FUV Irradiated Disk Atmospheres: Ly$\alpha$ and the Origin of Hot H${}_2$ Emission

Abstract

Protoplanetary disks are strongly irradiated by a stellar FUV spectrum that is dominated by Ly$\alpha$ photons. We investigate the impact of stellar Ly$\alpha$ irradiation on the terrestrial planet region of disks ($\lesssim 1$AU) using an updated thermal-chemical model of a disk atmosphere irradiated by stellar FUV and X-rays. The radiative transfer of Ly$\alpha$ is implemented in a simple approach that includes scattering by H I and absorption by molecules and dust. Because of their non-radial propagation path, scattered Ly$\alpha$ photons deposit their energy deeper in the disk atmosphere than the radially propagating FUV continuum photons. We find that Ly$\alpha$ has a significant impact on the thermal structure of the atmosphere. Photochemical heating produced by scattered Ly$\alpha$ photons interacting with water vapor and OH leads to a layer of hot (1500 - 2500 K) molecular gas. The temperature in the layer is high enough to thermally excite the H${}_2$ to vibrational levels from which they can be fluoresced by Ly$\alpha$ to produce UV fluorescent H${}_2$ emission. The resulting atmospheric structure may help explain the origin of UV fluorescent H${}_2$ that is commonly observed from classical T Tauri stars.

Accepted for publication in ApJ. 11 pages, 4 figures

References in corpus (9)

• Photoprocesses in protoplanetary disks
• Line Emission from Gas in Optically Thick Dust Disks around Young Stars
• Formation of Water in the Warm Atmospheres of Protoplanetary Disks
• Formation of simple organic molecules in inner T Tauri disks
• Water vapor distribution in protoplanetary disks
• Photodissociation of interstellar N2
• Shielding by Water and OH in FUV and X-ray Irradiated Protoplanetary Disks
• X-ray ionization rates in protoplanetary discs
• Photochemical Heating of Dense Molecular Gas

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• FUV Irradiation and the Heat Signature of Accretion in Protoplanetary Disk Atmospheres
• The UV Perspective of Low-Mass Star Formation
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• UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks
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• A Study of Ro-vibrational OH Emission from Herbig Ae/Be Stars
• The Mysterious Affair of the H${}_2$ in AU Mic
• Detection of H${}_2$ in the TWA 7 System: A Probable Circumstellar Origin
• Water UV-shielding in the terrestrial planet-forming zone: Implications from water emission