Astron. Astrophys. 332, 1035-1043 (1998)

5. Conclusion

We presented observational data from the NIR up to millimetre wavelengths of the cloud core M 17-North . The main issues discussed in the paper were the spectrum obtained by ISO including the UIBs and the fine structure lines and the analysis of the cloud core structure including the ground-based data and applying a radiative transfer code. This code also included the treatment of the emission from VSGs.

The detected features in the ISO spectrum can be summarized in the following way:

1. UIBs: The UIBs are carried by small aromatic hydrocarbon grains or large molecules transiently heated. They have a hydrogen coverage of 70% in M 17-North .
2. Fine structure lines: Using the line ratios we estimated an electron density and temperature of 100 cm^-3 and 5000 K, respectively. The electron temperature is relatively uncertain due to the insensibility of these fine structure lines to this parameter.
3. Continuum: It starts to dominate the spectrum at wavelengths longer than 15 µm and still originates from transiently heated grains. The continuum reaches its maximum at about 100 µm .

We detected an infrared source at NIR wavelengths, which is also "visible" at 10 µm . Though IRS1 might not be in the centre of the core of M 17-North , its detection at 10 µm and its colour index typical for an embedded object is the first direct sign of a young stellar object in M 17-North and already on-going star formation. With the RT model we tried to reproduce the continuum radiation observed by ISO and the other telescopes. The model shows that most of the extended FIR luminosity observed is provided by outside heating, namely by the exciting stars in the centre of M 17 . The outer radiation field cannot explain the NIR/MIR flux seen by ISO. We had to include a central source with a luminosity of 8000 . As this is only 10% of the total luminosity, this value is relatively uncertain.

Planned ISOCAM observations similar to the observations of M 17-SW (Cesarsky et al., 1996) will help to refine the analysis of M 17-North as we will be able to determine if the core is irradiated from the south or is powered from inside. The spatial variation of the UIBs, of the fine structure lines, and of the continuum will point to the energy source.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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