# Searching for the expelled hydrogen envelope in Type I supernovae via late-time H-alpha emission

## TL;DR

This study searches for late-time hydrogen emission in Type I supernovae to detect interaction with expelled hydrogen envelopes, revealing potential circumstellar material interaction years after explosion.

## Contribution

First observational survey to detect late-time H-alpha emission in hydrogen-poor supernovae, indicating possible ejecta-CSM interaction years post-explosion.

## Key findings

- H-alpha emission detected in 13 supernovae, including Type Ibc, IIb, and Ia.
- Temporal variation observed in some supernovae's H-alpha emission, suggesting ongoing interaction.
- Late-time H-alpha emission confirmed in SN 2014C, indicating strong ejecta-CSM interaction.

## Abstract

We report the first results from our long-term observational survey aimed at discovering late-time interaction between the ejecta of hydrogen-poor Type I supernovae and the hydrogen-rich envelope expelled from the progenitor star several decades/centuries before explosion. The expelled envelope, moving with a velocity of ~10 -- 100 km s$^{-1}$, is expected to be caught up by the fast-moving SN ejecta several years/decades after explosion depending on the history of the mass-loss process acting in the progenitor star prior to explosion. The collision between the SN ejecta and the circumstellar envelope results in net emission in the Balmer-lines, especially in H-alpha. We look for signs of late-time H-alpha emission in older Type Ia/Ibc/IIb SNe having hydrogen-poor ejecta, via narrow-band imaging. Continuum-subtracted H-alpha emission has been detected for 13 point sources: 9 SN Ibc, 1 SN IIb and 3 SN Ia events. Thirty-eight SN sites were observed on at least two epochs, from which three objects (SN 1985F, SN 2005kl, SN 2012fh) showed significant temporal variation in the strength of their H-alpha emission in our DIAFI data. This suggests that the variable emission is probably not due to nearby H II regions unassociated with the SN, and hence is an important additional hint that ejecta-CSM interaction may take place in these systems. Moreover, we successfully detected the late-time H-alpha emission from the Type Ib SN 2014C, which was recently discovered as a strongly interacting SN in various (radio, infrared, optical and X-ray) bands.