Astrochemistry

X-ray Photodesorption Of Complex Organic Molecules In Protoplanetary Disks — I. Acetonitrile CH3CN

By Keith Cowing

Status Report

astro-ph.IM

July 1, 2023

Filed under astro-ph.IM, astrochemistry, complex organic molecules, ices, Interstellar, organic chemistry, Protoplanetary Disks, X-ray photodesorption, X-rays

X-ray Photodesorption Of Complex Organic Molecules In Protoplanetary Disks — I. Acetonitrile CH3CN — TEYs in the N K edge region of a pure CH3CN ice at 15 K (top left panel; the inset shows the region near the N 1s → π ∗ resonance for an ice irradiated at 15 K and 90 K for the lower and upper curve, respectively; these curves are shifted vertically for more clarity), of a mixed H2O:CH3CN ice irradiated at 15 K with a dilution ratio of 1:1 and 10:1 (top and bottom right panel, respectively), and of a mixed 13CO:CH3CN ice irradiated at 15 K with a dilution ratio of 1:1 (bottom left panel). The photon fluence received by the ice before each TEY measurement is also displayed. The spectral width of the beam was set to 1.2 eV for all the TEY measurements, except for the one corresponding to the red curve in the bottom right panel, for a H2O:CH3CN ice having received a photon fluence of 1×1017 photons.cm−2 and for which the spectral width was 90 meV. The inset in the bottom right panel zooms into the TEY measured on the H2O:CH3CN (10:1) ice for a photon fluence of 1017 photons.cm−2 , where the vibrational structure of the core hole state of N2 formed near the ice surface can be seen near 401 eV. The ices have a total thickness of ∼ 100 ML. — astro-ph.IM

X-rays emitted from pre-main-sequence stars at the center of protoplanetary disks can induce nonthermal desorption from interstellar ices populating the cold regions. This X-ray photodesorption needs to be quantified for complex organic molecules (COMs), including acetonitrile CH3CN, which has been detected in several disks. We experimentally estimate the X-ray photodesorption yields of neutral species from pure CH3CN ices and from interstellar ice analogs for which CH3CN is mixed either in a CO- or H2O-dominated ice.

The ices were irradiated at 15 K by soft X-rays (400-600 eV) from synchrotron light (SOLEIL synchrotron). X-ray photodesorption was probed in the gas phase via quadrupole mass spectrometry. X-ray photodesorption yields were derived from the mass signals and were extrapolated to higher X-ray energies for astrochemical models. X-ray photodesorption of the intact CH3CN is detected from pure CH3CN ices and from mixed 13CO:CH3CN ices, with a yield of about 5×10^(-4) molecules/photon at 560 eV. When mixed in H2O-dominated ices, X-ray photodesorption of the intact CH3CN at 560 eV is below its detection limit, which is 10^(-4) molecules/photon. Yields associated with the desorption of HCN, CH4 , and CH3 are also provided.

The derived astrophysical yields significantly depend on the local conditions expected in protoplanetary disks. They vary from 10^(-4) to 10(-6) molecules/photon for the X-ray photodesorption of intact CH3CN from CO-dominated ices. Only upper limits varying from 5×10^(-5) to 5×10^(-7) molecules/photon could be derived for the X-ray photodesorption of intact CH3CN from H2O-dominated ices. X-ray photodesorption of intact CH3CN from interstellar ices might in part explain the abundances of CH3CN observed in protoplanetary disks. The desorption efficiency is expected to vary with the local physical conditions, hence with the disk region.

R. Basalgète, D. Torres-Díaz, A. Lafosse, L. Amiaud, G. Féraud, P. Jeseck, L. Philippe, X. Michaut, J.-H. Fillion, M. Bertin

Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2306.13048 [astro-ph.IM] (or arXiv:2306.13048v1 [astro-ph.IM] for this version)
Submission history
From: Romain Basalgete
[v1] Thu, 22 Jun 2023 17:17:00 UTC (171 KB)
https://arxiv.org/abs/2306.13048
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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