14.52$\pm$0.02 | 14.37$\pm$0.02 | 14.55$\pm$0.03
2020-06-02.18 | 29.08 | 15.39$\pm$0.09 | 14.78$\pm$0.08 | 14.54$\pm$0.13 | 14.26$\pm$0.10 | -
2020-06-02.24 | 29.14 | 15.35$\pm$0.09 | 14.76$\pm$0.03 | 14.54$\pm$0.02 | 14.34$\pm$0.03 | -
2020-06-04.27 | 31.17 | 15.50$\pm$0.05 | 14.83$\pm$0.02 | 14.56$\pm$0.02 | 14.38$\pm$0.03 | 14.60$\pm$0.02
2020-06-06.26 | 33.16 | 15.47$\pm$0.13 | 14.79$\pm$0.06 | 14.54$\pm$0.02 | 14.32$\pm$0.04 | -
2020-06-10.21 | 37.11 | 15.67$\pm$0.08 | 14.86$\pm$0.03 | 14.59$\pm$0.02 | 14.35$\pm$0.03 | -
2020-06-14.24 | 41.14 | 15.74$\pm$0.08 | 14.88$\pm$0.03 | 14.60$\pm$0.02 | 14.36$\pm$0.03 | -
2020-06-18.24 | 45.14 | 15.91$\pm$0.23 | 14.92$\pm$0.14 | 14.65$\pm$0.02 | 14.38$\pm$0.04 | -
2020-06-22.25 | 49.15 | 16.05$\pm$0.08 | 14.97$\pm$0.03 | 14.66$\pm$0.02 | 14.42$\pm$0.25 | -
2020-06-30.20 | 57.10 | 16.27$\pm$0.08 | 15.20$\pm$0.03 | 14.82$\pm$0.02 | 14.55$\pm$0.04 | -
2020-06-30.24 | 57.14 | 16.27$\pm$0.07 | 15.23$\pm$0.03 | 14.84$\pm$0.03 | 14.57$\pm$0.03 | 14.88$\pm$0.04
2020-07-01.21 | 58.11 | 16.34$\pm$0.07 | 15.29$\pm$0.03 | 14.91$\pm$0.03 | 14.63$\pm$0.04 | 14.99$\pm$0.04
2020-07-04.23 | 61.13 | 16.58$\pm$0.08 | 15.43$\pm$0.02 | 14.97$\pm$0.02 | 14.71$\pm$0.03 | 15.08$\pm$0.04
2020-07-05.20 | 62.10 | 16.78$\pm$0.10 | 15.57$\pm$0.03 | 15.08$\pm$0.03 | 14.78$\pm$0.03 | 15.20$\pm$0.04
2020-07-08.22 | 65.12 | 17.16$\pm$0.09 | 15.83$\pm$0.04 | 15.30$\pm$0.03 | 15.00$\pm$0.04 | 15.45$\pm$0.05
2020-07-10.20 | 67.10 | 17.44$\pm$0.13 | 16.18$\pm$0.05 | 15.57$\pm$0.05 | 15.30$\pm$0.05 | 15.76$\pm$0.06
2020-07-11.22 | 68.12 | 17.68$\pm$0.15 | 16.43$\pm$0.05 | 15.76$\pm$0.04 | 15.45$\pm$0.05 | 15.91$\pm$0.07
2020-07-18.20 | 75.10 | - | - | - | - | 16.55$\pm$0.20
2020-07-19.20 | 76.10 | 17.96$\pm$0.24 | 17.20$\pm$0.10 | 16.43$\pm$0.08 | 16.06$\pm$0.07 | 16.53$\pm$0.12
2020-07-20.20 | 77.10 | - | - | - | - | 16.49$\pm$0.13
2020-07-21.20 | 78.10 | 18.20$\pm$0.28 | 17.14$\pm$0.10 | 16.41$\pm$0.07 | 16.10$\pm$0.07 | 16.52$\pm$0.11
2020-07-23.20 | 80.10 | 18.26$\pm$0.38 | 17.18$\pm$0.15 | 16.37$\pm$0.11 | 16.15$\pm$0.09 | 16.55$\pm$0.14

a Phase with respect to the explosion epoch (MJD = 58973.1).

Table 11: Spectroscopic log of SN 2020jfo

Date | Phasea | Telescope
---|---|---
(UT) | (days) |
| Optical |
2020-05-06.90 | 2.80 | GTC
2020-05-07.54 | 3.44 | FTS
2020-05-08.55 | 4.45 | FTS
2020-05-12.36 | 8.26 | FTN
2020-05-16.49 | 12.39 | FTS
2020-05-17.27 | 13.17 | Shane
2020-05-19.50 | 15.40 | FTS
2020-05-23.00 | 18.90 | DOT
2020-05-24.00 | 19.90 | DOT
2020-05-24.46 | 20.36 | FTS
2020-05-25.33 | 21.17 | Shane
2020-05-28.48 | 24.38 | FTS
2020-05-29.29 | 25.19 | Shane
2020-06-05.48 | 32.38 | FTS
2020-06-13.30 | 40.20 | FTN
2020-06-26.27 | 53.17 | FTN
2020-07-06.42 | 63.32 | FTS
2020-07-09.20 | 66.10 | Shane
2020-07-14.26 | 71.16 | FTN
2020-07-18.20 | 75.09 | Shane
2020-07-23.25 | 80.15 | FTN
2020-07-28.19 | 85.09 | Shane
2020-08-01.35 | 89.25 | FTS
2021-01-13.00 | 253.90 | DOT
2021-01-15.00 | 255.90 | DOT
2021-05-04.30 | 365.20 | Shane
| NIR |
2020-05-13 | 9.27 | IRTF
2020-05-24 | 20.24 | IRTF
2020-06-15 | 42.19 | IRTF

a Phase with respect to the explosion epoch (MJD = 58973.1).