Very Early Ultraviolet and Optical Observations of the Type Ia Supernova
  2009ig
  Supernova (SN) 2009ig was discovered 17 hours after explosion by the Lick
Observatory Supernova Search, promptly classified as a normal Type Ia SN (SN
Ia), peaked at V = 13.5 mag, and was equatorial, making it one of the foremost
supernovae for intensive study in the last decade. Here, we present ultraviolet
(UV) and optical observations of SN 2009ig, starting about 1 day after
explosion until around maximum brightness. Our data include excellent UV and
optical light curves, 25 premaximum optical spectra, and 8 UV spectra,
including the earliest UV spectrum ever obtained of a SN Ia. SN 2009ig is a
relatively normal SN Ia, but does display high-velocity ejecta - the ejecta
velocity measured in our earliest spectra (v ~ -23,000 km/s for Si II 6355) is
the highest yet measured in a SN Ia. The spectral evolution is very dramatic at
times earlier than 12 days before maximum brightness, but slows after that
time. The early-time data provide a precise measurement of 17.13 +/- 0.07 days
for the SN rise time. The optical color curves and early-time spectra are
significantly different from template light curves and spectra used for
light-curve fitting and K-corrections, indicating that the template light
curves and spectra do not properly represent all Type Ia supernovae at very
early times. In the age of wide-angle sky surveys, SNe like SN 2009ig that are
nearby, bright, well positioned, and promptly discovered will still be rare. As
shown with SN 2009ig, detailed studies of single events can provide
significantly more information for testing systematic uncertainties related to
SN Ia distance estimates and constraining progenitor and explosion models than
large samples of more distant SNe.
