An international group of astronomers has reported the discovery of 12 new type II long-running supernovae as part of the Zwicky Transition Facility (ZTF) Local Universe Census (CLU). Discovery releases on preprint repository June 1st arXivnearly double the number of known supernovae of this subclass.

Type II supernovae (SNe) are the result of the rapid collapse and violent explosion of massive stars (masses greater than 8.0 solar masses). They differ from other SNes by the presence of hydrogen in their spectra. According to the shape of the light curves, they are generally divided into type IIL and type IIP. Type IIL SNe shows a steady (linear) decline after detonation, while Type IIPs show a slower decline (plateau) followed by a normal degradation.

Some Type II SNEs are characterized by unusually long peak increases lasting more than 40 days. Observations show that, in general, such long origin SNes originate from more compact (less than 100 solar radii), massive (about 20 solar masses) stars and have higher burst energies. However, thirty years have passed since the discovery of the first long eastern type II SNe, designated SN 1987A, only 16 eruptions of this subclass have been detected in the local universe.

Recently, a team of astronomers led by Tony Seet of Ohio State University (OSU) identified 13 far eastern Type II SNes (out of a total of 3,444 transients), of which 12 are new discoveries. Observations were made using the 48-inch Schmidt telescope (P48) at the Palomar Observatory as part of the ZTF CLU experiment.

The newly discovered type II SNe has collection times of 67 to 96 days and dome-shaped light curves in the r-band of photometry. All have peak absolute magnitudes in the r-band between -15.6 and -17.5, which is consistent with previous studies of known long eastern type II SNE.

The study found that all 12 SNes had spectra showing broad hydrogen alpha lines with velocities largely consistent with those reported in previous observations of Type II Sne. In addition, barium (Ba II) spectral lines were detected in the nine SNe reported in the paper.

The results show that the majority of SNe II in the new sample occurred in subsolar metallic environments, faint dwarf galaxies, and the outskirts of large star-forming galaxies. Two occurred in near or supersolar metallicity environments.

Based on a sample of CLU events, astronomers derive the long-standing growth rate of type II SNe for a completely collapsed supernova (CCSNe). This value has been calculated as approximately 1.4%.

“With our derived frequency of ≈1.4% of all CCSNes, we confirm that SN 1987A and similar SNe are indeed rare events,” the study authors wrote.