<p>I'm working in my final course project and I'm using the recently released <a href="http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/345" rel="nofollow noreferrer">GAIA DR2</a> data. I want to determine open clusters membership and I know this has mainly been done  using proper motions and positions, and maybe photometric data.<br>
Since now we have access to parallaxes I was wondering if one could also estimate the cluster membership from them, considering all cluster members must have approximately the same parallax value (if they are to be at the same distance from us). I guess the technique can't be much different to the usual one done with proper motions, but I can't work it out in a rigorous way. Any help is appreciated! Thanks in advance.</p>

CC BY-SA 3.0
2018-04-26T18:16:11.507Z
I'm not sure what your question is. Most open clusters are too far for even Gaia to get a good parallax measurement of, but you are welcome to try.
James K
CC BY-SA 3.0
2018-04-26T18:52:51.853Z
@JamesK The cluster distance has been estimated to be about 550 pc, so luckily it isn't very far. My question is: can the cluster members be distinguised using parallax data?  It seems like so, I just found [this article](https://arxiv.org/abs/1801.10042), in which the authors use parallax and proper motions to detect cluster members.
Lith
CC BY-SA 3.0
2018-04-26T21:38:18.467Z
The answer is yes and you have found a paper that tells you how it can be done.
ProfRob
CC BY-SA 3.0
2018-04-26T21:51:19.480Z
@RobJeffries Enlighten me.  How far can Gaia get good parallax, sufficient for deciding if a star is a cluster member or a background star? Then of the open clusters that are known, how many are close enough?
James K
CC BY-SA 3.0
2018-04-27T06:17:55.603Z
@JamesK You will see from the paper referred to by the OP that in Gaia DR1, parallaxes were used for 128 clusters out to 2kpc. The Gaia DR2 parallaxes are many times better and exist for stars that are 7 magnitudes fainter. Parallax can never be solely used to decide if something is a cluster member or background star. Most known open clusters (out to a few kpc) will have members with individual distance estimates in DR2. Those that don't - the issue will be crowding in the Galactic plane.
ProfRob
<parallax><star-cluster><gaia>
Open cluster membership determination via parallaxes
Lith
# Open cluster membership determination via parallaxes

(Asked by: username_0 on 2018-04-26)

I'm working in my final course project and I'm using the recently released [GAIA DR2](http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/345) data. I want to determine open clusters membership and I know this has mainly been done using proper motions and positions, and maybe photometric data.  
Since now we have access to parallaxes I was wondering if one could also estimate the cluster membership from them, considering all cluster members must have approximately the same parallax value (if they are to be at the same distance from us). I guess the technique can't be much different to the usual one done with proper motions, but I can't work it out in a rigorous way. Any help is appreciated! Thanks in advance.

**Tags:** parallax, star-cluster, gaia

> **Comments:**
> **username_1**: I'm not sure what your question is. Most open clusters are too far for even Gaia to get a good parallax measurement of, but you are welcome to try.  
> **username_0**: @JamesK The cluster distance has been estimated to be about 550 pc, so luckily it isn't very far. My question is: can the cluster members be distinguised using parallax data?  It seems like so, I just found [this article](https://arxiv.org/abs/1801.10042), in which the authors use parallax and proper motions to detect cluster members.  
> **username_2**: The answer is yes and you have found a paper that tells you how it can be done.  
> **username_1**: @RobJeffries Enlighten me.  How far can Gaia get good parallax, sufficient for deciding if a star is a cluster member or a background star? Then of the open clusters that are known, how many are close enough?  
> **username_2**: @JamesK You will see from the paper referred to by the OP that in Gaia DR1, parallaxes were used for 128 clusters out to 2kpc. The Gaia DR2 parallaxes are many times better and exist for stars that are 7 magnitudes fainter. Parallax can never be solely used to decide if something is a cluster member or background star. Most known open clusters (out to a few kpc) will have members with individual distance estimates in DR2. Those that don't - the issue will be crowding in the Galactic plane.  

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## 0 answers

