Molecules with ALMA at Planet-forming Scales (MAPS). VII. Substellar O/H and C/H and superstellar C/O in planet-feeding gas
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The elemental composition of the gas and dust in a protoplanetary disk influences the compositions of the planets that form in it. We use the Molecules with ALMA at Planet-forming Scales (MAPS) data to constrain the elemental composition of the gas at the locations of potentially forming planets. The elemental abundances are inferred by comparing source-specific gas-grain thermochemical models with variable C/O ratios and small-grain abundances from the DALI code with CO and C2H column densities derived from the high-resolution observations of the disks of AS 209, HD 163296, and MWC 480. Elevated C/O ratios (~2.0), even within the CO ice line, are necessary to match the inferred C2H column densities over most of the pebble disk. Combined with constraints on the CO abundances in these systems, this implies that both the O/H and C/H ratios in the gas are substellar by a factor of 4-10, with the O/H depleted by a factor of 20-50, resulting in the high C/O ratios. This necessitates that even within the CO ice line, most of the volatile carbon and oxygen is still trapped on grains in the midplane. Planets accreting gas in the gaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very little carbon and oxygen after reaching the pebble isolation mass. In the absence of atmosphere-enriching events, these planets would thus have a strongly substellar O/H and C/H and superstellar C/O atmospheric composition. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Bosman , A D , Alarcón , F , Bergin , E A , Zhang , K , van't Hoff , M L R , Öberg , K I , Guzmán , V V , Walsh , C , Aikawa , Y , Andrews , S M , Bergner , J B , Booth , A S , Cataldi , G , Cleeves , L I , Czekala , I , Furuya , K , Huang , J , Ilee , J D , Law , C J , Le Gal , R , Liu , Y , Long , F , Loomis , R A , Ménard , F , Nomura , H , Qi , C , Schwarz , K R , Teague , R , Tsukagoshi , T , Yamato , Y & Wilner , D J 2021 , ' Molecules with ALMA at Planet-forming Scales (MAPS). VII. Substellar O/H and C/H and superstellar C/O in planet-feeding gas ' , Astrophysical Journal Supplement Series , vol. 257 , no. 1 , 7 . https://doi.org/10.3847/1538-4365/ac1435
Astrophysical Journal Supplement Series
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DescriptionFunding: K.Z., J.B.B., I.C., J.H., and K.R.S. acknowledge support from NASA through NASA Hubble Fellowship grant Nos. HST-HF2-51401.001, HST-HF2-51429.001-A, HST-HF2-51405.001-A, HST-HF2-51460.001-A, and HST-HF2-51419.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. C.W. acknowledges financial support from the University of Leeds, STFC, and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, MR/T040726/1). J.D.I. acknowledges support from the Science and Technology Facilities Council of the United Kingdom (STFC) under ST/T000287/1.
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