Stellar activity and age relations are well investigated with open clusters in LAMOST DR5

A team of astronomers from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) have constructed so far the largest sample of open cluster member stars based on the LAMOST data. They studied the relations between the stellar activity and age in different effective temperature ranges, and uncovered the evolution of stellar activity with ages in detail, which also provides an important method to estimate the ages for field stars. The results have been published in The Astrophysical Journal (2019, ApJ, 887, 84).

Like people, young stars are also more active than old stars. Thus, there naturally exists a correlation between stellar activity and age. It is widely believed that the activity of FGK main-sequence stars decays with increasing age. Considering it is difficult to use isochrones to determine ages for these stars, the activity-age relation provides a solution of age estimation.

Open clusters are very good sample to study the stellar activity-age relations because of their well determined ages. The PhD student Jiajun Zhang, Prof. Jingkun Zhao and other co-authors built up a large spectral sample of open cluster member stars with high signal to noise ratiobased on the LAMOST data. According to the spectral characteristics, they redefined the activity indices to describe the stellar activity quantitatively, and further analyzed the relation between the activity indices and ages of the whole sample. Their results presume that the activity levels decrease slowly for relatively young stars while decrease rapidly for old stars, supporting that the activity-age relation is more significant among low-mass stars. It can be explained as that low-mass stars have relatively thicker convective zone and stronger magnetic fields.

In this study, two stellar activity-age relations were constructed, which could be used to roughly estimate the ages for field stars with accuracies of about 40% and 60%, respectively. Both of the two relations are applicable to main-sequence stars of a wide range of ages (from a few million years to 4 billion years), providing an important method to estimate the ages for these stars taking advantage of the LAMOST spectra. 


Figure: Cartoon diagram showing the relation between stellar activity and age. The black dots represent sunspots, which is an indicator of the stellar activity. The more sunspots a star has, the more active the star is.


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