IRS 9: The Case for a Dynamically-Ejected Star from the Galactic Center

Measuring stellar motions at the Milky Way’s Galactic center (GC) provides unique insight into the dynamical processes within galactic nuclei. We present proper motion measurements for 23 SiO-maser emitting stars within 45’’ of SgrA*, including four previously reported to have velocities exceeding their local escape velocities (i.e., they are “locally unbound” from the GC). Derived from 14 epochs of HST WFC3-IR observations (2010 - 2023), our measurements have a median precision of 0.038 mas/yr - up to ~100x more precise then previous constraints for some sources. By combining these proper motions with published radial velocities, we derive updated 3D velocities for the masers and find that only one is locally unbound (IRS 9; v3d = 370 +/- 1.2 km/s). Orbit integrations place the first constraints on the orbit of IRS 9, which is bound to the GC at larger radii with r_peri >= 0.100 +/- 0.005 pc and r_apo >= 5.25 +/- 0.18 pc. IRS 9’s high velocity relative to stars at similar radii in the Nuclear Star Cluster makes it a candidate to have experienced a strong dynamical interaction in order to place it on its orbit. We explore the Hills mechanism as a possible origin, but binary evaporation and ejection velocity limits indicate that IRS 9 is unlikely to have experienced such an event in the past 0.4 Myr (the timescale constrained by the orbit integrations). Alternative mechanisms that could produce IRS 9 include binary supernova disruption, two-body interactions, and stellar collisions. Identifying additional stars like IRS 9 will be essential for understanding these various dynamical processes.

💡 Research Summary

This paper presents a high‑precision astrometric study of SiO‑maser stars within 45 arcseconds (≈1.8 pc) of the Milky Way’s central supermassive black hole, Sgr A*. Using 14 epochs of Hubble Space Telescope WFC3‑IR imaging taken between 2010 and 2023, the authors measured proper motions for 23 maser sources with a median uncertainty of 0.038 mas yr⁻¹ (≈1.5 km s⁻¹), an improvement of up to two orders of magnitude over previous radio‑based measurements for sources beyond ~0.8 pc. By combining these proper motions with published line‑of‑sight velocities (radial velocities) from the literature, they derived three‑dimensional (3‑D) velocities for each star.

To assess whether any of the masers are “locally unbound” – i.e., moving faster than the escape speed at their projected distance from Sgr A* – the authors constructed a Galactic‑center mass model consisting of the central black hole (M_BH = 4.14 ± 0.16 × 10⁶ M⊙) and the Nuclear Star Cluster (NSC) described by an axisymmetric density profile (parameters q = 0.73 ± 0.04, γ = 0.71 ± 0.12, a_NSC = 5.9 ± 1.07 pc, M_NSC = 6.1 ± 0.3 × 10⁷ M⊙). Using a Monte‑Carlo approach (100 realizations) they propagated uncertainties in these parameters to obtain a distribution of the maximum escape velocity, v_esc,max, at each projected radius.

Only one source, IRS 9, exceeds its local escape speed with high significance: at a projected radius r₂d = 0.33 pc, IRS 9 has v₃D = 370.4 ± 1.2 km s⁻¹ compared to v_esc,max = 331.1 ± 6.5 km s⁻¹, a 5.9σ excess. The other three previously claimed “locally unbound” masers (SiO‑16, SiO‑21, SiO‑25) have HST proper motions that are more than 1000 km s⁻¹ lower than the radio values, placing them comfortably below the escape curve.

Although IRS 9 is locally unbound, the extended mass distribution of the NSC means it may still be gravitationally bound on larger scales. The authors integrated its orbit using the galpy package within a multi‑component potential that includes the SMBH, NSC, and the Nuclear Stellar Disk (Model 2 from Sormani et al. 2020). Assuming a line‑of‑sight distance of zero (i.e., the true distance equals the projected distance), the orbit integration yields a pericenter r_peri = 0.100 ± 0.005 pc, an apocenter r_apo = 5.25 ± 0.18 pc, and an eccentricity ≈ 0.96. Thus IRS 9 is on a highly elongated, bound orbit that brings it very close to the black hole before swinging out to several parsecs.

The paper then explores possible dynamical origins for such a high‑velocity star. The Hills mechanism—ejection of a star during the tidal breakup of a binary by the SMBH—was examined but found unlikely because binary evaporation and the required ejection velocity would demand an event within the last ~0.4 Myr, which is inconsistent with the orbital constraints. Alternative scenarios considered include: (1) binary supernova disruption, where a massive companion explodes and imparts a kick; (2) strong two‑body encounters in the dense NSC environment; and (3) direct stellar collisions. Given the high stellar density (∼10⁶ M⊙ pc⁻³) and the presence of massive, evolved stars, these mechanisms can plausibly generate the observed velocity.

Finally, the authors stress that identifying additional high‑velocity maser stars will be crucial for constraining the frequency and relative importance of these dynamical processes in the Galactic center. Such a census will improve our understanding of stellar dynamics in galactic nuclei, binary evolution under extreme tidal fields, and the role of supernova feedback in shaping the kinematic structure of nuclear star clusters.