The magnetic activity evolution of main sequence cool stars

  • Rachel Booth

Student thesis: Doctoral ThesisDoctor of Philosophy


Stellar ages are a difficult parameter to calculate as they are not directly observable but must be inferred via other methods. One potential method for cool main sequence stars is to determine a relationship between age and magnetic activity. However, the relationship is currently only reliable for ages younger than a gigayear. The work presented in this thesis aims to calibrate the age- activity relationship beyond a gigayear by using ages obtained through aster- oseismology.

The first study uses the X-ray luminosity as a stellar magnetic activity indicator and fourteen stars with detectable X-ray luminosities are used to calibrate the age-activity relationship. A relationship is found between stellar X-ray luminosity (normalised by the stellar surface area) and age that is steeper than the relationships found for younger stars, with an exponent of -2.80 ± 0.72. Given the reports of a flattening relationship between age and rotational period for old cool stars, one possible explanation is that a strong steepening of the relationship between activity and rotation is the cause behind the age- activity trend.

The second study reports on chromospheric Ca II H and K line measurements for 26 main-sequence cool stars with spectral types F and G by calculating the Rhk indicator. The Rhk indicator for the sample is found to be lower than predicted by common age-activity relations. These data suggests that old main-sequence stars follow a steeper decline in activity with age than expected. It is also reported that within the sample there is only a weak trend of R'hk stellar age, indicating that the Ca IIH & K line emission alone is not a strongly discriminating measurement to determine ages of cool stars beyond a gigayear.

Finally, literature rotation periods were collected and an investigation into the activity-rotation relationship is presented that finds no significant steepening of the relationship between activity and Rossby number. Therefore, alternative explanations are needed to understand the discrepancy between the magnetic activity and rotational evolution of stars older than a gigayear.
Date of AwardDec 2019
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorKatja Poppenhaeger (Supervisor) & Christopher Watson (Supervisor)

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