Research Highlights

An overly-luminous Type II supernova from a massive progenitor

Bose, S. et al., 2020

ASASSN-18am/2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae with a peak absolute magnitude of \(M_V \approx -20\) mag that is in between normal core-collapse SNe and superluminous SNe, which was also considered as a missing gap by some earlier studies. Only about five such supernovae have been identified so far, which also includes our previously discovered ASASSN-15nx (Bose, S. et al., 2018b, ApJ, 862, 107). The high intrinsic luminosity (without circumstellar interaction) of these supernovae makes it difficult or impossible to explain by widely accepted neutrino driven explosion mechanism. In this work, we examine various powering mechanisms, including radiative diffusion and magentar spindown models. Both of these models required a high synthesised \(\rm ^{56}Ni\) mass \(M_{\rm Ni} \sim0.3-0.4\,\rm M_\odot\) and very high kinetic energy of ejecta \(E_{\rm kin}= (3-9)\times10^{51}\) erg. The high \(\rm ^{56}Ni\) mass is consistent with strong iron-group nebular lines present in nebular spectra. Again, these estimated parameters, especially the high \(\rm ^{56}Ni\) mass can not be produced in recent modeling attempts for neutrino driven explosions.

The earliest spectrum shows "flash ionisation" features, which is also for the first time in these kind of supernovae, from which we estimate a mass-loss rate of \(\dot{M}\approx 2\times10^{-4} \, \rm M_\odot\,yr^{-1}\) and a wind velocity \(\lt 100\,\rm km\,s^{-1}\). Using these wind properties along with detected X-ray luminosity, we infer that the wind density is too low to power the luminous light curve by circumstellar interaction. We measure ejecta expansion velocities as high as \(17,000~\rm km\,s^{-1}\) for \(H\alpha\), which is remarkably high compared to other supernovae. We estimate an oxygen core mass of \(1.7 - 3.1\, \rm M_\odot\) using the \(\rm [O\,I]\) luminosity measured from nebular spectrum, implying a progenitor with an uncommonly high zero-age main-sequence mass of \(19-24\,\rm M_\odot\).

A bipolar core in dust forming envelope of a Normal Type IIP Supernova

Bose, S. et al., 2019, ApJL, 873, L3

ASASSN-16at/2016X exhibited features of normal type IIP supernova during most of its evolution until the early tail phase (<150 days). However, the late nebular spectra (observed until 740 days) showed a unique double peaked \(\rm H\,I\) emission, where each component is symmetrically positioned about the rest wavelength. This bifurcation in the nebular line indicates strong bipolarity in the distribution of radioactive 56Ni or inner ejecta. Such a prominent double-peaked profile has not been observed before in any other supernova nebular spectra. Additionally, ASASSN-16at we find evidence of dust formation during late phases, which adequately explain the evolution of double-peaked nebular emission. As more dust being formed with time, the redder component of the double-peaked structure is seen to progressively diminishing due to differential extinction from dust. The strong and increasing near-infrared excess during the post-photospheric phase also supports the scenario of dust formation. This is one of the finest example which shows the importance of late time observations in core-collapse supernovae, even if they show normal characteristics during early phases.

Nearest Superluminous Supernova in a Massive Metal-rich galaxy

Bose, S. et al., 2018a, ApJ, 853, 57

Gaia17biu/2017egm, which we identified to be a hydrogen-poor Superluminous supernova (SLSN-I) in a record-breaking low redshift of z=0.03 (distance of 400 million light-years) which is one third the distance for any previously discovered SLSNe-I. Interestingly the SN was found in a massive and metal rich spiral galaxy (NGC 3191), unlike hosts of previously observed SLSNe-I. We estimated the host galaxy mass of \(1.6\times 10^{10}\rm\,M_\odot\) and the metallicity near the location of the SN is \(12+\rm log[O/H]\approx 8.9\) (roughly solar metallicity). The host galaxy of Gaia17biu, has a very unusually high mass and metallicity compared to other SLSN-I hosts, although its properties are typical of the general population of star-forming galaxies and the hosts of core-collapse SNe. The low redshift of Gaia17biu, and the relative deficiency of low redshift (z<0.05) SLSNe-I with dwarf hosts implies that any suppression of SLSN-I production in metal-rich and massive hosts is likely weaker than previously thought.

Having the advantage of close proximity and low line-of-sight extinction, obtained very detailed obervations which is unprecendented for SLSN-I. Data were obtained in photometric, spectroscopic and spectropolarimetric modes, and spanning a wide wavelength range including X-rays, near ultraviolet, optical, near infreared and radio bands.