We present extensive photometric and spectroscopic observations of the Type
Ia supernova (SN Ia) 2000cx in the S0 galaxy NGC 524, which reveal it to b
e peculiar. Photometrically, SN 2000cx is different from all known SNe Ia,
and its light curves cannot be fitted well by the fitting techniques curren
tly available. There is an apparent asymmetry in the B-band peak, in which
the premaximum brightening is relatively fast (similar to that of the norma
l SN 1994D), but the postmaximum decline is relatively slow (similar to tha
t of the overluminous SN 1991T). The color evolution of SN 2000cx is also p
eculiar: the (B-V)(o) color has a unique plateau phase and the (V-R)(o) and
(V-I)(o) colors are very blue.
Although the premaximum spectra of SN 2000cx are similar to those of SN 199
1T-like objects (with weak Si ii lines), its overall spectral evolution is
quite different. The Si ii lines that emerged near maximum B-band brightnes
s stay strong in SN 2000cx until about 3 weeks past maximum. The change in
the excitation stages of iron-peak elements is slow. Both the iron-peak and
the intermediate-mass elements are found to be moving at very high expansi
on velocities in the ejecta of SN 2000cx.
We discuss theoretical models for SN 2000cx. SN 2000cx may be an overlumino
us object like SN 1991T, but with a larger yield of Ni-56 and a higher kine
tic energy in the ejecta. We also briefly discuss the implications of our o
bservations for the luminosity versus light-curve width relation.