Exchange Report by Fanyi Meng

Background information on my doctoral project

My thesis focuses on high-mass star formation (sub-project A6). By observational studies on the large scale structure of the Sagittarius B2 (SgrB2) molecular cloud complex, this research aims at understanding the most massive high-mass star forming region in our Galaxy. SgrB2 is located at a distance of 8.5 kpc, at only 100 pc from an extreme environment as it is the Galactic Center. This turns SgrB2 in an ideal object to study star formation in an environment that resembles starburst galaxies and active galactic centers that dominate star formation across the Universe. 

Previous studies on SgrB2 mainly focused on the two hot cores, SgrB2(N) and SgrB2(M). However, the large scale (~22 pc) envelope surrounding the hot cores may also be actively forming stars, which is suggested by structures of dense gas found in the envelope (Martin-Pintado et al 1999, ApJ, 519, 667) as well as by the recent ALMA observations on dust continuum and molecular lines (PI: Adam Ginsburg). I am characterizing the envelope of SgrB2 by studying the ionized gas, dense gas and dust content. For this, I have led three successfully accepted VLA observational projects (16A195, 16B031, 17A061). The VLA dataset makes use of all the four configurations of the array (A, B, C and D) and covers the frequency range from 4 to 12 GHz. The covered area of the sky is about 14 arcmin (i.e. the whole envelope of SgrB2). The usage of multiple configurations allows recovering all the scales from 0.3 arcsec to 14 arcmin (after combining with available single-dish GBT data). 

Exchange experiences

Support through the SFB 956 doctoral exchange program enabled my visit to NRAO, Socorro, New Mexico and Charlottesville Virginia, USA between July and September 2017 for improving my scientific skills. 

At NRAO, Socorro, New Mexico I was trained by Adam Ginsburg how to self-calibrate the Very Large Array (VLA) data. We developed a method to fit the spectral index from the continuum data taken by VLA, by which we are able to further investigate the synchrotron emission in SgrB2 – even in the vicinity of our Galactic center – and infer how such synchrotron emission interplays with the high-mass star formation in the Galactic center. Also, under Ginsburg’s instruction, we successfully processed the Radio Recombination Lines, which enables further studies on the kinematics of SgrB2.

Urvashi Rao Venkata, an expert on spectral index analysis, gave me some very valuable instructions on how to increase the credibility of our spectral index results when doing calibration and imaging. Kumar Golap kindly helped me about technical details related to the software we used for data reduction in CASA.  Due to a discussion with Dale A. Frail I have improved my understanding of shocks, magnetic fields and synchrotron emissions.

On September 6, 2017 I gave a talk entitled: The Newly Discovered Ionized Gas in the Envelope of Sagittarius B2. After the talk Ron Ekers, who was visiting Socorro, gave me some suggestions how to improve the scientific analysis of my data. At NRAO, Charlottesville, Virginia I met William Cotton, an expert on data reduction of VLA data. Upon our request he applied his tool – named Obit – to process our VLA data and gave me most helpful advice about data reduction and spectral energy distribution fitting. On September 26, 2017 I gave a talk entitled: The Newly Discovered Ionized Gas in the Envelope of Sagittarius B2 at NRAO, Charlottesville.

 

Conclusions

The stay abroad was an important step towards a successful completion of my doctoral project and for my future career. First of all, I gained an improved understanding on the VLA data reduction processes by applying CASA better after the stay at NRAO. Furthermore, working together with people who substantially contributed to the development of CASA and VLA data reduction offered a closer look into the software details and science of the developing facility. This is important because the techniques possessed by NRAO are not fully understandable from documentations alone and quite useful for specific tasks related to my doctoral project. Because we are collaborating with Adam Ginsburg at NRAO since the beginning of my doctoral research, this visit to NRAO was essential for my project and sitting next to his office made a real-time training on data reduction and computer programming possible. Additionally, learning William Cotton’s software Obit, would have not been that easy without his patient instructions. He really sat next to me and demonstrated how to use Obit commands line by line.

I learned not only important techniques, but also had outstanding fruitful discussions. If I could go for the exchange program again, I would like stay longer and learn more.

Finally, I would like to express my sincere appreciation, in particular, to DFG and all persons mentioned in this report, but also to those who contributed to this successful stay behind the scenes.