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 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.

Molecules with ALMA at Planet-forming Scales (MAPS). V. CO Gas Distributions

“…The C 17 O J = 1-0 rotational transition is split into three components that can be fitted to obtain constraints on the C 17 O column density, excitation temperature, and optical depth (Mangum & Shirley 2015). This method has been used recently in disk studies of C 2 H, HCN, and CN (Bergner et al 2019;Teague & Loomis 2020;Bergner et al 2021;Guzmán et al 2021). We present the first application of this method to C 17 O in protoplanetary disks.…”

“…Following the same method outlined in Bergner et al (2021), we generate model spectra for the C 17 O (1-0) hyperfine transitions to fit the observed spectra. We assume LTE and that all three hyperfine transitions have the same excitation temperature.…”

“…The free parameters in the fit are the total column density of C 17 O (N T , cm −2 ), the excitation temperature (T ex , K), the observed line width (σ obs , km s −1 ), and systematic velocity (V lsrk , km s −1 ). The σ obs parameter is larger than the local thermal+turbulence broadening, likely due to a combination of (1) emission contribution from the back and front sides of a flared disk, which have different projected velocities; (2) beam smearing, in which a wide range of Keplerian velocities are incorporated within the same beam; and (3) Hanning smoothing of the data, which effectively reduces the spectral resolution (Bergner et al 2021).…”

“…This includes a correction factor to account for beam smearing that will artificially lower the inferred optical depth. For full details of the fitting procedure, see Bergner et al (2021).…”

CO Line Emission Surfaces and Vertical Structure in Midinclination Protoplanetary Disks

“…We have shown an effective method for extracting CO emitting layers in a large sample of disks. Such a method can naturally be extended to comparable observations of CO isotopologue lines, which allows a full mapping of 2D disk structure and temperature (e.g., Pinte et al 2018;Law et al 2021a), or to other important molecular tracers of disk chemistry and structure (e.g., Teague & Loomis 2020;Bergner et al 2021). Higher-sensitivity CO line emission data are also necessary to better characterize the prevalence and nature of vertical substructures, and how they relate to other disk characteristics.…”