# Considering the Single and Binary Origins of the Type IIP SN 2017eaw

K. Azalee Bostroem Steward Observatory, University of Arizona, 933 North
Cherry Avenue, Tucson, AZ 85721-0065, USA  LSSTC Catalyst Fellow  Emmanouil
Zapartas Institute of Astrophysics, FORTH, N. Plastira 100, Heraklion, 70013,
Greece  IAASARS, National Observatory of Athens, Vas. Pavlou and I. Metaxa,
Penteli, 15236, Greece  Brad Koplitz School of Earth & Space Exploration,
Arizona State University, 781 Terrace Mall, Tempe, AZ 85287, USA  Benjamin F.
Williams Department of Astronomy, University of Washington, 3910 15th Avenue
NE, Seattle, WA 98195-0002, USA  Debby Tran Department of Astronomy,
University of Washington, 3910 15th Avenue NE, Seattle, WA 98195-0002, USA
Andrew Dolphin Steward Observatory, University of Arizona, 933 North Cherry
Avenue, Tucson, AZ 85721-0065, USA  Raytheon Technologies, 1151 East Hermans
Road, Tucson, AZ 85706, USA

###### Abstract

Current population synthesis modeling suggests that 30-50% of Type II
supernovae originate from binary progenitors, however, the identification of a
binary progenitor is challenging. One indicator of a binary progenitor is that
the surrounding stellar population is too old to contain a massive single
star. Measurements of the progenitor mass of SN 2017eaw are starkly divided
between observations made temporally close to core-collapse which show a
progenitor mass of 13-15 $M_{\odot}$ (final helium core mass $M_{\rm
He,core}=4.4-6.0~{}M_{\odot}$ \- which is a more informative property than
initial mass) and those from the stellar population surrounding the SN which
find $M\leq 10.8~{}M_{\odot}{}$ ($M_{\rm He,core}\leq 3.4~{}M_{\odot}$). In
this paper, we reanalyze the surrounding stellar population with improved
astrometry and photometry, finding a median age of $16.8^{+3.2}_{-1.0}$ Myr
for all stars younger than 50 Myr ($M_{\rm He,core}=4.7M_{\odot}$) and
$85.9^{+3.2}_{-6.5}$ Myr for stars younger than 150 Myr. $16.8$ Myr is now
consistent with the helium core mass range derived from the temporally near
explosion observations for single stars. Applying the combined constraints to
population synthesis models, we determine that the probability of the
progenitor of SN 2017eaw being an initially single-star is 65% compared to 35%
for prior binary interaction. $85.9$ Myr is inconsistent with any formation
scenarios. We demonstrate that combining progenitor age constraints with
helium core mass estimates from red supergiant SED modeling, late-time
spectra, and indirectly from light curve modeling can help to differentiate
single and binary progenitor scenarios and provide a framework for the
application of this technique to future observations.

Stellar populations(1622), Binary stars(154), Type II supernovae(1731),
Stellar evolutionary models(2046), Late stellar evolution(911), Massive
stars(732)

††software: astropy (Astropy Collaboration et al., 2018), binary population
synthesis code binary_c (Izzard et al., 2004, 2006, 2009), KEPLER (Weaver et
al., 1978), MATCH (Dolphin, 2002, 2012, 2013), MESA (Paxton et al., 2011,
2013, 2015, 2018, 2019), SNEC (Morozova et al., 2015), STELLA (Blinnikov et
al., 1998, 2000, 2006)

##  1 Introduction

In the canonical picture of single-star evolution, massive stars (M$\gtrsim$8
$M_{\odot}{}$) are thought to end their lives as core-collapse supernovae
(SNe). These stars and their explosive deaths strongly influence galaxy
evolution through ionizing flux, chemical enrichment, energy feedback, and
star formation (e.g. Kobayashi et al., 2020; Vogelsberger et al., 2014; Wang
et al., 2015; Hopkins et al., 2018). In the single-star paradigm, the hydrogen
envelope of the most massive stars is removed through strong stellar winds
which increase with progenitor mass (see Smith, 2014a, and references
therein). It is therefore expected that those SNe that show hydrogen in their
spectra originate from relatively lower mass massive stars
($M<25-30~{}M_{\odot}$), the majority of which become red supergiants (RSG)
prior to explosion (e.g., Heger et al., 2003).

This picture is complicated by the fact that $>$50% of O and B stars are
expected to form in binary systems (Sana et al., 2012; Kobulnicky et al.,
2014; Moe & Di Stefano, 2017). Although the effect of binary mass stripping to
hydrogen-poor SN is widely accepted (e.g., Eldridge et al., 2013), it should
also play a significant role for hydrogen-rich SNe (e.g., Podsiadlowski et
al., 1992; Podsiadlowski, 1992; Justham et al., 2014). In fact, Zapartas et
al. (2019) find that 30-50% of the progenitors of hydrogen-rich SNe interacted
with a companion over their lifetimes with the majority of these interactions
either gaining mass through Roche-lobe overflow or through mergers. Although
the possible effects of these binary paths, such as their role in the
diversity of Type II SNe (e.g., Eldridge et al., 2018a) or the explosion of
stars with initial masses less than 8 $M_{\odot}$ (Zapartas et al., 2017), are
being explored, the majority of the current literature still focuses on
single-star origins for these events.

While it is clear that binary evolution affects the progenitors of hydrogen-
rich SNe, it is challenging to identify individual SNe which have been
affected by binary evolution. This is because the observable effects are often
similar to those that could be explained by single-star evolution. If binary
mass accretion or merging occurred early enough in the evolution of a Type II
SN progenitor, no circumstellar material due to that interaction is expected
to linger in the vicinity during the SN. Similarly, the extra angular momentum
of a spun up binary accretor (de Mink et al., 2013) is lost when the
progenitor expands to become a RSG. In addition, although merging may form a
stellar product with an unusual core to envelope mass ratio compared to single
stars, the position in the HR diagram of the RSG that the product eventually
becomes is largely unaffected (Farrell et al., 2020). Finally, the light
curves of hydrogen-rich SNe are easily explained by both single and binary
progenitors (Eldridge et al., 2018b; Morozova et al., 2018).

Although it is hard to determine the binary origin of a progenitor star from a
single observable, it is possible that the combination of observables can
paint a more complete picture. Binary mass gainers and mergers can form higher
core masses than the stars of the same initial mass (e.g., Podsiadlowski et
al., 1992; Justham et al., 2014) allowing stars with initial masses below
$\sim$8 $M_{\odot}{}$ to explode. As the evolution of these binary products
will still be dictated by the long timescales of their initial masses,
Zapartas et al. (2021) predicted the existence of luminous hydrogen-rich RSG
progenitors, with high preSN core masses, in older stellar populations than
expected from single-star evolution. They thus suggested this discrepancy
among different observational methods about the inferred progenitor properties
(i.e. its initial mass) could be an indirect but clear signal of the binary
history of a Type II progenitor.

SN 2017eaw exploded in NGC 6946, colloquially known as the Fireworks Galaxy
due to its high number of SNe and was classified as a Type IIP SN (Xiang et
al., 2017). At $<$ 10 Mpc, a multitude of techniques were used to measure its
zero age main sequence (ZAMS) mass including pre-explosion observations with
the Hubble Space Telescope (HST) and Spitzer Space Telescope of the RSG
progenitor (Van Dyk et al., 2019; Kilpatrick & Foley, 2018; Rui et al., 2019),
light curve modeling (Morozova et al., 2020; Goldberg & Bildsten, 2020; Szalai
et al., 2019), late time spectra modeling (Szalai et al., 2019; Van Dyk et
al., 2019), and analysis of the star formation history of its stellar
neighborhood (Williams et al., 2018; Koplitz et al., 2021). The comparison of
these masses is striking: for all methods which evaluate mass based on
observations of the star at the end of it’s life, a progenitor of 13-15
$M_{\odot}{}$ is found. However, both Williams et al. (2018) and Koplitz et
al. (2021) failed to find evidence of young ($\lesssim$ 33 Myr) stars present
in the immediate vicinity, placing the progenitor mass at $\leq
10.8$$M_{\odot}{}$. Based on the significant discrepancy of these inferred
properties of the progenitor of the Type IIP SN 2017eaw, Zapartas et al.
(2021) pointed this source as the best candidate for a progenitor of a
hydrogen-rich SN with history of binary mass exchange.

In this paper we re-analyze all of these measurements, with particular
attention to the age (and therefore initial mass) derived from the stellar
populations surrounding SN 2017eaw. In Section 2, we explain our rationale for
using final He core mass rather than ZAMS mass throughout this paper. We
present an overview progenitor masses presented in the literature, derive He
core masses from these ZAMS masses, and detail assumptions used throughout the
paperabout distance and extinction in Section 3. In Section 4 we discuss
previous binary modeling and Section 5 presents a new analysis of the age of
the stellar population surrounding the SN site. We use binary evolution models
to interpret these results in Section 6, setting tight constraints on the
nature of the progenitor. Finally, Section 7 summarizes our results.

##  2 Final Helium Core Mass of 2017eaw

Although initial (i.e. ZAMS) mass is typically used to characterize supernova
progenitors, in our analysis we focus on the final helium core mass (i.e.
preSN core mass; $M_{He,core}$) rather than initial mass for a number reasons.
Firstly, for SN progenitors that experience binary mass accretion or merging,
the initial mass loses its meaning and its significance as a predictive
evolutionary property for the final state of the progenitor. In addition, the
direct progenitor imaging and the nebular spectrum observational techniques
directly probe the preSN core mass instead of the (arguably more intuitive)
initial mass through the preSN luminosity which is tightly correlated to the
core mass, e.g. see Fig 2 of Smartt et al. 2009 or the oxygen abundance
Jerkstrand et al. 2014a. And finally, it reduces the dependency of our results
on the uncertain mass-loss rates of these stars and the uncertainties in
physical processes inside the star during its evolution (e.g., convective
overshooting). We thus argue that the final helium core mass is a more
appropriate property to characterize the Type II SN progenitors, being
applicable even for binary progenitors (Zapartas et al., 2021). For the
remainder of this paper the final helium core mass should be considered the
primary description of progenitor mass and we provide the ZAMS mass only to
facilitate comparison to the literature.

##  3 Methods

Our study combines many different analyses that place various constraints on
the possible progenitor of SN 2017eaw all using single star evolutionary
models. Observations that originate temporally close to explosion directly
measure the final state of the progenitor from which an initial mass is
inferred. Alternatively, calculating the age of the stellar population at the
SN site directly probes the age (and therefore initial mass) of the progenitor
system. In theory, if the SN and its surrounding stellar population are
modeled correctly (and the assumption of non-binarity is correct), these
methods should agree. In this section we review assumptions about metalicity,
distance, and extinction as well as both the initial and final mass
measurements in the literature. Masses are summarized in Table 1. From each
initial mass we give a final He core mass based on the models used to infer
the ZAMS mass. Finally, for all He core masses, we derive a self consistent
ZAMS mass using Equation 1 in this paper.

Final Helium | Initial Mass [$M_{\odot}$] | Method | Reference
---|---|---|---
core mass [$M_{\odot}$] | reported (derived from final He core) |  |
4.4 | 15 (13.4) | SN light curve | Morozova et al. 2020
$4.6^{a}$, 6.8, 7.7 |  $14^{a}$ (13.9), 19 (19.5), 22 (21.7) | SN light curve | Goldberg & Bildsten 2020
$4.8$ |  $13\pm^{4}_{2}$ (14.5) | RSG SED | Kilpatrick & Foley 2018
$5.1$ |  $13-15$ (15.3) | RSG SED | Van Dyk et al. 2019
$6.0$ |  $12\pm 2$ (17.6) | RSG SED | Rui et al. 2019
4.4 | 15 (13.4) | SN late-time Spectra | Szalai et al. 2019
4.4 | 15 (13.4) | SN late-time Spectra | Van Dyk et al. 2019
$\leq 3.4^{b}$ |  $\leq 10.8$ (10.9) | Population age | Williams et al. 2018; Koplitz et al. 2021
$4.7^{b}$ |  $14$ (14.3) | Population age | This study
|  |  |
Table 1: The final helium core mass of 2017eaw is shown in column 1, based on
the observational method in column 3 and its reference at column 4. The
reported initial mass at that study (and the initial mass derived from the He
core mass based on Equation 1) are shown in column 2. For comparison we show
the initial mass value inferred from population age methods, discussed in
Section 3.2.4. ($a$): Preferred mass when the progenitor radius from pre-
explosion observations was considered. ($b$): From PARSEC stellar models
(Bressan et al., 2012).

###  3.1 Preliminary Assumptions

Many of the methods used to measure progenitor mass rely on an estimate of
either pre or post-explosion luminosity which depend on both distance and
extinction estimates. Additionally, metallicity affects both the mass-loss
rates of massive stars and the structure and evolution of single and binary
stars. In this section we describe the values we adopt for distance,
extinction, and metallicity.

Based on oxygen abundance measurements of three H II regions close to SN
2017eaw (Gusev et al., 2013), Van Dyk et al. (2019) adopt a slightly subsolar
metallicity of $Z=0.009-0.01$ ($[Fe/H]=-0.2$ to $-0.1$) while other authors
assume solar metallicity. When possible, we will use the subsolar metallicity
value in the analyses presented in this paper, however, we do not attempt to
correct different progenitor analyses for the minor metallicity differences
assumed by different authors.

The distance to NGC 6946 has been derived many times with many different
methods that yield significantly different distances (see Van Dyk et al.,
2019, and references therein for a through discussion). However, recent
estimates have converged (Anand et al., 2018; Murphy et al., 2018; Goldberg &
Bildsten, 2020; Eldridge & Xiao, 2019). We adopt a distance of $7.73\pm 0.78$
Mpc ($\mu=29.44$) from Van Dyk et al. (2019) which falls near the middle of
the range of these converging measurements.

An accurate estimate of extinction plays a key role in measuring luminosity.
Calculating extinction for SN 2017eaw is not straight forward. NGC 6946 lies
near the galactic plane and thus has a relatively high Milky Way extinction.
Additionally, there may have been more host extinction along the line of sight
to SN 2017eaw prior to explosion than is seen after explosion. This is because
RSGs may lose mass in the years preceding explosion which affects pre-
explosion observations. However, the SN explosion itself can destroy this
dust, leading to a different post explosion host extinction (Kochanek et al.,
2012a, b; Fraser et al., 2012).

Post-explosion host extinction is often calculated from the equivalent widths
of the Na ID and diffuse interstellar band (5870 Å) at the host redshift.
Unfortunately for this event we are unable to measure the equivalent width of
the host absorption features in low resolution spectra because the low
recession velocity of NCG 6946 (40 $\mathrm{km\,s^{-1}}$; Epinat et al., 2008)
causes the host dust absorption features to be blended with the strong Milky
Way dust features. Nonetheless, several authors analyze the Milky Way and host
extinction simultaneously, arriving at a range of measurements of host
extinction: $E(B-V)$=0–0.3 mag (Kilpatrick & Foley, 2018; Rui et al., 2019;
Szalai et al., 2019; Van Dyk et al., 2019). The only analysis based on high-
resolution spectroscopy finds no evidence for host extinction (Van Dyk et al.,
2019). Thus, we adopt the Milky Way extinction as E(B-V) = 0.30 mag(Van Dyk et
al., 2019) and consider there to be no post-explosion host extinction.

For pre-explosion host extinction, Van Dyk et al. (2019) find $A_{V}=1.2-4.7$
mag depending on whether the temperature was constrained to typical RSG
temperatures. Kilpatrick & Foley (2018) find $A_{V}=1.264^{+2.6}_{-0.94}$ mag.
Given the consistency of these measurements, we adopt the pre-explosion
luminosity and temperatures from Kilpatrick & Foley (2018), Van Dyk et al.
(2019), and Rui et al. (2019), adjusting only the luminosities to the distance
of Van Dyk et al. (2019).

###  3.2 Progenitor Mass Constraints

####  3.2.1 Light Curve Constraints

Progenitor and SN properties can be identified by modeling a SN light curve
using a variety of methods from semi-analytic models to radiation hydrodynamic
modeling of stellar evolution and the SN explosion. In these models, care must
be taken to differentiate between ZAMS mass and final mass, especially in the
case of the analytic models. It should also be kept in mind that this method
is inferring an initial mass from an ejecta mass (which is degenerate with
explosion energy, nickel mass, and progenitor radius Goldberg et al., 2019;
Goldberg & Bildsten, 2020). We consider the computational models of Goldberg &
Bildsten (2020) and Morozova et al. (2020).

Goldberg & Bildsten (2020) evolve a grid models with Modules for Experiments
in Stellar Astrophysics (MESA Paxton et al., 2011, 2013, 2015, 2018, 2019) to
iron core-collapse varying progenitor ZAMS mass, surface rotation, mixing
length, core overshooting, and wind efficiency. The SN explosion and
subsequent evolution are modelled with STELLA (Blinnikov et al., 1998, 2000,
2006) with a variety of explosion energies. Using Equation 1 of their paper
which relates explosion energy and progenitor mass to luminosity, nickel mass,
plateau length, and progenitor radius, they identify a set of $M_{ZAMS}=$14,
19, 22 $M_{\odot}{}$ models which are consistent with the observed light
curve, nickel mass, and plateau length. These masses are not exhaustive but
are rather demonstrative of the range of masses allowed by the degeneracy of
light curve modeling, which allow for a continuous range of masses. When they
include the progenitor radius from Kilpatrick & Foley (2018), they find only
the model with $M_{ZAMS}=14$ $M_{\odot}{}$ is consistent with the observations
of SN 2017eaw at a distance of 7.54 Mpc.

Morozova et al. (2020) model SN 2017eaw with a $M_{ZAMS}=15$ $M_{\odot}$
progenitor from the KEPLER code (Sukhbold et al., 2016). The SN explosion and
evolution are modeled with the Supernova Explosion Code (SNEC; Morozova et
al., 2015). They find that the 15 $M_{\odot}$ model matches the observed light
curve and no other progneitor mass was explored. However, as discussed above,
Goldberg et al. (2019); Goldberg & Bildsten (2020) detail the degeneracy
between progenitor mass, explosion energy, and progenitor radius in such
fitting without additional information such as progenitor radius from pre-
explosion observations. In this case, the relationship between $M_{ZAMS}$ and
progenitor radius is fixed by the KEPLER modeling and only degeneracy in
explosion energy and mass were explored.

SN light-curve modeling is performed using stellar evolution models from which
helium core masses can be drawn. Morozova et al. (2020) use the non-rotating,
solar metallicity models of KEPLER models of Sukhbold et al. (2016), that have
a helium core mass of 4.4 $M_{\odot}$. Goldberg & Bildsten (2020) provide
final helium core masses for each of their models in Table 1, giving masses of
4.6, 6.8, and 7.7 $M_{\odot}$, for the $M_{ZAMS}=14,\,19,$ and $22\,M_{\odot}$
models, respectively.

####  3.2.2 Pre-explosion Spectral Energy Distribution Constraints

Another method that can be used to derive progenitor mass is the direct
modeling of the spectral energy distribution (SED) of the progenitor star
prior to explosion. From pre-explosion images, the luminosity and temperature
can be derived and used to find the ZAMS mass of the progenitor star.

The site of SN 2017eaw was observed extensively with both HST and the Spitzer
Space Telescope, providing measurements from the ultraviolet through the
infrared - an unusually complete SED. Kilpatrick & Foley (2018) derive
$\log(L/L_{\odot})=4.9\pm 0.2$ and $\log(T_{\star})=3550^{+450}_{-250}$K which
they compare to evolutionary tracks to infer
$M_{ZAMS}=13^{+4}_{-2}~{}M_{\odot}{}$. We note that when a coarse grid of
models is used to determine a progenitor mass, the uncertainty in the ZAMS
mass can be dominated by the grid spacing. The set of models considered in
this analysis were ranged from $M_{ZAMS}$=9–17 $M_{\odot}{}$ in increments of
1 $M_{\odot}{}$ and thus 13 $M_{\odot}{}$ should be considered an approximate
value. We further note that the distance that they use for this analysis
$6.72\pm 0.15$ Mpc is less than the distance used in this paper. A greater
distance would increase the luminosity, and the inferred progenitor mass. We
will account for this fact later in this section.

Van Dyk et al. (2019) perform a similar analysis on the progenitor SED, They
derive $L_{bol}\approx(1.2\pm 0.2)\times 10^{5}L_{\odot}{}$ and
$T_{eff}\approx 2500-3300$ K. They find that the bolometric luminosity is only
consistent with a progenitor with $M_{ZAMS}=15~{}M_{\odot}$ from a grid of of
models with initial masses: 12, 15, and 18 $M_{\odot}{}$.

Rui et al. (2019) use the SED to derive an effective temperature of $T=3550\pm
100$ K. A luminosity of $log(L_{bol}/L_{\odot}{})=4.88\pm 0.20$ is determined
by applying a bolometric correction (Levesque et al., 2006) to the K-band
magnitude, resulting in $M_{ZAMS}=12\pm 2~{}M_{\odot}{}$. Here again a coarse
grid of 9, 12, 15, and 20 $M_{\odot}{}$ models were used which limit the
precision of the progenitor mass derived.

Although RSG luminosity is typically translated to initial mass, it can be
used to derive helium core mass at the end of carbon burning (e.g., Smartt et
al., 2009; Farrell et al., 2020). We correct the luminosities derived from
Kilpatrick & Foley (2018) and Rui et al. (2019) to the distance of Van Dyk et
al. (2019), finding $\log(L_{bol}/L_{\odot}{})=5.02$ and
$\log(L_{bol}/L_{\odot}{})=5.17$, respectively. We then use Equation 1 from
Farrell et al. (2020) to derive helium core masses of
$M_{He,core}=4.8~{}M_{\odot}{}$ (Kilpatrick & Foley, 2018),
$M_{He,core}=5.1~{}M_{\odot}{}$ (Van Dyk et al., 2019), and
$M_{He,core}=6.0~{}M_{\odot}{}$ (Rui et al., 2019).

####  3.2.3 Nebular Spectra Modeling

An alternate method to measure the final mass of a SN progenitor is based on
the strength of the [OI] ($\lambda\lambda 6300,6363$) emission line doublet in
spectra taken 200-500 days after explosion (Jerkstrand et al., 2014b; Dessart
& Hillier, 2020). At this time the outer layers of the SN ejecta are
transparent, revealing the inner nucleosyntheic products created during the
evolution of the star. In particular, the oxygen mass, which can be derived
from the oxygen luminosity, is strongly related to the helium core mass which
itself relates to the ZAMS mass (Woosley & Weaver, 1995).

One way to derive progenitor mass from nebular spectra is to model the
spectroscopic evolution of the [OI] emission doublet starting from the
explosion of an evolved RSG. Using spectra taken 220, 435, and 491 days after
explosion, Szalai et al. (2019) find the oxygen luminosity of SN 2017eaw is
best matched by the $\mathrm{M_{ZAMS}=15}~{}M_{\odot}{}$ model of Jerkstrand
et al. (2014b). Although Szalai et al. (2019) uses a different distance and
extinction, the empirical scaling accounts for these differences and no
modification to the derived mass is needed for this paper. Van Dyk et al.
(2019) compare a different set of late-time spectra (taken on days 213.9,
245.9, and 415.2) of SN 2017eaw to the same models (Jerkstrand et al., 2014b)
and also find them to be most consistent with an
$\mathrm{M_{ZAMS}=15}~{}M_{\odot}{}$ progenitor. In both cases, these masses
are derived from a grid of models with initial masses: 12, 15, 19, and 20
$M_{\odot}{}$.

The nebular spectra of SN 2017eaw are evaluated using the models of Jerkstrand
et al. (2014b). These models use the non-rotating solar metallicity KEPLER
models from Woosley & Heger (2007) as input. and are included in Sukhbold et
al. (2016) where the helium core masses are tabulated in Table 2. The 15
$M_{\odot}$ model has a final helium core mass of 4.35 $M_{\odot}$ which we
use as the final helium core mass derived from nebular spectra.

In summary, when considering all of the available constraints on the final
state of the progenitor and the spectroscopy of the event itself, the most
likely He core mass of the SN 2017eaw progenitor is 4.4–6.0 $M_{\odot}{}$
($M_{ZAMS}=$13.4–17.6 $M_{\odot}{}$).

####  3.2.4 Stellar Populations Constraints

Over the past decade, there have been many successful studies of SN progenitor
ages evaluated through studies of the star formation history (SFH) of the very
nearby surrounding resolved stars (Badenes et al., 2009; Gogarten et al.,
2009; Jennings et al., 2012; Williams et al., 2014). Here we discuss previous
attempts by Williams et al. (2018) and Koplitz et al. (2021) to apply this
technique to SNe in NGC 6946. Williams et al. (2018) measured a possible spike
in star formation around 30 Myr, corresponding to an initial mass of
$8.8^{+2.0}_{-0.2}$ $M_{\odot}$ and He core mass of $\sim$2.7 $M_{\odot}$
using PARSEC stellar isochrones (Bressan et al., 2012). Koplitz et al. (2021),
meanwhile, did not find evidence for any significant young population,
although their upper limit was consistent with the other result at that age.

There were a few key differences in the analyses of Koplitz et al. (2021);
Williams et al. (2018). First, they differ in whether or not a widespread
background stellar populations was simultaneously fit when deriving an age
from the SFH. Simultaneously fitting the background populations while fitting
the SFH adds more weight to populations unique to the SN region. In Koplitz et
al. (2021), the background stellar population is modeled as a contaminating
population consisting of the stars in an annulus from just outside the SN
region (in this case 50 pc) to 1000 pc, and is scaled to the size of the SN
region before fitting. Williams et al. (2018) also adopted a closer distance
(6.8 Mpc), which could lead to an overestimate of the age of any detected
young stars. Finally, the selection criteria for which stars were included in
the analysis were not the same between the analyses. We discuss our reanalysis
optimized for SN 2017eaw in Section 5.

##  4 Inclusion of binary scenarios for Type II SN progenitors

Zapartas et al. (2019, 2021) studied the expected age of the Type II SN
progenitors, employing single and binary population synthesis models. They
find that the delay-times between the birth of a binary progenitors and its
eventual core-collapse is on average higher than single star progenitors. In
fact, binary progenitors can have delay-times that surpass the maximum age
threshold of single star progenitors of around $50$ Myrs, reaching to ages of
200 Myrs (Zapartas et al., 2017). The long delay-time is predominantly due to
the low initial mass of the SN progenitor. Even though it eventually evolves
to a higher mass via binary evolution, these low mass progenitors have longer
evolutionary timescales, which dictate the evolutionary timescale of the whole
binary system.

The main binary scenarios that lead to Type II SNe involve the increase of
mass of the SN progenitor, during either a stable mass accretion phase or
during a faster merging process with its companion (e.g., Podsiadlowski et
al., 1992; Zapartas et al., 2019). In both cases, the final binary product has
a higher total and more importantly helium core mass at the end of its life,
compared to the one it would form without any mass gain from its binary
companion. The increase of the core mass of these binary Type II progenitors
is expected to lead to an increase in their preSN luminosity. Thus,
observational methods that directly probe the preSN core mass or luminosity
would infer a higher initial mass for the progenitor if they consider only a
single star evolution. However, as mentioned above, these binary progenitors
would have longer delay-times compared to a single star of the same initial
mass (as inferred from the final mass).

Based on these findings, Zapartas et al. (2021) suggested that a discrepancy
between the inferred initial mass of the progenitor from observational methods
that directly probe the progenitor’s core in its preSN state and the age
estimates of its surrounding population could be an indication of a binary
progenitor111Note that in their analysis the scenarios for Type II SN
progenitors from partially stripped donor stars was not found as a dominant
one, although it has been investigated in works employing detailed stellar
models (Eldridge et al., 2018a, 2019). Still, the latter path is not expected
to affect the total lifetime of the progenitor, so it would not be able to
explain a inferred mass discrepancy as discussed here..

In Section 6 we consider both single and binary evolution channels to
interpret our observational results. For this, we use the same set of binary
population synthesis models of core-collapse SNe from single and binary
progenitors as in Zapartas et al. (2019, 2021). These models employ the binary
population synthesis code binary_c (Izzard et al., 2004, 2006, 2009), based on
Hurley et al. (2000) fitting formulae for single stars and taking into account
physical processes in binary systems (Hurley et al., 2002, including
metallicity of $Z=0.008$, very close to the value of the SN 2017eaw region, no
rotation, convective overshooting according to Pols et al. (1998), and wind
mass-loss prescriptions throughout the evolution as described in Zapartas et
al. 2017).

For our population synthesis models with the closest metallicity to SN2017eaw
site (Z=$0.008$) we recover a relation to derive single-star initial masses
from final helium core masses of

$\frac{M_{\rm He-core,final}}{M_{\odot}}=0.34\frac{M_{\rm
initial}}{M_{\odot}}+0.002\big{(}\frac{M_{\rm
initial}}{M_{\odot}}\big{)}^{2}-0.56,$ (1)

very similar to Eq.1 of Zapartas et al. (2021) for solar metallicity.

##  5 Reanalysis of the Surrounding Population

With the broad agreement of final progenitor mass from a variety of methods
and observations, we reexamine the age of the SN region to uncover the source
of the discrepancy. Given that both Williams et al. (2018) and Koplitz et al.
(2021) included SN 2017eaw as part of a sample analysis where a uniform
analysis technique was applied to all SN sites in the sample, we reanalyzed
this region, optimizing our technique for the specific characteristics of this
progenitor location. The results of our new age analysis are summarized in
Table 2. Details of the fitting technique are provided below.

To update the color magnitude diagram (CMD) fitting results, we used the
package MATCH (Dolphin, 2002, 2012, 2013) which attempts to reproduce an
observed CMD using PARSEC stellar isochrones (Bressan et al., 2012). MATCH
returns the best-fit star formation rate as a function of look-back time as
well as the upper and lower uncertainties on this rate. The SN progenitor is
then assumed to belong to the median population younger than a chosen age
limit. The full details of the fitting and age-inference technique are
described in Koplitz et al. (2021). Here we adjust the area on the sky from
which we take our sample, the photometric cuts applied to the catalog, the age
limits of the populations considered, and the inclusion or exclusion of a
background component in the fitting.

Additionally, while Koplitz et al. (2021) focused on stellar populations
younger than 50 Myr, binary evolution can produce Type II SNe from stellar
populations as old as 200 Myr (Zapartas et al., 2017). Unfortunately,
comparing model isochrones to our CMD in Figure 1, the sensitivity of our
photometry limits our analysis to 150 Myr rather than 200 Myr. Still, the
expected SN rate from all binary scenarios with delay-times of 150-200 Myr is
of the order of $3\times 10^{-6}$ Myr-1 $M_{\odot}^{-1}$ (see eq. A2 of
Zapartas et al. 2017), consisting around 1% of all SNe. Thus, we encompass the
vast majority of all possible single and binary progenitor scenarios within
the 150 Myr limit from photometry.

Note that in our analysis, we only take into account the possibility of a
binary interaction of the progenitor of SN 2017eaw, not considering the effect
of binary products on the surrounding stellar population when we infer the age
of the population (something independent of the origin of the progenitor
itself). Binary products are expected to make a host region of a Type II SN
look slightly bluer, and thus “younger” (Xiao et al., 2019; Schady et al.,
2019).

Figure 1: CMD of the stars within 50 pc of SN 2017eaw shown as yellow circles
with the background populations shown in gray scale. Overplotted are
isochrones between 10 and 200 Myr with blue (red) representing younger (older)
ages. The progenitor star is highlighted in the top right corner of the figure
and is shown as a star. The red arrow shows the impact of reddening on our
photometry.

We start with the photometry catalog of Koplitz et al. (2021); however, we
correct the astrometry since there was a slight misalignment in their
analysis. The offset was small enough that many of the same stellar
populations were included in both their fit and ours. Additionally, we
loosened the quality cuts to signal-to-noise ratios above 3.5 and sharpness
squared values below 0.5, which were designed to produce the best photometry
for the crowded regions of the disk. Since SN 2017eaw resides in the outer
disk of NGC 6946, DOLPHOT is able to make good measurements at its location
resulting in the low crowding values. In fact, the crowding cut was found to
be unnecessary as every object that passed our signal-to-noise and sharpness
cuts had crowding values less than 1 (i.e. good stars). Our new catalog
includes more stars and increases our sensitivity to any young population.

MATCH finds total extinction (MW + host) by applying three types of reddening:
AV, a universal extinction for all stars, dAV, a differential extinction
calculated for each star individually, and AVY, an additional differential
extinction applied to stars younger than 100 Myr old. The best-fit AV and dAV
provide the range of possible total extinctions for the region, i.e. between
AV and AV \+ dAV (+AVY for young populations). The details of the different
types of extinction used by MATCH can be found in Koplitz et al. (2021). Most
of our fits found an AV value of $\sim$1.4 with little dAV ($<$0.4). This is
slightly higher than the Milky Way contribution toward NGC 6946
($A_{V}=0.938$; Schlafly & Finkbeiner 2011), but is consistent with little to
no host contribution, similar to what other techniques have found (Kilpatrick
& Foley, 2018; Van Dyk et al., 2019).

To understand how nearby objects affected the our age estimates, we ran MATCH
on a variety of photometric catalogs derived using different extraction radii.
In this way we account for the possibility that the progenitor of SN 2017eaw
traveled away from its birth place as a result of an ejection from a binary
system. These runaway progenitors can travel typically $\sim$ 100 pc until
their own explosions (Renzo et al., 2019). We thus conservatively considered
radii ranging from 50 to 200 pc in steps of 50 pc to encompass the population
at the birth site of a runaway progenitor. Figure 3 shows the location of SN
2017eaw, indicating the 50, 100, and 200 pc radii used in our analysis with
white circles. A nearby ($\sim$200 pc) young star cluster is shown with a red
circle. In Section 6.1.1 we argue that this cluster is unlikely to be the
origin of the progenitor of SN 2017eaw. As it dominated our fits with radii
$\geq$150 pc, we did not consider the 200 pc radius result for the remainder
of our analysis.

Additionally, we fit the SFH considering all stars near the SN site equally as
well as down weighting those consistent with a background populations. When
MATCH simultaneously fits the background population, it models and down
weights a background population before determining the SFH of the remaining
population. Not fitting a background component allows us to determine how much
overall SF is present while the fits with the background allow us to isolate
SF unique to the region.

Finally, we consider the effects of the inclusion of the progenitor of SN
2017eaw in our final catalog. To determine how the progenitor impacts our
inferred ages, we fit the CMD of each radius without it. When we fit a
separate background population simultaneously, the median age did not change
from the fit with the progenitor. In the fits without the background
populations, the median age did not change whether or not the progenitor was
included; however, the uncertainties were smaller without the progenitor.

Radius | Num. of | 50 Myr | 50 Myr | 150 Myr | 150 Myr
---|---|---|---|---|---
$\mathrm{(pc)}$ | Stars | Background | No Background | Background | No Background
50 | 50 | — | $16.8^{+3.2}_{-1.0}$ | — | $85.9^{+3.2}_{-6.5}$
100 | 210 | — | $16.6^{+3.4}_{-4.0}$ | $84.1^{+5.0}_{-27.9}$ | $148.8^{+9.7}_{-7.5}$
150 | 558 | $15.0^{+5.0}_{-2.4}$ | $22.9^{+2.2}_{-10.3}$ | $93.0^{+7.0}_{-13.6}$ | $142.0^{+16.5}_{-29.8}$
|  |  |  |  |
Table 2: Summary of our stellar population fitting. Column (1) shows the
radius of the region being considered, centered SN 2017eaw, in units of pc.
Column (2) is the number of stars used in the fit. Columns (3) and (5) are the
median age, in units of Myr, when we simultaneously fit the SN and background
populations with a 50 and 150 Myr cut off, respectively. Fits with no SF at
ages younger the cutoff are shown as dashed lines. Columns (4) and (6) are the
median age when the background population is not considered a separate
population with a 50 and 150 Myr cut off, respectively.  Figure 2: SFHs
measured with a 50 pc exaction radius. The red lines show the SFH when the
background populations were simultaneously fit. The fit without the background
populations are shown in blue for those younger than 50 Myr and green for
those older. The top panel shows the most recent 150 Myr of the SFHs and the
bottom panel zooms in on the first 55 Myr, highlighting the young populations
measured. The black dotted box in the top panel indicates the limits of the
bottom panel plot.

In addition to fitting multiple radii and with and without a background
population, we consider both a young stellar population only (upper age limit
of 50 Myrs, consistent with limits single star evolution) and an additional
older stellar population (upper age limit of 150 Myrs, consistent with limits
from binary evolution). Our fits with a 50 pc extraction radius and including
a background CMD found no populations younger than 150 Myr. Not finding young
SF above the background level near the SN site is not surprising given that
the location is extremely sparse while NGC 6946 has a high SF rate, between
3.2 and 12.1 M⊙yr-1 (Jarrett et al., 2013; Eldridge & Xiao, 2019). Given that,
had this analysis been applied to a SN remnant, with no knowledge of SN type,
Koplitz et al. (2021) likely would have concluded that it was a Type Ia SN.
Care should be given to the use of background populations in spare regions and
highly SF galaxies.

Fitting without the background CMD shows that there are young populations near
SN 2017eaw, with the 50 Myr cut off finding a median population consistent
with progenitor age estimates from near-explosion modeling.

Even with a 100 pc radius, our fits with a background component continue to
find no populations younger than 50 Myr, though a burst in SF is seen at 84
Myr when we expand the ages considered to 150 Myrs. With a 50 Myr cut off, our
fit without a background component measures the same median age for both a 50
and 100 pc radius. Thus, this age component of the population appears to be
widespread, partially explaining why it is considered a background population.
At larger radii and older ages (150 Myr), when there is no background
component included, a 150 Myr old population always dominates the fit.

Our fits including a background component only detect SF younger than 50 Myr
with a 150 pc radius, though it is a small amount ($\sim$1 M⊙ kyr-1). The
median age ($15.0^{+5.0}_{-2.4}$ Myr, corresponding to an He core mass of
$\sim$$4.5-4.7$ $M_{\odot}$; Bressan et al. 2012) is consistent with our
estimate with smaller radii without a background population and those from
near-explosion. These are therefore the same population, but it appears to be
spread out over a large enough area that it is hard to detect as separate from
the background at smaller radii.

Our progenitor age remains consistent when we only consider the stars within
150 pc, though the median is older. It is not surprising that the median age
is slightly older when no background component is included. Koplitz et al.
(2023) showed that progenitor ages from fits without a background component
were older than fits with one, likely because the background population is
more likely to be comprised of older stars that have had more time to
disperse.

In addition to the optical data presented above, Tran et al. (2023) derived a
spatially resolved SFH map of NGC 6946 from HST/WFC3 stellar photometry in the
NUV bands F275W and F336W for ages younger than $\sim$30 Myr. They used a
quadtree algorithm to divide their NUV ($F275W$ and $F336W$) HST photometry
into grids based on stellar density, with the smallest grids in regions of
highest stellar density. SN 2017eaw lies primarily in one large grid region of
647 square arcseconds ($\sim$ 937 pc x 937 pc) since it is located in an area
of low stellar density. The median age of the young ($\lesssim$30 Myr)
populations in that region was found to be $12.6^{+20.0}_{-12.6}$ Myr,
consistent with the 4.4–6.0 $M_{\odot}{}$ He core mass ($M_{ZAMS}=$13.4–17.6
$M_{\odot}{}$) found from near-explosion observations and our population age
derived from optical observation. Due to the multipeaked nature of the SFH
used to derive the median age of this population, the spread in the
uncertainty, which is derived from bootstrapping the SFH, obtained is very
high. In addition, the youngest time bin of the star formation history goes
from 0 to 4 Myr, which contributes to the large lower limit on uncertainty.

Figure 3: Our $F606W$ image of the region around SN 2017eaw. The white rings
indicate our extraction regions with 50, 100, and 200 pc radii. The red circle
indicates the young stellar cluster that seems to dominate our fits beyond 150
pc.

Given that when the background is considered, no star formation is found
within 50 pc of SN 2017eaw when either maximum age is used and within 100 pc
of SN 2017eaw when the 50 Myr cut off is used, we find it necessary to
consider that SN 2017eaw may have come from the background population, which
does appear to contain a young component, as found in both the Tran et al.
(2023) map and our new analysis here. Thus, the most likely progenitor age is
16.8 Myr (corresponding to a $M_{ZAMS}=14$ $M_{\odot}$ and a He core mass of
$\sim$$3.7-4.7$ $M_{\odot}$; Bressan et al. 2012) from our fit to the 50 pc
region with no background component.

While it is tempting to consider the progenitor of SN 2017eaw a single star
and declare the discrepancy solved with the population found at 50 pc when no
background is considered using a 50 Myr cut-off, it is still possible that the
progenitor of SN 2017eaw was a binary belonging either to the younger
population found with the 50 Myr cutoff or the older population found within
the same radius but with a 150 Myr cut off. Next, we will examine the
constraints placed by these ages.

Figure 4: Correlation of preSN helium core mass with the age of a Type II
progenitor systems. Black line is the unique correlation for our single star
models at $Z=0.008$. The 2D probability density of binary progenitors (red
density map) spreads to higher ages compared to single stars (for the same
final helium core mass). For SN 2017eaw, we show with grey the constraints
between $4.4-6.0~{}M_{\odot}$ helium core mass from prior preSN imaging, light
curve modelling, and nebular spectra. The two possible population ages found
in this work for the 50 pc region around SN 2017eaw, $16.8^{+3.2}_{-1.0}$
($85.9^{+3.2}_{-6.5}$) Myrs, allowing for star formation up to 50 (150) Myrs
with no simultaneous fitting of a separate background component, are shown
with blue (green) vertical shaded regions (first line of Table 2). The dotted,
black line is the prior age estimate for 2017eaw from Williams et al. (2018)
of $\sim 33$ Myrs without error bars.

##  6 Possible progenitors

We consider the following scenarios for the progenitor of SN 2017eaw: an
isolated initially single star, an interacting binary system, and a runaway
star.

###  6.1 The single or binary progenitor scenario

Figure 4 summarizes the results of our binary population synthesis and can be
used as a theoretical framework for our finding with the new analysis of SN
2017eaw. There is a unique anti-correlation between the preSN helium core
masses of single stars and their lifetimes prior to core collapse (solid black
line) which is given by Section 6.1.

$\displaystyle\frac{M_{\rm He-
core,final}}{M_{\odot}}=123.71-298.69\log_{10}\left(\frac{t}{\rm{Myr}}\right)+$
$\displaystyle+289.26\left[\log_{10}\left(\frac{t}{\rm{Myr}}\right)\right]^{2}-\,\,$
(2)
$\displaystyle-128.46\left[\log_{10}\left(\frac{t}{\rm{Myr}}\right)\right]^{3}+21.71\left[\log_{10}\left(\frac{t}{\rm{Myr}}\right)\right]^{4},$

valid for ages between roughly 8-50 Myrs (typical Type II single star
progenitor lifetimes). Single star progenitors of higher core masses are
expected to have shorter lifetimes. This relation is equivalent to our
Equation 1 for initial masses, but again only for single star progenitors of
Type II SN.

For binary progenitors the same general trend of shorter lifetimes for higher
core masses can be seen too (red 2D density distribution). However, the
different evolutionary scenarios break the unique correlation, leading to a
spread towards higher delay-times between the birth of the original stellar
system and the SN event itself.

Figure 5: Probability of each evolutionary scenario as a function of measured
population age, for progenitors of Type II SNe with final helium cores between
$M_{\rm He,core}=4.4-6.0\,M_{\odot}$, corresponding to the inferred core mass
range of SN 2017eaw progenitor. The total probability of all binary scenarios
(red line) is subdivided into mass gainers (yellow), MS mergers (pink) and
reverse mergers (brown). The gray shaded region shows the probability of a
single star progenitors, as a complement of binary probability, and is
concentrated to ages between $11-18.2$ Myrs. In white regions, no scenario can
reproduce a progenitor with that age. The two possible population ages found
in this work for the 50 pc region around SN 2017eaw, $16.8^{+3.2}_{-1.0}$
($85.9^{+3.2}_{-6.5}$) Myrs, allowing for star formation up to 50 (150) Myrs
with no simultaneous fitting of a separate background component, are shown
with blue (green) vertical shaded regions (first line of Table 2). The former,
blue, falling on the high side of the single-star age regime and the latter,
green, is inconsistent even with the long delay-time of binary scenarios.

From studies of preSN images, light curve modeling, and late nebular spectra
we inferred a helium core mass between $4.4-6.0~{}M_{\odot}$ for the SN
2017eaw progenitor (Section 2; Table 1). Combining these values with our
binary population synthesis results, the previous age estimate of SN 2017eaw
from Williams et al. (2018) of $\sim 33$ Myrs clearly falls outside the tight
single star relation and very comfortably in the range of binary progenitors.

In this work, we have found more robust age estimates, with a burst of star
formation at either $16.8^{+3.2}_{-1.0}$ (when a 50 Myr maximum age is
applied; blue shaded vertical regions) or at $85.9^{+3.2}_{-6.5}$ Myrs (when a
150 Myr maximum age is applied; green shaded vertical regions). Here we see
that combining the final mass constraints with the low age derived from 50 Myr
cutoff, the progenitor is consistent with both single and binary scenarios.
However, the longer age derived from 150 Myr cutoff combined with the
constraints from the final mass is inconsistent with both the single and
binary scenarios.

To further quantify these results, in Figure 5 we show the probability of
different evolutionary scenarios of forming a Type II SN progenitor with a
$4.4-6.0~{}M_{\odot}$ final helium core, as a function of their delay time
from birth. Single star models that form a final helium core in the range
above have a delay time between $\sim 18.2$ and $11$ Myrs, respectively (gray
area).

According to our models we find around 65% of progenitors with observationally
inferred delay-times between $16.8^{+3.2}_{-1.0}$ Myrs originate from single
stars, although this age range is at the older end of their $11-18.2$ Myrs
range mentioned above. This likelihood is based on the fact that the majority
of Type II SNe are expected to originate from single star progenitors
(Podsiadlowski et al., 1992; Zapartas et al., 2019, the latter based on the
same set of calculations as in this study). The $16.8^{+3.2}_{-1.0}$ Myr age
range corresponds to the lifetime of single stars that form helium core masses
between $4.0-4.9M_{\odot}$ (using Section 6.1) and initial masses of $\sim
13-15~{}M_{\odot}$ (Figure 6, grey). These masses are very consistent the
inferred masses of previous studies (Van Dyk et al., 2019; Kilpatrick & Foley,
2018; Szalai et al., 2019; Morozova et al., 2020; Goldberg & Bildsten, 2020;
Rui et al., 2019) for SN 2017eaw and the helium core masses we derive from
these studies. In fact, for single stars, the age of the population constrains
the helium core mass to the a small range at the low end of the values
inferred from those studies, and could be used to break some of the degeneracy
in light-curve modeling even when no pre-explosion imaging exists.

Interestingly, even with an age estimate in this range, a binary history for
the progenitor cannot be excluded, still being a significant possibility with
$\sim 35\%$ of the scenarios involving binary interaction (red line,
combination of all binary scenarios). Progenitors of these channels also form
a helium core mass of $4.0-4.9~{}M_{\odot}$, $16.8^{+3.2}_{-1.0}$ Myr after
birth. However, in these cases, due to prior mass accretion or merging, the
star that eventually explodes originates from a broader initial mass range of
$\sim 11-16~{}M_{\odot}$ (Figure 6, red bars), having a peak of the
distribution close to the equivalent single star.

These binary scenarios include the accretion of mass onto the Type II
progenitor from an initially more massive binary companion that has already
exploded and ejected from the system (yellow), or, slightly more likely, the
merging with a companion (pink). In the case of mass accretors and in $\sim
1/3$ of the cases of mergers, the progenitor star gains mass while still on
its MS, becoming $\sim 15~{}M_{\odot}$. For the remainder of the merger cases,
the progenitor has already evolved towards becoming a giant before triggering
a common envelope phase and leads to the merging with a main sequence (MS)
companion. In all cases, the binary product is expected to continue its
evolution in isolation until death, leading to a final helium core within the
constraint and with a total delay-time only slightly higher than that of a
single star of the same mass.

Figure 6: Initial mass distribution of Type II SN progenitors, with final
helium core, $M_{\rm He,core}=4.4-6.0~{}M_{\odot}$ and an age estimate between
$16.8^{+3.2}_{-1.0}$ Myrs, similar to SN 2017eaw-like events. The normalized
distribution of all binaries (red solid line) is subdivided into binary
scenarios (mass gainers or MS mergers), and includes both lower and higher
mass progenitors than the normalized distribution of single star progenitors.

For ages above $\sim 18.2$ Myrs, the probability of a single star progenitor
for SN 2017eaw drops to zero. This is because, according to our models, a
single star massive enough to produce a final $M_{\rm
He,core}=4.4-6.0~{}M_{\odot}$ cannot exceed this delay-time. Without adopting
different physical assumptions (e.g. high rotation velocity, higher convective
overshooting and/or lower metallicity), longer delay-times can only be reached
from progenitors that gained mass or merged with a binary companion during
their evolution.

Progenitors that accrete mass through stable binary mass transfer (yellow
distribution) result in a peak of the probability of this scenarios at around
30 Myrs. Scenarios involving the merging of the two MS and/or giant stars
(pink) have a similar distribution of ages. Higher ages of Type II SNe would
be reached from progenitors experiencing a reverse merger. These involve the
merging of a stripped degenerate carbon-oxygen or oxygen-neon-magnesium core
which is engulfed by the initial secondary star when the latter evolves to
become a giant. The initial mass of one or both stars is below the canonical
minimum threshold for a core-collapse SN, of $\sim 8~{}M_{\odot}$. However,
even this scenario struggles to reach a delay-time as high as
$85.9^{+3.2}_{-6.5}$ Myrs that we find when using the 150 Myr limit for
maximum age, although some extremely low mass reverse mergers can fall within
the uncertainty range, with delay-times of $\gtrsim 70$ Myrs. This means that
delay times of longer than $\sim 80$ Myrs would not be expected for Type II SN
2017eaw-like progenitors (with final cores more massive than
$4.4~{}M_{\odot}$), even taking into account binarity. Potentially a
combination of other stellar assumptions (e.g., higher core mass and/or higher
rotation) for intermediate mass stars that eventually experience a reverse
merger to form the progenitor, could explain such a long age, but cannot be
modelled with our setup up.

####  6.1.1 Runaway Scenario

One way to account for a discrepancy between the age of a SN and its inferred
final mass is with the hypothesis that the progenitor has moved away from its
parent population over it’s lifetime. Massive stellar progenitors may be
kicked out of their parent population through dynamical interactions in young
stellar clusters (Poveda et al., 1967) or due to a prior SN in a binary system
(Eldridge et al., 2011; Renzo et al., 2019).

We investigate whether the progenitor of SN 2017eaw could have originated from
the cluster seen in Figure 3 (shown in red). Our fits without simultaneous
fitting of a background CMD, considered out to 200 pc (the distance to the
cluster), indicate the cluster age is $\sim$5 Myr. This age implies the
progenitor would had to have been moving $\geq$40 km s-1 if it was expelled
from the cluster, which is in the low-probability tail for a SN ejection
(Renzo et al., 2019) but not unreasonable for a dynamical ejection (e.g.,
Leonard, 1991). However, the ejected star would have a maximum age of the
cluster (5 Myr) which in all cases would be too young to form a RSG and a Type
II progenitor (e.g. Georgy et al., 2012). Although we cannot exclude the
possibility of a fine-tuned scenario where a binary system is dynamically
ejected from the cluster (e.g. Perets & Šubr, 2012) and later merges,
producing a H-rich SN with a He core mass of of $4.4-6~{}M_{\odot}$ at 5 Myrs,
the scenario is considered extremely unlikely and as there is no further
evidence for this, we do not focus on it. Thus, we conclude that the
progenitor has probably not been ejected from the cluster.

There is still the possibility for an ejection of a companion from a prior SN
in the field, and this was taken into account in the selection of 50, 100 and
150 pc radii of our analysis (Section 5), as they include the typical distance
of $\sim 100$ pc (Renzo et al., 2019) that runaway progenitors travel since
the prior SN ejection.

###  6.2  Ejecta masses

Figure 7: Normalized distribution of the estimated ejecta mass of Type II SN
progenitors, with final helium core masses, $M_{\rm
He,core}=4.4-6.0~{}M_{\odot}$ and an age estimate between $16.8^{+3.2}_{-1.0}$
Myrs, similar to SN 2017eaw-like events. We assume that a compact remnant mass
of $1.4~{}M_{\odot}$ and the rest of the final, preSN mass is ejected. The
normalized distribution of all binaries (red solid line) is subdivided into
binary scenarios (mass gainers or MS mergers), and includes a broader spread
of progenitors masses than the normalized distribution of single star
progenitors. We also add the best fit inferred ejecta mass for SN2017eaw
according to Goldberg & Bildsten (2020) (vertical dashed magenta line)

Our analysis is mostly focused on methods that probe final core mass and the
derived age of the progenitor. In principle light curve method gives an extra
constraint on the ejecta mass during the explosion, although this information
is degenerate with the nickel mass produced, the explosion energy and the
preSN progenitor’s radius (Goldberg et al., 2019).

In Figure 7 we show the estimated ejecta mass of the SN 2017eaw-like
progenitors, where we define the ejecta mass as the final mass minus the
compact remnant. We assumed a typical $1.4~{}M_{\odot}$ mass of a neutron star
for the compact remnant, and that the rest of the final, preSN mass is ejected
during the SN. We find that for single star progenitors, the ejecta mass is
expected to be roughly $10~{}M_{\odot}$, originating from a single star of
$13-15~{}M_{\odot}$ initial mass as shown in Figure 6. Binary progenitors show
predominantly similar but slightly higher values of ejecta masses, due to
gaining mass from their companion. For comparison we also we also show the
$\sim 13.64-1.4=12.24~{}M_{\odot}$ ejecta mass inferred for 2017eaw from
Goldberg & Bildsten (2020), which at first glance is fairly consistent with
both single and binary scenarios.

Note that for stellar models, the uncertainty of the wind mass loss rate,
especially during the RSG phase (e.g., Smith, 2014b; Beasor et al., 2020; Yang
et al., 2023) limits the accuracy of the predicted final envelope (and thus
ejecta) masses. In addition. for progenitors that experienced binary
accretion, the poorly constrained mass transfer efficiency (de Mink et al.,
2007) as well as the mass lost during the common-envelope phase (e.g. Ivanova
et al., 2013) for merger progenitors are extra sources of significant
uncertainty. Given these caveats, we consider most of our single and binary
models consistent with the inferred ejecta mass and refrain from opting in
favour of a scenario. Nevertheless, we can still rule out some rare extreme
binary channels with almost complete stripping during the merging phase or of
very high accretion during the stable mass transfer of the progenitor. Still,
ejecta masses can, in general, provide an important constraint for future
studies, especially focusing on sub-classes of Type II SNe, such as short-
plateau ones (Eldridge et al., 2019; Hiramatsu et al., 2021).

###  6.3 Metallicity

Our analysis was based on a stellar population of slightly sub-solar
metallicity ($Z=0.008$), which is close but slightly lower than the inferred
metallicity of the host region of SN 2017eaw, estimated around $Z=0.009-0.011$
(Van Dyk et al., 2019). Repeating our analysis for a simulation of solar-
metallicity stellar population ($Z=0.0142$) resulted in similar qualitative
conclusions. However, it is important to note that at solar metallicity, the
age of single stars with our minimum assumed final core of 4.4 is lowered to
$\sim 17$ Myrs, increasing the probability of SN 2017eaw being a binary
progenitor even in the age window of $16.8^{+3.2}_{-1.0}$ Myrs above the 35%
limit of our default analysis. As metallicity effects can be important for the
application of this method to other SNe, especially in significantly sub-solar
metallicity environments, we show the correlation of age with final helium
core mass, as in Figure 4, for various metallicities in Appendix A.

##  7 Conclusion

In this paper we have combined progenitor measurements from the stellar
population at the SN site, pre-explosion imaging, and post-explosion imaging
and spectroscopy. We place tighter constraints on the progenitor of SN 2017eaw
and resolve a discrepancy in the literature between progenitor masses measured
from observations near explosion and those that measure the initial progenitor
mass.

From modeling the RSG progenitor prior to explosion, modeling the SN light
curve, and modeling the late-time spectra of SN 2017eaw, a ZAMS progenitor
mass between 12 and 15 $M_{\odot}{}$ was collected from the literature (Van
Dyk et al., 2019; Kilpatrick & Foley, 2018; Rui et al., 2019; Morozova et al.,
2020; Goldberg & Bildsten, 2020; Szalai et al., 2019). Additional progenitor
mass constraints were provided from the literature analysis of SFH of the
region around SN 2017eaw from Williams et al. (2018); Koplitz et al. (2021)
who find $M_{ZAMS}\leq 10.8$ $M_{\odot}$ ($M_{\rm He,core}\leq
3.4~{}M_{\odot}$).. We then translated the ZAMS progenitor masses derived from
observations near explosion to helium core masses resulting in a helium core
mass range of $M_{\rm He,core}=4.4-6.0~{}M_{\odot}$.

We revisit the analysis of the stellar population surrounding SN 2017eaw
making a number of significant changes. We tune the quality cuts to this
specific region, resulting in more stars in the CMD than previous analyses. We
then examine the SFH with and without a background population fit
simultaneously with the SN region. While there continue to be no young
populations when a separate background population is simultaneously fit with
the SN population, when we use the full photometric catalog as the SN
population, we find an age of $16.8^{+3.2}_{-1.0}$ Myr when a maximum age of
50 Myr is applied. This age is confirmed using the recent UV SFH of this
region by Tran et al. (2023).

When an upper age limit of 150 Myrs, which is more consistent with binary
channels for SN progenitors, is applied, the age of the stellar population
increases to $85.9^{+3.2}_{-6.5}$. This age is inconsistent with a single star
scenario for SN 2017eaw and is extreme even for the binary scenarios we
consider for this event.

The previous results consider a region with a 50 pc radius centered on SN
2017eaw. We also explored the impact of expanding the radius of the region
considered out to 200 pc, however, we find that beyond 150 pc, we include star
clusters which drive the age of the population but are unlikely to be the
birth population of SN 2017eaw, even if it had been kicked out of its natal
cluster.

Finally, we combine the final helium core masses and the new population ages
with single and binary population modeling to consider the single and binary
progenitor channels allowed by our analysis. We find the younger age of
$16.8^{+3.2}_{-1.0}$ Myr has a 65% chance of coming from a single star
progenitor and a 35% chance of having a prior binary mass accretion or merging
phase, with the latter being slightly more probable. For the most probable
single star scenario, we find that the progenitor of SN 2017eaw originates
from a $\sim 4.0-4.9~{}M_{\odot}$ helium core mass ($M_{ZAMS}\sim
13-15~{}M_{\odot}$), which is consistent with those derived from other
methods. When considering binary channels, a star that exploded with the same
helium core mass could originate from a wider initial mass range of $\sim
11-16~{}M_{\odot}$.

While we consider it important to recognize the maximum age limit imposed by
binary evolution (200 Myr) rather than single star evolution (50 Myr), in this
case we find that the median stellar population when a maximum age of 150 Myr
is used is inconsistent with any formation scenario. We encourage future works
to consider both upper age limits.

Our analysis of the SN 2017eaw can be seen as a case study of identifying
potential Type II SN progenitors with a history of binary mass accretion or
merging and demonstrates the power of combining different observational
techniques that probe the initial and preSN properties of the progenitor. We
encourage the community to use the theoretically expected correlation of age
with helium core mass, shown in Figure 4, as a framework applicable for
identifications of potential binary progenitors of nearby Type II SN events
for which a population age can be derived as well as a near explosion mass. As
the correlation is slightly sensitive to the metallicity of the stellar
population where the progenitor originated from, we provide the similar panels
for different metallicities in the Appendix.

We thank the referee for their careful reading of the paper and thoughtful
comments. Additional thanks to Jared Goldberg and Eric Bellm for fruitful
discussions regarding the SN analysis in this paper and Leo Girardi for
generating lookup tables between age and He core masses for stars
$>$6$~{}M_{\odot}$. E.Z. and K.A.B. acknowledge the organizers of IAUS 361
Massive Stars: Near and Far at which this paper was conceived of over a very
productive breakfast. E.Z. acknowledges funding support from the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme (Grant agreement No. 772086), as well as the Hellenic
Foundation for Research and Innovation (H.F.R.I.) under the “3rd Call for
H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project
Number: 7933). B.K. and B.F.W. acknowledge support for this work by NASA
through grants GO-15216, GO-14786, and AR-15042 from the Space Telescope
Science Institute, which is operated by AURA, Inc., under NASA contract NAS
5-26555. This work was supported by a grant from the Simons Foundation (CCA
928261, B.F.W.). K.A.B. is supported by an LSSTC Catalyst Fellowship; this
publication was thus made possible through the support of Grant 62192 from the
John Templeton Foundation to LSSTC. The opinions expressed in this publication
are those of the authors and do not necessarily reflect the views of LSSTC or
the John Templeton Foundation.

##  Appendix A Age vs core mass for different metallicities

To facilitate the application of this method to other SNe, in potentially
different metallicity environments, we provide Figure 4 for a range of
metallicities ($Z=0.0002-0.02$). For lower metallicities the lifetime of the
Type II progenitors, both singles and binaries, is longer. This is
predominantly due to the fact that at lower-Z, star of fixed final core mass
originate from lower initial masses, with longer evolutionary timescale (Pols
et al., 1998). Also for $Z=0.02$ we do not find Type II SN progenitors with
final helium-core masses higher than $\sim 8.5~{}M_{\odot}$, as the enhanced
wind mass loss prevents progenitors to form more massive cores.

Figure 8: Same as Figure 4, for different metallicities. Lower metallicity
shifts the distribution towards longer ages. For Z=0.02, due to stronger
stellar winds, there are no giant Type II progenitors with final core mass
above $\sim 8.5~{}M_{\odot}$.

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