Magnetic Fields And Outflows In Star Formation

The formation of stars plays a major role in the formation and evolution of galax-ies.Howerver,the detailed physics of star formation is still far from being under-stood.In particular,despite significant progresses in recent years,the role of mag-netic fields in star formation,especially in the early stage,is still under debates.It's also unclear how massive molecular outflows are driven.In this thesis,we use high sensitivity mm/submm polarization observations to study the magnetic field in three high-mass molecular clumps in an infrared dark cloud and in a low-mass starless core.Based on the results of 3D numerical simulations,we tested the reliability of the Davis-Chandrasekhar-Fermi(DCF)method on the estimation the magnetic field strength in massive star forming regions.We also studied the driving mechanism of a massive molecular outflow using multi-transition CO line observations.To study the role of magnetic fields in the initial phase of low-mass star formation,we report 850?m dust polarization observations of a low-mass(?12 M_?)starless core in the?Ophiuchus cloud,Ophiuchus C,made with the POL-2 instrument on the James Clerk Maxwell Telescope(JCMT)as part of the JCMT B-fields In STar-forming Region Observations(BISTRO)survey.We detect an ordered magnetic field projected on the plane of sky in the starless core.The magnetic field across the?0.1 pc core shows a pre-dominant northeast-southwest orientation centering between?40?to?100?,indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations.The polarization percentage(P)decreases with an increasing total intensity(I)with a power-law index of-1.03±0.05.We estimate the plane-of-sky field strength(B_pos)using modified Davis-Chandrasekhar-Fermi(DCF)methods based on structure function(SF),auto-correlation(ACF),and unsharp mask-ing(UM)analyses.We find that the estimates from the SF,ACF,and UM methods yield strengths of 103±46?G,136±69?G,and 213±115?G,respectively.Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical(i.e.unstable to collapse).The total magnetic energy calcu-lated from the SF method is comparable to the turbulent energy in Ophiuchus C,while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties.To investigate the role of magnetic fields in the early stages of massive star forma-tion,we present 1.3 mm ALMA dust polarization observations at a resolution of?0.02pc of three massive molecular clumps,MM1,MM4,and MM9,in the infrared dark cloud G28.34+0.06.With the sensitive and high-resolution continuum data,MM1 is resolved into a cluster of condensations.The magnetic field structure in each clump is revealed by the polarized emission.We found a trend of decreasing polarized emis-sion fraction with increasing Stokes I intensities in MM1 and MM4.Using the an-gular dispersion function method(a modified Davis-Chandrasekhar-Fermi method),the plane-of-sky magnetic field strength in two massive dense cores,MM1-Core1 and MM4-Core4,are estimated to be?1.6 m G and?0.32 m G,respectively.The virial parameters in MM1-Core1 and MM4-Core4 are calculated to be?0.76 and?0.37,re-spectively,suggesting that massive star formation does not start in equilibrium,which agrees with the competitive accretion model of massive star formation.Using the polarization-intensity gradient-local gravity method,we found that the local gravity is closely aligned with intensity gradient in the three clumps,and the magnetic field tends to be aligned with the local gravity in MM1 and MM4 except for regions near the emission peak,which suggests that the gravity plays a dominant role in regulating the gas collapse.Half of the outflows in MM4 and MM9 are found to be aligned within10?of the condensation-scale(<0.05 pc)magnetic field,indicating that the magnetic field could play an important role from condensation to disk scale in the early stage of massive star formation.We find that the fragmentation in MM1-Core1 cannot be solely explained by thermal Jeans fragmentation or turbulent Jeans fragmentation.The Davis-Chandrasekhar-Fermi(DCF)method is widely used to indirectly esti-mate the magnetic field strength from the plane-of-sky field orientation.In this work,we present a set of 3D MHD simulations and synthetic polarization images using radia-tive transfer of clustered massive star-forming regions.We apply the DCF method on the synthetic polarization maps to investigate its reliability in high-density molecular clumps and dense cores where self-gravity is significant.We investigate the validity of the assumptions of the DCF method step by step and compare the model and es-timated field strength to derive the correction factors for the estimated uniform and total(rms)magnetic field strength at clump and core scales.The correction factors in different situations are catalogued.We suggest that the turbulent magnetic energy is usually smaller than the turbulent kinetic energy in massive star-forming regions and the state is closer to energy equipartition for strong field cases.Thus,the magnetic field strength in weak field cases could be significantly overestimated by the DCF method due to energy none-equipartition.We investigate the accuracy of the angular dispersion function method(ADF,a modified DCF method)method on the effects that may affect the measured angular dispersion and find that the ADF method correctly account for the ordered field structure,the beam-smoothing,and the interferometric filtering,but may not correctly account for the signal integration along the line of sight.Our results suggest that the DCF methods should be avoided to be applied below the core scale.To explore the driving mechanism of the massive,parsec-sized,bipolar,and high velocity outflow in the massive star-forming region G240.31+0.07,we present Atacama Pathfinder Experiment(APEX)observations in the CO J=3–2,6–5,and 7–6 lines.The outflow is detected in all the lines,which allows us,in combination with the existing CO J=2–1 data,to perform a multi-line analysis of physical conditions of the outflowing gas.The CO 7–6/6–5,6–5/3–2,and 6–5/2–1 ratios are found to be nearly constant over a velocity range of?5–25 km s-¹for both blueshifted and redshifted lobes.We carry out rotation diagram and large velocity gradient(LVG)calculations of the four lines and find that the outflow is approximately isothermal with a gas temperature of?50 K and that the the CO column density clearly decreases with the outflow velocity.If the CO abundance and the velocity gradient do not vary much,the decreasing CO column density indicates a decline in the outflow gas density with velocity.By comparing with theoretical models of outflow driving mechanisms,our observations and calculations suggest that the massive outflow in G240.31+0.07 is being driven by a wide-angle wind and further support a disk mediated accretion at play for the formation of the central high-mass star.