Vertically extended and asymmetric CN emission in the Elias 2-27 protoplanetary disk^★

¹ European Southern Observatory, Karl-Schwarzschild-Str 2, 85748 Garching, Germany
e-mail: tpaneque@eso.org
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Departamento de Astronomia, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴ Max-Planck-Institut für extraterrestrische Physik, Gieβenbachstr. 1, 85748 Garching bei München, Germany
⁵ Núcleo Milenio de Formación Planetaria (NPF), Chile
⁶ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, Milano 20133, Italy

Received: 18 November 2021
Accepted: 16 July 2022

Abstract

Context. Cyanide (CN) emission is expected to originate in the upper layers of protoplanetary disks, tracing UV-irradiated regions. This hypothesis, however, has been observationally tested only in a handful of disks. Elias 2-27 is a young star that hosts an extended, bright, and inclined disk of dust and gas. The inclination and extreme flaring of the disk make Elias 2-27 an ideal target to study the vertical distribution of molecules, particularly CN.

Aims. Our aim is to directly trace the emission of CN in the disk around Elias 2-27 and compare it to previously published CO isotopolog data of the system. The two tracers can be combined and used to constrain the physical and chemical properties of the disk. Through this analysis we can test model predictions of CN emission and compare observations of CN in other objects to the massive, highly flared, asymmetric, and likely gravitationally unstable protoplanetary disk around Elias 2-27.

Methods. We analyzed CN N = 3–2 emission in two different transitions J = 7/2–5/2 and J = 5/2–3/2, for which we detect two hyperfine group transitions. The vertical location of CN emission was traced directly from the channel maps, following geometrical methods that had been previously used to analyze the CO emission of Elias 2-27. Simple analytical models were used to parameterize the vertical profile of each molecule and study the extent of each tracer. From the radial intensity profiles we computed radial profiles of column density and optical depth.

Results. We show that the vertical location of CN and CO isotopologs in Elias 2-27 is layered and consistent with predictions from thermochemical models. A north-south asymmetry in the radial extent of the CN emission is detected, which is likely due to shadowing on the north side of the disk. Combining the information from the peak brightness temperature and vertical structure radial profiles, we find that the CN emission is mostly optically thin and constrained vertically to a thin slab at z/r ~ 0.5. A column density of 10¹⁴ cm⁻² is measured in the inner disk, which for the north side decreases to 10¹² cm⁻² and for the south side to 10¹³ cm⁻² in the outer regions.

Conclusions. In Elias 2-27, CN traces a vertically elevated region above the midplane, very similar to that traced by ¹²CO. The inferred CN column densities, low optical depth (τ ≤ 1), and location near the disk surface are consistent with thermo-chemical disk models in which CN formation is initiated by the reaction of N with UV-pumped H₂. The observed north–south asymmetry may be caused by either ongoing infall or by a warped inner disk. This study highlights the importance of tracing the vertical location of various molecules to constrain the disk physical conditions.