Photometric and spectroscopic evolution of supernova SN 2009an: another case of a transitional Type Ia event

Abstract

We present optical UBVRI photometry and medium-resolution spectroscopy of a transitional Type Ia supernova, SN 2009an, over the period −6 to +150 d from the B maximum. With a Δm15(B) = 1.514 ± 0.132, SN 2009an declines faster than normal Type Ia events, but slower than the fast-declining, low-luminosity 1991bg-like events. The B-band absolute magnitude at maximum is −19.02 ± 0.20. The peak bolometric luminosity indicates that 0.41 M⊙ of 56Ni was synthesized during the explosion. The pre-maximum and early post-maximum spectral evolution of SN 2009an is very similar to that in the transitional Type Ia SN 2004eo. High-velocity features in the Ca ii near-infrared triplet are seen during the early phases. Similar to the other few objects belonging to this class, SN 2009an exhibits a higher value (0.4) of the Si ii line ratio R(Si ii). The velocity gradient of the Si ii 6355 Å line in the post-maximum epoch (v˙=60 km s−1 d−1) is at the boundary between the low-velocity-gradient and high-velocity-gradient groups.