Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

TL;DR

This study presents ultraviolet spectra and photometry of four Type Ia supernovae, revealing significant diversity in UV emission and its correlation with optical properties, suggesting factors like explosion asymmetry influence UV variability.

Contribution

The paper provides the first UV spectral time series for multiple SNe Ia, highlighting UV diversity and its potential link to explosion asymmetries and progenitor metallicity.

Findings

Significant diversity observed in near-maximum UV spectra of SNe Ia.

UV emission shows larger dispersion compared to optical, with weaker correlation to light-curve shape.

SN 2004dt is an outlier, brighter in UV than typical SNe Ia, indicating complex underlying factors.

Abstract

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This dataset provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near maximum-light spectra (~ 2000--3500 Angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in uvw1/F250W are found to correlate with the B-band light-curve shape parameter dm15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 < dm15 < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.