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Register a new userEvidence for multiple origins of fast declining Type II supernovae from spectropolarimetry of SN 2013ej and SN 2017ahn
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The origin of the diverse light-curve shapes of Type II supernovae (SNe), and whether they come from similar or distinct progenitors, has been actively discussed for decades. Here we report spectropolarimetry of two fast declining Type II (Type IIL) SNe: SN 2013ej and SN 2017ahn. SN 2013ej exhibited high continuum polarization from very soon after the explosion to the radioactive tail phase with time-variable polarization angles. The origin of this polarimetric behavior can be interpreted as the combination of two different aspherical structures, namely an aspherical interaction of the SN ejecta with circumstellar matter (CSM) and an inherently aspherical explosion. Aspherical explosions are a common feature of slowly declining Type II (Type IIP) SNe. By contrast, SN 2017ahn showed low polarization not only in the photospheric phase but also in the radioactive tail phase. This low polarization in the tail phase, which has never before been observed in other Type IIP/L SNe, suggests that the explosion of SN 2017ahn was nearly spherical. These observations imply that Type IIL SNe have, at least, two different origins: they result from stars that have different explosion properties and/or different mass-loss processes. This fact might indicate that 13ej-like Type IIL SNe originate from a similar progenitor to those of Type IIP SNe accompanied by an aspherical CSM interaction, while 17ahn-like Type IIL SNe come from a more massive progenitor with less hydrogen in its envelope.
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We present high-cadence, comprehensive data on the nearby ($Dsimeq33,rm{Mpc}$) Type II SN 2017ahn, discovered within $sim$1 day of explosion, from the very early phases after explosion to the nebular phase. The observables of SN 2017ahn show a significant evolution over the $simeq470,rm{d}$ of our follow-up campaign, first showing prominent, narrow Balmer lines and other high-ionization features purely in emission (i.e. flash spectroscopy features), which progressively fade and lead to a spectroscopic evolution similar to that of more canonical Type II supernovae. Over the same period, the decline of the light curves in all bands is fast, resembling the photometric evolution of linearly declining H-rich core-collapse supernovae. The modeling of the light curves and early flash spectra suggest a complex circumstellar medium surrounding the progenitor star at the time of explosion, with a first dense shell produced during the very late stages of its evolution being swept up by the rapidly expanding ejecta within the first $sim6,rm{d}$ of the supernova evolution, while signatures of interaction are observed also at later phases. Hydrodynamical models support the scenario in which linearly declining Type II supernovae are predicted to arise from massive yellow super/hyper giants depleted of most of their hydrogen layers.
We present optical and ultraviolet photometry, as well as optical spectra, for the type II supernova (SN) 2015bf. Our observations cover the phases from $sim 2$ to $sim 200$ d after explosion. The first spectrum is characterised by a blue continuum with a blackbody temperature of $sim 24,000$K and flash-ionised emission lines. After about one week, the spectra of SN 2015bf evolve like those of a regular SN II. From the luminosity of the narrow emission component of H$alpha$, we deduce that the mass-loss rate is larger than $sim 3.7times10^{-3},{rm M_odot,yr^{-1}}$. The disappearance of the flash features in the first week after explosion indicates that the circumstellar material is confined within $sim 6 times 10^{14}$ cm. Thus, we suggest that the progenitor of SN 2015bf experienced violent mass loss shortly before the supernova explosion. The multiband light curves show that SN 2015bf has a high peak luminosity with an absolute visual magnitude $M_V = -18.11 pm 0.08$ mag and a fast post-peak decline with a $V$-band decay of $1.22 pm 0.09$ mag within $sim 50$ d after maximum light. Moreover, the $R$-band tail luminosity of SN 2015bf is fainter than that of SNe~II with similar peak by 1--2 mag, suggesting a small amount of ${rm ^{56}Ni}$ ($sim 0.009,{rm M_odot}$) synthesised during the explosion. Such a low nickel mass indicates that the progenitor of SN 2015bf could be a super-asymptotic-giant-branch star that collapsed owing to electron capture.
We investigate the low-luminosity supernova SN 2016bkv and its peculiar early-time interaction. For that, we compute radiative transfer models using the CMFGEN code. Because SN 2016bkv shows signs of interaction with material expelled by its progenitor, it offers a great opportunity to constrain the uncertain evolutionary channels leading to low-luminosity supernovae. Our models indicate that the progenitor had a mass-loss rate of (6.0 +- 2.0) x 1e-4 Msun/yr (assuming a velocity of 150 km/s). The surface abundances of the progenitor are consistent with solar contents of He and CNO. If SN 2016bkvs progenitor evolved as a single star, it was an odd red supergiant that did not undergo the expected dredge up for some reason. We propose that the progenitor more likely evolved through binary interaction. One possibility is that the primary star accreted unprocessed material from a companion and avoided further rotational and convective mixing until the SN explosion. Another possibility is a merger with a lower mass star, with the primary remaining with low N abundance until core collapse. Given the available merger models, we can only put a loose constraint on the pre-explosion mass around 10-20 Msun, with lower values being favored based on previous observational constraints from the nebular phase.
We use natural seeing imaging of SN 2013ej in M74 to identify a progenitor candidate in archival {it Hubble Space Telescope} + ACS images. We find a source coincident with the SN in the {it F814W}-filter, however the position of the progenitor candidate in contemporaneous {it F435W} and {it F555W}-filters is significantly offset. We conclude that the progenitor candidate is in fact two physically unrelated sources; a blue source which is likely unrelated to the SN, and a red source which we suggest exploded as SN 2013ej. Deep images with the same instrument onboard {it HST} taken when the supernova has faded (in approximately two years time) will allow us to accurately characterise the unrelated neighbouring source and hence determine the intrinsic flux of the progenitor in three filters. We suggest that the {it F814W} flux is dominated by the progenitor of SN 2013ej, and assuming a bolometric correction appropriate to an M-type supergiant, we estimate that the mass of the progenitor of SN 2013ej was between 8 -- 15.5 M$_{odot}$.
We present optical and near-infrared photometric and spectroscopic observations of SN 2013ej, in galaxy M74, from 1 to 450 days after the explosion. SN 2013ej is a hydrogen-rich supernova, classified as a Type IIL due to its relatively fast decline following the initial peak. It has a relatively high peak luminosity (absolute magnitude M$_rm{V}$ = -17.6) but a small $^{56}$Ni production of ~0.023 M$_odot$. Its photospheric evolution is similar to other Type II SNe, with shallow absorption in the H{alpha} profile typical for a Type IIL. During transition to the radioactive decay tail at ~100 days, we find the SN to grow bluer in B - V colour, in contrast to some other Type II supernovae. At late times, the bolometric light curve declined faster than expected from $^{56}$Co decay and we observed unusually broad and asymmetric nebular emission lines. Based on comparison of nebular emission lines most sensitive to the progenitor core mass, we find our observations are best matched to synthesized spectral models with a M$_rm{ZAMS}$ = 12 - 15 M$_odot$ progenitor. The derived mass range is similar to but not higher than the mass estimated for Type IIP progenitors. This is against the idea that Type IIL are from more massive stars. Observations are consistent with the SN having a progenitor with a relatively low-mass envelope.
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