Globules and pillars in Cygnus X

IV. Velocity-resolved [OI] 63 μm map of a peculiar proplyd-like object

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁴ Physikalischer Verein, Gesellschaft für Bildung und Wissenschaft, Robert-Mayer-Str. 2, 60325 Frankfurt, Germany
⁵ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, 33615 Pessac, France
⁶ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51 y 216, 72000 Puebla, Mexico

^★ Corresponding author: nschneid@ph1.uni-koeln.de

Received: 12 September 2025
Accepted: 3 November 2025

Abstract

A proplyd is defined as a young stellar object (YSO) surrounded by a circumstellar disk of gas and dust and embedded in a molecular envelope undergoing photo-evaporation by external ultraviolet (UV) radiation. Since the discovery of the Orion proplyds, one question has arisen as to how inside-out photo-evaporation and external irradiation can influence the evolution of these systems. For such an investigation, it is essential to determine the molecular and atomic gas masses, as well as the photo-evaporation and free-fall timescales. Understanding the dynamics within the photo-dissociation regions (PDRs) of a potential envelope–disc system, as well as the surrounding gas in relation to photo-evaporative flows, requires spectrally resolved line observations. Thus, we chose to investigate an isolated, globule-shaped object (~0.37 pc × 0.11 pc at a distance of 1.4 kpc), located near the centre of the Cygnus OB2 cluster and named proplyd #7 in optical observations. In the literature, there is no consensus on the nature of this source. Observations point toward a massive star (with or without disc) with a H II region or a G-type T Tauri star with a photo-evaporating disc, embedded in a molecular envelope. We obtained a map of the [O I] line at 63 μm with 6″ angular resolution and employed archival data of the [C II] 158 μm line (14" resolution), using the upGREAT heterodyne receiver aboard SOFIA. We also collected IRAM 30m CO data at 1 mm (11″ resolution). All the lines were detected across the whole object. The peak integrated [O I] emission of ~5 K km s⁻¹ is located ~10″ west of an embedded YSO. The [O I] and [C II] data near the source show bulk emission at ~11 km s⁻¹ and a line wing at ~13 km s⁻¹, while the ¹²CO 2→1 data reveal additional blue-shifted high-velocity emission. The widespread [O I] emission prompts the question of its origin since the [O I] line can serve as a cooling line for a PDR or for shocks associated with a disc. From both local and non-local thermodynamic equilibrium (LTE and non-LTE) calculations, we obtained a column density of N_OI ≈ 10¹⁸ cm⁻² at a density of 4–8 × 10³ cm⁻³. The [O I] line is, thus, sub-thermally excited. The KOSMA-τ PDR model can explain the emissions in the tail with a low external UV field (<350 G_°, mostly consistent with our UV field estimates), but not at the location of the YSO. There, the high line intensities and increased line widths for all lines and a possible bipolar CO outflow suggest the presence of a protostellar disc. However, the existence of a thermal H II region, revealed by combining existing and new radio continuum data, points towards a massive star – and not a T Tauri-type one. The circumstellar environment of proplyd #7 consists mostly of molecular gas. We derived molecular and atomic gas masses of ~20 M_⊙ and a few M_⊙, respectively. The photo-evaporation (only considering external illumination) lifetime of 1.6 ± 10⁵ yr is shorter than the free-fall lifetime of 5.2 ± 10⁵ yr; thus, we find that proplyd #7 might not have had the time to produce many more stars. This viewpoint is supported by simulation results from the literature.