Early-Time Ultraviolet Spectroscopy and Optical Follow-up Observations of the Type IIP Supernova 2021yja

TL;DR

This study presents early ultraviolet and optical spectra of supernova 2021yja, revealing rapid UV cooling, a long plateau phase, and insights into explosion velocity, metallicity, and circumstellar interaction.

Contribution

It provides the first detailed early-time UV spectra of SN 2021yja, modeling its properties and constraining explosion parameters and environment.

Findings

UV spectra show rapid early cooling.

Long plateau with shallow decline observed.

UV emission confined near the photosphere.

Abstract

We present three epochs of early-time ultraviolet (UV) and optical HST/STIS spectroscopy of the young, nearby Type IIP supernova (SN) 2021yja. We complement the HST data with two earlier epochs of Swift UVOT spectroscopy. The HST and Swift UVOT spectra are consistent with those of other well-studied Type IIP supernovae (SNe). The UV spectra exhibit rapid cooling at early times, while less dramatic changes are seen in the optical. We also present Lick/KAIT optical photometry up to the late-time-tail phase, showing a very long plateau and shallow decline compared with other SNe IIP. Our modeling of the UV spectrum with the TARDIS radiative-transfer code produces a good fit for a high-velocity explosion, a low total extinction $E(B-V) = 0.07$ mag, and a subsolar metallicity. We do not find a significant contribution to the UV flux from an additional heating source, such as interaction with the circumstellar medium, consistent with the observed flat plateau. Furthermore, the velocity width of the Mg II $\lambda$2798 line is comparable to that of the hydrogen Balmer lines, suggesting that the UV emission is confined to a region close to the photosphere.