The accretion luminosity of Class I protostars

Context. The value of the accretion luminosity during the early phases of star formation is crucial information that aids in understanding how stars form, but it is still very difficult to obtain. Aims. We have developed a new methodology to measure accretion luminosity using mid-infrared hydrogen recombination lines and applied it to a limited sample of Class I protostars in the Taurus and Ophiuchus star-forming regions. Methods. We adopted the commonly used assumption that the properties of disk-protostar accretion in Class I objects is similar to the disk-star accretion in Class II objects. Using simultaneous observations of three hydrogen recombination lines, Brγ, Pfγ, and Brα, we derived the mean intrinsic line ratios, and we verified that these are constant across the probed range of photospheric and accretion properties. We established correlations between the line luminosities and accretion luminosity. Results. We measured the extinction toward the line emission regions in Class I protostars, comparing the observed line ratios to the Class II mean values. We then derived the Class I accretion luminosities from the established Class II correlations. We find that the accretion luminosity dominates the bolometric luminosity for the more embedded protostars, corresponding to lower values of the bolometric temperature. As the bolometric temperature increases above ~700 K, there is a sharp drop of the contribution of the accretion from the bolometric luminosity. Conclusions. Our findings are in qualitative agreement with numerical simulations of star formation. We suggest our methodology be applied to larger and more statistically significant samples of Class I objects to obtain a more detailed comparison. Our results also suggest that by combining multiple infrared line ratios, it will be possible to derive a more detailed description of the dust extinction law in protostellar envelopes.