Even though originally found in close association with diffuse HII regions, most of the interstellar H2 O masers (up to 80%) may not be associated with them (Palla et al. 1993). In fact, H2 O masers are now believed to trace the earliest evolutionary phases of massive stars, much before the onset of an ultra compact (UC) HII region (Codella et al. 1994; Codella & Felli 1995). This picture has been confirmed both by high resolution radio observations and high sensitivity near infrared (NIR) observations (Felli et al. 1996; Testi et al. 1994; Tofani et al. 1995) as well as by molecular line and 3mm continuum high resolution observations (Cesaroni et al. 1994a; 1994b; Codella et al. 1996; Olmi et al. 1996). The NIR observations are expected to trace not the stellar photosphere, which would be too extincted to be detectable, but the hottest layers of the dust cocoon in which the newborn star is embedded (Felli & Testi 1996).
H2 O masers are thus the ideal indicators of the youngest massive stars which have not yet swept out their parental cocoon, and are the best targets for the identification of regions of recent star formation.
Unfortunately, the best studied star forming regions are the most luminous ones and may contain several bright early type stars, making the interpretation more complex. The coexistence of objects in different evolutionary stages, and the presence of (moderately) evolved objects that have already strongly interacted with the surrounding medium, make it difficult to study the youngest and lower luminosity objects. Comparison of low resolution IRAS and radio observations may give only global results for the entire complex. Both high resolution and sensitivity are needed to separate young stars with different luminosities and in different evolutionary status (Felli et al. 1996; Hunter et al. 1995; Palla et al. 1995).
Although NIR imaging (especially at K m) has proved to be a useful tool to investigate these regions, mid and far infrared high resolution observations are urgently needed. In fact, it is in this range of wavelengths that the bulk of the emission from the youngest massive stars is expected. Even more important, isolated massive young stellar objects should be studied, in order to unveil the formation of the first generation of massive stars, without the difficulties and the complexities that arise in regions where other stars have already formed. When combined with near infrared and water maser unbiased surveys, high resolution and sensitivity ISOCAM 15µm observations (Omont 1996; Pérault et al. 1996) which purposely avoid the brightest IRAS sources (with ) are an unique tool to detect young early type stars.
In this paper we report the detection of one source which very
likely belongs to the class outlined above, together with three other
potential candidates in a region of about at
approximately , .
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998