Ultraviolet Spectroscopy of Type IIb Supernovae: Diversity and the Impact of Circumstellar Material

TL;DR

This study analyzes ultraviolet spectra of four Type IIb supernovae, revealing diversity in their features and suggesting that circumstellar material interaction influences UV emission and shock cooling behaviors.

Contribution

It provides the first comprehensive UV spectral comparison of multiple Type IIb supernovae and links UV excess to circumstellar interaction and shock cooling properties.

Findings

Diverse UV spectral features among Type IIb supernovae.

UV continuum excess likely caused by circumstellar material interaction.

Slower shock cooling correlates with stronger UV emission.

Abstract

We present new Hubble Space Telescope (HST) multi-epoch ultraviolet (UV) spectra of the bright Type IIb SN 2013df, and undertake a comprehensive anal- ysis of the set of four Type IIb supernovae for which HST UV spectra are available (SN 1993J, SN 2001ig, SN 2011dh, and SN 2013df). We find strong diversity in both continuum levels and line features among these objects. We use radiative-transfer models that fit the optical part of the spectrum well, and find that in three of these four events we see a UV continuum flux excess, apparently unaffected by line absorption. We hypothesize that this emission originates above the photosphere, and is related to interaction with circumstel- lar material (CSM) located in close proximity to the SN progenitor. In contrast, the spectra of SN 2001ig are well fit by single-temperature models, display weak continuum and strong reverse-fluorescence features, and are similar to spectra of radioactive 56Ni-dominated Type Ia supernovae. A comparison of the early shock-cooling components in the observed light curves with the UV continuum levels which we assume trace the strength of CSM interaction suggests that events with slower cooling have stronger CSM emission. The radio emission from events having a prominent UV excess is perhaps consistent with slower blast-wave velocities, as expected if the explosion shock was slowed down by the CSM that is also responsible for the strong UV, but this connection is currently speculative as it is based on only a few events.