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Search for Remnant Clouds Associated with the TW Hya Association
We report on a search for the parental molecular clouds of the TW Hyaassociation (TWA), using CO emission and NaI absorption lines. TWA isthe nearest young (˜50pc; ˜10Myr) stellar association; yet inspite of its youth there has been no detection of any associated natalmolecular gas, as is the case with other typical young clusters. Usinginfrared maps as a guide, we conducted a CO cloud survey toward a regionwith a dust extinction of E(B ? V) > 0.2mag, or AV> 0.6mag. CO emission was detected in the direction of three IR dustclouds, and we rejected one cloud out of the TWA, because nointerstellar Na absorption was detected in the nearby Hipparcos stars,implying that it is too distant to relate to the TWA. The two otherclouds exhibit only faint and small-scale CO emission. Interstellar NaIabsorptions of Hipparcos targets (HIP 57809, HIP 64837, and HIP 64925,at distances of 133, 81, and 101pc, respectively) by these clouds werealso detected. We conclude that only a small fraction of theinterstellar matter (ISM) toward the infrared (IR) dust cloud is locatedat a distance less than 100pc, which may be all that is left out of theremnant clouds of TWA; the remaining remnant cloud dissipated within thelast ˜1Myr. Such a short-dissipation timescale may be due to anexternal perturbation or kinematic segregation that has a large stellarproper motion relative to the natal cloud.
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