On the rotation of post-T Tauri stars in associations

Nearby associations or moving groups of post - T Tauri stars with ages between similar to 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 beta 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 0.1 M-. less than or equal to M < 1.5 M-. and 1.5 M ≤ M ≤ 2.6 M-., respectively. Because no observed rotational periods are known for these stars, we use a mathematical tool to infer representative equatorial rotation velocities v(0)(eq) from the observed distribution of projected rotational velocities (v sin i). 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 v(0)(eq) do not increase significantly. This insufficient increase of v(0)(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 v sin i) 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-b present 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.