The Role of Disks in the Angular Momentum Evolution of Young, Low-Mass Stars
Le Blanc, Thompson S.
Detailed studies of the spectral energy distributions (SEDs) of young stars can reveal much about their basic properties and their circumstellar material. Previous work suggests that the magnetic interactions of its circumstellar disk may be the result of the influence of a young stars angular momentum content and rotation rate. The generic prediction of these “disk-locking” theories is that a disk-locked star will be forced to rotate at the same Keplerian angular velocity of the inner edge of the disk; that is, the disks radius at which it is truncated or evacuated, should be equal to its co-rotation radius. Additional interpretations of these theories also suggest a correlation between the rotation period of a star and the structure of its circumstellar disk; slow rotators possess closely truncated disks that enforce the stars slow rotation, whereas rapid rotators possess disks that are largely evacuated or anemic, interpreted as being outside of the magnetic influence of the star, whereby the star is free to speed up as it continues its pre-main sequence contraction. As a test of the expectations of these theories, we model the spectral energy distributions of 33 young stars in the IC348 region with known rotation periods, as well as infrared excesses indicative of circumstellar disks. We match the observed SED for each star, sampling a range of 0.6-8.0m, to a grid of 200,000 pre-computed star+disk radiative transfer models, from which we then infer the circumstellar disks inner truncation radius. We then compare the inferred truncation radius to the disks co-rotation radius, calculated from the stars measured rotation period. We do not find any obvious differences in the disk truncation radii of slow versus rapid rotators. This holds true both at the level of whether close-in disk material is present at all, as well as analyzing the precise location of the inner edge of the disk relative to the co-rotation radius among the subset of stars with close-in disk material. One interpretation ￼￼ of these results is that the disk locking is unimportant for the IC 348 stars in our sample. Alternatively, if disk locking does operate, then it must operate on both slow and rapid rotators, potentially producing both spin-up and spin-down torques, and the transition from the disk-locked state to the disk-released state must occur more rapidly than the stellar contraction timescale.
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