Astron. Astrophys. 364, 712-722 (2000)

4. Conclusions

From our CCD photometric observations, we believe that we have convincingly detected evidence of extinction from the Draco cloud within our sample, and have some preliminary basis for estimating the distance to this object. In addition, our photometric catalog of stars is large and deep enough that we should have several included stars which are in the background of the cloud.

This work forms a basis for further observational studies of the Draco nebula, using more accurate spectroscopic and polarimetric techniques upon the stars in our catalog to constrain the distance and other properties of the cloud. We may summarize the results of this work below.

1. We have presented a photometric catalog which includes sufficient depth (V 19) to detect the Draco nebula if it is a galactic object.

2. A strong correlation between the photometric index and is found for our standard stars.

3. The use of the photometric index is a possible technique for photometric classification of faint, unreddened, chromospherically inactive early-type stars for which uvby photometry may be difficult, but high precision photometry and/or a reddening free index may be needed to accurately calculate the reddening toward the sightline.

4. Preliminary indications indicate that the Draco nebula is detected from photometric reddening of our sample, with a best fit distance of d= pc, and extinction A_V=2.5 magnitudes.

5. The predicted colors from our star count models are most consistent with a sample of stars with a limiting V magnitude of V=19 and a brighter B limiting magnitude of B = 18, which detects absorption from the Draco Nebula at a distance of 800 d 1100 pc, and with a cloud R_V = 1.5 , and extinction of A_V=2.5 magnitudes.

6. Our catalog of V, , and magnitudes for the Draco nebula should be a valuable tool for future spectroscopic and polarimetric studies of the cloud.

We also would like to note that with the above values of distance, R_V, and A_V, the Draco cloud is quite an exceptional object. The dense clumps in the cloud have angular sizes which range from 2 to 20 arcminutes, which correspond at d=1100 pc to linear dimensions of l=0.65 to 6.5 pc. Indeed the largest `plumes' of this cometary cloud are close to 2 degrees across, which would correspond to a linear dimension of 38 pc at the adopted distance. This would make the Draco cloud a very large object indeed, more comparable in size to galactic plane giant molecular clouds than typical high latitude clouds, and at a scale height = 640 pc which is well above the typical scale height of 90 pc for molecular clouds in the galaxy.

Additional observations and theoretical work are clearly needed to provide more accurate constraints on the possible mass, origin and internal conditions of the Draco nebula, which challenges many of our ideas about the structure of the galactic interstellar medium.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
helpdesk.link@springer.de