Search

Using the EXES instrument on SOFIA, we have obtained velocity-resolved spectra of several pure rotational lines of H2 toward shocked molecular gas within three Galactic sources: the supernova remnant (SNR) IC443 (Clump C), a protostellar outflow in the intermediate-mass star-forming region NGC 2071, and the SNR 3C391. These observations had the goal of searching for expected velocity shifts between ortho- and para-H2 transitions emitted by C-type shocks. In contrast in our previous similar study of HH7, the result of our search was negative: no velocity shifts were reliably detected. Several possible explanations for the absence of such shifts are discussed: these include a preshock ortho-to-para ratio that is already close to the high-temperature equilibrium value of 3 (in the case of IC443C), the more complex shock structures evident in all these sources, and the larger projected aperture sizes relative to those in the observations of HH7., Comment: 18 pages, including 9 figures. Accepted for publication in the Astrophysical Journal

(abridged) Together with gravity, turbulence, and stellar feedback, magnetic fields (B-fields) are thought to play a critical role in the evolution of molecular clouds and star formation processes. We aim to map the morphology and measure the strength of B-fields of the nearby molecular cloud, rho Ophiuchus-A ($\rho$ Oph-A), and then to understand the role of B-fields in regulating star formation and shaping the cloud. We have analyzed the far-infrared (FIR) polarization of thermal dust emission observed by SOFIA/HAWC$+$ at 89 and 154 $\mu$m toward the densest part of $\rho$ Oph-A, which is irradiated by the nearby B3/4 star, Oph-S1. The cloud exhibits well-ordered B-fields with magnetic orientations mainly perpendicular to the ridge of the cloud toward the densest region and B-field strengths are in the range of 0.2-2.5 mG, using the Davis-Chandrasekhar-Fermi method. The B-fields are strongest at the densest part of the cloud, which is associated with the starless core SM1, and decreases toward the outskirts of the cloud. By calculating the map of the mass-to-flux ratio, Alfv\'en Mach number, and plasma $\beta$ parameter in $\rho$ Oph-A, we find that the cloud is predominantly magnetically sub-critical, sub-Alfv\'enic, which implies that the cloud is supported by strong B-fields that dominate over gravity, turbulence, and thermal gas energy. Measured B-field strengths at two densest subregions using other methods that account for the compressible mode are relatively lower than that measured with the DCF method but do not significantly change our conclusions on the roles of B-fields relative to gravity and turbulence on star formation. A virial analysis suggests that the cloud is gravitationally unbound. We find that B-fields are sufficiently strong to support the cloud against radiative feedback and to regulate the shape of the cloud., Comment: Accepted to A&A

We study the kinematics of a pillar, namely G287.76-0.87, using three rotational lines of $^{12}$CO(5-4), $^{12}$CO(8-7), $^{12}$CO(11-10), and a fine structure line of [OI] $63\,\mu$m Southern Carina observed by SOFIA/GREAT. This pillar is irradiated by the associated massive star cluster Trumpler 16, which includes $\eta$~Carina. Our analysis shows that the relative velocity of the pillar with respect to this ionization source is small, $\sim 1\,\rm km\,s^{-1}$, and the gas motion in the tail is more turbulent than in the head. We also performed analytical calculations to estimate the gas column density in local thermal equilibrium (LTE) conditions, which yields $N_{\rm CO}$ as $(\sim 0.2 -5)\times 10^{17}\,\rm cm^{-2}$. We further constrain the gas's physical properties in non-LTE conditions using RADEX. The non-LTE estimations result in $n_{\rm H_{2}} \simeq 10^{5}\,\rm cm^{-3}$ and $N_{\rm CO} \simeq 10^{16}\,\rm cm^{-2}$. We found that the thermal pressure within the G287.76-0.87 pillar is sufficiently high to make it stable for the surrounding hot gas and radiation feedback if the winds are not active. While they are active, stellar winds from the clustered stars sculpt the surrounding molecular cloud into pillars within the giant bubble around $\eta$~Carina., Comment: 17 pages, 14 figures, accepted to ApJ

Dust grains are important in various astrophysical processes and serve as indicators of interstellar medium structures, density, and mass. Understanding their physical properties and chemical composition is crucial in astrophysics. Dust polarisation is a valuable tool for studying these properties. The Radiative Torque (RAT) paradigm, which includes Radiative Torque Alignment (RAT-A) and Radiative Torque Disruption (RAT-D), is essential to interpret the dust polarisation data and constrain the fundamental properties of dust grains. However, it has been used primarily to interpret observations at a single wavelength. In this study, we analyse the thermal dust polarisation spectrum obtained from observations with SOFIA/HAWC+ and JCMT/POL-2 in the OMC-1 region and compare the observational data with our numerical results using the RAT paradigm. In general, we show that the dense gas exhibits a positive spectral slope, whereas the warm regions show a negative one. We demonstrate that a one-layer dust (one-phase) model can only reproduce the observed spectra at certain locations and cannot match those with prominent V-shaped spectra (for which the degree of polarisation initially decreases with wavelength from 54 to $\sim$ 300$\,\mu$m and then increases at longer wavelengths). To address this, we improve our model by incorporating two dust components (warm and cold) along the line of sight, resulting in a two-phase model. This improved model successfully reproduces the V-shaped spectra. The best model corresponds to a mixture composition of silicate and carbonaceous grains in the cold medium. Finally, by assuming the plausible model of grain alignment, we infer the inclination angle of the magnetic fields in OMC-1. This approach represents an important step towards better understanding the physics of grain alignment and constraining 3D magnetic fields using dust polarisation spectra., Comment: 12 pages, 9 figures, 3 tables, accepted to A&A

Polarization of starlight and thermal dust emission caused by aligned dust grains is a valuable tool to characterize magnetic fields (B-fields) and constrain dust properties. However, the physics of grain alignment is not fully understood. To test the popular paradigm of radiative torque (RAT) theory, including RAT alignment (RAT-A) and disruption (RAT-D), we use dust polarization data observed by {\it Planck} and SOFIA/HAWC+ toward two filaments with contrasting physical conditions: Musca, a quiet filament, and OMC-1, a highly dynamic filament due to feedback. We analyze various relations of the observed polarization fraction, $P$, with gas column density, $\NHt$, dust temperature, $\Td$, and polarization angle dispersion function, $\S$. We found that $P$ decreases with increasing $\S$ and increasing $\NHt$, as expected from RAT-A. On the other hand, the $P-\Td$ relation is more complicated; it is a linear correlation at low $\Td$ but turns into an anti-correlation when $\Td$ reaches a certain high value. Next, we compute the polarization fraction on a pixel-by-pixel with B-fields in the plane of the sky using the DustPOL code based on RAT, incorporate the depolarization effect by B-field tangling using $\S$, and compare the realistic polarization model with observations of Musca and OMC-1. For Musca with well-ordered B-fields, our numerical model reproduces the decline of $P$ toward the filament spine (aka. polarization hole), having high $\NHt$ and low $\Td$, indicating the loss of grain alignment efficiency due to RAT-A. For OMC-1, with stronger B-field variations and higher $\Td$, our model can reproduce the observed $P-\Td$ and $P-N(\rm H_{2})$ relations only if the depolarization effect resulting from B-field tangling and RAT-D effect are taken into account. Our results provide more robust observational evidence for the RAT paradigm, particularly the recently discovered RAT-D., Comment: 13 figures, 24 pages

Magnetic fields (B-fields) in galaxies have recently been traced using far-infrared and sub-mm polarimetric observations with SOFIA, JCMT, and ALMA. The main assumption is that dust grains are magnetically aligned with the local B-field in the interstellar medium (ISM). However, the range of physical conditions of the ISM, dust grain sizes, and B-field strengths in galaxies where this assumption is valid has not been characterized yet. Here, we use the well-studied spiral galaxy M51 as a case study. We find that the timescale for the alignment mechanism arising from magnetically aligned dust grains (B-RAT) dominates over other alignment mechanisms, including radiative precession (k-RAT) and mechanical alignment (v-MAT), as well as the randomization effect (gas damping). We estimate the sizes of the aligned dust grain to be in the range of 0.009-0.182 $\mu$m and 0.019-0.452 $\mu$m for arms and inter-arms, respectively. We show that the difference in the polarization fraction between arms and interarms can arise from the enhancement of small dust grain sizes in the arms as an effect of the grain alignment disruption (RAT-D). We argue that the RAT-D mechanism needs to have additional effects, e.g., intrinsic variations of the B-field structure and turbulence, in the galaxy's components to fully explain the polarization fraction variations within the arms and inter-arms., Comment: 12 pages, 3 figures. Submitted to ApJ (comments are welcome)

We report 850 $\mu$m continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations (BISTRO) large program on the James Clerk Maxwell Telescope (JCMT). These data reveal a well-structured non-uniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30 deg from north to east. Field strengths estimates and a virial analysis for the major clumps indicate that NGC 2264C is globally dominated by gravity while in 2264D magnetic, gravitational, and kinetic energies are roughly balanced. We present an analysis scheme that utilizes the locally resolved magnetic field structures, together with the locally measured gravitational vector field and the extracted filamentary network. From this, we infer statistical trends showing that this network consists of two main groups of filaments oriented approximately perpendicular to one another. Additionally, gravity shows one dominating converging direction that is roughly perpendicular to one of the filament orientations, which is suggestive of mass accretion along this direction. Beyond these statistical trends, we identify two types of filaments. The type-I filament is perpendicular to the magnetic field with local gravity transitioning from parallel to perpendicular to the magnetic field from the outside to the filament ridge. The type-II filament is parallel to the magnetic field and local gravity. We interpret these two types of filaments as originating from the competition between radial collapsing, driven by filament self-gravity, and the longitudinal collapsing, driven by the region's global gravity., Comment: Accepted for publication in the Astrophysical Journal. 43 pages, 32 figures, and 4 tables (including Appendix)

Atacama Large Millimeter/Submillimeter Array (ALMA) has revolutionized the field of dust polarization in protoplanetary disks across multiple wavelengths. Previous observations and empirical modeling suggested multiple mechanisms of dust polarization toward HL Tau, including grain alignment and dust scattering. However, a detailed modeling of dust polarization based on grain alignment physics is not yet available. Here, using our updated POLARIS code, we perform numerical modeling of dust polarization arising from both grain alignment by Magnetically Enhanced Radiative Torque (MRAT) mechanism and self-scattering to reproduce the HL Tau polarization observed at three wavelengths 0.87, 1.3, and 3.1$\,$mm. Our modeling results show that the observed multi-wavelength polarization could be reproduced only when large grains contain embedded iron inclusions and those with slow internal relaxation must have wrong internal alignment (i.e., the grain's major axis parallel to its angular momentum). The abundance of iron embedded inside grains in the form of clusters is constrained to be $\gtrsim 16$%, and the number of iron atoms per cluster is $N_{\rm cl} \sim 9\times10^2$. Maximum grain sizes probed at wavelengths $\lambda$ = 0.87, 1.3, and 3.1$\,$mm are constrained at $\sim$ 60, 80, and 90$\,\mu$m, respectively., Comment: Published in ApJ

The structure of magnetic fields in galaxies remains poorly constrained, despite the importance of magnetism in the evolution of galaxies. Radio synchrotron and far-infrared (FIR) polarization and polarimetric observations are the best methods to measure galactic scale properties of magnetic fields in galaxies beyond the Milky Way. We use synthetic polarimetric observations of a simulated galaxy to identify and quantify the regions, scales, and interstellar medium (ISM) phases probed at FIR and radio wavelengths. Our studied suite of magnetohydrodynamical cosmological zoom-in simulations features high-resolutions (10 pc full-cell size) and multiple magnetization models. Our synthetic observations have a striking resemblance to those of observed galaxies. We find that the total and polarized radio emission extends to approximately double the altitude above the galactic disk (half-intensity disk thickness of $h_\text{I radio} \sim h_\text{PI radio} = 0.23 \pm 0.03$ kpc) relative to the total FIR and polarized emission that are concentrated in the disk midplane ($h_\text{I FIR} \sim h_\text{PI FIR} = 0.11 \pm 0.01$ kpc). Radio emission traces magnetic fields at scales of $\gtrsim 300$ pc, whereas FIR emission probes magnetic fields at the smallest scales of our simulations. These scales are comparable to our spatial resolution and well below the spatial resolution ($<300$ pc) of existing FIR polarimetric measurements. Finally, we confirm that synchrotron emission traces a combination of the warm neutral and cold neutral gas phases, whereas FIR emission follows the densest gas in the cold neutral phase in the simulation. These results are independent of the ISM magnetic field strength. The complementarity we measure between radio and FIR wavelengths motivates future multiwavelength polarimetric observations to advance our knowledge of extragalactic magnetism., Comment: Accepted for publication in the ApJ. Replaced to match final version. 35 pages, 17 figures, 4 tables

(Abridged) Context. Massive star formation is associated with energetic processes, which result in significant gas cooling via far-infrared (IR) lines. Velocity-resolved observations can constrain the kinematics of the gas, allowing the identification of the physical mechanisms responsible for gas heating. Aims. Our aim is to quantify far-infrared CO line emission toward high-mass star-forming regions, identify the high-velocity gas component associated with outflows, and estimate the physical conditions required for the excitation of the observed lines. Methods. Velocity-resolved SOFIA/GREAT spectra of 13 high-mass star forming clumps of various luminosities and evolutionary stages are studied using CO 11-10 and 16-15 lines. Results. All targets show strong high-J CO emission in the far-IR, characterized by broad line wings associated with outflows, thereby significantly increasing the sample of sources with velocity-resolved high-J CO spectra. The contribution of the emission in the line wings does not correlate with the envelope mass or evolutionary stage. Gas rotational temperatures cover a narrow range of 120-220 K for the line wings. The non-LTE radiative transfer models indicate gas densities of 1e5-1e7 cm-3 and N(CO) of 1e17- 1e18 cm-2, similar to physical conditions in deeply-embedded low- and high-mass protostars. The velocity-integrated CO line fluxes correlate with the bolometric luminosity over 7 orders of magnitude including data on the low-mass protostars, suggesting similar processes are responsible for the high-J CO excitation over a significant range of physical scales. Conclusions. Velocity-resolved line profiles allow the detection of outflows toward massive star-forming clumps spanning a broad range of evolutionary stages. The lack of clear evolutionary trends suggest that mass accretion and ejection prevail during the entire lifetime of star-forming clumps., Comment: 21 pages, 19 figures, accepted to A&A

We present observations of polarized dust emission at 850 $\mu$m from the L43 molecular cloud which sits in the Ophiuchus cloud complex. The data were taken using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope as a part of the BISTRO large program. L43 is a dense ($N_{\rm H_2}\sim 10^{22}$-10$^{23}$ cm$^{-2}$) complex molecular cloud with a submillimetre-bright starless core and two protostellar sources. There appears to be an evolutionary gradient along the isolated filament that L43 is embedded within, with the most evolved source closest to the Sco OB2 association. One of the protostars drives a CO outflow that has created a cavity to the southeast. We see a magnetic field that appears to be aligned with the cavity walls of the outflow, suggesting interaction with the outflow. We also find a magnetic field strength of up to $\sim$160$\pm$30 $\mu$G in the main starless core and up to $\sim$90$\pm$40 $\mu$G in the more diffuse, extended region. These field strengths give magnetically super- and sub-critical values respectively and both are found to be roughly trans-Alfv\'enic. We also present a new method of data reduction for these denser but fainter objects like starless cores., Comment: Accepted for publication in ApJ. 23 pages, 9 figures (7 main text, 2 appendix)

We present polarization observations of the young supernova remnant (SNR) Cas A using the High-resolution Airborne Wideband Camera-Plus (HAWC+) instrument onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The polarization map at 154 microns reveals dust grains with strong polarization fractions (5 - 30 percent), supporting previous measurements made over a smaller region of the remnant at 850 microns. The 154 microns emission and the polarization signal is coincident with a region of cold dust observed in the southeastern shell and in the unshocked central ejecta. The highly polarized far-IR emission implies the grains are large (greater than 0.14 microns) and silicate-dominated. The polarization level varies across the SNR, with an inverse correlation between the polarization degree and the intensity and smaller polarization angle dispersion for brighter SNR emission. Stronger polarization is detected between the bright structures. This may result from a higher collision rate between the gas and dust producing a lower grain alignment efficiency where the gas density is higher. We use the dust emission to provide an estimate of the magnetic field strength in Cas A using the Davis-Chandrasekhar-Fermi method. The high polarization level is direct evidence that grains are highly elongated and strongly aligned with the magnetic field of the SNR. The dust mass from the polarized region is 0.14+-0.04 Msun, a lower limit of the amount of dust present within the ejecta of Cas A. This result strengthens the hypothesis that core-collapse SNe are an important contributor to the dust mass in high redshift galaxies., Comment: MNRAS, accepted (18 pages with 14 figures)

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)., Comment: 14 pages, 5 figures. ApJ accepted

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.

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., Comment: 24 pages, 15 figures

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., Comment: 26 pages, 13 figures, ApJ accepted

Context: Supernovae (SNe) inject large amounts of energy and chemically enriched materials into their surrounding interstellar medium and, in some instances, into molecular clouds (MCs). The interaction of a supernova remnant (SNR) with a MC plays a crucial role in the evolution of the cloud's physical and chemical properties. Despite their importance, only a handful of studies have been made addressing the molecular richness in MCs impacted by SNRs. (Sub)millimter wavelength observations of SNR-MC can be used to build a census of their molecular richness, which in turn can motivate various chemical and physical models aimed at explaining the chemical evolution of the clouds. Aims: We carried out multi-molecule/multi-transition observations toward the region F abutting the SNR W28. We used the detected lines to constrain the physical conditions of this region. Methods: We used the APEX Telescope to observe molecular lines in the frequency from $213\rm{-}374\, \textrm{GHz}$. We used non-LTE RADEX modeling to interpret the observational data. Results: We detected emission from multiple molecular species, namely CH$_{3}$OH, H$_{2}$CO, SO, SiO, CN, CCH, NO, CS, HCO$^+$, HCN, HNC, N$_2$H$^+$, CO, and from isotopologues of some of them. We report the first detection of thermally excited (non-maser) CH$_{3}$OH emission toward a SNR. Employing non-LTE RADEX modeling of multiple H$_{2}$CO and CH$_{3}$OH lines, we constrain the kinetic temperature from 60 to 100$\,$K and the gas density from $9\times 10^{5}$ to $5\times 10^{6}\,\rm cm^{-3}$. We obtained an ortho-para ratio $\sim$2 for H$_{2}$CO, which indicates that formaldehyde is most likely formed on dust grain surfaces. Conclusions: Our results show that molecules as complex as H$_{2}$CO and CH$_{3}$OH can be detected in SNR-MC interactions. This could motivate chemical modelling to explore their formation pathways., Comment: 19 pages, 13 figures, 6 tables, accepted to A&A

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., Comment: This paper is accepted to the ApJ

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., Comment: 39 pages, 27 figures

Modern understanding of dust astrophysics reveals that RAdiative Torques (RATs) arising from the radiation-dust interaction can induce two fundamental effects, including grain alignment and rotational disruption. Here we review the recent progress in the theoretical development and observational testing of these effects using dust polarization observed toward star-forming regions (SFRs). We first review the basic theory of the RAT alignment and RAT disruption, which are referred to as RAT-A and RAT-D effects, respectively. We then briefly describe the numerical method we use to model polarized thermal dust emission by accounting for both RAT-A and RAT-D and theoretical predictions of dust polarization for observations. Next, we review our observational efforts to search for observational evidence of the RAT-A and RAT-D effects using thermal dust polarization toward SFRs. Finally, we discuss magnetic fields inferred from dust polarization observed toward these SFRs and implications of the RAT paradigm for different astrophysical conditions, including protostellar environments, dust evolution, and time-domain astrophysics., Comment: 44 pages, 15 figures, review article, accepted to Frontiers

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., Comment: 26 pages, 17 figures, 3 tables, accepted to ApJ

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., Comment: 43 pages, 17 figures, Table of notations is added. Version accepted

We present 850 $\mu$m polarimetric observations toward the Serpens Main molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT) as part of the B-fields In STar-forming Region Observations (BISTRO) survey. These observations probe the magnetic field morphology of the Serpens Main molecular cloud on about 6000 au scales, which consists of cores and six filaments with different physical properties such as density and star formation activity. Using the histogram of relative orientation (HRO) technique, we find that magnetic fields are parallel to filaments in less dense filamentary structures where $N_{H_2} < 0.93\times 10^{22}$ cm$^{-2}$ (magnetic fields perpendicular to density gradients), while being perpendicular to filaments (magnetic fields parallel to density gradients) in dense filamentary structures with star formation activity. Moreover, applying the HRO technique to denser core regions, we find that magnetic field orientations change to become perpendicular to density gradients again at $N_{H_2} \approx 4.6 \times 10^{22}$ cm$^{-2}$. This can be interpreted as a signature of core formation. At $N_{H_2} \approx 16 \times 10^{22}$ cm$^{-2}$ magnetic fields change back to being parallel to density gradients once again, which can be understood to be due to magnetic fields being dragged in by infalling material. In addition, we estimate the magnetic field strengths of the filaments ($B_{POS} = 60-300~\mu$G)) using the Davis-Chandrasekhar-Fermi method and discuss whether the filaments are gravitationally unstable based on magnetic field and turbulence energy densities., Comment: 18 pages, accepted for publication in ApJ

Asymptotic giant branch (AGB)stars create a rich inventory of molecules in their envelopes as they lose mass during later stages of their evolution. These molecules cannot survive the conditions in interstellar space, where they are exposed to ultraviolet photons of the interstellar radiation field. As a result, daughter molecules are the ones injected into space, and a halo of those molecules is predicted to exist around cool evolved stars. The most abundant molecule in the envelopes other than H2 is CO, which dissociates into C that is rapidly ionized in a halo around the star that is optically thin to the interstellar radiation field. We develop the specific predictions of the ionized carbon halo size and column density for the well-studied, nearby star IRC +10216. We compare those models to observations of the [C II] 157.7 micron far-infrared fine-structure line using SOFIA and Herschel. The combination of bright emission toward the star and upper limits to extended [C II] is inconsistent with any standard model. The presence of [C II] toward the star requires some dissociation and ionization in the inner part of the outflow, possibly due to a hot companion star. The lack of extended [C II] emission requires that daughter products from CO photodissociation in the outer envelope remain cold. The [C II] profile toward the star is asymmetric, with the blue-shifted absorption due to the cold outer envelope., Comment: accepted to ApJ (on 2021 Dec 7)

Molecular emission arising from the interactions of supernova remnant (SNR) shock waves and molecular clouds provide a tool for studying the dispersion and compression that might kick-start star formation as well as understanding cosmic ray enhancement in SNRs. Purely-rotational CO emission created by magneto-hydrodynamic shock in the SNR - molecular cloud interaction is an effective shock tracer, particularly for slow-moving, continuous shocks into cold inner clumps of the molecular cloud. In this work, we present a new theoretical radiative transfer framework for predicting the line profile of CO with the Paris-Durham 1D shock model. We generated line profile predictions for CO emission produced by slow, magnetized C-shocks into gas of density ~ 100,000 cm-3 with shock speeds of 35 and 50 km s-1. The numerical framework to reproduce the CO line profile utilizes the Large Velocity Gradient (LVG) approximation and the omission of optically-thick plane-parallel slabs. With this framework, we generated predictions for various CO spectroscopic observations up to J=16 in SNRs W28 and IC443, obtained with SOFIA, IRAM-30m, APEX, and KPNO. We found that CO line profile prediction offers constraints on the shock velocity and preshock density independent of the absolute line brightness and requiring fewer CO lines than diagnostics using an rotational excitation diagram., Comment: Accepted for publication in the Astrophysical Journal, Article ID: apjacb259

It has recently been suggested that planets can form by dust coagulation in the torus of active galactic nuclei (AGN) with low luminosity of $L_{\rm bol}\lesssim 10^{42} erg s^{-1}$, constituting a new class of exoplanets orbiting the supermassive black hole called \textit{blanets}. However, large dust grains in the AGN torus may be rotationally disrupted by the Radiative Torque Disruption (RATD) mechanism due to AGN radiation feedback, which would prevent the blanet formation. To test this scenario, we adopt the simple smooth and clumpy dust/gas distribution inside the torus region to study the effect of RATD on the evolution of composite dust grains in the midplane of the torus. We found that grain growth and then blanet formation are possible in the smooth torus model. However, in the clumpy torus model, grain growth will be strongly constrained by RATD, assuming the gas density distribution as adopted in Wada et al. We also found that icy grain mantles inside clumps are quickly detached from the grain core by rotational desorption, reducing the sticking coefficient between icy grains and coagulation efficiency. The grain rotational disruption and ice desorption occur on timescales much shorter than the growth time up to a factor of $\sim 10^{4}$, which are the new barriers that grain growth must overcome to form blanets. Further studies with more realistic AGN models are required to better constrain the effect of RATD on grain growth and blanet formation hypothesis around low luminosity AGN., Comment: 27 pages, 13 figures Accepted to ApJ

Circumstellar dust is formed and evolved within the envelope of evolved stars, including Asymptotic Giant Branch (AGB) and Red Supergiant (RSG). The extinction of stellar light by circumstellar dust is vital for interpreting RSG/AGB observations and determining high-mass RSG progenitors of core-collapse supernovae. Nevertheless, circumstellar dust properties are not well understood. Modern understanding of dust evolution suggests that intense stellar radiation can radically change the dust properties across the circumstellar envelope through the RAdiative Torque Disruption (RAT-D) mechanism. In this paper, we study the impacts of RAT-D on the grain size distribution (GSD) of circumstellar dust and model its effects on photometric observations of $\alpha$ Orionis (Betelgeuse). Due to the RAT-D effects, large grains formed in the dust formation zone are disrupted into smaller species of size $a < 0.5\,\rm\mu m$. Using the GSD constrained by the RAT-D effects, we model the visual extinction of background stars and Betelgeuse. We find that the extinction decreases at near-UV, optical, and infrared wavelengths while increasing at far-UV wavelengths. The resulting flux potentially reproduces the observation from the near-UV to near-IR range. Our results can be used to explain dust extinction and photometric observations toward other RSG/AGB stars., Comment: 21 pages, 18 figures, 2 tables, accepted to ApJ

We present the results of simultaneous 450 $\mu$m and 850 $\mu$m polarization observations toward the massive star forming region NGC 2071IR, a target of the BISTRO (B-fields in Star-Forming Region Observations) Survey, using the POL-2 polarimeter and SCUBA-2 camera mounted on the James Clerk Maxwell Telescope. We find a pinched magnetic field morphology in the central dense core region, which could be due to a rotating toroidal disk-like structure and a bipolar outflow originating from the central young stellar object, IRS 3. Using the modified Davis-Chandrasekhar-Fermi method, we obtain a plane-of-sky magnetic field strength of 563$\pm$421 $\mu$G in the central $\sim$0.12 pc region from 850 $\mu$m polarization data. The corresponding magnetic energy density of 2.04$\times$10$^{-8}$ erg cm$^{-3}$ is comparable to the turbulent and gravitational energy densities in the region. We find that the magnetic field direction is very well aligned with the whole of the IRS 3 bipolar outflow structure. We find that the median value of polarization fractions, 3.0 \%, at 450 $\mu$m in the central 3 arcminute region, which is larger than the median value of 1.2 \% at 850 $\mu$m. The trend could be due to the better alignment of warmer dust in the strong radiation environment. We also find that polarization fractions decrease with intensity at both wavelengths, with slopes, determined by fitting a Rician noise model, of $0.59 \pm 0.03$ at 450 $\mu$m and $0.36 \pm 0.04$ at 850 $\mu$m, respectively. We think that the shallow slope at 850 $\mu$m is due to grain alignment at the center being assisted by strong radiation from the central young stellar objects., Comment: 17 pages, 11 figures, 4 tables

We report the highest spatial resolution measurement of magnetic fields in M17 using thermal dust polarization taken by SOFIA/HAWC+ centered at 154 $\mu$m wavelength. Using the Davis-Chandrasekhar-Fermi method, we found the presence of strong magnetic fields of $980 \pm 230\;\mu$G and $1665 \pm 885\;\mu$G in lower-density (M17-N) and higher-density (M17-S) regions, respectively. The magnetic field morphology in M17-N possibly mimics the fields in gravitational collapse molecular cores while in M17-S the fields run perpendicular to the matter structure and display a pillar and an asymmetric hourglass shape. The mean values of the magnetic field strength are used to determine the Alfv\'enic Mach numbers ($\mathcal{M_A}$) of M17-N and M17-S which turn out to be sub-Alfv\'enic, or magnetic fields dominate turbulence. We calculate the mass-to-flux ratio, $\lambda$, and obtain $\lambda=0.07$ for M17-N and $0.28$ for M17-S. The sub-critical values of $\lambda$ are in agreement with the lack of massive stars formed in M17. To study dust physics, we analyze the relationship between the dust polarization fraction, $p$, and the thermal emission intensity, $I$, gas column density, $N({\rm H_2})$, and dust temperature, $T_{\rm d}$. The polarization fraction decreases with intensity as $I^{-\alpha}$ with $\alpha = 0.51$. The polarization fraction also decreases with increasing $N(\rm H_{2})$, which can be explained by the decrease of grain alignment by radiative torques (RATs) toward denser regions with a weaker radiation field and/or tangling of magnetic fields. The polarization fraction tends to increase with $T_{\rm d}$ first and then decreases when $T_ {\rm d} > 50$ K. The latter feature seen in the M17-N, where the gas density changes slowly with $T_{d}$, is consistent with the RAT disruption effect., Comment: Submitted to Apj

Located in the Large Magellanic cloud and mostly irradiated by a massive-star cluster R$\,$136, 30 Doradus is an ideal target to test the leading theory of the grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214$\,\mu$m using SOFIA/HAWC+. We analyse the variation of the dust polarization degree ($p$) with the total emission intensity ($I$), the dust temperature ($T_{\rm d}$), and the gas column density ($N_{\rm H}$) constructed from ${\it Herschel}$ data. The 30 Doradus complex is divided into two main regions relative to R$\,$136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R$\,$136. The first depolarization likely arises from the decrease of grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase of the gas density. The second trend (the increase of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory. The final trend (the decrease of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of the rotational disruption by RATs., Comment: 19 pages, 13 figures, 1 table, accepted to ApJ

The recent availability of high-resolution far-infrared (FIR) polarization observations of galaxies using HAWC+/SOFIA has facilitated studies of extragalactic magnetic fields in the cold and dense molecular disks.We investigate if any significant structural differences are detectable in the kpc-scale magnetic field of the grand design face-on spiral galaxy M51 when traced within the diffuse (radio) and the dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a complex scenario where radio and FIR polarization observations do not necessarily trace the same magnetic field structure. We find that the magnetic field in the arms is wrapped tighter at 154um than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with HI neutral gas. The star formation rate shows a clear correlation with the radio polarized intensity, which is not found in FIR, pointing to a small-scale dynamo-driven B-field amplification scenario. This work shows that multi-wavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multi-phase ISM., Comment: 35 pages, 25 figures, 5 tables. Accepted for publication in ApJ

We have obtained sensitive dust continuum polarization observations at 850 $\mu$m in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope (JCMT), as part of the BISTRO (B-fields in STar-forming Region Observations) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution ($\sim$2000 au or $\sim$0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38$\pm$14 $\mu$G, 44$\pm$16 $\mu$G, and 12$\pm$5 $\mu$G, respectively. These cores show distinct mean B-field orientations. B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field, and not well-correlated with other axes. In contrast, Miz-8b exhibits disordered B-field which show no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux., Comment: 20 pages (10 are main), 5 figures (3 are main), and 3 tables (2 are main); Accepted for its publication in ApJL

We report the first high spatial resolution measurement of magnetic fields surrounding LkH$\alpha$ 101, a part of the Auriga-California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 $\mu$m is found to be mostly associated with the red-shifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar-Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of $B_{\rm POS}\sim 115$ $\mathrm{\mu}$G. The mass-to-magnetic-flux ratio in critical value units, $\lambda\sim0.3$, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkH$\alpha$ 101 region is sub-critical and the magnetic field is strong enough to prevent gravitational collapse. The inferred $\delta B/B_0\sim 0.3$ implies that the large scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power-law with an index of $\alpha=0.82\pm0.03$, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkH$\alpha$ 101) in the region. The magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques are potential of explaining such a decreasing trend., Comment: 25 pages, 19 figures, Accepted for publication in The Astrophysical Journal

Dust polarization induced by aligned grains is widely used to study magnetic fields in various environments, including star-forming regions. However, the question of to what optical depth grain alignment still exists in a dense molecular cloud (MC) is unclear. In this paper, we aim to achieve analytical formulae for the minimum size of aligned grains ($a_{\rm align}$) and rotational disruption ($a_{\rm disr}$) by RAdiative Torques (RATs) as a function of the local physical parameters within dense MCs. We first find the analytical approximations for the radiation strength and the mean wavelength of the attenuated radiation field in a dense MC without and with embedded stars and then derive analytical formulae for $a_{\rm align}$ and $a_{\rm disr}$ as functions of the visual extinction $A_{V}$ and the gas density. We find that within a starless core of density $n_{\rm H}\sim 10^{5}\rm cm^{-3}$, grains of size $a<0.25\mu m$ can be aligned up to $A_{V}\sim 5$ by RATs, whereas micron-sized grains can still be aligned at $A_{V}\sim 50$. The increase in the alignment size with $A_{V}$ can explain the presence of polarization holes observed toward starless cores. For MCs with an embedded protostar, we find that the efficiency of both alignment and rotational disruption increases toward the protostar due to the increasing radiation strength. Such a disruption effect results in the decrease of the polarization degree with $A_{V}$ or emission intensity, which reproduces the popular polarization holes observed toward the location of protostars. Finally, we derive the formula for the maximum $A_{V}$ where grain alignment still exists in a starless core and discuss its potential for constraining grain growth., Comment: 18 pages, 12 figures, submitted to ApJ

Complex Organic Molecules (COMs) are believed to form in the ice mantle of dust grains and are released to the gas by thermal sublimation when grain mantles are heated to temperatures of $T_{\rm d}\gtrsim 100\,\rm K$. However, some COMs are detected in regions with temperatures below 100 K. Recently, a new mechanism of rotational desorption due to centrifugal stress induced by radiative torques (RATs) is proposed by Hoang & Tram 2020 that can desorb COMs at low temperatures. In this paper, we report observational evidence for rotational desorption of COMs toward the nearest massive star-forming region Orion BN/KL. We compare the abundance of three representative COMs which have very high binding energy computed by the rotational desorption mechanism with observations by ALMA, and demonstrate that the rotational desorption mechanism can explain the existence of such COMs. We also analyze the polarization data from SOFIA/HAWC+ and JCMT/SCUBA-2 and find that the polarization degree at far-infrared/submm decreases with increasing the grain temperature for $T_{\rm d}\gtrsim 71\,\rm K$. This is consistent with the theoretical prediction using the Radiative Torque (RAT) alignment theory and Radiative Torque Disruption (RATD) mechanism. Such an anti-correlation between dust polarization and dust temperature supports the rotational disruption as well as rotational desorption mechanism of COMs induced by RATs., Comment: 13 pages, 9 figures, 1 table, accepted to ApJ

We analyze the wavelength dependence of the far-infrared polarization fraction toward the OMC-1 star forming region using observations from HAWC+/SOFIA at 53, 89, 154, and 214 $\mu$m. We find that the shape of the far-infrared polarization spectrum is variable across the cloud and that there is evidence of a correlation between the slope of the polarization spectrum and the average line-of-sight temperature. The slope of the polarization spectrum tends to be negative (falling toward longer wavelengths) in cooler regions and positive or flat in warmer regions. This is very similar to what was discovered in $\rho$ Oph A via SOFIA polarimetry at 89 and 154 $\mu$m. Like the authors of this earlier work, we argue that the most natural explanation for our falling spectra is line-of-sight superposition of differing grain populations, with polarized emission from the warmer regions and less-polarized emission from the cooler ones. In contrast with the earlier work on $\rho$ Oph A, we do not find a clear correlation of polarization spectrum slope with column density. This suggests that falling spectra are attributable to variations in grain alignment efficiency in a heterogeneous cloud consistent with radiative torques theory. Alternative explanations in which variations in grain alignment efficiency are caused by varying gas density rather than by varying radiation intensity are disfavored., Comment: 21 pages, 10 figures. Accepted to ApJ. Major changes from first draft: 1. Additional discussion/introduction about ETAC/HCE, 2. More in-depth analysis on robustness of polarization slope/dust temperature correlation, 3. Updated notation on the spatial polarization ratio analysis, 4. Updated many figures for clarity and consistency

The alignment of dust grains with the ambient magnetic field produces polarization of starlight as well as thermal dust emission. Using the archival SOFIA/HAWC+ polarimetric data observed toward $\rho$ Ophiuchus (Oph) A cloud hosted by a B association star at 89 $\mu$m and 154 $\mu$m, we find that the fractional polarization of thermal dust emission first increases with the grain temperature and then decreases once the grain temperature exceeds $\simeq$ 25-32 K. The latter trend differs from the prediction of the popular RAdiative Torques (RATs) alignment theory which implies a monotonic increase of the polarization fraction with the grain temperature. We perform numerical modeling of polarized dust emission for the $\rho$ Oph-A cloud and calculate the degree of dust polarization by simultaneously considering the dust grain alignment and rotational disruption by RATs. Our modeling results could successfully reproduce both the rising and declining trends of the observational data. Moreover, we show that the alignment of only silicate grains or a mixture of silicate-carbon grains within a composite structure can reproduce the observational trends, assuming that all dust grains follow a power-law size distribution. Although there are a number of simplifications and limitations to our modeling, our results suggest grains in $\rho$ Oph-A cloud have a composite structure, and the grain size distribution has steeper slope than the standard size distribution for the interstellar medium. Combination of SOFIA/HAWC+ data with JCMT observations 450 $\mu$m and 850 $\mu$m would be useful to test the proposed scenario based on grain alignment and disruption by RATs., Comment: 14 pages, 12 figures, 1 table, accepted to ApJ

Prompt optical emission of gamma-ray bursts (GRBs) is known to have important effects on the surrounding environment. In this paper, we study rotational disruption and alignment of dust grains by radiative torques (RATs) induced by GRB afterglows and predict their signatures on the observational properties of GRB afterglows. We first study grain disruption using RAdiative Torque Disruption (RATD) mechanism and find that large grains (size $>0.1 \mu\rm m$) within a distance of $d< 40$ pc from the source can be disrupted into smaller grains. We then model the extinction curve of GRB afterglows and find that optical-NIR extinction is rapidly decreased, and UV extinction increases due to the conversion of large grains into smaller ones via RATD. The total-to-selective visual extinction ratio is found to decrease from the standard value of $R_{V}\sim 3.1$ to $\sim 1.5$ after disruption time $t_{\rm disr} \lesssim 10^{4}$ s. Next, we study grain alignment by RATs induced by GRB afterglows and model the wavelength-dependence polarization produced by grains aligned with magnetic fields. We find that polarization degree first increases due to enhanced alignment of small grains and then decreases when grain disruption by RATD begins. The maximum polarization wavelength $ \lambda_{\rm max}$ decreases rapidly from the standard value of $\sim 0.55 \mu\rm m$ to $\sim 0.15 \mu\rm m$ over alignment time of $t_{\rm align} \lesssim 30$ s due to enhanced alignment of small grains. Finally, we found that RATD induces a significant decrease in optical/NIR extinction, producing an optical re-brightening in the observed light curve of GRB afterglows. We show that our theoretical predictions can explain various observational properties of GRB afterglows, including steep extinction curves, time-variability of colors, and optical re-brightening of GRB afterglows., Comment: 19 pages, 16 figures

Supernovae from core-collapse of massive stars drive shocks into the molecular clouds from which the stars formed. Such shocks affect future star formation from the molecular clouds, and the fast-moving, dense gas with compressed magnetic fields is associated with enhanced cosmic rays. This paper presents new theoretical modeling, using the Paris-Durham shock model, and new observations, using the Stratospheric Observatory for Infrared Astronomy (SOFIA), of the H$_2$ S(5) pure rotational line from molecular shocks in the supernova remnant IC443. We generate MHD models for non-steady-state shocks driven by the pressure of the IC443 blast wave into gas of densities $10^3$ to $10^5$ cm$^{-3}$. We present the first detailed derivation of the shape of the velocity profile for emission from H$_2$ lines behind such shocks, taking into account the shock age, preshock density, and magnetic field. For preshock densities $10^3$-$10^5$ cm$^{-3}$, the the predicted shifts of line centers, and the line widths, of the H$_2$ lines range from 20-2, and 30-4 km/s, respectively. The a priori models are compared to the observed line profiles, showing that clumps C and G can be explained by shocks into gas with density 10$^3$ to $2\times 10^4$ cm$^{-3}$ and strong magnetic fields. For clump B2 (a fainter region near clump B), the H$_2$ spectrum requires a J-type shock into moderate density (~100 cm$^{-3}$) with the gas accelerated to 100 km/s from its pre-shock location. Clump B1 requires both a magnetic-dominated C-type shock (like for clumps C and G) and a J-type shock (like for clump B1) to explain the highest observed velocities. The J-type shocks that produce high-velocity molecules may be locations where the magnetic field is nearly parallel to the shock velocity, which makes it impossible for a C-type shock (with ions and neutrals separated) to form., Comment: accepted by ApJ (9/4/2019)

Radio observations of some Asymptotic Giant Branch (AGB) star envelopes show the excess emission at frequencies below 100 GHz which cannot be explained by thermal dust emission (hereafter anomalous microwave emission-AME). Moreover, AGB envelopes are a common place where gas molecules condense to form nanoparticles (e.g., polycyclic aromatic hydrocarbons) and large grains. In this paper, we will study whether electric dipole emission from rapidly spinning nanoparticles can reproduce the AME observed toward AGB stars. To properly model the size distribution of nanoparticles in the AGB envelope, we take into account both the increase of nanoparticles due to rotational disruption of large grains spun-up by radiative torques and the decrease of smallest nanoparticles due to rotational disruption driven by stochastic gas collisions. We then perform detailed modeling of microwave emission from rapidly spinning nanoparticles from both C-rich and O-rich AGB envelopes using the grain size distribution constrained by rotational disruption. We find that spinning dust emission is dominant over thermal dust emission at frequencies below 100 GHz. We attempt to fit the observational data of AME using our spinning dust model and demonstrate that spinning dust can reproduce the observed AME in six AGB stars. Finally, we discuss that microwave emission from spinning dust in AGB envelopes could be observed with high-resolution upcoming radio telescopes such as ngVLA and ALMA Band 1. This would be a major leap for understanding AGB envelopes, formation, evolution, and internal structures of dust. Observations would help to distinguish the carrier of AME from comparing C-rich to O-rich stars, because PAHs are formed in C-rich AGB stars while silicates are formed in O-rich stars., Comment: 15 pages, 10 figures, and 2 tables

Photometric and polarimetric observations toward type Ia supernovae (SNe Ia) frequently report an unusually low total-to-selective extinction ratio ($R_{\rm V} < 2$) and small peak wavelength of polarization ($\lambda_{\rm max}< 0.4 \mu m$). Recently, Hoang et al. proposed that the increase in the abundance of small grains relative to large grains near SNe Ia due to RAdiative Torque Disruption (RATD) can explain this puzzle. To test this scenario, we will perform detailed modeling of dust extinction and polarization of SNe Ia accounting for grain disruption by RATD and grain alignment by RAdiative Torques (RATs). For dust clouds at distance $d< 4$ pc from the source, we find that $R_{\rm V}$ decreases rapidly from the standard value of $3.1$ to $\sim 1.5$ after a disruption time $t_{\rm disr}\le 40$ days. We then calculate the observed SNe Ia light curve and find that the colors of SNe Ia would change with time due to time-varying extinction for dust clouds at distance of $d<4$ pc. We also calculate the wavelength-dependence polarization produced by grains aligned with the magnetic fields by RATs. We find that $ \lambda_{\rm max}$ decreases rapidly from $\sim 0.55 \mu$m to $\sim 0.15 \mu$m over an alignment time of $t_{\rm align}< 10$ days due to the enhanced alignment of small grains. By fitting the theoretical polarization curve with the Serkowski law, we find that the parameter $K$ from the Serkowski law increases when large grains are disrupted by RATD which can explain the $K$ vs. $\lambda_{\rm max}$ data observed for SNe Ia. Finally, we discover an anti-correlation between $K$ and $R_{\rm V}$ which might already be supported by SNe Ia observational data. Our results demonstrate the important effect of rotational disruption of dust grains by radiative torques on the time-dependent extinction, polarization, and colors of SNe Ia., Comment: 22 figures, 24 pages

Spectrally-resolved observations of three pure rotational lines of H$_2$, conducted with the EXES instrument on SOFIA toward the classic bow shock HH7, reveal systematic velocity shifts between the S(5) line of ortho-H$_2$ and the two para-H$_2$ lines [S(4) and S(6)] lying immediately above and below it on the rotational ladder. These shifts, reported here for the first time, imply that we are witnessing the conversion of para-H$_2$ to ortho-H$_2$ within a shock wave driven by an outflow from a young stellar object. The observations are in good agreement with the predictions of models for non-dissociative, C-type molecular shocks. They provide a clear demonstration of the chemical changes wrought by interstellar shock waves, in this case the conversion of para-H$_2$ to ortho-H$_2$ in reactive collisions with atomic hydrogen, and provide among the most compelling evidence yet obtained for C-type shocks in which the flow velocity changes continuously., Comment: 19 pages, 4 figures. Accepted for publication in the Astrophysical Journal Letters

Ice mantles on dust grains play a central role in astrochemistry. Water and complex organic molecules (COMs) are thought to first form on the ice mantles and subsequently are released into the gas phase due to star formation activity. However, the critical question is whether ice mantles can survive stellar radiation when grains are being heated from $T_{d}\sim 10\rm K$ to $\gtrsim 100$ K. In this paper, we first study the effect of suprathermal grain rotation driven by the intense radiation of young stellar objects (YSOs) on the ice mantles. We find that the entire ice mantles can be disrupted into small fragments by centrifugal stress before the water ice and COMs desorb via thermal sublimation. We then study the consequence of resulting ice fragments and find that tiny fragments of radius $a \lesssim 10$\AA~ exhibit transient release of COMs due to thermal spikes, whereas larger fragments can facilitate thermal sublimation at much higher rates than from the original icy grain or the same rate but with temperatures of $\sim 20-40$ K lower. We find that rotational desorption is efficient for hot cores/corinos from the inner to outer regions where the temperature drops to $T_{\rm gas}\sim 40\rm K$ and $n_{\rm H}\sim 10^{4}\rm cm^{-3}$. We discuss the implications of this mechanism for desorption of COMs and water ice in various environments, including outflow cavity walls, photodissociation regions, and protoplanetary disks. Finally, we show that very large aggregate grains can be disrupted into individual icy grains via rotational disruption mechanism, followed by rotational desorption of ice mantles., Comment: 17 pages, 11 figures; accepted to ApJ

In a previous work, Hoang and Tram discovered a new mechanism for destruction of nanoparticles due to suprathermal rotation of grains in stationary C-shocks, which is termed rotational disruption. In this paper, we extend our previous study for non-stationary shocks driven by outflows and young supernovae remnants that have dynamical ages shorter than the time required to establish a stationary C-shock, which is composed of a C-shock and a J-shock tail (referred as CJ-shock). For the C-shock component, we find that smallest nanoparticles (size $\lesssim 1$ nm) of weak materials (i.e., tensile strength $S_{\rm max} \lesssim 10^{9}\ \rm erg\ cm^{-3}$) can be rotationally disrupted due to suprathermal rotation induced by supersonic neutral drift. For the J-shock component, although nanoparticles are rotating thermally, the smallest ones can still be disrupted because the gas is heated to higher temperatures by J-shocks. We then model microwave emission from rapidly spinning nanoparticles where the grain size distribution has the lower cutoff determined by rotational disruption for the different shock models. We also calculate the spectral flux of microwave emission from a shocked region at distance of 100 pc from the observer for the different gas density, shock age, and shock velocities. We suggest that microwave emission from spinning dust can be used to trace nanoparticles and shock velocities in dense molecular outflows. Finally, we discuss a new way that can release molecules from the nanoparticle surface into the gas in the shocked regions, which we name rotational desorption., Comment: 19 pages, 15 figures. arXiv admin note: text overlap with arXiv:1810.12007

Polycyclic aromatic hydrocarbons (PAHs) and nanoparticles are expected to play an important role in many astrophysical processes due to its dominant surface area, including gas heating, chemistry, star formation , and anomalous microwave emission. In dense magnetized molecular clouds where C-shocks are present, PAHs and nanoparticles are widely believed to originate from grain shattering due to grain-grain collisions. The remaining question is whether these nanoparticles can survive in the dense and hot shocked regions, and how to constrain their size and abundance with observations. In this paper, we present a new mechanism to destroy nanoparticles in C-shocks based on centrifugal stress within rapidly spinning nanoparticles spun-up by stochastic atomic bombardment, which is termed rotational disruption. We find that, due to supersonic neutral gas-charged grain drift in C-shocks, nanoparticles can be spun-up to suprathermal rotation by stochastic torques exerted by supersonic neutral flow. The resulting centrifugal stress within suprathermally rotating nanoparticles can exceed the maximum tensile strength of grain material ($S_{\max}$), resulting in rapid disruption of nanoparticles smaller than $a\sim 1$ nm for $S_{\max}\sim 10^{9}\erg\cm^{-3}$. The proposed disruption mechanism is shown to be more efficient than thermal sputtering in controlling the lower cutoff of grain size distribution in C-shocks. We model microwave emission from spinning nanoparticles in C-shocks subject to supersonic neutral drift and rotational disruption. We find that suprathermally rotating nanoparticles can emit strong microwave radiation, and both peak flux and peak frequency increase with increasing the shock velocity. We suggest spinning dust as a new method to constrain nanoparticles and trace shock velocities in dense, shocked regions., Comment: 18 pages, 14 figures; Published in ApJ

Massive stars, supernovae, and kilonovae are among the most luminous radiation sources in the universe. Observations usually show near- to mid-infrared (NIR--MIR, $\lambda\sim 1-5~\mu$m) emission excess from H\,{\sc ii} regions around young massive star clusters (YMSCs). Early phase observations in optical to NIR wavelengths of type Ia supernovae also reveal unusual properties of dust extinction and dust polarization. The popular explanation for such NIR-MIR excess and unusual dust properties is the predominance of small grains (size $a\lesssim 0.05~\mu$m) relative to large grains ($a\gtrsim 0.1~\mu$m) in the local environment of these strong radiation sources. The question of why small grains are predominant in these environments remains a mystery. Here we report a new mechanism of dust destruction based on centrifugal stress within extremely fast-rotating grains spun-up by radiative torques, which we term the RAdiative Torque Disruption (RATD) mechanism. We find that RATD can disrupt large grains located within a distance of $\sim 1$ pc from a massive star of luminosity $L\sim 10^{4}L_{\odot}$ or a supernova. This effect increases the abundance of small grains relative to large grains and successfully reproduces the observed NIR-MIR excess and anomalous dust extinction/polarization. We apply the RATD mechanism for kilonovae and find that dust within $\sim$ 0.1 pc would be dominated by small grains. Small grains produced by RATD can also explain the steep far-UV rise in extinction curves toward starburst and high redshift galaxies, and the decrease of the escape fraction of Ly$\alpha$ photons from H\,{\sc ii} regions surrounding YMSCs., Comment: Published on 6 May, 2019