Stellar rotation as a new observable to test general relativity in the Galactic Center

TL;DR

This paper proposes using the rotation axis precession of S-stars near the Galactic Center as a novel method to test general relativity, specifically de Sitter precession, through spectral line width variations.

Contribution

It introduces a new observational approach based on stellar rotation axis reorientation to test relativistic effects in the Galactic Center, complementing orbital motion studies.

Findings

Line width variations of 20-76 km/s over 20-40 years are detectable.

S4714 and S62 are optimal candidates for this test due to their small pericenter distances.

Monitoring stellar rotation can also refine orbital and internal structure parameters.

Abstract

S-stars in the Galactic Center are excellent testbeds of various general relativistic effects. While previous works focus on modeling their orbital motion around Sgr A*--the supermassive black hole in the Galactic Center--here we explore the possibility of using the rotation of S-stars to test the de Sitter precession predicted by general relativity. We show that by reorienting the rotation axes of S-stars, de Sitter precession will change the apparent width of the absorption lines in the stellar spectra. Our numerical simulations suggest that the newly discovered S4714 and S62 are best suited for such a test because of their small pericenter distances relative to Sgr A*. Depending on the initial inclination of the star, the line width would vary by as much as $20-76\,{\rm km\,s^{-1}}$ within a period of $20-40$ years. Such a variation is comparable to the current detection limit. Since the precession rate is sensitive to the orbital eccentricity and stellar quadrupole structure, monitoring the rotation of S-stars could also help us better constrain the orbital elements of the S-stars and their internal structures.