Using Narrowband Photometry to Classify Stars and Brown Dwarfs

Abstract

We present a new system of narrowband filters in the near-infrared that can be used to classify stars and brown dwarfs. This set of four filters, spanning the H band, can be used to identify molecular features unique to brown dwarfs, such as H2O and CH4. The four filters are centered at 1.495 μm (H2O), 1.595 μm (continuum), 1.66 μm (CH4), and 1.75 μm (H2O). Using two H2O filters allows us to solve for the reddening of individual objects. This can be accomplished by constructing a color-color-color cube and rotating it until the reddening vector disappears. We created a model of predicted color-color-color values for different spectral types by integrating filter bandpass data with the spectra of known stars and brown dwarfs. We validated this model by making photometric measurements of seven known L and T dwarfs, ranging from L1 to T7.5. The photometric measurements agree with the model to within ±0.1 mag, allowing us to create spectral indexes for different spectral types. We can classify A through early M stars to within ±2 spectral types, late-type M and L dwarfs to within ±0.3 spectral types, and T dwarfs to within ±0.1 spectral types (1 σ). Thus, we can distinguish between a T1 and a T3 dwarf. The four physical bands can be converted into two reddening-free indexes μ1 and μ2 and an extinction AV for the individual objects. This technique, which is equivalent to extremely low resolution spectroscopy, can be used to survey large areas to provide rough spectral classifications for all the stars in the area, ranging down to the coolest brown dwarfs. It should prove particularly useful in young clusters where reddening can be high.