In my last publication in this section, I have introduced the concept/technique of Gyrochronology or how to date stars using their rotational periods. Gyrochronology is a technique with some weaknesses and recently put in doubt for aging old stars. But also has some interesting strengths. For example, it is a potentially good technique for dating groups of stars with the same but unknown age.
How do we know that a group of stars have the same age?
There are some cases where we are quite sure they do. For example, stellar clusters, that is, a group of stars gravitationally bounded (very close one to another and/or moving all in the same direction). We assume that all these stars were born in a single event (or group of events) almost at the same time. Therefore, we can analyze all the stars together for dating the complete group.
In a recent paper, Jason L. Curtis and collaborators have used Gyrochronology for dating the so-called Pisces-Eridanus Stellar Stream. In a previous publication, Meingast and collaborators discovered this stellar stream and claimed for very interesting and rare characteristics for it: it is very close to us (around 130 pc, 1 pc correspond to 3.26 light-years), and it has an age of around 1 Gyr. If confirmed, it will be the oldest stellar cluster within 300 pc from us, making further and more detailed studies possible. Very useful. In this work, Curtis et al. use recent data from the NASA satellite TESS where the rotational period, for many of the member in the cluster, has been measured and compared with the period distribution of other clusters with known ages.
Main results using Gyrochronology for dating the “Pisces-Eridanus” Stellar Stream
The main result is shown in the accompanying figure, taken from the original paper (Curtis et al., 2019; accepted for publication in the Astronomical Journal. arXiv:1905.10588).
In the left panel, we can see the rotational periods distribution as a function of the stellar temperature for different stellar clusters. This distribution changes with age. In general, the older the cluster, the larger the rotational periods.
If we overlap the period distribution obtained for Pisces-Eridanus, we obtain the right panel. It is clear that this distribution completely overlaps that of Pleiades. Therefore, this group must have an age similar to the Pleiades, that is, around 120 Myrs. Not 1 Gyr.
Besides the physical meaning of this new dating, it is a pleasure to see gyrochronology in action.