Effects of accretion on the structure and rotation of forming stars

Abstract

Context. Rotation period measurements of low-mass stars show that the spin distributions in young clusters do not exhibit the spin-up expected due to contraction in the phase when a large fraction of stars is still surrounded by accretion discs. Many physical models have been developed to explain this feature based on different types of star-disc interactions alone. In this phase, the stars accrete mass and angular momentum and may experience accretion-enhanced magnetised winds. The stellar structure and angular momentum content thus strongly depend on the properties of the accretion mechanism. At the same time, the accretion of mass and energy has a significant impact on the evolution of the stellar structure and the moment of inertia. Our understanding of the spin rates of young stars therefore requires a description of how accretion affects the stellar structure and angular momentum simultaneously. Aims. We aim to understand the role of accretion to explain the observed rotation-rate distributions of forming stars. Methods. We computed evolution models of accreting very young stars and determined in a self-consistent way the effect of accretion on stellar structure and the angular momentum exchanges between the stars and their disc. We then varied the deuterium content, the accretion history, the entropy content of the accreted material, and the magnetic field as well as the efficiency of the accretion-enhanced winds. Results. The models are driven alternatively both by the evolution of the momentum of inertia and by the star-disc interaction torques. Of all the parameters we tested, the magnetic field strength, the accretion history, and the deuterium content have the largest impact. The injection of heat plays a major role only early in the evolution. Conclusions. This work demonstrates the importance of the moment of inertia evolution under the influence of accretion for explaining the constant rotation-rate distributions that are observed during the star-disc interactions. When we account for rotation, the models computed with the recently calculated torque along with a consistent structural evolution of the accreting star are able to explain the almost constant spin evolution for the whole range of parameters we investigated, but it only reproduces a narrow range around the median of the observed spin rate distributions. Further development, including for example more realistic accretion histories based on dedicated disc simulations, are likely needed to reproduce the extremes of the spin rate distributions.