Exchange Report by Anika Schmiedeke

Supervising PI: Prof. Dr. Peter Schilke
SFB 956 project: A6
Duration of stay abroad: 15.09.2013 - 12.12.2013
Hosting institution: California Institute of Technology, USA Hosting supervisor: Dr. Darek Lis

Introduction

As part of my doctoral project, I am developing a routine that uses a detailed three-dimensional radiative transfer modeling approach to determine the physical structure, i.e. the density distribution, the gas and dust temperature, as well as the velocity and abundance fields of star forming regions by comparing the simulations to observed data.

This routine is used to characterize the well-known molecular cloud complex Sagittarius B2 (SgrB2) close to the Galactic Center. This very dense and massive cloud with a radius of 22.5 pc at a distance of 7.5 kpc contains two hot molecular cores, SgrB2-N and M, that are actively forming high-mass stars.

For SgrB2 full line surveys towards the hot cores have been obtained with the Herschel Observatory (HEXOS project, PI E. Bergin, U.Michigan). The SgrB2-M subprogram is coordinated by Peter Schilke, the SgrB2-N sub-program by Darek Lis, with close interaction between both programs. Interferometric maps towards both hot cores are available from the Submillimeter Array. Thus the HEXOS spectral surveys are providing the necessary variety both in molecules and covered wavelength range, while the complementary SMA data provides the necessary spatial resolution for this modeling approach. Before the exchange, both hot cores were treated separately, and I was only working on SgrB2-M as a complex test case for the modeling procedure. However, all attempts to model CO towards SgrB2-M already indicated, that the whole envelope would need to be included in the model. So the goal of the 10 week project was to extend the model to include both hot cores as well as the envelope. Determining the physical properties, especially the kinematics of the whole cloud complex will give us invaluable insights and help us determining the fate of the SgrB2 molecular cloud.

Experiences

To achieve the goal, the first task was to modify and extent the existing modeling routine. I in-cluded the possibility to (a) obtain sub-maps around and spectra towards several positions in the model and (b) specify a number of cluster centers where lowmass stars are sprinkled around. I then compiled a list of known Hii regions in SgrB2 from the literature and available catalogues and included them with their embedded high-mass star in the model. I iteratively adjusted the density distribution to match available maps from large scale continuum observations (ATLASGAL at 850 μm, HiGAL Spire at 500, 350 and 250 μm). One of the main conclusions from this step is, that SgrB2(N) must be placed ∼5 pc along the line of sight behind the plane containing SgrB2(M). Otherwise it is not possible to reproduce the continuum intensities of the hot cores at the different wavelengths properly.

Towards the end of the project, I already started obtaining the first CO spectra towards both hot cores. However, as one run took ∼ 30 h and the radiative transfer code was not yet parallelized at that time, only a few iterations were possible and effectively working on the velocity field was impossible. During my stay at Caltech, I have given a lunch seminar talk presenting the modeling approach and the first results for the complex model to the Caltech/JPL community. I received very useful feedback and various possibilities of the three-dimensional modeling were discussed, that helped me further developed means of presenting these highly complex three-dimensional models.

The Combined Array for Research in Millimeter-wave Astronomy (CARMA)

While being at Caltech, I got involved in the CARMA Orion project led by John Carpenter. This project aims at observing a one square degree map of Orion at 3 mm. I’ve been calibrating all incoming tracks from September 15th on and thus gained a lot of experience in interferometric data calibration and imaging. In order to gain a deeper under-standing of interferometric observations in general and CARMA specifically, I went to CARMA for one week in November.  

The interferometer is located in the Inyo Mountains in eastern California. In a collaboration of five US universities, three small arrays have been combined to build CARMA. It now consists of 23 antennas ranging from 3.5 meter to 10.4 meter, which can be operated in two separate subarrays, with one subarray containing the 15 larger antennas and the other subarray containing the eight 3.5 meter antennas. This is why some antennas are looking to the left side in the right picture and the other to the right side. 

There are always two observers on-site keeping an eye on everything and responding to problems. Normal operations are very much straight forward. The principal investigators, whose projects were accepted are providing scripts detailing the observations. So the observers simply have to add the script to the observing queue, write for how long the script should run (usually 4-8 hours) and press start.  So to keep the observers from having to monitor the computers actively in case anything goes wrong (and most of the time something does), the computer sets off an alarm throughout the building in case of an error. At the time of my shift, the alarm was set to play the song of "Two dragons" by Theodore Shappiro from the Starsky and Hutch Soundtrack. Since the observations are carried out 24-7, the alarms can go off at all times. The picture to the left was just taken because an alarm woke me at 6 am.

During this week we had a lot of different technical problems (and thus many alarms), e.g. an antenna got stuck in elevation, a correlator band dropped out due to a mal-functioning chassis and the impossibility of re-tuning the receivers without resetting the system due to a software bug in the operational system introduced after an software upgrade. While this is in general bad for normal operations, fixing each problem helped me further understand the hardware and the inner workings of the interferometer. The picture to the right shows a part of the correlator room.

Travels

On Thanksgiving, I've been visiting colleagues and friends in Flagstaff, Arizona. We went to see the Grand Canyon and were fascinated by the deep layer of clouds in the Canyon, that was due to a temperature inversion. In the picture below, you can see the North Rim just above the clouds.

Conclusions

Caltech provided a very focused, science oriented working environment. Working there for 10 weeks with Darek Lis was a very good experience for me and I gained a lot from discussions not only with him, but also with e.g. my office mate Michal Drahus and during the various talks and meetings held at Caltech. Observing at the CARMA interferometer was an excellent opportunity for me to further my knowledge about interferometry and to gain hands-on experience with observations and data calibration. Concerning the timing of the exchange, I was very lucky. I was staying at the student dormitory and since the semester started a week after I had arrived, it was especially easy to meet people. 

So altogether, the exchange was a very good start to go from small scales, i.e. the hot cores, to large scale, i.e. the whole cloud complex consistently with one single model setup, which is the basis for an important part of my doctoral thesis.