ANATOMY OF THE SAGITTARIUS-A COMPLEX .4. SGR-ASTERISK A AND THE CENTRAL CAVITY REVISITED

We present submm images of Sgr A and its surroundings obtained at lam bda 800, 600 and 450 mu m with the James Clerk Maxwell Telescope, JCMT (1), and derive flux densities of Sgr A at all three wavelengths. Com bined with upper limits by Gezari and associates at MIR wavelengths a time averaged radio spectrum is obtained which increases proportional to nu(1/3), attains a maximum at nu(max) greater than or similar to 60 0 GHz and must decrease rapidly at frequencies greater than or similar to 10(4) GHz. This spectrum allows for about 3 M. of 50K dust and ass ociated hydrogen in the telescope beam. While variations on time-scale s of a few months are now well established for the frequency range nu less than or similar to 100 GHz, our investigation of variability at h igher frequencies still yields only marginal results. The Circum-Nucle ar Disk (CND) extends over the central 12pc. At the galactocentric rad ius R similar to 1 pc dust and hydrogen column densities drop to low v alues and form the Central Cavity. Our submm images show that the bott om of this cavity is rather flat. Variations in the dust emission are just consistent with the detection of a 'Tongue' of similar to 200 M. of atomic hydrogen reported by Jackson et al. (1993) to be located bet ween the Northern and the Eastern Arm of the Minispiral. We present a revised submm/IR spectrum of the central 30 '' (R less than or similar to 0.6 pc) with flux densities corrected for an interstellar extincti on of A(upsilon) similar to 31 mag. This spectrum attains its maximum at lambda similar to 20 mu m and comes from dust with temperatures sim ilar to 170 - 400 K which is associated with the Eastern Arm and the E ast-West Bar. The integrated luminosity is similar to 5 106 Lo to whic h emission at lambda less than or similar to 30 mu m contributes simil ar to 80%. Heating of this dust is not provided by a central source bu t rather by a cluster of hot (T-eff similar to 3 - 3.5 10(4) K) and lu minous stars which could include the HeI/HI-stars detected by Krabbe e t al. (1991). Their total luminosity within R less than or similar to 0.6 pc must be similar to 2 10(7) L. and their Lyman continuum photon production rate N-Lyc similar to 1.3 10(51) s(-1). Outside the Central Cavity the principal sources of excitation appear to be medium mass s tars and late-type O-stars. IR- and Lyc-photon luminosities increase p roportional to R(1.2) while the dust temperature decreases proportiona l to R(-0.15). Within the Nuclear Bulge (R less than or similar to 250 pc) the IR-luminosity (similar to 10(9) L.), the production rate of L yman continuum photons (similar to 2 10(52) s(-1)) and the mass of mol ecular hydrogen (similar to 10(8) M.) amount to similar to 10% Of the corresponding characteristics in the Galactic Disk. Hence the formatio n rate of massive stars per unit mass of molecular hydrogen and the st ellar population in Bulge and Disk must be comparable. Comparison with model computations show that the formation of the Central Cavity is a consequence of the structure of the gravitational potential, which is dominated for R greater than or similar to 1 pc by stars of the Nucle ar Bulge and for R less than or similar to 1 pc by a compact object of a few 10(6) M.. Model computations for a black hole/accretion disk co nfiguration with an accretion rate of 10(-6...-7) M.yr(-1) indicate th at the accretion disk becomes gravitationally unstable for greater tha n or similar to 1.5 - 5 10(4) Schwarzschild radii. A more massive disk with a correspondingly higher accretion rate thus could account for t he formation of the central cluster of HeI/HI and - possibly - O-stars which appear to be responsible for most of the excitation of the cent ral pc.