Clustering Star Groups Into Known Structures

Like asmall child sorting toy cars, astronomers are intent on organizing things into groups, from types of galaxies to types of stars. A team of astronomers set out to try to see if newly discovered star groups areactually partof previouslyknown clusters by sorting them by their speed and location. Clustering Together and InteractingStars are born when giant clouds of gas and dust become so massive that they collapse in on themselves. If a cloud is large enough, it can form multiple stars with comparable ages, and those stars can travel through space together at similar velocities. These groups of stars can stay gravitationallybound to each other as an opencluster,ortheycan dissociate and travel through space together as a moving group or a stellar association. Groups of stars that traverse the galaxy together can also be the result of an interaction between two groups of stars.A team of astronomers led by Jonathan Gagn (Rio Tinto Alcan Planetarium, Canada) looked at roughly a dozen stellar groups to determine whether theyre associated with known structures such as open clusters. These results could open the door to new interpretations of how young stellar groups form and evolve.Ageing the SystemsA colormagnitude diagram showing the color of the star against its brightness. The different colors of lines show clusters with different ages. By plotting the new clusters on this map, the team could pinpoint the age of each Theia group. [Gagn et al. 2021]The team focused on the so-called Theiagroups,a large set of stellar structures recently identified withina distance of 10,000light-years.Todetermine whether these new stars belong to knowngroups,the team needed to get a better estimate of their ages. To do this, theauthorsused data from the Gaia Data Release 2, which was specifically designed to find stars traveling together by measuring their position and how fast they are moving through space.By using reference stars from five groups of different known ages,Gagn and collaboratorsfit each Theia group on a color-magnitude diagram, which plots brightness against color. Because the known clusters follow certain trends, placing the new groups on the diagram allowed theauthorsto find whichknowncluster, if any, theTheia groupswere associated with. From this, the authors couldestimate how old thegroupsare.Sorting into CategoriesAn example of how the team plotted their data for the Pleiades system. Left: the spatial extent of various groups compared to the Pleiades, showing Theia 369 is likely to be associated with it. Right: the velocities of the various clusters. This shows that Theia 369 contains members of the Pleiades and Theia 301 may constitute its trailing tail. [Adapted from Gagn et al. 2021]When presented with a group of toy cars, a child might sort them by model and color. In the case ofGagn and collaboratorsstellarclusters, once their ages were determined, the team sorted them by how fast they are moving and their sky location.Theauthorsplotted the velocities and spatial distributions ofboththe Theia groups and known structures,andtheyfound that many of the moving groups and Theia structuresare extensions of larger associations of stars or open clusters. This means that these groups are more extended than previously thought, and some might consist of tails that are co-moving with the cores of these clusters.Some groups may have similar velocities butarent physically close to each other, such as Theia 301, which may beextensions of current clusters. For example, Theia 301 and ABDoradusmay be trailing tidal tailsof the Pleiades.Additional data from Gaia and other missions will provide better constraints on velocities and ages so these groups can properly be sorted where they belong!CitationA Number of Nearby Moving Groups May Be Fragments of Dissolving Open Clusters, JonathanGagnetal2021ApJL915L29.doi:10.3847/2041-8213/ac0e9a