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Title:Radial Location Of Sgr B2 Measured From H3+ And H2o+ Absorption Spectra
Author(s):Oka, Takeshi
Contributor(s):Geballe, Thomas R.
Abstract:The giant molecular cloud Sgr B2 in the $\sim{150}$ pc radius Central Molecular Zone (CMZ) of the Galactic center has played a central role in molecular radio astronomical observations. In spite of its large distance, over 70 molecules have been first detected in Sgr B2 since the discoveries of NH$_3$ and H$_2$O. Nevertheless there seems to be no consensus on the depth within the CMZ of this well observed cloud. It has not even been known whether Sgr B2, which is 101 pc distant on the plane of the sky from the central black hole Sgr A*, is to the front or the rear of Sgr A*. A trigonometric parallax measurement by Reid et al. (2009)\footnote{Reid, M. J., Menten, K. M., Zheng, X. W., Bunthaler, A. \& Xu, Y. 2009, ApJ, 105, 7548} placed Sgr B2 130 pc in front of Sgr A* which would put it outside of the CMZ. Both Kruijssen et al. (2015) and Ridley et al. (2017) also placed Sgr B2 in front of Sgr A*. On the other hand Molinari et al (2011)\footnote{Molinari, S., Bally, J., Noriega-Crespo, A. et al. 2011, ApJ, 735, L33} placed Sgr B2 about 70 pc behind Sgr A* based on their dust continuum measurements. \vspace{0.05in} Recent observations of infrared lines of H$_3^+$ and CO \footnote{Oka, T. \& Geballe, T. R. 2020, ApJ, 902, 9} and the conclusion from the analysis of their velocity profiles that diffuse gas in the CMZ is expanding radially at a velocity of 150 km s$^{-1}$ with the front of the expansion 150 pc from Sgr A* provides a useful tool for determining the location of Sgr B2 within the CMZ. Although currently there is no known infrared star usable for observations of H$_3^+$ toward Sgr B2, the star $\iota$, 2M17470898-2829561, 16.6 pc to the west of the center of Sgr B2, is sufficiently close that it probes much of the same gas; the similarity of the velocity profile of the H$_3^+$ spectrum toward $\iota$ observed at the Gemini South Observatory and of H$_2$O$^+$ \footnote{Schilke, P., Comito, C. M\"{u}ller, H. S. P., et al. 2010, A\&A, 521, L11} by Herschel Observatory toward Sgr B2 attest to this. Using the velocity profiles of infrared absorption lines of H$_3^+$ toward the star $\iota$ and far infrared absorption lines of H$_2$O$^+$, both of which molecular ions reside in diffuse clouds, we conclude that Sgr B2 is behind Sgr A* by at least 70 pc.
Issue Date:2021-06-22
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Date Available in IDEALS:2021-09-24

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