Published online by Cambridge University Press: 22 February 2018
The statistics of stellar systems of multiplicity three and higher is reviewed. They are frequent, 0.15−0.25 of all stellar systems. Some 700 multiples are expected among the 3383 stars of spectral type F, G, and K within 50 pc, while only 76 of them are actually known. Many (if not all) close binaries have distant tertiary components, indicating that angular momentum exchange within multiple systems was probably critical in forming short-period binaries. The ratio of outer to inner periods in the best-studied nearby multiples and in low-mass pre-main sequence multiples does not exceed 104 at the formation epoch; larger ratios are produced by subsequent orbital evolution. All multiples with well-defined orbits are dynamically stable, the eccentricities of outer orbits obey the empirical stability limit Pout(1 – eout)3/Pin > 5 that is more strict than current theoretical limits. Relative orientation of orbits in triple stars shows some degree of alignment, especially in weakly-hierarchical systems. The statistics support the idea that most multiple stars originated from dynamical interactions in small clusters.
Se reseñan las propiedades estadísticas de los sistemas estelares de multiplicidad tres o mayor. Comprenden el 0.15 − 0.25 del total de los sistemas. Se espera que existan 700 múltiples entre las 3383 estrellas de tipos espectrales F, G y K más cercanas que 50 pc, pero sólo se conocen 76. Muchas de las binarias cerradas (y quizás todas) tienen componentes terciarias distantes, lo que indica que la transferencia de momento angular en los sistemas múltiples fue probablemente decisiva para la formación de binarias de corto período. El cociente entre el período largo y el corto en las múltiples major estudiadas y en las múltiples de baja masa pre-secuencia principal no excede 104 en la época de formación estelar; los cocientes mayores que 104 se producen por la evolución orbital subsecuente. Todas las múltiples conocidas con órbitas bien definidas son dinámicamente estables, y las excentricidades de las órbitas externas cumplen con el criterio empírico de estabilidad Pext(1 eext)3/-Pint > 5, el cual es más estricto que los límites teóricos actualmente aceptados. La orientación relativa de las órbitas en las estrellas triples muestra cierto grado de alineamiento, especialmente en los sistemas débilmente jerárquicos. Las estadísticas apoyan la idea de que la mayoría de las multiples se originaron por interacciones dinámicas en cúmulos pequeños.
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