Lower limit for differential rotation in members of young loose stellar associations^⋆

¹ INAF–Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
e-mail: elisa.distefano@oact.inaf.it
² University of Catania, Astrophysics Section, Dept. of Physics and Astronomy, via S. Sofia, 78, 95123 Catania, Italy
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 4 November 2015
Accepted: 5 April 2016

Abstract

Context. Surface differential rotation (SDR) plays a key role in dynamo models and determines a lower limit on the accuracy of stellar rotation period measurements. SDR estimates are therefore essential to constrain theoretical models and infer realistic rotation period uncertainties.

Aims. We measure a lower limit to SDR in a sample of solar-like stars belonging to young loose stellar associations with the aim of investigating how SDR depends on global stellar parameters in the age range (4 − 95 Myr).

Methods. The rotation period of a solar-like star can be recovered by analyzing the flux modulation caused by dark spots and stellar rotation. The SDR and the latitude migration of dark-spots induce a modulation of the detected rotation period. We employed long-term photometry to measure the amplitude of such a modulation and to compute the quantity ΔΩ_phot = 2π/P_min - 2π/P_max that is a lower limit to SDR.

Results. We find that ΔΩ_phot increases with the stellar effective temperature and with the global convective turn-over timescale τ_c, which is the characteristic time for the rise of a convective element through the stellar convection zone. We find that ΔΩ_phot is proportional to T_eff^2.18±0.65 in stars recently settled on the ZAMS. This power law is less steep than those found by previous authors, but closest to recent theoretical models. We investigate how ΔΩ_phot changes in time in a ~1 M_⊙ star. We find that ΔΩ_phot steeply increases between 4 and 30 Myr and that it is almost constant between 30 and 95 Myr. We find also that the relative shear increases with the Rossby number Ro. Although our results are qualitatively in agreement with hydrodynamical mean-field models, our measurements are systematically higher than the values predicted by these models. The discrepancy between ΔΩ_phot measurements and theoretical models is particularly large in stars with periods between 0.7 and 2 d. Such a discrepancy, together with the anomalous SDR measured by other authors for HD 171488 (rotating in 1.31 d), suggests that the rotation period could influence SDR more than predicted by the models.