3. Selection method
Our selection method of new YSO candidates is essentially based on the IR colours and magnitudes of the objects after de-reddening. However, before attempting to select new YSO candidates, we have first estimated the contamination by background and foreground stars and the influence of the luminosity classes on the star magnitudes. To derive this information, we have run the so called Besançon model (Robin & Creze, 1986) in the direction of the Cha I cloud. The model has been parametrised to take into account the DENIS limiting magnitudes and photometric errors. It neglects the cloud itself (extinction, star formation). Fig. 1 displays synthetic colour-magnitude diagrams obtained for the different classes of luminosity using this model. The brightest stars () correspond to highly red giants (), and the colour dispersion of the faintest stars results of the photometric errors. We must keep in mind these two points before using an infrared excess criterion to identify the YSOs. It is clear that we can identify only the objects exhibiting a strong infrared excess, i.e . The number of objects detected both in J and band within the covered area is 10 000, while the model yields only some 150 foreground stars, i.e. less than , assuming a distance of the cloud of 140 pc.
A previous investigation based on DENIS star counts in the J band on the Cha I cloud enabled us to draw an accurate extinction map (Cambresy et al., 1997) with a spatial resolution of . The peak of visual absorption that was measured is about 10 magnitudes. This map is used to deredden all the stars detected within the area that it encompasses. For those stars located inside the cloud, this reddening is, of course, an upper limit.
The YSO candidates are selected according to their colour and magnitude properties after this dereddening has been applied. Consequently, we introduce a bias in the selection of the reddest objects that is discussed below.
In practice, we have plotted the dereddened magnitudes in a colour-magnitude diagram together with the main sequence (Fig. 2a and b) and selected the stars which are separated from the main sequence by a distance corresponding to 8 magnitudes of visual extinction, at least. This provides 90 stars. Part of them cannot be shifted towards the main sequence just assuming an even larger extinction, and are likely to be intrinsically very red. This sample, still contains some unreliable sources because of photometric errors and, also, some red giant background stars. After eliminating these objects which are, basically, the bright and the faint ends of the sample, we are left with the 54 good candidates listed in Table 2. All of them have been carefully checked afterwards by visual inspection of the DENIS images to avoid possible misleading cross-identifications between the 3 DENIS channels, or optical artifacts such as ghosts produced by nearby bright star or bad pixels.
Since the exact value of the extinction suffered by each star cannot be accurately determined, we have assumed that it is the total extinction measured on the line of sight and taken it as an upper limit. Consequently, the constraints on the star colours depend on the location of the star with respect to the cloud. The criterion is more strict for stars in the front edge of the cloud than for stars just behind the cloud. In other words, the infrared excess can be hidden by an overestimate of the reddening actually suffered by the stars. The stars that we possibly missed should be, in average, brighter than the selected stars because they are less obscured. Fortunately, the near-infrared range is less sensitive to extinction than optical bands. The average visual extinction in the Cha I cloud is about 4 magnitudes, i.e. magnitudes of extinction. So, this effect does not introduce a significant bias in the luminosity function. We can crudely evaluate the maximum number of missed stars. Assuming that the extinction would be reduced by a factor 2, the total number of selected star with our infrared excess method would increase by a factor 1.7, i.e. additional stars. Most of them would probably be background stars because the extinction is now underestimated for these stars and thus, a residual infrared excess remains. Nevertheless, this provides an indication of the maximum number of stars that we can miss.
© European Southern Observatory (ESO) 1998
Online publication: September 17, 1998