Tracing snowlines and C/O ratio in a planet-hosting disk

ALMA molecular line observations towards the HD 169142 disk

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: abooth@strw.leidenuniv.nl; alice.booth@cfa.harvard.edu
² Center for Astrophysics – Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 23 May 2023
Accepted: 13 August 2023

Abstract

The composition of a forming planet is set by the material it accretes from its parent protoplanetary disk. Therefore, it is crucial to map the chemical make-up of the gas in disks to understand the chemical environment of planet formation. This paper presents molecular line observations taken with the Atacama Large Millimeter/submillimeter Array of the planet-hosting disk around the young star HD 169142. We detect N₂H⁺, CH₃OH, [CI], DCN, CS, C³⁴S, ¹³CS, H₂CS, H₂CO, HC₃N, and c–C₃H₂ in this system for the first time. Combining these data with the recent detection of SO and previously published DCO⁺ data, we estimated the location of H₂O and CO snowlines and investigate radial variations in the gas phase C/O ratio. We find that the HD 169142 disk has a relatively low N₂H⁺ flux compared to the disks around Herbig stars HD 163296 and MWC 480, indicating less CO freeze-out, and place the CO snowline beyond the millimetre disk at ≈150 au. The detection of CH₃OH from the inner disk is consistent with the H₂O snowline being located at the edge of the central dust cavity at ≈20 au. The radially varying CS/SO ratio across the proposed H₂O snowline location is consistent with this interpretation. Additionally, the detection of CH₃OH in such a warm disk adds to the growing evidence supporting the inheritance of complex ices in disks from the earlier colder stages of star formation. Finally, we propose that the giant HD 169142 b located at 37 au is forming between the CO₂ and H₂O snowlines where the local elemental make-up of the gas is expected to have C/O ≈ 1.0.