• Luhman, K. L.

Abstract

I present a comprehensive study of the MBM 12 young association (MBM 12A). By combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS) survey with new optical imaging and spectroscopy, I have performed a census of the MBM 12A membership that is complete to 0.03 M_solar (H~15) for a 1.75d×ばつ1.4d field encompassing the MBM 12 cloud. I find five new members with masses of 0.1-0.4 M_solar and a few additional candidates that have not been observed spectroscopically. From an analysis of optical and IR photometry for stars in the direction of MBM 12, I identify M dwarfs in the foreground and background of the cloud. By comparing the magnitudes of these stars to those of local field dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM 12 cloud; it is not the nearest molecular cloud and is not inside the local bubble of hot ionized gas as had been implied by previous distance estimates of 50-100 pc. I have also used Li strengths and H-R diagrams to constrain the absolute and relative ages of MBM 12A and other young populations; these data indicate ages of 2⁺³_-1 Myr for MBM 12A and ~10 Myr for the TW Hya and η Cha associations. MBM 12A may be a slightly evolved version of the aggregates of young stars within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).