Spin Down in Subgiant Stars: Tests of Physics and Main Sequence Braking Models

The prospect of precise, rotation-based stellar ages drives the development of magnetic braking models for Sun-like stars. The vast majority of these magnetic braking laws have been calibrated on and for main sequence stars. While rotation-age relations lack the same utility on the subgiant branch, subgiant rotational behavior remains an excellent test of the underlying stellar evolutionary models and the magnetic braking prescriptions themselves. I will extend two different main sequence braking models onto the subgiant branch: a “standard” model in which angular momentum loss occurs smoothly over the entire main sequence, and the more recent “weakened” magnetic braking model, in which spin down halts roughly midway through the main sequence. I will show that subgiant rotation rates are a discriminating test of whether weakened braking actually occurs on the main sequence, with period predictions that can differ by greater than 50% from the standard case.