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Current planet formation models are largely based on the observationalconstraint that protoplanetary disks have lifetime 3Myr. Recent studies,however, report the existence of PMS stars with signatures of accretion(strictly connected with the presence of circumstellar disks)andphotometrically determined ages of 30 Myr, or more. Here we present aspectroscopic study of two major age outliers in the ONC. We use broad band,intermediate resolution VLT/X-Shooter spectra combined with an accurate methodto determine the stellar parameters and the related age of the targets toconfirm their peculiar age estimates and the presence of ongoing accretion.Theanalysis is based on a multi-component fitting technique, which derivessimultaneously SpT, extinction, and accretion properties of the objects. Withthis method we confirm and quantify the ongoing accretion. From thephotospheric parameters of the stars we derive their position on the HRD, andthe age given by evolutionary models. Together with other age indicators likethe lithium equivalent width we estimate with high accuracy the age of theobjects. Our study shows that the two objects analyzed are not older than thetypical population of the ONC. Our results show that, while photometricdetermination of the photospheric parameters are an accurate method to estimatethe parameters of the bulk of young stellar populations, those of individualobjects with high accretion rates and extinction may be affected by largeuncertainties. Broad band spectroscopic determinations should thus be used toconfirm the nature of individual objects. Our analysis shows that this methodallows us to obtain an accurate determination of the photospheric parameters ofaccreting YSOs in any nearby star-forming region. We suggest that our detailed,broad- band spectroscopy method should be used to derive accurate properties ofcandidate old and accreting YSOs.

astronomy and astrophysics

Accurate determination of accretion and photospheric parameters in Young  Stellar Objects: the case of two candidate old disks in the Orion Nebula  Cluster