分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the internal and external alignment of carbonaceous grains, including graphite and hydrogenated amorphous carbon (HAC), in the interstellar medium (ISM) within the RAdiative Torque (RAT) paradigm. For internal alignment (IA), we find that HAC grains having nuclear paramagnetism due to hydrogen protons can have efficient nuclear relaxation, whereas both HAC and graphite grains can have efficient inelastic relaxation for grains aligned both at low$-J$ and high$-J$ attractors. For external alignment, HAC and graphite grains can align with the radiation direction ($k$-RAT) at low$-J$ attractors but cannot have stable alignment at high$-J$ attractors due to the suppression of radiative precession. HAC also has slow Larmor precession compared to the randomization by gas collisions and cannot be aligned with the magnetic field ($B$-RAT). Small HAC grains of $a<0.05\mu$m drifting through the diffuse ISM can be weakly aligned along the induced electric field ($E$-RAT) at high$-J$ attractors due to its fast precession. Paramagnetic relaxation by nuclear magnetism is found inefficient for HAC grains due to the rapid suppression of nuclear susceptibility when grains rotate at high$-J$ attractors. We then study the alignment of carbon dust in the envelope of a typical C-rich Asymptotic Giant Branch star, IRC+10216. We find that grains aligned at low$-J$ attractors can occur via $k$-RAT with the wrong IA in the inner region but via $B$-RAT in the outermost region. However, grains aligned at high$-J$ attractors have the right IA alignment via $k$-RAT due to efficient inelastic relaxation. The polarization pattern observed toward IRC+10216 by SOFIA/HAWC+ can reproduced when only grains at low$-J$ attractors are present due to removal of grains at high$-J$ attractors by the RAT disruption.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the physical processes inducing the alignment of the grain axis of maximum inertia moment with the angular momentum (${\bf J}$, i.e., internal alignment) and of ${\bf J}$ with the magnetic field (i.e., external alignment) of very large grains (VLGs, of radius $a>10\mu$m) using the grain alignment framework based on radiative torques (RATs) and mechanical torques (METs). We derive analytical formulae for critical sizes of grain alignment, assuming that grains are aligned at both low$-J$ and high$-J$ attractors by RATs (METs). For protostellar cores, we find that super-Barnett relaxation can induce efficient internal alignment for VLGs with large iron inclusions aligned at high$-J$ attractors by RATs (METs). In contrast, inelastic relaxation can be efficient for VLGs made of any composition. For external alignment, we find that VLGs with iron inclusions aligned at high$-J$ attractors can have magnetic alignment by RATs ($B-$RAT) or METs ($B-$ MET), enabling dust polarization as a reliable tracer of magnetic fields in such dense regions. Still, grains at low$-J$ attractors or grains without iron inclusions have alignment along the radiation direction ($k-$RAT) or gas flow ($v-$MET). For protostellar disks, we find that super-Barnett relaxation can be efficient for grains with large iron inclusions in the outer disk thanks to spinup by METs, but inelastic relaxation is inefficient. VLGs aligned at low-J attractors can have $k-$RAT ($v-$MET) alignment, but grains aligned at high$-J$ attractors have likely $B-$RAT ($B-$MET) alignment. Grain alignment by METs appears to be more important than RATs in protostellar disks.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Magnetic fields ($\textbf{B}$) are an important factor that controls the star formation process. The leading method to observe $\textbf{B}$ is using polarized thermal emission from dust grains aligned with $\textbf{B}$. However, in dense environments such as protostellar cores, dust grains may have inefficient alignment due to strong gas randomizations, so that using dust polarization to trace $\textbf{B}$ is uncertain. Hoang $\&$ Lazarian (2016) demonstrated that the grain alignment by RAdiative Torques is enhanced if dust grains contain embedded iron inclusions. Here we extend POLARIS code to study the effect of iron inclusions on grain alignment and thermal dust polarization toward a protostellar core, assuming uniform magnetic fields. We found that paramagnetic grains produce a low polarization degree of $p \sim 1\%$ in the envelope and negligible $p \ll 1\%$ in the central region due to the loss of grain alignment. In contrast, grains with a high level of iron inclusions can have perfect alignment and produce high $p \sim 40\%$ in the envelope and low $p \leq 10\%$ in the central region. Grains with a moderate level of iron inclusions induce the polarization flipping from $\textbf{P}$ $\parallel$ $\textbf{B}$ at millimeter to $\textbf{P}$ $\perp$ $\textbf{B}$ at submillimeter due to the change in the internal alignment caused by slow internal relaxation. The weak alignment of very large grains of $a \geq 10\mu m$ reduces the polarization by dichroic extinction at submillimeter wavelengths. We found a positive correlation between p and the level of iron inclusions, which opens a new window to constrain the abundance of irons locked in dust through dust polarimetry.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Rapidly spinning magnetic grains can acquire large magnetic dipole moments due to the Barnett effect. Here we study the new effect of Barnett magnetic dipole-dipole interaction on grain-grain collisions and grain growth, assuming grains spun up by radiative torques. We find that the collision rate between grains having embedded iron inclusions can be significantly enhanced due to Barnett magnetic dipole-dipole interaction when grains rotate suprathermally by radiative torques. We discuss the implications of enhanced collision rate for grain growth and destruction in the circumstellar envelope of evolved stars, photodissociation regions, and protostellar environments. Our results first reveal the importance of the dust magnetic properties and the local radiation field on grain growth and destruction.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Le Ngoc Tram
Lars Bonne
Yue Hu
Enrique Lopez-Rodriguez
Jordan A. Guerra
Pierre Lesaffre
Antoine Gusdorf
Thiem Hoang
Min-Young Lee
Alex Lazarian
B-G Andersson
Simon Coude
Archana Soam
William D. Vacca
Hyeseung Lee
Michael Gordon
摘要： The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R$\,$136 has been shown to be the main source of energy creating multiple pc-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B-fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214$\,\mu$m. The B-field morphology is complex, showing bending structures around R$\,$136. In addition, we use high spectral and angular resolution [\textsc{CII}] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with the B-field morphology and shows evidence of multiple expanding shells. Our B-field strength maps, estimated using the Davis-Chandrasekhar-Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350$\,\mu$G at 89, 154, and 214$\,\mu$m, respectively. We estimated that the majority of the 30 Doradus clouds are sub-critical and sub-Alfv\'enic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that the B-field is sufficient to hold the cloud structure integrity under feedback from R$\,$136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique (VGT) using [\textsc{CII}] and CO(2-1) is likely to confirm these results.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Mehrnoosh Tahani
Pierre Bastien
Ray S. Furuya
Kate Pattle
Doug Johnstone
Doris Arzoumanian
Yasuo Doi
Tetsuo Hasegawa
Shu-ichiro Inutsuka
Simon Coudé
Laura Fissel
Michael Chun-Yuan Chen
Frédérick Poidevin
Sarah Sadavoy
Rachel Friesen
Patrick M. Koch
James Di Francesco
Gerald H. Moriarty-Schieven
Zhiwei Chen
Eun Jung Chung
摘要： We study the HII regions associated with the NGC 6334 molecular cloud observed in the sub-millimeter and taken as part of the B-fields In STar-forming Region Observations (BISTRO) Survey. In particular, we investigate the polarization patterns and magnetic field morphologies associated with these HII regions. Through polarization pattern and pressure calculation analyses, several of these bubbles indicate that the gas and magnetic field lines have been pushed away from the bubble, toward an almost tangential (to the bubble) magnetic field morphology. In the densest part of NGC 6334, where the magnetic field morphology is similar to an hourglass, the polarization observations do not exhibit observable impact from HII regions. We detect two nested radial polarization patterns in a bubble to the south of NGC 6334 that correspond to the previously observed bipolar structure in this bubble. Finally, using the results of this study, we present steps (incorporating computer vision; circular Hough Transform) that can be used in future studies to identify bubbles that have physically impacted magnetic field lines.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Tao-Chung Ching
Keping Qiu
Di Li
Zhiyuan Ren
Shih-Ping Lai
David Berry
Kate Pattle
Ray Furuya
Derek Ward-Thompson
Doug Johnstone
Patrick M. Koch
Chang Won Lee
Thiem Hoang
Tetsuo Hasegawa
Woojin Kwon
Pierre Bastien
Chakali Eswaraiah
Jia-Wei Wang
Kyoung Hee Kim
Jihye Hwang
摘要： We present 850 $\mu$m dust polarization observations of the massive DR21 filament from the B-fields In STar-forming Region Observations (BISTRO) survey, using the POL-2 polarimeter and the SCUBA-2 camera on the James Clerk Maxwell Telescope. We detect ordered magnetic fields perpendicular to the parsec-scale ridge of the DR21 main filament. In the sub-filaments, the magnetic fields are mainly parallel to the filamentary structures and smoothly connect to the magnetic fields of the main filament. We compare the POL-2 and Planck dust polarization observations to study the magnetic field structures of the DR21 filament on 0.1--10 pc scales. The magnetic fields revealed in the Planck data are well aligned with those of the POL-2 data, indicating a smooth variation of magnetic fields from large to small scales. The plane-of-sky magnetic field strengths derived from angular dispersion functions of dust polarization are 0.6--1.0 mG in the DR21 filament and $\sim$ 0.1 mG in the surrounding ambient gas. The mass-to-flux ratios are found to be magnetically supercritical in the filament and slightly subcritical to nearly critical in the ambient gas. The alignment between column density structures and magnetic fields changes from random alignment in the low-density ambient gas probed by Planck to mostly perpendicular in the high-density main filament probed by JCMT. The magnetic field structures of the DR21 filament are in agreement with MHD simulations of a strongly magnetized medium, suggesting that magnetic fields play an important role in shaping the DR21 main filament and sub-filaments.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jihye Hwang
Jongsoo Kim
Kate Pattle
Chang Won Lee
Patrick M. Koch
Doug Johnstone
Kohji Tomisaka
Anthony Whitworth
Ray S. Furuya
Ji-hyun Kang
A-Ran Lyo
Eun Jung Chung
Doris Arzoumanian
Geumsook Park
Woojin Kwon
Shinyoung Kim
Motohide Tamura
Jungmi Kwon
Archana Soam
Ilseung Han
摘要： We present and analyze observations of polarized dust emission at 850 $\mu$m towards the central 1 pc $\times$ 1 pc hub-filament structure of Monoceros R2 (Mon R2). The data are obtained with SCUBA-2/POL-2 on the James Clerk Maxwell Telescope (JCMT) as part of the BISTRO (B-fields in Star-forming Region Observations) survey. The orientations of the magnetic field follow the spiral structure of Mon R2, which are well-described by an axisymmetric magnetic field model. We estimate the turbulent component of the magnetic field using the angle difference between our observations and the best-fit model of the underlying large-scale mean magnetic field. This estimate is used to calculate the magnetic field strength using the Davis-Chandrasekhar-Fermi method, for which we also obtain the distribution of volume density and velocity dispersion using a column density map derived from $Herschel$ data and the C$^{18}$O ($J$ = 3-2) data taken with HARP on the JCMT, respectively. We make maps of magnetic field strengths and mass-to-flux ratios, finding that magnetic field strengths vary from 0.02 to 3.64 mG with a mean value of 1.0 $\pm$ 0.06 mG, and the mean critical mass-to-flux ratio is 0.47 $\pm$ 0.02. Additionally, the mean Alfv\'en Mach number is 0.35 $\pm$ 0.01. This suggests that in Mon R2, magnetic fields provide resistance against large-scale gravitational collapse, and magnetic pressure exceeds turbulent pressure. We also investigate the properties of each filament in Mon R2. Most of the filaments are aligned along the magnetic field direction and are magnetically sub-critical.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Derek Ward-Thompson
Janik Karoly
Kate Pattle
Anthony Whitworth
Jason Kirk
David Berry
Pierre Bastien
Tao-Chung Ching
Simon Coude
Jihye Hwang
Woojin Kwon
Archana Soam
Jia-Wei Wang
Tetsuo Hasegawa
Shih-Ping Lai
Keping Qiu
Doris Arzoumanian
Tyler L. Bourke
Do-Young Byun
Huei-Ru Vivien Chen
摘要： We present BISTRO Survey 850 {\mu}m dust emission polarisation observations of the L1495A-B10 region of the Taurus molecular cloud, taken at the JCMT. We observe a roughly triangular network of dense filaments. We detect 9 of the dense starless cores embedded within these filaments in polarisation, finding that the plane-of-sky orientation of the core-scale magnetic field lies roughly perpendicular to the filaments in almost all cases. We also find that the large-scale magnetic field orientation measured by Planck is not correlated with any of the core or filament structures, except in the case of the lowest-density core. We propose a scenario for early prestellar evolution that is both an extension to, and consistent with, previous models, introducing an additional evolutionary transitional stage between field-dominated and matter-dominated evolution, observed here for the first time. In this scenario, the cloud collapses first to a sheet-like structure. Uniquely, we appear to be seeing this sheet almost face-on. The sheet fragments into filaments, which in turn form cores. However, the material must reach a certain critical density before the evolution changes from being field-dominated to being matter-dominated. We measure the sheet surface density and the magnetic field strength at that transition for the first time and show consistency with an analytical prediction that had previously gone untested for over 50 years (Mestel 1965).
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Tao-Chung Ching
Keping Qiu
Di Li
Zhiyuan Ren
Shih-Ping Lai
David Berry
Kate Pattle
Ray Furuya
Derek Ward-Thompson
Doug Johnstone
Patrick M. Koch
Chang Won Lee
Thiem Hoang
Tetsuo Hasegawa
Woojin Kwon
Pierre Bastien
Chakali Eswaraiah
Jia-Wei Wang
Kyoung Hee Kim
Jihye Hwang
摘要： We present 850 $\mu$m dust polarization observations of the massive DR21 filament from the B-fields In STar-forming Region Observations (BISTRO) survey, using the POL-2 polarimeter and the SCUBA-2 camera on the James Clerk Maxwell Telescope. We detect ordered magnetic fields perpendicular to the parsec-scale ridge of the DR21 main filament. In the sub-filaments, the magnetic fields are mainly parallel to the filamentary structures and smoothly connect to the magnetic fields of the main filament. We compare the POL-2 and Planck dust polarization observations to study the magnetic field structures of the DR21 filament on 0.1--10 pc scales. The magnetic fields revealed in the Planck data are well aligned with those of the POL-2 data, indicating a smooth variation of magnetic fields from large to small scales. The plane-of-sky magnetic field strengths derived from angular dispersion functions of dust polarization are 0.6--1.0 mG in the DR21 filament and $\sim$ 0.1 mG in the surrounding ambient gas. The mass-to-flux ratios are found to be magnetically supercritical in the filament and slightly subcritical to nearly critical in the ambient gas. The alignment between column density structures and magnetic fields changes from random alignment in the low-density ambient gas probed by Planck to mostly perpendicular in the high-density main filament probed by JCMT. The magnetic field structures of the DR21 filament are in agreement with MHD simulations of a strongly magnetized medium, suggesting that magnetic fields play an important role in shaping the DR21 main filament and sub-filaments.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Nguyen Bich Ngoc
Pham Ngoc Diep
Thiem Hoang
Le Ngoc Tram
Nguyen Chau Giang
Ngan Le
Thuong Duc Hoang
Nguyen Thi Phuong
Dieu D. Nguyen
Nguyen Minh Khang
Gia Bao Truong Le
摘要： We report the first measurement of polarized thermal dust emission toward the entire early and massive Infrared Dark Cloud G11.11$-$0.12 taken by the polarimeter SOFIA/HAWC+ at 214 $\mu m$ wavelength. Magnetic fields (B-fields) obtained from the polarized emission tend to be perpendicular to the filament's spine. We produce a map of B-field strengths for the center region of the filament. The strengths vary in the range of 100-600 $\mu\rm{G}$ and are strongest along the filament's spine. The central region is sub-Alfv\'enic and mostly sub-critical meaning that B-fields dominate over turbulence and are strong enough to resist gravitational collapse. The alignment and properties of dust grains are studied in the filament using the RAdiative Torque (RAT) theory. We find the decrease of polarization degree $P$ with emission intensity $I$, i.e., depolarization effect, of the form $P\propto I^{-\alpha}$ with $\alpha\sim$0.8-0.9, implying a significant loss of grain alignment in the filament's spine. The depolarization can be explained by the decrease in RAT alignment efficiency toward the denser regions with lower dust temperature, and cannot be explained by the B-field tangling. We study the effect of the enhanced magnetic relaxation by embedded iron inclusions on RAT alignment and find that the high polarization fraction $P\sim$20-30\% in the outer layer of the filament is potential evidence for the enhanced RAT alignment by magnetic relaxation. This is the first time this effect is evaluated in a filament. Based on the polarization fraction and RAT alignment theory, we find evidence for grain growth in the filament.
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