Description of source "G18.34+1.78SW" from Kang et al. (2016)

G18.34+1.78SW is one of the brightest methanol masers known in the Galaxy and rst detected in the KVN single-dish maser surveys (K. T. Kim et al. 2016, in preparation). It is associated with millimeter-core MM2 in a massive star-forming region, IRAS 18151-1208 (Marseille et al. 2008). It is imaged by the KaVA at an angular resolution of 2 mas (Matsumoto et al. 2014).

The single-dish total intensity spectra of G18.34+1.78SW show three maser features with peak velocities of +30.3 (A) +29.6 (B), and +31.4 km s1 (C in Figure 9(b))in both transitions, all of which show linearly polarized emission in both transitions (see also Figure 4(f)). The velocities of these three components agree well with the ndings of Matsumoto et al. (2014), although much of the total ux was missed in the VLBI image. The polarization properties of these three components are summarized in Table 6. We used a function with three Gaussian components to t the 0.1 km s1 resolution spectra. The line widths measured in the polarization prole range between 0.4 and 0.6 km s1, while it is 0.2 km s1 wider when measured in the total ux prole.

This object has P = 2.0%-6.8% in two transitions, and all three components show slightly higher polarization at 95 GHz, which is generally observed for other sources. The polarization angles of the brightest peak are similar at 44 and 95 GHz, i.e., 12 different, but the difference is beyond the measured error.

The angle difference ranges up to 43 in the second brightest peak. The fact that the polarization angle differences are not consistent for the three maser features at nearly the same position, at most 100 mas apart (Matsumoto et al. 2014), indicates that neither rotation measure nor the beam size can explain the difference in the polarization properties of the 44 and 95 GHz transitions. It may be a combination of the magnetic eld morphology and the saturation level difference between the two transition lines, as discussed in Section 4.2.

High-resolution line polarimetry using the VLA or ALMA will help us to understand the underlying physics