Authors: Namitha Issac, Anandmayee Tej, Tie Liu, and Yuefang Wu
First Author’s Institution: Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, Kerala, India
Status: Accepted for publication in MNRAS, open access on arXiv
In our own Galaxy and in many galaxies throughout the universe, stars are actively being born. However this process is complicated, requiring large reservoirs of molecular gas. When dense portions of large molecular clouds reach a critical mass or size, they are unstable to small perturbations. If such a perturbation occurs, they begin to collapse on themselves and eventually will form a star.
One spectacular way to perturb gas into conditions ripe for making stars is the collision of clouds of gas and dust. When this happens, the gas becomes compressed into dense filaments and cores. These dense cores accrete material from the surrounding clouds and collapse into protostars, the precursors of the stars we see in the night sky. In today’s paper, the authors study a particular system, G133.50+9.01 (hereafter G133), and the possibility that it is the result of such a cosmic collision.
A Multi-wavelength Snapshot
As star forming regions are enshrouded in thick clouds of dust, they are often nearly impossible to see in visible light. To get a complete picture of this complex region, the authors combine data from the radio, sub-millimeter, and infrared. This multi-wavelength view allows the authors to see different phases of the gas and dust in this region and get a complete physical picture.
First, the authors study the molecular gas of G133 using radio observations of carbon monoxide (CO), which is a tracer of the hydrogen gas from which stars are made. The maps of CO emission in three different wavelength ranges is shown in Figure 1. The morphologies seen in the CO gas show three main features. First, in panel (a) there is extended emission to the upper-left of the bright core. Second, there is an arc-like structure near the peak of the emission in panel (b). Finally, there is a “boomerang-like” feature extending from the lower-left to the upper right seen in panel (c). Panel (c) also shows a corresponding lack of emission in the region where emission is seen in panel (a).
Next, the authors look to the sub-millimeter to understand the dust context of G133. Using the 850 micron bandpass of JCMT and the Planck satellite, they find strong evidence for filamentary structures and dust cores, where stars may be forming. Figure 2 shows the sub-millimeter emission of this complex region. When compared to Figure 1, it is clear that structures in the dust have corresponding features in molecular gas. Additionally, the authors determine the direction of the magnetic field and find it to be perpendicular to the filaments.
Finally, the authors use the mid-infrared, from the WISE spacecraft, to probe the emission from newly-formed stars. These objects, aptly named young-stellar objects or YSOs, have distinct colors caused by the fact that they have disks of material surrounding them. The left panel of Figure 3 shows the color cuts used to identify these YSOs. Class I YSOs are deeply enshrouded by dust and have a sub-millimeter excess. Class II YSOs are less covered by dust, but still have an infrared excess. When these YSOs are plotted along with the dust cores and filaments, it is clear that the ongoing star formation occurs predominantly in the region of compressed gas.
A Cosmic Collision?
Through their multi-wavelength analysis, the authors have shown that the likely explanation for the star cluster seen in G133 is a collision between two clouds. Firstly, the gas morphology and dynamics suggest a merger between two clouds (called G133a and G133b). Secondly, the presence of filaments and dense dust cores, with a perpendicular magnetic field are consistent with theoretical predictions of a compressed gas. Finally, the locations of young stellar objects consistent with dense features in the gas and dust maps neatly links the star formation to the collision between the clouds.
Figure 4 shows a schematic diagram of what this interaction may have looked like. Going forward, the authors note that G133 is an ideal candidate for detailed follow-up and modeling to explain the observed features. It is exciting to have such a place to examine star formation and the processes that trigger it in detail. Since stars are one of the fundamental pieces of the universe we see around us, it is crucial not only to understand how they live, but how they were born.
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Featured image credit: ESA