2008A&A...479..503A

We present the results of a spectroscopic analysis of three young embedded sources (HH26 IRS, HH34 IRS, and HH46 IRS) belonging to different star-forming regions and displaying well-developed jet structures. The aim is to investigate the source accretion and ejection properties and their connection. We used VLT-ISAAC near-IR medium resolution (R∼9000) spectra (H and K bands) to derive, in a self-consistent way, parameters like the star luminosity, accretion luminosity, and the mass accretion rate. Mass ejection rates have also been estimated from analysing different emission features. The spectra present several emission lines but no photospheric features in absorption, indicating a large veiling in both the H and K bands. In addition to features commonly observed in jet driving sources ([FeII], H₂, HI, CO), we detect a number of emission lines due to permitted atomic transitions, such as NaI and TiI that are only 2-5 times weaker than the Brγ line. Some of these features remain unidentified. Emission from NaI 2.2 µm doublet is observed along with CO(2-0) band-head emission, indicating a common origin in an inner gaseous disc heated by accretion. We find that accretion provides about 50% and 80% of the bolometric luminosity in HH26 IRS and HH34 IRS, as expected for accreting young objects. Mass accretion and loss rates spanning 10^–6-10^–8M_☉/yr have been measured. The derived {dot}(M)_loss/{dot}(M)_acc is ∼0.01 for HH26 IRS and HH34 IRS, and >0.1 for HH46 IRS. These numbers are in the range of values predicted by MHD jet launching models and found in the most active classical T Tauri stars. Comparison with other spectroscopic studies performed on Class Is seems to indicate that Class Is actually having accretion-dominated luminosities are limited in number. Although the analysed sample is small, we can tentatively define some criteria to characterise such sources: they have K-band veiling larger than 2 and, in the majority of the cases, present IR features of CO and NaI in emission, although these do not directly correlate with the accretion luminosity. Class Is with massive jets have high L_acc/L_bol ratios but not all the identified accretion-dominated objects present a jet. As suggested by the SEDs of our three objects, the accretion-dominated objects could be in an evolutionary transition phase between Class 0 and I. Studies of the kind presented here, but on larger samples of possible candidates, should be performed in order to test and refine these criteria.