2013MNRAS.430..869S

We present optical UBVRI photometry and medium-resolution spectroscopy of a transitional Type Ia supernova, SN 2009an, over the period -6 to ∼ +150d 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 0.41M☉ 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 Caii 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 Siii line ratio R(Siii). The velocity gradient of the Siii 6355 Å line in the post-maximum epoch ({dot}v = 60km/s/d) is at the boundary between the low-velocity-gradient and high-velocity-gradient groups.