From the first two seasons of the Palomar Transient Factory , we identify three peculiar transients ( PTF 09dav , PTF 10iuv , PTF 11bij ) with five distinguishing characteristics : peak luminosity in the gap between novae and supernovae ( M _ { R } \approx - 15.5 to - 16.5 mag ) , rapid photometric evolution ( t _ { rise } \approx 12–15 days ) , large photospheric velocities ( \approx 6000 to 11000 km s ^ { -1 } ) , early spectroscopic evolution into nebular phase ( \approx 1 to 3 months ) and peculiar nebular spectra dominated by Calcium .
We also culled the extensive decade-long Lick Observatory Supernova Search database and identified an additional member of this group , SN 2007ke .
Our choice of photometric and spectroscopic properties was motivated by SN 2005E ( ) .
To our surprise , as in the case of SN 2005E , all four members of this group are also clearly offset from the bulk of their host galaxy .
Given the well-sampled early and late-time light curves , we derive ejecta masses in the range of 0.4–0.7 M _ { \odot } .
Spectroscopically , we find that there may be a diversity in the photospheric phase , but the commonality is in the unusual nebular spectra .
Our extensive follow-up observations rule out standard thermonuclear and standard core-collapse explosions for this class of “ Calcium-rich gap ” transients .
If the progenitor is a white dwarf , we are likely seeing a detonation of the white dwarf core and perhaps even shock-front interaction with a previously ejected nova shell .
In the less likely scenario of a massive star progenitor , a nonstandard channel specific to a low-metallicity environment needs to be invoked ( e.g. , ejecta fallback leading to black hole formation ) .
Detection ( or the lack thereof ) of a faint underlying host ( dwarf galaxy , cluster ) will provide a crucial and decisive diagnostic to choose between these alternatives .