Phakoseismology: Asteroseismology of gravitationally microlensed stars
- 1Department of Physics and Astronomy, Aarhus University
Abstract
Context. Asteroseismology relies on disk-integrated photometry or radial velocity, where geometric cancellation suppresses oscillation modes of high spherical degree l ( l ≥ 4). This cancellation restricts observations to deeply penetrating, low-degree oscillations, leaving the outer stellar envelope inaccessible.
Aims. We investigate whether a high-magnification gravitational microlensing event can momentarily circumvent this limitation by differentially magnifying the surface of an oscillating source star.
Methods. We modelled the disk-integrated visibility of solar-like oscillations during microlensing events, weighting the stellar surface by limb darkening and by the spatially varying magnification. We computed the relative visibility of angular multiplets and the distribution of power among their azimuthal components, and constructed a likelihood model for recovering the three-dimensional orientation of the stellar spin axis from resolved rotational multiplets.
Results. When the projected lens–source separation becomes comparable to the angular radius of the source, the magnification gradient acts as a transient spatial filter that breaks the symmetry of disk integration, which normally causes high-degree modes to cancel out. The visibilities of high-degree modes then rise by up to three orders of magnitude. The same non-axisymmetric weighting makes the relative power of the azimuthal components depend on both the inclination and the sky-projected azimuth of the spin axis, imprinting the full spin orientation onto the power spectrum. With relative-power measurement uncertainties of 5%, we are able to recover the input orientation to σ i = 2.9° and σ γ = 11.5°.
Conclusions. Microlensing can act as a transient spatial filter for unresolved stellar oscillations, doing more than amplifying distant stars: the magnification pattern itself becomes a tool for spatially selective asteroseismology. The effect is strongest for rare close encounters and is unlikely to support large samples. Moreover, the newly visible high-degree modes will overlap with rotationally split and, in evolved stars, mixed modes, making mode identification an observational challenge. Nevertheless, individual high-quality events may open a new window onto envelope physics and spin geometries for stars in the Galactic bulge and beyond.
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BibTeX
@article{marcussen2026phakoseismology,
title = {{Phakoseismology: Asteroseismology of gravitationally microlensed stars}},
author = {Marcus L. Marcussen and Mikkel N. Lund},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202662677},
url = {https://www.aanda.org/10.1051/0004-6361/202662677},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Phakoseismology: Asteroseismology of gravitationally microlensed stars
AU - Marcus L. Marcussen
AU - Mikkel N. Lund
PY - 2026
DA - 2026/10/29
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202662677
UR - https://www.aanda.org/10.1051/0004-6361/202662677
AB - Context. Asteroseismology relies on disk-integrated photometry or radial velocity, where geometric cancellation suppresses oscillation modes of high spherical degree l ( l ≥ 4). This cancellation restricts observations to deeply penetrating, low-degree oscillations, leaving the outer stellar envelope inaccessible. Aims. We investigate whether a high-magnification gravitational microlensing event can momentarily circumvent this limitation by differentially magnifying the surface of an oscillating source star. Methods. We modelled the disk-integrated visibility of solar-like oscillations during microlensing events, weighting the stellar surface by limb darkening and by the spatially varying magnification. We computed the relative visibility of angular multiplets and the distribution of power among their azimuthal components, and constructed a likelihood model for recovering the three-dimensional orientation of the stellar spin axis from resolved rotational multiplets. Results. When the projected lens–source separation becomes comparable to the angular radius of the source, the magnification gradient acts as a transient spatial filter that breaks the symmetry of disk integration, which normally causes high-degree modes to cancel out. The visibilities of high-degree modes then rise by up to three orders of magnitude. The same non-axisymmetric weighting makes the relative power of the azimuthal components depend on both the inclination and the sky-projected azimuth of the spin axis, imprinting the full spin orientation onto the power spectrum. With relative-power measurement uncertainties of 5%, we are able to recover the input orientation to σ i = 2.9° and σ γ = 11.5°. Conclusions. Microlensing can act as a transient spatial filter for unresolved stellar oscillations, doing more than amplifying distant stars: the magnification pattern itself becomes a tool for spatially selective asteroseismology. The effect is strongest for rare close encounters and is unlikely to support large samples. Moreover, the newly visible high-degree modes will overlap with rotationally split and, in evolved stars, mixed modes, making mode identification an observational challenge. Nevertheless, individual high-quality events may open a new window onto envelope physics and spin geometries for stars in the Galactic bulge and beyond.
SN - 0004-6361
SN - 1432-0746
ER -