The Tai Chi in Star Formation by Hua-bai Li

Author:Hua-bai Li
Language: eng
Format: epub
ISBN: 9781681742939
Publisher: IOP Publishing
Published: 2017-10-07T00:00:00+00:00

Figure 3.9. The plane-of-sky magnetic field morphology (arrows) of a sun spot based on Zeeman linear polarization. Here the Zeeman measurement is made with the absorption line of Fe at 6302.5 Å. The frequency splitting allows one to estimate the line-of-sight field strength (greyscale) varying between ±2500 Gauss. (Reproduced with permission. Copyright NAOJ.).

The same strategy, unfortunately, does not apply to molecular clouds, because the Zeeman line splitting, , is much smaller than the line broadening, Δv, due to turbulence and thus it is impossible to separate the π and σ components. Note that the elliptically polarized σ components have a linearly polarized portion in the direction perpendicular to the B-field projection, which is opposite to the π component. The net linear polarization of the merged π and σ components can be either parallel or perpendicular to the B-field projection, which is called the Goldreich–Kylafis effect (Goldreich and Kylafis 1981), depending on γ and the different opacities due to the frequency difference ∆f. While the 90 degree ambiguity prevents it from mapping field morphologies, Li and Henning (2011) showed that the Goldreich–Kylafis effect can still help in distinguishing the morphologies between those with a random and with a well-defined mean direction. Their argument is that a random field will still appear random with the 90 degree ambiguity, while an ordered field will appear with two directional distribution peaks 90 degrees apart. They observed line polarization from six molecular cloud complexes in Galaxy M33 (figure 3.1) and found that the CO polarization tends to be either aligned or perpendicular to the spiral arms (figure 3.10). Based on the argument above, they concluded that the spiral arm B-field is anchored into molecular clouds. This is in agreement with Milky Way observations (figure 3.7) but more informative, because the spiral arm direction is not observable from our edge-on point of view of the Milky Way.

Figure 3.10. Top: CO(2–1) maps and polarization vectors. The cloud complexes are from Galaxy M33 (positions shown in figure 3.1). The contours are 90%, 80%, …, 10% of the peak intensity of each cloud (from dark to light grey). The red vectors show the polarization detections, and the thick grey vectors show the tangents of the local spiral arms. The ellipses indicate the Submillimeter Array synthetic beams. The coordinate (right ascension, declination) offsets are in arcseconds. Bottom: distribution of the CO polarization-arm offsets. The offsets are from the difference between the red and grey vectors in the upper panel. N is the number of offsets falling in each bin. Contributions from different giant molecular clouds (GMCs) are indicated by color. The distribution can be fitted by a double-Gaussian function with a standard deviation of 20.7° ± 2.6° and peaks at −1.9° ± 4.7° and 91.1° ± 3.7° (peaks shown by vertical black lines). The directions of synchrotron polarization from the regions near the GMCs (within one beam size, as shown in figure 3.1) are also shown as dashed lines, with the same color code as the GMCs. Reproduced from Li and Henning (2011) by permission of Springer.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.