|
Lifetimes for embedded protostars and prestellar cores are critical
constraints on the physics of collapse and accretion but cannot be
measured directly. The established method relies on the ratio of
embedded protostars to T Tauri stars, determining the ages of the
latter from evolutionary tracks. Likewise the ratio of starless to protostellar cores
gives an estimate of the lifetime of the prestellar phase, once the
lifetime of the embedded protostars is known.
Current estimates vary by an order of magnitude and place few
constraints upon theory. With current and upcoming surveys (Spitzer,
SCUBA, SCUBA-2, Herschel, UKIDSS) largely removing the uncertainties
due to small number statistics, the time is ripe to reexamine the
assumptions and selection effects involved in making lifetime
estimates for these early stages of star formation. From the
viewpoint of our own SCUBA survey of Perseus, I
consider the selection biases in the mm/submm, the assumption of a
constant star formation rate, whether submillimetre cores form
multiple stars, and how to select consistent ranges of mass at each
evolutionary stage. More fundamentally, I present initial evidence to
suggest that pre- and protostellar core lifetimes may depend on mass.
I also compare other measures of timescales, such as depletion
chemistry and mass infall/outflow rates.
|
