Discovery and Evolution of an Unusual Luminous Variable Star in NGC 3432 (Supernova 2000ch)

We present photometric and spectroscopic observations of SN 2000ch, anunusual 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 LickObservatory Supernova Search, at an unfiltered magnitude of about 17.4.Pre-discovery images obtained in 1997, 1998, and 2000 April show the objectwith R = 19.2-19.5 mag. Optical spectra obtained beginning on 2000 May 6 show asmooth, flat continuum and strong, broad hydrogen Balmer emission lines atwavelengths consistent with the catalogued redshift of NGC 3432, strengtheningthe association of the variable with the galaxy. Photometric monitoring revealsa complex and erratic light curve over a time span of ~10 days. Subsequentoptical spectra over the next ~3 months continued to show strong Balmeremission lines with a mean full-width at half-maximum intensity ~1550 km/s anda distinct red asymmetry. A spectrum obtained 9 months after the outburst issimilar to the previous spectra, but the integrated flux in H-alpha is nearlyhalf that observed during the outburst. The object's photometric behavior,spectrum, and luminosity suggest that it is a very massive and luminousvariable star and might be related to some luminous blue variable stars such aseta Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude impliesan absolute magnitude of approximately -12.7 at the distance of NGC 3432, avalue which is comparable to eta Carinae during its outburst in themid-nineteenth century.Comment: 18 pages, 9 figures, PASP, in pres