Some supernovae (SNe), such as Type IIn SNe, are powered by collision of the
SN ejecta with a dense circumstellar matter (CSM). Their emission spectra show
characteristic line shapes of combined broad emission and narrow P-Cyg lines,
which should closely relate to the CSM structure and the mass-loss mechanism
that creates the dense CSM. We quantitatively investigate the relationship
between the line shape and the CSM structure by Monte Carlo radiative transfer
simulations, considering two representative cases of dense CSM formed by steady
and eruptive mass loss. Comparing the Hα emission between the two cases,
we find that a narrow P-Cyg line appears in the eruptive case while it does not
appear in the steady case, due to the difference in the velocity gradient in
the dense CSM. We also reproduce the blue-shifted photon excess observed in
some SNe IIn, which is formed by photon transport across the shock wave and
find the relationship between the velocity of the shocked matter and the amount
of the blue shift of the photon excess. We conclude that the presence or
absence of narrow P-Cyg lines can distinguish the mass loss mechanism, and
suggest high-resolution spectroscopic observations with λ/Δλ≳104 after the light curve peak for applying this diagnostic
method.Comment: Submitted to Ap