The physical and chemical structure of Sagittarius B2

Abstract

Context. Sagittarius B2 (Sgr B2) is a giant molecular cloud complex in the central molecular zone of our Galaxy hosting several sites of high-mass star formation. The two main centers of activity are Sgr B2(M) and Sgr B2(N), which contain 27 and 20 continuum sources, respectively. Our analysis aims to be a comprehensive modeling of each core spectrum, where we take the complex interaction between molecular lines, dust attenuation, and free-free emission arising from H II regions into account. In this work, which is the first of two papers on the complete analysis, we determine the dust and, if H II regions are contained, the parameters of the free-free thermal emission of the ionized gas for each core, and derive a self-consistent description of the continuum levels of each core. Aims. Using the high sensitivity of ALMA, we characterize the physical and chemical structure of these continuum sources and gain better insight into the star formation process within the cores. Methods. We used ALMA to perform an unbiased spectral line survey of all 47 sources in ALMA band 6 with a frequency coverage from 211 to 275 GHz. In order to model the free-free continuum contribution of a specific core, we fit the contained recombination lines to obtain the electron temperatures and the emission measures, where we use an extended XCLASS program to describe recombination lines and free-free continuum simultaneously. In contrast to previous analyses, we derived the corresponding parameters here not only for each core, but also for their local surrounding envelope, and determined their physical properties. Results. The distribution of recombination lines we found in the core spectra closely fits the distribution of H II regions described in previous analyses. In Sgr B2(M), the three inner sources are the most massive, whereas in Sgr B2(N) the innermost core A01 dominates all other sources in mass and size. For the cores we determine average dust temperatures of around 236 K (Sgr B2(M)) and 225 K (Sgr B2(N)), while the electronic temperatures are located in a range between 3800 and 23 800 K. Conclusions. The self-consistent description of the continuum levels and the quantitative description of the dust and free-free contributions form the basis for the further analysis of the chemical composition of the individual sources, which is continued in the next paper. This detailed modeling will give us a more complete picture of the star formation process in this exciting environment.