edented sensitivity and resolution, and can be compared with high-resolution maps obtained at near-IR and radio wavelengths. In particular, while the substructure of IRc2 is consistent with high-resolution NIR observations, we confirmed that the radio source I is not associated with any MIR object. This is therefore a very embedded HII region, and possibly the real engine of the KL nebula. IRc2, IRc7, and IRc4/5 could be only the illuminated portions of a toroid surrounding source I.

In Fig. 6 are shown the 10 µm and 20 µm images of the giant HII region W51 (Ligori, Robberto, and Herbst 1999). Once again, the high quality of these images is evident when comparing them with near-IR images obtained recently (e.g. the picture on the back page of the autumn 2000 issue of the UKIRT newsletter). Thermal IR images of embedded HII regions can provide information on the distribution of dust grains associated with the ionized gas and, assuming plausible values for the grain size and composition, on their temperature. Of course, it is also possible to detect a number of YSOs which are not sufficiently evolved or massive to be able to ionize the surrounding material.
The most interesting, and active, region in our images of W51 is IRS 2. The combination of near-IR, mid-IR, mm and radio

References

Herbst T.M., Robberto M., and Beckwith S.V.W., 1997, AJ, 114, 744
Jewitt D., and Kalas P., 1998, ApJ, 499, L103
Ligori S., Robberto M., and Herbst, T.M., 1999, Proc. Conf. “Star formation 1999”, Nagoya, Japan
Robberto M., and Herbst, T.M., 1998, Proc. SPIE, 3354, 711
Robberto M., and Herbst, T.M., 1998, ApJ, 498, 400
Bertero M., Boccacci P., and Robberto M., 2000, PASP, 112, 1121
Robberto M., Ligori S., Beckwith S.V.W., Herbst T.M., Custo A., Bertero, M., Boccacci P., 2000, Proc. Conf. “Ionized Gaseous Nebulae”, Mexico City

Fig 7: The galactic center region at 10 µm

observations shows a complex scenario, with a sequence of objects in different evolutionary states: a cold core is detected with mm observations, while the mid-IR data show other stellar objects at positions not coincident with the core. Only a part of these objects are associated with ionized gas emission observed at radio, while the brightest object is found in a region with weak radio emission.

The Galactic center

The Galactic center
Finally, in Fig. 7 a beautiful image of the Galactic center region, obtained at 10 µm is shown. A number of compact sources can be seen for the first time at these wavelengths, and the image is rich in detail. The work on this data is still underway.