2012A&A...538A..64C

Characterising stellar and circumstellar properties of embedded young stellar objects (YSOs) is mandatory for understanding the early stages of the stellar evolution. This task requires the combination of both spectroscopy and photometry, covering the widest possible wavelength range, to disentangle the various protostellar components and activities. As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey On NTT), we present a multi-wavelength spectroscopic and photometric investigation of embedded YSOs in L1641, aimed to derive the stellar parameters and evolutionary stages and to infer their accretion properties. Our multi-wavelength database includes low-resolution optical-IR spectra from the NTT and Spitzer (0.6-40µm) and photometric data covering a spectral range from 0.4 to 1100µm, which allow us to construct the YSOs spectral energy distributions (SEDs) and to infer the main stellar parameters (visual extinction, spectral type, accretion, stellar, bolometric luminosity, mass accretion, and ejection rates). The NTT optical-NIR spectra are rich in emission lines, which are mostly associated with YSO accretion, ejection, and chromospheric activities. A few emission lines, prominent ice (H₂O and CO₂), and amorphous silicate absorption features have also been detected in the Spitzer spectra. The SED analysis allows us to group our 27 YSOs into nine Class I, eleven Flat, and seven Class II objects. However, on the basis of the derived stellar properties, only six Class I YSOs have an age of ∼10⁵yr, while the others are older (5x10⁵-10⁶yr), and, among the Flat sources, three out of eleven are more evolved objects (5x10⁶-10⁷yr), indicating that geometrical effects can significantly modify the SED shapes. Inferred mass accretion rates ({dot}(M)_acc) show a wide range of values (3.6x10^–9 to 1.2x10^–5M_☉/yr), which reflects the age spread observed in our sample well. Average values of mass accretion rates, extinction, and spectral indices decrease with the YSO class. The youngest YSOs have the highest {dot}(M)_acc, whereas the oldest YSOs do not show any detectable jet activity in either images and spectra. Apart from the outbursting source #25 and, marginally, #20, none of the remaining YSOs is accretion-dominated (L_acc>L_*). We also observe a clear correlation among the YSO {dot}(M)_acc, M_*, and age. For YSOs with t>10⁵yr and 0.4M_☉≤M_*≤1.2M_☉, a relationship between {dot}(M)_acc and t ({dot}(M)_acc ∝t^–1.2) has been inferred, consistent with mass accretion evolution in viscous disc models and indicating that the mass accretion decay is slower than previously assumed. Finally, our results suggest that episodic outbursts are required for Class I YSOs to reach typical classical T Tauri stars stellar masses.