Modelling the emergence and evolution of the rotation-activity relation

TY - JOUR

T1 - Modelling the emergence and evolution of the rotation-activity relation

AU - Stuart, Kieran

AU - Gregory, Scott

N1 - © 2025 The Author(s).

PY - 2025/5

Y1 - 2025/5

N2 - Main-sequence stars follow a well-defined rotation-activity relation. There are two primary regimes: saturated, where the fractional X-ray luminosity is approximately constant, and unsaturated, where the fractional X-ray luminosity decreases with increasing Rossby number (or decreasing rotation rate). Pre-main-sequence (PMS) stars have a larger scatter in than main-sequence stars, are observed to have saturated levels of X-ray emission, and do not follow the rotation-activity relation. We investigate how PMS stars evolve in the rotation-activity plane and the time-scale over which the X-ray rotation-activity relation emerges. Using observational data of 600 stars from four PMS clusters, stellar internal structure models, a rotational evolution model, and observed X-ray luminosity trends with age, we simulate the evolution of the PMS stars in the rotation-activity plane up to ages of 100 Myr. Our model reproduces the rotation-activity relation found for main-sequence stars, with higher mass stars beginning to form the unsaturated regime from around 10 Myr. After 25 Myr, the gradient of the unsaturated regime matches that found for main-sequence stars. For stars of mass greater than 0.6 M, the maximum age by which a star has left the saturated regime correlates with when the star leaves the PMS. We find that an intracluster age spread is a key factor in contributing to the observed scatter in, particularly for ages < 10 Myr.

AB - Main-sequence stars follow a well-defined rotation-activity relation. There are two primary regimes: saturated, where the fractional X-ray luminosity is approximately constant, and unsaturated, where the fractional X-ray luminosity decreases with increasing Rossby number (or decreasing rotation rate). Pre-main-sequence (PMS) stars have a larger scatter in than main-sequence stars, are observed to have saturated levels of X-ray emission, and do not follow the rotation-activity relation. We investigate how PMS stars evolve in the rotation-activity plane and the time-scale over which the X-ray rotation-activity relation emerges. Using observational data of 600 stars from four PMS clusters, stellar internal structure models, a rotational evolution model, and observed X-ray luminosity trends with age, we simulate the evolution of the PMS stars in the rotation-activity plane up to ages of 100 Myr. Our model reproduces the rotation-activity relation found for main-sequence stars, with higher mass stars beginning to form the unsaturated regime from around 10 Myr. After 25 Myr, the gradient of the unsaturated regime matches that found for main-sequence stars. For stars of mass greater than 0.6 M, the maximum age by which a star has left the saturated regime correlates with when the star leaves the PMS. We find that an intracluster age spread is a key factor in contributing to the observed scatter in, particularly for ages < 10 Myr.

KW - stars: activity

KW - stars: rotation

KW - stars: evolution

KW - stars: pre-main-sequence

KW - X-rays: stars

UR - https://www.scopus.com/pages/publications/105003679622

U2 - 10.1093/mnras/staf589

DO - 10.1093/mnras/staf589

M3 - Article

SN - 0035-8711

VL - 539

SP - 1922

EP - 1943

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

IS - 3

ER -