Are There Age Spreads in Star Forming Regions?

TL;DR

This paper reviews the evidence for luminosity spreads in young star forming regions' HR diagrams and examines whether these spreads indicate large age spreads, suggesting instead that they likely reflect short-term accretion effects rather than prolonged star formation.

Contribution

The paper critically assesses whether luminosity spreads in HR diagrams truly indicate age spreads, proposing rapid accretion as an alternative explanation.

Findings

Luminosity spreads are genuine and not due to observational errors.

There is little evidence for age spreads larger than a few million years.

Rapid accretion during early stellar phases may explain luminosity variations.

Abstract

A luminosity spread at a given effective temperature is ubiquitously seen in the Hertzsprung-Russell (HR) diagrams of young star forming regions and often interpreted in terms of a prolonged period (>=10 Myr) of star formation. I review the evidence that the observed luminosity spreads are genuine and not caused by astrophysical sources of scatter. I then address whether the luminosity spreads necessarily imply large age spreads, by comparing HR diagram ages with ages from independent clocks such as stellar rotation rate, the presence of circumstellar material and lithium depletion. I argue that whilst there probably is a true luminosity dispersion, there is little evidence to support age spreads larger than a few Myr. This paradox could be resolved by brief periods of rapid accretion during the class I pre main-sequence phase.