Disparity between H\alpha\ and H\beta\ in SN 2008in: inhomogeneous external layers of type IIP supernovae?

Abstract

We study disparity between H\alpha\ and H\beta\ in early spectra of the type IIP supernova SN 2008in. The point is that these lines cannot be described simultaneously in a spherically-symmetric model with the smooth density distribution. It is shown that an assumption of a clumpy structure of external layers of the envelope resolves the problem. We obtain estimates of the velocity at the inner border of the inhomogeneous zone (\approx6100 km s^{-1}), the filing factor of inhomogeneities (\leq0.5), and the mass of the inhomogeneous layers (~ 0.03 M_{\odot}). The amplitude of flux fluctuations in the early spectrum of H\alpha\ (\Delta F/F ~ 10^{-2}) imposes a constraint on the size of inhomogeneities (\leq 200 km s^{-1}). A detection of fluctuations in the early H\alpha\ of type IIP supernovae might become an observational test of the inhomogeneous structure of their envelopes. We propose also the indirect test of the clumpy structure of external layers: the study of properties of the initial radiation outburst due to the shock breakout. The inhomogeneous structure of external layers of type IIP supernovae could be an outcome of density perturbations and density inversion in outer convective layers of presupernova red supergiant.