Having grown up in the belief that most stars in the galaxy are binaries, Centauri Dreams has found a recent paper by Charles Lada fascinating. Lada (Harvard-Smithsonian Center for Astrophysics) argues persuasively that we have misunderstood the distribution of binary systems because of a key assumption: that the frequency of binary pairs is roughly the same in all stellar types.
A bit of history: William Herschel’s early work on binary stars produced hundreds of visual pairs in the early 19th Century, an introduction to the tens of thousands later catalogued. In the late 20th Century, studies of main sequence F and G type stars indicated that a high percentage (as many as 80 percent) were members of binary or multiple star systems. From this came the conclusion that most stars followed the pattern established by F and G stars; the Sun, in other words, was an anomaly as a G-type star that is also single.
But Lada argues that two things have now changed our view. First, we’ve learned from work on the stellar initial mass function (IMF) that most stars in the galactic disk are M stars (the initial mass function describes the distribution of masses into which stars are formed).
Second, we have begun to see that binary star frequency may relate directly to spectral type. In fact, M stars have a relatively low percentage (30 percent) of binaries, while L and T stars (near and below the hydrogen burning limit) are as much as a factor of 2 lower still. From this follows Lada’s argument: Most stellar systems in the galaxy consist of single stars.
Centauri Dreams‘ take: The disruptive effects of binary systems on planet formation have long been discussed, although recent studies have shown stable orbits in the habitable zones of systems like the Centauri A and B pairing. Lada’s conclusions imply that planetary systems similar to our own Solar System (in terms of architecture and stability) may be more common than previously assumed around M stars, there being far fewer binary systems to worry about. Once again, we’re left with the conclusion that M stars, once considered relatively uninteresting from the standpoint of life-bearing worlds, could emerge as a major focus in exoplanetary studies.
The paper is “Stellar Multiplicity and the IMF: Most Stars are Single Born,” submitted to the Astrophysical Journal Letters, and available online. For more on the initial mass function, one recent paper of note is Larson, Richard B., “The Stellar Initial Mass Function and Beyond,” ASP Conference Series 287 (2003), p. 65, also available here.
