Universität zu Köln

Fanyi, Meng ORCID: 0000-0002-5927-2049 (2020). Multi-scale observational study of Sagittarius B2. PhD thesis, Universität zu Köln.

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Abstract

The giant molecular cloud Sagittarius B2 (SgrB2) is the most massive (~ 10^7 Msun) region with ongoing high-mass star formation in the Galaxy. SgrB2 has a higher density (>10^5 cm^{-3}) and dust temperature (~50--70 K) compared to other star forming regions in the Galactic plane. Additionally, SgrB2 is located at a projected distance of only ~100 pc to the Galactic center. These features make SgrB2 an excellent case to study high-mass star formation in an extreme, high-pressure environment. Such an environment resembles nearby starburst galaxies. Understanding the structure of the SgrB2 molecular cloud complex is necessary to comprehend the most massive star forming region in our Galaxy, which at the same time provides an unique opportunity to study in detail the nearest counterpart of the extreme environments that dominate star formation in the Universe. At small scales (0.01--0.1 pc), dense cores appear spread throughout the whole region of SgrB2, embedded within a large scale (~20 pc) envelope with two main sites of high-mass star formation activity at the center of the cloud: SgrB2(M) and SgrB2(N). In order to characterize the properties of SgrB2 from small to large scales, I started an observational project covering spatial scales from 0.004 pc (1000 au) up to 20 pc (the size of the envelope of SgrB2). At small scales, I studied the physical properties of 308 dense cores distributed throughout the entire SgrB2 cloud. I combined mm wavelength data and cm wavelength data to characterize the properties of dense cores and their associated Hii regions. Among the 308 compact dust cores that are identified, 58 are found associated with Hii regions, and 49 are associated with outflows. The cores have a mean mH2 of 150--2500 Msun, by assuming various dust properties and gas temperatures. Most of the 58 Hii regions are ionized by B0 stars. At intermediate scales, I studied the SgrB2(DS) Hii region, which is located in the southern part of the envelope. Using VLA data from 4 to 12 GHz, I derived a spectral index between -1.2 and -0.4, suggesting that SgrB2(DS) is a mixture of thermal and non-thermal emission at radio wavelengths. The thermal free-free emission is likely tracing an Hii region ionized by an O7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. A model of the Sgr B2(DS) region was developed, which reveals that first-order Fermi acceleration can reproduce the observed flux density and spectral index. At the largest scales, I characterized the physical properties of all the Hii regions in the envelope as well as the kinematic structure of the envelope, using newly acquired VLA cm wavelength data and ALMA HC3N line data. The volume filling factor of ionized gas in the envelope of SgrB2 is ~0.03-0.01. The Hii regions in the envelope are more extended than those in SgrB2(M) and SgrB2(N), suggesting that the Hii regions in the envelope may be older than those in the central regions SgrB2(M) and SgrB2(N), and therefore, indicating that high-mass star formation might have started throughout the envelope before than in the central regions. The envelope is also filled with arcs and bubbles, which are traced by HC3N. In summary, this study reveals the properties of SgrB2 from the scale of dense dust cores to the large envelope. The dense cores show various evolutionary stages in terms of high-mass star formation activity. The envelope resembles a ``swiss cheese'' and hosts star forming activities that may have started before those in the central parts of SgrB2. Additionally, for the first time, a model reveals that synchrotron emission from Hii regions can be due to locally produced relativistic electrons accelerated by shocks with moderate velocities.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Fanyi, Meng meng@ph1.uni-koeln.de orcid.org/0000-0002-5927-2049 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-300034
Date:	15 December 2020
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language High-mass star formation English ISM English Sgr B2 English Radio continuum English Molecular lines English Interstellar dust English
Date of oral exam:	14 September 2020
Referee:	Name Academic Title Schilke, Peter Prof. Dr. Ossenkopf-Okada, Volker PD Dr.
Funders:	Collaborative Research Centre 956, sub-projects A6, Deutsche Forschungsgemeinschaft (DFG) - project ID 184018867
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/30003