This WISE multicolor image shows NGC 1605, a binary open cluster located 8,480 light-years away in the constellation of Perseus. It was first discovered on December 11, 1786 by the German-born British astronomer William Herschel. Image credit: NASA / WISE / Camargo, arXiv: 2109.14664.
Open clusters are groups of a few tens to a few thousand stars weakly bound by gravity.
These objects are so-named due to the fact that individual stars are easily resolved through a telescope.
Some examples such as the Hyades and Pleiades are so close that the individual stars can be clearly distinguished by the naked eye.
Stars in an open cluster have a common origin — they formed from the same initial giant molecular cloud.
Open clusters have been observed with a range of ages — they may be made up of young stars or older stars.
Owing to their open and diffuse structure, open clusters are not particularly stable, and their constituent stars might disperse after a few million years.
For this reason, they are found in spiral and irregular galaxies, where new stars are being formed, but not in elliptical galaxies, which are not forming stars and in which any open clusters would have long since broken up.
Over 1,000 open clusters — some of which are young binary clusters — have so far been discovered within our Milky Way Galaxy.
“Most known open cluster pairs are young with some few intermediate-age exceptions and no one old survivor, which suggest that such a systems are short-lived,” Professor Camargo said.
“In this sense, the survival timescale for binary clusters appears to be in the range 10-100 million years, with less than 17% of the Galactic multiple systems surviving for more than 25 million years.”
“Binary clusters may form after the gravitational collapse of multiple massive and dense gas clumps inside giant molecular clouds, with collapsing clumps forming clusters of similar ages and very close to each other,” he explained.
“On the other hand, occasionally, a cluster pair may be formed by tidal capture during a close encounter between two clusters typically with different ages.”
In the new study, Professor Camargo analyzed images of the NGC 1605 open cluster captured by NASA’s Wide-field Infrared Survey Explorer (WISE) as well as photometric data gathered by the Two Micron All Sky Survey (2MASS) and ESA’s Gaia spacecraft.
He found that NGC1605 is the by-product of a high-energy close encounter between two old open clusters.
These binary members — designated NGC 1605a and NGC 1605b — are two billion years old and 600 million years old, respectively.
In addition, these clusters are undergoing a merger; both are located at the same distance from the Sun of 8,480 light-years (2,600 parsecs) and with a projected separation of only 5.6 light-years (1.8 parsecs) between their central cores.
A stellar bridge connecting NGC1605a and NGC1605b suggests a possible mass transfer.
“Close encounters between star clusters are rare, obviously the subsequent formation of binary clusters is even rarer, and the evolution to a merger event is extremely unlikely, which makes the discovery extremely important,” Professor Camargo said.
“NGC 1605a and NGC 1605b become the first old binary open cluster known in the Galaxy and, if that was not enough, they appear to be undergoing a merger during a close encounter, leaving streams populated by bound substructures.”
“In this work I suggest that during the close encounter stellar groups are been pulled out of the merging clusters by tidal interactions and these substructures are forming new star clusters distributed along tidal streams that trace the two clusters path.”
“The study represents a paradigm shift by demonstrating that during a close encounter of two open clusters, in addition to the evaporation of individual stars from these clusters, stellar groups can be pulled out of them by tidal effects, forming tidal debris distributed along of tidal streams and, surprisingly, some of them may survive as bound stellar systems.”
Denilso Camargo. 2021. NGC1605a and b: an old binary open cluster in the Galaxy. ApJ, in press; arXiv: 2109.14664