Gyrochronology, a technique for estimating the age of stars based on their rotation, has expanded from stars in clusters to stars in remote locations, revealing consistent aging patterns and expanding the range of stellar age determinations.
Scientists from the Leibniz Institute for Astrophysics Potsdam (AIP) and Boston University have succeeded in establishing a link between the rotation rates of stars within a cluster and stars outside the cluster (i.e. field stars), allowing them to deduce the age of the latter. The results show that the gyrochronological method can be applied not only to stars in clusters, but also to field stars, allowing the age of many more stars to be determined.
How old is a star? It's a tough question, one that's easier to answer for stars that live in star clusters. This is because all the stars in the cluster - regardless of size - have the same origin and therefore the same age. By studying the collective properties of stars in a cluster and their current stage of evolution, a good estimate of their age can be obtained.
Researchers are currently exploring the new field of gyrochronology, which can determine the age of individual stars. It establishes a relationship between a star's rotation and color and age.
The period of a star's rotation around its axis can be determined by observing its brightness: many stars have dark spots on their surface, like the sun's sunspots. As the star rotates, the star spot moves into the observer's field of view, and the star's brightness decreases by a small amount.
By measuring these small spots in the star's light intensity, and when they occur repeatedly, for example using data from the Kepler satellite, the star's rotation period can be measured.
Rotational evolution in star clusters
Studies of low-mass dwarf stars in star clusters show that stars spin more and more slowly as they age. Comparing the rotation periods of the stars in the cluster with their colors in the star map reveals a unique pattern: the curves of star formation in the cluster collectively define a skeleton of rotational evolution, with each rib of the skeleton corresponding to a cluster of a specific age, with older clusters defining higher ribs in turn. Each rib is a curve of equal age. By plotting the cluster stars on the diagram, you can use these lines to deduce its age. However, because this method was developed on the basis of star clusters, it was not clear until now whether this method of dating would also work for stars outside star clusters, which make up the vast majority of stars in our galaxy.
Apply computational methods to extracluster stars
This is where recent research comes in. The authors used a sample of more than 300 wide binary stars. This is a system of two stars orbiting each other far enough apart that they do not interact and therefore do not interfere with their normal rotational evolution. Broad binaries are field stars, but their common origin allows an assumption also used for cluster stars - that they are of the same age. This means that if the evolution of field stars is actually the same as that of cluster stars, then if the two stars in the broad binary were placed on the cluster skeleton, they would appear consistent. In other words, if one star in a broad binary is on a rotating rib of a star cluster, is the other star also on the same rotating rib? The study's authors found that this was indeed the case.
In fact, the authors found that they could divide the binaries they studied into a series of subgroups, each associated with a cluster of corresponding ages. "When we started comparing all the broad binary star systems to the cluster skeleton, we were surprised to see how well coordinated they were," said David Gruner, first author of the study and a doctoral student in AIP's Stellar Activity Group. "Even star systems with very different masses showed remarkable consistency in their positions in the diagram, to the point where they were virtually indistinguishable from the cluster."
Therefore, it can be speculated that the few stars located above the collection of cluster ribs are older than the clusters measured to date. Furthermore, the authors show that in the vast majority of systems studied, the rotational age of one component matches that of the other component. Because the broad binary sample is very diverse both in terms of distribution across the sky and other stellar properties such as metallicity, this result means that gyrochronology is likely to be reliable for extracluster stars.
Implications for future research
Dr. Sydney Barnes, head of the AIP Stellar Activity Group, added: "This work to a certain extent ensures that reliable ages of more field stars can be obtained from rotation rates in the future. This result is of great significance to the PLATO satellite mission, which aims not only to discover a large number of planet-hosting stars, but also to provide their ages, allowing people to glimpse the evolutionary history of exoplanets for the first time."
Compiled source: ScitechDaily