On the Structure of Disk-shaped Extragalactic Nebulae. III. On the Formation of Multiple Systems

Abstract

The plan of the present paper is: (i) To give a general picture of the formation of binary systems. It is supposed that the stars forming a binary condense in a disk-shaped nebula of the type discussed in Papers I and II (that is, a nebula in which the main contribution to the mass is due to interstellar hydrogen). The way in which stars in such a nebula may come to form binary systems is discussed in Section 2 where it is shown that stars are unlikely to form a binary unless they condensed from interstellar material that initially occupied two neighbouring regions of the nebula. (ii) An independent line of investigation is followed in Sections 3 and 4. Firstly it is shown in Section 3 that the angular momentum of a binary can be determined within fairly narrow limits in terms of the initial masses of the components. Then, in Section 4, by a comparison of the observed angular momenta of binaries with the calculations of Section 3 an estimate is made for the initial masses of a selection of binary systems (Tables I and II). It is found that the initial masses so determined are consistently less, by a factor of order 10, than the observed masses of the components. This result is interpreted as strong evidence in favour of the theory that the components have increased significantly in mass due to accretion of interstellar hydrogen, during the time that has elapsed since the formation of the binary systems. (iii) The work of Sections 2 and 4 is applied in Section 5 to a consideration of the general properties of binary systems with special reference to the observational data given by Aitken. ⋆ (iv) In Section 6 the general picture of the formation of binaries is extended to cover the formation of triple and quadruple systems. It is shown that when the component stars in such systems undergo accretion of interstellar material at a sufficiently rapid rate, then (a) almost all triple systems will consist of a relatively close pair together with a comparatively distant companion. (b) About 5/7 of quadruple systems will consist of two close pairs separated by a comparatively large distance, whilst the other 2/7 of quadruple systems will consist of (relatively close pair) + (comparatively distant companion) + (still more distant companion). These considerations can be extended to systems containing more than four components.