  We present the early UV and optical light curve of Type IIP supernova (SN)
2010aq at z=0.0862, and compare it to analytical models for thermal emission
following SN shock breakout in a red supergiant star. SN 2010aq was discovered
in joint monitoring between the Galaxy Evolution Explorer (GALEX) Time Domain
Survey (TDS) in the NUV and the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the
g, r, i, and z bands. The GALEX and Pan-STARRS1 observations detect the SN less
than 1 day after shock breakout, measure a diluted blackbody temperature of
31,000 +/- 6,000 K 1 day later, and follow the rise in the UV/optical light
curve over the next 2 days caused by the expansion and cooling of the SN
ejecta. The high signal-to-noise ratio of the simultaneous UV and optical
photometry allows us to fit for a progenitor star radius of 700 +/- 200 R_sun,
the size of a red supergiant star. An excess in UV emission two weeks after
shock breakout compared to SNe well fitted by model atmosphere-code synthetic
spectra with solar metallicity, is best explained by suppressed line blanketing
due to a lower metallicity progenitor star in SN 2010aq. Continued monitoring
of PS1 MDS fields by the GALEX TDS will increase the sample of early UV
detections of Type II SNe by an order of magnitude, and probe the diversity of
SN progenitor star properties.
