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F. Eisenhauer, T. Ott, R. Genzel, R. Schödel

The Distance to the Galactic Center.

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F. Eisenhauer, T. Ott, R. Genzel, R. Schödel

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  1. The Distance to the Galactic Center The distanceto the Galactic center(R0) is a fundamentalparameter for determining thestructure of the MilkyWay. Through its impacton the calibration ofstandard candles, such asRR Lyrae stars, Cepheids,and giants, the Galacticcenter distance also holdsan important role inestablishing the extragalactic distancescale. We report here thefirst primary distance measurementto the Galactic centerwith an uncertainty ofonly 5%. This determinationhas become possible throughthe advent of precisionmeasurements of proper motionsand line-of-sight velocities ofthe star S2. Thisstar is orbiting themassive black hole andcompact radio source SgrA* that is locatedprecisely at the centerof the Milky Way. The essenceof the method isthat the star's line-of-sightmotion is measured viathe Doppler shift ofits spectral features interms of an absolutevelocity, whereas its propermotion is measured interms of an angularvelocity. The orbital solutionties the angular andabsolute velocities, thereby yieldingthe distance to thebinary. Position measurements of S2in the infrared astrometricframe. Crosses (denoting 1 error bars) withdates mark the differentposition measurements of S2,taken with the MPEspeckle camera SHARP onthe NTT (between 1992and 2001) and withNACO on the VLT(in 2002 and 2003).The continuous curve showsthe best-fit Kepler orbit, whose focusis marked as asmall error circle. Thefocus of the ellipseis within a fewmilliarcseconds at the positionof the compact radiosource, which is markedby a large circledcross. The size ofthe cross denotes the±10 mas positional uncertaintyof the infrared relativeto the radio astrometricreference frame. H I Br absorption spectra of S2,obtained on 2003 April8/9 with SPIFFI(upper two panels), on May 8/9(lower left panel), and on June11/12 (lower right panel) with NACOat the VLT. TheSPIFFI spectrum is sky-subtractedand in a 0 1× 0 2 aperture; theNACO spectrum is slit-noddedand in a 0 086× 0 1 aperture. Thesedifferences account for thefact that the minispiralemission features between -400and +400 km s-1 LSR are visible inthe SPIFFI data butnot in the NACOspectrum. published as: Eisenhauer et al., ApJL 597, 191 (2003) F. Eisenhauer, T. Ott, R. Genzel, R. Schödel

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