Lithium and the evolution of intermediate-mass T Tauri and Herbig stars. Rotation, accretion, and planets

Interior models predict that stellar envelopes change from convective to radiative during the pre-main-sequence (pre-MS) evolution of intermediate-mass stars. Although the amount of surface lithium (Li) is a direct probe of mixing in stellar interiors, analyses focused on this type of source are practically absent. We contribute to our understanding of the evolution of young intermediate-mass stars by providing a comprehensive analysis of their Li content. A sample of 71 intermediate-mass T Tauri (IMTT) and Herbig stars within the mass range 1.5 -- 3.5 M_⊙ was carefully selected for the analysis. Metallicities, rotational velocities, and accretion rates were obtained from spectra. The curves of growth for stars hotter than 8000 K were built to infer the Li abundances, which were interpreted considering standard models of stellar interiors and non-standard processes affecting Li depletion. Li is generally less strongly depleted in intermediate-mass stars than in their lower-mass counterparts, as expected from standard evolution models. However, Li abundances significantly below the cosmic value are observed in 25 -- 30$%$ of intermediate-mass stars. It is also unexpected that the results show no significant difference between the 1.5 --2.5 M_⊙ and 2.5 -- 3.5 M_⊙ subsamples. Evidence is provided showing that disk-locking works in young intermediate-mass stars. This constitutes independent support for the hypothesis that magnetospheric accretion scenario operates in these sources. We found that disk-locking is effective for a timescale that is about twice shorter than for lower-mass stars, before magnetospheres reduce their sizes during the transition from the IMTT to the Herbig regime. This contraction of the magnetosphere can explain the increase in rotation by a factor of about 3 and in accretion by a factor of about 4 that is observed during this transition. We propose a complex scenario linking rotation, accretion, and the surface Li abundance. Finally, we tentatively suggest that the known relation between the presence of planets and Li depletion might also be present in intermediate-mass MS stars and might originate in the pre-MS. We provide the most complete Li analysis and database focused on IMTT and Herbig stars to date. However, we emphasize the need of additional observations and non-standard models such as those available for their lower-mass analogs.