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 Ia) 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 <= 10 days and a peak absolute
B magnitude of -15.4 +/- 0.2 at most, making it one of the most subluminous SN
Ia 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, similar to that of SN 2008ha but
about 4000 and 7000 km/s less than that of SN 2002cx and normal SN Ia,
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.
