The physical and chemical structure of Sagittarius B2

VI. UCHii regions in Sgr B2^★

¹ University of Chinese Academy of Sciences, Beijing 100049, PR China
e-mail: mengfanyi@ucas.ac.cn;meng@ph1.uni-koeln.de
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Department of Astronomy, University of Florida, PO Box 112055 Gainesville, FL, USA
⁴ NRAO, 520 Edgemont Rd, Charlottesville, VA 22903, USA
⁵ Agnes Scott College, 141 E. College Ave., Decatur, GA 30030, USA
⁶ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁷ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 30 March 2022
Accepted: 8 July 2022

Abstract

Context. The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact Hii (UCHii ) regions were identified in SgrB2’s central hot cores, SgrB2(M) and SgrB2(N).

Aims. Our aim is to characterize the properties of the Hii regions in the entire SgrB2 cloud. Comparing the Hii regions and the dust cores, we aim to depict the evolutionary stages of different parts of SgrB2.

Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (~6GHz) to observe the whole SgrB2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCHii regions and their dense gas environment.

Results. We identify 54 UCHii regions in the 6 GHz image, 39 of which are also detected at 22.4 GHz. Eight of the 54 UCHii regions are newly discovered. The UCHii regions have radii between 0.006 pc and 0.04 pc, and have emission measure between 10⁶ pc cm⁶ and 10⁹ pc cm⁶. The UCHii regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of ~10⁶–10⁹ cm³ around the UCHii regions. The pressure of the UCHii regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHii regions is determined to be ~10⁴–10⁵ yr. The percentage of the dust cores that are associated with Hii regions are 33%, 73%, 4%, and 1% for SgrB2(N), SgrB2(M), SgrB2(S), and SgrB2(DS), respectively. Two-thirds of the dust cores in SgrB2(DS) are associated with outflows.

Conclusions. The electron densities of the UCHii regions we identified are in agreement with that of typical UCHii regions, while the radii are smaller than those of the typical UCHii regions. The dust cores in SgrB2(M) are more evolved than in SgrB2(N). The dust cores in SgrB2(DS) are younger than in SgrB2(M) or SgrB2(N).