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

The Astrophysical Journal (Impact Factor: 6.73). 10/2011; DOI: 10.1088/0004-637X/749/2/126
Source: arXiv

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.

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    ABSTRACT: High-quality observations of $B$ and $V$ light curves obtained at Las Campanas Observatory for local Type Ia Supernovae (SNe Ia) show clear evidence that SNe Ia with the same brightness decline or stretch may have systematic and independent deviations at times < 5 days before and at times > 30 days after maximum light. This suggests the existence of two independent secondary parameters which control the shape of SN Ia light curves in addition to the brightness decline relation. stretch. The differences are consistent in morphology of the time dependence and size with predictions by models within the delayed detonation scenario. The secondary parameters may reflect two independent physical effects caused by variations in the progenitor and accretion rates, and link the LC variations in shape with the intrinsic, absolute brightness. Comment: 32 pages, 9 Figures, ApJ accepted
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