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Published November 20, 2015 | Published + Submitted
Journal Article Open

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


We present a detailed analysis of narrow Na i and K i absorption resonance lines toward nearly 40 T Tauri stars in Taurus with the goal of clarifying their origin. The Na i λ5889.95 line is detected toward all but one source, while the weaker K i λ7698.96 line is detected in about two-thirds of the sample. The similarity in their peak centroids and the significant positive correlation between their equivalent widths demonstrate that these transitions trace the same atomic gas. The absorption lines are present toward both disk and diskless young stellar objects, which excludes cold gas within the circumstellar disk as the absorbing material. A comparison of Na i and CO detections and peak centroids demonstrates that the atomic gas and molecular gas are not co-located, the atomic gas being more extended than the molecular gas. The width of the atomic lines corroborates this finding and points to atomic gas about an order of magnitude warmer than the molecular gas. The distribution of Na i radial velocities shows a clear spatial gradient along the length of the Taurus molecular cloud filaments. This suggests that absorption is associated with the Taurus molecular cloud. Assuming that the gradient is due to cloud rotation, the rotation of the atomic gas is consistent with differential galactic rotation, whereas the rotation of the molecular gas, although with the same rotation axis, is retrograde. Our analysis shows that narrow Na i and K i absorption resonance lines are useful tracers of the atomic envelope of molecular clouds. In line with recent findings from giant molecular clouds, our results demonstrate that the velocity fields of the atomic and molecular gas are misaligned. The angular momentum of a molecular cloud is not simply inherited from the rotating Galactic disk from which it formed but may be redistributed by cloud–cloud interactions.

Additional Information

© 2015 The American Astronomical Society. Received 2015 July 18; accepted 2015 October 5; published 2015 November 12. The authors thank D. E. Welty for providing the high-resolution optical spectra toward HD 27778 and J. L. Pineda for sharing the Herschel spectra. I.P. would like to thank T. Koskinen and P. Lavvas for stimulating discussions on giant exoplanet atmospheres and L. Hartmann and T. Megeath for helpful discussions on star formation processes. This work was partially supported by an NSF Astronomy & Astrophysics Research Grant (ID: 1312962). Facility: Keck - .

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Published - Pascucci_2015.pdf

Submitted - 1510.02022v1.pdf


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August 20, 2023
October 25, 2023