Molecules with ALMA at Planet-forming Scales (MAPS) XI: CN and HCN as Tracers of Photochemistry in Disks

Abstract

UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been proposed to trace the UV field in various astrophysical objects, however to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN N=1-0 transitions at 0.3'' resolution towards five disk systems. All disks show bright CN emission within $\sim$50-150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions, and also means that CN is the dominant carrier of the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

References in corpus (23)

• The NumPy array: a structure for efficient numerical computation
• Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview and Highlights
• Measuring turbulent motion in planet-forming disks with ALMA: A detection around DM Tau and non-detections around MWC 480 and V4046 Sgr
• Molecules with ALMA at Planet-forming Scales (MAPS) V: CO gas distributions
• Molecules with ALMA at Planet-forming Scales (MAPS) IV: Emission Surfaces and Vertical Distribution of Molecules
• Dust Density Distribution and Imaging Analysis of Different Ice Lines in Protoplanetary Disks
• Molecules with ALMA at Planet-forming Scales (MAPS) XIV: Revealing disk substructures in multi-wavelength continuum emission
• A survey of C2H, HCN, and C18O in protoplanetary disks
• Polarized disk emission from Herbig Ae/Be stars observed using Gemini Planet Imager: HD 144432, HD 150193, HD 163296, and HD 169142
• CN rings in full protoplanetary disks around young stars as probes of disk structure
• Chemistry and line emission from evolving Herbig Ae disks
• Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO
• Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC${}_3$N, CH${}_3$CN, and $c$-C${}_3$H${}_2$
• Stacking Spectra in Protoplanetary Disks: Detecting Intensity Profiles from Hidden Molecular Lines in HD 163296
• Multi-epoch Ultraviolet HST Observations of Accreting Low-mass Stars
• Molecules in the disk orbiting the twin young suns of V4046 Sgr
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• Consistent dust and gas models for protoplanetary disks IV. A panchromatic view of protoplanetary disks
• The Excitation Conditions of CN in TW Hya
• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• Chemical Evolution in a Protoplanetary Disk within Planet Carved Gaps and Dust Rings
• Disk Illumination and Jet Variability of the Herbig Ae Star HD 163296 Using Multi-Epoch HST/STIS Optical, Near-IR, and Radio Imagery and Spectroscopy
• Tracing Molecular Stratification within an Edge-on Protoplanetary Disk

Cited by in corpus (28)

• Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview and Highlights
• Molecules with ALMA at Planet-forming Scales (MAPS) V: CO gas distributions
• Molecules with ALMA at Planet-forming Scales (MAPS). VII. Sub-stellar O/H and C/H and super-stellar C/O in planet feeding gas
• Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO
• Directly tracing the vertical stratification of molecules in protoplanetary disks
• Herbig Stars: A Quarter Century of Progress
• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• A JWST/MIRI analysis of the ice distribution and PAH emission in the protoplanetary disk HH 48 NE
• Chemical footprints of giant planet formation. Role of planet accretion in shaping the C/O ratio of protoplanetary disks
• The edge-on protoplanetary disk HH 48 NE I. Modeling the geometry and stellar parameters
• Chemical modeling of Orion Nebula Cluster disks: evidence for massive, compact gas disks with ISM-like gas-to-dust ratios
• Vertically extended and asymmetric CN emission in the Elias 2-27 protoplanetary disk
• A major asymmetric ice trap in a planet-forming disk IV. Nitric oxide gas and a lack of CN tracing sublimating ices and a C/O ratio $<1$
• Radial and vertical constraints on the icy origin of H${}_2$CO in the HD 163296 Protoplanetary Disk
• Cold Deuterium Fractionation in the Nearest Planet-Forming Disk
• Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems
• PRODIGE -- Planet-forming disks in Taurus with NOEMA. I. Overview and first results for 12CO, 13CO, and C18O
• Vertical and radial distribution of atomic carbon in HD 163296
• A nearly constant CN/HCN line ratio in nearby galaxies: CN as a new tracer of dense gas
• Circumplanetary Disk Candidate in the Disk of HD 163296 Traced by Localized Emission from Simple Organics
• UV-processing of icy pebbles in the outer parts of VSI-turbulent disks
• Edge-On Disk Study (EODS) III: Molecular Stratification in the Flying Saucer Disk
• Molecules with ALMA at Planet-forming Scales (MAPS) XIX. Spiral Arms, a Tail, and Diffuse Structures Traced by CO around the GM Aur Disk
• Molecules with ALMA at Planet-forming Scales (MAPS) III: Characteristics of Radial Chemical Substructures
• Molecules with ALMA at Planet-forming Scales (MAPS) II: CLEAN Strategies for Synthesizing Images of Molecular Line Emission in Protoplanetary Disks
• Protoplanetary Disk Chemistry
• First Detection of HC5N in a Class II Disk around TW Hya
• CO Line Emission Surfaces and Vertical Structure in Mid-Inclination Protoplanetary Disks