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 297 is presented. The spectrum is dominated by a strong continuum with superimposed emission lines and features both in absorption and in emission. In particular, 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 H2O at 2.96 mum have been observed. The ISO-SWS spectrum has been combined with 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 parametrized by density and temperature power laws, deriving suitable values for the spectral type (B2), the visual extinction (7.5 mag) and the distance (280 pc). Consistent determination of the extinction and estimates of both the source mass loss rate (9-10(-7) M-circle dot yr(-1)) and the size of the emitting ionized region (30 stellar radii) have been derived by the analysis of the HI recombination lines of the Brackett, Pfund and Humphreys series observed by ISO-SWS together with Paschen and Brackett lines observed from the ground. Some peculiarities have been observed in the ISO-SWS spectrum of this source: i) the ratio between the column density of the solid CO2 and H2O (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 the extended dusty cloud in which the star is embedded and their relative strengths could be evidence that they are composed by processed grains.