2011A&A...534A..32A

We present the results of the analysis of low-resolution optical-near IR spectroscopy (0.6-2.4µm) of a sample (47 sources) of Class I and Class II young stellar objects in the Chamaeleon I and II star-forming clouds. These data are part of the POISSON project (Protostellar Optical-Infrared Spectral Survey On NTT). The aim of the observations is to determine the accretion luminosity (L_acc) and mass accretion rate ({dot}(M)_acc) of the sources through the analysis of the detected emission features. Taking advantage of the wide wavelength range covered by our spectra, we also aim at verifying the reliability and consistency of the existing empirical relationships connecting emission line luminosity and L_acc. We employ five different tracers ([OI]λ6300, Hα, CaIIλ8542, Paβ, and Brγ) to derive the accretion luminosity, and critically discuss the various determinations in the light of the source properties. The tracers provide L_acc values characterised by different scatters when plotted as a function of L_*. The Brγ relation appears to be the most reliable, because it gives the minimum dispersion of L_acc over the entire range of L_*, whereas the other tracers, in particular Hα, provide much more scattered L_acc results, which are not expected for the homogeneous sample of targets we are observing. The direct comparison between L_acc(Brγ) and the accretion luminosity obtained from the other four tracers also shows systematic differences in the results provided by the empirical relationships. These may probably be ascribed to different excitation mechanisms that contribute to the line emission, which may vary between our sample and those where the relationships have been calibrated, which were mostly based on observations in Taurus. Adopting the accretion luminosities estimates derived from the Brγ line, we infer L_acc in the range 0.1L_*-1L_* for all sources, and {dot}(M)acc of the order 10^–7-10^–9M_☉/yr, in the range of values commonly obtained for Class II objects. The mass accretion rates derived in Cha I are roughly proportional to M_*², in agreement with the results found in other low-mass star-forming regions. We find that the discrepancies observed in the case of L_acc(Brγ) and L_acc(Paβ) can be related to different intrinsic Paβ/Brγ ratios. The derived ratios point to the existence of two different emission modalities, one that agrees with predictions of both wind and accretion models, the other suggesting optically thick emission from relatively small regions (10²¹-10²² cm²) with gas at low temperatures (<4000K), the origin of which needs additional investigation.