Abstract
My thesis project aims at understanding the natures of low mass protostars and proto-brown dwarfs at early evolutionary stage. A new category of Young Stellar Objects (YSOs), Very Low Luminosity Objects (VeLLOs), are newly discovered and defined by their low internal luminosity (Lint<0.1Lsun). The low luminosity infers that VeLLOs host a substellar mass central object at the present time and thus can be interpreted as a very young protostar or low mass protostar. This low luminosity also provides a constraint on their accretion luminosity as well as the accretion rate, suggesting VeLLOs are at a quiescent accretion phase. This thesis consists of four papers: (i) For the first paper, we develop a new identification method of YSOs to identify faint protostars in order to find more VeLLOs. Applying this method to the Spitzer archival data in five nearby molecular clouds, we identify 322 (28%) new YSO toward the lower end of the luminosity function compared with previous works. Based on this method, we further find seven VeLLOs in these molecular clouds. (ii) We study the chemical and dynamical properties of 15 selected Low Luminosity Objects (Lint<0.2Lsun) through molecular line emissions at millimeter/submillimeter wavelengths. We found that LLOs could have wide variety in their parent cores and tend to be at an very early evolutionary stage. (iii) We present a detailed investigation on the protostellar outflow driven by a unique VeLLO, IRAS 16253--2429. Our results suggest that IRAS 16253--2429 is very likely a proto-brown dwarf binary system. (iv) Using CFHT, we make an infrared outflow survey toward 20 LLOs. The outflows are detected in 12 LLOs (including 4 from Spitzer) and the outflow opening angles are widening as the core evolves as expected.