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I'm trying to plot H-R Diagrams of open clusters using data from Gaia DR3. My workflow is something like this:

  • Plot apparent magnitude vs log(distance) in pc. An overdensity would indicate the presence of a cluster around that distance. enter image description here
  • Filter out stars near this overdensity and generate a histogram against distance. enter image description here
  • Fit the histogram to the gaussian and apply a $3\sigma$ cut to pick stars likely in the cluster. enter image description here enter image description here
    The corresponding HR Diagram looks like this: enter image description here I have plotted HR Diagrams for many other clusters too: enter image description here The horizontal branch of stars( in the diagram, not the one that follows the red giant phase) is so pronounced, it worries me since the open clusters are not very old to contain so many white dwarfs(for example, when I plotted the HR Diagram of Omega Centauri, the stars were mostly in the red giant region, and Omega Centauri is definitely older than these). Note that the Wishing Well Cluster doesn't even have a distinctive main sequence. I have tried playing with the ADQL Queries I used, but either I don't see a distinct Gaussian, or I face the issue described above.
    What could have gone wrong? Is there a more efficient way to pick out the stars? Any help would be appreciated. (Let me know if representative codes are required at any point).
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  • $\begingroup$ You need to say exactly how you produced the plot. There is clearly an error since the main sequence has an absolute magnitude of around zero at a colour of zero and there are relatively few stars more luminous. Cluster colour magnitude diagrams don't look like this $\endgroup$
    – ProfRob
    Jul 8 at 7:14
  • $\begingroup$ @ProfRob Can you please tell me what specifics I need to mention? Will it help if I include the code? $\endgroup$
    – Ambica Govind
    Jul 8 at 7:23
  • $\begingroup$ Plot the HR diagram for THAT cluster. You have stars at 480 pc (did you use the same distance for all stars), so a distance modulus of 8.4. If you've ignored extinction, and from your first diagram the members are between apparent magnitudes of 8 and 19, then their absolute magnitudes should be between -0.4 and 10.6. Clearly that isn't the case, so you've made an error. $\endgroup$
    – ProfRob
    Jul 8 at 12:42
  • $\begingroup$ and why does the upper envelope of the first diagram get brighter for more distant stars? Faint magnitudes in the Gaia catalogue are dominated by more distant stars and there certainly isn't a sharply defined cut-off. I don't think you have fully explained what you;ve done to the data. $\endgroup$
    – ProfRob
    Jul 8 at 12:47
  • $\begingroup$ @ProfRob I have added a parallax cut to the data such as parallax over the error in parallax is greater than 5. Which is why you see the sharp decline at the upper envelope. $\endgroup$
    – Ambica Govind
    Jul 8 at 15:34

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You don't have a horizontal branch, which would have an absolute magnitude of $\sim 2$ and you don't have a massive population of white dwarfs, which have a sequence that parallels the main sequence but about 10 magnitudes fainter at the same colour. The Trapezium and Alpha Per are too young to have a white dwarf population too. The Wishing Well (NGC 3532), Beehive (Praesepe) do have a few white dwarfs and The "King Cobra" cluster (M67) has a large number of white dwarfs, but they have an absolute magnitude $>10$ and you would struggle to see them in Gaia data (Richer et al. 1998)

What you have is a bunch of objects at the faint end of the distribution in each cluster that have very poor $B_p$ photometry. You should have a look at section 6 of Riello et al. (2021), where some problems are discussed in relation to the photometry of faint sources and possible filtering steps suggested.

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