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PTF09dav is a peculiar subluminous type Ia supernova (SN) discovered by thePalomar Transient Factory (PTF). Spectroscopically, it appears superficiallysimilar to the class of subluminous SN1991bg-like SNe, but it has severalunusual features which make it stand out from this population. Its peakluminosity is fainter than any previously discovered SN1991bg-like SN Ia (M_B-15.5), but without the unusually red optical colors expected if the faintluminosity were due to extinction. The photospheric optical spectra have veryunusual strong lines of Sc II and Mg I, with possible Sr II, together withstronger than average Ti II and low velocities of ~6000 km/s. The host galaxyof PTF09dav is ambiguous. The SN lies either on the extreme outskirts (~41kpc)of a spiral galaxy, or in an very faint (M_R>-12.8) dwarf galaxy, unlike other1991bg-like SNe which are invariably associated with massive, old stellarpopulations. PTF09dav is also an outlier on the light-curve-width--luminosityand color--luminosity relations derived for other sub-luminous SNe Ia. Theinferred 56Ni mass is small (0.019+/-0.003Msun), as is the estimated ejectamass of 0.36Msun. Taken together, these properties make PTF09dav a remarkableevent. We discuss various physical models that could explain PTF09dav. Heliumshell detonation or deflagration on the surface of a CO white-dwarf can explainsome of the features of PTF09dav, including the presence of Sc and the lowphotospheric velocities, but the observed Si and Mg are not predicted to bevery abundant in these models. We conclude that no single model is currentlycapable of explaining all of the observed signatures of PTF09dav.

The Subluminous and Peculiar Type Ia Supernova PTF09dav