Low-mass star formation: from the core mass function to the mass accretion in young stars

The modern picture of the star formation scenario is far from explaining all the physical mechanisms and observational features of forming stars. One of the most compelling unsolved questions is how low-mass stars (M★ < 2 M☉) collect their masses. The goal of this Thesis is to contribute to answering this question through observations of low-mass star-forming systems in their prestellar, protostellar, and pre-main sequence phases. Low-mass stars form from the collapse of gravitationally bound cores (or prestellar cores), which result from the fragmentation process of the Giant Molecular Clouds (GMCs), in an evolutionary sequence that leads to the formation of a protostellar object (the so-called Class 0/I young stellar objects, YSOs) accreting in mass from a dense circumstellar disk. YSOs gradually evolve into optically visible pre-main sequence (PMS) stars following the dissipation of their envelope and disk (Class II or T-Tauri stars, and Class III YSOs). Although the general picture is rather well defined, several questions remain unsolved, especially during the first stages of the evolution from cloud cores to protostars, when sources are so cold and/or deeply embedded in their parental cloud that they can be observed only at infrared (IR) and sub-millimetric wavelengths. In the work presented in this Thesis, two key aspects of the star formation process are approached to help understand how low-mass stars reach their final masses. Firstly, the distribution and physical properties of prestellar cores in the Serpens/Aquila star-forming cloud are investigated through the analysis of far-infrared (FIR, 70–500 µm) images acquired by the Herschel Space Observatory in the framework of the Gould Belt key program. In particular, the prestellar core mass distribution (core mass function, CMF) is derived and compared with the stellar initial mass function (IMF) to determine whether the present distribution of stellar masses is already defined prior to collapse. This leads to a complete census of the starless cores population of the cloud, to assess whether they are gravitationally bound and will eventually collapse to form stars. As a second part of the work, a spectroscopic survey in the near-IR (1–2.4 µm) of a sample of Class I and II YSOs in the NGC 1333 young (< 1 Myr) cluster is presented, conducted with the ESO/VLT-KMOS facility. The aim of this survey is to investigate the inner star-disk interaction region (< 1 AU), where accretion from the disk onto the protostar, and the consequent ejection of matter in the form of jets, takes place. In particular, stellar parameters such as visual extinction, stellar mass, radius, and luminosity are computed, and the disk accretion luminosity is obtained from the luminosity of HI emission lines (e.g., Paβ and Brγ). This allows for the determination of the mass accretion rates of the NGC 1333 YSOs population and for a comparison with the results from older (2–3 Myr old) star-forming regions such as Lupus and Chamaeleon I. The main original results of this Thesis are the following: (i) it demonstrates that the CMF is not universal but depends on the star-forming region properties, and its shape is not always compatible with the one of the IMF; (ii) the determination of the accretion and stellar properties of a population of embedded YSOs, conducted here for the first time, has demonstrated that Class I sources have higher mass accretion rates than Class II objects, but not high enough to justify the observed masses, assuming a steady-accretion framework.