Absorption-Line Survey of H₃⁺ toward the Galactic Center Sources. III. Extent of Warm and Diffuse Clouds

We present a follow-up observation on Geballe and Oka (2010, ApJ, 709, L70), who found high column densities of H₃⁺ ~ 100 pc apart from the Galactic center on the lines of sight to 2MASS J17432173$-$2951430 (J1743) and 2MASS J17470898$-$2829561 (J1747). The wavelength coverages on these sightlines have been extended in order to observe two key transitions of H₃⁺ — R(3, 3)ˡ and R(2, 2)ˡ — which constrain the temperature and density of the environment. The profiles of the H₃⁺ R(3, 3)ˡ line, which is due only to gas in the Galactic center, closely match the differences between the H₃⁺ R(1, 1)ˡ and CO line profiles, just as they do for previously studied sightlines in the Galactic center. Absorption in the R(2, 2)ˡ line of H₃⁺ is present in J1747 at velocities of between $-$60 and $+$100 km s⁻¹. This is the second clear detection of this line in the interstellar medium after GC IRS 3 in the Central cluster. The temperature of the absorbing gas in this velocity range is 350 K, significantly warmer than that of diffuse clouds in other parts of the Central Molecular Zone. This indicates that the absorbing gas localizes on the Sgr B molecular cloud complex. The warm and diffuse gas revealed by Oka et al. (2005, ApJ, 632, 882) apparently extends to ~100 pc, but there is a hint that its temperature is somewhat lower in the line of sight to J1743 than elsewhere in the Galactic center. The observation of H₃⁺ toward J1747 is compared with the recent Herschel observation of H₂O⁺ toward Sgr B2, and their chemical relation and remarkably similar velocity profiles are discussed.