On the Rotation of Post-T Tauri Stars in Associations
Abstract
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 0.1Msolar<=M<1.5Msolar and 1.5Msolar<=M<=2.6Msolar, 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 LX and LX/Lb 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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- October 2004
- DOI:
- 10.1086/423439
- arXiv:
- arXiv:astro-ph/0406412
- Bibcode:
- 2004AJ....128.1812D
- Keywords:
-
- Galaxy: Open Clusters and Associations: Individual: Name: TW Hydrae;
- open clusters and associations: individual (β Pictoris);
- open clusters and associations: individual (Tucana);
- Galaxy: Open Clusters and Associations: Individual: Name: Horologium;
- Stars: Pre-Main-Sequence;
- Stars: Rotation;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 37 pages, 10 figures, 4 tables. Accepted by the Astronomical Journal