extinction and that previous studies have found them to be fairly typical objects with no obvious veiling by a circumstellar accretion disc (see above references). While the 1-2 Myr calibrators are more massive than the ONC PMOs, the Lyon DUSTY isochrones predict that they also have larger radii, such that their surface gravities are very similar.

![image](1Myr_010813.pdf){height="4.35in" width="2.in"}

![image](10Myr_010813.pdf){height="4.35in" width="2.in"}

![image](Field_010813.pdf){height="4.35in" width="2.in"}

![image](1Myr_010813_NaI.pdf){height="4.35in" width="2.in"}

![image](10Myr_010813_NaI.pdf){height="4.35in" width="2.in"}

![image](Field_010813_NaI.pdf){height="4.35in" width="2.in"}

Observations & Data Reduction
=============================

Observations of the ten calibrators were made over eight nights between 23 August 2008 - 29 October 2009. Observations of the PMOs were made on 16 October 2009. Observations were made using the 8m Gemini Telescope at Gemini North with NIFS [@mcgregor03]. The latter was fitted with the K band grating, centred at 2.2$\mu$m. Observations were normally made in an ABBA pattern to facilitate the removal of the sky background and dark current. Some characteristics of the brown dwarf calibrators and of the observed PMO, together with some observational notes including the S/N range of the NaI absorption feature for each object are shown in Table 1. These ranges were chosen as they are typical of the S/N for each observation.

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