# JWST-TST Proper Motions: I. High-Precision NIRISS Calibration and Large Magellanic Cloud Kinematics

## TL;DR

This paper presents high-precision calibration methods for JWST NIRISS, enabling detailed proper motion studies of stars in the Large Magellanic Cloud with accuracy surpassing Gaia for faint stars, and demonstrates JWST's potential for advanced astrometry.

## Contribution

We developed and validated calibration techniques for JWST NIRISS that enable high-precision astrometry, and applied these to measure proper motions in the LMC, including the first absolute PM of a star cluster.

## Key findings

- Achieved median PM uncertainty of ~13 μas/yr for G~20 stars.
- First measurement of the absolute proper motion of the star cluster OGLE-CL LMC 407.
- Detected kinematic substructure and measured velocity dispersions consistent with or refining Gaia results.

## Abstract

We develop and disseminate effective point-spread functions and geometric-distortion solutions for high-precision astrometry and photometry with the JWST NIRISS instrument. We correct field dependencies and detector effects, and assess the quality and the temporal stability of the calibrations. As a scientific application and validation, we study the proper motion (PM) kinematics of stars in the JWST calibration field near the Large Magellanic Cloud (LMC) center, comparing to a first-epoch Hubble Space Telescope (HST) archival catalog with a 16-yr baseline. For stars with G~20, the median PM uncertainty is ~13 $\mu$as yr$^{-1}$ (3.1 km s$^{-1}$), better than Gaia DR3 typically achieves for its very best-measured stars. We kinematically detect the known star cluster OGLE-CL LMC 407, measure its absolute PM for the first time, and show how this differs from other LMC populations. The inferred cluster dispersion sets an upper limit of 24 $\mu$as yr$^{-1}$ (5.6 km s$^{-1}$) on systematic uncertainties. Red-giant-branch stars have a velocity dispersion of 33.8 $\pm$ 0.6 km s$^{-1}$, while younger blue populations have a narrower velocity distribution, but with a significant kinematical substructure. We discuss how this relates to the larger velocity dispersions inferred from Gaia DR3. These results establish JWST as capable of state-of-the-art astrometry, building on the extensive legacy of HST. This is the first paper in a series by our JWST Telescope Scientist Team (TST), in which we will use Guaranteed Time Observations to study the PM kinematics of various stellar systems in the Local Group.