Starspot Rotation Research Articles

Weakened magnetic braking supported by asteroseismic rotation rates of Kepler dwarfs

Studies using asteroseismic ages and rotation rates from star-spot rotation have indicated that standard age-rotation relations may break down roughly half-way through the main sequence lifetime, a phenomenon referred to as weakened magnetic braking. While rotation rates from spots can be difficult to determine for older, less active stars, rotational splitting of asteroseismic oscillation frequencies can provide rotation rates for both active and quiescent stars, and so can confirm whether this effect really takes place on the main sequence. We obtained asteroseismic rotation rates of 91 main sequence stars showing high signal-to-noise modes of oscillation. Using these new rotation rates, along with effective temperatures, metallicities and seismic masses and ages, we built a hierarchical Bayesian mixture model to determine whether the ensemble more closely agreed with a standard rotational evolution scenario, or one where weakened magnetic braking takes place. The weakened magnetic braking scenario was found to be 98.4% more likely for our stellar ensemble, adding to the growing body of evidence for this stage of stellar rotational evolution. This work represents the largest catalogue of seismic rotation on the main sequence to date, opening up possibilities for more detailed ensemble analysis of rotational evolution with Kepler.

Time evolution and rotation of starspots on CoRoT-2 from the modelling of transit photometry

CoRoT-2, the second planet-hosting star discovered by the CoRoT satellite, is a young and active star. A total of 77 transits were observed for this system over a period of 135 days. Small modulations detected in the optical light curve of the planetary transits are used to study the position, size, intensity, and temporal evolution of the photospheric spots on the surface of the star that are occulted by the planetary disk. We apply a spot model to these variations and create a spot map of the stellar surface of CoRoT-2 within the transit band for every transit. From these maps, we estimate the stellar rotation period and obtain the longitudes of the spots in a reference frame rotating with the star. Moreover, the spots temporal evolution is determined. This model achieves a spatial resolution of 2\circ. Mapping of 392 spots vs. longitude indicates the presence of a region free of spots, close to the equator, reminiscent of the coronal holes observed on the Sun during periods of maximum activity. With this interpretation, the stellar rotation period within the transit latitudes of -14.\circ 6 \pm 10 \circ is found to be 4.48 days. This rotation period is shorter than the 4.54 days as derived from the out-of-transit light modulation. Since the transit data samples a region close to the stellar equator, while the period determined from out-of-transit data reflects the average rotation of the star, this is taken as an indication of a latitudinal differential rotation of about 3% or 0.042 rad/d.

Magnetic Activity–related Radial Velocity Variations in Cool Stars: First Results from the Lick Extrasolar Planet Survey

The discovery of the radial velocity (v -->r) signatures of planets around several solar-like stars highlights the importance of exploring the sources of v -->r variations intrinsic to the stars themselves. We study the stars in the Lick planetary survey for v -->r variations related to stellar activity: the rotation of starspots and convective inhomogeneities and their temporal evolution. We study the relationships between the weighted v -->r dispersion, ?'v (which has first been corrected for the orbital contribution from known planets and the mean internal error), and spectral type, rotation, and activity (as measured by Ca II H and K). We find that the largest ?'v values occur among both the coolest (dMe) and the warmest (active F) stars. Values of ?'v increase with H and K emission and scale proportional to vsin${r sin}$ -->${r sin}$ -->i in G and K stars and proportional to (vsin${r sin}$ -->${r sin}$ -->i) -->1.3 in F stars. For a G star with vsin${r sin}$ -->${r sin}$ -->i ? 8-10 km s -->?1 (age ~0.3 Gyr), for example, 20 m s -->?1 ?'v 45 m s -->?1, roughly consistent with the predicted ?'v levels due to magnetic activity (Saar & Donahue). All the stars with proposed planetary companions show ?'v values typical for their spectral type, activity, and/or rotation. However, before the planetary v -->r perturbations are removed, these stars show significantly enhanced ?'v values. We develop a simple model that can predict the ?'v expected for a given star (within ? 40%) as a function of vsin${r sin}$ -->${r sin}$ -->i, spectral type, photometric variability, and macroturbulent velocity. The implications for extrasolar planet searches are discussed.