KITP: nbody

While I was galavanting about the southland yesterday, the program moved on and
John compared N-body and Monte Carlo (video and podcast)
and Sverre and Rainer continued with the theme (video and podcast)

today we move onto triples
'cause you got to know that if two stars are good, three stars are more better...

theoretically a lot of stars are binaries, (cf Matthew's latest)

Hogeveen Monte Carlo'd observed binaries to try to invert the q distribution and concluded the secondary mass function is a steep function of q, suggesting a lot of unobserved low mass secondaries
someone needs to redo this with modern large consistent samples

pretty pictures from Leicester

observationally, there are a lot of binaries, and triples, quads etc
for solar like primary stars in the field, the binary fraction is about 50% for
secondary masses > 0.1 times the primary mass, or so.
Higher mass primary stars do seem to trend towards higher multiplicity, and lower mass primaries to lower - however, because the luminosity function is steep, there is a strong selection bias against seeing low mass secondaries (with q (=M2/M1) ~< 0.1 or so)

Kobulnicky and Fryer (2007) have an interesting study of high mass primaries.
Mazeh et al (1998) have some interesting numbers on low mass primaries suggesting a rising secondary mass function (as q decreases) but not very steep - see also Goldberg et al (2003)

we also have to remember that a lot of low mass stars are secondaries ot high mass stars and don't get counted as primaries, though because the mass function is quite steep this is not that big a correction

so - all O stars have companions, many M stars do not (not counting low mass brown dwarf or planetary mass companions, which we arguably should do, but we can't, yet, so there)

we need to be consistent in our definition of what is a binary - in particular mass ratio and orbital period range being discussed
we glory in our inconsistencies

UPDATE: I was asked to recover some of the references on binary fractions from our ad hoc list

Kouwenhoven (PhD thesis 2006) and A&A 474 77 (2007)

Correia et al A&A 459 909 (2006) - triples and quads

Umbreit looked at low mass secondaries and brown dwarf bias

Halbwach - but ADS doesn't show anything - ah, Halbwachs!
multiple papers - cf A&A 397 159 (2003)

I can't decipher the handwriting to reconstruct the other observations