The Evolution of Angular Momentum of Intermediate Mass Stars: From the Birthline to the Main Sequence

Abstract

Projected rotational velocities (vsini ) have been measured for a sample of 145 stars with masses between 0.4 and >10 M ⊙ (median mass 2.1 M ⊙ ) located in the Orion star-forming complex. These measurements have been supplemented with data from the literature for Orion stars with masses as low as 0.1 M ⊙ . The primary finding from analysis of these data is that the upper envelope of the observed values of angular momentum per unit mass (J/M) varies as M 0.25 for stars on convective tracks having masses in the range ∼0.1 to ∼ 3 M ⊙ . This power law extends smoothly into the domain of more massive stars (3 to 10 M ⊙ ), which in Orion are already on the ZAMS. This result stands in sharp contrast to the properties of main sequence stars, which show a break in the power law and a sharp decline in J/M with decreasing mass for stars with M < 2 M ⊙ . A second result of our study is that this break is seen already among the PMS stars in our Orion sample that are on radiative tracks, even though these stars are only a few million years old. A comparison of rotation rates seen for stars on either side of the convective-radiative boundary shows that stars do not rotate as solid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used to constrain the processes that control early stellar angular momentum, we show that the broad trends in the data can be accounted for by simple models that posit that stars: 1) lose angular momentum before they are deposited on the birthline, plausibly through star-disk interactions; 2) undergo additional braking as they evolve down their convective tracks; and 3) are subject to core-envelope decoupling during the convective-radiative transition.