The 1999aa-like Type Ia Supernova iPTF14bdn in the Ultraviolet and Optical

TL;DR

This paper presents the first UV spectral series of a 1999aa-like Type Ia supernova, revealing unique early-time spectral features and temperature effects linked to iron group element distribution, enhancing understanding of supernova diversity.

Contribution

It provides the first UV spectral series of a 1999aa-like SN and identifies the origin of early blue colors as a temperature effect caused by iron group elements.

Findings

Early UV colors are due to less absorption between 2800-3200 Å.

High temperatures and iron group elements suggest mixing of Ni in outer ejecta.

The I-band secondary maximum is caused by Fe III to Fe II decay.

Abstract

We present ultraviolet (UV) and optical photometry and spectra of the 1999aa-like supernova (SN) iPTF14bdn. The UV data were observed using the Swift Ultraviolet/Optical Telescope (UVOT) and constitute the first UV spectral series of a 1999aa-like SN. From the photometry we measure $\Delta m_{15}({\it B})\,=\,0.84 \pm0.05$ mag and blue UV colors at epochs earlier than $-5$ days. The spectra show that the early-time blue colors are the result of less absorption between $2800 - 3200 \,\AA~$ than is present in normal SNe Ia. Using model spectra fits of the data at $-10 $ and $+10 $ days, we identify the origin of this spectral feature to be a temperature effect in which doubly ionized iron group elements create an opacity 'window'. We determine that the detection of high temperatures and large quantities of iron group elements at early epochs imply the mixing of a high Ni mass into the outer layers of the SN ejecta. We also identify the source of the I-band secondary maximum in iPTF14bdn to be the decay of Fe III to Fe II, as is seen in normal SNe Ia.