Speaker
Description
We investigate the ultraviolet (UV) spectral properties of a large sample of Type Ia supernovae (SNe~Ia) observed with the {\it Swift} UltraViolet and Optical Telescope (UVOT) grism. This sample is much larger than our previous works and includes more early-time UV spectra. We investigate how the UV emission relates to $\Delta m_{15}(B)$, host-galaxy properties, and ejecta characteristics. We recover the previously reported correlation between UV flux levels and $\Delta m_{15}(B)$ at near-peak phase, and find the relation to also hold at early times. In contrast, we do not confirm earlier claims of a metallicity dependence at near-peak and find only a weak trend at early times, with no significant correlations observed with other global host-galaxy parameters such as stellar mass, age, star-formation rate, and specific star-formation rate. The UV properties show a strong connection with ejecta properties such as the pseudo-equivalent width of \si\ at both early and near-peak phases, while only a weak association with \si\ ejecta velocity at early times. Furthermore, we investigate SNe~Ia exhibiting early-time photometric excesses and find no corresponding enhancement in their UV spectra, suggesting that these excesses are unlikely to be driven by circumstellar interactions. Finally, we compare the UV spectra of several SNe~Ia subtypes and find distinct UV signatures, consistent with their optical properties. These results indicate that the UV diversity of SNe~Ia is primarily driven by explosion and ejecta properties rather than by their global host-galaxy environment. The UV spectra used in this work are publicly available on \texttt{GitHub}.
| Participate the oral/poster presentation award competition | Yes |
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