CCD PHOTOMETRY OF 3 TYPE-IA SUPERNOVAE - V, R, AND I LIGHT CURVES

Cousins system VRI photometry has been obtained for the Type Ia supern ova 1992G with CCDs attached to small telescopes at Leuschner Observat ory (LO), Van Vleck Observatory (VVO), and Whitin Observatory (WO). Si milar equipment, observing and reduction procedures have been adopted at all three sites and there is generally good agreement among the dat a sets. Two other Type Ia supernovae, 1991M and 1991T, were observed o nly at VVO. The shapes of the light curves of all three supernovae, in cluding the spectroscopically peculiar SN 1991T, are quite similar, an d the combined light curve in V matches Leibundgut's [Ph.D. thesis, Un iversity of Basel (1988)] template reasonably well. Since very little R and I photometry of supernovae has been done previously, our data pr ovide the first real definition of SN Ia templates in those bands. The I template is of particular interest since it shows a secondary maxim um 23 +/- 1 days after the primary peak, similar to what is seen in no rmal Type Ia supernovae at J, H, and K wavelengths. The behavior in R is intermediate between V and I. The existence of substantial structur e in the R and I light curves of normal Type Ia supernovae (and its ab sence in the peculiar Type Ia SN 1991bg) enhances their utility as cos mological probes and provides an observational challenge to models of the event. The observed colors of SN 1991M and SN 1991T as a function of time are quite similar, indicating that they suffer about the same amount of reddening. SN 1992G is clearly more heavily reddened than th ese two, by about 0.1 mag in R - I. The intrinsic colors of these Type Ia supernovae at V(max) are V - R = 0.0 and R - I = - 0.3, with an un certainty of around 0.05 mag, if the reddening of SN 1991T is E (B - V ) = 0.13. If Type Ia supernovae are assumed to be good standard candle s, then NGC 4527, the parent galaxy of SN 1991T, must have a peculiar velocity of 1200 +/- 150 km s-1 relative to the Hubble flow and direct ed away from the Galaxy. If the peculiar velocity of NGC 4527 is less than this, then SN 1991T must be an overluminous SN Ia. Tully & Shaya (1984) regard NGC 4527 as a near-side member of the Virgo cluster fall ing in at approximately 1000 km s-1. Our data alone yield a Hubble con stant of 75 +/- 15 km s-1 Mpc-1 if M(V) at maximum light is - 18.7 mag , and 50 +/- 10 km s-1 Mpc-1 if it is - 19.6 mag.