Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

We propose new diagnostics for circumstellar interaction in Type IIPsupernovae by the detection of high velocity (HV) absorption features in Halphaand He I 10830 A lines during the photospheric stage. To demonstrate themethod, we compute the ionization and excitation of H and He in supernovaejecta taking into account time-dependent effects and X-ray irradiation. Wefind that the interaction with a typical red supergiant wind should result inthe enhanced excitation of the outer layers of unshocked ejecta and theemergence of corresponding HV absorption, i.e. a depression in the blueabsorption wing of Halpha and a pronounced absorption of He I 10830 A at aradial velocity of about -10,000 km/s. We identify HV absorption in Halpha andHe I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to thiseffect. The derived mass loss rate is close to 10^{-6} Msun/yr for bothsupernovae, assuming a wind velocity 10 km/s. We argue that, in addition to theHV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN1999em show a HV notch feature that is formed in the cool dense shell (CDS)modified by the Rayleigh-Taylor instability. The CDS results from both shockbreakout and radiative cooling of gas that has passed through the reverse shockwave. The notch becomes dominant in the HV absorption during the latephotospheric phase, ~60 d. The wind density deduced from the velocity of theCDS is consistent with the wind density found from the HV absorption producedby unshocked ejecta.Comment: 38 pages, 12 figures, ApJ, in pres