Summary

In summarising these results, we must emphasise that brown dwarf binaries should be considered as part of a continuum, stretching from O-type systems through solar-type stars to ultracool brown-dwarf/brown-dwarf binaries, rather than as a distinct category unto themselves. Taking that perspective, there are three broad characteristics of binary systems:

- The overall multiplicity fraction decreases from early to late spectral types (where we use the spectral type of the primary to characterise the system);

- The proportion of equal-mass systems increases at later spectral types; and

- The distribution of component separations becomes more compressed at later spectral types.

The data underpinning the last conclusion are shown in Fig. 5.11 (adapted from Reid & Walkowicz, 2006), which plots the total system mass, Mt = Mi + M2, and the mass ratio, q, as a function of component separation, A. Reid et al (2001) originally pointed out that the maximum separation of low-mass binaries appeared to scale logarithmically with Mt (i.e. \og(Amax) « Mt). Burgasser et al (2003) subsequently demonstrated that the outer envelope is better matched by Amax « Mf at Mt > 0.3M©. As noted in Sect. 5.4.3, the handful of low-mass binaries that violate these limits are generally younger than ~ 108 years.

Fig. 5.12 offers some clues as to how we might understand these results. Here, we plot the mass/separation diagram for stellar, brown dwarf and planetary companions to a volume-complete sample of nearby solar-type stars. This clearly illustrates the high frequency of planetary companions and dearth of brown dwarf companions at small separations (< 10AU). More importantly, it shows that the brown dwarf desert extends well into the stellar mass regime, with only a handful of companions with M2 sin i < 0.7M©. This is in stark contrast to the mass distribution of wide companions (>100 AU), which extends to brown dwarf masses and, indeed, closely matches the mass distribution of single stars.

Rephrasing these results, Fig. 5.12 shows that there is a clear preference for near-equal mass systems at small separations among solar-type stars. The inner region presumably reflects the effects of competitive accretion: big stars don't let small stars form close to them. Wide (A > 100AU) systems form through the gravitational association of independent protostellar cores, and therefore span a greater range of mass ratios.

Separation (AU)

a"

0.1 1 10 100 1000 104 Separation (AU)

Fig. 5.11. Total system masses and mass ratios as a function of separation (adapted from Reid & Walkowicz, 2006): crosses mark stellar binaries; solid points are systems with ultracool companions; the dashed line and solid line in the mass/separation diagram are from Burgasser et al (2003).

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Suppose that this dichotomy holds over the full mass range of primary stars. It seems likely that the boundary between the inner and outer regions will scale with the gravitational potential, i.e. the mass of the primary. Subsequent dynamical interactions are likely to be more effective at disrupting wide low-mass binaries. Removing those systems decreases the total binary fraction, and preferentially eliminates low-g systems, leading to a higher proportion of equal-mass systems and the mass/separation diagram shown in Fig. 5.11.

Close binaries are clearly a "bad thing" (to quote Sellars & Yateman) for the formation and survival of planetary systems, since gravitational interactions are liable to truncate and even disrupt the protostellar disk. Taken at face value, the decrease in the fraction of stellar or brown dwarf companions with decreasing primary mass suggests that the environment around low-mass stars and brown dwarfs may be more suitable for planet formation. However, if post-formation disruption plays a significant role in constricting the separation distribution plotted in Fig. 5.11, then the present-day multiplicity fraction may lead to an overestimate of the number of low-mass dwarfs with undisturbed planetary systems. From the brown dwarf perspective, Figs. 5.11 and 5.12 suggest that higher mass (M > 0.7M©) AFGK stars l o.i o.oooi o.oooi

10 100 1000 io4

Fig. 5.12. The mass/separation diagram for known stellar, brown dwarf and planetary companions of the volume-complete sample of 479 solar-type stars (4 < Mv < 7) within 25 parsecs of the Sun. Notice that there are only a handful of companions with 0.01 < jM^ < 0.5 and S < 10AU - the brown dwarf desert extends through the M dwarf regime.

10 100 1000 io4

Fig. 5.12. The mass/separation diagram for known stellar, brown dwarf and planetary companions of the volume-complete sample of 479 solar-type stars (4 < Mv < 7) within 25 parsecs of the Sun. Notice that there are only a handful of companions with 0.01 < jM^ < 0.5 and S < 10AU - the brown dwarf desert extends through the M dwarf regime.

are the best targets for direct imaging surveys, since they offer the best prospects for harbouring sub-stellar companions at wide separations.

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