Expanding shells around young clusters -- S 171/Be 59

TL;DR

This work investigates why some HII regions around young clusters exhibit expansion velocities that challenge existing simulations, using Sharpless 171 and Berkeley 59 as a test case. The authors combine high-resolution optical spectroscopy of 27 cluster stars and extensive $^{13}$CO($J=1-0$) mapping to characterize the cluster’s mean RV and the velocity pattern of the surrounding molecular shell, identifying three distinct shell components with expansion velocities of roughly $v_{\rm exp} \approx 4$ km s$^{-1}$, $\approx 12$ km s$^{-1}$, and intermediate values. They develop a multi-stage modelling approach: (i) a multiphysics SPH wind-bubble simulation that captures the formation of a shell and detached cloudlets, (ii) simplified momentum-conserving wind-bubble models (Weaver and Lancaster) to infer plausible ambient ISM densities and system ages, and (iii) a novel cloudlet model to study the propagation of dense fragments through the ISM. The results qualitatively show that winds from the Be 59 cluster can drive a structured shell while fragmentation generates high-velocity, low-mass cloudlets that travel farther than the main shell, though quantitative fitting remains sensitive to the chosen physics and numerical method. The study highlights the importance of incorporating clumpiness and advanced hybrid hydrodynamics to accurately reproduce complex shell kinematics and underscores that the inferred system age and ambient density depend on the momentum-transfer model used.

Abstract

Some HII regions that surround young stellar clusters are bordered by molecular shells that appear to expand at a rate inconsistent with our current model simulations. In this study we focus on the dynamics of Sharpless 171 (including NGC 7822), which surrounds the cluster Berkeley 59. We aim to compare the velocity pattern over the molecular shell with the mean radial velocity of the cluster for estimates of the expansion velocities of different shell structures, and to match the observed properties with model simulations. Optical spectra of 27 stars located in Berkeley 59 were collected at the Nordic Optical Telescope, and a number of molecular structures scattered over the entire region were mapped in $^{13}$CO(1-0) at Onsala Space Observatory. We obtained radial velocities and MK classes for the cluster's stars. At least four of the O stars are found to be spectroscopic binaries, in addition to one triplet system. From these data we obtain the mean radial velocity of the cluster. From the $^{13}$CO spectra we identify three shell structures, expanding relative to the cluster at moderate velocity (4 km/s), high velocity (12 km/s), and in between. The high-velocity cloudlets extend over a larger radius and are less massive than the low-velocity cloudlets. We performed a model simulation to understand the evolution of this complex. Our simulation of the Sharpless 171 complex and Berkeley 59 cluster demonstrates that the individual components can be explained as a shell driven by stellar winds from the massive cluster members. However, our relatively simple model produces a single component. Modelling of the propagation of shell fragments through a uniform interstellar medium demonstrates that dense cloudlets detached from the shell are decelerated less efficiently than the shell itself. They can reach greater distances and retain higher velocities than the shell.

Figures (18)

• Figure 1: Optical image from the Digitized Sky Survey (left) and mid-IR image from Wide-field Infrared Survey Explorer (WISE, right), featuring the Hii region S 171 in relation to the cluster Berkeley 59, which is located in the central white box. The northern part of the nebula is called NGC 7822. The large circle has a radius of 1.6$\degr$ and shows the approximate extent of the Hii region. The green contours show the 4 mm (70 GHz) continuum map from Planck. The smaller circles in the optical image mark the positions of the hottest members of the extended Cep OB 4 association; light blue circles represent spectral types O9 and B0, and yellow circles types B1 and B2. North is up, and east to the left. The WISE image is a colour composition of red (22 $\mu$m), green (4.6 $\mu$m), and blue (3.4 $\mu$m). Figure \ref{['fig:field']} is a zoomed-in view of the area in the white box.
• Figure 2: Locations of program stars in the Berkeley 59 region according to the designations in Table \ref{['tab:stars']} on an image from the Digitized Sky Survey. The cluster centre is put between the bright stars A (BD+66$\degr$1674) and B (BD+66$\degr$1675). The white arrow points at BD+66$\degr$1673, V747 Cep. The red arrow marks the position of a bright IR source. North is up, and east is to the left.
• Figure 3: Examples of normalised spectra obtained for some program stars over two selected wavelength regions. Certain spectral features discussed in the text are marked. The spectra are ordered in steps of 0.3 (intensity scale) in the left panel and 1.0 in the right panel and with a smaller spacing between the third and fourth spectra; the double-lined spectroscopic binary BD+66$\degr$1674 is shown at two different phases. To improve the S/N for the faint IR star, its left spectrum is a combination of many observations, Doppler-corrected for the binary motion, which smears out the DIBs.
• Figure 4: Phase diagrams for (from the top): V747 Cep for a period of 5.33146 days; BD+66$\degr$1675, period of 74 days; the IR star, period of 9.51 days; and P16, period of 13 days.
• Figure 5: Map of the fields we obtained CO spectra in. Left: Large region, 4.4$\times$4.4 degrees, surrounding the emission nebula S 171. The arrow points at the position of the central cluster. The northern arc of bright nebulosity is NGC 7822. In this panel the more peripheral areas observed for $^{13}$CO emission are marked and labelled. The circles (Enorth and Esouth) mark observations at selected positions along two dusty striations that extend to the east from the centre. The dashed box marks the central region shown in the right panel. Right: Image spanning 1.9$\times$1.2 degrees. The core of the stellar cluster is inside the dashed box. The position of an elephant trunk, called the 'Dancing Queen' is marked with a black circle. North is up and east to the left in both images, which are from the red Deep Sky Survey.