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Register a new userMagnetic fields in the early stages of massive star formation as revealed by ALMA
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Junhao Liu
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We present 1.3 mm ALMA dust polarization observations at a resolution of $sim$0.02 pc 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 emission fraction with increasing Stokes $I$ intensities in MM1 and MM4. Using the angular 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 $sim$1.6 mG and $sim$0.32 mG, respectively. textbf{The ordered magnetic energy is found to be smaller than the turbulent energy in the two cores, while the total magnetic energy is found to be comparable to the turbulent energy.} The total virial parameters in MM1-Core1 and MM4-Core4 are calculated to be $sim$0.76 and $sim$0.37, respectively, suggesting that massive star formation does not start in equilibrium. Using the polarization-intensity gradient-local gravity method, we found that the local gravity is closely aligned with intensity gradient in the three clumps, and the magnetic field tends to be aligned with the local gravity in MM1 and MM4 except for regions near the emission peak, which suggests that the gravity plays a dominant role in regulating the gas collapse. Half of the outflows in MM4 and MM9 are found to be aligned within 10$^{circ}$ of the condensation-scale ($<$0.05 pc) magnetic field, indicating that the magnetic field could play an important role from condensation to disk scale in the early stage of massive star formation. We also found that the fragmentation in MM1-Core1 cannot be solely explained by thermal Jeans fragmentation or turbulent Jeans fragmentation.
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Here we present new ALMA observations of polarized dust emission from six of the most massive clumps in W43-Main. The clumps MM2, MM3, MM4, MM6, MM7, and MM8, have been resolved into two populations of fragmented filaments. From these two populations we extracted 81 cores (96 with the MM1 cores) with masses between 0.9 Msun to 425 Msun and a mass sensitivity of 0.08 M$_{odot}$. The MM6, MM7, and MM8 clumps show significant fragmentation, but the polarized intensity appears to be sparse and compact. The MM2, MM3, and MM4 population shows less fragmentation, but with a single proto-stellar core dominating the emission at each clump. Also, the polarized intensity is more extended and significantly stronger in this population. From the polarized emission, we derived detailed magnetic field patterns throughout the filaments which we used to estimate field strengths for 4 out of the 6 clumps. The average field strengths estimations were found between 500 $mu$G to 1.8 mG. Additionally, we detected and modeled infalling motions towards MM2 and MM3 from single dish HCO$^{+}(J=4 rightarrow 3)$ and HCN$(J=4 rightarrow 3)$ data resulting in mass infall rates of $dot{mathrm{M}}_{mathrm{MM2}} = 1.2 times 10^{-2}$ Msun yr$^{-1}$ and $dot{mathrm{M}}_{mathrm{MM3}} = 6.3 times 10^{-3}$ Msun yr$^{-1}$. By using our estimations, we evaluated the dynamical equilibrium of our cores by computing the total virial parameter $alpha_{mathrm{total}}$. For the cores with reliable field estimations, we found that 71% of them appear to be gravitationally bound while the remaining 29% are not. We concluded that these unbound cores, also less massive, are still accreting and have not yet reached a critical mass. This also implies different evolutionary time-scales, which essentially suggests that star-formation in high mass filaments is not uniform.
With a mass of $sim$1000 $M_odot$ and a surface density of $sim$0.5 g cm$^{-2}$, G023.477+0.114 also known as IRDC 18310-4 is an infrared dark cloud (IRDC) that has the potential to form high-mass stars and has been recognized as a promising prestellar clump candidate. To characterize the early stages of high-mass star formation, we have observed G023.477+0.114 as part of the ALMA Survey of 70 $mu$m Dark High-mass Clumps in Early Stages (ASHES). We have conducted $sim$1.2 resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm in dust continuum and molecular line emission. We identified 11 cores, whose masses range from 1.1 $M_odot$ to 19.0 $M_odot$. Ignoring magnetic fields, the virial parameters of the cores are below unity, implying that the cores are gravitationally bound. However, when magnetic fields are included, the prestellar cores are close to virial equilibrium, while the protostellar cores remain sub-virialized. Star formation activity has already started in this clump. Four collimated outflows are detected in CO and SiO. H$_2$CO and CH$_3$OH emission coincide with the high-velocity components seen in the CO and SiO emission. The outflows are randomly oriented for the natal filament and the magnetic field. The position-velocity diagrams suggest that episodic mass ejection has already begun even in this very early phase of protostellar formation. The masses of the identified cores are comparable to the expected maximum stellar mass that this IRDC could form (8-19 $M_odot$). We explore two possibilities on how IRDC G023.477+0.114 could eventually form high-mass stars in the context of theoretical scenarios.
The dominant mechanism leading to the formation of brown dwarfs (BDs) remains uncertain. The most direct keys to formation, which are obtained from younger objects (pre-BD cores and proto-BDs), are limited by the very low number statistics available. We aim to identify and characterize a set of pre- and proto-BDs as well as Class II BDs in the Lupus 1 and 3 molecular clouds to test their formation mechanism. We performed ALMA band 6 (1.3 mm) continuum observations of a selection of 64 cores previously identified from AzTEC/ASTE data (1.1 mm), along with previously known Class II BDs in the Lupus 1 and 3 molecular clouds. Surveyed archival data in the optical were used to complement these observations. We expect these ALMA observations prove efficient in detecting the youngest sources in these regions, since they probe the frequency domain at which these sources emit most of their radiation. We detected 19 sources from 15 ALMA fields. Considering all the pointings in our observing setup, the ALMA detection rate was $sim$23% and the derived masses of the detected sources were between $sim$0.18 and 124 $mathrm{M_{Jup}}$. We classified these sources according to their spectral energy distribution as 5 Class II sources, 2 new Class I/0 candidats, and 12 new possible pre-BD or deeply embedded protostellar candidates. We detected a promising candidate for a Class 0/I proto-BD source and inferred the disk dust mass of a bona fide Class II BD. The pre-BD cores might be the byproduct of an ongoing process of large-scale collapse. The Class II BD disks follow the correlation between disk mass and the mass of the central object that is observed at the low-mass stellar regime. We conclude that it is highly probable that the sources in the sample are formed as a scaled-down version of low-mass star formation, although disk fragmentation may be responsible for a considerable fraction of BDs.
Here we present the first results from ALMA observations of 1 mm polarized dust emission towards the W43-MM1 high mass star forming clump. We have detected a highly fragmented filament with source masses ranging from 14Msun to 312Msun, where the largest fragment, source A, is believed to be one of the most massive in our Galaxy. We found a smooth, ordered, and detailed polarization pattern throughout the filament which we used to derived magnetic field morphologies and strengths for 12 out of the 15 fragments detected ranging from 0.2 to 9 mG. The dynamical equilibrium of each fragment was evaluated finding that all the fragments are in a super-critical state which is consistent with previously detected infalling motions towards W43-MM1. Moreover, there are indications suggesting that the field is being dragged by gravity as the whole filament is collapsing.
We study the core mass function (CMF) of the massive protocluster G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array via 1.3~mm continuum emission at a resolution of 1.0arcsec (2500~au). We have mapped a field of 5.3arcmin$times$5.3arcmin centered on the protocluster clump. We measure the CMF in the central region, exploring various core detection algorithms, which give source numbers ranging from 60 to 125, depending on parameter selection. We estimate completeness corrections due to imperfect flux recovery and core identification via artificial core insertion experiments. For masses $Mgtrsim1:M_odot$, the fiducial dendrogram-identified CMF can be fit with a power law of the form ${rm{d}}N/{rm{d}}{rm{log}}Mpropto{M}^{-alpha}$ with $alpha simeq1.24pm0.17$, slightly shallower than, but still consistent with, the index of the Salpeter stellar initial mass function of 1.35. Clumpfind-identified CMFs are significantly shallower with $alphasimeq0.64pm0.13$. While raw CMFs show a peak near $1:M_odot$, completeness-corrected CMFs are consistent with a single power law extending down to $sim 0.5:M_odot$, with only a tentative indication of a shallowing of the slope around $sim1:M_odot$. We discuss the implications of these results for star and star cluster formation theories.
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