A VLA search for compact radio sources in the explosive molecular outflows DR 21 and G5.89

TL;DR

This study uses high-angular-resolution VLA Ku-band imaging (12–18 GHz) to search for compact radio sources that could be runaway stars associated with explosive molecular outflows in DR 21 and G5.89. Thirteen DR 21 CRSs (nine in the core) reveal a mix of thermal and nonthermal emission, with arc-shaped ionization features whose parabolic fits and line intersections point toward a likely main ionizing star and a refined explosion center from all CO streamers. In G5.89, the bright shell suppresses weak CRSs, yielding only two outer detections and a single parabolic arc, yet arc analyses still provide insight into ionizing sources. Overall, arc structures emerge as effective tracers of ionizing sources and possible links between EMOs and H II regions, highlighting the need for future astrometric and multiwavelength follow-up to confirm runaway-star associations and unify the EMO–stellar feedback picture.

Abstract

We present high-angular-resolution ($\sim0\rlap{.}''1$) VLA Ku-band (12--18 GHz) observations of two explosive molecular outflows (EMOs), DR 21 and G5.89, in a search for runaway stars related to these explosive events. In DR 21, we identified 13 compact radio sources (CRS), 9 located in the DR 21 core and near the CO streamer ejection region. The radio properties of the CRSs show that three are nonthermal radio emitters, likely magnetically active stars, while the nature of the remaining CRSs cannot be conclusively identified. All detected CRSs are good candidates for follow-up proper motion studies to confirm whether they are runaway stars. We also identify multiple ionized arc-shaped structures that can be fitted with parabolas whose symmetry axes converge to a position coincident with CRSs #11, raising the possibility that this source is the main ionizing star. A re-analysis of the 18 molecular outflow streamers refines the center of the explosive event, which aligns closely with the position indicated by the arcs convergence point, supporting a common stellar origin for the EMOs and the HII-region. In G5.89, the observations reveal a shell with a square-like morphology. The strong extended emission from this HII region prevents the detection of weak compact radio sources inside the shell; only two were identified well beyond the shell, and a single parabolic arc was fitted within this region. Overall, arc structures in ionized regions seem to be good tracers of the origin of the ionizing sources.

Figures (11)

• Figure 1: VLA radio images of DR 21 (top) and G5.89 (bottom) at K$_u$ band (12--18 GHz). Dots mark the blue- and redshifted streamers detected with ALMA by guzman2024 and zapata2020 respectively.
• Figure 2: VLA radio images of DR 21 at a mean frequency of 15 GHz. Left: Image combines the data of the observed session and use the full band and all baselines. Contour levels are at --3, 3, 5, 10, 30, and 60 times the noise level near to the arcs, 56 $\mu$Jy beam$^{-1}$. The white square indicates the area shown in the right panel. Right: Image obtained combining the data of the observed session, using the full band and baselines $>300\,{\rm k}\lambda$. Contour levels are at --5 (red), 5, 10 and 15 times the noise level close to the arcs, 30 $\mu$Jy beam$^{-1}$. The diamonds indicate the position of detected compact radio sources. The corresponding synthesized beam is shown in the bottom left corner of each panel.
• Figure 3: VLA radio images of G5.89 at a mean frequency of 15 GHz. Left: Image combines the data of the observed session and use the full band and all baselines. Contour levels are at --25, 25, 50,100 and 200 times the noise level of the image, 692 $\mu$Jy beam$^{-1}$. The white square indicates the area shown in the right panel. The diamonds indicate the position of detected compact radio sources. Right: Image obtained combining the data of the observed session, using the full band and baselines $>300\,{\rm k}\lambda$. Contour levels are at --100, 100, 200 and 300 times the noise level, 44 $\mu$Jy beam$^{-1}$. The corresponding synthesized beam is shown in the bottom left corner of each panel.
• Figure 4: Detailed view of the parabola fit for each arc in DR21 cm image in order from 1 to 9. The black points are the data taken from the observations. The magenta parabola shows the best fit for each arc, the magenta line reveals the orientation of the parabola, and the magenta cross presents the parabola focus $x_f$, $y_f$. The yellow lines indicate the different possibilities found using MCMC ensemble sampling. Contours are 3, 5, 7, 9, 11, 15 times 56$~\mu$Jy beam$^{-1}$.
• Figure 5: Projection of the symmetry axis of each parabola fitted to each of the identified arcs in DR21. Cyan dots indicate the intersection point of each pair of lines. The yellow transparent ellipse is centered in the mean position of the line intersection points, and the size reflects the standard deviation. The white circle shows the central position of the explosion reported by Zapata et al. (2013), with its size corresponding to the uncertainties. Similarly, the red ellipse represents the central position of the explosion recently suggested by guzman2024. Upper and right panel are the distribution histograms of the intersection points in the R.A. and Declination coordinates, respectively.