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Astron. Astrophys. 318, L13-L16 (1997)

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4. Conclusions and future work

We have presented preliminary results obtained by the ISOGAL team during the 15µm survey of the inner galaxy. An infrared source with ([FORMULA] - [FORMULA]) colour index greater than 10 is found and is associated with a newly discovered H2 O maser. This finding confirms that ISO can detect young massive (proto-)stars in their earliest evolutionary stages. In these stages, massive stars have not yet developed an HII region around them, and, being deeply embedded in their parental dusty cocoon, can be studied only at infrared and sub-millimiter wavelength.

In the same field we observed more than 200 point sources with ([FORMULA] - [FORMULA]) [FORMULA], three of which with a colour greater than 9. They could be young stars still embedded in dusty envelopes. These results confirm the finding of Pérault et al. (1996), who found [FORMULA] objects with similar colours per square degree in a field at [FORMULA]. In our case, the association of the source with the largest colour index with a water maser confirms that we are indeed detecting a YSO. The low 15 [FORMULA] m fluxes of these objects (when compared with bright IRAS YSOs) suggests that, at least statistically, they might be intermediate luminosity YSOs not detectable by IRAS.

These findings prove the effectiveness of the ISOGAL observations in detecting new very young stars away from the luminous, well known star forming regions, when compared with deep K-band observations.

Clearly the objects presented here, and in particular that associated with the H2 O maser, deserve to be studied in detail at other wavelengths with high resolution and sensitivity, in order to ascertain their nature. ISOCAM observations at other wavelengths would also be advisable, due to the high resolution and sensitivity achievable in spectral regions not easily accessible from the ground.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998