The physical and chemical structure of Sagittarius B2

Abstract

Context. The giant molecular cloud Sagittarius B2 (hereafter Sgr B2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of Sgr B2, we encounter the Sgr B2(DS) region, which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended H II region. Hints of non-thermal emission have been found in the H II region associated with Sgr B2(DS). Aims. We seek to characterize the spatial structure and the spectral energy distribution of the radio continuum emission in Sgr B2(DS). We aim to disentangle the contribution from the thermal and non-thermal radiation, as well as to study the origin of the non-thermal radiation. Methods. We used the Very Large Array in its CnB and D configurations, and in the frequency bands C (4–8 GHz) and X (8–12 GHz) to observe the whole Sgr B2 complex. Continuum and radio recombination line maps are obtained. Results. We detect radio continuum emission in Sgr B2(DS) in a bubble-shaped structure. From 4 to 12 GHz, we derive a spectral index between − 1.2 and − 0.4, indicating the presence of non-thermal emission. We decomposed the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium but stimulated by the non-thermal emission. Conclusions. Sgr B2(DS) shows a mixture of thermal and non-thermal emission at radio wavelengths. The thermal free–free emission is likely tracing an H II region ionized by an O 7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the Sgr B2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.

Self-modulation of cosmic rays in molecular clouds: Imprints in the radio observations

“…While its average column density is about N CMZ H2 , in the central region of the Sgr B2 within 5 pc × 2.5 pc we have N B2 H2 = 10 24 − 10 25 cm −2 (Schmiedeke et al 2016), i.e., 1-2 orders of magnitude higher than the average CMZ value. Protheroe et al (2008) (see also Jones et al 2011;Meng et al 2019) found that the radio emission from the central region of Sgr B2 is dominated by thermal emission of HII gas at frequencies above ∼ 0.3 GHz, as shown in Figure 2. Thus, unlike the CMZ, no indications of synchrotron-like emission from the Sgr B2 region were observed.…”

Magnetism Science with the Square Kilometre Array

“…Magnetic fields in the ISM surrounding star-forming regions both influence the evolution of and may also be impacted by H II regions [129,170]. The SKA will be able to probe the detailed physical conditions in and around H II regions [171,172]. Supernova shock waves appear to amplify the magnetic fields of the upstream ISM significantly.…”

CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

“…A dense gas CRIR similar to that inferred in diffuse gas may necessitate a source of low-energy CRs within the gas. Some of these sources have already been posited, such as protostellar jets (Padovani et al 2016), protostellar accretion shocks (Padovani et al 2016;Gaches & Offner 2018), HII regions (Meng et al 2019;Padovani et al 2019) and embedded stellar winds (Yang & Wang 2020). These are localized sources of CRs acceleration that are expected to induce rather inhomogeneous distribution of CRs within molecular clouds.…”