Recently there has been quite an evolution in the derivation of interstellar absorption in molecular clouds from star and color counts, e.g. Cambrésy (1999). The treatment of the counts have become much more tractable with the digital form of the data extracted from well calibrated CCD exposures or from digitized sky surveys. A problem with classical counts has been, at least when distance estimates are concerned, that the accuracy depends on the scatter in absolute magnitude of the stellar types counted. The peaked, almost temperature independent, distribution of near infrared colors like (H-K)0 for dwarfs and giants have made the estimate of individual color excesses of higher accuracy possible, e.g. Alves et al. (1998) and references therein. Distances must still be based on a comparison of the actual counts to the estimates from a galactic model, a comparison made difficult by the "degeneracy" of the (H-K)0 color. A variation of this theme was adopted by Thoraval et al. (1997) replacing (H-K) with (V-I). The use of (V-I) counts includes (V-I) measurements in a nearby reference field presumed to be free from reddening comparable to that caused by the cloud under study. The reference region is chosen to have the same galactic latitude as the cloud field. The reddening influence from the diffuse part of the interstellar medium may not be avoided but can in fact have a confusing effect on the interpretation of the extinction in the cloud region. The reference field is supposed to be without molecular clouds, this means that the diffuse medium is sampled over longer lines of sight than the cloud field assuming similar limiting magnitudes. The sight lines in the cloud field are shortened by the absorption in the molecular cloud. With a random distribution of diffuse interstellar clouds this implies that the reddening distribution, exclusively originating in the diffuse ISM, will be wider for the reference field than for the cloud field. Likewise the mean diffuse reddening in the reference will probably exceed the mean diffuse reddening, not the total mean, of the cloud field. Both these effects tend to lower the color count estimate of the cloud extinction.
Our original intention for this study was to apply (V-I) data for counts in the CG 30 region and to complement the counts with uvby measurements in the cloud field as well as in the reference field - sort of calibrating the color counts. With a favorable distribution of stars with respect to the globules' tails we may even have a possible indication of an upper distance limit and the uvby data from the reference field may refine the accuracy of the absorption zero point and standard deviation in the (V-I) distribution as a function of distance. Presently we limit ourselves to a discussion of the V, V-I data in cloudy fields in what may be a new way to use such data, and as we will demonstrate with a promising outcome.
The data were taken with the DFOSC and the Danish 1.54 m telescope on La Silla in March 1997. One frame of the V and I fields have a dimension of 13:0513:05 and the mosaic covers an area 26´26´. The resulting surface distribution of sight lines with V and (V-I) data are shown in Fig. 1. This figure contains 6168 sight lines. The location of the cometary globules are obvious. For their designation Fig. 4 of Nielsen et al. (1998) may be consulted. The accumulated V and I distributions show that the photometry is complete to about V = 205 and I = 185-190 and the formal errors of V and (V-I) are typically 003 and 004 respectively all the way down to the limiting magnitude, this accuracy is essential for our proposed method to work. The limiting V magnitude is at 21th mag as may be seen from Fig. 2. The bulk of stars being rather red implies that the I magnitudes are generally much more accurate than the V magnitudes. Details of the data taking, reduction and transformation will be discussed in more detail in a forthcoming paper.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999