THE FREQUENCY AND FORMATION MECHANISM OF B2-B5 MAIN-SEQUENCE BINARIES

We obtained about 20 coudé spectra of each of 74 B2-B5 IV or V stars and combined the results of their radial velocities with a previous study of 42 similar stars. We list new, improved, or published orbital elements for nine known double-lined and 26 single-lined (13 new, 13 known) spectroscopic binaries. In addition we list data for 49 visual companions (two with orbital elements, four showing orbital motion, six occultation or speckle companions, and 37 common proper motion pairs). Seven deletions are made because the systems met the selection criterion (B ≤ 6.00 mag) only because they are double. Attempts are made to allow for undetected systems with secondary masses ≥ 1 M⊙. We find that systems with periods of 10-2 to 105 yr have secondary frequencies that fit the Salpeter luminosity function (but not the van Rhijn function), and therefore the systems were formed primarily by capture. For systems with periods shorter than 10-2 yr = 3.6 days, the separations of components are only a few stellar radii, so they have probably undergone mass transfer and their secondary masses tell us nothing directly about the formation mechanism. In the reduced sample of 109 stars we detected 32 spectroscopic and 49 visual companions, implying 0.7 observed companions per primary. After allowance for incompleteness, we find at least 0.8 companions of M2 ≥ 2 M⊙ primary or 1.9 companions of M2 > 1 M⊙ per primary. These are minimal values because new components are still being discovered, especially with speckle interferometry and occasionally among the remaining variable velocity stars. Consideration of these results for the B2-B5 stars and published results on Am and F3-G2 stars show secondary luminosity or mass functions that can be explained entirely in terms of a capture mechanism for binary formation. That is, the widely spaced (visual) binaries in each case follow the Salpeter (or van Rhijn) function as one would expect in first- or early generation captures. For the closely spaced (spectroscopic) binaries with ages less than about one free-fall time (i.e., the B2-B5 stars), multiple capture processes have not had time to occur, so the secondary mass function is also a Salpeter function. But for the older stars (Am, F3-G2) that have probably existed in clusters for many free-fall times, the multiple captures have allowed the formation of hard binaries with mass ratios near 1. Thus binary formation by fission or bifurcation, which was proposed earlier but has been difficult to achieve theoretically for compressible viscous gases, may not occur frequently or ever.