Abstract
The Primordial Group hypothesis states that only sufficiently young open clusters (OCs) can be multiple, and old OCs are essentially isolated. We tested this postulate through four different studies using a manual search of Gaia EDR3 and extensive literature. First, we revisited the work of de La Fuente Marcos and de La Fuente Marcos (2009), which states that only ca. 40% of OC pairs are of primordial origin. However, no plausible binary system among their proposed OC pairs having at least one member older than 0.1 Gyr was found. Second, we researched the OCs < 0.01 Gyr old in Tarricq et al. (2021) and found that ca. 71% of them remain in their primordial groups. Third, a similar study of the oldest OCs (age > 4 Gyr) showed that they are essentially alone. Forth, the well-known case of the double cluster in Perseus and some other binary systems described in the literature were also shown to accommodate the title hypothesis. A simplified bimodal model allows for retrieval of the overall fraction of related OCs (approximately 12–16%) from our results, assuming that young clusters remain associated at ~0.04 Gyr. The obtained results further support that OCs are born in groups (Casado 2021).
Highlights
Facultad de Ciencias, Universidad Autónoma de Barcelona, 08193 Bellaterra, Barcelona, Spain; Abstract: The Primordial Group hypothesis states that only sufficiently young open clusters (OCs) can be multiple, and old OCs are essentially isolated
A reasonable overall fraction of associated OCs is obtained by assuming that OCs younger than 0.04 Gyr are still most likely associated with their primordial relatives, while older OCs are most likely isolated as their siblings have been separated or disintegrated by tidal forces in the Galaxy and close encounters with giant molecular clouds [65]
We formulated and tested the Primordial Group hypothesis, which states that OCs are born in primordial groups that disperse through the galactic disc in a
Summary
We conducted a more in-depth study of some of the proposed OC pairs in DFM, using data from the extensive literature and Gaia EDR3 (see Section 3). Following the criteria of previous studies, the obtained groups were refined by discarding OCs that were more than 100 pc away from any other member [9,11,24,25], assuming that all members are at the average distance (d) of the group. This cutoff was an order of magnitude and an unrestrictive maximum, as other studies have used more restrictive limits (e.g., DFM used 30 pc). The latter condition implies that the differences in tangential velocities are less than.
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