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Detection of Late-Time Circumstellar Interaction of SN 2017eaw in NGC 6946

SN 2017eaw, the tenth supernova observed in NGC 6946, was a normal Type II-P supernova with an estimated 11 - 13 Msun supergiant progenitor. Here we present nebular phase spectra of SN 2017eaw at +545 and +900 days post-max, extending approximately 50 - 400 days past the epochs of previously published spectra. While the +545 day spectra is similar to spectra taken between days +400 and +493, the +900 day spectrum shows dramatic changes both in spectral features and emission line profiles. The Halpha emission is flat-topped and box-like with sharp blue and red profile velocities of ~ -8000 and +7500 km/s. These late-time spectral changes indicate strong circumstellar interaction with a mass-loss shell, expelled ~ 1700 years before explosion. SN 2017eaw's +900 day spectrum is similar to those seen for SN 2004et and SN 2013ej observed 2 - 3 years after explosion. We discuss the importance of late-time monitoring of bright SNe II-P and the nature of pre-supernova mass-loss events for SN II-P evolution.


INTRODUCTION
Supernovae (SNe) play an important role for chemical enrichment of the interstellar medium and in their host galaxies evolution including the creation of neutron stars and stellar-mass black holes. Core-collapse SNe arise from the death of massive stars (≥ 8 M ) and those that have hydrogen in their spectrum are classified as Type II events (Filippenko 1997;Arcavi 2017;Gal-Yam 2017).
Further sub-classifications are based on light curve evolution. Historically, Type II-plateau SNe (SNe II-P) have a plateau phase following peak brightness which lasts of order 2 -3 months before a linear decline, while Type II-linear SNe (SNe II-L) decline linearly almost immediately after reaching peak brightness. However, recent studies have suggested a more continuous distribution of Type II SNe based on light curve decay rates rather than a bimodal distribution, where the length plateau is related to the mass of the progenitor, with lower mass progenitors having longer plateaus (Anderson et al. 2014;Valenti et al. 2016;Gutiérrez et al. 2017).
Archival images showing the progenitors of supernovae prior to explosion have led to the identification of red supergiants (RSGs) for the progenitors of SNe II-P (e.g. Van Dyk et al. 2003;Smartt et al. 2004;Fraser et al. 2012Fraser et al. , 2014Maund et al. 2014a,b). The initial masses of these RSG progenitors are typically inferred to be between 9.5 -16.5 M (e.g., Smartt et al. 2009;Smartt 2015). This mass range is supported by models of SN II-P nebular phase spectra (Jerkstrand et al. 2012(Jerkstrand et al. , 2014. As SNe evolve, they become more optically transparent due to decreased ejecta density, which leads to an increasingly emission-line dominated nebular phase starting ∼ 150 -200 days after peak brightness. This nebular phase can provide valuable insights into the kinematic and elemental properties of a SN's ejecta, as well as information about the SN's circumstellar (CSM) environment. Bright supernovae are especially valuable for these studies as they can be observed longer in the nebular phase.
One of the brightest recent SN II-P was SN 2017eaw in NGC 6946, the 10th supernova in this galaxy in the past century. SN 2017eaw was discovered on 2017 May 14.238 (UT) (Wiggins 2017;Dong & Stanek 2017) and reached a peak V magnitude of 12.8. Its light curve and spectroscopic evolution followed that of a normal SN II-P (Cheng et al. 2017;Xiang et al. 2017;Tomasella et al. 2017;Tsvetkov et al. 2018;Szalai et al. 2019;Van Dyk et al. 2019;Buta & Keel 2019).
Archival Hubble Space Telescope (HST) and Large Binocular Telescope images of NGC 6946 enabled the identification of SN 2017eaw's progenitor as an estimated 11 -13 M RSG Kilpatrick & Foley 2018;Johnson et al. 2018;Rui et al. 2019). In addition, archival Spitzer imaging also showed that the progenitor was surrounded by a dusty T = 960 K shell at 4000 R (Kilpatrick & Foley 2018).
Late-time, nebular phase CSM interactions have been reported for a few SNe II-P. The optical signature of a SN's interaction with surrounding CSM is the appearance of a box-like or horned Hα emission feature as the outer layers of hydrogen-rich ejecta are heated by radiation from the forward shock colliding with the CSM (Chevalier & Fransson 2003Milisavljevic et al. 2012). Box-like or flat-topped emission profiles result from ejecta colliding with shells of CSM, while horned profiles result from collisions with CSM disks (Gerardy et al. 2000;Jerkstrand 2017). The timescale at which the collision occurs allows one to determine the radius of the CSM shell or disk, while the Hα velocity width gives information about the CSM density, as denser CSM will decelerate the ejecta more resulting in narrower emission (e.g., Kotak et al. 2009;Andrews & Smith 2018).
In this paper we present optical spectroscopy of SN 2017eaw at +545 and +900 days after explosion. We found significant changes in spectral evolution between these two epochs indicative of late-time ejecta-CSM interaction. The observations are presented in §2 with the results and discussion presented in §3. Our conclusions are summarized in §4.

OBSERVATIONS
As part of an optical survey of recent core-collapse SNe in nearby galaxies, we obtained photometric and spectroscopic observations of SN 2017eaw in NGC 6946. Using the Hiltner 2.4m telescope at MDM Observatory equipped with the Ohio State Multi-Object Spectrograph (OSMOS; Martini et al. 2011) and an ITL 4064 × 4064 CCD, R-band photometry was obtained on 7 November 2018, 14 December 2018 and 3 May 2019. Additional photometry observations using a filter matching the HST WFPC2 F675W filter were obtained on 30 September and 28 October 2019. The F675W is a broadband red continuum filter covering the majority of the same emission features as a standard R band filter but with greater sensitivity of the weak, broad [Ca II] and [O II] emission blend. Below we treat F675W images as roughly equivalent (±0.25 mag) to images taken in R band. Observations were reduced using the OS-MOS 1 imaging reduction pipeline in Astropy (Astropy Collaboration et al. 2013. Standard methods of aperture photometry were followed in combination with data from Buta & Keel (2019) to calibrate our images and measure magnitudes of SN 2017eaw.
Optical spectra of SN 2017eaw were taken on 7 -9 November 2018 using OSMOS and a 1 wide slit with a red VPH grism. Exposure times were 3000 s or 4000 s. Observations were reduced using the OSMOS spectral reduction pipeline in Astropy (Astropy Collaboration et al. 2013 and calibrated using Hg and Ne lamps and spectroscopic standard stars (Oke 1974;Massey & Gronwall 1990). To improve the signal to noise of the final spectrum, three individual observations were combined to a single 11,000 second exposure. We adopted 4000 5000 6000 7000 8000 9000 Rest Wavelength (Å)  There was little evidence of CSM interaction for SN 2017eaw in all previously published results, which cover SN 2017eaw's evolution up to day +594. A small and brief magnitude increase was observed in the optical light curve of SN 2017eaw between 6 -10 days after explosion, similar to that observed in SN 2012aw, which has been attributed to CSM interaction (Szalai et al. 2019). There was also a weak narrow Hα emission feature which only appeared in the 1.4 day spectrum (Rui et al. 2019).
The +545 day nebular phase spectrum of SN 2017eaw shown in Figure 1  Further evidence for dust in SN 2017eaw's ejecta is indicated by Spitzer infrared observations up to +560 days post explosion, from which dust masses of 10 −4 M of silicate dust and 10 −6 M carbonaceous dust were estimated (Tinyanont et al. 2019). This amount of dust is 100 to 10,000 smaller than model predictions for SNe II-P. However, more dust may form over time and/or the dust may have formed quickly and currently be obscured in optically thick regions so that the observed dust mass would increase as the ejecta becomes optically thin (Tinyanont et al. 2019).

Ejecta-Circumstellar Interaction
Our +900 day spectrum shown in Figure 1 (2019) Figure 2. The boxy Hα profile is strong evidence of ejecta-CSM interaction. The emission extends between approximately −8000 and +7500 km s −1 . The fact that this box-like feature is so broad (HWZI ∼ 8000 km s −1 ) suggests that the highest velocity ejecta are running into high density CSM before the ejecta have become optically thin. Further, the flat-top of this feature indicates the CSM is a thick shell rather than a disk or torus which would have horn-like emission profiles that with increased emission at the highest velocities (Gerardy et al. 2000;Jerkstrand 2017).
The larger bumps in the otherwise flat-topped Hα profile appear to be real as they are stronger and wider than simply noise on top of the flat emission profile. These features may indicate either clumpy ejecta interacting with a thick shell of CSM material, or smooth ejecta interacting with a clumpy shell of CSM emission (Gerardy et al. 2000;Jerkstrand 2017).
We can constrain the approximate epoch for ejecta-CSM interaction via the R-band light curve evolution, as shown in Figure 3. The late-time nebular phase R-band evolution between days +290 and +564 followed a decay rate of 1.469 ± 0.031 mag (100 d One can calculate the electron density, n e , of the ionized hydrogen ejecta, from the recombination time τ r = (α A n e ) −1 , where α A is the recombination coefficient of hydrogen. Assuming a temperature of 10, 000 K, a recombination time of 100 days, the electron density is 2.77 × 10 5 cm −3 . We choose a recombination time of 100 days, as the light curve data indicates a flattening between day +581 and +721. However, as we have no spectral data between day +545 and +900, the recombination time could be somewhat longer.
We can also estimate the mass within the ionized hydrogen using where m p is the mass of the proton, F c (Hα) is the extinction corrected Hα flux, hν Hα is the energy of an Hα photon, and α ef f Hα (H 0 , T e ) is the effective recombination coefficient of Hα (Melnik & Copetti 2013). Using the electron density found from recombination, the extinction corrected flux of 2.3 ± 0.3 × 10 −14 erg s −1 cm −2 , a distance to NGC 6946 of 7.73±0.78 Mpc (Van Dyk et al. 2019), we estimate ∼ 1.9 M of ionized hydrogen ejecta. This estimate is in agreement with SNe II-P/II-L hydrogen mass envelope estimates which range from 1 -10 M (Cappellaro & Turatto 2001), while more recent classifications have defined SN II-P explosions as having hydrogen envelope masses as low as M H ≥ 0.3 M (Limongi 2017). This is a lower limit on the mass of the hydrogen envelope as we assumed the entire hydrogen envelope was fully ionized to make the initially density estimate. If, however, it is only partially ionized, then the mass of the hydrogen envelope would be higher.

Similarities to SN 2004et and SN 2013ej
A few other SNe II-P observed late in the nebular phase have also shown box-like Hα profiles indicating ejecta-CSM interactions.

2018; Sollerman et al. 2019). Box-like emission profiles
were first observed on day +823 for SN 2004et, on day +807 for SN 2013ej (Mauerhan et al. 2017), and on day +1153 for iPTF14hls (Andrews & Smith 2018).   (Mauerhan et al. 2017) and iPTF14hls, (not shown in these figures) had a HWZI of ∼ 1500 km s −1 (Andrews & Smith 2018). We note that SN 2017eaw's boxy Hα emission also resembles the boxy emission from the Type IIb SN 1993J at day +975 (Matheson et al. 2000a,b) as shown in Figure 6. Compared to SN 2017eaw, SN 1993J's oxygenrich ejecta emission are weaker compared to its Hα emission. For SN 1993J the boxy Hα emission was clearly observed as early as day +300 (Finn et al. 1995), when [O I] and Hα emission were equal in strength. The [O I] emission faded more rapidly compared to the Hα emission in SN 1993J. The Hα emission of SN 1993J is slightly wider than that observed for SN 2017eaw, but also relatively flat. We note that for SN 1993J the box-like Hα emission feature has been modelled having significant contribution from [N II] at these late-times due to We now focus on comparing SN 2017eaw with SN 2004et and SN 2013ej which show comparable expansion velocities and time frames for CSM interaction. Unlike what is observed for both SN 2004et and SN 2013ej, the Hα profile of SN 2017eaw appears virtually flat, with no hint of decreased flux on the receding red side compared to those on the near blue facing side (e.g., Mauerhan et al. 2017). This suggests that there is not a significant amount of dust in the CSM or ejecta which would absorb the emission from the far side of the supernova. Furthermore, SN 2017eaw does not show a multi-peaked Hα profile like that observed for SN 2004et and SN 2013ej (Kotak et al. 2009;Mauerhan et al. 2017).
The Hα luminosity of SNe II-P can be used to determine the ejecta nickel mass based on the well-modeled exponential decay rate in the nebular phase (e.g., Chugai 1990). Following the plateau phase, the Hα luminosity of SN 2017eaw, SN 2004et and SN 2013ej exponentially declined until CSM interaction is observed in their spectra, between +800 and +900 days, see Figure 7. The Hα luminosities were measured from previously published spectra of SN 2004et (Sahu et al. 2006;Kotak et al.  the Hα flux at +900 days is about 90 percent of the flux at +545 days, which does not follow the exponential decay predicted and previously observed throughout the rest of the nebular phase. Infrared (IR) studies of SN 2017eaw between days +100 to +600 showed a similar evolution to SN 2004et which experienced an IR rebrightened at day +1000 shortly after showing CSM interaction in the optical (Tinyanont et al. 2019). SN 2013ej also showed a rebrightening around +800 days, approximately the same time as the broadening and flattening of the Hα profile (Mauerhan et al. 2017). The IR brightening in SN 2004et and SN 2013ej cannot be explained solely with IR echos from preexisting dust in the CSM. The more likely scenario is that the MIR increase is from heated dust formed in the CDS as a result of the ejecta-CSM collision (Kotak et al. 2009;Mauerhan et al. 2017). Follow-up IR imaging of SN 2017eaw would likely show a similar rebrightening as the ejecta interacts with the CSM.

Characteristics of the Circumstellar Material
We can estimate the radius for the CSM shell surrounding SN 2017eaw by assuming the maximum observed ejecta velocity at day +900 has not greatly changed since the start of CSM interaction. The radius of the CSM shell is then given by r shell = vt, where v is the ejecta velocity and t is the date of initial ejecta-CSM interaction. From R-band photometry, the ejecta-CSM interaction occurred on or before day +721, and, and using the Hα emission velocity from our +900 day spectrum as the velocity of ejecta 8500 km s −1 , the radius of the CSM shell is approximately 5.3 × 10 16 cm or 0.017 pc. Interestingly, the size of this CSM shell is similar to the 8 × 10 16 cm shell estimated for SN 2004et (Kotak et al. 2009). Assuming typical red supergiant (RSG) wind speeds of 10 km s −1 , the mass-loss episode responsible for this material would have had to occur ∼ 1700 yr before SN 2017eaw exploded.
Due to long term monitoring of NGC 6946 for SNe or failed supernovae, there exists archival optical and infrared images of SN 2017eaw's progenitor for nine years prior to explosion. The progenitor showed little variability, ruling out eruptive mass loss episodes for approximately a decade prior to explosion (e.g., Johnson et al. 2018;Tinyanont et al. 2019). The CSM around SN 2017eaw was likely sculpted by photoionized trapped RSG wind with similar to structure to that observed around Betelgeuse, with estimated mass-loss ratesṀ ∼ (0.9 − 5) × 10 −6 M yr −1 (Kilpatrick & Foley 2018;Tinyanont et al. 2019).
In order to exhibit box-like emission features, the cooling time must be shorter than the adiabatic time scale to enable the formation of the cold dense shell behind the reverse shock (Chevalier & Fransson 2003Kotak et al. 2009). From an analysis of SN 2017eaw, like that done for SN 2004et, we estimate the cooling time as t c = 4.6 × 10 −3 (Ṁ −5 /u w1 ) −1 V 5.34 s4 t 2 days days, wherė M −5 is the mass loss rate in units of 10 −5 M yr −1 , u w1 is the wind velocity in units of 10 km s −1 and V s4 is the ejecta velocity in units of 10 4 km s −1 (Chevalier & Fransson 2003;Kotak et al. 2009). Since the box-like emission was present in SN 2017eaw's day +900 spectrum, the shock must still be radiative. Using the width of the Hα emission V s ≈ 10 4 km s −1 and the typical RSG wind speeds of u w = 10 km s −1 , we estimate the mass loss rate of the progenitor asṀ ≈ 3 × 10 −6 M yr −1 in agreement with previous results derived from X-ray luminosities shortly after outburst and progenitor SED analysis.

Frequency of SNe II-P Showing Late-Time CSM Interactions
Analysis of 38 SNe II-P in the nebular phase up to day +500 did not find any showing box-like emission profiles (Silverman et al. 2017). This raises the question of whether bright Type II-P objects like SN 2004et, SN 2013ej, and SN 2017eaw which have observations past day +500 are unique in showing interaction or if there is simply an underlying observational bias of too few very late-time observations.
Since the majority of SNe II-P are not observed 2 -3 years after explosion, some or maybe most may undergo similar CSM interactions and we simply have not realized it due to a lack of very late-time observations. To answer the question more SNe II-P events need to be studied in the 2 -3 year time frame. We note that all three cases where late-time CSM interactions were observed, the SNe II-P were relatively bright; SN 2017eaw reached V = 12.8 mag, SN 2013ej reached 12.5 mag, and SN 2004et reached 12.6 mag.
Consequently, it is worthwhile to photometrically and spectroscopically follow the next bright SN II-P event. Moreover, since SN 2004et is still visible (Long et al. 2019), other bright SN II-P events with similar ages should be looked at again to investigate if they might also still be visible. Although any late-time emission, if present, will be quite faint, such additional late-time detections will give us useful information to better understand the CSM environments around SNe II-P.

CONCLUSIONS
SN 2017eaw's optical spectrum dramatically evolved between day +545 and +900, developing a box-like, flattopped Hα emission profile, indicative of ejecta-CSM interaction. The Hα emission at day +900 dominated the spectrum relative to the oxygen-rich ejecta emission which was strong in earlier phase spectra. SN 2017eaw is just the third SN II-P with observed ejecta-CSM inter-action with box-like emission profiles after +500 days. However, unlike like the other late-time SNe II-P spectra, SN 2017eaw showed a fairly flat-topped profile indicating little dust is present in the CSM shell.
Given the similarity of SN 2017eaw's evolution with that of SN 2004et up to day +900, we can predict some of SN 2017eaw's future spectroscopic evolution. Spectra of SN 2004et taken at 10.2 years resembled its day +933 spectrum, in that broad, box-like Hα emission was still observed while oxygen emission increased in relative strength to the Hα (Long et al. 2019). It is therefore likely that the broad Hα profile observed in the +900 day spectrum of SN 2017eaw will also still be sustained for several years to come. In any case, even if SN 2017eaw's luminosity fades below practical spectroscopic levels, continued photometric monitoring of its evolution would be worthwhile.