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

TL;DR

This paper tests whether SiO maser stars near SgrA* are locally unbound from the Galactic Center and what dynamical processes might eject them. It uses 14 epochs of HST WFC3-IR astrometry to produce proper motions for 23 masers within $r_{2d}\leq45''$, combines them with literature radial velocities to yield updated $v_{3d}$, and compares against $v_{esc,max}$ to identify unbound cases. IRS 9 stands out as the only source with $v_{3d}>v_{esc,max}$, and orbit integrations place it on a highly eccentric, bound orbit with $r_{peri}\ge0.100\pm0.005$ pc and $r_{apo}\ge5.25\pm0.18$ pc, implying a past dynamical interaction; Hills mechanism is unlikely within the last ${\sim}$0.4 Myr. The work assesses alternative channels (binary SN disruption, close two-body interactions, stellar collisions) and notes that additional high-velocity NSC stars are needed to constrain these mechanisms. Together, the study provides a precise census of GC maser kinematics and informs the assembly history of the NSC and its interaction with the central SMBH.

Abstract

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.

Figures (6)

• Figure 1: The 23 SiO masers examined in this study, overlaid on an HST F153M image. The green square represents IRS 9, which is found to exhibit a velocity larger than the maximum escape velocity at its radius (i.e., it is locally unbound from the GC). The green arrow shows the direction of its HST proper motion (not to scale). The yellow squares correspond to three masers proposed to be locally unbound (SiO-16, SiO-21, and SiO-25), but the new HST proper motions indicate that they do not exceed their maximum escape velocities. The red circles show the positions of the other masers in the sample. All sources have projected distances of R $\leq$ 45" ($\sim$1.8 pc) from SgrA* (cyan x).
• Figure 2: Maser $v_{3d}$ as a function of $r_{2d}$. A model for $v_{esc, max}$ and the corresponding 1$\sigma$ uncertainty in that model is shown by the red line and shaded region. Previously reported masers with $v_{3d} > v_{esc, max}$ are highlighted by blue markers: IRS 9 (blue square), SiO-16 (blue triangle), SiO-21 (blue diamond), and SiO-25 (blue plus sign). When the HST proper motions are used, only IRS 9 has $v_{3d} > v_{esc, max}$.
• Figure 3: The orbit of IRS 9, integrated for the past 0.4 Myr and assuming d$_{los}$ = 0 pc. In each panel, the black lines represent orbits drawn from an MC simulation over the measured properties of IRS 9, and so the spread of lines reveals the uncertainty in the orbit. Top left: A "top-down" view of the orbit from the North Galactic Pole (with Earth at negative LOS distance). The solid black square represents the current position of IRS 9 while the faded black circles represent the end point of each orbit (i.e., 0.4 Myr in the past). SgrA* is represented by the red "x". Top right: An "edge-on" view of the orbit from the Galactic Plane, constructed in the same manner as the top-left panel. Bottom left: The distance of IRS 9 from SgrA* as a function of time. Bottom right: The $v_{3d}$ of IRS 9 as a function of radius, compared to $v_{esc, max}$ (red line with uncertainty shown as shaded region). IRS 9 is locally unbound when $v_{3d}$$>$$v_{esc, max}$, as is the case at the present-day (at $r_{2d}$ = 0.33 pc). However, the star is bound to the GC at larger radii as $v_{3d}$ drops below $v_{esc, max}$.
• Figure 4: The observed $v3d$ distribution of 32 stars with projected radii within $\pm 1"$ of IRS 9 (black histogram), compared to the $v3d$ of IRS 9 itself (blue star). The expected $v3d$ distribution of the NSC is taken to be a Maxwellian distribution with a scale parameter equal to the velocity dispersion at this radius derived from a dynamical model of the NSC Chatzopoulos:2015lq. From this distribution, we calculate that the probability of observing a star with a $v3d$ greater or equal to IRS 9 is $\sim$1%.
• Figure 5: Binary disruption timescale (left) and $v_{\infty,smbh}$ (right) as a function of Hills disruption radius for hypothetical binary systems containing IRS 9. The allowed ranges for a $M_{irs9}$ / $M_{comp}$ = 1 system (green hashed region) and a $M_{irs9}$ / $M_{comp}$ = 0.1 system (blue hashed region) are shown, as well as the current values for IRS 9 (red points). Assuming that the Hills disruption occurred at the minimum periapse of IRS 9's current orbit ($r_{peri}$ = 0.1 pc), the disruption timescales of the binary systems are $\lesssim$30 Myr. Given IRS 9's current age of $\gtrsim$ 3 Gyr, the binary systems could not survive long enough to disrupt within the past 0.4 Myr. Further, the binary systems cannot achieve the minimum $v_{\infty,smbh}$ of IRS 9 (134 $\pm$ 18 km s$^{-1}$, calculated assuming $|d_{los}|$ = 0 pc) via the Hills mechanism at a disruption radius of 0.1 pc.