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From: Nemesio Rodriguez Fernandez  nemesio.rodriguez@obspm.fr 
To: gcnews@zia.aoc.nrao.edu
Subject: submit paper.tex A&A in press



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\title{ISO observations of the Galactic center Interstellar Medium:
}

\subtitle{neutral gas and dust}


\author{N. J. Rodríguez-Fernández \inst{1}
          \and J. Martín-Pintado \inst{2}
      \and A. Fuente \inst{3}
       \and T. L. Wilson \inst{4}
   }

   \offprints{N. J. Rodríguez-Fernández}

   \institute{LERMA (UMR 8112), Observatoire de Paris, 61 Av de l'Observatoire,
   F-75014 Paris, France\\
       \email{nemesio.rodriguez@obspm.fr}
       \and
        DAMIR, Instituto de Estructura de la Materia, Consejo Superior de
        Investigaciones Científicas (CSIC), Serrano 121, E-28006 Madrid, Spain
       \and
       Observatorio Astronómico Nacional, Instituto Geográfico Nacional,
       Apdo. 1143, E-28800 Alcalá de Henares, Spain
       \and
       Max-Planck-Institut für Radio Astronomie, Auf dem Hügel 69,
       D53121 Bonn, Germany
     }
   \date{Received ; accepted }


\abstract{ The 500 central pc of the Galaxy (hereafter GC) exhibit a 
widespread gas component with a kinetic temperature of 100-200~K. The 
bulk of this gas is not associated to the well-known thermal radio 
continuum or far infrared sources like Sgr A or Sgr B. How this gas is 
heated has been a longstanding problem. With the aim of studying the 
thermal balance of the neutral gas and dust in the GC, we have observed 
18 molecular clouds located at projected distances far from thermal 
continuum sources with the Infrared Space Observatory (ISO). In this 
paper we present observations of several fine structure lines ([\OI] 63 
and 146 \mum, [\CII] 158 \mum, [\SiII] 35 \mum, [\SI] 25 \mum ~ and 
[\FeII] 26 \mum), which are the main coolants of the gas with kinetic 
temperatures of several hundred K. We also present the full continuum 
spectra of the dust between 40 and 190 \mum. All the clouds exhibit a 
cold dust component with a temperature of $\sim 15$ K. A warmer dust 
component is also required to fit the spectra. The temperature of this 
dust component changes between 27 and 42 K from source to source. We 
have compared the gas and the dust  emission with the predictions from 
J-type and C-type shocks and photodissociation region (PDRs) models. We 
conclude that the dust and the fine structure lines observations are 
best explained by a PDR with a density of  10$^3$ \cmmt ~ and an 
incident far-ultraviolet field 10$^3$ times higher than the local 
interstellar radiation field. The fine structure line emission arises in 
PDRs in the interface between a diffuse ionized gas component and the 
dense molecular clouds. The [\CII] 158 \mum~ and [\SiII] 35 \mum ~ lines 
also have an important contribution from the ionized gas component. PDRs 
can naturally explain the discrepancy between the gas and the dust 
temperatures. However, these PDRs can only account for 10-30$\%$ of the 
total H$_2$ column density with a temperature of $\sim 150$ K. We 
discuss other possible heating mechanisms for the rest the warm 
molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs)
or the dissipation of supersonic turbulence.
}