Most Stars Are Single ! C. J. Lada ApJ, 640, L63 (2006)

1. From observations: a low binary fraction for late-type stars2. 2/3 of all disk MS stellar systems are composed of single star3. 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%)• 1991 ： for G stars 2/3 are multiple

But is the BSF for G stars representative of all stars ?

SSF- Single Star FractionBSF- 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 literatureConclusion:

• 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 IMF78% for Miller-Scalo IMF;84% for PDMF

The Total Single Star Fraction

• SSF<M =43% (characterized for G stars)• SSFM=74%• MTF=73% or 78%

SSF<M – SSF for earlier than M typeSSFM – SSFfor M type stars

MTF – fraction of all stars that are M-type starsETF=1-MTF – early type fraction

SST(total) = SSF<M×ETF ＋ SSFM×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 dynamicaldecay 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. 2001

• Most stars could harbor planetary systems unperturbed by binary companions !?

BSF