The Great Escape: Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A* (1907.11725v2)

Abstract: We present the serendipitous discovery of the fastest Main Sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a $\sim 2.35$ M$\odot$ A-type star located at a distance of $\sim 9$ kpc from the Sun and has a heliocentric radial velocity of $1017\pm 2.7$ km/s without any signature of velocity variability. The current 3-D velocity of the star in the Galactic frame is $1755\pm50$ km/s. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of $\sim 1800$ km/s and travelled for $4.8$ Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion $V{y,\odot}= 246.1\pm 5.3$ km/s or position $R_0=8.12\pm 0.23$ kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disk of young stars at the Galactic centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.

The Ejection and Implications of S5-HVS1: A Study on Galactic Center-originated Hyper-velocity Stars

The discovery of S5-HVS1, an A-type star with a mass of roughly 2.35 M$_\odot$, signifies an important contribution to the understanding of the dynamics and ejection mechanisms linked to the supermassive black hole (SMBH) at the core of the Milky Way. This paper by Koposov et al. provides a thorough exploration of this star, ejected from the Galactic Center, with a reported velocity of approximately 1700 km/s, connecting it unambiguously to Sgr A*.

The star's current position, around 9 kpc from the Sun, combined with precise measurements of its radial velocity, makes S5-HVS1 the fastest known hyper-velocity star (HVS). Its velocity was calculated to be nearly twice that of other known HVSs associated with the Galactic Center, raising questions about whether its ejection involved unique mechanisms or temporal fluctuations in ejection velocities from Sgr A*. The trajectory analysis, as detailed in the paper, positions its ejection point consistently with the Galactic Center, reinforcing the hypothesis of a Hills mechanism ejection scenario. This model describes a system where a stellar binary interacts with a SMBH, potentially resulting in the dramatic acceleration of one star to velocities exceeding 1000 km/s. The association of S5-HVS1 thus serves as robust evidence supporting Hills' ejection mechanism.

The analysis brings forth fascinating implications regarding the Milky Way's kinematic properties, including providing constraints on the Solar motion and Galactic parameters like the Sun's position relative to the center. Notably, measurements indicate a Solar velocity of $V_{y,\odot} = 246.1\pm 5.3$ km/s and position $R_0 = 8.12\pm 0.23$ kpc. This paper contributes a noteworthy inference, albeit with S5-HVS1 alone, the precision might be insufficient to spatially place constraints on the Milky Way's dark matter halo flattening, as its ∼4.8 Myr flight from the GC limits potential interactions with the gravitational field across its brief trajectory.

Additionally, the ejection velocity and vector orientation may hint at possible relations with the disk of young stars surrounding Sgr A*. As observed, the trajectory aligns well with the orbital plane of these stars, suggesting commonalities in their formation history. Such alignment could mean concurrent formation processes associated with past accretion events at the Galactic Center, positing fascinating avenues for further exploration about star formation and dynamics near SMBHs.

Looking towards the future, the promise of more discoveries akin to S5-HVS1 is palpable. With improvements in observational precision and the advancing capabilities of surveys such as 4MOST and DESI, capturing more hyper-velocity stars and analyzing their trajectories will help refine understanding of Galactic dynamics, potentially offering "natural probes" of substructure within the Milky Way. New hyper-velocity stars could aid in disentangling complex interactions at play in galactic centers, particularly with possible IMBH dynamics involved.

Ultimately, this paper expands the framework of inquiry around Galactic dynamics and offers an exemplar in S5-HVS1 for studying high-velocity ejections, enriching our grasp of both small-scale encounters and broader Galactic phenomena.