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

Abstract

We present photometric and spectroscopic observations of SN 2000ch, an\nunusual and extremely luminous variable star located in the galaxy NGC 3432.\nThe object was discovered on 2000 May 3.2 during the course of the Lick\nObservatory Supernova Search, at an unfiltered magnitude of about 17.4.\nPre-discovery images obtained in 1997, 1998, and 2000 April show the object\nwith R = 19.2-19.5 mag. Optical spectra obtained beginning on 2000 May 6 show a\nsmooth, flat continuum and strong, broad hydrogen Balmer emission lines at\nwavelengths consistent with the catalogued redshift of NGC 3432, strengthening\nthe association of the variable with the galaxy. Photometric monitoring reveals\na complex and erratic light curve over a time span of ~10 days. Subsequent\noptical spectra over the next ~3 months continued to show strong Balmer\nemission lines with a mean full-width at half-maximum intensity ~1550 km/s and\na distinct red asymmetry. A spectrum obtained 9 months after the outburst is\nsimilar to the previous spectra, but the integrated flux in H-alpha is nearly\nhalf that observed during the outburst. The object's photometric behavior,\nspectrum, and luminosity suggest that it is a very massive and luminous\nvariable star and might be related to some luminous blue variable stars such as\neta Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude implies\nan absolute magnitude of approximately -12.7 at the distance of NGC 3432, a\nvalue which is comparable to eta Carinae during its outburst in the\nmid-nineteenth century.\n