「第三の星」が連星を引き寄せる(”Third Wheel” Star Brings Companions Closer Together)

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2025-07-08 カリフォルニア工科大学(Caltech)

カルテックの研究により、白色矮星と伴星から成る爆発変光星「カタクリズミック・バリアブル(CV)」の形成に、第三の星の存在が重要であることが明らかになった。Gaia衛星のデータで特定された50の三重星系を解析した結果、約20%で第三星の重力が伴星の軌道を縮小させCV化を促進し、60%では共通包絡殻進化の開始に関与していた。実際のCVの約40%が三重星由来と推定され、観測バイアスにより過小評価されている可能性があるとされた。

「第三の星」が連星を引き寄せる(”Third Wheel” Star Brings Companions Closer Together)This artwork depicts a triple-star system in which two of the stars are locked in a tight gravitational orbit. The bright star in the foreground on the right is a white dwarf, which is stealing mass from its stellar companion. Eventually, this building up of mass on the white dwarf will trigger periodic explosions. Together, the two stars form an object called cataclysmic variable. New Caltech-led research has shown that a third star in triple-star systems (like the star depicted in the background here), can gravitationally influence its neighboring stars and lead to the formation of cataclysmic variables.Credit: Caltech/R. Hurt (IPAC)

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三重星における激変変数:形成モデルと新たな発見 Cataclysmic Variables in Triples: Formation Models and New Discoveries

Cheyanne Shariat, Kareem El-Badry, Smadar Naoz, Antonio C. Rodriguez, and Jan van Roestel
Publications of the Astronomical Society of the Pacific
DOI:10.1088/1538-3873/add5a1

Abstract

The formation of cataclysmic variables (CVs) has long been modeled as a product of common envelope evolution (CEE) in isolated binaries. However, a significant fraction of intermediate-mass stars—the progenitors of the white dwarfs (WDs) in CVs—are in triples. We therefore investigate the importance of triple star dynamics in CV formation. Using Gaia astrometry and existing CV catalogs, we construct a sample of ∼50 CVs in hierarchical triples within 1 kpc of the Sun, containing main-sequence and WD tertiaries at separations of 100–30,000 au. We infer that at least 10% of CVs host wide tertiaries. To interpret this discovery, we evolve a population of 2000 triples using detailed three-body simulations, 47 of which become CVs. We predict that 20% of CVs in triples form without ever experiencing CEE, where the WD and donor are brought together by the eccentric Kozai-Lidov mechanism after the formation of the WD. These systems favor larger donor stars and longer birth orbital periods (8–20 hr) than typical CVs. Among systems that do undergo CEE, about half would not have interacted without the presence of the tertiary. Triple formation channels both with and without CEE require initially wide inner orbits (≳1 au), which in turn require larger tertiary separations to be stable. Consistent with this prediction, we find that the observed Gaia CV triples have wider separations on average than normal wide binaries selected in the same way. Our work underscores the importance of triples in shaping interacting binary populations including CVs, ultracompact binaries, and low-mass X-ray binaries.

1701物理及び化学
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