Deformation and differential rotation in slowly rotating young intermediate-mass stars

Abstract

Asteroseismology, the study of stellar vibrations, is a method which can probe the structure deformation and internal rotation of stars. Salient among the seismic inferences of rotation from TESS observations are TIC 408165734, whose equatorial rotation rate is 10\% faster than the pole, and TIC 307930890, which has significant radial shear and shows a decreasing spin rate outward through its envelope. We also measure structural deformation in fifteen stars, nine of which are oblate, a finding consistent with expectations for relatively fast-rotating, non-magnetic stars. The difference between polar and equatorial radii in TIC 47639058 is 130 times larger than that for the Sun. The remaining six stars display splittings consistent with a prolate shape (surprisingly), possibly indicating the presence of equatorial toroidal magnetic fields. These inferences provide constraints for numerical simulations and new insights to guide theories of $\delta$ Scuti structure and rotation.