Seismic constraints on the spin evolution of slowly rotating young intermediate-mass stars

Abstract

$\delta$ Scuti stars are hot, rapid rotators and are a poorly understood class of pulsators. Asteroseismology provides the only means with which to probe their interior dynamics. However, their complex and unexplained oscillation patterns restrict analyses to only a small fraction with interpretable pulsations. Here, we identify 5381 $\delta$ Scuti stars from 63 sectors of TESS observations, of which 300 had interpretable oscillations, with 24 showing rotational splittings. We inferred compositions and ages ($\tau$) for the 300 stars finding them in near-ZAMS states (Bedding et al. 2020), and measured the mean envelope rotation rates ($< f_{rot} >$) for 24 of them. Analyzing their age-dependent rotation, we found these stars essentially exhibit weak-to-no spindown, while evolving past the ZAMS across a narrow time-span during which they show regular pulsations. A quantitative fit to their spin-evolution results in a trend $f_{rot} (d^{-1}) \propto (\tau/{Gyr})^{-0.048 \pm 0.016}$, much slower than the spindown of cooler late-type stars due to magnetic braking (Skumanich's law: $f_{rot} (d^{-1}) \propto (\tau/{Gyr})^{-0.5}$). Based on stellar evolution calculations, we show this weak spindown is consistent with the gradual increase in their moment-of-inertia.