THE SUBLUMINOUS SUPERNOVA 2007qd: A MISSING LINK IN A FAMILY OF LOW-LUMINOSITY TYPE Ia SUPERNOVAE

We present multi-band photometry and multi-epoch spectroscopy of the peculiar Type Ia supernova (SN la) 2007qd, discovered by the SDSS-II Supernova Survey. It possesses physical properties intermediate to those of the peculiar SN 2002cx and the extremely low-luminosity SN 2008ha. Optical photometry indicates that it had an extraordinarily fast rise time of less than or similar to 10 days and a peak absolute B magnitude of -15.4 +/- 0.2 at most, making it one of the most subluminous SN la ever observed. Follow-up spectroscopy of SN 2007qd near maximum brightness unambiguously shows the presence of intermediate-mass elements which are likely caused by carbon/oxygen nuclear burning. Near maximum brightness, SN 2007qd had a photospheric velocity of only 2800 km s(-1), similar to that of SN 2008ha but about 4000 and 7000 km s(-1) less than that of SN 2002cx and normal SN la, respectively. We show that the peak luminosities of SN 2002cx like objects are highly correlated with both their light-curve stretch and photospheric velocities. Its strong apparent connection to other SN 2002cx like events suggests that SN 2007qd is also a pure deflagration of a white dwarf, although other mechanisms cannot be ruled out. It may be a critical link between SN 2008ha and the other members of the SN 2002cx like class of objects.