Chemistry in the GG Tau A Disk: Constraints from H2D+, N2H+, and DCO+ High Angular Resolution ALMA Observations

Resolved molecular line observations are essential for gaining insight into the physical and chemical structure of protoplanetary disks, particularly in cold, dense regions where planets form and acquire their chemical compositions. However, tracing these regions is challenging because most molecule...

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Published in:arXiv.org 2024-09
Main Authors: Kashyap, Parashmoni, Majumdar, Liton, Dutrey, Anne, Guilloteau, Stéphane, Willacy, Karen, Chapillon, Edwige, Teague, Richard, Semenov, Dmitry, Henning, Thomas, Turner, Neal, Sahai, Raghvendra, Kóspál, Ágnes, Coutens, Audrey, Piétu, V, Gratier, Pierre, Ruaud, Maxime, Phuong, N T, E Di Folco, Chin-Fei, Lee, Y -W Tang
Format: Article
Language:English
Subjects:
Accretion disks
Angular resolution
Astrochemistry
Chemical composition
Chemistry
Cold
Constraints
Cosmic rays
Emission
Freezing
Ionization
Low temperature
Molecular chemistry
Molecular structure
Planet formation
Planetary composition
Protoplanetary disks
Rotating disks
Vapor phases
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Summary:Resolved molecular line observations are essential for gaining insight into the physical and chemical structure of protoplanetary disks, particularly in cold, dense regions where planets form and acquire their chemical compositions. However, tracing these regions is challenging because most molecules freeze onto grain surfaces and are not observable in the gas phase. We investigated cold molecular chemistry in the triple stellar T Tauri disk GG Tau A, which harbours a massive gas and dust ring and an outer disk, using ALMA Band 7 observations. We present high angular resolution maps of N2H+ and DCO+ emission, with upper limits reported for H2D+, 13CS, and SO2. The radial intensity profile of N2H+ shows most emission near the ring outer edge, while DCO+ exhibits double peaks, one near the ring inner edge and the other in the outer disk. With complementary observations of lower-lying transitions, we constrained the molecular surface densities and rotation temperatures. We compared the derived quantities with model predictions across different cosmic ray ionization (CRI) rates, carbon-to-oxygen (C/O) ratios, and stellar UV fluxes. Cold molecular chemistry, affecting N2H+, DCO+, and H2D+ abundances, is most sensitive to CRI rates, while stellar UV flux and C/O ratios have minimal impact on these three ions. Our best model requires a low cosmic ray ionization rate of 1e-18 s-1. However, it fails to match the low temperatures derived from N2H+ and DCO+, 12 to 16 K, which are much lower than the CO freezing temperature.
ISSN:2331-8422