Interstellar H(3)(+)

Abstract

Protonated molecular hydrogen, H(3)(+), is the simplest polyatomic molecule. It is the most abundantly produced interstellar molecule, next only to H(2), although its steady state concentration is low because of its extremely high chemical reactivity. H(3)(+) is a strong acid (proton donor) and initiates chains of ion-molecule reactions in interstellar space thus leading to formation of complex molecules. Here, I summarize the understandings on this fundamental species in interstellar space obtained from our infrared observations since its discovery in 1996 and discuss the recent observations and analyses of H(3)(+) in the Central Molecular Zone near the Galatic center that led to a revelation of a vast amount of warm and diffuse gas existing in the region.

Fig. 1.

A schematic diagram showing relations of H₂, CO, and H₃⁺ number densities, n(X), versus cloud density, n(H). Note that n(H₂) and n(CO) scale with n(H), whereas n(H₃⁺) is independent of n(H).

Fig. 2.

The first detection of interstellar H₃⁺ at the UKIRT toward two young stellar objects that are deeply embedded in their natal dense molecular clouds. The doublet at 3.66808 μm and 3.66852 μm are the R(1, 1)^u and R(1, 0) transitions of para- and ortho-H₃⁺, respectively. (Reproduced from ref. .)

Fig. 3.

Observed H₃⁺ column densities versus color excess E(B − V) for dense clouds (Upper) and diffuse clouds (Lower). Note that the two categories of clouds have comparable N(H₃⁺), although E(B − V), and hence the amount of gas in the sightline, is 10 times lower for diffuse clouds than from dense clouds. (Reproduced from refs. and .)

Fig. 4.

Rotational levels of H₃⁺ in the ground vibrational state. In the CMZ so far observed, most H₃⁺ are populated in three levels: the (1, 1) ground level (para-H-₃⁺), the (1, 0) ortho-ground level, and the (3, 3) ortho-metastable level. Blue arrows indicate spontaneous emissions. See ref. for more detail.

Fig. 5.

Spectra of H₃⁺ (Upper) and CO (Lower) observed by the Subaru Telescope toward GCS 3-2, the brightest infrared source near the Galactic center. The R(3, 3)^l line (in red) shows the presence of a vast amount of metastable H₃⁺, which signifies high temperature. (Reproduced from ref. .)

Fig. 6.

Observed H₃⁺ (top three traces) R(1, 1)^l, R(3, 3)^l, and R(2, 2)^l lines and CO 2-0 overtone R(1) line toward GCS 3-2 observed by the Gemini South Telescope and the UKIRT. The vertical scaling of the R(3, 3)^l and R(2, 2)^l spectra is multiplied by a factor of 2 and that of the CO spectrum is divided by 2 for clarity. (Reproduced from ref. .)

Fig. 7.

Location of observed infrared stars indicated by crosses. They all showed the R(3, 3)^l spectrum, signifying high temperature, and extremely weak, if any, R(2, 2)^l spectrum, signifying low density, indicating the widespread presence of hot and diffuse gas. The background is the VLA 20-cm image by Yusef-Zadeh and Morris (60). (Reproduced from ref. .)