<p>You could try selecting them in an <em>absolute</em> magnitude versus colour diagram.</p>

<p>e.g. Use the apparent G magnitude and parallax to find absolute G magnitude and plot this versus a colour (<span class="math-container">$B_p-R_p$</span>).</p>

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

<p>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.</p>

CC BY-SA 4.0
2019-02-03T03:00:48.297Z
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.
NeutronStar
CC BY-SA 4.0
2019-02-03T09:01:37.920Z
@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?
ProfRob
CC BY-SA 4.0
2019-02-04T00:03:44.723Z
No, Gaia DR2 has *estimated* dereddening.  The "e_bp_min_rp_val" in https://gea.esac.esa.int/archive/documentation/GDR2/Gaia_archive/chap_datamodel/sec_dm_main_tables/ssec_dm_gaia_source.html
NeutronStar
CC BY-SA 4.0
2019-02-04T07:42:32.057Z
@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?
ProfRob
CC BY-SA 4.0
2019-02-05T00:20:27.947Z
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.
NeutronStar
CC BY-SA 4.0
2019-02-02T08:11:07.727Z
2019-02-02T08:11:07.727Z
ProfRob
<p>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.</p>

<star><milky-way>
How many horizontal branch stars are within 2 kpc of the sun?
NeutronStar
# How many horizontal branch stars are within 2 kpc of the sun?

(Asked by: username_0 on 2019-01-31)

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.

**Tags:** star, milky-way

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## 1 answers

### Answer by username_1 on 2019-02-02. Score: 1

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.

> **Comments:**
> **username_0**: 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.  
> **username_1**: @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?  
> **username_0**: No, Gaia DR2 has *estimated* dereddening.  The "e_bp_min_rp_val" in https://gea.esac.esa.int/archive/documentation/GDR2/Gaia_archive/chap_datamodel/sec_dm_main_tables/ssec_dm_gaia_source.html  
> **username_1**: @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?  
> **username_0**: 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.  

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