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

6. Red clump giants

As discussed in Sect. 5 giants probably do not pose a problem for the identification of the (200, 36) feature by means of dwarf stars exclusively. In Fig. 9 the lower dotted line represents a reddening line of a M_V = +10 star located at 2.6 kpc. We have assumed that the bluest value of (V-I)₀ for a M_V = +10 giant is 08, see Egret et al. (1997). Recall that 2.6 kpc and A_V = 28 defines the blue envelope of the CG 31 tail box displayed in Fig. 3. This reddening line shifted to the bright side by V = 2^m magnitudes fits the upper envelope in Fig. 9. It seems most unlikely that a bright confinement of a color magnitude diagram should have the exact slope of the reddening vector by coincidence. This is a very interesting situation: a group of stars exactly spread out along a reddening line means that they are of a very well defined type, located at the same distance but experiencing a large range of reddening. Measured along the upper confinement (V-I) ranges from 10 to 32. We suggest that this string of stars must belong to a most ordinary stellar type and since it cannot be LC V, the most frequent class next to the main sequence stars are the red clump giants. Furthermore very well defined with M_V = +10 and (V-I)₀ = 1.0 either with a very small spread, see e.g. Fig. 3.5.5 and 3.5.6 in the Hipparcos and Tycho Catalogues Vol. 1 (ESA 1997). If this identification is correct the clump giants are located almost exactly 1 kpc away and their absorption ranges from nil to A_V = 62. Stars with V within 03 of the upper red envelope do show a noticeable concentration in Fig. 1, they are mainly found in a 7´7´ box shown in Fig. 1. At 1 kpc this angular extent means that they are located within a diameter of 2.3 pc. Incidentally 1000 pc is almost the distance to the Gum Nebula center. We are pretty convinced that the giant identification is right with a rather high degree of probability. Of the 14 stars within 03 from the reddest envelope nine have complete uvby colors from our Strömgren survey of the region. Of these seven are classified as giants only two seems to be late type dwarfs, their c₁ values are much too small for giants. In Fig. 9 the two dwarfs are located at (V-I,V) = (1.315,12.120) and (1.788,13.371) respectively. The success rate for identifying red clump giants along the red confinement is accordingly more than 50.

If the giant interpretation is right the reddest stars between the two dotted lines in Fig. 9 might sample some of the most obscured lines of sight. We select stars with (V-I) 30 and V 188 which if they are red clump stars will be absorbed by more than 55 magnitudes in V. Their location are plotted as the large filled circles in Fig. 1. Apart from two directions they seem closely associated to the cometary globules' edges or their tails. The two "discrepant" sight lines are however pointed in a direction where Fig. 1 indicates a very low number of stars per unit area: (X, Y) (1400, -200). Possibly suggesting a minor hitherto unrecognized globule.

From inspection of Fig. 2 or Fig. 9 the bulk of stars seems to have a rightmost confinement apparently parallel to the reddening vector. In Fig. 9 the thick dashed line shows the reddening line for red clump giants at 9.3 kpc from the Sun, with R_GC = 8.5 kpc putting red clump giants 14.5 kpc from the Milky Way center. We note the dramatic difference of the number of stars to the bright and faint side of this line. In Fig. 10 we show a contour plot of the stellar density sampled in (V-I,V) boxes sized 01 and 05 in (V-I) and V respectively. In Fig. 10 a reddening vector is fitted to the 12 stars/box contour, the line has its bluest point approximately at the intrinsic color of the red clump giants. The V-M_V shift is 1425 and as mentioned since the red clump giants have a strongly peaked M_V distribution at M_V = +1^m such a reddening line sample clump stars stars at identical distance but with very different amounts of reddening. The stars along the 12 stars/box contour are thus 7.1 kpc from the Sun and those along the 30 stars/box contour also nearly parallel to the reddening vector but located at 8.9 kpc. Assuming R_GC = 8.5 kpc red clump giants located on the two reddening lines are at 13 and 14 kpc from the Galaxy's center respectively. The absorption range for stars along either reddening line is from A_V 0 to A_V V_lim-16 5^m. One could wonder why a reddening line fits the slope of an iso-density contour? Why are there identical numbers of stars with any absorption between 0 and 5? Perhaps not so strange after all if we postulate that only the diffuse medium of the disk causes the absorption of these remote stars, if molecular matter was the absorbing agent the absorption would probably be larger, as discussed in the previous sections and the stars accordingly much nearer. If the absorption is diffuse it seems evenly distributed between a minimum and maximum value as shown in Jonch-Sorensen (1994). If the structure of the local diffuse medium is representative on a scale of several kpcs the reddening distribution broadens with distance and any reddening between the minimum and maximum values observed, have the same probability. The local version of this trend may be seen within 400 pc in Fig. 1 of Knude (1979), where stars between 250 and 400 pc have a completely flat distribution and those between 30 and 80 pc have a peaked distribution.

Fig. 10. Stellar density in the color - magnitude diagram, sampled in 0.1 and 0.5 magnitude bins in (V-I) and V respectively. The dashed line indicates a reddening line adapted to the 12 star per (V-I,V) box iso-contour by a shift V - MV = 1425, the 30 stars per (V-I,V) box is fitted with a similar 1475 shift

The 12-30 star/box contours may in fact indicate the edge of the Milky Way. Stars nearer than 7-9 kpc, we mean red clump giants exclusively, will be on a line shifted upwards whereas more remote ones will be on a line shifted downwards. If the dust disk is exponential the reddening taking place in the outer parts becomes less, this means that if we observe red clump giants with A_V 0 at 13-14 kpc we have an equal probability to encounter red clump giants with A_V 0 at larger distances from the center - that is if they exist. But as Fig. 9 shows virtually no stars with (V-I) 0.8-0.9 are observed for V 16^m indicating that the central distance for disk stars has an upper limit at 13-14 kpc. 13-14 kpc may accordingly estimate the optical disk radius. A rather interesting thought that the disk size may be estimated, model independent, from deep V and I observations, only requirement is the solar distance from the Galactic center.

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999
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