Interferometric Observations Of Dense Cores In High-mass Star Forming Regions

High-mass(M>8 M(?))stars play a pivotal role in galaxies.They dominate the stellar luminosities in star clusters in Milky Way and in extra-galaxies;the stellar winds and the supernova explosion dramatically change the ingredients and energetics of the interstellar medium;they also disseminate the heavy elements(Fe and above)into the space that are essential for the creation of organic life.Understanding the formation of high-mass stars therefore is of utmost importance.In the last decades,much progress has been achieved in the observational and theoretical studies of high-mass star for-mation.We have known that high-mass stars form in dense cores deeply embedded in molecular clouds,and tend to be in a clustered environment.However,several critical questions concerning these dense cores remain unsettled.For instance,are the dense cores in virial equilibrium?What are the roles of turbulence and magnetic field in the formation and subsequent collapse of dense cores?How do the extreme physical con-ditions in the Galactic Center region,such as strong turbulence,affect the evolution of dense cores?In this thesis,we make use of the high angular resolution observations toward high-mass star forming regions in the Galaxy at(sub)millimeter and radio wavelengths,to tackle three issues concerning high-mass star formation:statistical properties of high-mass dense cores;fragmentation of clumps and dynamical status of dense cores at very early evolutionary phases;physical properties of dense cores in giant molecular clouds nearby the Galactic Center.These issues attack the same question of the formation and collapse of high-mass dense cores from different perspectives.The three observation projects are:· a Very Large Array survey of 62 high-mass star forming regions to obtain their ammonia inversion lines,in order to obtain statistics of temperatures,linewidths,masses,and virial parameters of the dense cores;· a comprehensive study of the infrared dark cloud G28.53-0.25 with ammonia inversion line observations from the Very Large Array and Green Bank Telescope,H2O and CH3OH maser observations from the Expanded Very Large Array,and dust continuum and molecular line observations from the Submillimeter Array,to understand the fragmentation of massive clumps and the dynamical status and associated high-mass star formation of dense cores;· a mini-survey of 6 giant molecular clouds in the Central Molecular Zone of the Galaxy using ammonia inversion lines,H20 maser,and radio continuum from the Karl G.Jansky Very Large Array,and(sub)millimeter continuum and molecular lines from the Submillimeter Array,to study the distribution of dense gas and embedded star formation.Through these observations,we find that when 1-pc scale clumps(or filaments)fragment into cores,the thermal Jeans mass is usually a few Mc,much smaller than observed dense core masses.Once taking the turbulent support into account,the Jeans mass becomes consistent with observed masses,which makes it possible to form super-thermal-Jeans-mass dense cores.One the other hand,the molecular clouds in the Cen-tral Molecular Zone lack embedded compact sources thus show little star formation,which suggests strong turbulence cloud hinder fragmentation of gas hence formation of dense cores.In either case,observations confirm that turbulence plays a vital role in the formation of high-mass dense cores.We also find that in a simplified occasion where only turbulence and thermal mo-tions support the collapse of cores,the virial parameter(virial-to-gas mass ratio)is usu-ally smaller than the critical value of 2,suggesting that they are gravitationally bound.In particular,the more massive cores tend to have a virial parameter much smaller than 1,indicating that they may be far from virial equilibrium,or extra support from mag-netic field is needed against the self-gravity.The latter scenario has been proved to be consistent with star formation activities in massive clumps in one of our samples.This may suggest that magnetic field and turbulence are both essential in supporting dense cores and hence controlling the embedded high-mass star formation.