Volume 534, October 2011
|Number of page(s)||18|
|Section||Stellar structure and evolution|
|Published online||27 September 2011|
I. Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources⋆
INAF-Osservatorio Astronomico di Roma, via di Frascati 33
Monte Porzio Catone,
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
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 (Lacc) and mass accretion rate (Ṁacc) 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 Lacc.
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 Lacc 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 Lacc over the entire range of L∗, whereas the other tracers, in particular Hα, provide much more scattered Lacc results, which are not expected for the homogeneous sample of targets we are observing. The direct comparison between Lacc(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 Lacc in the range 0.1 L∗–1 L∗ for all sources, and Ṁacc 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 Lacc(Brγ) and Lacc(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.
Key words: stars: evolution / techniques: spectroscopic / accretion, accretion disks / stars: formation / surveys
© ESO, 2011
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