29 June 2000 Is the galactic center source, IRS 21, as large as it appears?
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Abstract
We present diffraction limited 2-25 micrometers images, obtained with the W.M. Keck 10-m telescopes that spatially resolve the cool Galactic Center source IRS 21, an enigmatic object that has alluded classification. Modeled as a Gaussian, the azimuthally averaged intensity profile of IRS 21, an enigmatic object that has alluded classification. Modeled asa a Gaussian, the azimuthally averaged intensity profile of IRS 21 has a HWHM radius of 740 +/- 30 AU at 2.2 micrometers and an average HWHM radius of 1540 +/- 90 AU at mid-IR wavelength. These sizes along with its color temperature favor the hypothesis that IRS 21 is self-luminous rather than an externally heated dust clump. Based on the size alone, the remaining possible dust geometries are (1) an intrinsic inflow or outflow or (2) an extrinsic dust distribution, in which case IRS 21 could be simply embedded in the Northern Arm. A simple SED model of the IR photometry from the literature and our mid-IR images reveal that the near-IR radiation is scattered light from an unknown embedded source while the mid-IR radiation is the remaining re-radiated light. The agreement between the 2.2 micrometers polarization angle for IRS 21 and the 12.5 micrometers polarization angle at the position of IRS 21, the symmetric shape of its intensity profiles, as well as the similarity of the observed properties of all the Northern Arm sources, lead us to conclude that the scattering dust around IRS 21 is extrinsic to the central source and is associated with the Northern Arm.
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Angelle M. Tanner, Andrea M. Ghez, Mark Morris, Eric E. Becklin, A. Cotera, Michael E. Ressler, "Is the galactic center source, IRS 21, as large as it appears?", Proc. SPIE 4005, Discoveries and Research Prospects from 8- to 10-Meter-Class Telescopes, (29 June 2000); doi: 10.1117/12.390142; https://doi.org/10.1117/12.390142
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