Absorption Line Survey of H3+ toward the Galactic Center Sources I. GCS 3-2 and GC IRS3

We present high-resolution (R = 20000) spectroscopy of H3+ absorption towardthe luminous Galactic center sources GCS 3-2 and GC IRS 3. With the efficientwavelength coverage afforded by Subaru IRCS, six absorption lines of H3+ havebeen detected in each source from 3.5 to 4.0 um, three of which are new. Inparticular the 3.543 um absorption line of the R(3, 3)^l transition arisingfrom the metastable (J, K) = (3, 3) state has been tentatively detected for thefirst time in the interstellar medium, where previous observations of H3+ hadbeen limited to absorption lines from the lowest levels: (J, K) = (1, 0) ofortho-H3+ and (1, 1) of para-H3+. The H3+ absorption toward the Galactic centertakes place in dense and diffuse clouds along the line of sight as well as themolecular complex close to the Galactic nucleus. At least four kinematiccomponents are found in the H3+ absorption lines. We suggest identifications ofthe velocity components with those of HI, CO, and H2CO previously reported fromradio and infrared observations. H3+ components with velocities that matchthose of weak and sharp CO and H2CO lines are attributed to diffuse clouds. Ourobservation has revealed a striking difference between the absorption profilesof H3+ and CO, demonstrating that the spectroscopy of H3+ provides informationcomplementary to that obtained from CO spectroscopy. The tentative detection ofthe R(3, 3)^l line and the non-detection of spectral lines from other J > 1levels provide observational evidence for the metastability of the (3, 3)level, which is theoretically expected. This suggests that other metastable J =K levels with higher J may also be populated.Comment: 18 pages, 7 figures, To appear in Publications of the Astronomical Society of Japan vol. 54 no.