Discovery and evolution of an unusual luminous variable star in NGC 3432 (Supernova 2000ch)

We present photometric and spectroscopic observations of SN 2000ch, an unusual and extremely luminous variable star located in the galaxy NGC 3432. The object was discovered on 2000 May 3.2 during the course of the Lick Observatory Supernova Search, at an unfiltered magnitude of about 17.4. Prediscovery images obtained in 1997, 1998, and 2000 April show the object with R = 19.2-19.5 mag. Optical spectra obtained beginning on 2000 May 6 show a smooth, flat continuum and strong, broad hydrogen Balmer emission lines at wavelengths consistent with the cataloged redshift of NGC 3432, strengthening the association of the variable with the galaxy. Photometric monitoring reveals a complex and erratic light curve over a time span of ∼10 days. Subsequent optical spectra over the next ∼3 months continued to show strong Balmer emission lines with a mean full width at half-maximum intensity ∼1550 km s^-1 and a distinct red asymmetry. A spectrum obtained 9 months after the outburst is similar to the previous spectra, but the integrated flux in Hα is nearly half that observed during the outburst. The object's photometric behavior, spectrum, and luminosity suggest that it is a very massive and luminous variable star and might be related to some luminous blue variable stars such as η Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude implies an absolute magnitude of M_v ≈ -12.7 at the distance of NGC 3432, a value that is comparable to η Car during its outburst in the mid-19th century.