Search for Remnant Clouds Associated with the TW Hya Association

Abstract

Abstract We report on a search for the parental molecular clouds of the TW Hya association (TWA), using CO emission and NaI absorption lines. TWA is the nearest young ($\sim$50 pc; $\sim$10 Myr) stellar association; yet in spite of its youth there has been no detection of any associated natal molecular gas, as is the case with other typical young clusters. Using infrared maps as a guide, we conducted a CO cloud survey toward a region with a dust extinction of $E$($B-V$) $\gt$ 0.2 mag, or $A_{V}$$\gt$ 0.6 mag. CO emission was detected in the direction of three IR dust clouds, and we rejected one cloud out of the TWA, because no interstellar Na absorption was detected in the nearby Hipparcos stars, implying that it is too distant to relate to the TWA. The two other clouds exhibit only faint and small-scale CO emission. Interstellar NaI absorptions of Hipparcos targets (HIP 57809, HIP 64837, and HIP 64925, at distances of 133, 81, and 101 pc, respectively) by these clouds were also detected. We conclude that only a small fraction of the interstellar matter (ISM) toward the infrared (IR) dust cloud is located at a distance less than 100 pc, which may be all that is left out of the remnant clouds of TWA; the remaining remnant cloud dissipated within the last $\sim$1 Myr. Such a short-dissipation timescale may be due to an external perturbation or kinematic segregation that has a large stellar proper motion relative to the natal cloud.