Globules and pillars in Cygnus X IV. Velocity-resolved [OI] 63 mu map of a peculiar proplyd-like object

TL;DR

The paper investigates proplyd #7 in Cygnus X using velocity-resolved [O I] 63 μm data from SOFIA/upGREAT, along with archival [C II] 158 μm and IRAM CO 2→1 observations, to dissect the gas dynamics, PDR structure, and the nature of the embedded source. The data reveal multiple kinematic components, including a central 11 km s$^{-1}$ feature, a red wing near 13 km s$^{-1}$, and a CO outflow, suggesting complex interactions between external UV irradiation and possible internal activity. PDR modelling with KOSMA-τ shows inconsistencies between line intensities and FIR continuum, implying that simple external-FUV-driven PDRs cannot fully explain the observations; a thermal H II region and potentially shock-excited gas likely play significant roles. The inferred gas masses (~20 M$_\odot$ total) and short photo-evaporation timescale ($t_{photo} \approx 1.6\times10^5$ yr) indicate that external feedback may disperse the globule before extensive secondary star formation, though an embedded disc around a massive star cannot be completely ruled out. Overall, proplyd #7 appears more consistent with an irradiated globule with outflow activity than a classical protoplanetary disc, highlighting the need for higher-resolution follow-up (e.g., JWST/NIR) to resolve its true nature.

Abstract

We investigated an isolated, globule-shaped object (0.37x0.11 pc), located near the centre of the Cygnus OB2 cluster and named proplyd #7 in optical observations. The source can be a massive star (with or without disc) with a HII region or a G-type T Tauri star with a photo-evaporating disc, embedded in a molecular envelope. We obtained a map of the OI line at 63 micron with 6" angular resolution and employed archival data of the CII 158 micron line (14" resolution), using the upGREAT heterodyne receiver aboard SOFIA. We also collected IRAM 30m CO data at 1mm (11" resolution). All the lines were detected across the whole object. The peak integrated OI emission of ~5 K km/s is located ~10" west of an embedded YSO. The OI and CII data near the source show bulk emission at ~11 km/s and a line wing at ~13 km/s, while the 12CO 2-1 data reveal additional blue-shifted high-velocity emission. The KOSMA-tau PDR model can explain the emissions in the tail with a low external UV field (<350 Go, 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 HII region, revealed by combining existing and new radio continuum data, points towards a massive star - and not a T Tauri-type one. We derived molecular and atomic gas masses of ~20 Msun and a few Msun, respectively. The photo-evaporation (only considering external illumination) lifetime of 1.6x10^5 yrs is shorter than the free-fall lifetime of 5.2x10^5 yrs; thus, we find that proplyd #7 might not have had the time to produce many more stars.

Figures (13)

• Figure 1: False-colour image of the central Cygnus X region Schneider2016a and a zoom onto the region indicated by the red dashed box around Proplyd #7 (right). The R and G channels correspond to Spitzer 8 $\mu$m, while the B channel corresponds to Herschel (PACS 70 $\mu$m) emission. The proplyd-like objects detected by Wright2012 and Schneider2016a are labelled in yellow ( pr1-11). Proplyd #7 ( pr 7) is located in the upper centre region, close to some massive stars of the Cyg OB2 association. These are marked in grey, following the nomenclature in Wright2015.
• Figure 2: [O I] 63 $\mu$m, [C II] 158 $\mu$m, and $^{12}$CO 2$\to$1 spectra of proplyd #7. To increase the S/N, the [O I] data were smoothed to an angular resolution of 10$"$ and re-gridded to 10$"$ (original resolution 6$"$ on a $\sim$2$"$ sampling grid). The velocity resolution was degraded to $\sim$0.4 km s$^{-1}$. The [C II] (CO) spectra are on their nominal angular resolution of 14$"$ (12$"$) and velocity resolution of 0.2 (0.3) km s$^{-1}$. The displayed main beam brightness temperature and velocity ranges are indicated in the small lower panels. The star indicates the approximate location of the YSO.
• Figure 3: Example of an [O I] spectrum and fit to the data. The black histogram shows the observed [O I] spectrum closest to the central YSO (position with offset $-$4$"$,0), smoothed to an angular resolution of 10$"$. The blue Gaussian lines represent the fit to this spectrum with two components. Centre velocity, temperature, and FWHM values are given in the panel. The red line gives the resulting fit.
• Figure 4: Channel map of $^{12}$CO 2$\to$1 line emission. Each channel covers a velocity range of 0.3 km s$^{-1}$. Contours go from 2 to 34 K km s$^{-1}$ in steps of 4 K km s$^{-1}$ and the green star indicates the position of the YSO.
• Figure 5: Distribution of [O I] line centre velocity and FWHM. Overlays of [O I] line integrated (v=9 to 15 km s$^{-1}$) emission (contour levels 5, 6, 7, 8, and 8.5 K km s$^{-1}$) on Herschel/PACS 70 $\mu$m emission in color. The left panel shows the line centre velocities in km s$^{-1}$ of the two components (first component in black, second in grey) and the right panel the linewidth in km s$^{-1}$. The values (Table \ref{['tab1']}) were determined from Gaussian fits to the spectra from Fig. \ref{['spectra']}.