Open cluster remnant

Last updated April 29, 2019

In astronomy, an open cluster remnant (OCR) is the final stage in the evolution of an open star cluster.

Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A branch of astronomy called cosmology is the study of the Universe as a whole.

Theory

Viktor Ambartsumian (1938) and Lyman Spitzer (1940) showed that, from a theoretical point of view, it was impossible for a star cluster to evaporate completely; furthermore, Spitzer pointed out two possible final results for the evolution of a star cluster: evaporation provokes physical collisions between stars, or evaporation proceeds until a stable binary or higher multiplicity system is produced.

Lyman Strong Spitzer, Jr. was an American theoretical physicist, astronomer and mountaineer. As a scientist, he carried out research into star formation, plasma physics, and in 1946, conceived the idea of telescopes operating in outer space. Spitzer invented the stellarator plasma device and is the namesake of NASA's Spitzer Space Telescope. As a mountaineer, he made the first ascent of Mount Thor, with Donald C. Morton.

Star clusters are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Observations

Using objective-prism plates, Lodén (1987, 1988, 1993) has investigated the possible population of open cluster remnants in our Galaxy under the assumption that the stars in these clusters should have similar luminosity and spectral type. He found that about 30% of the objects in his sample could be catalogued as a possible type of cluster remnant. The membership for these objects is ≥ 15. The typical age of these systems is about 150 Myr with a range of 50-200 Myr. They show a significant density of binaries and a large number of optical binaries. The stars of these OCRs have a trend to be massive and hence early-type (A-F) stars although this observational method includes a noticeable selection effect because bright early-type spectra are easier to detect than fainter and later ones. In fact, almost no stars with spectral type later than F appear among his objects. On the other hand, his results were not fully conclusive because there are known regions in the sky with many stars of the same spectral type but in which it is difficult to find two stars with the same proper motions or radial velocity. A striking example of this fact is Upgren 1; initially, it was suggested that this small group of seven F stars was the remnant of an old cluster (Upgren & Rubin 1965) but later, Gatewood et al. (1988) concluded that Upgren 1 is only a chance alignment of F stars resulting from the close passage of members of two dynamically different sets of stars. Very recently, Stefanik et al. (1997) have shown that one of the sets is formed by 5 stars including a long-period binary and an unusual triple system.

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power.

The radial velocity of an object with respect to a given point is the rate of change of the distance between the object and the point. That is, the radial velocity is the component of the object's velocity that points in the direction of the radius connecting the object and the point. In astronomy, the point is usually taken to be the observer on Earth, so the radial velocity then denotes the speed with which the object moves away from or approaches the Earth.

Simulations

Regarding numerical simulations, for systems with some 25 to 250 stars, von Hoerner (1960, 1963), Aarseth (1968) and van Albada (1968) suggested that the final outcome of the evolution of an open cluster is one or more tightly bound binaries (or even a hierarchical triple system). Van Albada pointed out several observational candidates (σ Ori, ADS 12696, ρ Oph, 1 Cas, 8 Lac and 67 Oph) as being OCRs and Wielen (1975) indicated another one, the Ursa Major moving group (Collinder 285).

Sebastian Rudolf Karl von Hoerner was a German astrophysicist and radio astronomer.

Sverre Johannes Aarseth, is a research scientist at the Institute of Astronomy at the University of Cambridge. Although retired, Aarseth is still an active researcher. He has dedicated his career to the development of N-body codes. He is the author of the NBODY family of codes, the current iteration is NBODY7. His current areas of research include the effects of stellar evolution in N-body codes, the influence of black holes on stellar systems, the evolution of globular clusters, and the use of GPUs to increase the speed of his codes.

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References

Aarseth, S. J.; 1968, Bull. Astron. Ser., 3, 3, 105
van Albada, T. S.; 1968, Bull. Astron. Inst. Neth., 19, 479
Ambartsumian, V. A.; 1938, Ann. Len. State Univ., # 22, 4, 19 (English translation in: Dynamics of Star Clusters, eds. J. Goodman, P. Hut, (Dordrecht: Reidel) p. 521)
Gatewood, G.; De Jonge, J. K.; Castelaz, M.; et al., 1988, ApJ, 332, 917
von Hoerner, S.; 1960, Z. Astrophys., 50, 184
von Hoerner, S.; 1963, Z. Astrophys., 57, 47
Lodén, L. O.; 1987, Ir. Astron. J., 18, 95
Lodén, L. O.; 1988, A&SS, 142, 177
Lodén, L. O.; 1993, A&SS, 199, 165
Spitzer, L.; 1940, MNRAS, 100, 397
Stefanik, R. P.; Caruso, J. R.; Torres, G.; Jha, S.; Latham, D. W.; 1997, Baltic Astronomy, 6, 137
Upgren, A. R.; Rubin V. C.; 1965, PASP, 77, 355
Wielen, R.; 1975, in: Dynamics of Stellar Systems, ed. A. Hayli, (Dordrecht: Reidel) p. 97

Further reading

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