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

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1. Introduction

Quantification of several physical parameters pertinent for star formation and to phenomenae related to star formation like HH objects and jets depend on determining the distance to the star forming region. For the nearby star forming regions an indication of the distance may be obtained directly from the Hipparcos and Tycho Catalogues (ESA 1997, e.g. Knude & Hog 1998) if they are extended themselves or are part of a larger structure with a well defined distance. Since the distance to the CG 30 complex is supposed to be of the order of 400-450 pc, e.g. Henning & Launhardt (1998), Hodapp & Ladd (1995) the limiting magnitude of Hipparcos does apparently not permit a direct accurate distance estimate. The high column density and small size of globules make distance determination with classical methods an uncertain undertaking. Instead distances are often derived from the globules' association with objects of presumed known distances like molecular clouds or stellar associations, e.g. Launhardt & Henning (1997). In the case of CG 30 the distance of the Vela OB2 association or the distance to the Vela sheet has been adopted. The cometary globules may, however, present a slightly more tractable case than the regular globules since their tails seem less massive and thus could offer extinction measurements of background stars shining through the tails.

Nielsen et al. (1998) proposed that the gas of CG 30 might be distributed in two major distinct globules with a mass ratio 4:1 and showing a significant velocity difference. One could speculate whether such a fragmentation plays a role in the angular momentum balance. To evaluate this the linear size of the separation must be known with some precision. The important evaluation of the dynamical timescale of the molecular outflow from the protostellar source associated with HH 120 in CG 30 of course also depends on the distance estimate. CG 30 may be of particular interest since it may harbor one of the better Class 0 candidates, Henning & Launhardt (1998).

The CG 30 distance could furthermore be representative for all the globules forming the Vela - CG - Annulus, Bourke et al. (1995).

For the uvby[FORMULA] part of our program, to be presented elsewhere, we also obtained exposures in the 150 Å wide [FORMULA] band centered on the Balmer line [FORMULA]. These exposures reveal very detailed structure in Balmer line emission from the ionized gas on the surface of the globules delineating distinct structures and showing that the mass distribution of the complex may be much more fragmented than anticipated. We may be able to identify individual small fragments since the optical resolution is an order of magnitude better than the single dish mm resolution. An accurate distance estimate will thus permit a determination of the distribution of the linear sizes of the fragments not only the angular sizes and if mass densities may be provided from the mm data a mass spectrum of the fragments can be produced, a spectrum that could be useful in a dynamical study of the region.

Our present discussion of the (V-I), V diagram introduces another view on distance and extinction determination than star and color counts by searching for sets of main sequence stars at approximately similar distance and reddened by comparable amounts of dust. Such a set forms a locus in the color - magnitude diagram with a shape as the intrinsic color - absolute magnitude relation valid for the main sequence. A locus like that is generally difficult, or even impossible to recognize except when it forms confinements to the blue or the red of features or at times it may fit an isolated internal feature in the color - magnitude diagrams. Deep CCD exposures resulting in frames with a high surface density allow us to search rather small areas - where the identification of a shifted main sequence may be feasible - for the effects of interstellar dust.

The interpretation of the (V-I) - V diagram requires a (V-I)0 - MV relation. We produce one from nearby, presumably unreddened Hipparcos stars. The relation we propose do not pay any attention neither to the effects of varying metallicity on color and absolute magnitude nor to the evolution across the main sequence band. Statistically most of the stars observed are presumably disk stars only slightly evolved and of solar like composition.

The second most frequent stellar type of the fairly sampled Hertzsprung - Russell diagram is the red clump giants. Their prevalence suggests that they should be identifiable in the color - magnitude diagram but their sharply defined (V-I)0 and MV values imply that they do not form a locus like the main sequence stars but that they may be found along a properly shifted reddening line. We discuss two such reddening lines: the red confinement of the color - magnitude diagram and perhaps most interestingly the red clump giants situated at the edge of the Milky Way's optical disk.

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999