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Astron. Astrophys. 350, 985-996 (1999)

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Distance and absorption of the tails in the CG 30/CG 31/CG 38 complex *

An application of a (V-I)0 - MV main sequence relation derived from the Hipparcos and Tycho catalogues

J. Knude, H. Jonch-Sørensen and A.S. Nielsen

Niels Bohr Institute for Astronomy, Geophysics and Physics, Juliane Maries Vej 32, 2100 Kobenhavn O, Denmark (indus@astro.ku.dk)

Received 15 April 1999 / Accepted 9 September 1999


From a mosaic of rather deep V and I images of the CG 30/CG 31 region we report distances and absorptions of these and other interesting interstellar features, the distances and absorptions estimates are based on shifting a new (V-I)0 - MV relation obtained from nearby, presumably unreddened Hipparcos stars with luminosity class V classification to fit confining trends in the (V-I) - V diagram. The possible success of the fit is due to the combination of the rather faint limiting magnitude of the V, I data and the inclusion of absolute magnitude in the range [1[FORMULA]10] and not least to the fact that MV does not depend linearly on (V-I)0 but has a gentle wavy appearance. Malmquist bias corrections are found not to be required to our intrinsic relation.

Generally speaking the blue confinements are relevant for the most distant, less reddened stars whereas the red confinements pertain to the nearest most reddened stars.

At 660 pc we find an absorption AV = 2.8 mag, at [FORMULA]400 pc a possible feature with AV [FORMULA] 0[FORMULA]5, and finally at 200 pc we propose a feature with AV = 3.6 mag. We identify this latter feature with the windblown cometary tail of CG 31 and propose that 200 pc could be the common distance of the association of cometary globules in the Vela - CG - Annulus conversely to the generally assumed value of 400 - 450 pc. The CG 30/CG 31/ CG 38 complex may consequently be [FORMULA]200 pc removed from the Vela OB2 association. In the tail region of CG 31 the most distant main sequence stars included in the sample are observed at 2.6 kpc with AV = 2[FORMULA]8, at b = [FORMULA] 2.6 kpc does not bring us beyond one scale height of the disk gas. In the less reddened part of the region we reach 4.4 kpc and AV = 1[FORMULA]7.

Because of the excellent precision of the Hipparcos MV determination and the good photometric accuracy of our V and I data the accuracy of [FORMULA] is 10-20[FORMULA] if individual absorption features may be fitted with [FORMULA](V-MV) = 0[FORMULA]5-1[FORMULA]0 and [FORMULA](V-I)] = 0[FORMULA]1 - 0[FORMULA]2 which seems possible. The uncertainty in the [FORMULA] ratio introduces another 5[FORMULA] uncertainty in [FORMULA].

We further propose to identify the stars along the red confinement of the complete color - magnitude diagram as red clump giants at a constant distance and with absorption ranging from almost nothing to AV [FORMULA] 6[FORMULA]2. Nine of these 14 stars are concentrated in a small region at 1 kpc only 2.3 pc across. In the color - magnitude diagram we find another feature being exactly parallel to the reddening vector and located where the stellar densities seem to wane. The iso-density contours with 12-30 stars per (V-I,V) bin are parallel to a shifted reddening line. Assuming that these contours mainly are populated by red clump giants and that the solar distance from the Galactic Center is 8.5 kpc the shift locates these stars at 13-14 kpc, so maybe we have a direct measure of the Milky Way radius.

Key words: stars: Hertzsprung – Russel (HR) and C-M diagrams – ISM: clouds – ISM: dust, extinction – ISM: individual objects: CG 30; CG 31; CG 38

* Based on observations ESO, La Silla

Send offprint requests to: J. Knude

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999