A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions

Abstract

We present a comparative study of the near-infrared (NIR) H${}_2$ line emission from five regions near hot young stars: Sharpless 140, NGC 2023, IC 63, the Horsehead Nebula, and the Orion Bar. This emission originates in photodissociation or photon-dominated regions (PDRs), interfaces between photoionized and molecular gas near hot (O) stars or reflection nebulae illuminated by somewhat cooler (B) stars. In these environments, the dominant excitation mechanism for NIR emission lines originating from excited rotational-vibrational (rovibrational) levels of the ground electronic state is radiative or UV excitation (fluorescence), wherein absorption of far-UV photons pumps H${}_2$ molecules into excited electronic states from which they decay into the upper levels of the NIR lines. Our sources span a range of UV radiation fields ($G_0=10^2$-$10^5$) and gas densities ($n_H=10^4$-$10^6$ cm${}^{-3}$), enabling examination of how these properties affect the emergent spectrum. We obtained high-resolution ($R\approx 45,000$) spectra spanning $1.45$-$2.45$~$\mu$m on the 2.7m Harlan J. Smith Telescope at McDonald Observatory with the Immersion Grating INfrared Spectrometer (IGRINS), detecting up to over 170 transitions per source from excited vibrational states ($v=1$-$14$). The populations of individual rovibrational levels derived from these data clearly confirm UV excitation. Among the five PDRs in our survey, the Orion Bar shows the greatest deviation of the populations and spectrum from pure UV excitation, while Sharpless 140 shows the least deviation. However, we find that all five PDRs exhibit at least some modification of the level populations relative to their values under pure UV excitation, a result we attribute to collisional effects.

21 pages, 10 figures, 4 tables, Published in ApJ. Tables 2, 3, & 4 are available in machine readable form in the arxiv source (click Other formats)

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Cited by in corpus (14)

• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
• High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2
• PDRs4All XI. Detection of infrared CH${}^{+}$ and CH${}_3^{+}$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. II. Application to interstellar PDRs
• The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging
• PROJECT-J: the shocking H2 outflow from HH46
• Quantifying the informativity of emission lines to infer physical conditions in giant molecular clouds. I. Application to model predictions
• 10 Years of Archival High-Resolution NIR Spectra: The Raw and Reduced IGRINS Spectral Archive (RRISA)
• The shocked molecular layer in RCW 120
• Analysis of the first infrared spectrum of quasi-bound H2 line emission in Herbig-Haro 7
• H3+ in irradiated protoplanetary disks: Linking far-ultraviolet radiation and water vapor