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Astron. Astrophys. 315, L269–L272 (1996)

ASTRONOMY AND ASTROPHYSICS

SWS observations of the Galactic center?
D. Lutz1 , H. Feuchtgruber1;2 , R. Genzel1 , D. Kunze1 , D. Rigopoulou1 , H.W.W. Spoon1 , C.M. Wright1 , E. Egami1 , R. Katterloher1, E. Sturm1 , E. Wieprecht1, A. Sternberg3 , A.F.M. Moorwood4 , and Th. de Graauw5
1 2 3 4 5

Max-Planck-Institut f¨ r extraterrestrische Physik,Postfach 1603, D-85740 Garching, Germany u ISO Science Operations Center, Astrophysics Division of ESA, P.O. Box 50727, E-28080 Villafranca/Madrid, Spain School of Physics and Astronomy and The Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel European Southern Observatory, Karl-Schwarzschildstraße 2, D-85748 Garching, Germany Space Research Organization of the Netherlands and KapteynInstitute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 17 July 1996 / Accepted 28 August 1996

Abstract. We present a 2.4–45 m spectrum of the center of our Galaxy obtained with the Short Wavelength Spectrometer (SWS) on board ISO. The wide range of ionic fine structure lines observed yields an average effective temperature for the ionizing stars of 35000 K, with a smallcontribution of significantly hotter stars, consistent with ageing of an active period of massive star formation that took place a few million years ago. Several absorption features from the foreground cold ISM are detected for the first time. The extinction law towards the Galactic center lacks the expected deep minimum in the 4–8m range. From the detection of OH 34.6m absorption, we infer that radiativepumping is likely the major excitation mechanism for OH emission from the Galactic center. We discuss the rich spectrum of iron and nickel and conclude that the [Fe II] spectrum is inconsistent with pure collisional excitation. This calls for caution in interpretation of [Fe II] emission in sources, such as starburst galaxies, which contain intense radiation fields. Key words: Galaxy: center –ISM: general – dust: extinction – Infrared: ISM: lines and bands

SWS interactive analysis system (IA), using calibration files as of June 25 and additional tools to improve flatfielding and to remove fringes. The aperture was centered on Sgr A , with the long axis oriented at position angle -1.4 . The 1400  2000 aperture for the short wavelengths also included some of the well-known mid-infraredsources like IRS3 and IRS1, much of the ionized ‘bar’ including the ‘mini-cavity’ region, and part of the ionized northern arm. For longer wavelengths, more of the ionized minispiral is covered by the larger (2000  3300 ) aperture. We did not attempt to correct for this in the spectrum displayed in Fig. 1, leading to jumps in the spectrum where the aperture size changes. The aperture size washowever taken into account by dividing by 2 all fluxes for lines beyond 30m when interpreting line ratios.

2. Ionizing continuum and stellar population The detection of fine structure lines of [Ne II], [Ne III], [S III], [S IV], [Ar II], [Ar III], [O IV] allows a detailed diagnostic of the ionized medium and the exciting continuum. We have used the photoionization code CLOUDY version 84.12 (Ferland1993) updated with the [Ar II] collision strength of Pelan and Berrington (1995) to infer the average effective temperature of the ionizing stars from the observed ratios of lines from different ionization stages of the same element. We have adopted an ionization parameter U  1 constrained by the density of 3000 cm 3 obtained from the [S III] and [Ne III] line ratios, the Galactic centerionizing flux of 31050 s 1 (see e.g. Genzel and Townes 1987) and a typical distance of 0.5 pc to the ionized clouds. To estimate the impact of the adopted stellar spectra, we have used stellar atmosphere models of both Kurucz (1992) and Sellmaier et al. (1996). Using this and the three independent ratios [SIV]/[SIII], [ArIII]/[ArII], [NeIII]/[NeII] we infer that the average effective temperature is...
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