When I saw this plot earlier this week I thought: "Why is it so different from an HR diagram?" Is the color index J-Ks reversed from the visible light one? Is the main sequence that growing slant? Where are the white dwarves? Where are the giants? I didn't have time to look into these matters, but they lurk there. Rather, here.
I suppose, the difference you refer to is the familiar narrow
correlation between the absolute visual magnitude M_V and the B-V (or V-I) color index [or also the original HR variables: Luminosity vs. Temperature] along the Main Sequence. Accompanied by the wider, separated patches corresponding to White Dwarfs, Giants and SuperGiants below and above the Main Sequence. Say, as a reference, a behaviour like depicted here e.g. for all stars that can be seen with the unaided eye:
In the VVV paper, the overall correlation between Ks magnitude and J - Ks color was very much "fatter" and thus less well defined.
If very dim stars (up to mag ~ 19, say) are included like in the VVV analysis, also the usual HR/CMD (main sequence) correlations look much "fatter". With 84 million of (mainly dim) stars the CMD displays will necessarily look much more "stuffed", given the relatively small dynamical range of such plots. More "stuffed" HR diagrams one finds e.g. also in case of deep Hubble-based CMDs of globular clusters (GCs). And that despite the fact that for GCs there is the additional beneficial feature that all stars of a given GC have about the same age! If the stars of GCs of widely different ages are overlaid, again a much fatter CMD correlation appears, mainly concerning the RedGiant branch (RGB)..
Here is how I interpret the VVV findings:
1) Not to forget, perhaps: The VVV measurements lie in the Near Infrared (with average filter wavelengths of J <-> 1.254 mu, H <-> 1.646 mu and Ks <-> 2.149 mu
), hence quite a bit away from the visual regime that one is familiar with. Moreover, near the galactic center, there is a very high amount of extinction, crowding and reddening which certainly affects (i.e. adds to wash out) the correlations we may expect. The reddening varies between E(B − V) < ~0.2 away from the galactic center (b = -10deg) and as much as E(B − V) ~10 (!) near the GC!
Finally bulge and disk stars partially overlap. The bulge is centered on the stellar disk of the Milky Way, where most of the stars, gas, and dust
of our Galaxy is concentrated.
2) The most instructive breaking down of the findings was performed in the paper, by slicing the total amount of bulge sources into 15 areas, which allows to study the behaviour of the bulge CMD differentially
, as function of the location.
These 15 areas are depicted here:
The color coding (along with its explanation above the diagram) illustrates the dramatic differences in star densities that one encounters in the various areas. We see that the density variations mainly depend on galactic latitude b
and only little on galactic longitude l.
Qualitatively, I would expect a very washed out CMD near the galactic center (areas #10, #11 and #12), while in the lowest density areas (#01, #02, #03) I'd expect a CMD that is somewhat closer to more familiar prejudices.
This expectation is confirmed in the 15 associated CMDs. The area numbers are displayed in the various sub-diagrams. Compare the 2nd row from the top (b ~ 0) with the last one (b~-9.5 deg)!
3) For a finer localization and examination of the various components (Red Clump Giants, dwarf sequence etc) a comparison with specific stellar population synthesis models was necessary and performed in the paper. Notably the BesanÃ§on model
seems to describe the data well and allows to identify all the expected types of sources. One finds a predominance of main-sequence and giant stars in the outer bulge, which belong both to the bulge and halo. Thin- and thick-disk stars are also present in fewer numbers. In the inner bulge the CMD is dominated by bulge giants
, which contribute up to 46% of the total sources
at b ∼ −3◦ The analysis of the outermost bulge area reveals a well-defined sequence of late K and M dwarfs.
A crucial display is this one from the outermost region in the VVV bulge area (b = -9.5 deg)
Main-sequence stars are depicted in orange, subgiants in blue and giants
are red dots. Supergiants, bright giants, and white dwarfs are also present, but there are only a few identifications.
Given the considerable overall pattern complexity a more detailed study of the discussion in the VVV paper seems unavoidable.