Flash Spectroscopy: Emission Lines from the Ionized Circumstellar
  Material around $<10$-Day-Old Type II Supernovae
  Supernovae (SNe) embedded in dense circumstellar material (CSM) may show
prominent emission lines in their early-time spectra ($\leq 10$ days after the
explosion), owing to recombination of the CSM ionized by the shock-breakout
flash. From such spectra ("flash spectroscopy"), we can measure various
physical properties of the CSM, as well as the mass-loss rate of the progenitor
during the year prior to its explosion. Searching through the Palomar Transient
Factory (PTF and iPTF) SN spectroscopy databases from 2009 through 2014, we
found 12 Type II SNe showing flash-ionized (FI) signatures in their first
spectra. All are younger than 10 days. These events constitute 14\% of all 84
SNe in our sample having a spectrum within 10 days from explosion, and 18\% of
SNe~II observed at ages $<5$ days, thereby setting lower limits on the fraction
of FI events. We classified as "blue/featureless" (BF) those events having a
first spectrum which is similar to that of a black body, without any emission
or absorption signatures. It is possible that some BF events had FI signatures
at an earlier phase than observed, or that they lack dense CSM around the
progenitor. Within 2 days after explosion, 8 out of 11 SNe in our sample are
either BF events or show FI signatures. Interestingly, we found that 19 out of
21 SNe brighter than an absolute magnitude $M_R=-18.2$ belong to the FI or BF
groups, and that all FI events peaked above $M_R=-17.6$ mag, significantly
brighter than average SNe~II.
