Formation Of Contact Binaries Via A Magnetized Wind, A Parameter-Free Model

Abstract

Based on the model of angular momentum loss (AML) via a magnetized wind (Mestel, 1984) an empirical formula was derived and calibrated for the AML rate of single, solar type stars (Stepień, 1988, 1991):

$$\frac{{d{P_{rot}}}}{{dt}} = \left( {6\pm 3} \right) \times {10^{ - 9}}{\operatorname{P} _{rot}}^{e - 0.2{\operatorname{P} _{rot}}}$$

(1)

where time t is in years and P_rot in days. Note that, apart from empirical uncertainty, the above formula contains no adjustable free parameters. It reproduces well the Skumanich law P_rot ∼ t^1/2 for ages 0.1 Gyr ≲ t ≲ 10 Gyr and predicts correctly the presently observed spin-down rate of the Sun.