New class II methanol masers in W3(OH)

We report interferometric observations of nine class II methanol maser candidate lines toward W3(OH). Narrow maser emission spikes at ν_LSR = -43.1 km s^-1 are present in three of the lines: 3₁-4₀ A⁺, 7₂-6₃ A⁺, and 7₂-6₃ A^-. For all three lines the maser position is near the northern edge of the W3(OH) ultracompact H II region (maser emission is also seen near the southern edge in the 3₁-4₀ A⁺ line). For the remaining six lines there is no obvious counterpart to the narrow maser spike at -43.1 km s^-1. Additional spatially extended emission is present in all nine lines over the range from -41 to -48 km s^-1. By comparing our observed flux densities with an extensive set of model calculations, we infer physical characteristics of the maser region. In these calculations the methanol is excited by infrared radiation from warm dust, and this excited gas amplifies the free-free background emission from the ultracompact H II region. The gas forming the narrow maser spikes appears to have both high kinetic temperature, T_kin ≥ 110 K, and high density, n_H2 ≈ 10⁷ cm^-3. Low-temperature solutions are ruled out by the observed line ratios and low-density solutions by the unphysically large path length that would be required. The gas is rich in methanol (2N_M = N_A + N_E ≥ 10^-6N_H2), and the methanol column density in the tangential direction for each symmetry species (divided by line width) is N_M/ΔV ≈ 10¹² cm^-3 s. Somewhat lower values of n_H2 and N_M/ΔV are also acceptable. The size of the region emitting the maser spike is of order 100 × 1000 AU. In most of the lines the broad emission from -41 to -48 km s^-1 can also be attributed to weak maser action, produced in gas with similar physical conditions (high density and temperature). It differs from the narrow spike emission mainly through a beaming factor that can be interpreted as an elongation factor for clumps of maser gas. The combination of narrow and broad emission can arise naturally from an ensemble of clumps of different elongations and orientations. In this unified picture the best fit to the data is provided by n_H2 ≈ 2 × 10⁶ cm^-3 and N_M/ΔV ≈ 4 × 10¹¹ cm^-3 s, somewhat lower than the values obtained for just the spike component. The methanol maser clumps may be present in an expanding shell surrounding the H II region, similar to the material producing OH maser emission in this source.