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We present the optical photometric and spectroscopic observations of the nearby Type Ia supernova (SN) 2021hpr. The observations covered the phase of âˆ'14.37 to +63.68 days relative to its maximum luminosity in the B band. The evolution of multiband light/color curves of SN 2021hpr is similar to that of normal Type Ia supernovae (SNe Ia) with the exception of some phases, especially a plateau phase that appeared in the V âˆ' R color curve before peak luminosity, which resembles that of SN 2017cbv. The first spectrum we observed at t ∼ âˆ'14.4 days shows a higher velocity for the Si ii λ 6355 feature (∼21,000 km sâˆ'1) than that of other normal velocity (NV) SNe Ia at the same phase. Based on the Si ii λ 6355 velocity of ∼12,420 km sâˆ'1 around maximum light, we deduce that SN 2021hpr is a transitional object between high velocity (HV) and NV SNe Ia. Meanwhile, the Si ii λ 6355 feature shows a high velocity gradient (HVG) of about 800 km sâˆ'1 dayâˆ'1 from roughly âˆ'14.37 to âˆ'4.31 days relative to the B -band maximum, which indicates that SN 2021hpr can also be classified as an HVG SN Ia. Despite SN 2021hpr having a higher velocity for the Si ii λ 6355 and Ca ii near-IR (NIR) triplet features in its spectra, its evolution is similar to that of SN 2011fe. Including SN 2021hpr, there have been six supernovae observed in the host galaxy NGC 3147; the supernovae explosion rate in the last 50 yr is slightly higher for SNe Ia, while for SNe Ibc and SNe II it is lower than expected rate from the radio data. Inspecting the spectra, we find that SN 2021hpr has a metal-rich (12 + log(O/H) ≈ 8.648) circumstellar environment, where HV SNe tend to reside. Based on the decline rate of SN 2021hpr in the B band, we determine the distance modulus of the host galaxy NGC 3147 using the Phillips relation to be 33.46 ± 0.21 mag, which is close to that found by previous works. [ABSTRACT FROM AUTHOR]

We present optical and ultraviolet photometry, as well as optical spectra, for the type II supernova (SN) 2015bf. Our observations cover the phases from ∼2 to ∼200 d after explosion. The first spectrum is characterized by a blue continuum with a blackbody temperature of ∼24 000 K and flash-ionized emission lines. After about 1 week, the spectra of SN 2015bf evolve like those of a regular SN II. From the luminosity of the narrow emission component of H α, we deduce that the mass-loss rate is larger than |${\sim}3.7\times 10^{-3}\, {\rm M_\odot \, yr^{-1}}$|⁠. The disappearance of the flash features in the first week after explosion indicates that the circumstellar material is confined within ∼6 × 1014 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 MV  = −18.11 ± 0.08 mag and a fast post-peak decline with a V -band decay of 1.22 ± 0.09 mag within ∼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 56Ni (⁠|${\sim}0.009\, {\rm M_\odot }$|⁠) synthesized 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. [ABSTRACT FROM AUTHOR]

ASASSN-14ms may represent the most luminous Type Ibn supernova (SN Ibn) ever detected, with an absolute U-band magnitude brighter than −22.0 mag and a total bolometric luminosity >1.0 × 1044 erg s−1 near maximum light. The early-time spectra of this SN are characterized by a blue continuum on which are superimposed narrow P Cygni profile lines of He i , suggesting the presence of slowly moving (∼1000 km s−1), He-rich circumstellar material (CSM). At 1–2 months after maximum brightness, the He i line profiles become only slightly broader, with blueshifted velocities of 2000–3000 km s−1, consistent with the CSM shell being continuously accelerated by the SN light and ejecta. Like most SNe Ibn, the light curves of ASASSN-14ms show rapid post-peak evolution, dropping by ∼7 mag in the V band over three months. Such a rapid post-peak decline and high luminosity can be explained by interaction between SN ejecta and helium-rich CSM of 0.9 M⊙ at a distance of ∼1015 cm. The CSM around ASASSN-14ms is estimated to originate from a pre-explosion event with a mass-loss rate of 6.7 M⊙ yr−1 (assuming a velocity of ∼1000 km s−1), which is consistent with abundant He-rich material violently ejected during the late Wolf–Rayet (WN9-11 or Opfe) stage. After examining the light curves for a sample of SNe Ibn, we find that the more luminous ones tend to have slower post-peak decline rates, reflecting that the observed differences may arise primarily from discrepancies in the CSM distribution around the massive progenitors. [ABSTRACT FROM AUTHOR]

We present extensive observations of SN 2018zd covering the first ∼450 d after the explosion. This SN shows a possible shock-breakout signal ∼3.6 h after the explosion in the unfiltered light curve, and prominent flash-ionization spectral features within the first week. The unusual photospheric temperature rise (rapidly from ∼12 000 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 by 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 (⁠|$L_{\rm max} = [1.36\pm 0.63] \times 10^{43}\, \rm erg\, s^{-1}$|⁠) of SN 2018zd. The luminous peak and low expansion velocity (v ≈ 3300 km s−1) make SN 2018zd like a member of the LLEV (luminous SNe II with low expansion velocities) events originating 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 56Ni (⁠|$0.013\!-\!0.035 \rm M_{\odot }$|⁠), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star that collapsed owing to electron capture. [ABSTRACT FROM AUTHOR]

We presented a detailed analysis of progenitor properties of type IIP supernova 2017eaw in NGC 6946, based on the pre-explosion images and early-time observations obtained immediately after the explosion. An unusually red star, with M F814W  = −6.9 mag and m F606W − m F814W = 2.9 ± 0.2 mag, can be identified at the SN position in the pre-discovery Hubble Space Telescope (HST) images taken in 2016. The observed spectral energy distribution of this star, covering the wavelength of 0.6–2.0  |$\ \mathrm{\mu m}$|⁠, matches that of an M4-type red supergiant (RSG) with a temperature of about 3550 K. These results suggest that SN 2017eaw has a RSG progenitor with an initial mass of 12 ± 2 M⊙. The absolute F814W-band magnitude of this progenitor star is found to evolve from −7.2 mag in 2004 to −6.9 mag in 2016. Such a dimming effect is, however, unpredicted for an RSG in its neon/oxygen burning phase when its luminosity should modestly increase. The spectrum of SN 2017eaw taken a few hours after discovery clearly shows a narrow Hα emission feature blueshifted by ∼160 km s−1. This narrow component disappeared in the spectrum taken two days later, suggesting the presence of a circumstellar material (CSM) shell (i.e. at a distance of <2.1–4.3 × 1014 cm). Combining the inferred distance with the expansion velocity of the CSM, we suggest that the progenitor of SN 2017eaw should have experienced violent mass-loss at about 1–2 yr prior to explosion, perhaps invoked by pulsational envelop ejection. This mechanism may help explain its luminosity decline in 2016 as well as the lack of detections of RSGs with relatively higher initial mass as progenitors of SNe IIP. [ABSTRACT FROM AUTHOR]

We present extensive spectroscopic observations for one of the closest Type Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 d before to 473.2 d after B -band maximum light. The diffuse interstellar band features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as A v ∼ 1.9 ± 0.6 mag. Spectroscopically, SN 2014J can be put into the high-velocity (HV) subgroup in Wang's classification with a velocity of Si  ii λ 6355 at maximum light of |$v$| 0 = 1.22 ± 0.01 × 104 km s−1 but has a low velocity gradient (LVG, following Benetti's classification) of |$\dot{v}=41\pm 2$| km s−1 d−1, which is inconsistent with the trend that HV SNe Ia generally have larger velocity gradients. We find that the HV SNe Ia with LVGs tend to have relatively stronger Si  iii (at ∼4400 Å) absorptions in early spectra, larger ratios of S  ii λ 5468 to S  ii λ 5640, and weaker Si  ii 5972 absorptions compared to their counterparts with similar velocities but high velocity gradients. This shows that the HV+LVG subgroup of SNe Ia may have intrinsically higher photospheric temperature, which indicates that their progenitors may experience more complete burning in the explosions relative to the typical HV SNe Ia. [ABSTRACT FROM AUTHOR]

Optical and ultraviolet observations for the nearby type II plateau supernova (SN IIP) 2013am in the nearby spiral galaxy M65 are presented in this paper. The early spectra are characterized by relatively narrow P-Cygni features, with ejecta velocities much lower than observed in normal SNe IIP (i.e., ∼2000 km s–1 versus ∼5000 km –1 in the middle of the plateau phase). Moreover, prominent Ca II absorptions are also detected in SN 2013am at relatively early phases. These spectral features are reminiscent of those seen in the low-velocity and low-luminosity SN IIP 2005cs. However, SN 2013am exhibits different photometric properties, having shorter plateau phases and brighter light curve tails if compared to SN 2005cs. Adopting RV = 3.1 and a mean value of total reddening derived from the photometric and spectroscopic methods (i.e., E(B – V) = 0.55 ± 0.19 mag), we find that SN 2013am may have reached an absolute V-band peak magnitude of –15.83 ± 0.71 mag and produced an 56Ni mass of M☼ in the explosion. These parameters are close to those derived for SN 2008in and SN 2009N, which have been regarded as “gap-filler” objects linking the faint SNe IIP to the normal ones. This indicates that some low-velocity SNe IIP may not necessarily result from the low-energetic explosions. The low expansion velocities could be due to a lower metallicity of the progenitor stars, a larger envelope mass ejected in the explosion, or the effect of viewing angle where these SNe were observed at an angle away from the polar direction. [ABSTRACT FROM AUTHOR]