2004AJ....128.1812D

Nearby associations or moving groups of post-T Tauri stars with ages between ∼10 and 30 Myr are excellent objects for the study of the initial spin-up phase during the pre-main-sequence evolution. An empirical approach is adopted here for the first time with these stars to infer their rotations, properties, and relations to X-ray emission. Three nearby associations with distances less than 100 pc are considered: the TW Hydrae association (TWA) with an age of 8 Myr, the β Pictoris moving group (BPMG) with an age of 12 Myr, and a combination of Tucana and Horologium associations (Tuc/HorA; 30 Myr). Two low- and high-rotation modes are considered for each association, with stellar masses of 0.1M_☉≤M<1.5M_☉and 1.5M_☉≤M≤2.6M_☉, respectively. Because no observed rotational periods are known for these stars, we use a mathematical tool to infer representative equatorial rotation velocities v₀(eq) from the observed distribution of projected rotational velocities (vsini). This is done for each mode and for each association. A spin-up is found for the high-rotation mode, whereas in the low-rotation mode the v₀(eq) do not increase significantly. This insufficient increase of v₀(eq) is probably the cause of a decrease of the total mean specific angular momentum for the low-mass stars between 8 and 30 Myr. However, for the high-mass stars, where a sufficient spin-up is present, the specific angular momentum is practically conserved in this same time interval. A two-dimensional (mass and vsini) K-S statistical test yields results compatible with a spin-up scenario. By supposing that the distribution of the masses of these three associations follows a universal mass function, we estimate the number of members of these associations that remain to be detected. The analysis of rotational and stellar masses using the luminosity X-ray indicators L_X and L_X/L_bpresent similar properties, as does the dependence on stellar mass and rotation, at least for the younger associations TWA and BPMG, to those obtained for T Tauri stars in the Orion Nebula Cluster (1 Myr). A strong desaturation effect appears at ∼30 Myr, the age of Tuc/HorA, measured essentially by the early-G and late-F type stars. This effect seems to be provoked by the minimum configuration of the stellar convection layers, attained for the first time for the higher mass stars at ∼30 Myr. The desaturation appears to be independent of rotation at this stage.