Accurate determination of accretion and photospheric parameters in young stellar objects: The case of two candidate old disks in the Orion Nebula Cluster

摘要

Context. Current planet formation models are largely based on the observational constraint that protoplanetary disks have a lifetime of ~3 Myr. Recent studies, however, report the existence of pre-main-sequence stars with signatures of accretion (strictly connected with the presence of circumstellar disks) and photometrically determined ages of 30 Myr or more. Aims. Here, we present a spectroscopic study of two major age outliers in the Orion Nebula Cluster. We use broadband, intermediate resolution VLT/X-shooter spectra combined with an accurate method to determine the stellar parameters and the related age of the targets to confirm their peculiar age estimates and the presence of ongoing accretion. Methods. The analysis is based on a multicomponent fitting technique, which derives simultaneously spectral type, extinction, and accretion properties of the objects. With this method, we confirm and quantify the ongoing accretion. From the photospheric parameters of the stars, we derive their position on the H-R diagram and the age given by evolutionary models. With other age indicators like the lithium-equivalent width, we estimate the age of the objects with high accuracy. Results. Our study shows that the two objects analyzed are not older than the typical population of the Orion Nebula Cluster. While photometric determination of the photospheric parameters are an accurate method to estimate the parameters of the bulk of young stellar populations, our results show that those of individual objects with high accretion rates and extinction may be affected by large uncertainties. Broadband spectroscopic determinations should thus be used to confirm the nature of individual objects. Conclusions. The analysis carried out shows that this method allows us to obtain an accurate determination of the photospheric parameters of accreting young stellar objects in any nearby star-forming region. We suggest that our detailed, broadband spectroscopy method should be used to derive accurate properties of candidate old and accreting young stellar objects in star-forming regions. We also discuss how a similarly accurate determination of stellar properties can be obtained through a combination of photometric and spectroscopic data.