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Register a new userSN 2018zd: An Unusual Stellar Explosion as Part of the Diverse Type II Supernova Landscape
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We present extensive observations of SN 2018zd covering the first $sim450$,d after the explosion. This SN shows a possible shock-breakout signal $sim3.6$,hr after the explosion in the unfiltered light curve, and prominent flash-ionisation spectral features within the first week. The unusual photospheric temperature rise (rapidly from $sim 12,000$,K to above 18,000,K) within the earliest few days suggests that the ejecta were continuously heated. Both the significant temperature rise and the flash spectral features can be explained with the interaction of the SN ejecta with the massive stellar wind ($0.18^{+0.05}_{-0.10}, rm M_{odot}$), which accounts for the luminous peak ($L_{rm max} = [1.36pm 0.63] times 10^{43}, rm erg,s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity ($v approx 3300$ km s$^{-1}$) make SN 2018zd to be like a member of the LLEV (luminous SNe II with low expansion velocities) events originated due to circumstellar interaction. The relatively fast post-peak decline allows a classification of SN 2018zd as a transition event morphologically linking SNe~IIP and SNe~IIL. In the radioactive-decay phase, SN 2018zd experienced a significant flux drop and behaved more like a low-luminosity SN~IIP both spectroscopically and photometrically. This contrast indicates that circumstellar interaction plays a vital role in modifying the observed light curves of SNe~II. Comparing nebular-phase spectra with model predictions suggests that SN 2018zd arose from a star of $sim 12,rm M_{odot}$. Given the relatively small amount of $^{56}$Ni ($0.013 - 0.035 rm M_{odot}$), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star which collapsed owing to electron capture.
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We present spectroscopic and photometric observations of the Type IIn supernova (SN) 2008iy. SN 2008iy showed an unprecedentedly long rise time of ~400 days, making it the first SN to take significantly longer than 100 days to reach peak optical luminosity. The peak absolute magnitude of SN 2008iy was M_r ~ -19.1 mag, and the total radiated energy over the first ~700 days was ~2 x 10^50 erg. Spectroscopically, SN 2008iy is very similar to the Type IIn SN 1988Z at late times, and, like SN 1988Z, it is a luminous X-ray source (both supernovae had an X-ray luminosity L_ X > 10^41 erg/s). The Halpha emission profile of SN 2008iy shows a narrow P Cygni absorption component, implying a pre-SN wind speed of ~100 km/s. We argue that the luminosity of SN 2008iy is powered via the interaction of the SN ejecta with a dense, clumpy circumstellar medium. The ~400 day rise time can be understood if the number density of clumps increases with distance over a radius ~1.7 x 10^16 cm from the progenitor. This scenario is possible if the progenitor experienced an episodic phase of enhanced mass-loss < 1 century prior to explosion or the progenitor wind speed increased during the decades before core collapse. We favour the former scenario, which is reminiscent of the eruptive mass-loss episodes observed for luminous blue variable (LBV) stars. The progenitor wind speed and increased mass-loss rates serve as further evidence that at least some, and perhaps all, Type IIn supernovae experience LBV-like eruptions shortly before core collapse. We also discuss the host galaxy of SN 2008iy, a subluminous dwarf galaxy, and offer a few reasons why the recent suggestion that unusual, luminous supernovae preferentially occur in dwarf galaxies may be the result of observational biases.
We present high-cadence ultraviolet (UV), optical, and near-infrared (NIR) data on the luminous Type II-P supernova SN 2017gmr from hours after discovery through the first 180 days. SN 2017gmr does not show signs of narrow, high-ionization emission lines in the early optical spectra, yet the optical lightcurve evolution suggests that an extra energy source from circumstellar medium (CSM) interaction must be present for at least 2 days after explosion. Modeling of the early lightcurve indicates a ~500R$_{odot}$ progenitor radius, consistent with a rather compact red supergiant, and late-time luminosities indicate up to 0.130 $pm$ 0.026 M$_{odot}$ of $^{56}$Ni are present, if the lightcurve is solely powered by radioactive decay, although the $^{56}$Ni mass may be lower if CSM interaction contributes to the post-plateau luminosity. Prominent multi-peaked emission lines of H$alpha$ and [O I] emerge after day 154, as a result of either an asymmetric explosion or asymmetries in the CSM. The lack of narrow lines within the first two days of explosion in the likely presence of CSM interaction may be an example of close, dense, asymmetric CSM that is quickly enveloped by the spherical supernova ejecta.
The supernovae of Type Ibc are rare and the detailed characteristics of these explosions have been studied only for a few events. Unlike Type II SNe, the progenitors of Type Ibc have never been detected in pre-explosion images. So, to understand the nature of their progenitors and the characteristics of the explosions, investigation of proximate events are necessary. Here we present the results of multi-wavelength observations of Type Ib SN 2007uy in the nearby ($sim$ 29.5 Mpc) galaxy NGC 2770. Analysis of the photometric observations revealed this explosion as an energetic event with peak absolute R band magnitude $-18.5pm0.16$, which is about one mag brighter than the mean value ($-17.6pm0.6$) derived for well observed Type Ibc events. The SN is highly extinguished, E(B-V) = 0.63$pm$0.15 mag, mainly due to foreground material present in the host galaxy. From optical light curve modeling we determine that about 0.3 M$_{odot}$ radioactive $^{56}$Ni is produced and roughly 4.4 M$_{odot}$ material is ejected during this explosion with liberated energy $sim 15times10^{51}$ erg, indicating the event to be an energetic one. Through optical spectroscopy, we have noticed a clear aspheric evolution of several line forming regions, but no dependency of asymmetry is seen on the distribution of $^{56}$Ni inside the ejecta. The SN shock interaction with the circumburst material is clearly noticeable in radio follow-up, presenting a Synchrotron Self Absorption (SSA) dominated light curve with a contribution of Free Free Absorption (FFA) during the early phases. Assuming a WR star, with wind velocity $ga 10^3 {rm km s}^{-1}$, as a progenitor, we derive a lower limit to the mass loss rate inferred from the radio data as $dot{M} ga 2.4times10^{-5}$ M$_{odot}$, yr$^{-1}$, which is consistent with the results obtained for other Type Ibc SNe bright at radio frequencies.
In the transitional mass range ($sim$ 8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova having strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition, explosion energy, light curve, and nucleosynthesis. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically-enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained with the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis, and remnant populations in the transitional mass range.
We present optical and near-infrared observations of the rapidly evolving supernova (SN) 2017czd that shows hydrogen features. The optical light curves exhibit a short plateau phase ($sim 13$ days in the $R$-band) followed by a rapid decline by $4.5$ mag in $sim 20 mathrm{days}$ after the plateau. The decline rate is larger than those of any standard SNe, and close to those of rapidly evolving transients. The peak absolute magnitude is $-16.8$ mag in the $V$-band, which is within the observed range for SNe IIP and rapidly evolving transients. The spectra of SN 2017czd clearly show the hydrogen features and resemble those of SNe IIP at first. The H$alpha$ line, however, does not evolve much with time and it becomes similar to those in SNe IIb at decline phase. We calculate the synthetic light curves using a SN IIb progenitor which has 16 M$_{odot}$ at the zero-age main sequence and evolves in a binary system. The model with a low explosion energy ($5times 10^{50}$ erg) and a low ${}^{56}$Ni mass ($0.003 mathrm{M}_{odot}$) can reproduce the short plateau phase as well as the sudden drop of the light curve as observed in SN 2017czd. We conclude that SN 2017czd might be the first identified weak explosion from a SN IIb progenitor. We suggest that some rapidly evolving transients can be explained by such a weak explosion of the progenitors with little hydrogen-rich envelope.
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