POISSON project

I. Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources^⋆

¹ INAF-Osservatorio Astronomico di Roma, via di Frascati 33 00040 Monte Porzio Catone, Italy
e-mail: simone.antoniucci@oa-roma.astro.it
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁵ Laboratoire d’Études du Rayonnement et de la Matière en Astrophysique (LERMA), Observatoire de Paris, ENS, UPMC, UCP, CNRS, 61 Avenue de l’Observatoire, 75014 Paris, France
⁶ Dublin Institute for Advanced Studies, School of Cosmic Physics, 31 Fitzwilliam Place, Dublin 2, Ireland
⁷ Institut de Planetologie et d’Astrophysique de Grenoble (IPAG), UMR 5571, BP 53, 38041 Grenoble Cedex 09, France

Received: 9 June 2011
Accepted: 11 August 2011

Abstract

Context. We present the results of the analysis of low-resolution optical-near IR spectroscopy (0.6–2.4 μm) 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).

Aims. The aim of the observations is to determine the accretion luminosity (L_acc) and mass accretion rate (Ṁ_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.

Methods. We employ five different tracers ([O i] λ6300, Hα, Ca ii λ8542, Paβ, and Brγ) to derive the accretion luminosity, and critically discuss the various determinations in the light of the source properties.

Results. 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 0.1 L_∗–1 L_∗  for all sources, and Ṁ_acc  of the order 10^-7−10^-9 M_⊙ yr^-1, 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 (<4000 K), the origin of which needs additional investigation.