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Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission

Bosman, Arthur D. and Banzatti, Andrea and Bruderer, Simon and Tielens, Alexander G. G. M. and Blake, Geoffrey A. and van Dishoeck, Ewine F. (2019) Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission. Astronomy and Astrophysics, 631 . Art. No. A133. ISSN 0004-6361. https://resolver.caltech.edu/CaltechAUTHORS:20200309-142220044

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Abstract

Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk. Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars. Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data. Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (N_(CO) < 10¹⁸ cm⁻²); no CO is detected within or at the inner dust rim, due to dissociation at high temperatures. In contrast, explaining the narrow lines with high vibrational ratios requires an inner cavity of a least 5 AU in both dust and gas, followed by a cool (100–300 K) molecular gas reservoir with gas-to-dust ratios greater than 10 000 (N_(CO) > 10¹⁸ cm⁻²) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (T_(gas) ~ T_(dust)). Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1051/0004-6361/201935910DOIArticle
https://arxiv.org/abs/1909.02031arXivDiscussion Paper
ORCID:
AuthorORCID
Bosman, Arthur D.0000-0003-4001-3589
Banzatti, Andrea0000-0003-4335-0900
Blake, Geoffrey A.0000-0003-0787-1610
van Dishoeck, Ewine F.0000-0001-7591-1907
Additional Information:© 2019 ESO. Article published by EDP Sciences. Received 17 May 2019; Accepted 4 September 2019; Published online 06 November 2019. We thank Antonio Garufi for helpful discussions on imaging of Herbig disks, Inga Kamp and Daniel Harsono for help with the CO collisional rate coefficients and Paul Molliere for providing the results to the chemical equilibrium calculations. Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA). This work is partly basedon observations obtained with iSHELL under program 2016B049 at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. This work is partly based on observations made with CRIRES on ESO telescopes at the Paranal Observatory under programs 179.C-0151, 093.C-0432, 079.C-0349, 081.C-0833, 091.C-0671. This work is partly based on observations obtained with NIRSPEC at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. This project has made use of the SciPy stack (Jones et al. 2001), including NumPy (Oliphant 2006) and Matplotlib (Hunter 2007).
Group:Astronomy Department
Funders:
Funding AgencyGrant Number
Nederlandse Onderzoekschool Voor Astronomie (NOVA)UNSPECIFIED
NASANNH14CK55B
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:astrochemistry – protoplanetary disks – line: formation – molecular processes
Record Number:CaltechAUTHORS:20200309-142220044
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200309-142220044
Official Citation:Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission. Arthur D. Bosman, Andrea Banzatti, Simon Bruderer, Alexander G. G. M. Tielens, Geoffrey A. Blake and Ewine F. van Dishoeck. A&A, 631 (2019) A133; DOI: https://doi.org/10.1051/0004-6361/201935910
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101793
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:09 Mar 2020 22:36
Last Modified:09 Mar 2020 22:36

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