SIMBAD references

We present submm images of Sgr A* and its surroundings obtained at λ800, 600 and 450µm with the James Clerk Maxwell Telescope, JCMT, and derive flux densities of Sgr A* at all three wavelengths. Combined with upper limits by Gezari and associates at MIR wavelengths a time averaged radio spectrum is obtained which increases ∝ν^1/3, attains a maximum at ν_max>600GHz and must decrease rapidly at frequencies >10⁴GHz. This spectrum allows for about 3M_☉ of 50K dust and associated hydrogen in the telescope beam. While variations on time-scales of a few months are now well established for the frequency range ν≲100GHz, our investigation of variability at higher frequencies still yields only marginal results. The Circum-Nuclear Disk (CND) extends over the central 12pc. At the galactocentric radius R∼1pc dust and hydrogen column densities drop to low values and form the Central Cavity. Our submm images show that the bottom of this cavity is rather flat. Variations in the dust emission are just consistent with the detection of a `Tongue' of ∼200M_☉ of atomic hydrogen reported by Jackson et al. (1993ApJ...402..173J) to be located between the Northern and the Eastern Arm of the Minispiral. We present a revised submm/IR spectrum of the central 30" (R≲0.6pc) with flux densities corrected for an interstellar extinction of A_v∼31mag. This spectrum attains its maximum at λ∼20µm and comes from dust with temperatures ∼170-400K which is associated with the Eastern Arm and the East-West Bar. The integrated luminosity is ∼5x10⁶L_☉ to which emission at λ≲30µm contributes ∼80%. Heating of this dust is not provided by a central source but rather by a cluster of hot (T_eff∼3-3.5x10⁴K) and luminous stars which could include the HeI/HI-stars detected by Krabbe et al. (1991ApJ...382L..19K). Their total luminosity within R≲0.6 pc must be ∼2x10⁷L_☉ and their Lyman continuum photon production rate N_Lyc∼1.3x10⁵¹s^–1. Outside the Central Cavity the principal sources of excitation appear to be medium mass stars and late-type O-stars. IR- and Lyc-photon luminosities increase ∝R^1.2 while the dust temperature decreases ∝R^–0.15. Within the Nuclear Bulge (R≲250pc) the IR-luminosity (∼10⁹L_☉), the production rate of Lyman continuum photons (∼2x10⁵²s^–1) and the mass of molecular hydrogen (∼10⁸M_☉) amount to ∼10% of the corresponding characteristics in the Galactic Disk. Hence the formation rate of massive stars per unit mass of molecular hydrogen and the stellar 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>1pc by stars of the Nuclear Bulge and for R≲1pc by a compact object of a few 10⁶M_☉. Model computations for a black hole/accretion disk configuration with an accretion rate of 10^–6...–7M_☉yr^–1 indicate that the accretion disk becomes gravitationally unstable for >1.5-5x10⁴ Schwarzschild radii. A more massive disk with a correspondingly higher accretion rate thus could account for the formation of the central cluster of HEi/HI and - possibly- O-stars which appear to be responsible for most of the excitation of the central pc.