S55 (star)

S55 (also known as S0–102) is a star that is located very close to the centre of the Milky Way, near the radio source Sagittarius A*, orbiting it with an orbital period of 12.8 years. Until 2019, when the star S62 became the new record holder, it was the star with the shortest known period orbiting the black hole at the centre of the Milky Way. This beat the record of 16 years previously set by S2. The star was identified by a University of California, Los Angeles team headed by Andrea M. Ghez.[1] At its periapsis, its speed reaches 1.7% of the speed of light. At that point it is 246 astronomical units (34 light hours, 36.7 billion km) from the centre,[2][3] while the black hole radius is only a small fraction of that size (the Schwarzschild radius is about 0.082 au).[4] It passed that point in 2009 and will be there again in 2022.

S55
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 17h 45m 40.0409s
Declination −29° 0′ 28.118″
Characteristics
Astrometry
Distance26,674±42 ly
(8,178±13 pc)
Orbit
CompanionSagittarius A*
Period (P)12.8±0.1 yr
Semi-major axis (a)0.1078±0.0010″
Eccentricity (e)0.721±0.008
Inclination (i)150±2°
Longitude of the node (Ω)325±4°
Periastron epoch (T)2009.34±0.04
Argument of periastron (ω)
(secondary)		332±4°
Other designations
[GKM98] S0–102.
Database references
SIMBADdata

Its position in the sky has been monitored from 2000 to 2012 using the W. M. Keck telescope and from 2002 to 2016 with the VLT. One complete orbit has been observed. From Earth's current perspective, it travels in a clockwise direction. Having observed two stars orbiting through complete periods around the centre (S55 and S2), the gravitational potential of SgrA* could be established.[5] It is also possible that there is a great deal of dark matter around the orbits of these stars.[6] Also general relativistic effects due to gravitational redshift should become observable.[5]

References
  1. Stuart Wolpert (4 October 2012). "UCLA astronomers discover star racing around black hole at center of our galaxy".
  2. Gillessen, S.; et al. (February 2017). "An Update on Monitoring Stellar Orbits in the Galactic Center". The Astrophysical Journal. 837 (1): 30. arXiv:1611.09144. Bibcode:2017ApJ...837...30G. doi:10.3847/1538-4357/aa5c41. S2CID 119087402.
  3. Næss, S. (4 October 2019). "Galactic center S-star orbital parameters".
  4. The GRAVITY collaboration (April 2019). "A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty". Astronomy & Astrophysics. 625: L10. arXiv:1904.05721. Bibcode:2019A&A...625L..10G. doi:10.1051/0004-6361/201935656. S2CID 119190574.
  5. Meyer, L.; Ghez, A. M.; Schödel, R.; Yelda, S.; Boehle, A.; Lu, J. R.; Do, T.; Morris, M. R.; Becklin, E. E.; Matthews, K. (2012). "The Shortest-Known-Period Star Orbiting Our Galaxy's Supermassive Black Hole". Science. 338 (6103): 84–87. arXiv:1210.1294. Bibcode:2012Sci...338...84M. doi:10.1126/science.1225506. PMID 23042888. S2CID 6029405.
  6. Dokuchaev, V.I.; Eroshenko, Yu.N.; Klimkov, K.S. (2015). "Precession of Fast S0 Stars in the Vicinity of Supermassive Black Hole in the Galactic Center". Physics Procedia. 74: 292–296. Bibcode:2015PhPro..74..292D. doi:10.1016/j.phpro.2015.09.243.
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