An Update on Monitoring Stellar Orbits in the Galactic Center

Abstract

Using 25 years of data from uninterrupted monitoring of stellar orbits in the Galactic Center, we present an update of the main results from this unique data set: A measurement of mass of and distance to Sgr A*. Our progress is not only due to the eight year increase in time base, but also due to the improved definition of the coordinate system. The star S2 continues to yield the best constraints on the mass of and distance to Sgr A*; the statistical errors of 0.13 × 10 6 M and 0.12 kpc have halved compared to the previous study. The S2 orbit fit is robust and does not need any prior information. Using coordinate system priors, also the star S1 yields tight constraints on mass and distance. For a combined orbit fit, we use 17 stars, which yields our current best estimates for mass and distance: M = 4.28 ± 0.10| stat. ± 0.21| sys × 10 6 M and R 0 = 8.32 ± 0.07| stat. ± 0.14| sys kpc. These numbers are in agreement with the recent determination of R 0 from the statistical cluster parallax. The positions of the mass, of the near-infrared flares from Sgr A* and of the radio source Sgr A* agree to within 1 mas. In total, we have determined orbits for 40 stars so far, a sample which consists of 32 stars with randomly oriented orbits and a thermal eccentricity distribution, plus eight stars for which we can explicitly show that they are members of the clockwise disk of young stars, and which have lower eccentricity orbits.

Sgr A*, the best-sampled of all AGN?

“…6 and 7), and from dozens of other stars [Gillessen et al, 2017], which deviate slightly from Newtonian behaviour. In particu-lar, when deriving the mass M, and distance d of its orbital center Sgr A* from the orbit of star S2, during the years between 2001 and 2007, I noticed that their values rose monotonicly during increasing approach -from 10 6.41 M to 10 6.63 M , and from 8.0 kpc to 8.33 kpc respectivelyand stayed constant thereafter, e.g.…”

Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center

“…We choose only images (33 in total) in which Sgr A* was flaring in the infrared in order to locate it directly in our coordinate system and have a good control of its possible motion with respect to the stellar cluster. In addition to the three candidate stars (S2, S38, and S55/S0-102), five stars (S7, S10, S26, S30, and S65) in their vicinity of them are selected to verify their reported positions and motions (Gillessen et al 2009b;Plewa et al 2015;Gillessen et al 2017). This allowed us to validate the quality of the reference frame on an image-by-image basis.…”

“…S2, S38, and S55/S0-102 are shown with orange circles and are labeled. The semimajor axis and the eccentricity of all the S-stars except S2, S38, and S55/S0-102 have been taken from Gillessen et al (2017, Table 3). …”

“…These results could be improved using other high-velocity Sstars in the vicinity of Sgr A* (e.g. Ghez et al 2008;Gillessen et al 2009b;Boehle et al 2016;Gillessen et al 2017). So far the orbits of 40 of these stars at distances between 1 and 100 milliparsecs (mpc) from Sgr A* have been determined using NIR imaging and spectroscopy (Gillessen et al 2017).…”

“…However, it is presumed that there are many more fainter stars in the innermost region around Sgr A* to be discovered . The star S2 has a short orbital period of about 16.2 yr which has enabled us to observe it long enough to determine its motion with high accuracy (Ghez et al 2003;Gillessen et al 2009bGillessen et al , 2017. Its velocity at periapse in units of the speed of light is ∼0.02, and if we consider a semimajor axis of 0 .124, an eccentricity of 0.88, and a BH mass of 4 × 10 6 M , then the periapse shift to the lowest order will be around 10 .8 which is large enough to be observable (Jaroszynski 1998;Fragile & Mathews 1999;Rubilar & Eckart 2001;Weinberg et al 2005).…”