Dr. XU Jinlong from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) and his collaborators revealed that the first buried cluster in California is formed by dismemberment after being hit by another giant molecular cloud. The findings were published on April.8 in Astrophysical Journal Letters. 

The study was based on the observation data of the 13.7-meter Millimeter Wave Telescope of Purple Mountain Observatory (PMO) of CAS and the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) of NAOC, as well as the archive data of other wavebands.

"Most of the stars in the universe form in the form of clusters, while the clusters are formed in the giant molecular cloud. After the parent molecular cloud is dispersed, the clusters visible in the optical band are left behind," said Dr. XU. "In the early stage of evolution, the age of stars in clusters is very similar and they are almost formed at the same time, and it requires a large number of gases to accumulate rapidly in the formation position of clusters."

In addition, the traditional model of astronomy suggested that the only reason for the formation of star clusters is the gravitational collapse of giant molecular clouds.

But what event triggered the formation of star clusters? Especially in large-scale giant molecular clouds, more than one cluster can be formed, so how is the first cluster formed? The formation of the first cluster will cause the formation of other stars and clusters in the whole giant molecular cloud like a spark.

"The California giant molecular cloud is the largest in mass within 500 pc of our solar system, and it has similar shape and scale with the famous Orion A giant molecular cloud. However, at present, there is only one cluster (LKHα 101) and several B-type massive stars in California, and these B-type massive stars are also located in the cluster," said Dr. XU. 

The researchers used the observation data of the PMO 13.7-meter Millimeter Wave Telescope to observe the molecular spectrum of carbon monoxide (CO) at the cluster location. They found that a new fibrous giant cloud interacted with the California, and the cluster was located at the intersection of the two giant clouds. 

At the same time, they used FAST to observe the radio hydrogen (H) and carbon (c) RRLs in the HII region associated with the cluster, and ruled out the possibility that the cluster only overlaps with the cross position in the visual direction through the detected composite line system speed, so as to directly confirm that the first buried cluster in the California giant molecular cloud is dismembered after being hit by another giant cloud Form.

The team concluded that the "kindling" of the first cluster in the giant molecular might be caused by the impact of foreign gases. The results of this study will be used for reference in the study of the origin of clusters. 

Upper: CO molecular gas distribution and cluster position; Lower: the hydrogen (H) and carbon (c) RRLs from the FAST observation (Image by XU Jinlong)