PTF 09dav is a peculiar subluminous type Ia supernova ( SN ) discovered by the Palomar Transient Factory ( PTF ) . Spectroscopically , it appears superficially similar to the class of subluminous SN1991bg-like SNe , but it has several unusual features which make it stand out from this population . Its peak luminosity is fainter than any previously discovered SN1991bg-like SN Ia ( M _ { B } \sim - 15.5 ) , but without the unusually red optical colors expected if the faint luminosity were due to extinction . The photospheric optical spectra have very unusual strong lines of Sc ii and Mg i , with possible Sr ii , together with stronger than average Ti ii and low velocities of \sim 6000 km s ^ { -1 } . The host galaxy of PTF09dav is ambiguous . The SN lies either on the extreme outskirts ( \sim 41 kpc ) of a spiral galaxy , or in an very faint ( M _ { R } \geq - 12.8 ) dwarf galaxy , unlike other 1991bg-like SNe which are invariably associated with massive , old stellar populations . PTF 09dav is also an outlier on the light-curve-width–luminosity and color–luminosity relations derived for other sub-luminous SNe Ia . The inferred ^ { 56 } \mathrm { Ni } Â mass is small ( 0.019 \pm 0.003 M _ { \odot } ) , as is the estimated ejecta mass of 0.36 M _ { \odot } . Taken together , these properties make PTF 09dav a remarkable event . We discuss various physical models that could explain PTF 09dav . Helium shell detonation or deflagration on the surface of a CO white-dwarf can explain some of the features of PTF 09dav , including the presence of Sc and the low photospheric velocities , but the observed Si and Mg are not predicted to be very abundant in these models . We conclude that no single model is currently capable of explaining all of the observed signatures of PTF 09dav .