Explain the evolution of binary frequency across young stellar classes

Explain how the observed decline in binary frequency from Class I to Class II to Class III young stellar objects can arise despite substantial overlap in the ages of low-veiling Class I, Class II, and Class III populations.

Background

The paper discusses observations showing that the wide-separation binary fraction decreases steeply with decreasing spectral index: Class I young stellar objects have a higher binary fraction than Class II objects, which in turn have a higher binary fraction than Class III objects. Earlier work interpreted this pattern as evidence that the binary fraction declines with stellar age.

However, the paper’s gravity-based age estimates indicate substantial age overlap among low-veiling Class I, Class II, and Class III young stellar objects. The authors therefore identify an unresolved tension between the apparent class-dependent decline in binary frequency and the absence of a simple one-to-one correspondence between evolutionary class and age. They suggest that circumstellar-envelope mass and whether a system remains within its natal cloud may help retain loosely bound wide companions, but do not establish this explanation.