Red eminence: The intermediate-luminosity red transient AT 2022fnm

TL;DR

This paper reports on the detailed five-month observation of AT 2022fnm, an unusual transient with properties of both LRNe and ILRTs, suggesting a weak stellar eruption or low-energy supernova-like explosion.

Contribution

It provides the first comprehensive observational analysis of AT 2022fnm, highlighting its hybrid features and modeling its light curve to infer physical parameters.

Findings

No second peak in light curve, unlike LRNe.

Spectral properties resemble ILRTs.

Estimated ejecta mass of about 4 solar masses.

Abstract

We present results from a five-month-long observing campaign of the unusual transient AT 2022fnm, which displays properties common to both luminous red novae (LRNe) and intermediate-luminosity red transients (ILRTs). Although its photometric evolution is broadly consistent with that of LRNe, no second peak is apparent in its light curve, and its spectral properties are more reminiscent of ILRTs. It has a fairly rapid rise time of 5.3$\pm$1.5 d, reaching a peak absolute magnitude of $-12.7\pm$0.1 (in the ATLAS $o$ band). We find some evidence for circumstellar interaction, and a near-infrared excess becomes apparent at approximately +100 d after discovery. We attribute this to a dust echo. Finally, from an analytical diffusion toy model, we attempted to reproduce the pseudo-bolometric light curve and find that a mass of $\sim$4 M$_\odot$ is needed. Overall, the characteristics of AT 2022fnm are consistent with a weak stellar eruption or an explosion reminiscent of low-energy type IIP supernovae, which is compatible with expectations for ILRTs.