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