Physical and chemical variations within the W3 star-forming region. I. SO2, CH3OH and H2CO.

Submillimeter observations of SO2, CH3OH and H2CO towards three positions in the W3 star-forming molecular cloud (IRS4, IRS5 and W3(H2O)) are presented. Significant differences in the physical and chemical conditions between the three sources are found. The gas temperature appears highest with T approximately equals 220 K towards W3(H2O), followed by T approximately equals 100 K towards IRS5 and T approximately equals 55 K towards IRS4. Densities are 10⁶/cu cm or larger. The chemical composition of W3(H2O) with strong CH3OH lines resembles that of a 'hot core' type region. In contrast, W3IRS5 appears at an earlier evolutionary stage where complex molecules are still depleted onto grains, but where energetic events have produced strong SO2 emission. W3IRS4 is the most evolved region with a chemistry similar to that found in quiescent molecular clouds. Here the time scale since star formation has probably been long enough for the gas-phase ion-molecule chemistry to become dominant again.