分类： 天文学 >> 天文学 提交时间： 2023-02-19
Hsi-Wei Yen
Patrick Koch
Chin-Fei Lee
Naomi Hirano
Nagayoshi Ohashi
Jinshi Sai
Shigehisa Takakuwa
Ya-Wen Tang
Ken'ichi Tatematsu
Bo Zhao
摘要： To study transportation of magnetic flux from large to small scales in protostellar sources, we analyzed the Nobeyama 45-m N2H+ (1-0), JCMT 850 um polarization, and ALMA C18O (2-1) and 1.3 mm and 0.8 mm (polarized) continuum data of the Class 0 protostar HH 211. The magnetic field strength in the dense core on a 0.1 pc scale was estimated with the single-dish line and polarization data using the Davis-Chandrasekhar-Fermi method, and that in the protostellar envelope on a 600 au scale was estimated from the force balance between the gravity and magnetic field tension by analyzing the gas kinematics and magnetic field structures with the ALMA data. Our analysis suggests that from 0.1 pc to 600 au scales, the magnetic field strength increases from 40-107 uG to 0.3-1.2 mG with a scaling relation between the magnetic field strength and density of $B \propto \rho^{0.36\pm0.08}$, and the mass-to-flux ratio increases from 1.2-3.7 to 9.1-32.3. The increase in the mass-to-flux ratio could suggest that the magnetic field is partially decoupled from the neutral matter between 0.1 pc and 600 au scales, and hint at efficient ambipolar diffusion in the infalling protostellar envelope in HH 211, which is the dominant non-ideal magnetohydrodynamic effect considering the density on these scales. Thus, our results could support the scenario of efficient ambipolar diffusion enabling the formation of the 20 au Keplerian disk in HH 211.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Jets can facilitate the mass accretion onto the protostars in star formation. They are believed to be launched from accretion disks around the protostars by magneto-centrifugal force, as supported by the detections of rotation and magnetic field in some of them. Here we report a radial flow of the textbook-case protostellar jet HH 212 at the base to further support this jet-launching scenario. This radial flow validates a central prediction of the magneto-centrifugal theory of jet formation and collimation, namely, the jet is the densest part of a wide-angle wind that flows radially outward at distances far from the (small, sub-au) launching region. Additional evidence for the radially flowing wide-angle component comes from its ability to reproduce the structure and kinematics of the shells detected around the HH 212 jet. This component, which can transport material from inner to outer disk, could account for the chondrules and Ca-Al-rich inclusions detected in the Solar System at large distances.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shih-Ying Hsu
Sheng-Yuan Liu
Tie Liu
Dipen Sahu
Chin-Fei Lee
Kenichi Tatematsu
Kee-Tae Kim
Naomi Hirano
Yao-Lun Yang
Doug Johnstone
Hongli Liu
Mika Juvela
Leonardo Bronfman
Huei-Ru Vivien Chen
Somnath Dutta
David J. Eden
Kai-Syun Jhan
Yi-Jehng Kuan
Chang Won Lee
Jeong-Eun Lee
摘要： The presence of complex organic molecules (COMs) in the interstellar medium (ISM) is of great interest since it may link to the origin and prevalence of life in the universe. Aiming to investigate the occurrence of COMs and their possible origins, we conducted a chemical census toward a sample of protostellar cores as part of the ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) project. We report the detection of 11 hot corino sources, which exhibit compact emissions from warm and abundant COMs, among 56 Class 0/I protostellar cores. All the hot corino sources discovered are likely Class 0 and their sizes of the warm region ($>$ 100 K) are comparable to 100 au. The luminosity of the hot corino sources exhibits positive correlations with the total number of methanol and the extent of its emissions. Such correlations are consistent with the thermal desorption picture for the presence of hot corino and suggest that the lower luminosity (Class 0) sources likely have a smaller region with COMs emissions. With the same sample selection method and detection criteria being applied, the detection rates of the warm methanol in the Orion cloud (15/37) and the Perseus cloud (28/50) are statistically similar when the cloud distances and the limited sample size are considered. Observing the same set of COM transitions will bring a more informative comparison between the cloud properties.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Hsi-Wei Yen
Patrick Koch
Chin-Fei Lee
Naomi Hirano
Nagayoshi Ohashi
Jinshi Sai
Shigehisa Takakuwa
Ya-Wen Tang
Ken'ichi Tatematsu
Bo Zhao
摘要： To study transportation of magnetic flux from large to small scales in protostellar sources, we analyzed the Nobeyama 45-m N2H+ (1-0), JCMT 850 um polarization, and ALMA C18O (2-1) and 1.3 mm and 0.8 mm (polarized) continuum data of the Class 0 protostar HH 211. The magnetic field strength in the dense core on a 0.1 pc scale was estimated with the single-dish line and polarization data using the Davis-Chandrasekhar-Fermi method, and that in the protostellar envelope on a 600 au scale was estimated from the force balance between the gravity and magnetic field tension by analyzing the gas kinematics and magnetic field structures with the ALMA data. Our analysis suggests that from 0.1 pc to 600 au scales, the magnetic field strength increases from 40-107 uG to 0.3-1.2 mG with a scaling relation between the magnetic field strength and density of $B \propto \rho^{0.36\pm0.08}$, and the mass-to-flux ratio increases from 1.2-3.7 to 9.1-32.3. The increase in the mass-to-flux ratio could suggest that the magnetic field is partially decoupled from the neutral matter between 0.1 pc and 600 au scales, and hint at efficient ambipolar diffusion in the infalling protostellar envelope in HH 211, which is the dominant non-ideal magnetohydrodynamic effect considering the density on these scales. Thus, our results could support the scenario of efficient ambipolar diffusion enabling the formation of the 20 au Keplerian disk in HH 211.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qiuyi Luo
Tie Liu
Kenichi Tatematsu
ShengYuan Liu
Pak Shing Li
James di Francesco
Doug Johnstone
Paul F. Goldsmith
Somnath Dutta
Naomi Hirano
ChinFei Lee
Di Li
KeeTae Kim
Chang Won Lee
JeongEun Lee
Xunchuan Liu
Mika Juvela
Jinhua He
ShengLi Qin
HongLi Liu
摘要： During the transition phase from a prestellar to a protostellar cloud core, one or several protostars can form within a single gas core. The detailed physical processes of this transition, however, still remain unclear. We present 1.3 mm dust continuum and molecular line observations with the Atacama Large Millimeter/submillimeter Array (ALMA) toward 43 protostellar cores in the Orion Molecular Cloud Complex ($\lambda$ Orionis, Orion B, and Orion A) with an angular resolution of $\sim$ 0.35" ($\sim$ 140 au). In total, we detect 13 binary/multiple systems. We derive an overall multiplicity frequency (MF) of 28$\%$ $\pm$ 4$\%$ and a companion star fraction (CSF) of 51$\%$ $\pm$ 6$\%$, over a separation range of 300-8900 au. The median separation of companions is about 2100 au. The occurrence of stellar multiplicity may depend on the physical characteristics of the dense cores. Notably, those containing binary/multiple systems tend to show higher gas density and Mach number than cores forming single stars. The integral-shaped filament (ISF) of Orion A giant molecular cloud (GMC), which has the highest gas density and hosts high-mass star formation in its central region (the Orion Nebula cluster), shows the highest MF and CSF among the Orion GMCs. In contrast, the $\lambda$ Orionis Giant Molecular Cloud (GMC) has a lower MF and CSF than the Orion B and Orion A GMCs, indicating that feedback from HII regions may suppress the formation of multiple systems. We also find that the protostars comprising a binary/multiple system are usually at different evolutionary stages.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Dipen Sahu
Sheng-Yuan Liu
Doug Johnstone
Tie Liu
Neal J. Evans
Naomi Hirano
Kenichi Tatematsu
James Di Francesco
Chin-Fei Lee
Kee-Tae Kim
Somnath Dutta
Shih-Ying Hsu
Shanghuo Li
Qiu-Yi Luo
Patricio Sanhueza
Hsien Shang
Alessio Traficante
Mika Juvela
Chang Won Lee
David J. Eden
摘要： Starless cores represent the initial stage of evolution toward (proto)star formation, and a subset of them, known as prestellar cores, with high density (~ 10^6 cm^-3 or higher) and being centrally concentrated are expected to be embryos of (proto)stars. Determining the density profile of prestellar cores, therefore provides an important opportunity to gauge the initial conditions of star formation. In this work, we perform rigorous modeling to estimate the density profiles of three nearly spherical prestellar cores among a sample of five highly dense cores detected by our recent observations. We employed multi-scale observational data of the (sub)millimeter dust continuum emission including those obtained by SCUBA-2 on the JCMT with a resolution of ~5600 au and by multiple ALMA observations with a resolution as high as ~480 au. We are able to consistently reproduce the observed multi-scale dust continuum images of the cores with a simple prescribed density profile, which bears an inner region of flat density and a r^-2 profile toward the outer region. By utilizing the peak density and the size of the inner flat region as a proxy for the dynamical stage of the cores, we find that the three modeled cores are most likely unstable and prone to collapse. The sizes of the inner flat regions, as compact as ~500 au, signify them being the highly evolved prestellar cores rarely found to date.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Dipen Sahu
Sheng-Yuan Liu
Tie Liu
Neal J. Evans
Naomi Hirano
Ken'ichi Tatematsu
Chin-Fei Lee
Kee-Tae Kim
Somnath Dutta
Dana Alina
Leonardo Bronfman
Maria Cunningham
David J. Eden
Guido Garay
Paul F. Goldsmith
Jinhua He
Shih-Ying Hsu
Kai-Syun Jhan
Doug Johnstone
Mika Juvela
摘要： Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar phase, to harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high density (sub)structures on the thousand-au scale in a sample of dense prestellar cores. Through our recent ALMA observations towards the Orion molecular cloud, we have found five extremely dense prestellar cores, which have centrally concentrated regions $\sim$ 2000 au in size, and several $10^7$ $cm^{-3}$ in average density. Masses of these centrally dense regions are in the range of 0.30 to 6.89 M$_\odot$. {\it For the first time}, our higher resolution observations (0.8$'' \sim $ 320 au) further reveal that one of the cores shows clear signatures of fragmentation; such individual substructures/fragments have sizes of 800 -1700 au, masses of 0.08 to 0.84 M$_\odot$, densities of $2 - 8\times 10^7$ $cm^{-3}$ and separations of $\sim 1200$ au. The substructures are massive enough ($\gtrsim 0.1~M_\odot$) to form young stellar objects and are likely examples of the earliest stage of stellar embryos which can lead to widely ($\sim$ 1200 au) separated multiple systems.
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