SN 2008in - Bridging the gap between normal and faint supernovae of type IIP

We present optical photometric and low-resolution spectroscopic observations of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts of the nearly face-on spiral galaxy M61. Photometric data in the X-ray, ultraviolet, and near-infrared bands have been used to characterize this event. The SN field was imaged with the ROTSE-IIIb optical telescope about seven days before the explosion. This allowed us to constrain the epoch of the shock breakout to JD = 2454825.6. The duration of the plateau phase, as derived from the photometric monitoring, was ∼98 days. The spectra of SN 2008in show a striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D and 1999br. A comparison of ejecta kinematics of SN 2008in with the hydrodynamical simulations of Type IIP SNe by Dessart et al. indicates that it is a less energetic event (∼5 × 10⁵⁰ erg). However, the light curve indicates that the production of radioactive ⁵⁶Ni is significantly higher than that in the low-luminosity SNe. Adopting an interstellar absorption along the SN direction of A_V ∼0.3 mag and a distance of 13.2 Mpc, we estimated a synthesized ⁵⁶Ni mass of ∼0.015 M _⊙. Employing semi-analytical formulae derived by Litvinova and Nadezhin, we derived a pre-SN radius of ∼126 R_⊙, an explosion energy of 5.4 × 10⁵⁰ erg, and a total ejected mass of ∼16.7 M_⊙. The latter indicates that the zero-age main-sequence mass of the progenitor did not exceed 20 M_⊙. Considering the above properties of SN 2008in and its occurrence in a region of sub-solar metallicity ([O/H] ∼ 8.44 dex), it is unlikely that fall-back of the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore favor a low-energy explosion scenario of a relatively compact, moderate-mass progenitor star that generates a neutron star.