The luminosity of SN 1999by in NGC 2841 and the nature of "peculiar" Type Ia supernovae

We present UBVRIJHK photometry and optical spectroscopy of the so-called peculiar Type Ia supernova 1999by in NGC 2841. The observations began 1 week before visual maximum light, which is well defined by daily observations. The light curves and spectra are similar to those of the prototypical subluminous event SN 1991bg. We find that maximum light in B occurred on 1999 May 10.3 UT (JD 2; 451; 308.8 +/- 0.3) with B = 13.66 +/- 0.02 and a color of B-max - V-max = 0.51 +/- 0.03. The late-time color implies minimal dust extinction from the host galaxy. Our photometry, when combined with the recent Cepheid distance to NGC 2841, gives a peak absolute magnitude of M-B = - 17.15 +/- 0.23, making SN 1999by one of the least luminous Type Ia events ever observed. We estimate a decline rate parameter of Deltam(15)(B) = 1.90 mag, versus 1.93 for SN 1991bg, for which 1.10 is typical for so-called normal events. We compare SN 1999by with other subluminous events and find that the B-max - V-max color correlates strongly with the decline rate and may be a more sensitive indicator of luminosity than the fading rate for these objects. We find a good correlation between luminosity and the depth of the spectral feature at 580 nm, which had been attributed solely to Si II. We show that in cooler photospheres the 580 nm feature is dominated by Ti II, which provides a simple physical explanation for the correlation. Using only subluminous Type Ia supernovae, we derive a Hubble parameter of H-0 = 75(-11)(+12) km s(-1) Mpc(-1), consistent with values found from brighter events.