We present a sample of 34 normal SNe II detected with the Zwicky Transient
Facility, with multi-band UV light-curves starting at tβ€4 days after
explosion, as well as X-ray detections and upper limits. We characterize the
early UV-optical colors and provide prescriptions for empirical host-extinction
corrections. We show that the t>2days UV-optical colors and the blackbody
evolution of the sample are consistent with the predictions of spherical phase
shock-cooling (SC), independently of the presence of `flash ionization"
features. We present a framework for fitting SC models which can reproduce the
parameters of a set of multi-group simulations without a significant bias up to
20% in radius and velocity. Observations of about half of the SNe II in the
sample are well-fit by models with breakout radii <1014cm. The other
half are typically more luminous, with observations from day 1 onward that are
better fit by a model with a large >1014cm breakout radius. However,
these fits predict an early rise during the first day that is too slow. We
suggest these large-breakout events are explosions of stars with an inflated
envelope or a confined CSM with a steep density profile, at which breakout
occurs. Using the X-ray data, we derive constraints on the extended
(βΌ1015 cm) CSM density independent of spectral modeling, and find most
SNe II progenitors lose <10β4Mββyrβ1 a few years before
explosion. This provides independent evidence the CSM around many SNe II
progenitors is confined. We show that the overall observed breakout radius
distribution is skewed to higher radii due to a luminosity bias. We argue that
the 66β22+11β% of red supergiants (RSG) explode as SNe II with breakout
radii consistent with the observed distribution of field RSG, with a tail
extending to large radii, likely due to the presence of CSM.Comment: Submitted to ApJ. Comments are welcome at [email protected] or
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