PROTOSTARS · A photometric and spectral survey of young stars in nearby star-forming regions: towards a revised evolutionary sequence based on quantitative accretion/ejection diagnostics

A photometric and spectral survey of young stars in nearby star-forming regions: towards a revised evolutionary sequence based on quantitative accretion/ejection diagnostics

Summary: Young Stellar Objects (YSOs) are characterized by their Spectral Energy Distribution (SED), i.e. the energy they radiate over the electromagnetic spectrum (see Figure 1). In particular YSOs emitting more infrared and/or millimeter radiation than optical are thought to be very young. While this is true in general, our study shows that YSO emission line features, indicative of accretion and/or outflows, are better indicators of age. Moreover accretion rates are consistent with viscous accretion disk models. Introduction Our working model of low mass star formation arises from a combination of the empirical classification of the YSO SEDs (Lada & Wilking 1984; Lada 1987) with a theoretical picture of YSO formation, which involves the collapse of an isolated rotating dense core, forming an accreting protostellar core and a disc (Adams & Shu 1986; Adams et al. 1987). The empirical evolutionary sequence goes from Class 0 to III objects. Class 0 YSOs are the youngest sources (~104 yr), while Class III (the so-called Weak T Tauri Stars) are the oldest ones (107 yr) (Figure 1). Our understanding of the YSO early evolutionary stages (Class 0 and I), mostly relies on SED analysis or, indirectly, on studies of their jets and outflows. Recent theoretical studies on SEDs (e.g. Robitaille et al. 2007) have shown that the SED analysis alone is not sufficient to disentangle the YSO evolutionary stage, and NIR spectroscopy is needed to characterise the embedded accreting protostar. For example, geometrical effects may produce SED mis-classifications, i.e. old objects observed edge-on might show Class I shapes, or young YSOs face-on may appear older, showing Class II SED shapes. Thus, a correct classification can be only obtained from the combined analysis of the YSO SED and the characterisation of the embedded stellar object and its activity. Quantitative information on the various phenomena characterising the environment of young stars can be derived from NIR spectroscopic studies, which allow us to observe embedded objects, and investigate processes occurring in regions that are spatially unresolved. Observations and Sample Selection: In this framework, we have undertaken a combined photometric and spectroscopic survey on a flux-limited sample of selected Class I/II sources, located in six different nearby clouds (Cha I, Cha II, L 1641, Serpens, Lupus, and Corona Australis). This survey combines optical and near-infrared spectra (0.6-2.4 μm), taken with the EFOSC2/SOFI instruments on the New Technology Telescope (NTT) of the European Southern Observatory, mid-infrared spectra from the Space Spitzer Telescope (5-40 μm), and photometric data which cover the entire SED from 0.4 to 1100 (see Figure 2). Results: The collected dataset allows us to construct the YSOs spectral energy distributions and to infer the main stellar parameters (visual extinction, spectral type, accretion, stellar, and bolometric luminosities, mass accretion and ejection rates). Our study shows that YSO emission line features, indicative of accretion and/or outflows, are better indicators of age than SEDs. Moreover we found accretion rates are consistent with viscous accretion disk models.

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