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

摘要

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 data set provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near-maximum-light spectra (approx. 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 the uvw 1/F250W filter are found to correlate with the B-band light-curve shape parameter Delta(m)(15)(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., approx. 0.4 mag versus approx. 0.2 mag for those with 0.8 mag is less than Deltam(15)(B) is less than 1.7 mag). SN 2004dt is found as an outlier of this correlation (at is greater than 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by approx. 0.9 mag and approx. 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.