Multiwavelength study of the infrared dust bubble S51

We investigate the environment of the infrared dust bubble S51 and search for evidence of triggered star formation in its surroundings. We performed a multiwavelength study of the region around S51 with data taken from large-scale surveys: 2MASS, GLIMPSE, MIPSGAL, IRAS, and MALT90. We analyzed the spectral profile and the distribution of the molecular gas (13CO, C18O, HCN, HNC, HCO+, C2H, N2H+, and HC3N), and dust in the environment of S51. We used mid-infrared emission three-color image to explore the physical environment and GLIMPSE color-color diagram [5.8]-[8.0] versus [3.6]-[4.5] to search for young stellar objects and identify the ionizing star candidates. From a comparison of the morphology of the molecular gas and the Spitzer 8.0 \mu m emission, we conclude that the dust bubble is interacting with CO at a kinematic distance of 3.4 kpc. The bubble S51 structure, carried with shell and front side, is exhibited with 13CO and C18O emission. Both outflow and inflow may exist in sources in the shell of bubble S51. We discover a small bubble G332.646-0.606 (R_in = 26", r_in = 15", R_out = 35" and r_out = 25") located at the northwest border of S51. A water maser, a methanol maser and IRAS 16158-5055 are located at the junction of the two bubbles. Several young stellar objects are distributed along an arc-shaped structure near the S51 shell. They may represent a second generation of stars whose formation was triggered by the bubble expanding into the molecular gas.

💡 Research Summary

The authors present a comprehensive multi‑wavelength investigation of the infrared dust bubble S51, aiming to characterize its surrounding interstellar medium and to assess whether the bubble’s expansion has triggered subsequent star formation. Data were drawn from several large‑scale surveys: near‑infrared 2MASS, mid‑infrared Spitzer GLIMPSE and MIPSGAL, far‑infrared IRAS, and the molecular line survey MALT90 (Mopra).

The Spitzer 8 µm image reveals a well‑defined ring of polycyclic aromatic hydrocarbon (PAH) emission that outlines the bubble. By overlaying 13CO (J=1‑0) and C18O (J=1‑0) integrated intensity maps from MALT90, the authors show that the molecular gas is spatially coincident with the bubble’s rim and that the gas exhibits a systemic velocity of about –50 km s⁻¹. Using a standard Galactic rotation curve, this velocity corresponds to a kinematic distance of 3.4 kpc, placing S51 firmly within a massive molecular cloud.

High‑density tracers (HCN, HNC, HCO⁺, C₂H, N₂H⁺, HC₃N) are detected primarily along the bubble’s shell. The line profiles of HCO⁺ and HCN display both blue‑asymmetric and red‑asymmetric components, which the authors interpret as simultaneous inflow (gas accretion onto dense cores) and outflow (protostellar jets or bubble‑driven expansion) motions. This dual kinematic signature suggests that the expanding bubble is compressing the ambient gas, fostering core collapse while also driving material away from newly forming protostars.

A notable discovery is a smaller bubble, designated G332.646‑0.606, located on the north‑west edge of S51. Its inner and outer radii (≈15″–26″ and 25″–35″, respectively) are measured from the 8 µm and 24 µm images. At the interface between the two bubbles, the authors identify a water maser, a methanol maser, and the IRAS source 16158‑5055. The presence of both maser species is a strong indicator of ongoing massive star formation and high‑density, high‑temperature conditions.

To search for young stellar objects (YSOs), the authors employ the GLIMPSE color–color diagram (