Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as Tracers of Photochemistry in Disks - Archive ouverte HAL Access content directly
Journal Articles The Astrophysical Journal Supplement Series Year : 2021

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

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Jennifer B. Bergner
  • Function : Author
Karin I. Öberg
  • Function : Author
Viviana V. Guzmán
  • Function : Author
Charles J. Law
  • Function : Author
Ryan A. Loomis
  • Function : Author
Gianni Cataldi
  • Function : Author
Arthur D. Bosman
  • Function : Author
Yuri Aikawa
Sean M. Andrews
  • Function : Author
Edwin A. Bergin
  • Function : Author
Alice S. Booth
  • Function : Author
L. Ilsedore Cleeves
  • Function : Author
Ian Czekala
  • Function : Author
Jane Huang
  • Function : Author
John D. Ilee
  • Function : Author
Feng Long
  • Function : Author
Hideko Nomura
Chunhua Qi
Kamber R. Schwarz
  • Function : Author
Richard Teague
Takashi Tsukagoshi
Catherine Walsh
  • Function : Author
David J. Wilner
  • Function : Author
Yoshihide Yamato
  • Function : Author

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 toward five disk systems. All disks show bright CN emission within ~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.

Dates and versions

insu-03672369 , version 1 (19-05-2022)

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Jennifer B. Bergner, Karin I. Öberg, Viviana V. Guzmán, Charles J. Law, Ryan A. Loomis, et al.. Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as Tracers of Photochemistry in Disks. The Astrophysical Journal Supplement Series, 2021, 257, ⟨10.3847/1538-4365/ac143a⟩. ⟨insu-03672369⟩
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