A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions
| Autor: | Daniel T. Jaffe, Kyle F. Kaplan, Harriet L. Dinerstein, Hwihyun Kim |
|---|---|
| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Předmět: |
Physics
Nebula 010504 meteorology & atmospheric sciences Reflection nebula Photodissociation FOS: Physical sciences Astronomy and Astrophysics Rotational–vibrational spectroscopy Astrophysics Astrophysics::Cosmology and Extragalactic Astrophysics 01 natural sciences Astrophysics - Astrophysics of Galaxies Spectral line 3. Good health Stars 13. Climate action Space and Planetary Science Astrophysics of Galaxies (astro-ph.GA) Excited state 0103 physical sciences Astrophysics::Solar and Stellar Astrophysics Emission spectrum 010303 astronomy & astrophysics Astrophysics::Galaxy Astrophysics 0105 earth and related environmental sciences |
| Popis: | 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. Comment: 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) |
| Databáze: | OpenAIRE |
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