# The Red Supergiant Progenitor of Type II Supernova 2024ggi

Danfeng Xiang Department of Physics, Tsinghua University, Beijing 100084,
China  Jun Mo  Department of Physics, Tsinghua University, Beijing 100084,
China  Xiaofeng Wang Department of Physics, Tsinghua University, Beijing
100084, China  Lingzhi Wang Chinese Academy of Sciences, South America Center
for Astronomy (CASSACA), National Astronomical Observatories, CAS, Beijing
100101, China  Jujia Zhang Yunnan Observatories (YNAO), Chinese Academy of
Sciences (CAS), Kunming, 650216, China  International Centre of Supernovae,
Yunnan Key Laboratory, Kunming 650216, China  Key Laboratory for the Structure
and Evolution of Celestial Objects, CAS, Kunming, 650216, China  Han Lin
Yunnan Observatories (YNAO), Chinese Academy of Sciences (CAS), Kunming,
650216, China  International Centre of Supernovae, Yunnan Key Laboratory,
Kunming 650216, China  Key Laboratory for the Structure and Evolution of
Celestial Objects, CAS, Kunming, 650216, China  Liyang Chen  Department of
Physics, Tsinghua University, Beijing 100084, China  Cuiying Song Department
of Physics, Tsinghua University, Beijing 100084, China  Liang-Duan Liu
Institute of Astrophysics, Central China Normal University, Wuhan 430079,
China  Key Laboratory of Quark and Lepton Physics (Central China Normal
University), Ministry of Education, Wuhan 430079, China  Zhenyu Wang  Yunnan
Observatories (YNAO), Chinese Academy of Sciences (CAS), Kunming, 650216,
China  International Centre of Supernovae, Yunnan Key Laboratory, Kunming
650216, China  University of Chinese Academy of Sciences, Beijing 100049,
China  Gaici Li  Department of Physics, Tsinghua University, Beijing 100084,
China

###### Abstract

We present a detailed analysis of the progenitor and its local environment for
the recently discovered type II supernova (SN) 2024ggi at a distance of about
6.7 Mpc, by utilizing the pre-explosion images from the Hubble Space Telescope
(HST) and Spitzer Space Telescope. The progenitor is identified as a red,
bright variable star, with absolute $F814W$-band magnitudes being $-$6.2 mag
in 1995 to $-$7.2 mag in 2003, respectively, consistent with that of a normal
red supergiant (RSG) star. Combining with the historical mid-infrared light
curves, a pulsational period of about 379 days can be inferred for the
progenitor star. Fitting its spectral energy distribution with stellar
spectral models yields the stellar parameters of temperature, radius and
bolometric luminosity as $T_{*}=3290_{-27}^{+19}$ K, $R_{*}=887_{-51}^{+60}$
$\mathrm{R_{\odot}}$, and log($L$/L⊙)$=4.92_{-0.04}^{+0.05}$, respectively.
The above parameters indicate that the progenitor of SN 2024ggi is consistent
with the stellar evolutionary track of a solar-metallicity massive star with
an initial mass of $13_{-1}^{+1}$ $\mathrm{M_{\odot}}$. Moreover, our analysis
indicates a relatively low mass loss rate (i.e., $<3\times 10^{-6}$
$\mathrm{M_{\odot}}$ yr-1) for the progenitor compared to that inferred from
the flashed spectra and X-ray detection (i.e., $10^{-2}$$-$$10$-5
$\mathrm{M_{\odot}}$ yr-1), implying a significant enhancement in mass loss
within a few years prior to the explosion.

stellar evolution (1599); Type II supernovae (1731); Red supergiant
stars(1375); stellar mass loss (1613)

††facilities: HST (WFPC2, ACS, WFC3), Spitzer/IRAC (IRSA, 2022), VISTA-Vircam,
LJT. ††software: astropy (Astropy Collaboration et al., 2013, 2018), emcee
(Foreman-Mackey et al., 2013), SExtractor (Bertin & Arnouts, 1996), IRAF
(Tody, 1986, 1993), DOLPHOT (http://americano.dolphinsim.com/dolphot/), DUSTY
version 2.0 (Ivezic & Elitzur, 1997). ††thanks: These authors contributed
equally to this work.††thanks: These authors contributed equally to this
work.††thanks: E-mail<EMAIL_ADDRESS>
##  1 Introduction

Type II Supernovae (SNe II) are thought to result from core-collapse
explosions of red supergiants (RSGs), which have initial masses of 8–25
$\mathrm{M_{\odot}}$ (Heger et al., 2003). These stars retain most of their
hydrogen envelopes before core collapse, producing supernovae with persistent
and prominent hydrogen lines in their spectra. With pre-discovery images,
mainly thanks to space-based telescopes such as the Hubble Space Telescope
(HST), progenitors have been identified for dozens of SNe II (Smartt, 2015)
and more recently for SN 2017eaw (Kilpatrick & Foley, 2018; Van Dyk et al.,
2019; Rui et al., 2019), SN 2022acko (Van Dyk et al., 2023) and SN 2023ixf
(Kilpatrick et al., 2023; Jencson et al., 2023; Soraisam et al., 2023; Niu et
al., 2023; Xiang et al., 2024). These observations and analysis have confirmed
the connections between RSGs and SNe IIP/IIL. Some of the progenitors were
found to hold thick circumstellar (CS) materials or dust shells which led to
suppressed observed flux in optical bands but enhanced one in infrared (IR)
bands, e.g., SN 2012aw, SN 2017eaw, and SN 2023ixf. Among them, the dust mass
around the progenitor star of SN 2023ixf can even be comparable to that of
asymptotic giant branch stars, causing its colors in the IR bands to deviate
remarkably from those of ordinary RSGs. CS materials locating close to the
precursor star can also produce observational spectral features in young SNe
II, typically narrow emission lines in early spectra, the so-called “flash”
lines (Pooley et al., 2002; Quimby et al., 2007; Rui et al., 2019; Zhang et
al., 2023). Moreover, interaction of SN ejecta with CS materials can also
result in higher temperature and fast rise in the early light curves. Most
interestingly, the serendipitous capture of pre- and after-explosion of SN
2023ixf have witnessed that the resulted shock passed through the dust shell,
leading to a quick change of shock radiation from red to blue light (Li et
al., 2024). It is possible that the close-by CS materials are common for SNe
IIP/IIL, but the mass loss rate derived from their spectra or light curves are
far higher than that from their RSG progenitors. It remains unknown on how the
mass loss of RSGs is enhanced in their final years prior to explosion.

SN 2024ggi is another type II SN discovered within about 7 Mpc after SN
2023ixf over the past decade. It was discovered by ATLAS in the nearby spiral
galaxy NGC 3621 ($z=0.002435$) on Apr. 11.14, 2024 (Srivastav et al., 2024).
The earliest spectrum obtained at about 11 hours after discovery showed
prominent narrow emission lines of H, HeI, HeII, CIII, CIV and NIII (Zhai et
al., 2024), and fast evolution of these narrow features was revealed by later
high-resolution spectra (Pessi et al., 2024). X-ray emission was also detected
by the Follow-up X-ray Telescope (FXT) on board Einstein Probe (EP) (Zhang et
al., 2024b).

Based on the HST archival images, Srivastav et al. (2024) reported possible
detection of a red progenitor of SN 2024ggi. Yang et al. (2024) and Pérez-
Fournon et al. (2024) also reported the $griz$-, J- and Ks-band photometry at
the SN site, where the progenitor candidate is found to be bright in near-
infrared (NIR) bands.

In this letter, we present measurements of HST archived pre-explosion images
around the site of SN 2024ggi, including the progenitor candidate. We also
checked the $Spitzer$/IRAC observations, although they have lower spacial
resolutions. With these data, we analyzed the progenitor environment of SN
2024ggi and estimated the properties of its progenitor star. A distance
modulus to the SN of $\mu=$29.14$\pm$0.06 mag ($D_{\mathrm{L}}=$6.72$\pm$0.19
Mpc) (Tully et al., 2013) is adopted throughout this paper.

##  2 Observation Data and Reduction

###  2.1 HST Pre-explosion Images

We searched the pre-explosion HST images from Mikulski Archive for Space
Telescopes (MAST)111http://archive.stsci.edu/ and the Hubble Legacy Archive
(HLA)222http://hla.stsci.edu/, and found publicly available images in various
bands, covering the phases from late 1994 to 2003.

To get accurate positions of SN 2024ggi in the pre-discovery images, we made
use of a drizzled ACS/WFC F814W image in 2003 obtained from HLA as a pre-
explosion image, and an image combined from 3 unfiltered 3-second images
obtained by the Lijiang 2.4-m Telescope (LJT; Wang et al., 2019) on Apr. 12
2024 as a post-explosion image. We first chose 10 common stars that appeared
on the LJT and HST images and then got their positions in each image using
SExtractor (Bertin & Arnouts, 1996). A second-order polynomial geometric
transformation function is applied using the IRAF333IRAF is distributed by the
National Optical Astronomy Observatories, which were operated by the
Association of Universities for Research in Astronomy, Inc., under cooperative
agreement with the National Science Foundation (NSF). geomap task to convert
their coordinates in the post-explosion image to those in the pre-explosion
images. Based on the IRAF geoxytran task, this established the transformation
relationship between the coordinates of SN 2024ggi in the post-explosion image
and those in the HST pre-explosion images. The uncertainties of the
transformed coordinates are a combination of the uncertainties of the SN
position and the geometric transformation. The identified progenitor candidate
in the pre-explosion images is shown in Fig. 1(a). The point spread function
(PSF) of the progenitor candidate in the ACS-F814W image has a full width at
half maximum (FWHM) of $\sim$8.5 pixels ($\sim$0.43″). The locations of the
progenitor in other band images are obtained by either visually matching the
images to the F814W image or transforming from the F814W image using similar
coordinates transform procedures. In Fig. 1(b)$\sim$(c) we show the pseudo-
color images created from the HST pre-explosion images obtained in 1995 and
2003.

Figure 1: (a): The ACS-F814W image centered on the location the progenitor
candidate (marked by a red cross). The green ellipse shows the transformed SN
position with the size presenting the 1-$\sigma$ error
($\sigma_{\mathrm{RA}}$=0.087″, $\sigma_{\mathrm{Decl.}}$=0.118″);
(b)$\sim$(c): Pseudo-color images created from the HST pre-explosion images
obtained in 1995 and 2003. The progenitor candidate is marked by white ticks.
(d)$\sim$(e): The field of SN 2024ggi on the Spitzer CH1 and CH2 images. The
red circle in each panel is centered on the SN site with a radius of 1.2″,
which is the aperture of the photometry. North is up and east is to the left.

DOLPHOT444http://americano.dolphinsim.com/dolphot/ 2.0 was used to get
photometry of the progenitor on the pre-explosion images. The photometry is
performed on the bias- and flat-corrected C0M FITS images from the WFPC2/WFC
instrument, and bias-, flat-, CTE-corrected FLC FITS images from the ACS/WFC
and ACS/WFC instruments, all obtained from the MAST archive. Choosing the
F814W image as a reference image, DOLPHOT is run simultaneously on multiple-
band images taken on the same day. The progenitor is not detected in the
WFC3/UVIS images in all bands and the WFPC2/WFC images in F336W, F439W and
F658N bands. Magnitudes and their uncertainties of the progenitor candidate
are extracted from the output of DOLPHOT. The photometric results are listed
in Tab. 2.

Multiple observations were conducted by WFPC2 in F555W and F814W bands during
the period from Dec. 27 1994 to Feb. 12 1995. And observations on Feb. 3 2003
repeated several times in ACS F435W, F555W and F814W bands. For these images,
we also performed photometry on the combined images of each band. These
results are given in Tab. 3.

As shown in Tab. 3, from 1995 to 2003, the progenitor brightened by
0.99$\pm$0.02 mag and 0.80$\pm$0.39 mag in F814W and F555W bands,
respectively. As discussed in Appendix B, the variability in F814W band is not
due to any artificial effect but intrinsic to pulsational RSG progenitor,
while the result shown in F555W band is less credible.

A reddening of $E(B-V)$ = 0.113$\pm$0.023 mag can be inferred for the host
galaxy from the Na I D lines in the mid-resolution spectra of SN 2024ggi
(Zhang et al., 2024a, in Prep.). Including the Galactic reddening of
$E(\mathrm{B-V})=$0.072 mag (Schlafly & Finkbeiner, 2011), the total reddening
to SN 2024ggi is given as $E(B-V)=0.185\pm 0.023$ mag. The total extinction is
consistent with that derived by Pessi et al. (2024), which is $E(B-V)$ =
0.16$\pm$0.02. Applying the distance to NGC 3621 and the above total
reddening, we get absolute magnitudes in each bands. The progenitor is very
red with $M_{\mathrm{F555W}}=-1.71\pm 0.67$, $M_{\mathrm{F814W}}=-6.21\pm
0.08$ mag, F555W$-$F814W$=4.50\pm 0.66$ mag in 1995, and
$M_{\mathrm{F555W}}=-2.53\pm 0.26$, $M_{\mathrm{F814W}}=-7.20\pm 0.07$ mag,
F555W$-$F814W$=4.67\pm 0.25$ mag in 2003, respectively. Such a large color
index immediately reminds us of the most striking SN II in 2023: SN 2023ixf,
whose progenitor was found to have F555W$-$F814W$=4.28\pm 1.23$ mag, but with
a much fainter $M_{\mathrm{F814W}}$ of $-4.84\pm 0.05$ mag. Therefore, the
severely suppressed flux in blue bands is also likely due to that the
surrounding dust obscuring the progenitor.

Close inspections of the progenitor environment on the HST images reveals that
there are several bright stars near the progenitor, among which the closest
one is about 0.39″ away. Given a distance of 6.7 Mpc to NGC 3621, this
corresponds to a spacial distance of $\sim$12.7 pc. However, this neighbor
star appears much bluer (with a blackbody temperature of over 10,000 K) than
the progenitor candidate. The Legacy Survey pre-explosion $griz$ images (Yang
et al., 2024) have a resolution of 0.1″/pixel, but photometry (with
$g-r\approx 0$ mag) at the SN site is mainly attributed to the neighbor star
rather than the progenitor candidate.

###  2.2 Spitzer-IRAC observations and data reduction

The SN 2024ggi field in NGC 3621 was observed with the Spitzer Infrared Array
Camera (IRAC) in mid-infrared (MIR) bands before its explosion. The
observations were monitored by several programs covering from 2004 to 2019, by
PI R.Kennicutt (program ID: 159) and M. Kasliwal (program IDs: 10136, 11063,
13053, 14089). The level 2 post-BCD (Basic Calibrated Data) images were
obtained from the Spitzer Heritage Archive
(SHA)555http://irsa.ipac.caltech.edu/applications/Spitzer/SHA/, which were
reduced by the Spitzer pipeline and resampled onto 0.6″/pixel. A point-like
source can be detected in $CH1$ ($3.6\mu$m) and $CH2$ ($4.5\mu$m) images near
the site of SN 2024ggi with a $2\sigma$ threshold (Fig. 1 (d)$\sim$(e)).

Aperture photometry was performed on the pre-explosion images of the SN field
with an aperture radius of 2 pixels (1.2″). Aperture corrections were applied
following the IRAC Data Handbook. The level 2 post-BCD images have been
calibrated in an absolute surface-brightness unit of MJy/sr, which can be
transformed into units of $\mu$Jy/pixel2 by a conversion factor of 8.4616 for
the angular resolution of the IRAC images 0.6″/pixel. The measured fluxes in
CH1 and CH2 bands are listed in Tab. 4.

However, we caution that the PSF of the progenitor candidate has an FWHM of
about 2 pixels, i.e. $\sim$1.2″, which is about 5.6 times that of the star in
the HST images. In this radius range, several bright stars are found on the
HST images, and resolved photometry of the progenitor on the Spitzer images is
not possible (as shown in Fig. 6 and the magnitudes of the bright stars are
tabulated in Tab. 5). On the other hand, Pérez-Fournon et al. (2024) reported
photometric results of $m_{J}=19.74\pm 0.17$ mag and $m_{\mathrm{Ks}}=18.44\pm
0.30$ mag for the progenitor candidate using the pre-explosion images of VISTA
Hemisphere Survey (VHS, McMahon et al., 2013) with an aperture radius of 1″.
The VHS images have better spacial resolution than the Spitzer ones.

##  3 Data analysis

###  3.1 Infrared emission of the progenitor star

As noted in Sec. 2.2, measurements of the flux in the pre-explosion Spitzer
images at the SN site are probably contributed by several stars including the
progenitor of SN 2024ggi. Inspecting the colors and magnitudes of stars within
1.2″ around the progenitor candidate, we found that several stars may
contribute to the flux in the IR bands. As shown in Fig. 7, although the
progenitor is the brightest and reddest star, there are at least 2 other
bright stars with comparable red colors. Nonetheless, it is still possible to
estimates the contribution of the progenitor in IR bands. As discussed in
Appendix C, the measured NIR and MIR fluxes mainly come from the contribution
of the progenitor. The nearby stars contribute only $\lesssim$12% and
$\lesssim$30% to the total fluxes in NIR and MIR bands, respectively.

Phase-averaged flux in CH1 and CH2 bands are $f_{\mathrm{CH1}}=10.36\pm 0.96$
$\mu$Jy, $f_{\mathrm{CH2}}=9.25\pm 0.56$ $\mu$Jy, giving
$m_{\mathrm{CH1}}=18.56\pm 0.10$ mag, $m_{\mathrm{CH1}}=18.21\pm 0.07$ mag.
The corresponding absolute magnitudes are $M_{\mathrm{CH1}}=-10.62\pm 0.12$
mag and $M_{\mathrm{CH2}}=-10.96\pm 0.09$ mag, respectively. Then, after
subtraction of the field stars, the $M_{\mathrm{CH1}}$ and $M_{\mathrm{CH2}}$
values of the progenitor are $-10.43\pm 0.21$ mag and $-10.77\pm 0.12$ mag,
respectively. Using the results from Pérez-Fournon et al. (2024), we obtain
$M_{\mathrm{J}}=-9.47\pm 0.20$ mag and $M_{\mathrm{Ks}}=-10.72\pm 0.32$ mag
for the progenitor star. These absolute magnitudes and colors are well within
the range of RSGs (e.g. Yang et al., 2020).

###  3.2 Periodic variability in Mid-infrared and optical bands

As discussed in Sec. 3.1, the MIR flux measured from Spitzer CH1 and CH2
images is mainly contributed by the progenitor of SN 2024ggi. The Spitzer
light curves show prominent temporal variation (top panel of Fig. 2). We
searched for periodic variability by applying the Lomb-Scargle method (Lomb,
1976; Scargle, 1982) with the python routine LombScargleMultiband in the
package astropy.timeseries (VanderPlas et al., 2012; VanderPlas & Ivezić,
2015) on the MIR light curves. A period of 378.5$\pm$29.4 days (log$P\sim
2.58$) can be found by fitting the light curves in CH1 and CH2 bands
simultaneously. Amplitudes of the light curves are $\sim$0.49 mag and
$\sim$0.28 mag in CH1 and CH2 bands, respectively. This variability is most
likely due to radial pulsation of the progenitor star.

Figure 2: Periodic analysis of the Spitzer CH1 and CH2 light curves. Upper:
Light curves in CH1 and CH2 bands, the fitted periodic light curves with
$P=379$ days are plotted as solid lines; Lower: The phase-folded MIR light
curves overplotted with the HST magnitudes of the progenitor.

If we assume that the variation in MIR bands is attributed to the pulsation of
the progenitor of SN 2024ggi, with empirical period-luminosity ($P-L$)
relations of RSGs, we are able to estimate its absolute magnitudes in IR
bands. However, as noted by Xiang et al. (2024), dusty RSGs may deviate from
the common $P-L$ relation of RSG groups at wavelengths shorter than CH1 due to
possible dust absorption. Absolute magnitude in CH1 band is derived as
$-10.14$ mag or $-10.38$ mag if the $P-L$ relation of RSGs in M33 or M31 is
applied (see Tab. 6 and 7 of Ren et al., 2019). This value is very close to
that derived in Sec. 3.1. We also checked the magnitudes in other bands, and
found that they all obey the $P-L$ relation of RSGs. This suggests that the
progenitor of SN 2024ggi be a normal RSG in IR bands.

Temporal variation is also seen in HST optical bands (see Tab. 2). The HST
phases and magnitudes are plotted in Fig. 2. As shown in Fig. 2, changes in
optical bands are synchronized with those seen in the IR bands. The epoch of
observations in 2003 corresponds to a phase near 0.75, and the epochs in 1995
correspond to phases near the minimum of periodic variation. Therefore, the
amplitudes in optical bands are roughly the differences of the magnitudes
between 1995 and 2003, i.e., 0.80$\pm$0.39 mag and 0.99$\pm$0.03 mag in F555W
and F814W band, respectively. The variation amplitudes decrease with
increasing wavelengths, consistent with other long period variables (e.g.
Reiter et al., 2015; Yang et al., 2018).

###  3.3 Local environment of the progenitor star

The host galaxy of SN 2024ggi is a face-on spiral galaxy with an extended disk
(Koribalski et al., 2004). The site of SN explosion is 110″ ($\sim$3.6 kpc)
away from the galactic center. According to the metallicity measurement of NGC
3621, the local metallicity of SN 2024ggi has a range of
[Fe/H]=$-$0.18$\sim$0.16 with different methods adopted (Bresolin et al.,
2012).

When looking into the stellar population within 100 pc around the SN site in
Fig. 3, we found that the stellar population around the SN site seems to cover
a large age range. Compared with MESA Isochrones and Stellar Tracks (MIST;
Choi et al., 2016; Dotter, 2016), the bluest stars belong to a young stellar
population of $t=$10 Myr, but most stars are red and faint and belong to a
much older population of $t=$200 Myr. Such multiple age components were also
found in the surrounding stellar populations of some SNe IIP (Maund, 2017).
Among the red stars, the progenitor is the reddest and distinct from any
populations. Higher metallicity may solve this problem. With [Fe/H]=0.25, the
closest population to the progenitor has an age of 16 Myr, which is
approximately the life time of the progenitor of a type II SN. Thus, we assume
solar metallicity in our analysis below.

Figure 3: The extinction-corrected color and absolute magnitude diagram of
stars within 3″ (100 pc) around the progenitor. The progenitor is marked by a
red filled square in each panel. Left: F435W$-$F555W vs. $M_{\mathrm{F555W}}$.
Faint stars with $m_{\mathrm{F435W}}>$29 mag or $m_{\mathrm{F555W}}>$28 mag
are plotted as gray squares; Right: F555W$-$F814W vs. $M_{\mathrm{F814W}}$.
Faint stars with $m_{\mathrm{F555W}}>$28 mag or $m_{\mathrm{F814W}}>$28 mag
are plotted as gray squares. For comparison, MIST isochrones with [Fe/H]=0
(solid lines) and [Fe/H]=0.25 (dotted lines) at different ages are
overplotted.

###  3.4 Properties of the progenitor star

With the measured photometric data of the progenitor through optical to MIR
bands, we are able to fit the SED of the progenitor. First we check dust-free
stellar spectral models. The MARCS spectra models666https://marcs.oreme.org/
(Gustafsson et al., 2008; Van Eck et al., 2017) are compared with the SED of
the progenitor. As shown in Fig. 4, the SED of the progenitor can be well fit
by the spectral model with a relatively low temperature of
$T_{\mathrm{eff}}=3200$ K. The corresponding stellar radius is about 962
$\mathrm{R_{\odot}}$. Such a temperature, however, is relatively low among
other SN IIP progenitors (Smartt, 2015).

Figure 4: Upper: Spectral energy distribution of the progenitor of SN 2024ggi (black squares) and the best-fit DUSTY+MARCS model with log g = $-$0.5, $R_{\mathrm{out}}/R_{\mathrm{in}}$ = 2.0. A dust-free MARCS stellar model with [Fe/H]$=0$, $T_{\mathrm{eff}}=3200$ K is also plotted as a grey line. Lower: Location of the progenitor of SN 2024ggi on the HRD. Overplotted are the MIST stellar evolutionary tracks of [Fe/H]$=0$, $v/v_{\mathrm{crit}}=0.4$. The filled dots marked by the initial masses (in solar mass) represent the ending points of the MIST tracks. Table 1: MCMC fitting parameters for the SED of the progenitor of SN 2024ggi with DUSTY+MARCS models. The mass loss rate is calculated by assuming a wind velocity of 50 km s-1. The lower and upper limits are given as the values at 16%, 84% of the posterior probability distribution of the MCMC sampling. The last column presents the least $\chi^{2}$ of the best-fit model.  [Fe/H] | log g | $R_{\mathrm{out}}/R_{\mathrm{in}}$ | $T_{*}$ | $\tau_{V}$ | log($L$/L⊙) | $R_{d}$ | $R_{*}$ | $\dot{M}$ | $M_{\mathrm{w}}$ | $\chi^{2}$
---|---|---|---|---|---|---|---|---|---|---
|  |  | (K) |  |  | (1016 cm) | ($\mathrm{R_{\odot}}$) | (10-6 $\mathrm{M_{\odot}}$ yr-1) | (10-5 $\mathrm{M_{\odot}}$) |
0.00 |  $-$0.5 | 2 | 3290${}_{-27}^{+16}$ | 0.26${}_{-0.19}^{+0.40}$ | 4.92${}_{-0.02}^{+0.03}$ | 1.00${}_{-0.97}^{+30.53}$ | 887${}_{-26}^{+35}$ | 0.83${}_{-0.63}^{+1.63}$ | 7.18${}_{-6.97}^{+564.20}$ | 2.816
0.00 |  $-$0.5 | 10 | 3289${}_{-28}^{+19}$ | 0.22${}_{-0.16}^{+0.32}$ | 4.92${}_{-0.02}^{+0.03}$ | 1.45${}_{-1.40}^{+35.93}$ | 892${}_{-28}^{+35}$ | 1.01${}_{-0.76}^{+1.88}$ | 92.47${}_{-91.06}^{+4682.00}$ | 2.815
0.00 | 0.0 | 2 | 3276${}_{-28}^{+34}$ | 0.55${}_{-0.43}^{+0.76}$ | 4.95${}_{-0.03}^{+0.03}$ | 2.52${}_{-2.48}^{+44.10}$ | 926${}_{-33}^{+38}$ | 1.96${}_{-1.55}^{+3.36}$ | 63.72${}_{-63.31}^{+2278.00}$ | 4.255
0.00 | 0.0 | 10 | 3276${}_{-30}^{+35}$ | 0.40${}_{-0.32}^{+0.56}$ | 4.95${}_{-0.03}^{+0.03}$ | 2.59${}_{-2.51}^{+44.13}$ | 926${}_{-33}^{+39}$ | 2.05${}_{-1.70}^{+3.71}$ | 376.90${}_{-374.60}^{+10840.00}$ | 4.322

Similar to SN 2023ixf, the flash lines in the early spectra of SN 2024ggi
reveal possible presence of dust shell surrounding its progenitor star. We
then fit the SED with stellar spectral models with dust. The fitting method is
the same as in Xiang et al. (2024) (see also Kilpatrick & Foley, 2018; Wang et
al., 2021). First, we use DUSTY (Ivezic & Elitzur, 1997) to calculate the
output flux of a star with spectral models from the MARCS spectral library
with [Fe/H]=0. We assume that the dust is 100% graphite and a gas to dust
ratio $\psi=200$. The model parameters of DUSTY have optical depth of
$\tau_{V}=10^{-4}\sim 10$. The ratio of the outer and inner radius of the dust
shell $R_{\mathrm{out}}/R_{\mathrm{in}}$ = 2, 10. For RSGs, log g = $-$0.5 and
0.0 are adopted. The fitting results are reported in Tab. 1, and the best-fit
model (with least $\chi^{2}$) is shown in the upper panel of Fig. 4.

As shown in Tab. 1, models with log g = $-$0.5 have the minimum $\chi^{2}$.
According to the fitting, the progenitor has a radius $R_{*}=887_{-51}^{+60}$
$\mathrm{R_{\odot}}$, $T_{*}=3290_{-27}^{+19}$ K, and
log($L$/L⊙)$=4.92_{-0.04}^{+0.05}$.777Uncertainty of the distance is included
in the uncertainties of $R_{*}$ and log($L$/L⊙).  Comparing the location of
the progenitor with MIST single star evolutionary tracks on the
Hertzsprung–Russell diagram (HRD), the progenitor star is estimated to have an
initial mass of $13_{-1}^{+1}$ $\mathrm{M_{\odot}}$ (see the lower panel of
Fig. 4).

The optical depth $\tau_{V}\approx 0.3$ implies that the dust shell is
optically thin. Even though the fitted model can match the observation well,
we note that the posterior probability distribution of $\tau_{V}$ is maximized
at the lower limit of the model parameter grid. This indicates that dust has
very limited influence on the SED of the progenitor. The mass loss rate
yielded for the progenitor has an upper limit of 3$\times 10^{-6}$
$\mathrm{M_{\odot}}$ yr-1. This rate is much less than that derived from the
SN flash lines ($\sim 10^{-2}$ $\mathrm{M_{\odot}}$ yr-1; Jacobson-Galán et
al., 2024) or X-ray light curves ($\sim 10^{-5}$ $\mathrm{M_{\odot}}$ yr-1;
Yan et al., 2024, In Prep.). Similar to SN 2023ixf, this implies that the
progenitor of SN 2024ggi also had an enhanced mass loss prior to the
explosion.

##  4 Conclusion

SN 2024ggi is another nearby H-rich supernova which presents strong flash
lines in its early spectra after SN 2023ixf. The SN is located in the inner
disk of the spiral galaxy NGC 3621. We studied the progenitor candidate of SN
2024ggi with pre-explosion HST images in optical bands and Spitzer images in
mid-infrared (MIR) bands.

The progenitor star is clearly resolved on the pre-explosion HST images
obtained over 20 years before the explosion. The progenitor candidate is the
reddest star around the SN site and the brightest among the red stars. A
bright point-like source can be seen at the SN site on the Spitzer/IRAC images
in CH1 and CH2 bands. Although the progenitor can not be resolved from the
Spitzer/IRAC images, over 70% of the flux measured by aperture photometry is
contributed by the progenitor star. The MIR fluxes show periodic variation
with a period of 379 days, amplitudes of 0.49 mag and 0.28 mag in CH1 and CH2
bands, respectively. Similar temporal variation can be found in HST light
curves. Applying the period found from MIR light curves, variation amplitudes
in F555W and F814W bands are estimated to be $\sim$0.8 mag and $\sim$1 mag,
respectively. All these observational properties agree well with red
supergiants (RSGs).

Further study of the progenitor star with stellar spectral models resulted in
stellar parameters of $T_{*}\approx 3290$ K, $R_{*}\approx 887$
$\mathrm{R_{\odot}}$, and log($L$/L⊙)$\approx 4.92$. The possible dust shell
surrounding the progenitor is found to be very thin ($\tau_{V}\sim 0.3$),
implying a much thinner circumstellar shell than that of SN 2023ixf. The
derived mass loss rate is much lower than that deduced from the early
observations of the SN, indicating that the progenitor might have experienced
a significantly enhanced mass loss when approaching its death. The location of
the progenitor on the Hertzsprung–Russell diagram is consistent with stellar
evolutionary tracks with an initial mass of $13_{-1}^{+1}$
$\mathrm{M_{\odot}}$ at solar metallicity.

This work is supported by the National Natural Science Foundation of China
(NSFC grants 12288102, 12033003, 11633002, and 12303047) and the Tencent
Xplorer Prize. L.W. is sponsored (in part) by the Chinese Academy of Sciences
(CAS), through a grant to the CAS South America Center for Astronomy (CASSACA)
in Santiago, Chile. J.Z. is supported by the National Key R&D Program of China
with No. 2021YFA1600404, the National Natural Science Foundation of China
(12173082), the Yunnan Province Foundation (202201AT070069), the Top-notch
Young Talents Program of Yunnan Province, the Light of West China Program
provided by the Chinese Academy of Sciences, and the International Centre of
Supernovae, Yunnan Key Laboratory (No. 202302AN360001). We acknowledge the
support of the staff of the LJT. Funding for the LJT has been provided by the
CAS and the People’s Government of Yunnan Province. The LJT is jointly
operated and administrated by YNAO and the Center for Astronomical Mega-
Science, CAS.

##  Appendix A Photometric results of HST and Spitzer/IRAC images

Photometric results of the pre-explosion HST images of the progenitor star of
SN 2024ggi are presented in Tab. 2 and Tab. 3. The results on the Spitzer
images of the progenitor star are presented in Tab. 4.

Table 2: Photometric results of the pre-explosion HST images at the site of SN 2024ggi. All magnitudes are in the Vega system. Obs. date | Instrument | Filter | mag/limit |  1-$\sigma$ error
---|---|---|---|---
1994-12-27 | WFPC2/WFC | F814W | 23.264 | 0.050
1994-12-27 | WFPC2/WFC | F814W | 23.251 | 0.049
1995-01-04 | WFPC2/WFC | F555W | 27.807 | 0.865
1995-01-04 | WFPC2/WFC | F555W | 27.635 | 1.026
1995-01-04 | WFPC2/WFC | F814W | 23.276 | 0.050
1995-01-04 | WFPC2/WFC | F814W | 23.342 | 0.055
1995-01-15 | WFPC2/WFC | F555W | 28.957 | 2.429
1995-01-17 | WFPC2/WFC | F555W | 27.612 | 0.717
1995-01-20 | WFPC2/WFC | F555W | 27.411 | 0.575
1995-01-20 | WFPC2/WFC | F555W | 29.894 | 5.840
1995-01-24 | WFPC2/WFC | F555W | 30.127 | 6.698
1995-01-24 | WFPC2/WFC | F555W | 27.050 | 0.437
1995-01-24 | WFPC2/WFC | F814W | 23.275 | 0.049
1995-01-28 | WFPC2/WFC | F555W | 28.907 | 2.263
1995-01-28 | WFPC2/WFC | F555W | 27.627 | 0.696
1995-01-28 | WFPC2/WFC | F555W | 27.217 | 1.417
1995-01-28 | WFPC2/WFC | F814W | 23.367 | 0.139
1995-02-12 | WFPC2/WFC | F555W | 29.019 | 2.649
1995-02-12 | WFPC2/WFC | F814W | 23.252 | 0.047
1995-02-12 | WFPC2/WFC | F814W | 23.253 | 0.047
1995-02-25 | WFPC2/WFC | F555W | 27.982 | 1.150
2003-02-03 | ACS/WFC | F435W | 28.749 | 1.367
2003-02-03 | ACS/WFC | F555W | 27.899 | 0.706
2003-02-03 | ACS/WFC | F814W | 22.275 | 0.017
2003-02-03 | ACS/WFC | F435W | 27.793 | 0.578
2003-02-03 | ACS/WFC | F555W | 26.785 | 0.278
2003-02-03 | ACS/WFC | F814W | 22.309 | 0.017
2003-02-03 | ACS/WFC | F435W | 28.802 | 1.332
2003-02-03 | ACS/WFC | F555W | 27.149 | 0.368
2003-02-03 | ACS/WFC | F814W | 22.272 | 0.017
2003-02-03 | ACS/WFC | F814W | 22.289 | 0.017
Table 3: Photometric results of the progenitor candidate on the combined pre-explosion HST images. All magnitudes are in the Vega system. Detection limits are given at 5-$\sigma$. Obs. date range | Instrument | Filter | mag/limit |  1-$\sigma$ error
---|---|---|---|---
1994-12-27/1995-02-12 | WFPC2/WFC | F814W | 23.274 | 0.019
1995-01-04/1995-02-25 | WFPC2/WFC | F555W | 28.013 | 0.302
2003-02-03/2003-02-03 | ACS/WFC | F435W | 28.464 | 0.661
2003-02-03/2003-02-03 | ACS/WFC | F555W | 27.209 | 0.245
2003-02-03/2003-02-03 | ACS/WFC | F814W | 22.286 | 0.008
1995-01-04/1995-01-04 | WFPC2/WFC | F336W |  $>$24.7 |
1995-01-04/1995-01-24 | WFPC2/WFC | F439W |  $>$26.0 |
2009-01-17/2009-01-17 | WFPC2/WFC | F658N |  $>$22.8 |
2019-12-13/2019-12-13 | WFC3/UVIS | F336W |  $>$26.3 |
2019-12-13/2019-12-13 | WFC3/UVIS | F275W |  $>$25.5 |
2023-05-19/2023-05-19 | WFC3/UVIS | F300X |  $>$26.3 |
Table 4: The CH1\- and CH2-band aperture photometry on the pre-explosion Spitzer/IRAC images at the site of SN 2024ggi. An aperture radius of 1.2″ is adopted in the measurement. MJD | CH1 flux | $\sigma$ | CH2 flux | $\sigma$
---|---|---|---|---
(days) |  ($\mu$Jy) |  ($\mu$Jy) |  ($\mu$Jy) |  ($\mu$Jy)
53166.08 | 7.04 | 4.27 | 8.17 | 4.21
53152.49 | 3.98 | 3.54 | 4.62 | 3.39
56889.94 | 5.43 | 3.89 | 7.52 | 4.07
56769.85 | 13.36 | 5.36 | 9.10 | 4.31
56740.32 | 11.61 | 5.03 | 10.19 | 4.50
57111.58 | 15.04 | 5.53 | 12.11 | 4.84
57118.96 | 16.03 | 5.72 | 12.09 | 4.83
57139.42 | 18.08 | 6.13 | 11.66 | 4.77
57271.89 | 9.21 | 4.64 | 6.17 | 3.71
57484.19 | 13.10 | 5.23 | 10.79 | 4.61
57491.65 | 13.58 | 5.30 | 10.03 | 4.46
57506.37 | 15.88 | 5.69 | 10.95 | 4.63
57672.07 | 8.44 | 4.51 | 9.97 | 4.56
57847.87 | 13.38 | 5.29 | 11.14 | 4.70
58012.19 | 7.76 | 4.31 | 6.15 | 3.68
58260.46 | 15.94 | 5.80 | 11.54 | 4.73
58389.03 | 8.42 | 4.52 | 8.87 | 4.31
58598.27 | 14.37 | 5.50 | 9.72 | 4.34
58633.65 | 14.36 | 5.50 | 11.42 | 4.72
58749.71 | 7.05 | 4.26 | 11.11 | 4.81
58785.52 | 9.81 | 4.83 | 9.93 | 4.54

##  Appendix B Variability of the progenitor and field stars in HST bands

The magnitudes of the progenitor varied by about 1 mag from 1995 to 2003. To
examine whether this variability is intrinsic, we analyzed the properties of
the field stars around the SN site. Fig. 5 shows the magnitude variations of
the progenitor candidate and the stars within a radius of 5″ and in F555W and
F814W bands. In F555W band, the brightest stars (m$<$26 mag) have little
variation, while faint stars (including the progenitor) all show large
magnitude changes. Thus, the magnitude change in F555W may not be intrinsic
but due to incorrect zeropoints of the two images, or that measurements of
these faint objects are less reliable. In F814W band, the progenitor is one of
the two brightest stars, and only one of the other bright stars (m$<$24 mag)
also has large magnitude change. Thus, the variability seen for the progenitor
candidate in F814W and MIR bands should be true.

Figure 5: Magnitude changes of the surrounding stars within 5″ around the
progenitor candidate in F555W (upper) and F814W (lower) bands from 1995 to
2003. The red point in each panel represents the progenitor candidate of SN
2024ggi.

##  Appendix C Estimation of contribution of nearby stars in infrared bands

The Spitzer/IRAC MIR images and VHS NIR images have resolutions much lower
than that of the HST images. Photometry was done with an aperture radii of
1.2″ and 1.0″ on Spitzer/IRAC and VHS images, respectively. Besides the
progenitor candidate, several other bright stars are found within the
photometric aperture of the IR images. Thus, contribution from other stars
which were included in the aperture of IR images should be subtracted from the
measured IR fluxes for the progenitor.

As shown in Fig. 7, the progenitor is the brightest in F814W-band (marked by
blue squares). The magnitudes of these stars are listed in Tab. 5. Among these
bright stars, stars 1, 3 and 6 are quite blue, so we ignored their
contribution in IR bands. Stars 2 and 5 are fainter than the progenitor and
have slightly bluer colors. Thus, for these two stars, we assume that
contributions of the IR fluxes to their total fluxes should not be larger than
that of the progenitor. In other words, the magnitude difference between the
two stars and the progenitor in IR bands should not be less than the that in
the F814W band. Star 2 is 1.3 mag fainter than the progenitor in F814W. That
means, the flux of star 2 is less than $\sim$30% that of the progenitor in IR
band. Similarly, star 5 contributes to the emission of the progenitor by
$\lesssim$14% in IR bands. Other stars are too faint so that they contribute
little to the total IR fluxes. In conclusion, in CH1 and CH2 bands, the
progenitor’s contribution is 70%–100%, and in J and Ks bands, the progenitor’s
contribution is 88%–100%, since star 2 is not included in the VHS aperture.

Figure 6: Field around the progenitor candidate (marked by red circle) on the ACS-F814W image in 2003. The yellow circles mark the brightest 9 stars, and the red circle marks the progenitor. The white circle has a radius of 1.2″ centered on the progenitor, while the green circle has a radius of 1″. Table 5: Magnitudes and 1-$\sigma$ error of the bright stars on the HST/ACS pre-explosion images in 2003. Stars are sorted by the magnitude in F814W. Positions of the stars are shown in Fig. 6. Number | F435W | F555W | F814W
---|---|---|---
progenitor | 28.46$\pm$0.66 | 27.21$\pm$0.24 | 22.29$\pm$0.01
1 | 23.04$\pm$0.01 | 22.82$\pm$0.01 | 22.37$\pm$0.01
2 | 28.54$\pm$0.66 | 27.61$\pm$0.32 | 23.61$\pm$0.02
3 | 24.68$\pm$0.03 | 24.49$\pm$0.03 | 24.14$\pm$0.03
4 | 27.69$\pm$0.32 | 26.05$\pm$0.09 | 24.43$\pm$0.04
5 | - | 28.29$\pm$0.63 | 24.46$\pm$0.04
6 | 25.79$\pm$0.07 | 25.34$\pm$0.06 | 24.65$\pm$0.04
7 | 28.79$\pm$0.81 | 27.35$\pm$0.27 | 25.00$\pm$0.06
8 | 28.24$\pm$0.77 | 27.20$\pm$0.25 | 25.10$\pm$0.06
9 | - | 27.55$\pm$0.36 | 25.21$\pm$0.06
Figure 7: The observed colors and magnitudes of stars within 1.2″ around the
progenitor candidate of SN 2024ggi. The blue and black points are field stars,
and the red points are the progenitor candidate in each panel. Upper:
F435W$-$F555W vs. F555W on the combined 2003 image of HST; Middle:
F555W$-$F814W vs. F814W on the combined 1995 image; Lower: F555W$-$F814W vs.
F814W on the combined 2003 image.

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