TheYBand at 1.035 Microns: Photometric Calibration and the Dwarf Stellar/Substellar Color Sequence

We define and characterize a photometric bandpass (called "Y") that iscentered at 1.035 um, in between the traditionally classified ``optical'' and``infrared'' spectral regimes. We present Y magnitudes and Y-H and Y-K colorsfor a sample consisting mostly of photometric and spectral standards, spanningthe spectral type range sdO to T5V. Deep molecular absorption features in thenear-infrared spectra of extremely cool objects are such that the Y-H and Y-Kcolors grow rapidly with advancing spectral type especially from late M throughmid L, substantially more rapidly than J-H or H-K which span a smaller totaldynamic range. Consistent with other near-infrared colors, however, Y-H and Y-Kcolors turn blueward in the L6-L8 temperature range with later T-type objectshaving colors similar to those of warmer M and L stars. Use of the Y-bandfilter is nonetheless promising for easy identification of low-mass stars andbrown dwarfs, especially at young ages. The slope of the interstellar reddeningvector within this filter is A_Y = 0.38 x A_V. Reddening moves stars nearlyalong the YHK dwarf color sequence making it more difficult to distinguishunambiguously very low mass candidate brown dwarf objects from higher massstars seen, e.g. through the galactic plane or towards star-forming regions.Other diagrams involving the Y-band may be somewhat more discriminating.Comment: accepted at PAS