Astron. Astrophys. 324, L5-L8 (1997)

4. Results

In order to optimize the resolution, we choose the J band and a number stars to estimate the local density. The extinction is characterized at each point by the mean distance to the 20 nearest stars. In the J band the resulting spatial resolution is for low extinction () and for . In the I band we find more than and the band is not sensitive enough. The final result is the extinction map presented in Fig. 3 with the IRAS 100 emission isocontours in which a warm contribution has been subtracted (see below). This map in false colours results from the recombination of the 4 wavelet planes.

Fig. 3. Extinction map derived from J band stellar counts and cold IRAS 100 emission isocontours at 20, 25, 30, 35 MJy.sr-1 (see text)

The standard deviation of the map is magnitude. The resolution for high extinction allows to clearly identify 4 distinct maxima greater than 7 which were not resolved by the Schmidt plate analysis. The maximum is . There is no evidence of saturation. Using the relation between visual extinction and column density (Savage and Mathis, 1979) we derive the mass M of the cloud for :

where is the angular size in square radian, d the distance to the cloud, and µ the mean particle mass. To calculate µ we consider a gas composed of of hydrogen (H₂ +HI) and of helium. The mass cannot be estimated in a straightforward way for because the slow variation of the extinction and the lower sensitivity in the J band induce important consequences on the value of the solid angle which delineates the absorbing region, but the larger uncertainty on the mass determination comes from the distance estimate.

The extinction map that we have drawn out and the 100 map of IRAS have a comparable spatial resolution. It is therefore tempting to cross-correlate the 2 maps to derive some properties of the dust grains. According to Laureijs et al. (1991) the IRAS 100 flux consists of a cold and a warm components which can be split off into two components using the 60/100 colour temperature. Following Boulanger et al. (1997), the cold contribution can be written:

Fig. 3 shows that our extinction map and the cold 100 emission are in very good agreement. Note that the two areas with no IRAS contour () are due to the presence of peculiar objects such as the Infrared Nebula (Schwartz & Henize, 1983) and very young stellar objects with outflows (Jones et al., 1985). This excellent correlation suggests that the J extinction and the cold 100 emission have the same origin, a result in agreement with the Désert at al. (1990) dust model which shows that the 100 emission and the near infrared extinction are both caused by big grains. The warm component contribute strongly to emission, but not much to the extinction. A plot of the cold IRAS emission versus visual extinction is presented in Fig. 4.

Fig. 4. Variation of cold IRAS emission with visual extinction. Each point corresponds to a position on the map spaced of in both directions

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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