2019MNRAS.483.3762K

We examine flash spectroscopy of a circumstellar medium (CSM) ionized by the hard radiation pulse produced by the emerging shock of a supernova (SN). We first find that the rise and fall times of the H α emission constrains the location of the CSM with a peak at t_peak ≃R_*sqrt(2/c v_s) for a star of radius R_* and a shock velocity of v_s. The dropping temperature of the transient emission naturally reproduces the evolution of lines with different ionization energies. Secondly, for red supergiants (RSGs), the shock break out radiatively accelerates the CSM to produce broad, early-time line wings independent of the Thomson optical depth of the CSM. Finally, the CSM recombination rates in binaries can be dominated by a dense, cool, wind collision interface like those seen in Wolf-Rayet binaries rather than the individual stellar winds. Combining these three results, the flash spectroscopy observations of the normal Type IIP iPTF13dqy (SN 2013fs) are naturally explained by an RSG with a normal, Thomson optically thin wind in a binary with a separation of ∼10^4R_☉without any need for a pre-SN eruption. Similarly, the broad line wings seen for the Type IIb iPTF13ast (SN 2013cu), whose progenitors are generally yellow supergiants in binaries, are likely due to radiative acceleration of the CSM rather than a pre-existing, Wolf-Rayet-like wind.