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

We present optical UBVRI photometry and medium-resolution spectroscopy of a transitional Type Ia supernova, SN 2009an, over the period -6 to similar to+ 150 d from the B maximum. With a Delta m(15)(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-circle dot of Ni-56 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 (similar to 0.4) of the Si II line ratio R(Si II). The velocity gradient of the Si II 6355 angstrom line in the post-maximum epoch ((v) over dot = 60 km s(-1) d(-1)) is at the boundary between the low-velocity-gradient and high-velocity-gradient groups.