We present spectral observations of 130 young stellar objects ( YSOs ) in the Serpens Cloud Core and NGC 1333 embedded clusters .
The observations consist of near-IR spectra in the H and K -bands , from SpeX on the IRTF and far-red spectra ( 6000 - 9000 Å ) from Hectospec on the MMT .
These YSOs were identified in previous Spitzer and Chandra observations , and the evolutionary classes of the YSOs were determined from the Spitzer mid-IR photometry .
With these spectra , we search for corroborating evidence for the pre-main sequence nature of the objects , study the properties of the detected emission lines as a function of evolutionary class , and obtain spectral types for the observed YSOs .
The temperature implied by the spectral types are combined with luminosities determined from the near-IR photometry to construct HR diagrams for the clusters .
By comparing the positions of the YSOs in the HR diagrams with the pre-main sequence tracks of Baraffe ( 5 ) , we determine ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks .
The apparent isochronal ages of the YSOs in both clusters range from less than 1 Myr to 10 Myr , with most objects below 3 Myr .
The observed distribution of ages for the Class II and Class III objects are statistically indistinguishable .
We examine the spatial distribution and extinction of the YSOs as a function of their isochronal ages .
We find the sources < 3 Myr to be concentrated in the molecular cloud gas while the older sources are spatially dispersed and are not deeply embedded .
Nonetheless , the sources with isochronal ages > 3 Myr show all the characteristics of young stellar objects in their spectra , their IR spectral energy distributions , and their X-ray emission ; we find no evidence that they are contaminating background giants or foreground dwarfs .
However , we find no corresponding decrease in the fraction of sources with infrared excess with isochronal age ; this suggests that the older isochronal ages may not measure the true age of the > 3 Myr YSOs .
Thus , the nature of the apparently older sources and their implications for cluster formation remain unresolved .