Complex organic molecules in protoplanetary disks

By C. Walsh, T. Millar, H. Nomura, E. Herbst, S. W. Weaver, Y. Aikawa, J. Laas, Anton I. Vasyunin Leiden Observatory, The Netherlands., S. O. Mathematics and 15 more. 2013

187 citations recorded by the source corpus.

Context. Protoplanetary disks are vital objects in star and planet formation, possessing all the material, gas and dust, which may form a planetary system orbiting the new star. Small, simple molecules have traditionally been detected in protoplanetary disks; however, in the ALMA era, we expect the molecular inventory of protoplanetary disks to significantly increase. Aims. We investigate the synthesis of complex organic molecules (COMs) in protoplanetary disks to put constraints on the achievable chemical complexity and to predict species and transitions which may be observable with ALMA. Methods. We have coupled a 2D steady-state physical model of a protoplanetary disk around a typical T Tauri star with a large gas-grain chemical network including COMs. We compare the resulting column densities with those derived from observations and perform ray-tracing calculations to predict line spectra. We compare the synthesised line intensities with current observations and determine those COMs which may be observable in nearby objects. We also compare the predicted grain-surface abundances with those derived from cometary comae observations. Results. We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances ~10-6–10-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, ~10-12–10-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H2CO observed towards T Tauri star-disk systems. There is poor agreement with HC3N lines observed towards LkCa 15 and GO Tau and we discuss possible explanations for these discrepancies. The synthesised line intensities for CH3OH are consistent with upper limits determined towards all sources. Our models suggest CH3OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging, even with ALMA “Full Science” capabilities. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun’s natal disk.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/00a8b0f834db151a8099d35baf51d842325549dd

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The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. D. Wilner
2. E. Dishoeck
3. S. Andrews
4. K. Oberg
5. E. Bergin
6. R. Garrod
7. Simon Bruderer
8. C. Qi

In Atmospheric Science

1. R. Garrod
2. E. Bergin
3. E. Dishoeck
4. Simon Bruderer
5. H. Cuppen
6. S. Ioppolo
7. C. Walsh
8. K. Oberg

In Spectroscopy

1. D. Wilner
2. R. Garrod
3. E. Bergin
4. E. Dishoeck
5. S. Andrews
6. K. Oberg
7. C. Qi
8. Simon Bruderer

Citations in the corpus

• Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546 (2016, methodology): “…freeze out of CO and other carrier molecules (Öberg et al. 2011); the transformation of this icy reservoir into complex organic molecules in cold regions (van Dishoeck 2008; Walsh et al. 2014b); and the vertical transport and oxidation of carbonaceous solids in warm gas (Lee et al. 2010).”
• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “Because the methodology is covered in detail in a series of previous publications (see, e.g., Nomura & Millar 2005; Nomura et al. 2007; Walsh et al. 2010, 2012, 2014), here, we highlight the important parameters only.”
• Molecules with ALMA at Planet-forming Scales (MAPS). I. Program Overview and Highlights (2021, background): “Walsh et al. (2014) report gas and grain surface column densities for their fiducial disk as a function of radius, and find H2CO and CH3CN ice-to-gas ratios of ∼103, and a HC3N ratio of 4 at 20 au (taken here to be representative for the disk interior to 50 au).”
• Formation of ethylene glycol and other complex organic molecules in star-forming regions (2016, background): “Although Walsh et al. (2014) raised some doubts about the efficiency of this process due to the low mobility produced by the OH-bonds, the recent experimental work by Butscher et al. (2015) has shown that this mechanism seems to be a very efficient pathway to form EG.”
• Grain Surface Models and Data for Astrochemistry (2017, background): “…all, the width of the barrier, a, in the simple approximation mentioned above, is often not known and therefore commonly approximated as 1–2 Å (Garrod and Herbst 2006; Furuya et al. 2013; Walsh et al. 2014), initiated by older literature such as Tielens and Hagen (1982) and Hasegawa et al. (1992).”