Walsh, C orcid.org/0000-0001-6078-786X, Nomura, H, Millar, TJ et al. (1 more author) (2012) Chemical Processes in Protoplanetary Disks. II. On the Importance of Photochemistry and X-Ray Ionization. Astrophysical Journal, 747 (2). 114. ISSN 0004-637X
Abstract
We investigate the impact of photochemistry and X-ray ionization on the molecular composition of, and ionization fraction in, a protoplanetary disk surrounding a typical T Tauri star. We use a sophisticated physical model, which includes a robust treatment of the radiative transfer of UV and X-ray radiation, and calculate the time-dependent chemical structure using a comprehensive chemical network. In previous work, we approximated the photochemistry and X-ray ionization; here, we recalculate the photoreaction rates using the explicit UV wavelength spectrum and wavelength-dependent reaction cross sections. We recalculate the X-ray ionization rate using our explicit elemental composition and X-ray energy spectrum. We find that photochemistry has a larger influence on the molecular composition than X-ray ionization. Observable molecules sensitive to the photorates include OH, HCO+, N2H+, H2O, CO2, and CH3OH. The only molecule significantly affected by the X-ray ionization is N2H+, indicating that it is safe to adopt existing approximations of the X-ray ionization rate in typical T Tauri star-disk systems. The recalculation of the photorates increases the abundances of neutral molecules in the outer disk, highlighting the importance of taking into account the shape of the UV spectrum in protoplanetary disks. A recalculation of the photoreaction rates also affects the gas-phase chemistry due to the adjustment of the H/H2 and C+/C ratios. The disk ionization fraction is not significantly affected by the methods adopted to calculate the photochemistry and X-ray ionization. We determine that there is a probable "dead zone" where accretion is suppressed, present in a layer, Z/R \lesssim 0.1-0.2, in the disk midplane, within R ≈ 200 AU.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2012, The American Astronomical Society. Reproduced in accordance with the publisher's self-archiving policy. |
Keywords: | Astrochemistry; ISM: molecules; Protoplanetary disks; Stars: formation |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Engineering & Physical Sciences (Leeds) > School of Physics and Astronomy (Leeds) > Astrophysics (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 14 May 2019 08:43 |
Last Modified: | 14 May 2019 08:43 |
Status: | Published |
Publisher: | American Astronomical Society |
Identification Number: | 10.1088/0004-637X/747/2/114 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:107150 |