2000A&A...362..199F

The 7 and 15µm observations of the Galaxy obtained by the ISOGAL program offer an unique possibility to investigate and separate the different populations of stars in the Galactic Plane, in particular to study the population of low flux density YSOs which could not be detected with IRAS. Considering the results obtained by ISOCAM in nearby star forming regions and in other test fields in the Galactic Plane, as well as theoretical indications, we establish criteria of general validity that can be used to select YSOs from the much larger population of Post Main Sequence (Post-MS) stars present in the ISOGAL fields. The selection is based primarily on the position of the sources in the [15] - [7]-[15] diagram, which involves only ISOGAL data and allows to select objects with IR excess as possible YSOs using the survey data alone. The criteria are applied to five ISOGAL fields centered at l~+45° and b∼0°, covering a total area of 0.504deg². The total number of point sources detected in both filters above the confidence limits of [7]<9.5 and [15]<8 are 386. The ISOGAL results are compared with radio observations in order to identify high luminosity YSOs out of the larger population of lower luminosity ones. We find 3 high luminosity YSOs plus 3 diffuse sources which are associated with HII regions and 35 lower luminosity YSOs. For low flux densities we find a consistent number of sources (73) detected only at 15µm which, according to the colour upper limit, are classified as candidate YSOs. The validity of the adopted criteria for selecting possible YSOs in the ISOGAL fields are also supported by two independent results: 1) the sources classified as YSOs and the candidate YSOs are often associated with diffuse nebulosities visible at 15µm, landmarks of recent star forming activity, and 2) the numbers of high luminosity and low luminosity YSOs that we have found compare well with the expectations.