Stars are born in molecular clouds through gravitational contraction. What is the physical mechanism that allows
mass accretion to occur against centrifugal forces and magnetic fields? What determines their final stellar
masses? Despite our devotion over many decades, we astronomers and astrophysicists have not been able to find
clear answers.
In this talk I will focus on the following two key topics: (A) the physical mechanism of accretion in and from
the circumstellar disk; and (B) the role of episodic and violent mass accretion in developing final stellar
masses. We are tackling these issues with long-term monitoring observations of mass accretion and jet ejection
toward a few pre-main sequence stars (using Gemini, CFHT etc.), and imaging observations of young stars with
extremely active mass accretion at near-IR and millimeter wavelengths (using Subaru, ALMA etc.), respectively.
Our observations to date, and numerical simulations by members of the team, suggest the following: (1) steady
mass accretion is more important than episodic mass accretion for the formation of many stars; and (2) a jet
plays an important role for accretion from the inner disk edge to the star. However, further studies are
required for us to reach convincing conclusions.
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