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

5. Conclusion

The extinction map of the Chamaeleon I cloud has been significantly improved for extinction greater than with respect to previous maps obtained from star counts on Schmidt plates. Four distinct maxima are detected and we reach 9 visual magnitudes of extinction without degradation of the resolution. This result has been obtained both by exploiting the massive star counts in the J band provided by DENIS, and by applying a variant of the classical star count method which is adapted to large variations of extinction and a wavelet analysis of the extinction map. Moreover, DENIS give us the opportunity to investigate the cloud at a larger scale than the earlier investigations which were limited to small regions around the visible reflection nebulae.

The comparison with the cold IRAS 100 is striking. Each of the 3 most important extinction maxima corresponds to a peak of cold IRAS emission. The IRAS flux is therefore a good indicator of extinction. We plan further investigations of the relation between the cold IRAS emission and extinction when new CO observations at a comparable spatial resolution will be available.

Finally we stress the fact that our map has been derived from J star counts converted to visual extinction with . This value of 3.1 is the standard estimate for diffuse interstellar medium, but it can reach 5.5 in dense molecular cloud cores (Whittet et al., 1987). According to Cardelli et al. (1989):

So, we obtain 0.282 for and 0.334 for . This means that we overestimate the extinction by a factor 1.18 if we choose rather than 5.5. Extinction is generally expressed in visual magnitudes. This choice is not the best because the extinction law in the visible range depends on the composition of the medium. On the other hand, the extinction law in the infrared seems universal. Therefore, it should be better to refer the extinction to near infared magnitudes. Lastly, it appears that J DENIS data with a detection limit of magnitude are better adapted to investigate the obscuration of regions where the extinction is larger than 4 magnitudes, rather than the Schmidt plate even at a usual detection limit of magnitudes.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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