The search for YSOs from ISOGAL data - Application to the 1=+45 field

The 7 and 15 mum observations of the Galaxy obtained by the ISOGAL program offer an unique possibility to investigate and separate the different popul ations of stars in the Galactic Plane, in particular to study the populatio n of low flux density YSOs which could not be detected with IRAS. Considering the results obtained by ISOCAM in nearby star forming regions a nd in other test fields in the Galactic Plane, as well as theoretical indic ations, we establish criteria of general validity that can be used to selec t YSOs from the much larger population of Post Main Sequence (Post-MS) star s present in the ISOGAL fields. The selection is based primarily on the pos ition of the sources in the [15] - [7]-[15] diagram, which involves only IS OGAL data and allows to select objects with IR excess as possible YSOs usin g the survey data alone. The criteria are applied to five ISOGAL fields centered at 1 similar to +45 degrees and b similar to 0 degrees, covering a total area of 0.504 deg(2). The total number of point sources detected in both filters above the confi dence limits of [7] < 9.5 and [15] < 8 are 386. The ISOGAL results are comp ared with radio observations in order to identify high luminosity YSOs out of the larger population of lower luminosity ones. We find 3 high luminosit y 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 o f sources (73) detected only at 15 mum which, according to the colour upper limit, are classified as candidate YSOs. The validity of the adopted criteria for selecting possible YSOs in the ISO GAL fields are also supported by two independent results: 1) the sources cl assified as YSOs and the candidate YSOs are often associated with diffuse n ebulosities visible at 15 mum, landmarks of recent star forming activity, a nd 2) the numbers of high luminosity and low luminosity YSOs that we have f ound compare well with the expectations.