The prevalence and influence of circumstellar material around hydrogen-rich supernova progenitors

TL;DR

This study systematically investigates hydrogen-rich supernovae, revealing that over a third exhibit circumstellar material signatures shortly after explosion, which are typically short-lived and do not significantly affect early brightness.

Contribution

First large-scale survey confirming that circumstellar material presence is common around Type II supernova progenitors and characterizing its observational signatures and durations.

Findings

Over 36% of early Type II SNe show flash-ionization features.

Most flash features last about 5 days, with some persisting over 10 days.

CSM interaction does not significantly alter early SN brightness or color.

Abstract

Narrow transient emission lines (flash-ionization features) in early supernova (SN) spectra trace the presence of circumstellar material (CSM) around the massive progenitor stars of core-collapse SNe. The lines disappear within days after the SN explosion, suggesting that this material is spatially confined, and originates from enhanced mass loss shortly (months to a few years) prior to explosion. We performed a systematic survey of H-rich (Type II) SNe discovered within less than two days from explosion during the first phase of the Zwicky Transient Facility (ZTF) survey (2018-2020), finding thirty events for which a first spectrum was obtained within $< 2$ days from explosion. The measured fraction of events showing flash ionisation features ($>36\%$ at $95\%$ confidence level) confirms that elevated mass loss in massive stars prior to SN explosion is common. We find that SNe II showing flash ionisation features are not significantly brighter, nor bluer, nor more slowly rising than those without. This implies that CSM interaction does not contribute significantly to their early continuum emission, and that the CSM is likely optically thin. We measured the persistence duration of flash ionisation emission and find that most SNe show flash features for $\approx 5 $ days. Rarer events, with persistence timescales $>10$ days, are brighter and rise longer, suggesting these may be intermediate between regular SNe II and strongly-interacting SNe IIn.