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K. Krisciunas, C. Stubbs (UW), A. Diercks (Caltech), E. Magnier (CFHT), B. Skiff (Lowell Obs.), K. Gloria, N. C. Hastings, R. McMillan (APO), A. Riess (STScI)
We present optical photometry of seven Type Ia supernovae: SNe 1998es, 1999aa, 1999cc, 1999cl, 1999cp, 1999da, and 1999dk. We also obtained near infrared photometry of three of these. A more direct route to determining AV (and hence the correct distance to a host glaxy) is to use the V-K color excess rather than E(B-V). One often assumes that the unreddened B-V color of a Type Ia SN is 0.00 at the time of B-band maximum, so the {\em observed} B-V color at that time is taken as the color excess. This is not true for all Type Ia SNe, especially the rapidly declining ones. [The maximum brightness and rate of decline of Type Ia SNe can be characterized by the parameter \Delta (Riess, Press, & Kirshner 1996; Riess et al. 1998, Appendix). More luminous (\Delta < 0) SNe are slow decliners. Less luminous (\Delta > 0) SNe are faster decliners.] The B-V color excess of Type Ia SNe can also be determined from the photometry between 30 and 90 days {\em after} light maximum (Phillips et al. 1999, astro-ph/9907052). We find that the V-K color near light maximum for Type Ia SNe unreddened in their host galaxies (SNe 1999cp, 1980N, 1981B, 1981D, and 1983R) implies a ``universal'' pair of linear relationships that intersect at 6 days after maximum. If we let t be the number of days since B-band maximum, for -10 \leq t \leq +6, V-K = -0.68 - 0.055 \; t, while for +6 \leq t \leq +25, V-K = -1.51 + 0.075 \; t. The RMS deviation of the fits is ± 0.1 mag. This is based on SNe with -0.2 \leq \Delta \leq 0.2 which have been corrected for small amounts of reddening in {\em our} Galaxy (Schlegel, Finkbeiner, & Davis 1998). Perhaps there are families of V-K curves as a function of \Delta. Since AV \approx 3.1 E(B-V) scales any error in E(B-V) by a factor of three, while AV \approx 1.13 E(V-K), AV might be more easily determined from the V-band light curve of a Type Ia SN (which can be easily interpolated) and a small amount of K-band data obtained near maximum light.
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