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For my research purposes, I am wondering how many horizontal branch (HB) stars are within 2 kpc of the sun? I tried to find some with the Gaia DR2 data with a magnitude cut that leaves HB stars out if they're beyond ~2 kpc, and there were surprisingly few unambiguous HB stars, so I'm just wondering how many there are and/or how to go about estimating that.

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You could try selecting them in an absolute magnitude versus colour diagram.

e.g. Use the apparent G magnitude and parallax to find absolute G magnitude and plot this versus a colour ($B_p-R_p$).

The HB stars occupy a reasonably distinct region (box?) in this diagram.

Your problem is that this distinct box is blurred by differences in composition and by extinction/reddening. You could sidestep the latter to some extent by avoiding low Galactic latitudes.

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  • $\begingroup$ That's the thing though, I'm selecting them based on absolute magnitude vs. (dereddened) color, but I'm not finding any. I wanted a check to see if 0 is a reasonable expectation. $\endgroup$ – NeutronStar Feb 3 '19 at 3:00
  • $\begingroup$ @Joshua then you are not fully explaining what dataset you have, or what you are doing to it, since dereddening requires something additional to Gaia Dr2? $\endgroup$ – Rob Jeffries Feb 3 '19 at 9:01
  • $\begingroup$ No, Gaia DR2 has estimated dereddening. The "e_bp_min_rp_val" in gea.esac.esa.int/archive/documentation/GDR2/Gaia_archive/… $\endgroup$ – NeutronStar Feb 4 '19 at 0:03
  • $\begingroup$ @Joshua so you are restricting your analysis to stars with a measured Teff in DR2? You need to explain all the details of what you are doing and show your absolute CMD and selection box. There are of course many HB stars within 2kpc. You could find some examples and put them on your CMD? $\endgroup$ – Rob Jeffries Feb 4 '19 at 7:42
  • $\begingroup$ My question, though, isn't how what I'm doing with the Gaia data is wrong, but rather what the number of HB stars within 2 kpc is (as a check on what I'm doing with the Gaia data). I believe we're getting off track here. $\endgroup$ – NeutronStar Feb 5 '19 at 0:20

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