# Scalar field effects on the orbit of S2 star

## TL;DR

This paper investigates how a hypothetical scalar field, representing ultra-light dark matter, could influence the orbit of the S2 star around SgrA*, and how such effects might be detectable with current astronomical instruments.

## Contribution

It models the impact of a scalar field dark matter halo on S2's orbit and assesses its detectability with the GRAVITY instrument, introducing a novel way to constrain dark matter properties.

## Key findings

- Scalar field effects cause detectable orbital precession.
- Orbital variations depend on scalar field mass and coupling.
- Constraints on scalar field parameters can be derived from observations.

## Abstract

Precise measurements of the S-stars orbiting SgrA* have set strong constraints on the nature of the compact object at the centre of the Milky Way. The presence of a black hole in that region is well established, but its neighboring environment is still an open debate. In that respect, the existence of dark matter in that central region may be detectable due to its strong signatures on the orbits of stars: the main effect is a Newtonian precession which will affect the overall pericentre shift of S2, the latter being a target measurement of the GRAVITY instrument. The exact nature of this dark matter (e.g., stellar dark remnants or diffuse dark matter) is unknown. This article assumes it to be an scalar field of toroidal distribution, associated with ultra-light dark matter particles, surrounding the Kerr black hole. Such a field is a form of "hair" expected in the context of superradiance, a mechanism that extracts rotational energy from the black hole. Orbital signatures for the S2 star are computed and shown to be detectable by GRAVITY. The scalar field can be constrained because the variation of orbital elements depends both on the relative mass of the scalar field to the black hole and on the field mass coupling parameter.