From astrobites: “Is S0-2 a Binary Star?”

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Astrobites

Sep 26, 2017
Philipp Plewa

Title: Investigating the Binarity of S0-2: Implications for its Origins and Robustness as a Probe of the Laws of Gravity around a Supermassive Black Hole
Authors: D. S. Chu, T. Do, A. Hees, A. Ghez, S. Naoz, G. Witzel, S. Sakai, S. Chappell, A. K. Gautam, J. R. Lu, K. Matthews
First Author’s Institution: Department of Physics and Astronomy, University of California, Los Angeles

Status: Submitted to The Astrophysical Journal, open access

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S0–102 is a star that is located very close to the centre of the Milky Way, near the radio source Sgr A*, orbiting it with an orbital period of 11.5 years. As of 2012 it is the star with the shortest known period orbiting the black hole at the centre of the Milky Way. This beat the record of 15 years previously set by S0–2. The star was identified by a University of California, Los Angeles team headed by Andrea M. Ghez.

Andrea Ghez, UCLA

At its periapsis, its speed exceeds 1% of the speed of light.[3] At that point it is 260 astronomical units (36 light hours, 38.9 billion km) from the centre, while the black hole radius is less than one thousandth of that size (11 million km). It passed that point in 2009 and will be there again in 2020.

The most exciting discoveries in astronomy all have something in common: They let us marvel at the fact that nature obeys laws of physics. The discovery of S0-2 is one of them. S0-2 (also known as S2) is a fast-moving star that has been observed to follow a full elliptical, 16-year orbit around the Milky Way’s central supermassive black hole, precisely according to Kepler’s laws of planetary motion. Serving as a test particle probe of the gravitational potential, S0-2 provides some of the best constraints on the black hole’s mass and distance yet, being the brightest of the S-stars, which are a group of young main-sequence stars concentrated within the inner 1” (0.13 ly) of the nuclear star cluster.

The next time S0-2 will reach its closest approach to the black hole, in 2018, there will exist a unique opportunity to detect a deviation from Keplerian motion, namely the relativistic redshift of S0-2’s radial (line-of-sight) velocity, in a direct measurement. In anticipation of this event, the authors of today’s paper investigate possible consequences of S0-2 not being a single star, but a spectroscopic binary, which would complicate this measurement.

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Figure 1: Top: Radial velocity measurements of S0-2 over time. Bottom: Residual velocities after subtraction of the best-fit model for the orbital motion.

To search for any periodicity in S0-2’s radial velocity curve that would indicate the presence of a companion star, the authors combine their most recent velocity measurements with previous ones obtained as part of monitoring programs carried out at both the WMKO in Hawaii and the VLT in Chile.


Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level

ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

The resulting data set consists of 87 measurements in total, which are spread over 17 years of observations and have a typical uncertainty of a few 10 km/s (Figure 1, top panel). When S0-2 passes the black hole, the relativistic redshift of its radial velocity is predicted to amount to roughly 200 km/s at closest approach, while the radial velocity is expected to change from +4000 to -2000 km/s. S0-2’s actual speed at this time will be close to 8000 km/s, about 2.7% of the speed of light.

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