Runaway stars from young star clusters as probes of cluster formation

One of the major questions astronomers currently face is "how and when do stars form?" Our own Sun is one of billions of stars that have formed in the 13-billion year lifetime of our Galaxy. When we look at these stars we see a wide range of sizes, masses and temperatures. Some stars appear isolated, others live in a binary system with another star, and some are found in groups of stars known as clusters. The vast majority of young stars are observed in groups or clusters of some sort, suggesting that the majority of stars might form in such clusters. However, since even the early phases of a star's life are much longer than human lifetimes, we cannot directly observe how these clusters form and evolve, and instead must infer how these processes occur indirectly.

There are two competing models for how a star cluster forms. The first is the model of "monothlic collapse", in which a dense cloud of gas collapses under its own gravity and then forms stars in a clustered distribution. The second model is that of "hierarchical mergers", in which the stars form over a larger volume and at lower densities, but then gather together, due to gravity, to form a cluster of stars. Distinguishing between these models is important because they imply very different conditions for star formation and different environments under which young planetary systems form and evolve. Determining which model is closer to the truth is difficult because very young star clusters are often obscured within the molecular cloud that they formed from.

Fortunately, these competing models for star cluster formation result in a number of tell-tale signatures, one of which is the number and speeds of stars that get ejected from the cluster early in its life. Such "runaway" stars have been known about for many decades, but discoveries have typically been limited to the most luminous runaway stars. It is only recently, thanks to data from the Gaia satellite, that astronomers have been able to identify the more numerous low-mass, low-luminosity runways. It is these stars that probe the conditions within a star cluster at the time the stars were ejected.

The main limiting factor for existing studies of runaway low-mass stars (which are typically based on apparent motion on the plane of the sky) is our ability to reliably confirm that the runaway star is young (as opposed to one of the many older stars found in our galaxy) and determine the three dimensional (3D) trajectory of the star to verify that it has been ejected from the star cluster in question. To do this requires spectroscopy of hundreds of candidate runaway stars to determine their youth and measure line-of-sight velocities.

This programme will combine 2D motions from the European Space Agency's Gaia satellite with spectroscopy from the new WEAVE multi-object spectrograph on the William Herschel Telescope, both facilities that STFC supports. This combination of data will provide both confirmation of youth and the 3D motion of runaway star candidates. The number and speed of runaway stars ejected from a cluster will be compared with the predictions from numerical simulations of the two cluster formation models outlined above to determine how star clusters form.

In addition, by measuring both the distance between the star and cluster, and the 3D motion of the runaway star we can calculate the time since the star was ejected from the cluster. This provides a lower limit for the age of the star, since the star cannot be younger than the time spent in motion away from its birth cluster. This method of determining the age of a star is valuable because it does not depend on assumptions about the properties of the star or models for how stars change as they age, and therefore is an important tool for calibrating models for how stars evolve.