Evolução do momento angular de estrelas de baixa massa

Abstract

The study of the evolution of the angular velocity of low-mass stars is important to understand the stellar dynamo process, since it is correlated with magnetic activity. Recent works indicate that the angular momentum loss of low-mass stars is connected to the Rossby number, Ro, the ratio between the rotation period and the local convective turnover time. Based on this, we investigated the relation between the disk lifetime of young stars, τdisk , and the Rossby number. According to the traditional disk-locking mechanism, stars with ages smaller than τdisk , transfer angular momentum to the disk by torquing the stellar magnetic field that connects the star to the disk. This phenomenon occurs during the contraction phase of the star, and apparently one process compensates for the other, since the angular velocity of the star, Ω, remains almost constant, Ω = Ω initial . When the stellar age becomes larger than τdisk , the star decouples from the disk and its angular velocity varies with time. Initially, the decrease in the moment of inertia due to stellar contraction outweighs the loss of angular momentum through winds and the angular velocity of the star increases. Later, when the star is on the main sequence, its angular velocity reaches a saturation value, the loss of angular momentum through magnetized winds then dominates and the angular velocity of the star’s decreases. To investigate the relationship between Ro and the initial coupling between the star and the disk, we consider that the star is connected to its disk until a Rossby number is reached, defined as the critical Rossby number for disk-star decoupling, Rocrit,D . We modeled the evolution of the angular velocity, starting from different initial values of the stellar angular velocity and using different values of τdisk (equivalent to Rocrit,D ). We compared our models in the Ω vs. age plane of the star with observational data available in the literature for stars of various cluster masses of different ages, with the aim of determining a possible relationship between Rocrit,D and the time the star remains coupled to the disk. Our results suggest that the critical Rossby number decreases with age in the mass range analyzed (from 0.1 to 1.5 M⊙ ). In addition, the results indicate that lower mass stars have higher Rocrit,D values and lower disk lifetime values than the higher mass stars in our sample. This relationship was observed for stars defined as slow, medium and fast rotators. The tendency for the disk lifetime to increase with the mass of the star is in line with the results described in the literature for slow and medium rotators.