Most Stars Are Single ! C. J. Lada ApJ, 640, L63 (2006) 1.From observations: a low binary fraction for late-type stars 2.2/3 of all disk MS stellar systems are composed of single star 3.Some implications for star & planet formation

What were the observations and assumptions Systematic observations  Mid-late 20th century: for MS F,G stars  very high BSF (~70% － 80%)BSF 1991 ： for G stars  2/3 are multiple But is the BSF for G stars representative of all stars ? SSF- Single Star Fraction BSF- Binary Star Fraction

BSF to be a function of Sp  only ~30% for M stars IMF (field & embedded clusters) peak broadly between 0.1 － 0.5Msun  most disk stars are M-type 2 recent developmets:

SSF vs. spectral type -- compiled from literature Conclusion: SSF is a function of Sp, 43% for G (Duquennoy1991) to ~74% for M (Delfosse2004)

Field IMF: broad peak between 0.1~0.5 Msun  M type stars -Muench et al 2002

Fraction of all stars above H-burning limit that are M stars: 73% for Muench’s IMF 78% for Miller-Scalo IMF; 84% for PDMF

The Total Single Star Fraction SSF <M =43% (characterized for G stars) SSF M =74% MTF=73% or 78% SSF <M – SSF for earlier than M type SSF M – SSFfor M type stars MTF – fraction of all stars that are M-type stars ETF=1-MTF – early type fraction SST(total) = SSF <M ×ETF ＋ SSF M ×MTF = 66% or 67% 2/3 of all MS primary stars currently residing in the Galactic disk are single stars

Some discussions From observations  one-to-one mapping of molecular cloud core mass to stellar mass, modified by a SFE of 30%-50%, is possible ! BSF as function of stellar mass  clues to physical process of star formation “if protostellar cores further fragment and produce small-N clusters, the dynamical decay of these clusters into binary and single stars can in certain circumstances produce a BSF that declines with decreasing primary mass, similar to what is observed” -Durisen et al Most stars could harbor planetary systems unperturbed by binary companions !?

BSF