C. Briceno, Th. Preibisch, W. Sherry, E. Mamajek, R. Mathieu, F. Walter,
The Low-mass Populations in OB Associations
Review chapter in the book Protostars & Planets V,
eds. B. Reipurth, D. Jewitt, & K. Keil,
University of Arizona Press, Tucson, p. 345--360 (2007)
Low-mass stars (0.1 <~ M <~ 1 M_sun)
in OB associations are key to addressing some of
the most fundamental problems in star formation.
The low-mass stellar populations of OB associations
provide a snapshot of the fossil
star-formation record of giant molecular cloud complexes.
Large scale surveys have identified hundreds
of members of nearby OB associations, and revealed that low-mass stars
exist wherever high-mass stars have recently formed. The spatial
distribution of low-mass members of OB associations demonstrate the existence
of significant substructure ("subgroups").
This "discretized" sequence of stellar groups is consistent with an origin
in short-lived parent molecular clouds within a Giant Molecular Cloud
Complex. The low-mass population in each subgroup within an OB
association exhibits little evidence for significant age spreads
on time scales of ~10 Myr or greater, in
agreement with a scenario of rapid star formation and cloud
The Initial Mass Function (IMF) of the stellar populations in OB associations
in the mass range 0.1 <~ M <~ 1 M_sun is largely
consistent with the field IMF, and
most low-mass pre-main sequence stars in the solar vicinity are in
OB associations. These findings agree with early suggestions
that the majority of stars in the Galaxy were born in OB associations.
The most recent work further suggests that a significant fraction of
the stellar population may have their origin in the more spread
out regions of OB associations, instead of all being born in dense clusters.
Ground-based and space-based (Spitzer
Space Telescope) infrared studies have provided robust evidence that
primordial accretion disks around low-mass stars dissipate on
timescales of a few Myr. However, on close inspection
there appears to be great variance in the
disk dissipation timescales for stars of a given mass in OB
associations. While some stars appear to lack disks at ~1 Myr,
a few appear to retain accretion disks up to ages of
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