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We present a detailed analysis of narrow of NaI and KI absorption resonancelines toward nearly 40 T Tauri stars in Taurus with the goal of clarifyingtheir origin. The NaI 5889.95 angstrom line is detected toward all but onesource, while the weaker KI 7698.96 angstrom line in about two thirds of thesample. The similarity in their peak centroids and the significant positivecorrelation between their equivalent widths demonstrate that these transitionstrace the same atomic gas. The absorption lines are present towards both diskand diskless young stellar objects, which excludes cold gas within thecircumstellar disk as the absorbing material. A comparison of NaI and COdetections and peak centroids demonstrates that the atomic and molecular gasare not co-located, the atomic gas is more extended than the molecular gas. Thewidth of the atomic lines corroborates this finding and points to atomic gasabout an order of magnitude warmer than the molecular gas. The distribution ofNaI radial velocities shows a clear spatial gradient along the length of theTaurus molecular cloud filaments. This suggests that absorption is associatedwith the Taurus molecular cloud. Assuming the gradient is due to cloudrotation, the rotation of the atomic gas is consistent with differentialgalactic rotation while the rotation of the molecular gas, although with thesame rotation axis, is retrograde. Our analysis shows that narrow NaI and KIabsorption resonance lines are useful tracers of the atomic envelope ofmolecular clouds. In line with recent findings from giant molecular clouds, ourresults demonstrate that the velocity fields of the atomic and molecular gasare misaligned. The angular momentum of a molecular cloud is not simplyinherited from the rotating Galactic disk from which it formed but may beredistributed by cloud-cloud interactions.

Narrow Na and K Absorption Lines Toward T Tauri Stars - Tracing the Atomic Envelope of Molecular Clouds