brown dwarf calibrators at each age bin. The ages of the youngest objects are based on estimates in McGovern et al. 2004. The ages of the field dwarfs are speculative, but they are known to be much older than the other objects in this sample. $\sigma$ Orionis 51’s pEW is more typical of a younger object. The cyan circles are the EWs of field dwarfs described in C05 and R09](EW2_260313.pdf)

. \[fig:fig2\]

### Age Spreads

Does the spread of values of pEW in each age category represent a real spread in ages? The spectrum either side of the NaI feature contains molecular absorption features which deepen with decreasing T$_{eff}$ (see Section 8.3). These features depress the local pseudo-continuum, leading to lower values of pEW. If our sample contained a wide range of spectral types, this is a possible cause of the spread of values of pEW. However, with one exception, our sample are spectral types M8.5$-$L0. An ($I, I-J$) colour-magnitude study of 35 $\sigma$ Orionis cluster members found evidence of a spread in ages from 1-7 Myr [@kenyon01]. A later survey of the $\sigma$ Orionis cluster found a large spread in measured values of Li pEW [@kenyon05], indicating a spread 