Our most important result is the demonstration of the usefulness of a (V-I) - V diagram for the general field. The presence of absorbing features may be studied by shifting the (V-I)0 - MV relation obtained from the Hipparcos and Tycho Catalogues. We suggest that the CG 30/CG 31 complex may be as close as 200 pc compared to the previously preferred 400 pc. This smaller distance is possibly corroborated by Ca H and K spectroscopy of a sample of stars in the direction of the Vela supernova remnant, located inside the Vela - CG - Annulus, Cha et al. (1999). There are thus some prospects of deriving distances to nearby molecular clouds from V and I photometry if they are associated with column densities not exceeding 3 to 4 in AV and if the V, I surveys each goes down to the 20th magnitude. The main sequence fitting depends much on stellar types with large spatial density like stars with MV 8m. Fainter surveys may either be used to locate more distant clouds or to measure larger column densities, possibly even density profiles in parts of the nearby molecular clouds.
Broad band V and I photometry may accordingly be used both for studying the extinction variation in a molecular cloud and to estimate the cloud distance.
The suggested reduction of the distance from the assumed 400-450 pc to only 200 pc may have consequences for understanding the physics of the optical jets associated with Herbig - Haro objects. If the distance 200 pc does apply for the complete Vela - CG - Annulus the transverse velocities of jets and outflows computed for other HH objects than HH 120 in CG 30 from proper motions and distances may of course be reduced with the same factor approximately equal to 2. Another consequence is of course that the inclination estimated from tangential and radial velocities has to be corrected in accordance with the new distance estimate, the size of this correction depends on the ratio of the tangential and radial velocity. The proposed reduction of the distance from about 400 pc to 200 pc may accordingly have consequences for the interpretation of the data pertaining to the prominent jet system of HH 46/47 (e.g. Raymond et al. 1994).
Another important issue is that some individual red clump giant candidates may be recognized in the color magnitude diagram and their distance and reddening estimated. The number of proposed red clump giants in our small 26´ 26´ region exceeds 10. The knowledge of the distance and absorption provided by V and I photometry eases the interpretation of e.g. any additional intermediate band photometry; the known reddening may permit more accurate estimates of the clump giants chemical composition. Assuming that the conditions in our small region is representative of the galactic disk V and I photometry down to the 21th and 19.5th magnitude respectively in a o wide band along the galactic equator could provide more than 8.000 red clump giant candidates from the field.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999