INTERSTELLAR ABSORPTION-LINES TOWARD NGC-2264 AND AFGL-2591 - ABUNDANCES OF H-2, H-3+, AND CO

High-resolution infrared spectra of NGC 2264 IRS reveal interstellar absorption lines of CO near 2.3 μm wavelength and provide upper limits on the strengths of lines of H2 at 2.12 and 2.22 μm and of H3+ at 3.72 μm. Toward NGC 2264, the interstellar CO exhibits a uniform rotational excitation temperature, Trot = 28-4+6 K for levels J = 0-6 and has a total column density N(CO) = (5.0 ± 1.2) × 1018 cm-2. The true abundance of CO is N(CO)/N(H2) > 5 × 10-5 if the hydrogen is thermalized at T = 28 K, and the abundance of H3+ is less than 3 × 10-4 times that of CO. Similar spectra of AFGL 2591 provide an upper limit on the column density of H3+ of 5 × 1015 cm-2. These limits on H3+ and measurements of CO and H2 are in harmony with theoretical expectations if the clouds are exposed to cosmic-ray ionization rates of the order of 10-16 s-1 or less. The limit for NGC 2264 is particularly interesting in relation to the tentative identification of H2D+ in submillimeter emission in that direction. If the H2D+ detection is real, then our limit on H3+ implies H2D+/H3+ > 0.05.