On the MBM 12 Young Association

Abstract

I present a comprehensive study of the MBM 12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-SkySurvey (2MASS) survey with new optical imaging and spectroscopy,I have performed a census of the MBM 12A membership that is completeto 0.03 M-circle dot (H similar to 15) for a 1.degrees 75 x 1.degrees4field encompassing the MBM 12 cloud. I find five new members withmasses of 0.1-0.4 M-circle dot and a few additional candidates thathave not been observed spectroscopically. From an analysis of opticaland IR photometry for stars in the direction of MBM 12, I identifyM dwarfs in the foreground and background of the cloud. By comparingthe magnitudes of these stars to those of local field dwarfs, I arriveat a distance modulus 7.2 +/-0.5 (275 pc) to the MBM 12 cloud; itis not the nearest molecular cloud and is not inside the local bubbleof hot ionized gas as had been implied by previous distance estimatesof 50-100 pc. I have also used Li strengths and H-R diagrams to constrainthe absolute and relative ages of MBM 12A and other young populations;these data indicate ages of 2(-1)(+3) Myr for MBM 12A and similarto 10 Myr for the TW Hya and eta Cha associations. MBM 12A may bea slightly evolved version of the aggregates of young stars withinthe Taurus dark clouds (similar to1 Myr) near the age of the IC 348cluster (similar to2 Myr).