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The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth
We have obtained the first detection of spectral absorption lines in oneof the high-velocity stars in the vicinity of the Galaxy's centralsupermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126μm) are seen in absorption in S0-2 with equivalent widths (2.8+/-0.3and 1.7+/-0.4 Å) and an inferred stellar rotational velocity(220+/-40 km s-1) that are consistent with that of an O8-B0dwarf, which suggests that it is a massive (~15 Msolar) young(less than 10 Myr) main-sequence star. This presents a major challengeto star formation theories, given the strong tidal forces that prevailover all distances reached by S0-2 in its current orbit (130-1900 AU)and the difficulty in migrating this star inward during its lifetimefrom farther out where tidal forces should no longer preclude starformation. The radial velocity measurements (z>=-510+/-40km s-1) and our reported proper motions for S0-2 stronglyconstrain its orbit, providing a direct measure of the black hole massof 4.1(+/-0.6)×106(R0/8 kpc)3Msolar. The Keplerian orbit parameters haveuncertainties that are reduced by a factor of 2-3 compared to previouslyreported values and include, for the first time, an independent solutionfor the dynamical center; this location, while consistent with thenominal infrared position of Sgr A*, is localized to a factor of 5 moreprecisely (+/-2 mas). Furthermore, the ambiguity in the inclination ofthe orbit is resolved with the addition of the radial velocitymeasurement, indicating that the star is behind the black hole at thetime of closest approach and counterrevolving against the Galaxy. Withfurther radial velocity measurements in the next few years, the orbit ofS0-2 will provide the most robust estimate of the distance to theGalactic center.
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