Peculiar core-collapse supernova breaks the mold with a long, dim plateau

Astronomers from the Chinese Academy of Sciences (CAS) have employed the Lijiang 2.4-m telescope to perform optical photometric and spectroscopic observations of a core-collapse Type IIP supernova designated SN 2024abfl. Results of the observational campaign, published April 2 on the arXiv, preprint server, deliver essential information regarding the origin of this peculiar supernova.

Type II supernova classification

Based on the shape of light curves, astronomers generally divide type II supernovae (SNII) into two classes. Type II-Linear supernovae (SNe IIL) have a fairly rapid, linear decay after maximum light, while type II-Plateau supernovae (SNe IIP) remain bright (on a plateau) for an extended period of time after maximum. This plateau in the light curve of a standard SN IIP typically lasts about 100 days.

It is assumed that SNe IIP originate from precursor stars that retain a substantial amount of their hydrogen layers (greater than three solar masses) before exploding as core-collapse supernovae (CCSNe). Although many studies of SNe IIP have been conducted in the last two decades, some of their properties are still not well understood.

A nearby Type IIP supernova

SN 2024abfl is a low-luminosity Type IIP supernova at a redshift of approximately 0.003. Its host galaxy is NGC 2146 and the supernova is characterized by a durable plateau and unusually low expansion velocities.

A team of astronomers led by Luhan Li of CAS has employed the Lijiang 2.4-m and Xinglong 2.16-m telescopes to further investigate SN 2024abfl, hoping to get more insights into its origin.

Distance estimates and properties

First of all, the researchers tried to calculate the distance to the host galaxy of SN 2024abfl, which is crucial for constraining the supernova’s progenitor mass, absolute magnitude, and explosion energy.

They estimate that NGC 2146 is located some 28.5–32.4 million light years away; however, previous studies point to a distance of 31–54.4 million light years. These discrepancies are due to the galaxy’s disturbed morphology, likely driven by tidal interactions with a low–surface brightness companion, which complicates distance determinations.

The study found that SN 2024abfl is intrinsically faint—its inferred plateau luminosity is only about 100 duodecillion erg/s. It turns out that even adopting the largest distance estimates, it still remains at the faint end of the low-luminosity Type IIP supernovae population. The light curve of SN 2024abfl showcases a long-lasting plateau of approximately 110 days.

Furthermore, it was found that SN 2024abfl has an unusually small synthesized mass of radioactive nickel (⁵⁶Ni), which is estimated to be 0.002–0.004 solar masses. In addition, the spectra of this supernova show exceptionally low expansion velocities, with the velocity of iron lines (Fe II) of around 1,200 km/s at 50 days after the explosion, which is significantly lower when compared to other observed SNe IIP.

The origin of SN 2024abfl

Therefore, based on the findings, the authors of the paper assume a very low-mass core-collapse origin of SN 2024abfl.

“Considering the plateau color and duration, the magnitude drop between plateau and tail, and the spectroscopic properties, the event is more consistent with a low-mass CC origin than with an EC [electron capture] SN scenario. A robust distance measurement will be crucial to refine the explosion parameters and further test this interpretation,” the scientists conclude.

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