S2 (star)

S2, also known as S0–2, is a star in the star cluster close to the supermassive black hole Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours (18 Tm or 120 au) – an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun. The mass when the star first formed is estimated by the European Southern Observatory (ESO) to have been approximately 14 M☉.[5] Based on its spectral type (B0V ~ B3V), it probably has a mass of 10 to 15 solar masses.

S2

Image of the Galactic Center showing the position of S2
Credit: ESO / MPE / Gillessen et al.
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 17h 45m 40.0442s[1]
Declination −29° 00′ 27.975″[1]
Characteristics
Spectral type B0-2 V[2]
Astrometry
Distance7,940±420[3] pc
Orbit[3]
CompanionSagittarius A*
Period (P)16.0518[4] yr
Semi-major axis (a)0.12540 ± 0.00018″
Eccentricity (e)0.88466 ± 0.00018
Inclination (i)133.818 ± 0.093°
Longitude of the node (Ω)227.85 ± 0.19°
Periastron epoch (T)2018.37974 ± 0.00015
Argument of periastron (ω)
(secondary)		66.13 ± 0.12°
Other designations
[CRG2004] 13, [GKM98] S0–2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0–2.
Database references
SIMBADdata

Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. The accumulating evidence points to Sgr A* as being the site of such a black hole. By 2008, S2 had been observed for one complete orbit.[6] In 2020, partway through its next orbit, the GRAVITY collaboration released an analysis showing full agreement with Schwarzschild geodesics.[7]

A team of astronomers, mainly from the Max Planck Institute for Extraterrestrial Physics, used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the Galactic Center. They determined it to be 7.94 ± 0.42 kiloparsecs, in close agreement with prior determinations by other methods.[3][8]

S2 was precisely tracked during its May 2018 close approach to Sgr A*, with results in accord with general relativity predictions.[9]

Nomenclature

The designation S0–2 was first used in 1998. S0 indicates a star within one arc-second of Sgr A*, indicating the galactic centre, and S0–2 was the second closest star seen at the time of the measurements.[10] The star had been catalogued simply as S2 a year earlier, the second of eleven infrared sources near the galactic centre, numbered approximately anti-clockwise.[11] It is a coincidence that the star is numbered "2" in both lists; other catalogs number it differently.[10]

Orbit

The highly eccentric orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects.[12] These effects reached a maximum at closest approach, which occurred in mid-2018.[13][14] Given a recent estimate of 4.31 million M☉ for the mass of the Sgr A* black hole and S2's close approach, this makes S2's the fastest known ballistic orbit, reaching speeds exceeding 5,000 km/s (11,000,000 mph, or 1⁄60 the speed of light) and acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity).[15]

The motion of S2 is also useful for detecting the presence of other objects near to Sgr A*. It is believed that there are thousands of stars, as well as dark stellar remnants (stellar black holes, neutron stars, white dwarfs) distributed in the volume through which S2 moves. These objects will perturb S2's orbit, causing it to deviate gradually from the Keplerian ellipse that characterizes motion around a single point mass.[16] So far, the strongest constraint that can be placed on these remnants is that their total mass comprises less than one percent of the mass of the supermassive black hole.[17]

2018 pericentre passage

In 2018, S2 made its closest approach to Sgr A*, reaching 7650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1400 times its Schwarzschild radius.[18][19] S2 reached its pericenter on May 19, 2018, while its velocity in the line of sight from Earth peaked in April, and later hit its minimum in late August and early September.[18]

Independent analyses by the GRAVITY collaboration[20][18][21][22] (led by Reinhard Genzel) and the KECK/UCLA Galactic Center Group[23][24] (led by Andrea Ghez) revealed a combined transverse Doppler and gravitational redshift up to 200 km/s/c, in agreement with general relativity predictions.

Additional analysis has revealed a Schwarzschild precession of 12 arcminutes (0.2 degrees) in S2's orbit caused by the close passage, fully consistent with general relativity.[25][26][27]

S0–102
Main article: S0–102

In 2012, a star called S0–102 (or S55) was found to be orbiting even closer to the Milky Way's central supermassive black hole than S0–2 does. At one-sixteenth the brightness of S0–2, S0–102 was not initially recognized because it required many more years of observations to distinguish it from its local infrared background. S0–102 has an orbital period of 11.5 years, even shorter than that of S0–2. Of all the stars orbiting the black hole, only these two have their orbital parameters and trajectories fully known in all three dimensions of space.[28] The discovery of two stars orbiting the central black hole so closely with their orbits fully described is of extreme interest to astronomers, as the pair together will allow much more precise measurements on the nature of gravity and general relativity around the black hole than would be possible from using S0–2 alone.

A still closer star S62 has since been discovered with an orbital period of 9.9 years.[29]

  • Artist's annotation of S2 passing supermassive black hole (note black hole is not to scale) at center of Milky Way, confirming gravitational red shift
    Artist's annotation of S2 passing supermassive black hole (note black hole is not to scale) at center of Milky Way, confirming gravitational red shift[30]
  • Orbits of S0–2 and S0–102 around the Milky Way galaxy's super­massive black hole
    Orbits of S0–2 and S0–102 around the Milky Way galaxy's super­massive black hole
  • Inferred orbits of S2 and five other stars around supermassive black hole candidate Sgr A* at the Milky Way galactic centre
    Inferred orbits of S2 and five other stars around supermassive black hole candidate Sgr A* at the Milky Way galactic centre[31]
  • Observations showing the discovery of the orbit of S2 about the galactic centre
    Observations showing the discovery of the orbit of S2 about the galactic centre

See also

References
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  2. Paumard, T.; Genzel, R.; Martins, F.; Nayakshin, S.; Beloborodov, A.M.; Levin, Y.; Trippe, S.; Eisenhauer, F.; Ott, T.; Gillessen, S.; Abuter, R.; Cuadra, J.; Alexander, T.; Sternberg, A. (2006). "The two young star disks in the central parsec of the galaxy: Properties, dynamics, and formation". The Astrophysical Journal. 643 (2): 1011–1035. arXiv:astro-ph/0601268. Bibcode:2006ApJ...643.1011P. doi:10.1086/503273. S2CID 14440768.
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  4. Hees, A. (2017). "Testing General Relativity with stellar orbits around the supermassive black hole in our galactic center". Physical Review Letters. 118 (21): 211101. arXiv:1705.07902. Bibcode:2017PhRvL.118u1101H. doi:10.1103/PhysRevLett.118.211101. PMID 28598651. S2CID 206291276.
  5. Habibi, M.; et al. (2017). "Twelve years of spectroscopic Monitoring in the galactic center: The closest look at S-stars near the black hole". The Astrophysical Journal. 847 (2): 120. arXiv:1708.06353. Bibcode:2017ApJ...847..120H. doi:10.3847/1538-4357/aa876f. S2CID 119078556.
  6. A short documentary on Sagittarius A* on YouTube
  7. Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Cardoso, V.; Clénet, Y.; De Zeeuw, P. T.; Dexter, J.; Eckart, A.; Eisenhauer, F.; Förster Schreiber, N. M.; Garcia, P.; Gao, F.; Gendron, E.; Genzel, R.; Gillessen, S.; Habibi, M.; Haubois, X.; Henning, T.; Hippler, S.; Horrobin, M.; Jiménez-Rosales, A.; Jochum, L.; Jocou, L.; Kaufer, A.; Kervella, P.; Lacour, S.; et al. (GRAVITY Collaboration) (16 April 2020). "Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole". Astronomy & Astrophysics. 636 (L5): L5. arXiv:2004.07187. Bibcode:2020A&A...636L...5G. doi:10.1051/0004-6361/202037813. S2CID 215768928.
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  12. "Black hole as peep-hole". ScienceNews.
  13. Ghez, Andrea M. (speaker) (19 April 2016). Black Holes @ 100 Workshop: Galactic Center (video lecture). Harvard University: Black Hole Initiative. Remarks beginning at 31:55. Archived from the original on 2021-12-20. ... we are '2018 or bust' these days, because at that moment your orbital determination becomes so much better
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