An Updated Reference Frame for the Galactic Inner Parsec

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Darling_2023_ApJ_955_117.pdf (2.81 MB)

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https://iopscience.iop.org/article/10.3847/1538-4357/acec71

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https://doi.org/10.3847/1538-4357/acec71
 http://hdl.handle.net/11603/32924

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UMBC Center for Space Sciences and Technology (CSST) / Center for Research and Exploration in Space Sciences & Technology II (CRSST II)

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https://orcid.org/0000-0002-7517-9223

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2023-09-25

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journal articles
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Darling, Jeremy, Jennie Paine, Mark J. Reid, Karl M. Menten, Shoko Sakai, and Andrea Ghez. “An Updated Reference Frame for the Galactic Inner Parsec.” The Astrophysical Journal 955, no. 2 (September 2023): 117. https://doi.org/10.3847/1538-4357/acec71.

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Abstract

Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr⁻¹ and 0.12 mas yr⁻¹, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s⁻¹. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8 μas yr⁻¹ (0.30 km s⁻¹) in R.A. and 11 μas yr⁻¹ (0.44 km s⁻¹) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.