An ISO investigation of the MWC 297 circumstellar region

The ISO-SWS full grating spectrum (2.3-45 mum) of the Herbig Be star MWC 29 7 is presented. The spectrum is dominated by a strong continuum with superi mposed emission lines and features both in absorption and in emission. In p articular, we detect in emission 23 HI recombination lines of the Brackett, Pfund and Humphreys series and four PAH features, while in absorption two broad silicate bands at 9.7 and 16.4 mum, solid CO2 at 4.27 mum and solid H 2O at 2.96 mum have been observed. The ISO-SWS spectrum has been combined w ith ISO-LWS data and ground based photometry to derive the spectral energy distribution (SED) from optical to radio wavelengths. The observed SED has been fitted with a model that assumes a spherical dusty envelope parametriz ed by density and temperature power laws, deriving suitable values for the spectral type (B2), the visual extinction (7.5 mag) and the distance (280 p c). Consistent determination of the extinction and estimates of both the so urce mass loss rate (9-10(-7) M-circle dot yr(-1)) and the size of the emit ting ionized region (30 stellar radii) have been derived by the analysis of the HI recombination lines of the Brackett, Pfund and Humphreys series obs erved by ISO-SWS together with Paschen and Brackett lines observed from the ground. Some peculiarities have been observed in the ISO-SWS spectrum of t his source: i) the ratio between the column density of the solid CO2 and H2 O (2.0 +/-1.5) higher than the values usually observed and ii) the presence of a silicate broad absorption band at 16.4 mum stronger than the 9.7 mum absorption. The observed silicate absorption features are probably due to t he extended dusty cloud in which the star is embedded and their relative st rengths could be evidence that they are composed by processed grains.