2008ApJ...680..580S

UBVRI photometry and medium-resolution optical spectroscopy of the peculiar Type Ia supernova SN 2005hk are presented and analyzed, covering the premaximum phase to around 400 days after explosion. The supernova is found to be underluminous compared to ``normal'' Type Ia supernovae. The photometric and spectroscopic evolution of SN 2005hk is remarkably similar to the peculiar Type Ia event SN 2002cx. The expansion velocity of the supernova ejecta is found to be lower than normal Type Ia events. The spectra obtained ≳200 days since explosion do not show the presence of forbidden [Fe II], [Fe III], and [Co III] lines, but are dominated by narrow, permitted Fe II, NIR Ca II, and Na I lines with P Cygni profiles. The thermonuclear explosion model with Chandrasekhar mass ejecta and a kinetic energy smaller (E_K=0.3x10⁵¹ ergs) than that of canonical Type Ia supernovae is found to well explain the observed bolometric light curve. The mass of ⁵⁶Ni synthesized in this explosion is 0.18 M_☉. The early spectra are successfully modeled with this less energetic model, with some modifications of the abundance distribution. The late spectrum is explained as a combination of a photospheric component and a nebular component.