Comments on Inferences of Star Formation Histories and Birthlines

Abstract

Palla & Stahler have recently argued that star formation in Taurus and other nearby molecular clouds extends over a period of at least 10 Myr, implying quasi-static cloud evolution and star formation. Their conclusions contradict other recent results indicating that molecular clouds are transient objects and star formation proceeds rapidly. The Palla & Stahler picture implies that most molecular clouds should have extremely low rates of star formation, and that in such inactive stages the stellar initial mass function should be strongly skewed toward producing stars with masses $\gtrsim 1 \msun$; neither prediction is supported by observations. I show that the Palla & Stahler conclusions for Taurus depend almost entirely on a small number of stars with masses $\gtrsim 1 \msun$; the lower-mass stars show no evidence for such an extended period of star formation. I further show that most of the stars apparently older than 10 Myr in the direction of Taurus are probably foreground non-members. I also present birthline calculations which support the idea that the ages of the stars with masses $\gtrsim 1 \msun$ have been systematically overestimated because ``birthline'' age corrections have been underestimated; such birthlines would eliminate the need to postulate skewed initial mass functions. The simplest and most robust explanation of current observations characterizing the vast majority of young stars in molecular clouds is that cloud and star formation is rapid and dynamic.