Solar System Encounters with Open Star Clusters
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r System Encounters with Open Star Clusters M. D. Sizovaa, *, S. V. Vereshchagina, B. M. Shustova, and N. V. Chupinaa a Institute
of Astronomy, Russian Academy of Sciences, Moscow, 119017 Russia *e-mail: [email protected]
Received March 24, 2020; revised April 15, 2020; accepted April 30, 2020
Abstract—The time and minimum distances of approaches were calculated more than 100 open star clusters (OSCs) near the Solar System over the past 5 Myr. It was shown that the closest proximity of the centers of the OSCs to the Solar System usually exceeded 60 pc. The Hyades cluster approached the Solar System at ~25 pc. The effect of such encounters on the comet motion in the outer regions of the Oort cloud is comparable and may even exceed the effect of encounters with individual stars. The celestial positions of the cluster antiapex were compared with the aphelia of long-period comets. Cases of its close positions were revealed for some comets and the interstellar asteroid 1I/2017 U1 (Oumuamua). The frequency of OSC passages in the vicinity of the Solar System was estimated. Encounters at a distance of 25 pc occur at a rate of ~20 times per billion years. DOI: 10.1134/S106377292009005X
1. INTRODUCTION The Oort cloud is the source of long-period comets (see, e.g., [1]). Since comets in the Oort cloud have weak gravitational connection with the Sun, their orbits can undergo relatively strong perturbations from encounters with various compact gravitating masses (molecular clouds, star clusters, stars, or interstellar bodies of prestellar mass, such as brown dwarfs and interstellar planets). Some of these encounters may cause, in particular, a temporary increase in the stream of comets coming into the inner Solar System (the so-called “comet showers”). The influence of the so-called galactic tides on the Solar System are not considered, although in a general sense it is also significant (see, e.g., the recent study [2] and references therein) and has a substantial effect on the parameters of cometary orbits (see, e.g., [3]). The galactic tidal forces change little on a short time scale (few million years), while in this study, the focus is on noticeable and rather frequent local perturbations of the gravitational field. Furthermore, the encounters with molecular clouds were not addressed. Under ordinary conditions, this effect is not so significant as compared to stellar encounters [4, 5], although in certain cases (e.g., the motion of the Solar System through a dense giant molecular cloud among numerous massive clumps), the effect of such encounters can be considerable [6]. As for brown dwarfs and interstellar planets, the observational statistics of such objects is still very incomplete. It is possible that their number is relatively small compared to stars [7]. It is also important to note that the masses of such objects are relatively small, and only short-distance encounters can have a significant
effect. However, such encounters are very rare. For this reason, we confined ourselves to considering encounters with stars a
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