Observational study of the role of magnetic fields in star formation

Hakobian, Nicholas

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https://hdl.handle.net/2142/45513 Copy

Description

Title

Observational study of the role of magnetic fields in star formation

Author(s)

Hakobian, Nicholas

Issue Date

2013-08-22T16:42:45Z

Director of Research (if dissertation) or Advisor (if thesis)

Crutcher, Richard M.

Doctoral Committee Chair(s)

Crutcher, Richard M.

Committee Member(s)

• Gammie, Charles F.
• Looney, Leslie W.
• Troland, Thomas

Department of Study

Astronomy

Discipline

Astronomy

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• astronomy
• star formation
• magnetic fields
• molecular clouds

Abstract

"This project is a multi-faceted approach to establish a link between proposed theories of star formation and direct observation. Some key factors explored include: if magnetic fields cause significant support against gravitational collapse, which physical parameters are sampled by different tracer molecules, and what these results tell us about the structure and development of the regions observed. Until recently, only ambipolar diffusion theory had numerical models that simulated possible physical results that could be compared to observational data. These theories interpret the models in terms of a physical parameter: the ratio of the mass to the magnetic flux (M/$\Phi$). Performing measurements of the magnetic field, to determine the magnetic flux ($\Phi$), is complicated. It is only possible to obtain direct measurements of the strength of the line-of-sight component of the magnetic field through the normal Zeeman effect. Only a few molecules have Zeeman splitting factors large enough to be successfully used to measure magnetic fields. Of these few, OH traces lower density molecular species, while CN is believed to trace higher density regions. Using the Green Bank Telescope (GBT) we mapped the magnetic fields of cores and envelopes of dark cloud cores using OH as a tracer molecule. From this, the ratio of M/$\Phi$ between the cores and envelopes were computed, and were consistently determined to be < 1. This is inconsistent with published ambipolar diffusion theory which expects this ratio to be > 1. This study can be extended to other types of objects by using different tracer molecules. CN can be used to probe hot dense regions; however, it requires high resolution mapping currently only obtainable with an interferometer. Using CARMA, we obtained maps of 6 high mass star formation regions with a spatial resolution of approximately 2"" by combining data from the C, D, and E arrays. CARMA's correlator was used to sample several spectral lines simultaneously in order to compare the structure of the CN emission with emission of other tracer molecules. We determined that CN is a good high density tracer which correlates well with other tracers such as HCO+ and HCN. From this, we concluded that these regions can be observed at high resolution and long integration times with an instrument capable of performing CN Zeeman measurements, when such an interferometer array becomes available. A study was additionally performed to apply the Li & Houde method of estimating magnetic field strengths from linewidth differences of ion and neutral molecular species. We were unable to replicate the previously published results; however, there were several differences in the datasets that may contribute to the non-detection of this effect. Several possible reasons for this discrepancy were determined, and further investigation may be able to determine whether this analysis technique holds significant merit."

Graduation Semester

2013-08

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http://hdl.handle.net/2142/45513

Copyright and License Information

Copyright 2013 Nicholas Szandor Hakobian

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