So last time we established that the putative middle-aged, nearby cluster Lodén1 is neither middle-aged, nor nearby, nor a cluster.

(That, by the way, was basically the title of our paper, but the referee really disliked it (they thought it was confusing to call it a cluster then say it isn’t a cluster). Then the copyeditor went and changed all of our “amongst”s to “among”s.  Ah, well.)

We still need to take a look at NGC 2240 just in case, but frankly it’s pretty obviously not a real cluster, so it’s a low priority.  Also, Gaia is coming, and it will make this whole endeavor much easier.

The rest of this post is some thoughts by Jason Curtis on the topic:

While Lodén 1 does not appear to be a real cluster, the existence and recent realization that Ruprecht 147 is the oldest nearby cluster suggests that similar clusters might have also gone overlooked. As we say in our paper, “the utility of such clusters for stellar astrophysics demands that we find them.” This fall, the European Space Agency will begin releasing high-precision position and velocity data produced by its Gaia mission, starting with the brightest 2 million stars that were observed by the Hipparcos satellite in the early 1990s.

This first catalog will reach stars like the Sun at distances of up to 1000 light years, including the majority of the proposed candidates of Lodén 1 and the membership of Ruprecht 147. We expect that the improved proper motion precision will enhance our ability to determine the nature of unproven clusters like Lodén 1 and improve the membership identification of established clusters like Ruprecht 147.

Artist’s impression of Gaia

The stars that make up a cluster travel together through the galaxy, with little spread in position (e.g., clusters can span 10—20 light years) and velocity (typically 500 m/s, whereas the cluster itself can travel at tens of km/s relative to neighboring stars). Within a few years, Gaia will release 3D positions (coordinates and parallaxes) and 3D space motions (proper motions and radial velocities) for some 150 million stars, while fainter stars will still receive 3D positions and 2D proper motions. Even sparse star clusters like Ruprecht 147 are 2—3 times denser than the Solar neighborhood. This is not a huge contrast; however, the inclusion of ultra-high-precision proper motions from Gaia (which will see a 100—200x increase in precision over existing proper motions) will make finding sparse clusters easy!

Gaia’s measurements will let us….

1. “Weigh” clusters by measuring velocity dispersion from proper motions
2. Resolve internal structure of nearby clusters with parallaxes, and eliminate distance uncertainty for others
3. Enable easy cluster identification by looking for over-densities in 5D phase space (3D positions—including parallax— and proper motions). Basically, stars are distributed throughout the Solar neighborhood and nearby galactic environment with a characteristic density or spacing. Clusters, even sparse clusters, should appear as over-densities given the expected high precision of the parallaxes. Furthermore, the stars in a given few parsec volume do not exhibit coherent proper motion. Together, these 5 position/motion components should yield new clusters, and new members of known clusters.

Lots to do, but first we should search for bound and moderately rich (N>20) systems that can immediately be characterized and targeted by space missions like TESS.

Our paper is now on the arXiv and will soon appear in the Astronomical Journal.