J. Cernicharo 1 D. Cesarsky 2 S. Pérez-Martínez 1 F. Najarro 1 A. Jones 2 B. Lefloch 1
1 CSIC, Instituto de Estructura de la Materia, Dpto. Fisica Molecular, C/Serrano 121, 28006 Madrid, Spain
2 IAS, Paris, France
The ionizing star of the Trifid nebula, an O7V star, has been observed with ISOCAM-CVF and a pixel of 3 arcsec. The observed flux at all wavelengths is much stronger and has a different shape than the expected spectral energy distribution of a O7V star at 1.6kpc. We have used the theoretical PSF to remove the contribution of the central object and to study the distribution of the dust emission around it. The CVF spectrum in the direction of the star shows the silicate band in emission and a adjacent continuum that can be fitted by dust at 600 and 150 K. This source of emission has a half-power size of 2.5 arcsec. Far from the central object the silicates are seen in absorption while in the adjacent pixels warm dust at 600 K is clearly observed.
The silicate emission has, in addition to the broad emission feature, several narrow features that we interpret as due to crystalline silicates. We present a model in which the central O7V star heats a sourrounding cocoon of dust and produces a gradient temperature as a function of the distance. In the inner part silicates are crystalline while in the colder external layers (dust temperature around 150 K) are essentially amorphous. The model has been computed using the expected stellar emerging spectrum and the available optical properties of silicates. The Trifid Nebula must be extremely young as the O7V star has not dissipate yet the cocoon in which it was formed.