Phakoseismology: Asteroseismology of gravitationally microlensed stars

Abstract

Asteroseismology relies on disk-integrated photometry or radial velocities, in which geometric cancellation suppresses modes of high spherical degree ($l\geq4$). Observations are thus limited to deeply penetrating, low-degree modes, leaving the outer envelope inaccessible. We investigate whether a high-magnification microlensing event can temporarily lift this limitation by differentially magnifying the surface of an oscillating source star. We modelled the visibility of solar-like oscillations during microlensing, weighting the surface by limb darkening and the local magnification. We computed multiplet visibilities and the power distribution among azimuthal components, and built a likelihood model for recovering the 3D spin-axis orientation from resolved rotational multiplets. When the lens-source separation becomes comparable to the source's angular radius, the magnification gradient acts as a transient spatial filter that breaks the symmetry of disk integration, raising high-degree mode visibilities by up to three orders of magnitude. The same non-axisymmetric weighting makes the relative power of azimuthal components depend on both the inclination and sky-projected azimuth of the spin axis, imprinting the full spin orientation onto the power spectrum. With $5\%$ relative-power uncertainties, we recover the orientation to $σ_i=2.9^\circ$ and $σ_γ=11.5^\circ$. Microlensing thus does more than amplify distant stars: its magnification pattern enables spatially selective asteroseismology. The effect is strongest for rare close encounters and unlikely to support large samples, and the newly visible high-degree modes will overlap with rotationally split and mixed modes, complicating mode identification. Nevertheless, individual high-quality events may open a window on envelope physics and spin geometries for stars in the Galactic bulge and beyond.