Main sequence companions to white dwarfs II: the age-activity-rotation relation from a sample of Gaia common proper motion pairs

TL;DR

This study uses Gaia data to analyze the age-activity-rotation relationship in main sequence stars paired with white dwarfs, revealing how magnetic activity and rotation evolve over billions of years and identifying exceptions possibly due to past accretion events.

Contribution

It provides new empirical evidence on the age-activity-rotation relation using a large sample of Gaia-identified pairs, highlighting deviations from standard gyrochronology models.

Findings

Stars older than 5 Gyr are inactive unless tidally locked by companions.

Rotational velocities decrease with age, supporting gyrochronology.

Moderately old stars show unexpected activity, suggesting weakened magnetic braking or past accretion.

Abstract

Magnetic activity and rotation are related to the age of low-mass main sequence stars. To further constrain these relations, we study a sample of 574 main sequence stars members of common proper motion pairs with white dwarfs, identified thanks to Gaia astrometry. We use the white dwarfs as age indicators, while the activity indexes and rotational velocities are obtained from the main sequence companions using standard procedures. We find that stars older than 5 Gyr do not display Halpha nor Caii H&K emission unless they are fast rotators due to tidal locking from the presence of unseen companions and that the rotational velocities tend to decrease over time, thus supporting the so-called gyrochronology. However, we also find moderately old stars (~2-6 Gyr) that are active presumably because they rotate faster than they should for their given ages. This indicates that they may be suffering from weakened magnetic braking or that they possibly evolved through wind accretion processes in the past. The activity fractions that we measure for all stars younger than 5 Gyr range between ~10-40 per cent. This is line with the expectations, since our sample is composed of F, G, K and early M stars, which are thought to have short (<2 Gyr) activity lifetimes. Finally, we observe that the Halpha fractional luminosities and the R'HK indexes for our sample of (slowly rotating) stars show a spread (-4>log(LHalpha/Lbol); log(R'HK)> -5) typically found in inactive M stars or weakly active/inactive F, G, K stars.