S62 and S4711: Indications of a population of faint fast moving stars inside the S2 orbit -- S4711 on a 7.6 year orbit around Sgr~A*

Abstract: We present high-pass filtered NACO and SINFONI images of the newly discovered stars S4711-S4715 between 2004 and 2016. Our deep H+K-band (SINFONI) and K-band (NACO) data show the S-cluster star S4711 on a highly eccentric trajectory around Sgr~A* with an orbital period of 7.6 years and a periapse distance of 144 AU to the super massive black hole (SMBH). S4711 is hereby the star with the shortest orbital period and the smallest mean distance to the SMBH during its orbit to date. The used high-pass filtered images are based on co-added data sets to improve the signal to noise. The spectroscopic SINFONI data let us determine detailed stellar properties of S4711 like the mass and the rotational velocity. The faint S-cluster star candidates, S4712-S4715, can be observed in a projected distance to Sgr~A* of at least temporarily $\leq$ 120 mas. From these stars, S4714 is the most prominent one with an orbital period of 12 years and an eccentricity of 0.985. The stars S4712-S4715 show similar properties with comparable magnitudes and stellar masses to S4711. The MCMC simulations determine confidently precise uncertainties for the orbital elements of S62 and S4711-S4715. The presence of S4711 in addition to S55, S62, and the also newly found star S4714 implies a population of faint stars that can be found at distances to Sgr~A* that are comparable to the size of our solar system. These short orbital time period stars in the dense cluster around the SMBH in the center of our Galaxy are perfect candidates to observe gravitational effects such as the periapse shift.

• The paper identifies fast-moving star S4711 orbiting Sgr A* with a 7.6-year period and a periapsis of 144 AU.
• The paper details the discovery of additional faint stars in proximity to Sgr A*, enhancing our understanding of Galactic center dynamics.
• The study employs MCMC simulations to precisely determine orbital parameters, offering insights into relativistic effects near the supermassive black hole.

An Examination of the Population of Faint Fast-Moving Stars Inside the S2 Orbit

The paper entitled "S62 and S4711: Indications of a Population of Faint Fast Moving Stars Inside the S2 Orbit" by Florian Peißker et al. documents the discovery and analysis of previously unidentified stars located close to the supermassive black hole (SMBH) in the Galactic Center. This study leverages observations made with the NACO and SINFONI instruments, focusing particularly on the faint stars S4711-S4715, adding significant insight into the understanding of stellar dynamics near Sagittarius A* (Sgr~A*).

Key Discoveries

1. Identification of S4711: The star S4711 orbits Sgr~A* with an unprecedentedly short orbital period of 7.6 years and a minimal periapsis distance of 144 AU. This makes S4711 one of the nearest stars to the SMBH with the shortest known orbital period, marking it as a further subject for studying general relativistic effects.
2. Detection of Additional Stars: In addition to S4711, four other stars—S4712, S4713, S4714, and S4715—were identified within close proximity to Sgr~A*. Notably, S4714 exhibits an orbit with high eccentricity (0.985) similar to other known S-cluster stars, supporting the hypothesis of a dense, dynamic stellar environment near the core.
3. Orbital Parameters and Stellar Characteristics: Detailed analysis using MCMC simulations provided precise uncertainties for the orbital elements of the observed stars. S4711, in particular, was found to have a mean spatial orbital velocity of approximately 0.5% the speed of light, with its spectroscopic analysis suggesting a spectral type of B8/9-V and a low rotational velocity.

Implications

The detection of such faint stars presents a tantalizing opportunity to probe the relativistic regime near a SMBH. The stars newly discovered in this study provide an experimental framework to further analyze effects predicted by the general theory of relativity, including the Schwarzschild precession and the gravitational redshift. S4711 and S4714, due to their trajectories and proximity to Sgr~A*, offer a valuable chance to observe these effects directly.

Moreover, the presence of these stars might lead to insights into the population dynamics and formation history of the Galactic center. The high eccentricity and close periapsis suggest possible origins via the Hills mechanism or previous interactions with other massive perturbers in the region. Consequently, understanding their origins could illuminate broader questions about star formation in extreme gravitational environments and the subsequent evolution of such systems.

Future Prospects

The observation of such faint stars would benefit significantly from ultra-precise instruments such as those that will be available on the upcoming Extremely Large Telescope (ELT). The ELT, with its advanced adaptive optics and unprecedented resolution, is expected to uncover even finer details about the dynamics of these stars near the SMBH, expanding upon the foundation laid by this study.

In conclusion, the discovery and analysis of faint stars very close to Sgr~A* such as S4711 and S4714 are crucial for the continued exploration of gravitational physics outside the standard Newtonian framework, providing observational verification of relativistic predictions while enhancing our understanding of the complex star formation and evolutionary processes in the unique environment of the Galactic center.