Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

Abstract

We propose new diagnostics for circumstellar interaction in Type IIP supernovae (SNe IIP) by the detection of high-velocity (HV ) absorption features in H and He i 10830 8 lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account timedependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e., a depression in the blue absorption wing of H and a pronounced absorption of He i 10830 8 at a radial velocity of about À10 4 km s À1 . We identify HV absorption in H and He i 10830 8 lines of SN 1999em and in H of SN 2004dj as being due to this effect. The derived mass-loss rate is close to 10 À6 M yr À1 for both supernovae, assuming a wind velocity 10 km s À1 . We argue that in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em 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 shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, k60 days. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.

Moderately luminous Type II supernovae

“…At this stage also SN 2009bw andSN 1999em show HV Hα at ∼7300 km s −1 (Inserra et al 2012a) and ∼8200 km s −1 (Chugai et al 2007), respectively. The weak HV Hα components of SN 2009dd and SN 1996W appear at velocity ∼13800 and ∼11500 km s −1 , respectively, remaining steady during the early phases.…”

“…Recently also a few SNe with overall normal spectral features have shown weak but unequivocal evidence of ejecta-CSM interaction from early times, reviving the interest for the studies of interacting SNe. Among them we recall the type IIP SNe 1999em and 2004dj (Chugai et al 2007) or the atypical type II SN 2007od (Inserra et al 2011) and SN 2009bw (Inserra et al 2012a), the last two belonging to the bright tail of the type IIP SN luminosity distribution.…”

Dust formation in the ejecta of the type II-P supernova 2004dj

“…Applying Eq. (10) of Chugai et al (2007), the expected radius of the continously growing CDS is 1.9−7.0 × 10 16 cm between +267 and +1246 days in a self-similar model. These values agree well with the size of the cold component involved in our models (see Table 6), which strengthens the assumption that CDS plays a role in producing the MIR SEDs.…”

SN 1999ga: a low-luminosity linear type II supernova?

“…This could be caused by a temporary peak in the density profile of the hydrogen layer. A flat-topped profile may be also produced in a cool dense shell (CDS) that forms at the interface between the SN ejecta and the wind produced by the progenitor (Chugai et al 2007). …”

“…The Hα feature is narrower and, in contrast to the December 29 spectrum, has evolved developing a rounded profile. In the ejectawind interaction scenario of Chugai et al (2007), this evolution of Hα is expected to be accompained by an absorption component that progessively shifts toward redder wavelengths. This is not observed in SN 1999ga, making the CDS scenario quite unlikely.…”