The Serpens Molecular Cloud is one of the most active star-forming regions within 500pc, with over 1000 young stellar objects (YSOs) at different evolutionary stages. The ages of the member stars inform us about the star formation history of the cloud. In this paper, we develop a spectral energy distribution (SED) fitting method for nearby evolved (diskless) young stars from members of the Pleiades to estimate their ages, with a temperature scale adopted from APOGEE spectra. When compared with literature temperatures of selected YSOs in Orion, the SED fits to cool (<5000K) stars have temperatures that differ by an average of <=50K and have a scatter of ~210K for both disk-hosting and diskless stars. We then apply this method to YSOs in the Serpens Molecular Cloud to estimate ages of optical members previously identified from Gaia DR2 astrometry data. The optical members in Serpens are concentrated in different subgroups with ages from ~4 to ~22Myr; the youngest clusters, W40 and Serpens South, are dusty regions that lack enough optical members to be included in this analysis. These ages establish that the Serpens Molecular Cloud has been forming stars for much longer than has been inferred from infrared surveys.

Cone search capability for table J/ApJ/933/77/table5 (Fits of Serpens members)

Cone search capability for table J/ApJ/933/77/regions (Proper motions and radial velocities of Serpens groups and Collinder 359 (Table 8) and ages of Serpens groups (Table 6))