Kazuki Tokuda
Department of Physical Science, Osaka Prefecture University

Abstract
Observational studies of the earliest stage of star formation is a key to investigate the initial conditions of star formation, e.g., the way of the fragmentation and the dynamical collapse. One of the observational difficulties is to appropriately identify such high-density cores just around the moment of protostar formation because the timescale is quite short. We performed ALMA observations toward a protostellar core, MC27/L1521F, in the Taurus molecular clouds with an angular resolution of ~20 au. We revealed various types of complex structures, such as arc-like gas, high-density starless condensations, and warm (>15-60 K) filamentary/clumpy components with the size scale of ~10-1000 au (Tokuda et al. 2014; 2016; 2018). We also resolved the rotation of the tiny disk (Rdisk ~10 au) around the associated protostar and revealed that the stellar mass gets already ~0.2 Mo without signs of mass accretion (Tokuda et al. 2017). These results suggest that the gas motion around the protostar is highly dynamical (turbulent) and these features may be relevant to the formation of (misalignment) multiple system, very tiny disk, and stopping mass accretion on to protostars.
We are also performing ALMA-ACA (Atacama Compact Array) surveys toward prestellar cores to search for well-developed structures prior to protostar formation. We found that a very low-mass (~0.2-0.4 Mo) core, MC5-N, in Taurus has a highly concentrated density distribution with possible substructures (Tokuda et al. 2019 PASJ, in press.). We have concluded that this core is a unique source as a pre-stellar core on the verge of brown dwarf or very low-mass star formation.

References:
Tokuda, K., et al. 2014, ApJL, 789, L4
Tokuda, K., et al. 2016, ApJ, 826, 26
Tokuda, K., et al. 2017, ApJ, 849, 101
Tokuda, K., et al. 2018, ApJ, 862, 8
Tokuda, K., et al. 2019, PASJ, in press. (arXiv:1904.05490)

2019 June 26, 13:30

IA/U.Porto
Centro de Astrofísica da Universidade do Porto (Auditorium)
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