We chose the molecular clouds in the Gould Belt for this study.They are nearby (150–500 pc; Loinard et al. 2011), thus providing the highest possible spatial resolution, and most of them are located at high Galactic latitudes (|b| > 10?). Since stellar polarization samples the B-fields along the entire line-of-sight weighted by dust density, it is important to disentangle the molecular clouds in question from physically unrelated material along the line-of-sight. At high Galactic latitudes, the contribution to the total polarization from dust that is not related to the target cloud is relatively small (see Section 4.1 for more discussion about optical polarimetry) simply because there is less dust at higher latitudes. Our sample clouds are listed in Table 1. We attempted to study all the 13 clouds1 involved in the Herschel Gould Belt Survey (Andr′e et al. 2010), but there is no data available for the B-fields of Polaris flare. As wewant to test the cloud formation scenarios by comparing the orientations of clouds and ambient ICM B-fields, in the following we describe how these orientations are determined.
By with FANG Min
Upper panel: an illustration of the fact that galactic B-fields (red lines) follow spiral arms (dark dashed arrow) and anchor clouds (Li et al. 2009; Li & Henning 2011), but have rich structures perpendicular to the galactic disc, as shown in Figs 1 and 2. Compared to line of sight 1, line of sight 2 goes through less galactic mass other than one particular cloud. Stars with larger distance provide averaged field directions for larger scales. Lower panel: we sample the stars with reliable polarization detections from the 100-pc bins centred at distance 100, 300, 700, 1500 and 2500 pc. The plots show that the Galactic B-field is more coherent at larger scales (>700 pc), but is almost random at the scale ～100 pc. Also plotted is the distribution of the B-fields from cloud cores (dashed line) at pc to subpc scales probed with thermal dust emission (Dotson et al. 2010). The core field distribution is very similar to that of Galactic fields at 100–300 pc scales, the size of the accumulation length of a typical molecular cloud.
The work by Hua-bai Li, Min Fang,etc. has been accepted to be published in the MNRAS (December 21, 2013) 436 (4): 3707-3719. please see (http://mnras.oxfordjournals.org/content/436/4/3707.full.pdf+html).
