RI Photometry of 2MASS‐selected Late M and L Dwarfs

Abstract

ABSTRACTWe report R and I band observations for 204 late M and L dwarfs selected on the basis of Two Micron All Sky Survey JHKs and photographic red colors, made with a telescope of modest aperture. It is shown that deep surveys covering these red bands can provide data sets that complement JHKs, permitting a fairly good photometric classification system for L dwarfs. Due primarily to the disappearance of strong TiO opacities, R - I reaches a maximum at late M type and turns blueward for subtypes M9 to about L3. Apart from a small plateau at L0–L2, the I - Ks color remains as a monotonic measure of spectral type or temperature over this range, and likewise for M dwarfs. For late L types, both colors probably get redder again, although the accuracy of our data and number of objects do not give us robust conclusions by L6–L8. It is also interesting to look at the dispersions of the IJKs bands at a given spectral type. It is widely believed that this dispersion is caused by object‐to‐object variations in the amount, location, or other properties of dust or clouds. We find a moderately larger spread for the J - Ks color than for I - J.