How old are the stars outside the cluster?


July 20, 2023

(Nanowerk News) Scientists at the Leibniz Institute for Astrophysics Potsdam (AIP) and Boston University have managed to establish a relationship between the rotation rates of stars within a star cluster and those outside it, called field stars, which makes it possible to determine the age of the star cluster. Results (Astronomy & Astrophysics, “The wide binary shows the consistency of rotational evolution between open clusters and field stars”) showed that the gyrochronology method can be applied not only to star clusters, but also works well for field stars, and thus the ages of more stars can be determined.

How old is the star? This is a tricky question, and one that is easier to answer for stars in star clusters. This is because all the stars in a star cluster – regardless of size – have the same origin and the same age. By studying the collective properties of the stars in a cluster and their current stage of evolution, one can get a good estimate of their age.

Researchers are now exploring a new field of gyrochronology that allows the age of individual stars to be determined. This establishes the relationship between the star’s rotation and its color and age. The rotation period of a star rotating around its axis can be determined from observations of its brightness: many stars have dark spots on their surface, like sunspots.

As the star rotates and the starspots move into the observer’s field of view, the star’s brightness decreases slightly. By measuring the small dips in a star’s light intensity and when they repeat, for example with data from the Kepler satellite, as used here, a star’s rotation period can be measured. ageline framework for open star clusters in rotation vs.  color chart Outline star cluster agelines open in rotation vs. color chart. Each row represents a certain star cluster and age. Binary stars (circles and diamonds with connecting lines) follow the age line. (Images: D. Gruner, SA Barnes, KA Janes)

Studies of low-mass dwarf stars in clusters have shown that stars rotate more slowly with age. Plotting the rotational periods of the stars against their colors in a diagram, a characteristic pattern emerges: star clusters form a curve that collectively determines the rotational evolutionary skeleton, with each rib of the skeleton corresponding to a specific age group, and successively older age groups determining the taller rib. Each rib then becomes a curve with the same age.

By plotting a star cluster in a diagram, its age can be derived using these lines. However, because this method was developed based on star clusters, until recently it was unclear whether this method of dating also works for stars outside the cluster, and which make up the majority of stars in our Galaxy.

This is where the latest work comes in. The author worked with a sample of over 300 wide binary stars. This is a system of two stars orbiting each other and far enough from each other not to interact and disturb their normal rotational evolution. Broad binary stars are field stars but their similar origin allows one assumption to also be used for cluster stars – that they are the same age. This means that if field stars really do evolve in the same way as cluster stars, then two stars from a wide binary will give a consistent picture if placed in the cluster frame. In other words, if one star in a wide binary is located on a rotational rib of a particular cluster, will the other stars also be located on the same rib? The study authors found that this is indeed the case.

In fact, the authors found that they were able to divide the binary stars examined into a series of subgroups, each associated with a corresponding age-appropriate group. David Gruner, lead author of the study and PhD student at AIP in the Stellar Activity group, stated: “It was surprising to see how well all of our broad binary systems performed when we started comparing them to the cluster framework. Even systems with stars of very different masses show extraordinary coherence at their locations on the diagram, to the point that they are almost indistinguishable from the cluster.”

Some of the stars above the cluster’s later cluster of ribs can be considered to be older than the cluster that has been measured to date. Furthermore, the authors show that for most of the systems studied, the rotational age of one component corresponds to the rotational age of the other. Because the broad sample of binaries varies widely, both in their distribution across the sky and in other stellar properties, such as metal content, the results imply that gyrochronology is likely to be used reliably for field stars.

Dr Sydney Barnes, head of the Stellar Activity group at AIP, added: “This work provides assurance that in the future a reliable age for a much larger number of field stars can be obtained from their rotation rate.” These results will be important for the PLATO satellite mission, which aims not only to find a large number of stars inhabiting planets, but also to provide their ages, allowing the first glimpses into the evolutionary history of exoplanets.


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