The Y Band at 1.035 Microns: Photometric Calibration and the Dwarf Stellar/Substellar Color Sequence

We define and characterize a photometric bandpass (called "Y") that is centered near 1.035 μm, in between the traditionally classified "optical" and "infrared" spectral regimes. We present Y magnitudes and Y−H and Y−K colors for a sample consisting mostly of photometric and spectral standards, spanning the spectral type range sdO to T5 V. Deep molecular absorption features in the near‐infrared spectra of extremely cool objects are such that the Y−H and Y−K colors grow rapidly with advancing spectral type especially from late M through mid‐L, substantially more rapidly than J−H or H−K, which span a smaller total dynamic range. Consistent with other near‐infrared colors, however, Y−H and Y−K colors turn blueward in the L6–L8 temperature range, with later T‐type objects having colors similar to those of warmer M and L stars. Y−J colors remain constant at 1.0 ± 0.15 mag from early‐L through late‐T dwarfs. The slope of the interstellar reddening vector within this filter is A_Y = 0.38A_V. Reddening moves stars nearly along the YHK dwarf color sequence, making it more difficult to distinguish unambiguously very low mass candidate brown dwarf objects from higher mass stars seen, e.g., through the Galactic plane or toward star‐forming regions. Other diagrams involving the Y band may be somewhat more discriminating.