An improved age–activity relationship for cool stars older than a gigayear

Abstract

Stars with convective envelopes display magnetic activity, which decreases over time due to the magnetic braking of the star. This age-dependence of magnetic activity is well-studied for younger stars, but the nature of this dependence for older stars is not well understood. This is mainly because absolute stellar ages for older stars are hard to measure. However, relatively accurate stellar ages have recently come into reach through asteroseismology. In this work we present X-ray luminosities, which are a measure for magnetic activity displayed by the stellar coronae, for 24 stars with well-determined ages older than a gigayear. We find 14 stars with detectable X-ray luminosities and use these to calibrate the age-activity relationship. We find a relationship between stellar X-ray luminosity, normalized by stellar surface area, and age that is steeper than the relationships found for younger stars, with an exponent of −2.80 ± 0.72. Previous studies have found values for the exponent of the age-activity relationship ranging between −1.09 to −1.40, dependent on spectral type, for younger stars. Given that there are recent reports of a flattening relationship between age and rotational period for old cool stars, one possible explanation is that we witness a strong steepening of the relationship between activity and rotation.

Chromospheric emission of solar-type stars with asteroseismic ages

“…The very low activity of the studied sample is in line with previous work on coronal X-ray activity for a sample of old cool stars with a variety of age-determination methods (Booth et al 2017). We discussed the observed discrepancy with moderately fast rotation periods for similarly old stars with asteroseismic ages (van Saders et al 2016) in the context of both detectability questions for the physical quantities involves, and a genuine departure from the parallel decrease in rotation rate and magnetic activity which may be present in old main-sequence stars.…”

“…Therefore, a possible physical explanations is that the evolution of rotation with age and activity with age do not go hand in hand any longer after the star reaches an old age, as has been suggested by Booth et al (2017). This would mean a change in the rate of rotational braking.…”

“…In recent years, the availability of stellar ages from asteroseismology has fuelled new investigations into the nature of rotation and magnetic activity-age relationships for old stars (van Saders et al 2016;Booth et al 2017). van Saders et al (2016) investigated the rotation-age relationship using data from Kepler and asteroseismic ages.…”

“…This technique provides information about the interior of stars through observations of stellar oscillations and has proved to be the most accurate age-dating method for old field stars. Asteroseismology has opened up the possibility of stellar age investigations for stars older than a gigayear with recent studies using the technique to study rotation (van Saders et al 2016) and coronal X-ray activity (Booth et al 2017).…”

The XUV environments of exoplanets from Jupiter-size to super-Earth

“…We note that our sample has a trend with slow rotators being more X-ray luminous than predictions, suggesting their activity may not drop as quickly as predicted. Booth et al (2017) recently found a steeper ageactivity slope for old, cool stars to previous studies. They suggested that in the context of the findings of van Saders et al (2016), which found evidence for weaker magnetic breaking in field stars older than 1 Gyr, this could point to a steepening of the rotation-activity relationship, in contrast to our measurements.…”