NIR H_2_ line em. from 5 photodissociation regions

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 H2 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~45000) 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).