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Appendix A: Detection of methanol

The residuals of the fit (presented in the bottom of Fig. A.1) clearly show that in addition to the two recombination lines, there is a narrow feature in the −77.4 km s^-1 channel. The width of the line is unresolved (≤7.4 km s^-1), suggesting that the emission comes from a molecular species. The upper limit to the width of the line is consistent with the values in the range of the 3 to 5 km s^-1 typically observed in molecular cores with massive star formation (Garay et al. 2010; Qiu et al. 2011). If we assume that this emission has the same LSR radial velocity as the ionized gas, it then has a rest frequency of 40.6346 GHz, in agreement with the expected frequency for torsionally excited CH₃OH (14_3,11–13_4,10)(v_t = 0) line emission, whose rest frequency is 40.6351 GHz (Xu & Lovas 1997). Emission of the CH₃OH molecule has been previously reported for this UC H ii region (see, e.g., Wyrowski et al. 1999; and Harvey-Smith & Cohen 2006). Since the emission from this line seems to be dominating the emission in the −77.4 km s^-1 channel, we plot the intensity map of this single channel in Fig. A.2. We see that the emission seems to be concentrated mainly in an extended component on the west side of W3(OH). Filamentary CH₃OH maser emission was found at similar positions inside W3(OH) by Harvey-Smith & Cohen (2006).

	Fig. A.1 Residuals of the fit, with a narrow feature evident at −77.4 km s-1.
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	Fig. A.2 W3(OH) moment 0 image of the radio emission from the −77.4 km s-1 channel. The contours are at −20, 20, 40, 60, and 80% of the peak level of 26.3 mJy beam-1 km s-1. Filled squares show the position of the CH3OH masers reported by Harvey-Smith & Cohen (2006) and coincides with the observed extended CH3OH emission.
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	Fig. A.3 W3(OH) moment 0 image of the radio emission from the CH3OH (143,12–134,9) transition. The image corresponds to the channel with VLSR = −47.65 km s-1. The contours are at −20, 20, 40, 60, and 80% the peak level of 29.5 mJy beam-1 km s-1. Filled squares show the position of the CH3OH masers reported by Harvey-Smith & Cohen (2006) and coincides with the observed extended CH3OH emission.
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To support the tentative identification of this methanol transition, we inspected our data for signs of emission from other methanol transitions. Our 2 GHz bandwidth includes the transitions CH₃OH (14_3,12–13_4,9), with rest frequency 40.40537 GHz, and CH₃OH (9_−4,6–10_−3,8) with rest frequency 41.11004 GHz. We proceed in a similar fashion to what we did for the recombination lines. Some emission was found for the CH₃OH (14_3,12–13_4,9) transition (see Fig. A.3), but emission from the transition CH₃OH (9_−4,6–10_−3,8) was not detected. The emission peaks of the CH₃OH (14_3,12–13_4,9) detection coincide with those of the excess emission at −77.4 km s^-1 in the H54α recombination line, suggesting a similar origin. Both CH₃OH (14_3,11–13_4,10) and CH₃OH (14_3,12–13_4,9) are methanol transitions of A-type, while the CH₃OH (9_−4,6–10_−3,8) is an E-type transition. For radiative transport purposes, these A- and E-type symmetry states can be considered as two different molecules (Leurini et al. 2004). Thus, this could explain why we detect similar intensities for the CH₃OH (14_3,11–13_4,10) and CH₃OH (14_3,12–13_4,9) lines, but not CH₃OH (9_−4,6–10_−3,8). We conclude that the residual line emission is due to the CH₃OH molecule.

Appendix B: Structure of the H54α and He54α emission

To show the spatial distribution of the emission of both, the H54α and the He54α lines, we plotted the moment 0 intensity maps for the H54α and He54α radio recombination lines in Figs. B.1 and B.2, respectively. The structure of the emission of both lines seems to be similar to the continuum map. However, the He54α emission is only marginally detected in this high angular resolution image.

We also obtained maps of the intensity-weighted velocity (moment 1) for the H54α line from the image cube with the full baselines (Fig. B.3). The velocity gradient for the whole W3(OH) UC H ii region is consistent with a champagne-like expansion.

The moment 0 for the highest resolution image of the H54α emission is shown in Fig. B.4. The emission from the compact source is clearly detected. Finally, the spectrum for the compact source and the least-squares fit to it are shown in Fig. B.5.

	Fig. B.1 W3(OH) moment 0 image of the radio emission from the H54α recombination line. Gray scales and contour scales images are superposed to highlight those zones with less significant emission inside the UC H ii region. The contours are at −20, 20, 40, 60, and 80% the peak level of 301 mJy beam-1 km s-1.
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	Fig. B.2 W3(OH) moment 0 image of the radio emission from the He54α recombination line. The contours are at −20, 20, 40, 60, and 80% the peak level of 50.7 mJy beam-1 km s-1.
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	Fig. B.3 W3(OH) moment 1 image of the radio emission from the H54α recombination line. The color bar to the right indicates the radial LSR velocity. The solid line shows the position and orientation of the slice where the position–velocity diagram shown in Fig. 3 was made.
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	Fig. B.4 W3(OH) moment 0 image of the radio emission from the H54α recombination line from the spectral image cutting baselines shorter than 6 km. The contours are at 20, 40, 60, and 80% the peak level of 111 mJy beam-1 km s-1.
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	Fig. B.5 Radio recombination line spectrum (solid line) for the compact source near the center of W3(OH). The parameters of the fit (dashed line) are discussed in the main text.
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© ESO, 2014