The Pleiades form an intermediate age (~120Myr) nearby (125pc) cluster. Visible by the naked eye in the Taurus constellation, they have been studied extensively over the past 100 years. Photograph Credit: Marco Lorenzi
We search the astronomical archives and retrieved wide field images obtained at various telescopes:
More than 16,500 images were retrieved and processed on our super-computer. The photograph on the left shows the footprint of the various datasets overplotted on the sky over Mauna Kea observatories (Subaru, Keck I & II, and UKIRT, from left to right). The area covered encompasses more than 80 square degrees.
The figure shows a so-called "vector-point diagram". It represents the proper motion in declination vs the proper motion in right ascension. A large clump randomly distributed around zero corresponds to the field stars.
A smaller clump, located around (+20,-40), also appears, and corresponds to the Pleiades members.
For comparison, the same diagram drawn using the UKIDSS DR9 proper motion measurements over the exact scame area are represented. The Pleiades clump is barely visible, making it hard to select members.
These proper motion measurements and the multi-wavelength photometry were fed to our proto-type selection software and the membership probability of every single source in the data was computed.
The figure on the left shows a (i, i-K) color-magnitude diagram with the final sequence of the Pleiades members found in our study overplotted. The color scale represents the membership probability. We reach well beyond Gaia's limit of sensitivity, down to ~25MJup. The sequence is extremely clean, and one can see the binary sequence just 0.75mag above the single-star sequence.
We applied our novel selection techniques to the old Tycho-2 and Hipparcos catalogues. We identified 207 candidates (in red) out of which 83 were new! Some historical candidates (in green in the figure) were also rejected.
The figure shows a zoom on the core of the cluster. In blue are represented the members discovered over more than 150 years of studies. We all re-discovered them in our analysis. In red are represented the new previously unknown members.
We almost doubled the number of high probability members, and multiply by five the number of substellar (brown dwarfs) members, leading to great improvements on the mass function of the cluster.
(Photograph Credit: Rogelio Bernal Andreo)
The figure shows the mass function of 3 important clusters:
- blue: the Trapezium cluster, 1 very young (1Myr) and rather massive cluster in Orion
- red: the Pleiades (120Myr), as measured in our study
- yellow: the Hyades, a 600Myr cluster
The Pleiades is often described as a snapshot of the Trapezium’s future and of the Hyades’ past. The proportion of very low mass stars and brown dwarfs is decreasing over time and the peak of the mass function is shifting from 0.2 M⦿ at the age of the Trapezium and of the Pleiades to 0.5~1 M⦿ by the age of the Hyades. The flattening of the Pleiades below 0.05M⦿ is observed in the Trapezium as well, reinforcing the idea that the over abundance of low mass brown dwarfs is due to additional formation mechanisms (photo-evaporation, disk instabilities, competitive accretion, ejections?) rather than late dynamical evolution.
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© Last Update: 02-05-2017 by H. Bouy