WSRT AND VLA OBSERVATIONS OF THE 6 CENTIMETER AND 2 CENTIMETER LINES OF H_2CO IN THE DIRECTION OF W3

摘要

Formaldehyde absorption toward the H Ⅱ regions in W3 has been imaged at 6 cm with the West-erbork Synthesis Radio Telescope and at 2 cm with the Very Large Array with an angular resolution of ～5″ and a velocity resolution of ～0.6 km s~(-1). The opacities in both transitions are low, ranging between 0.05 and 0.3. The LSR velocities are around -39 km s~(-1) toward the eastern component of the molecular core which includes the H Ⅱ regions W3 A and W3 B and between -45 and -42 km s~(-1) toward the western component which includes W3 C and W3 D. The typical line widths are 1-3 km s~(-1) except toward the H Ⅱ region W3 D (FWHM ～ 5 km s~(-1)). For this source, a "champagne flow" at the edge of the molecular cloud may explain both the wide formaldehyde lines and the gradient in the recombination lines. Typical molecular hydrogen densities are ～10~5 cm~(-3), in agreement with earlier estimates from observations of other molecules. The abundance of formaldehyde relative to molecular hydrogen is (1-2) x 10~(-9) except toward W3 D where it is ≥ 5 x 10~(-9). The formaldehyde absorption toward the shell-like H Ⅱ region W3 A is strongest toward the south/ southwest edge of the continuum source and appears to arise from a thin, compressed layer. This layer may have been compressed by the expansion of the ionized gas into part of the eastern molecular core.