Volume 630, October 2019
|Number of page(s)||14|
|Section||Interstellar and circumstellar matter|
|Published online||24 September 2019|
The physical and chemical structure of Sagittarius B2
V. Non-thermal emission in the envelope of Sgr B2
I. Physikalisches Institut, Universität zu Köln,
Zülpicher Str. 77,
2 INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
3 Laboratoire Univers et Particules de Montpellier, UMR 5299 du CNRS, Université de Montpellier, place E. Bataillon, cc072, 34095 Montpellier, France
4 Jansky Fellow of the National Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro, NM 87801, USA
5 Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
6 Agnes Scott College, 141 E. College Ave., Decatur, GA 30030, USA
Accepted: 15 August 2019
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.
Key words: stars: formation / ISM: individual objects: Sgr B2 / radio continuum: ISM / radio lines: ISM / ISM: clouds / stars: massive
© ESO 2019
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.