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Astron. Astrophys. 348, 768-782 (1999)

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5. The spatially averaged continuum spectrum of the central 30" ([FORMULA]1.25 pc)

K band emission relates to stars. Due to the large distance of the Nuclear Bulge ([FORMULA] kpc) and the high dust opacity in the direction of the Galactic Center ([FORMULA] mag) most of the sources above our detection limit of [FORMULA]Jy are either early-type MS stars or Giants and Supergiants (see Appendix D and Fig. D1). While the low- and medium mass MS stars can be traced by their K band continuum emission the distribution and luminosity of more massive MS stars and Giants can be estimated from the emission of dust and ionized gas located in the Nuclear Bulge. In this section we derive the radio through NIR spectrum of the central 30" and decompose it into the contributions from free-free emission, warm and hot dust emission and stellar emission.

Fig. 5 shows the dereddened spectrum of the central 30" ([FORMULA] 1.25 pc for [FORMULA]kpc). Data with [FORMULA] Hz are from Zylka et al. (1995); additional data for [FORMULA] Hz have been obtained from the K band observations discussed here and from an H band mosaic obtained in a similar way which will be presented and discussed in a later paper. Observed and dereddened flux densities are given in Table 1 a. The spectrum has been decomposed into three characteristic dust components and two characteristic stellar components as shown in Fig. 5. Corresponding fit parameters are given in Table 1 b (see Mezger 1994 and references therein). For [FORMULA]m the spectrum is dominated by stellar emission. The stellar population within the central 30" consists of a mixture of hot and luminous stars, cool Giants and Supergiants and a large number of low-mass, low-luminosity stars which should account for most of the stellar mass of [FORMULA] (see footnote 5). Based on work by Eckart, Genzel and collaborators (see also MDZ96, Sect. 5.3.2) we attribute of the total observed K band flux density of [FORMULA] Jy, [FORMULA] Jy to 24 hot stars, [FORMULA] Jy to cool but luminous stars with [FORMULA] mag and [FORMULA] Jy to low-mass low-luminosity stars with [FORMULA] mag. For the hot stars we adopt [FORMULA] K (Najarro et al., 1994 and 1997) and for the cool stars [FORMULA] K. With these assumptions we obtain the two Planck spectra attributed to stellar emission. The corresponding luminosity of 9.2[FORMULA]107 [FORMULA] (Table 1 b) is well above the (corrected) dust luminosity of 7.5[FORMULA]107 [FORMULA] given in Table 1 c. The cool Giants and Supergiants whose progenitors were medium-mass stars together with the low-mass, low-luminosity stars account for comparable luminosities of [FORMULA] but their contribution to the total stellar luminosity is negligible.

Also shown in Fig. 5 is part of the spectrum extending from [FORMULA]2.66 to 11.56 µm which we observed with higher resolution using ISOPHOT-PHT-S (see Lemke et al., 1996). The data were reduced with the PHT interactive analysis package PIA V7.0.2p(e) (Gabriel et al., 1996) in the standard way using the drift recognition, the orbital dependent dark current and the default detector responses. Strong absorptions are found in the wavelength range [FORMULA]m and [FORMULA]m. These features are shown in Fig. 5 for the short wavelength range of ISOPHOT-PHT-S but have not been considered in the continuum fit.

The first three components in Table 1 b relate to dust. For wavelengths [FORMULA] the dust is opaque. L are the luminosities of the dust components and [FORMULA] are - for a metallicity [FORMULA] - the associated hydrogen masses, both given in solar units. [FORMULA] is the solid angle of a Gaussian source of FWHP [FORMULA]. Hence, the 40 K dust appears to fill the central [FORMULA] completely. From [FORMULA]m we derive an average visual extinction of this dust component of [FORMULA] mag, which is - within the rather large error margins - close to [FORMULA] mag estimated for the extinction between Galactic Center and Sun. The 40 K dust is, however, not the dust located between Sun and Galactic Center which is too extended to be seen in emission in submm/FIR images of size [FORMULA]. Furthermore, a dust temperature of 40 K is typical for extended envelopes of Galactic Center molecular clouds in the Nuclear Bulge (e.g., Gordon et al. 1993) but much too high for dust located in the Galactic Disk. We conclude that the 40 K dust must be located close to, but not in front of the Galactic Center, otherwise the total extinction towards the central cluster would be [FORMULA] mag, and therefore deny NIR observations of the Nuclear Bulge.

The hydrogen mass associated with all dust components is [FORMULA] (Table 1 b). The mass of ionized hydrogen contained in the HII region Sgr A West is [FORMULA] (Table 7 of MDZ96). The central [FORMULA] account for 7 Jy of a total free-free flux density of 27 Jy at [FORMULA] cm of Sgr A West encircled by the Circum-Nuclear Disk. The scaled mass of HII contained in the central [FORMULA] is [FORMULA]. It thus appears likely that all of the 150 K dust and [FORMULA] of the 40 K dust are mixed with the ionized gas and hence with the central star cluster. The remaining part of the 40 K dust could be located in the neutral gas of the Sgr A East core GMC, but at the remote ionization front of Sgr A West (see MDZ96, Figs. 17a,b). The 300 K component is probably contributed by circumstellar dust.

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Online publication: August 13, 199