Aperture synthesis imaging of partially ionized gas in W48

ONELLO, J. S.; PHILIPS, J. A.; BENAGLIA, P.; GOSS, W. M.; TERZIAN, Y.

ASTROPHYSICAL JOURNAL

Chicago University Press

Lugar: Chicago; Año: 1994 vol. 426 p. 249 - 253

0004-637X

We present the results of VLA observations of cool gas surrounding the H II region W48 at the H168 alpha and C168 alpha transition frequencies. Previous single-dish observations of W48 showed that the 168 alpha recombination spectrum was composed of a broad line from ionized hydrogen (H(+)) within the H II region, as well as strong, narrow lines (H(0), C(+), and S(+)) from gas outside the Stromgren sphere. High spatial resolution imaging of W48 reveals that the strong C(+) and S(+) lines arise from the stimulated amplification of the background H II region. Like the C(+) and S(+) lines, the narrow line from partially ionized hydrogen (H(0)) is strongest near the continuum peak of the H II region and suggests stimulated amplification of the line. The width of the H(0) line constrains the temperature of the partially ionized medium to be less than 200 K. However, the gas in which the H(0) line originates is found not to be spatially conincident with the carbon-emitting region. We suggest that the H(0) emission arises in the advancing ionization front of the H II region, while the C(+) and S(+) lines originate in a more extended region of the ambient molecular cloud partially ionized by photons with lambda greater than 912 A escaping the boundary of the Stromgren sphere.