Protostellar Interferometric Line Survey of the Cygnus-X region (PILS-Cygnus)

The role of the external environment in setting the chemistry of protostars^★

¹ Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K., Denmark
e-mail: sarel.vanderwalt@nbi.ku.dk
² Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87–100 Torun, Poland
³ Departments of Astronomy and Chemistry, University of Virginia, Charlottesville, VA 22904, USA

Received: 14 October 2022
Accepted: 25 June 2023

Abstract

Context. Molecular lines are commonly detected towards protostellar sources. However, to get a better understanding of the chemistry of these sources we need unbiased molecular surveys over a wide frequency range for as many sources as possible to shed light on the origin of this chemistry, particularly any influence from the external environment.

Aims. We present results from the PILS-Cygnus survey of ten intermediate- to high-mass protostellar sources in the nearby Cygnus-X complex, through high angular resolution interferometric observations over a wide frequency range.

Methods. Using the Submillimeter Array (SMA), a spectral line survey of ten sources was performed in the frequency range 329–361 GHz, with an angular resolution of ~1″.5, or ~2000 AU at a source distance of 1.3 kpc from the Sun. Spectral modelling was performed to identify molecular emission and determine column densities and excitation temperatures for each source. Emission maps were made to study the morphology of emission. Finally, emission properties were compared across the sample.

Results. We detect CH₃OH towards nine of the ten sources, with CH₃OCH₃ and CH₃OCHO towards three sources. We further detect CH₃CN towards four sources. Towards five sources the chemistry is spatially differentiated, meaning that different species peak at different positions and are offset from the peak continuum emission. Low levels of deuteration are detected towards four sources in HDO emission, whereas deuterated complex organic molecule emission is detected towards one source (CH₂DOH towards N63). The chemical properties of each source do not correlate with their position in the Cygnus-X complex, nor do the distance or direction to the nearest OB associations. However, the five sources located in the DR21 filament do appear to show less line emission compared to the five sources outside the filament.

Conclusions. This work shows how important wide frequency coverage observations are combined with high angular resolution observations for studying the protostellar environment. Furthermore, based on the ten sources observed here, the external environment appears to only play a minor role in setting the chemical environment on these small scales (<2000 AU).