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An Update on Monitoring Stellar Orbits in the Galactic Center S Draft version November 29, 2016 Preprint typeset using LATEX style emulateapj v. 5/2/11 AN UPDATE ON MONITORING STELLAR ORBITS IN THE GALACTIC CENTER S. Gillessen1, P.M. Plewa1, F. Eisenhauer1, R. Sari2, I. Waisberg1, M. Habibi1, O. Pfuhl1, E. George1, J. Dexter1, S. von Fellenberg1, T. Ott1, R. Genzel1,3 Draft version November 29, 2016 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 6 constraints on the mass of and distance to Sgr A*; the statistical errors of 0:13 × 10 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 6 and distance: M = 4:28 ± 0:10jstat: ± 0:21jsys × 10 M and R0 = 8:32 ± 0:07jstat: ± 0:14jsys kpc. These numbers are in agreement with the recent determination of R0 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. 1. INTRODUCTION Galactic Center, R0, is important for many branches of The near-infrared regime is a sweet spot for studying astronomy. R0 is one of the fundamental parameters of the gravitational potential in the Galactic Center. For any model of the Milky Way, and its value determines measuring the latter, one would like to have as high a mass and size of the galaxy. This ties R0 into the resolution as possible, and have access to the emission of cosmological distance ladder, since galactic variables objects compact and bright enough that they can serve serve as zero point for the period-luminosity relations as test particles for the potential. The optimum band determined usually in the Large Magellanic Cloud. The is around 2 µm wavelength, where the extinction screen mass of the massive black hole is equally important. amounts to less than 3 mag (Fritz et al. 2011), and where Using this value, one can place the Milky Way onto adaptive optics at 8 m-class telescopes is performing scaling relations (Kormendy & Ho 2013). Knowing the well for typical atmospheric conditions. The intrinsic mass of and distance to Sgr A* is the reason why the resolution of around 50 mas allows measuring stellar Galactic Center is a unique testbed concerning massive orbits with semi-major axes of similar size, corresponding black holes and their vicinities for numerous models to orbital periods around a decade for a black hole of in many branches of astrophysics (Yuan & Narayan 4 million solar masses in 8 kpc distance. 2014). The Milky Way also serves as a check for mass 25 years of near-infrared observations of the Galactic measurements in other galaxies, since the true black hole Center have shown that a wealth of fundamental mass is known and one can simulate observations at lower astrophysical and physical questions can be addressed resolution (Feldmeier et al. 2014). with these measurements, ranging from star formation, The most profound result of the orbital work is to stellar dynamics, to testing general relativity (Genzel the proof of existence of astrophysical massive black et al. 2010). The outstanding, main result is that the holes. This opens up a new route of testing general radio source Sgr A* is a massive black hole in the center relativity, at a mass scale and a field curvature that have not been accessible so far. Since the fundamental arXiv:1611.09144v1 [astro-ph.GA] 28 Nov 2016 of the Milky Way. The key to this result is that one can measure Sgr A*'s mass from tracing individual stellar parameters are known for Sgr A*, one can think of orbits around it. If a sufficiently large part of an orbit more ambitious experiments using the black hole. Most is sampled, one can deduce information on the potential notably, in the near future two observations might through which the star is moving. In particular, one can become feasible: (i) Using the motions of stars and/or determine the central mass and the distance to it. Due that of plasma radiating very close to the event horizon, to its proximity, the Galactic Center is the only galactic one might be able to measure the spin of the black nucleus where such an experiment is currently feasible. hole. The instrumental route to that goal is near-infrared A geometric determination of the distance to the interferometry (Eisenhauer et al. 2011). (ii) A global radio-interferometric array operating at around 1 mm 1 Max-Planck-Institut f¨ur Extraterrestrische Physik, 85748 should be able to resolve Sgr A*, i.e. to deliver an actual Garching, Germany image of the black hole's shadow (Luminet 1979; Falcke 2 Racah Institute of physics, The Hebrew University, Jerusalem 91904, Israell et al. 2000; Doeleman et al. 2008). Additionally, the most 3 Astronomy & Physics Departments, University of California, stringent tests of general relativity would be possible, if Berkeley, CA 94720, USA a pulsar representing a perfect clock in a short-period 2 Gillessen et al. orbit around Sgr A* were found (Psaltis et al. 2015). S27 S48 S53 Here, we report on updates to our ongoing, long-term S28 S26 program of monitoring of stellar orbits around Sgr A*. S29 The first orbit determination dates back to 2002 (Sch¨odel S37 S30 et al. 2002; Ghez et al. 2003). A few years later, orbits S12 S52 for a handful of stars had been determined (Ghez et al. S39 S36 2005; Eisenhauer et al. 2005), and again a few years S47 S64 S59 S5 S13 S14 later, the number of known orbits exceeded 20 (Gillessen S2 S60 et al. 2009b). To date, the number of known orbits has S4 risen to around 40, and both statistical and systematic S6 S31 S55 S62 S20 S38 errors of our measurements are much reduced compared S17 S175 to previous measurements. S21 S43 S7 S63 S54 S18 S57 S23 2. DATA S19 S44 This work is an update and improvement over our S22 S42 S51 previous orbital study (Gillessen et al. 2009b). The two S1 S32 S8 S25 main improvements are: S10 S9 S41 S33 S35 • Since the previous work we have added eight more years of data, using the adaptive optics (AO) S34 S24 S50 S45 imager NACO on the VLT (Lenzen et al. 1998; S46 S58 Rousset et al. 1998) and the AO-assisted integral S11 field spectrograph SINFONI (Eisenhauer et al. Figure 1. Mock image of the central arcsecond for our reference 2003; Bonnet et al. 2003). This extends our time epoch 2009.0, constructed from the measured motions and base from 17 to 25 years for the imaging and from magnitudes of the stars, assuming a PSF size and pixel sampling as five to 13 years for the spectroscopy. We add (in in our NACO data. Stars with spectral identification have colored labels, blue for early-type stars (Br-γ absorption line detected), the best cases) 78 epochs of imaging and 19 epochs red for late-type stars (CO band heads detected). The yellow cross of spectroscopy. denotes the position of Sgr A*. • We implement the improved reference frame (ten epochs) show that we can reconstruct the stellar described in Plewa et al. (2015). This greatly positions to the same level of precision as before. Also, improves our prior knowledge, where we expect an there is no systematic mismatch between the positions orbit fit to reconstruct the mass responsible for the obtained before and after. We therefore do not need to orbital motion of the S-stars. The new calibration apply any corrections related to the interruption. links to the International Celestial Reference Frame For orbits fits using a combined data set of both VLT- (ICRF) in a two-step procedure, and compared to and Keck-based observations, we replace the data points our previous work does not rely on the assumption of Ghez et al. (2008) by the newer publication of the that the mean motion of a large sample of stars same team from Boehle et al. (2016). observed around Sgr A* is zero. 3. THE GRAVITATIONAL POTENTIAL IN THE GALACTIC CENTER The other steps of the analysis are identical, and we refer the reader to Gillessen et al. (2009b) for more 3.1. Orbit fitting details. In particular, the following critical issues are Orbit fitting has a relatively large number of free treated identical: parameters. While in its most simple form the potential has only one free physical parameter (the central mass The assignment of statistical errors to individual • M), we do not know a priori where the mass is located data points. and how it moves. Hence six additional parameters • The relative weight between the earlier Speckle need to be determined simultaneously: The distance to data (1992-2001, Hofmann et al.
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