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≥4). 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 σi=2.9∘ and σγ=11.5∘. 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.
Related
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BibTeX
@misc{marcussen2026phakoseismology,
title = {{Phakoseismology: Asteroseismology of gravitationally microlensed stars}},
author = {Marcus L. Marcussen and Mikkel N. Lund},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.28005},
eprint = {2610.28005},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.28005},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Phakoseismology: Asteroseismology of gravitationally microlensed stars
AU - Marcus L. Marcussen
AU - Mikkel N. Lund
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2610.28005
UR - https://arxiv.org/abs/2610.28005
AB - Asteroseismology relies on disk-integrated photometry or radial velocities, in which geometric cancellation suppresses modes of high spherical degree (l≥4). 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^∘ and σ_γ=11.5^∘. 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.
ER -