Free Access
Issue
A&A
Volume 650, June 2021
Article Number A174
Number of page(s) 11
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/202039068
Published online 25 June 2021

© ESO 2021

1. Introduction

A supernova (SN) is the most spectacular way a star can end its life, where progenitors more massive than 8−9 M (see, e.g., Heger et al. 2003; Smartt 2009) are expected to explode as core-collapse (CC) SNe.

Supernovae interacting with a dense circumstellar medium (CSM) can produce a wide range of observables, resulting in a large heterogeneity of photometric and/or spectroscopic properties. The classification of interacting transients is typically based on the presence of narrow emission features in their spectra, with Type IIn (Schlegel 1990) or Ibn (Pastorello et al. 2016; Hosseinzadeh et al. 2019) SNe being those showing prominent narrow H or He lines, respectively.

The current picture for the most common narrow-lined interacting SNe is that of fast-moving ejecta colliding with a slow-moving dense CSM. In the shocked regions, a characteristic “forward-reverse” shock structure forms (e.g., Chevalier 1982; Chevalier & Fransson 1994), and energetic photons can ionize the surrounding medium giving rise to the structured, multi-component emission line profiles usually observed in SNe IIn (see, e.g., Turatto et al. 1993; Chugai & Danziger 1994; Taddia et al. 2020). In this context, narrow emission lines (full-width-at-half-maximum (FWHM) of a few 102 km s−1) are recombination lines produced in the slow-moving, unshocked CSM.

This requires the presence of a dense CSM produced by the progenitor star prior to its explosion, and seems to suggest massive luminous blue variables (LBVs, see, e.g., Trundle et al. 2008; Gal-Yam & Leonard 2009), red supergiants (RSG) with super-winds (see, e.g., Smith et al. 2009; Yoon & Cantiello 2010), or Wolf-Rayet (WR) stars in binary systems (e.g., Maund et al. 2016) as candidate progenitors for SNe IIn and Ibn. Such stars are all able to provide the environment required to produce the signatures of interaction (see Humphreys & Davidson 1994; Crowther 2007). This scenario is also supported by observations of eruptive episodes occurring weeks to years prior to the explosion of the star as an interacting SN (Foley et al. 2007; Pastorello et al. 2007, 2013; Ofek et al. 2014a; Tartaglia et al. 2016).

On the other hand, ejecta-CSM interaction can occur in many kinds of explosions or stellar outbursts and may prevent the observer from gaining insight into the intrinsic nature of the transient, including the explosion mechanism triggering the SN explosion. This is the case for the subclass of interacting transients known as SNe Ia-CSM (see, e.g., Silverman et al. 2013), which are believed to be thermonuclear explosions embedded in a dense H-rich medium.

Signatures of interaction, occasionally delayed with respect to the SN explosion, have also been observed in stripped-envelope (SE) SNe, optical transients typically showing a lack of H (SNe IIb and Ib) or both H and He features (SNe Ic) in their optical spectra. A few recent examples of SE SNe showing this evolution are SNe 2014C (Milisavljevic et al. 2015; Margutti et al. 2017), 2017dio (Kuncarayakti et al. 2018), 2017ens (Chen et al. 2018), and SNe 2019tsf and 2019oys (Sollerman et al. 2020). These can be interpreted as CC explosions of SE SNe within a dense, H-rich medium, with a possible progenitor scenario being that of a massive CSM produced during the RSG stage or by a stellar companion (e.g., Milisavljevic et al. 2015).

In this context, we present the discovery of SN 2018ijp and discuss the results of our follow-up campaign. We discovered SN 2018ijp as part of the Zwicky Transient Facility (ZTF; Graham et al. 2019; Bellm et al. 2019; Masci et al. 2019) during the first year of operations. The transient was discovered in the host SDSS J102137.72+085554.1 (Fig. 1) on 2018 November 7.41 UT and labeled ZTF18aceqrrs1. The photometric and spectroscopic follow-up campaigns were triggered soon after discovery through the GROWTH ‘Marshal’ (Kasliwal et al. 2019). A description of the facilities used and the reduction steps performed to obtain final light curves and spectra are described in Sect. 2. SN 2018ijp shows a relatively fast photometric evolution with double-peaked g- and r-band light curves. The spectra show strong signatures of delayed interaction with a dense pre-existing H-rich CSM in the form of narrow H lines in emission increasing their strength with time and a spectral continuum becoming significantly bluer with time, as described in Sect. 3. While interaction features dominate the evolution of SN 2018ijp at later times, we note that the spectrum around the first peak resembles those typical of a subclass of broad-lined Type Ic SNe (Ic-BL SNe; see, e.g., Taddia et al. 2019, and references therein), with a good match with several Type Ic/Ic-BL SNe. In addition, modeling the first peak in the context of a radioactively powered light curve gives 56Ni and total ejected masses comparable with those obtained for the Type Ic-BL SN iPTF15dqg (Taddia et al. 2019). In Sects. 3 and 4, we therefore discuss the observables of SN 2018ijp in the context of a massive, SE star within a dense H-rich medium. A brief summary of our main conclusions is reported in Sect. 5.

thumbnail Fig. 1.

Color image of the field of SN 2018ijp, obtained combining g-, r-, and i-band frames obtained on 2019 January 26 with LT. SN 2018ijp is the blue source in the middle of the inset.

In the following, we adopt a foreground Galactic extinction E(B − V) = 0.029 mag along the line of sight of SN 2018ijp, as estimated by Schlafly & Finkbeiner (2011) using a standard extinction law (Cardelli et al. 1989) with RV = 3.1. We did not include any additional contribution from the local environment to the total extinction, because we could not identify strong NaID features at the redshift of the host in the spectra of SN 2018ijp (see Sect. 3.2). The distance to SN 2018ijp was computed from the redshift derived using host lines (see Sect. 3.2) assuming a standard cosmology with H0 = 73 km s−1 Mpc−1, ΩM = 0.27 and ΩΛ = 0.73, resulting in a luminosity distance DL = 373 Mpc2.

2. Observations and data reduction

Photometry of the transient was mostly obtained using the Samuel Oschin telescope (P48) with the ZTF camera (Dekany et al. 2020) in g and r bands. Additional photometry was obtained with the Nordic Optical Telescope (NOT) using the Alhambra Faint Object Spectrograph and Camera (ALFOSC3). P48 frames were obtained through the NASA/IPAC Infrared Science Archive4, while magnitudes for these data were obtained using the dedicated pipeline SNOOPY5 performing point-spread-function (PSF) photometry on template-subtracted images. Templates and magnitudes of the reference stars were obtained from the Sloan Digital Sky Survey Data Release 14 (DR14; Abolfathi et al. 2018) available through the SDSS Catalog Archive Server (CAS6). Three additional gri epochs were obtained using the Liverpool Telescope (LT; Steele et al. 2004) with the optical imaging component of the infrared-optical (IO) suite of instruments (IO:O7). Reduction of the LT data was performed through a dedicated pipeline, using PSF photometry obtained on template-subtracted images. Templates and magnitudes of the photometric standards used were provided by the Pan-STARRS1 (PS1) survey (Tonry et al. 2012). One point of i-band photometry (on 2018 December 24.36 UT) was obtained using the Palomar 60-inch telescope (P60) with the Spectral Energy Distribution (SED) Machine (SEDM; Ben-Ami et al. 2012; Blagorodnova et al. 2018) and reduced using the FPIPE pipeline (Fremling et al. 2016).

A log of the spectroscopic observations is reported in Table 1, including the names of the instruments used and basic information about the spectra. The classification spectrum, along with three additional spectra, were obtained with the Keck-I telescope using the Low Resolution Imaging Spectrograph (LRIS; Oke et al. 1994) and reduced using the automated pipeline LPIPE (Perley 2019). Three additional spectroscopic observations were performed using the NOT with ALFOSC, reduced using FOSCGUI8. An additional intermediate-resolution spectrum was obtained using the ESO Very Large Telescope (VLT) with the X-shooter (Vernet et al. 2011) echelle spectrograph9 and was reduced using the ESO dedicated pipeline through the ESOREX v3.2.0 (ESO CPL Development Team 2015) and GASGANO environments.

Table 1.

Log of the spectroscopic observations of SN 2018ijp.

3. Analysis and discussion

3.1. Photometry

Pre-SN observations of the field of SN 2018ijp were obtained by ZTF from 2018 March 31.3 UT, resulting in no detections down to average magnitudes of ≃21 mag in both g and r bands. Last nondetection limits were obtained on 2018 November 4.5 UT (corresponding to g > 20.6 and r > 21.4 mag), roughly three days before the first g- and r-band detections. We therefore adopt 2018 November 6.0 UT (JD = 2458428.5 ± 1.5, the mid-point between the first detection and the last nondetection limit) as an estimate of the explosion epoch of SN 2018ijp and refer to phases with respect to this date. Magnitudes at rest frame epochs were obtained computing k-corrections using the spectra of SN 2018ijp and following the prescriptions of Hogg et al. (2002, see their Eq. (13)), adopting a recessional velocity of cz = 25540 km s−1, as derived from the redshift estimated from the host lines in the X-shooter spectrum (z = 0.0852; see Sect. 3.2.2). The resulting gri light curves are shown in Fig. 2 (left panel), along with an estimate of the bolometric luminosity of SN 2018ijp (right panel), which is discussed below.

thumbnail Fig. 2.

Left: gri light curves of SN 2018ijp, k-corrected apparent magnitudes versus rest frame days. Blue ticks at the bottom mark the epochs of spectroscopic observations. Right: bolometric light curve of SN 2018ijp estimated following the prescriptions of Lyman et al. (2014) (red points) and by fitting the SEDs obtained from the spectra (solid blue points). An estimate of the early peak obtained matching the luminosities obtained at ≃ + 34 d is also shown (open blue points). The inset shows the fit of the model of Arnett (1982) to the early luminosity evolution of SN 2018ijp, resulting in MNi ≃ 0.3 M and τm ≃ 4.9 d, corresponding to Ek ≃ 3.3 × 1051 erg and Mej ≃ 0.7 M.

The early photometric evolution is fast, with both g- and r-band light curves rapidly rising to a first maximum within ≃10 d from the SN explosion. At ≃ + 34 d both the g- and r-band light curves show a further rise to a second and broader peak (lasting ≃25 d), while the i-band light curve does not reveal the same “double-peaked” shape because of a lack of early observations in this band. After the second peak (t ≳ 60 d), the photometric evolution is slower in all bands, with decline rates of ≃0.012, 0.011, and 0.013 mag d−1 in g-, r-, and i-band, respectively. The rise times in g and r were computed fitting the early evolution in each band with a second-order polynomial in order to estimate the epoch of the maximum in each band. These were computed as tmax − texpl, giving trise, g = 7.9 ± 1.5 and trise, r = 10.7 ± 1.5 d, where the errors are completely dominated by the uncertainty on the explosion epoch. These rise times are slightly faster than the average r-band rise time inferred by Taddia et al. (2015, 2019) and for their samples of SNe Ic-BL (≃14.7 and ≃15 d, respectively), although still comparable with the faster end of the SN Ic-BL iPTF rise-time distribution presented in Taddia et al. (2019, their Fig. 17). We note, in addition, that the photometric evolution during the rise is well reproduced by power laws of the form Lg ∝ t0.29 and Lr ∝ t0.41 erg s−1, with fluxes in g and r computed using the zero-points for the ZTF filters reported by the Spanish Virtual Observatory (SVO10; Rodrigo et al. 2012).

The g − r early (i.e., at t ≲ 10 d) color evolution is relatively fast, with the color index increasing from ≃0.15 to ≃0.60 mag. At t ≳ 10 d, the g − r index evolves toward bluer colors until ≃ + 80 d, thereafter remaining roughly constant (≃0.1 mag) throughout the rest of the photometric evolution of SN 2018ijp. At t ≳ +60 d, we note an almost linear decline in r − i, with the color index becoming progressively bluer with time, as reflected by the evolution of the pseudo-continuum observed in the spectra of SN 2018ijp (see Sect. 3.2).

Absolute magnitudes were obtained, after correcting apparent values for the Galactic reddening and adopting a distance modulus μ = 37.85 mag (see Sect. 1). The resulting Mr, peak falls within the brighter end of the distribution of peak magnitudes presented in Taddia et al. (2019).

The early (t ≲ +30 d) bolometric light curve of SN 2018ijp was obtained following the prescriptions of Lyman et al. (2014, see their Eq. (6) for their sample of SE SNe), allowing an estimate of the g-band bolometric corrections from the evolution of the g − r color. The resulting light curve peaks at ≃1.24 × 1043 erg s−1, with the maximum occurring at d. The total radiated energy is ≃2.1 × 1049 erg within the first 34 d.

An alternative estimate of the luminosity can be obtained using the information on the SED available through the analysis of the spectra at t ≥ +24 d. We computed BVRI and gri synthetic photometry using the CALCPHOT task available through the IRAF/STSDAS Synthetic Photometry (SYNPHOT) package and fitted black body (BB) functions to the resulting SEDs. Final luminosities were then obtained integrating the fluxes in each band excluding the spectral region at wavelengths shorter than 2000 Å, where the flux is expected to be suppressed by line blanketing (see, e.g., Nicholl et al. 2017). Assuming a power-law decline at t ≳ 30 d (see Fig. 2; right panel), and interpolating the bolometric light curves at the same phases, we estimated an offset of 2.85 × 1042 erg s−1 between the two methods, as computed at +34 d. Applying this offset to the early light curve would give a peak luminosity of ≃1.5 × 1043 erg s−1 with a total radiated energy of ≃2.8 × 1049 erg within the first 34 d and ≃1.4 × 1050 erg during the 410 d covered by our follow-up campaign.

A comparison of the +24 d spectrum with the SN templates included in the SuperNova IDentification (SNID11; Blondin & Tonry 2007) tool and the GEneric cLAssification TOol (GELATO12; Harutyunyan et al. 2008) gives a good match with SNe Ic/Ic-BL, with a particularly good match with SNe 2004aw and 1998bw around peak (see Sect. 4). In the following, we therefore compare the main observables of SN 2018ijp at t ≲ 20 d with quantities inferred from samples of SNe Ic-BL, and conduct simple modeling of such an early light curve. Following Lyman et al. (2016, their Eq. (4)), the average peak luminosity obtained with the methods described above already suggests a relatively high mass of 56Ni expelled by the SN explosion (MNi ≃ 0.57 M), although comparable to the average value found by Drout et al. (2011) for their sample of SNe Ic-BL.

The total mass and the kinetic energy of the ejecta can be derived following the prescriptions of Arnett (1982) (see also the formulation of Wheeler et al. 2015 of the analytical model applied to a sample of SE SNe). The model assumes spherical symmetry, a constant optical opacity κopt, small initial radius (R0 ≪ 1014 cm) and homologous expansion of the optically thick ejecta R(t) = R0 + vsct, with vsc being the scale expansion velocity (see Arnett 1982). Under these assumptions, the characteristic time-scale can be defined, with τ0 and τh being the diffusion and hydrodynamical times, respectively (see Wheeler et al. 2015).

The evolution of the bolometric luminosity can be expressed as a function of the kinetic energy of the ejecta Ek, the 56Ni mass M56Ni, and the total mass of the ejecta Mej as follows (see Chatzopoulos et al. 2012):

(1)

where x ≡ t/τm, ϵCo = 6.78 × 109 erg s−1 g−1 and ϵNi = 3.90 × 1010 erg s−1 g−1 (see, e.g., Cappellaro et al. 1997) and τCo, τNi are the radioactive decay times of 56Co and 56Ni (111.3 and 8.8 d, respectively; see, e.g., Nadyozhin 1994). Assuming a constant optical opacity κopt = 0.07 cm2 g−1 (Chugai 2000) and fitting Eq. (1) to the bolometric light curve of SN 2018ijp gives M56Ni = 0.33 ± 0.05 M and τm = 4.89 ± 0.80 d. Assuming a uniform density within the ejecta, τm can also be expressed as follows:

(2)

where β is an integration constant (β ≃ 13.8, as in Wheeler et al. 2015). The degeneracy between the kinetic energy and the total mass of the ejecta Ek = 1/2Mejv2⟩, with ⟨v2⟩ being the mean squared expansion velocity, can be broken assuming uniform density within the expanding ejecta and hence (Arnett 1982), where vph is the photospheric velocity. An estimate of ⟨v⟩ can be obtained measuring the minima of the P Cygni absorption profiles of Fe II or O I lines around maximum light (see, e.g., Dessart et al. 2016), which in the case of SN 2018ijp corresponds to km s−1 at ≃ + 24 d (see Sect. 3.2). Following Dessart et al. (2016), this corresponds to ⟨v⟩≃21 200 km s−1. Taking this value for the mean expansion velocity, Eq. (2) gives Mej = 0.73 ± 0.05 M and Ek = (3.26 ± 0.17)×1051 erg for the total mass and the kinetic energy of the ejecta.

The derived values are similar to those found by Taddia et al. (2019) for their sample of SNe Ic-BL, with an ejected mass comparable to that found for PTF11lbm using a similar approach. However, the 56Ni mass is higher, corresponding to ∼45% of Mej. While these values do not make of SN 2018ijp the most extreme case (see, e.g., the values found for iPTF16asu), the relative amount of nickel is unreconcilably high and might suggest a non-negligible contribution of ejecta-CSM interaction to the total luminosity also around the first peak of SN 2018ijp.

The bolometric light curve, as constructed from the spectra, can be reproduced by a “broken power-law” starting from +24 d (Fig. 2, right panel), with the break occurring at ≃ + 120 d. A similar behavior is observed in interacting transients, where the SN shock is expected to break through a dense and extended pre-existing CSM (see, e.g., Fransson et al. 2014; Ofek et al. 2014b; Tartaglia et al. 2020, and references therein). The total radiated energy up to +410 d, as well as the prominent narrow Hα line visible at all phases also support a CSM interaction interpretation for the second peak of SN 2018ijp. In the case of SN 2018ijp, at least a fraction of the luminosity output during the first peak could also be powered by interaction of the SN ejecta. A more complex modeling, beyond the scope of this paper, is probably required to properly model the evolution of SN 2018ijp in order to estimate its explosion parameters.

3.2. Spectroscopy

3.2.1. Low-resolution spectroscopy

Low- and medium-resolution optical spectra, calibrated against photometry obtained at the closest epochs, are shown in Fig. 3. At +24 d, the spectrum shows unresolved Balmer lines in emission (Hα and Hβ) on top of shallower broader features. The lack of other narrow emission lines typically associated with H II regions (e.g., [O III], [O II] and [N III]) would suggest that these are recombination lines arising from an unshocked CSM, although the signal-to-noise ratio (S/N) of the spectrum is not sufficient to rule out the presence of such lines (see Sect. 3.2.2). Blends of Fe II lines are likely responsible for the “bumps” observed between 4000 and 5000 Å (multiplets 26, 27, 28, 37, and 38) and at ∼5300 Å (multiplets 42, 48, and 49), making direct estimate of the ejecta photospheric velocity (through the Fe IIλ5169 line; Dessart & Hillier 2005) difficult. On the other hand, at λ ≳ 7000 Å the spectrum shows broader features of O I 7772−7775 Å. From the minimum of the O I P-Cygni absorption we inferred an expansion velocity of ≃12 400 km s−1, with the blue wing extending to ≃2 × 104 km s−1, which, following the discussion in Dessart et al. (2016), corresponds to a mean expansion velocity of ≃21 200 km s−1. At the same epoch, and throughout the spectroscopic evolution of SN 2018ijp covered by our follow-up campaign, we also detect the near infrared (NIR) Ca II triplet (see Fig. 3). The +65 d spectrum reveals significant evolution, with the continuum becoming apparently much bluer. The total luminosity of Hα (measured in the 6000−7000 Å range) at +65 d (≃1.1 × 1041 erg s−1) also shows a drastic increase with respect to the previous epoch (LHα, +24 d ≃ 2.8 × 1040 erg s−1), subsequently remaining roughly constant up to +137 d.

thumbnail Fig. 3.

Spectral sequence of SN 2018ijp. Rest frame phases refer to the estimated epoch of the explosion. The +165 d spectrum has been degraded to the resolution of the +105 d spectrum to facilitate the comparison. The +24 d spectrum has been degraded to one-third of its resolution and plotted in black, while the original spectrum is reported in gray.

While blue excesses can be generally associated with the contribution of fluorescence from numerous blended Fe lines (see, e.g., Tartaglia et al. 2020, and references therein), an increase in the temperature of the pseudo-continuum can also be interpreted as a result of ongoing ejecta–CSM interaction. This interpretation would also be supported by the shape of the bolometric light curve (Fig. 2, right panel), showing a “broken power-law” shape typical of interacting SNe. A “delayed” interaction might be explained by the presence of a confined dense shell surrounding the progenitor star of SN 2018ijp. Based on the evolution of the bolometric luminosity and the results discussed in Sect. 3.1, as well as on the Hα evolution discussed below, we can assume that the onset of ejecta-CSM interaction is at t ≳ +24 d. A constant expansion velocity of v ≃ 21 200 km s−1 (see above) would place the shell at a distance of ≃4.4 × 1015 cm from the progenitor star of SN 2018ijp. This estimate is of the same order of magnitude as that inferred from the BB fit performed at +65 d (≃1015 cm). Assuming that the SN shock breaks through the confined shell roughly at t ≃ 120 d (i.e., when the break in the power law describing the light curve occurs), this would imply an external radius of the shell of ≃2.2 × 1016 cm. Such a detached H-rich shell might be produced by a single massive progenitor and hence expelled 20−100 yr before CC, assuming a vwind ≃ 70 km s−1, as estimated in Sect. 3.2.2 (see also Margutti et al. 2017, for a similar interpretation for SN 2014C). Another explanation could be the last eruptive episode of a Wolf-Rayet star in its transitional phase from the LBV phase (see, e.g., the case of the Type Ibn SN 2006jc; Pastorello et al. 2007). However, it could also originate from a H-rich companion through a stationary wind or an eruptive event (see Sect. 5).

In Fig. 4 we show the evolution of the spectral region around Hα, revealing a structured and asymmetric profile throughout the spectroscopic evolution of the transient. At +24 d, the overall profile is well reproduced by a broad, blueshifted component with a FWHM ≃ 104 km s−1 and a narrow unresolved component (FWHM < 70 km s−1) centered at Hα rest wavelength, although a comparison with early spectra of the Type Ic-BL SN 1998bw might suggest a different interpretation for this feature (see Sect. 4). A multi-Gaussian fit between +65 and +165 d reveals two broad components: a blueshifted component with a FWHM of ≃2 × 104 km s−1 and a redder one with a FWHM slowly decreasing from ≃104 to ≃3 × 103 km s−1, with a third narrow and unresolved component. While the redder component is likely due to the wings of the typical electron scattering profile observed in high-resolution spectra of interacting transients (see, e.g., Huang & Chevalier 2018), the corresponding velocity width is also consistent with those observed in shocked regions of dense media typically surrounding the progenitors of SNe IIn. A clumpy (e.g., Chugai & Danziger 1994) or highly asymmetric (e.g., Smith et al. 2014) CSM could explain the simultaneous presence of a broad component, which would then be produced by the outer ionized layers of the freely expanding SN ejecta, and an intermediate component arising from the shocked CSM, with the narrow emission feature possibly produced in the ionized unshocked CSM (see, e.g. Turatto et al. 1993, although see Sect. 3.2.2 for a possible different interpretation of the Hα profile observed at t ≥ +65 d). By +410 d, the shape of Hα has changed significantly, showing a narrow component on top of a boxy, flat-topped line profile reminiscent of the overall profile observed in H-rich interacting SNe IIn (see, e.g., Taddia et al. 2020, and the discussion in Sect. 5), and is not well reproduced by a combination of Gaussian profiles.

thumbnail Fig. 4.

Evolution of the Hα profile, along with a multi-component fit.

3.2.2. The X-shooter spectrum

Medium-resolution spectra were obtained with X-shooter on 2019 May 4.06 UT (JD = 2458607.56, t = +165 d). The observations consist of four exposures per arm, median combined to produce a single spectrum covering the 350−2000 nm wavelength range13. Each observation was obtained at airmass ≲1.5, with an average seeing of , and we therefore take the nominal values of resolution for each arm (R ≡ λλ = 5400, 8900 and 5600 in the UVB, VIS, and NIR arm, respectively, for slit widths of in UVB and in VIS and NIR)14. These are consistent with the resolutions measured from the available sky lines. The resulting spectra are shown in Fig. 5.

thumbnail Fig. 5.

X-shooter spectra of SN 2018ijp obtained at +165 d. Top panel: insets with zooms into the [O II]λλ3726, 3729 and [O III]λ4363 regions. Middle and bottom panels: insets of the Hα, He Iλ10830, and Paα regions in velocity space. The NIR spectrum has been re-rebinned to one-fifth of its resolution to facilitate the identification of the main emission features.

The Hα region shows a structured profile with a narrow (FWHM ≃ 70 km s−1) and an “intermediate” (FWHM ≃ 7400 km s−1) component, both centered at Hα rest wavelength, on top of a broader (FWHM ≃ 21 500 km s−1) blueshifted (vshift ≃ 10 600 km s−1) component (see also Fig. 4). A much shallower broad component is observed in Hβ, with a FWHM of ≃2130 km s−1 and an unresolved narrow component (FWHM < 60 km s−1).

The measured Hδ/Hβ and Hγ/Hβ line ratios of the narrow components are ≃0.2 and 0.5, roughly corresponding to the predicted Balmer decrement in a Case B recombination scenario (assuming T = 104 K and ne = 102 cm−3; see Osterbrock & Ferland 2006), consistent with negligible contribution of the local environment to the total extinction in the direction of SN 2018ijp. The electron density was inferred from the [O II] line ratio (j3729/j3726 = 1.26, corresponding to ne ≃ 102 cm−3, see Osterbrock & Ferland 2006), as well as from the [S II]j6716/j6732 ratio, giving the same result. The narrow Hα/Hβ ratio of ≃3.5 is somewhat higher than that predicted in the case B scenario, which might suggest some contribution from the transient to Hα. A number of other narrow lines are also detected, including [O II]λλ3726, 3729, [O III]λλ4363, 4958 and 5007, [Ne III]λλ3868, 3967, [S II]λλ6716, 6732 and [N II]λλ6548, 6583, which, along with the narrow Balmer lines are all below the resolution limit, with the exception of the Hα narrow component, showing a FWHM ≃ 70 km s−1.

Following the line identification of Fransson et al. (2002), we also detect narrow, unresolved, high-ionization lines, such as [Fe XI]λ7892, [Fe X]λ6375, [Fe VI]λ5720, 6087, and 5276 as well as narrow He Iλ3889 and 7065.

As highlighted in Figs. 3 and 5, the nondetection of such features at earlier phases is most likely due to the lower S/N and resolution of the spectra. As in SN 1995N (see also Sollerman et al. 2020), these features can be interpreted as arising from the dense circumstellar gas. These lines are significantly fainter at +410 d where the S/N and resolution of the spectrum is still sufficient to detect such features.

The [O III]λ4363 is clearly detected in the X-shooter spectrum. This line is usually faint compared to [O III]λλ4959, 5007, with (j5007 + j4959)/j4363 ≳ 50 in typical H II regions. The inferred value (j5007 + j4959)/j4363 = 7.2 implies very high temperatures and densities for the emitting gas (e.g., Te ≳ 2.7 × 104 K for electron densities ne ≳ 106 cm−3 in a five-level atom approximation; see De Robertis et al. 1987; Shaw & Dufour 1995), suggesting a circumstellar origin for the [O III] lines as well. This could imply a [O III] flux arising from different regions (i.e., both from an underlying H II region and the CSM). Alternatively, narrow features, such as [O II], [O III], and [S II], might all have a circumstellar origin, along with Fe high-ionization features, with different lines mapping regions with different densities. A similar behavior is observed in H II regions, where [O II] lines are thought to arise from outer regions of the nebula, whereas [O III] features are produced from more uniformly distributed gas (see, e.g., Nicholls et al. 2014, and references therein).

In order to further explore these different scenarios, we inspected the 2D X-shooter frames, which suggested a common origin for at least the [O II] and the narrow Balmer lines, with spatially extended emission suggesting contamination from the environment of SN 2018ijp. The [O III] and high-ionization Fe lines, on the other hand, do not show similar spatially extended emission, favoring a circumstellar origin. The evolution of the narrow lines over the remaining ∼300 d of our spectroscopic coverage also seems to support a circumstellar origin at least for the high-ionization Fe lines, with fluxes which decrease significantly from +165 to +410 d.

4. The nature of SN 2018ijp

As spectra at t ≥ +65 d are dominated by interaction, we use the spectrum obtained at +24 d (i.e., 16 d after the first r-band peak) to investigate the nature of the explosion of SN 2018ijp. Our approach is based on automatic comparisons of the observed spectrum with the databases of SNID (based on the correlation techniques of Tonry & Davis 1979) and GELATO tools. SNID, in particular, is able to clip narrow emission lines at a specified redshift, which allows spurious correlation peaks to be avoided. Using this option to avoid contamination from the narrow Hα component, we get the best match with the Type Ic SN 2004aw (Taubenberger et al. 2006) 12 d after maximum, although the comparison also suggests many other Type Ic/Ic-BL SNe as good matches to the spectral features of SN 2018ijp, including the Type Ic-BL SN 1998bw (see Fig. 6, where we also include a comparison with the Type Ia-CSM PTF11kx around peak, disfavoring such a classification for SN 2018ijp). A similar result was obtained using GELATO, giving the best match with the Type Ic-BL SN 2003jd (Valenti et al. 2008) around maximum light. These comparisons thus suggest a SE SN classification for SN 2018ijp. In Fig. 7 (also including the peculiar Ic SNe 1997ef Iwamoto et al. 2000; Mazzali et al. 2000), on the other hand, we show that the first peak of SN 2018ijp resembles the photometric evolution of PTF11kx (see the inset), with similar absolute magnitudes and a fast rise to maximum, although SN 1998bw also gives a good match at t ≳ −10 d (with respect to the r-band peak). Around the second peak, SN 2018ijp shows a different evolution with respect to all other comparison objects with the possible exception of PTF11kx, although its sampling is not as good as the one provided by our follow-up campaign between 20 d ≲ t ≲ 220 d. Based on this similarity, and because Ia-CSM SNe can also produce spectra similar to SN 2018ijp around the first peak, we further investigate the nature of SN 2018ijp by extending the comparison to transients of a different nature, as described below.

thumbnail Fig. 6.

+24 d spectrum (16 d after r-band maximum, shown in gray) compared to “best-match” templates obtained with the SNID (Type Ic SN 2004aw and Ic-BL SN 1998bw) and GELATO (Ic SN 2003jd) classification tools. Spectra of SNe 2004aw and 1998bw are the “flattened” templates included in the SNID database. Phases relative to maximum light of each object are reported in the legend. The spectral continua of SNe 2018ijp and 2003jd (not included in the SNID template database) were estimated through spline functions and subtracted. The spectrum of SN 2018ijp has been smoothed in order to highlight its broad features.

thumbnail Fig. 7.

Comparison of the absolute r-band light curve of SN 2018ijp with those of a number of other SNe. The sample was chosen based on comparison of the +24 d spectrum (see Fig. 6). The choice of the reported bands is given in the legend and was made based on the available photometric data for each object. In the inset, we show a comparison of the early r-band light curve of SN 2018ijp with those of SNe 2003jd, 2004aw, 1998bw, and PTF11kx.

Following Aldering et al. (2006), we compared the +24 d spectrum of SN 2018ijp (i.e., ≃16 d after maximum) with “diluted” spectra of SN 1991T (Filippenko et al. 1992; Mazzali et al. 1995; Gómez & López 1998; Silverman et al. 2012) around peak. This choice is based on the claim that SNe Ia-CSM seem to show an association with 91T-like transients (see, e.g., Hamuy et al. 2003; Aldering et al. 2006; Prieto et al. 2007; Taddia et al. 2012). Spectra of SN 1991T around maximum were diluted using a BB in order to simulate the effects of interaction – including a constant factor to account for the different distances and luminosities of the two objects – and the “best-fit model” was determined through a simple χ2 minimization routine. A similar approach was adopted by Leloudas et al. (2015) to test how interaction would affect the appearance of SN spectra and their classification, although we did not include narrow Balmer lines (i.e., Hα) in our fit. In Fig. 8, we show the results of this fitting procedure, giving a good match with SNe 2004aw and 1998bw and thus confirming the results obtained with SNID. On the other hand, the best-fit spectrum obtained with SN 1991T does not seem to reproduce well the broad features observed in SN 2018ijp. We note that a good match with SN 2004aw is obtained even without including a BB in the fitting procedure, suggesting that the interaction does not significantly shape the pseudo-continuum of SN 2018ijp around the first peak. This is also the rationale for our light-curve analysis in Sect. 3.1.

thumbnail Fig. 8.

Best-fit models obtained using spectra of the Type Ic SNe 2004aw, Ic-BL 1998bw, and Ia 1991T, adding a BB to the comparison objects. Blue scaled spectra are those of the comparison objects, while green lines are the BBs required to get the “best-fit” diluted spectra (see the main text for details). The spectrum of SN 2018ijp is obtained at +24 d (≃16 d after maximum, in the rest frame). Phases refer to maximum light.

At later phases (i.e. t ≥ +65 d), we could not find a good match with any template spectrum either using SNID or GELATO, although, after clipping the narrow Hα component, GELATO suggests a marginal similarity with the Type Ib SN 2009er. The same result was obtained also diluting spectra of SNe 1991T, 1998bw, and 2004aw, as detailed above. At the same time, we also note that the late spectral evolution of SN 2018ijp is different from that of the aforementioned transients (see Fig. 9), although at +410 d, we note a remarkable similarity with a late spectrum of PTF11k obtained at a similar phase (see Fig. 10). This evidence supports our claim that later spectra are dominated by strong signatures of interaction (namely a very blue continuum, and the presence of several Fe II multiplets, with a prominent and structured Hα line in emission; see Fig. 4).

thumbnail Fig. 9.

Comparison of spectra of SN 2018ijp at selected epochs and spectra of the SNe discussed in the main text at similar phases. Phases refer to the epoch of maximum light.

thumbnail Fig. 10.

Comparison of the late spectrum of SN 2018ijp obtained at +410 d (≃400 d after r-band maximum) with spectra of the SNe discussed in the main text. Phases refer to the epochs of maximum light.

5. Summary and conclusions

We present the peculiar photometric and spectroscopic evolution of SN 2018ijp and discuss its observables. The transient shows double-peaked g- and r-band light curves with a fast-evolving first peak showing an evolution similar to those observed in the Type Ic-BL, SN 1998bw, and the Ia-CSM, PTF11kx.

A comparison of the first spectrum, obtained at +24 d (around the first peak), with spectral templates included in commonly used classification tools (such as SNID and GELATO) favor a SE SN, providing good matches with the peculiar Type Ic SN 2004aw and the Ic-BL SN 1998bw (see Fig. 3). We could not find a good match with any SN Ia, either with SNID or GELATO, or modeling the effects of ongoing ejecta-CSM interaction with a BB (see Sect. 4 and Fig. 8), which did not give a good match with a 91T-like SN, generally associated to SNe Ia-CSM (e.g., Leloudas et al. 2015). We note that the “template” spectrum of SN 2004aw, ≃22 d after maximum, resembles that of SN 2018ijp even without introducing an extra component. Based on this early spectral similarity, we model the early evolution of the bolometric luminosity of SN 2018ijp using the simple analytical model of Arnett (1982), suggesting a mass of expelled radioactive 56Ni of ≃0.3 M with respect to a total ejected mass of 0.7 M and kinetic energy of 3.3 × 1051 erg. This makes a reasonable match to SE SNe, with inferred values that are consistent, overall, with those found by Taddia et al. (2019) for their sample of SNe Ic-BL, and the mean nickel mass of the iPTF Ic SN sample (0.2 M; Barbarino et al. 2021). We remark that this analysis is only meant to provide rough estimates and the resulting values have to be considered as approximations of the real quantities. In addition, the very high fraction of radioactive 56Ni to total ejected mass inferred for SN 2018ijp might suggest that interaction also plays a role in its early evolution, as possibly indicated by the presence of narrow emission lines superimposed on Ic-BL spectral features. In order to explain simultaneous signatures of interaction and SN features observed in the +24 d spectrum, we need to invoke a peculiar geometrical configuration for the CSM (e.g., a highly asymmetric explosion), a “clumpy” CSM (see, e.g., Chugai & Danziger 1994), a moderate amount of CSM (with respect to those expected in SNe IIn and Ibn), or a combination of the three. However, this would affect the results of our modeling of the first peak of SN 2018ijp, although it still gives reasonable results compared to those obtained by Taddia et al. (2019) for their sample of SNe Ic-BL.

Inspection of the spectral region around the Hα line (Fig. 4) suggests the presence of a very broad blueshifted component, marginally visible from +24 d (see Fig. 3). Identifying this broad feature as H would be at odds with a SE SN origin, although the comparisons with SNe Ic-BL discussed in Sect. 4 (see, e.g., Fig. 6) reveal that broad features at these wavelengths are also observed in SE SNe. At t > +24 d, the spectral evolution of SN 2018ijp is clearly dominated by signatures of interaction, as also highlighted by its late photometric evolution. In this context, the very high velocities inferred for the broad Hα component (see Fig. 4) at these phases could be explained with a high optical depth by incoherent electron scattering in the post-shock region. A combined effect of a high optical depth and shock velocity was discussed by Taddia et al. (2020, see their Fig. 22) in order to explain the similarly structured Hα profile of SN 2013L. In this scenario, while the slope of the blue wing of Hα is strongly affected by the optical depth of the CSM, the suppression of the flux at redder wavelengths is caused by the efficient thermalization (obscuration) of the Hα photons in the SN ejecta. This scenario is also supported by the shape of Hα at +410 d, showing a boxy, flat-topped profile similar to that of SN 2013L, although with lower velocities, suggesting a lower expansion velocity of the shocked material. The match with the absolute light curve of PTF11kx at later phases, the dramatic evolution in the observed spectral continuum from +24 and +65 d (see Sect. 3.2 and Fig. 3), and the shape of the bolometric light curve at t ≳ 25 d (see Fig. 2, right panel) all seem to support a scenario where the late-time evolution of SN 2018ijp is dominated by an ejecta–CSM interaction.

We therefore invoke two main phases to explain the overall evolution of SN 2018ijp, a first one dominated by radioactive decays, and an interaction-dominated phase where the SN ejecta collide with a H-rich pre-existing CSM (see Sect. 3.1), although we cannot exclude an additional contribution of CSM interaction during the first phase. In this scenario, the early spectral features observed in SN 2018ijp, with high expansion velocities measured in the +24 d spectrum as well as the relatively high mass of radioactive 56Ni, seem to suggest a stripped massive star, with the CSM either produced during a previous evolutionary stage or by a H-rich companion.

The location of the H-rich shell with respect to the center of the explosion and its estimated expansion velocity, on the other hand, is difficult to reconcile with an outburst of a massive H-rich star subsequently exploding as a SE SN. A constant expansion velocity of 21 200 km s−1 for the SN ejecta would give an inner radius of ≃4 × 1015 cm, with an outer layer extending up to ≃2 × 1016 (assuming vwind ≃ 70 km s−1, as inferred in Sect. 3.2.2). This would suggest that the mass-loss episode started ∼100 yr and lasted up to ∼20 yr before the explosion of the progenitor star. While a more accurate modeling of the observables is needed, a simple scenario might involve the presence of a H-rich companion as a possible source of the dense CSM needed to produce the observables of SN 2018ijp.


2

Derived using COSMOCALC (Wright 2006) available at: http://www.astro.ucla.edu/~wright/CosmoCalc.html

9

Obtained under programme 0102.D-0221 (P.I. Sollerman).

Acknowledgments

The Oskar Klein Centre is funded by the Swedish Research Council. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. SED Machine is based upon work supported by the National Science Foundation under Grant No. 1106171 The data presented here were partly obtained with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. This research has made use of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. This work was supported by the GROWTH project funded by the National Science Foundation under Grant No 1545949. Funding for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS website is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. L.T. acknowledges support from MIUR (PRIN 2017 grant 20179ZF5KS). Y.-L.K. has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement No. 759194 USNAC). This research has made use of the SVO Filter Profile Service (http://svo2.cab.inta-csic.es/theory/fps/) supported from the Spanish MINECO through grant AYA2017-84089 IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. SNOOPy is a package for SN photometry using PSF fitting and/or template subtraction developed by E. Cappellaro. A package description can be found at http://sngroup.oapd.inaf.it/snoopy.html. FOSCGUI is a graphic user interface aimed at extracting SN spectroscopy and photometry obtained with FOSC-like instruments. It was developed by E. Cappellaro. A package description can be found at http://sngroup.oapd.inaf.it/foscgui.html.

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All Tables

Table 1.

Log of the spectroscopic observations of SN 2018ijp.

All Figures

thumbnail Fig. 1.

Color image of the field of SN 2018ijp, obtained combining g-, r-, and i-band frames obtained on 2019 January 26 with LT. SN 2018ijp is the blue source in the middle of the inset.

In the text
thumbnail Fig. 2.

Left: gri light curves of SN 2018ijp, k-corrected apparent magnitudes versus rest frame days. Blue ticks at the bottom mark the epochs of spectroscopic observations. Right: bolometric light curve of SN 2018ijp estimated following the prescriptions of Lyman et al. (2014) (red points) and by fitting the SEDs obtained from the spectra (solid blue points). An estimate of the early peak obtained matching the luminosities obtained at ≃ + 34 d is also shown (open blue points). The inset shows the fit of the model of Arnett (1982) to the early luminosity evolution of SN 2018ijp, resulting in MNi ≃ 0.3 M and τm ≃ 4.9 d, corresponding to Ek ≃ 3.3 × 1051 erg and Mej ≃ 0.7 M.

In the text
thumbnail Fig. 3.

Spectral sequence of SN 2018ijp. Rest frame phases refer to the estimated epoch of the explosion. The +165 d spectrum has been degraded to the resolution of the +105 d spectrum to facilitate the comparison. The +24 d spectrum has been degraded to one-third of its resolution and plotted in black, while the original spectrum is reported in gray.

In the text
thumbnail Fig. 4.

Evolution of the Hα profile, along with a multi-component fit.

In the text
thumbnail Fig. 5.

X-shooter spectra of SN 2018ijp obtained at +165 d. Top panel: insets with zooms into the [O II]λλ3726, 3729 and [O III]λ4363 regions. Middle and bottom panels: insets of the Hα, He Iλ10830, and Paα regions in velocity space. The NIR spectrum has been re-rebinned to one-fifth of its resolution to facilitate the identification of the main emission features.

In the text
thumbnail Fig. 6.

+24 d spectrum (16 d after r-band maximum, shown in gray) compared to “best-match” templates obtained with the SNID (Type Ic SN 2004aw and Ic-BL SN 1998bw) and GELATO (Ic SN 2003jd) classification tools. Spectra of SNe 2004aw and 1998bw are the “flattened” templates included in the SNID database. Phases relative to maximum light of each object are reported in the legend. The spectral continua of SNe 2018ijp and 2003jd (not included in the SNID template database) were estimated through spline functions and subtracted. The spectrum of SN 2018ijp has been smoothed in order to highlight its broad features.

In the text
thumbnail Fig. 7.

Comparison of the absolute r-band light curve of SN 2018ijp with those of a number of other SNe. The sample was chosen based on comparison of the +24 d spectrum (see Fig. 6). The choice of the reported bands is given in the legend and was made based on the available photometric data for each object. In the inset, we show a comparison of the early r-band light curve of SN 2018ijp with those of SNe 2003jd, 2004aw, 1998bw, and PTF11kx.

In the text
thumbnail Fig. 8.

Best-fit models obtained using spectra of the Type Ic SNe 2004aw, Ic-BL 1998bw, and Ia 1991T, adding a BB to the comparison objects. Blue scaled spectra are those of the comparison objects, while green lines are the BBs required to get the “best-fit” diluted spectra (see the main text for details). The spectrum of SN 2018ijp is obtained at +24 d (≃16 d after maximum, in the rest frame). Phases refer to maximum light.

In the text
thumbnail Fig. 9.

Comparison of spectra of SN 2018ijp at selected epochs and spectra of the SNe discussed in the main text at similar phases. Phases refer to the epoch of maximum light.

In the text
thumbnail Fig. 10.

Comparison of the late spectrum of SN 2018ijp obtained at +410 d (≃400 d after r-band maximum) with spectra of the SNe discussed in the main text. Phases refer to the epochs of maximum light.

In the text

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