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Milky Way Galaxy

Orbital Parameters of Stars Orbiting Sgr A*

A number of stars orbiting within several hundred AU of the black hole candidate Sagittarius A* have been observed in the infrared K band. Instruments that make use of adaptive optics and interferometry are able to image these stars with a resolution of 0.025 arc-seconds or better. From these precise measurements, under the assumption that these stars are orbiting a massive black hole, orbital parameters for several of these stars have been obtained. In one study, velocities along the line of sight are derived from Doppler shift measurements, which further constrains the orbits. This page gives the orbital parameters for the stars with the best-determined orbital parameters.

The data in the following table comes from two papers, the paper by Eisenhauer et al. (2005), and the paper by Ghez et al. (2005).[1,2] I include only the stars that have small errors relative to the measured quantities. This means 3 of 7 stars from Ghez are listed, while all of the stars from Eisenhauer are listed. Eisenhauer et al. derive orbits from positional data and a small amount of redshift data. Ghez et al. derive orbits from positional data alone. Both groups assume that Sgr A* has a distance of 8 kpc; Eisenhauer et al. derives a direct value of 7.62 kpc for this distance from the orbital results; they incorporate the error in this distance into the errors for the orbital parameters.

Comparing the two sets of results, one sees that they differ by more than their statistical errors. Clearly these studies have some systematic errors to work out. Over time, as each star is observed for a full orbit or more, these errors should shrink. Already the star S2 has been observed for a full orbit, and the differences in the orbital parameters of this star are not very great.

The stars in this list generally have more than one name, because they were observed and cataloged in more than one survey. The stars with names prepended with a single “S” are from Genzel et al. (1997).[3] Stars with names prepended with “S0” are from Ghez et al. (1998).[4] In the table, the name of a star from Genzel et al. is given in the first column, and the alias of a star from Ghez et al. is given in the second column. The remaining values are the standard Keplerian orbital parameters: the semimajor axis a, listed in units of AU under the assumption of an 8 kpc distance to the Galactic center and in units of arc second for the results of Eisenhauer et al., the eccentricity e, the orbital period P in years, and the date T0 when the star was last at its closest approach to Sgr A*.

Eisenhauer et al. give the semimajor axis a in terms of seconds of arc, while Ghez gives semimajor axis in AU for the 8 kpc distance. The corresponding values in AU are derived for the Eisenhauer results for the 8 kpc distance. For the derived distance of 7.62±0.32 kpc, the size in AU is smaller by the factor 0.95, which is the ratio of the two distances. The relationship between length in AU and angle in arc-seconds is simple: multiply the angle in arc seconds by the distance in parsecs to get the length in AU. So, with a distance of 8 kpc, 1 equals 8,000 AU, and with a distance of 7.62 kpc, 1 equals 7,620 AU.

Orbital Parameters

Star

Alias

Reference

a (″)

a (AU)

e

P (years)

T0 (date)

S1

S0-1

[1]

0.412±0.024

3300±190

0.358±0.036

94.1±9.0

2002.6±0.6

S2

S0-2

[1]

0.1226±0.0025

980±20

0.8760±0.0072

15.24±0.36

2002.315±0.012

[2]

919±23

0.8670±0.0046

14.53±0.65

2002.308±0.013

S8

S0-4

[1]

0.329±0.018

2630±140

0.927±0.019

67.2±5.5

1987.71±0.81

S12

S0-19

[1]

0.286±0.012

2290±100

0.9020±0.0047

54.4±3.5

1995.628±0.016

[2]

1720±110

0.833±0.018

37.3±3.8

1995.758±0.050

S13

S0-20

[1]

0.219±0.058

1750±460

0.395±0.032

36±15

2006.1±1.4

S14

S0-16

[1]

0.225±0.022

1800±180

0.9389±0.0078

38±5.7

2000.156±0.052

[2]

1680±510

0.974±0.016

36±17

2000.201±0.025

[1] Eisenhauer, F., et al. “Sinfoni in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month.” The Astrophysical Journal 628 (20 July 2005): 246–259.

[2] Ghez, A.M., Hornstein, S.D., Tanner, A., Lu, J.R., Morris, M., Becklin, E.E., Duchêne, G. “Stellar Orbits Around the Galactic Center Black Hole.” The Astrophysical Journal 620 (20 February 2005): 744–757.

[3] Genzel, R., Eckart, A., Ott, T., and Eisenhauer, F. “On the Nature of the Dark Mass in the Centre of the Milky Way.” Monthly Notices of the Royal Astronomical Society 291 (1997): 219–234.

[4] Ghez, A.M., Klein, B. L., Morris, M., and Becklin, E.E. “High Proper-Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy.” The Astrophysical Journal 509 (20 December 1998): 678–686.

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