654 GHz continuum and C 18O(6-5) observations of G240.31+0.07 with the submillimeter array

Abstract

We report a dual-band observation at 223 and 654 GHz (460 μm) toward an ultracompact (UC) H II region, G240.31+0.07, using the Submillimeter Array. With a beam size of 1.5″ × 0.8″, the dust continuum emission is resolved into two clumps, with clump A well coincident with an H 2O maser and the UC H II region. The newly discovered clump, B, about 1.3″ (≃8.3 × 10 3 AU) to the southwest of clump A, is also associated with H 2O masers and may be a more recent star-forming site. The continuum flux densities imply an opacity spectral index of β = 1.5 ± 0.3, suggestive of a value lower than the canonical 2.0 found in the interstellar medium and in cold, massive cores. The presence of hot (≃100 K) molecular gas is derived by the brightness ratio of two H 2CO lines in the 223 GHz band. A radial velocity difference of 2.5 ± 0.4 km s -1 is found between the two clumps in C 18O(6-5) emission. The total (nebular and stellar) mass of roughly 58 M ⊙ in the central region is close to, but not much larger than, the minimum mass required for the two clumps to be gravitationally bound for binary rotation. Our continuum data do not suggest a large amount of matter associated with the H 2 knots that were previously proposed to arise from a massive disk or envelope. © 2007. The American Astronomical Society. All rights reserved.