2 millimeter dust around NGC 7538-IRS 1, 2, and 3: I. Unfilled structure of dust cloud

A 2-mm continuum mapping observation around NGC 7538 - IRS 1, 2, and 3 was made by the Nobeyama Bolometer Array (NOBA) with a spatial resolution of 12 ". An extended source of similar to 32" x 44" (Delta alpha x Delta delta), having a hardly resolved central core structure, was observed. The core giv es a flux density of 3.1 Jy +/- Jy, and is clearly identified with the free -free emission from the compact H II regions. A smooth and extended source gives an integrated flux density of 5 Jy +/- 1.5 Jy, and an intensity spect rum of nu (3)-nu (3.5) in the millimeter wave region. The fitting of a gray body function to the observational data shows that an absorption index, be ta, of 1.8-2.0, a dust filling factor, eta, of 0.3-0.5, and a dust temperat ure, T-d, of 34 K-45 K are reasonable parameters for this 2-mm extended dus t. Astronomical silicate grains were examined for a dust model. The total d ust mass and luminosity were obtained as M-d similar or equal to 0.85 x 10( 2) M-circle dot and L-d similar or equal to 2.2 x 10(5) L-circle dot, respe ctively. The extent of 2-mm dust emission partly coincides with those of th e 100-mum map, of the 57-mum map, and of the total intensity of the wing pa rts of the CO (J = 1-0) line emission of the region. The absorbing silicate dust layer must be distinct from the extended 2-mm dust, and must arise on smaller scales than the extended 2-min dust.