of ages. The authors did not reach any conclusions as to the cause of this spread in Li pEW, but noted that similar spreads in Li pEW have been observed among low mass objects in the Chamaeleon I cluster (@joergens01; @natta04). A study of the luminosity spread in the HRD of the ONC argued that there was little evidence to support age spreads greater than a few Myr [@jeffries11].

There is some debate whether star formation is a slow process, taking place over several Myr, or whether it takes only as much time as is required for a sound wave to cross a molecular cloud. The sound crossing time depends on the radius of the cloud and on the sound speed, but @elmegreen00 has suggested that star formation could take as little as 1 Myr or less.

In the case of the $\sigma$ Orionis objects, $\sigma$ Orionis 51’s pEW is 1.5$\sigma$ from the mean pEW of its siblings which might indicate an age spread. The scatter in the pEWs of the 1-2 Myr objects is also slightly greater than would be expected from the 1$\sigma$ uncertainties. However, the scatter in the data could have other causes. For example, magnetic activity 