4. Discrete sources
The SADAM software library was used for object extraction and photometry, employing the multi-resolution method developed by Starck et al. (1997). Specifying conservative thresholds, of order 80 discrete sources were listed in the LW2 image and two dozens in the LW3 image. Their flux densities, and , were computed, and their positions derived. No `colour correction' was applied to the photometry, i.e. the flux densities were derived assuming source spectra with constant across the filter bands. For YSO-like spectra this may cause inaccuracies of 10 % in the derived fluxes, since LW2 and LW3 are broadband filters. However, for the present observations background subtraction is the dominant factor determining the total number of sources extracted, and the uncertainty in the fluxes derived for each source.
In the areas with the lowest background levels in the LW2 image the weakest detected discrete sources have 5-10 mJy. In areas with brighter extended emission the detection limit is a factor 2 higher, depending on the `compactness' of the source. In the LW3 image the detection limits are higher, since almost the entire image has high background levels.
The LW2 discrete sources appear generally to coincide with relatively bright stars in the NIR, but only rarely are corresponding sources found in our LW3 data. Sources outside the extent of the pillars are expected to have LW3 counterparts only if embedded. Two such objects, labelled I (isolated object N of the head of ) and the already mentioned object B, do appear as LW3 sources.
We report the detection of a discrete object close to the tip of a narrow extended protrusion on the SW side of , this object apparently coincides with M16-E31. Its peak flux is 50 % higher than that of the head of in both bands, it is unresolved, and in the NIR there is only a faint object visible in this position. Its derived flux densities are 50 mJy and 175 mJy. Forming the colour index log( / ) we obtain 0.5; thus it is a very `red' object indeed. We tentatively identify it (cf. Nordh et al. 1996) as an embedded YSO. Assuming an emissivity law the colour temperature corresponding to the observed / ratio is 250 K.
From extensive ISOCAM observations (also using the LW2 and LW3 filters) of the Chamaeleon region (Nordh et al. 1996), and incorporating IRAS data to take colder source components into account, Olofsson et al. (1997) have established an empirical linear relationship between log() and log() for YSOs (defined as having ) valid over 3-4 magnitudes of luminosity. Using their preliminary calibration and assumed distance of 150 pc for Chamaeleon, and adopting a distance to M 16 of 2 kpc (Hi93), it predicts 2 ( /5 mJy) for M 16. Extrapolating to luminosities just above their highest, we derive a bolometric luminosity of for the YSO connected to M16-E31.
The bright peak `below' M16-E31 in the LW2 map we identify with source 367B of Currie et al. (1996). It has 100 mJy, however, it is difficult to assign a LW3 flux, thus we cannot positively identify this object as a YSO.
The two other sources detected in both filters mentioned are I and B, both located in areas of low background emission. We derive 15 mJy and 40 mJy giving 0.4 and 6 for I, and 75 mJy and 45 mJy giving and (extrapolated) 30 for B, provided that these objects indeed are embedded YSOs.
We have attempted to correlate discrete sources in our LW2 image with features in the HST image, in particular with the EGGs discussed in He96. To our levels of angular resolution (1.22 /D = 2.8 and 6.0 corresponding to approximately 6000 AU and 12000 AU) and sensitivity we do not, in general, find such a correlation.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998