arXiv subjects

Bao Truong

Publications and source records attributed to Bao Truong.

11 recordsLinked to original sources

arXiv2026

Grain alignment and dust evolution physics with polarisation (GRADE-POL). II. On the physical basis of Serkowski and super-Serkowski polarisation spectra

Optical-to-near-infrared interstellar polarisation, induced by aligned dust grains, generally follows a convex wavelength dependence, known as the Serkowski relation. However, observations in the ultraviolet (UV) and at [mid-]infrared wavelengths have indicated that some of the spectra do not follow this relation. Specifically, about 25% show an excess in the degree of polarisation at mid-UV wavelengths ($λ^{-1} > 3\,\rm μm^{-1}$), referred to as the super-Serkowski polarisation. For this study, we re-examined both the Serkowski and super-Serkowski spectra based on the joint effect of paramagnetic relaxation, known as the Davis-Greenstein (DG) and radiative torque (RAT) alignment. We used the observational data for HD 30614, HD 204827, HD 37903 and HD 161056 to constrain our modelling. We examined two types of radiation fields: one derived from the scaled interstellar radiation field and the other originating from a B-type star. For the super-Serkowski spectra of HD 30614 and HD 204827, our model demonstrates that RAT alignment enhanced by radiation produced from a B-type star below the Lyman limit ($λ=912Å$) can reasonably explain the observations and that a combination with the DG alignments results in a better fit for $λ^{-1}\geq 5.5\,\rm μm^{-1}$. For the Serkowski spectra in HD 37903 and HD 161056, only the RAT alignment by itself under the typical interstellar radiation field above the Lyman limit, within a typical cold neutral medium, can account for the observed spectra, with a combination of a very inefficient DG alignment. The capacity of our model to predict the starlight polarisation spectrum from infrared to far-UV is thus a promising tool for interpreting future missions that observe spectrophotometry in the UV bands.

astro-ph.GA↗

arXiv2026

Galactic Dust Polarization in Turbulent Multiphase ISM: On the Origin of the $EE/BB$ Asymmetry

Polarized thermal emission from Galactic dust is the dominant foreground for CMB polarization measurements at high frequencies, with its statistical properties shaped by the interplay between turbulence and magnetic fields in the multiphase interstellar medium (ISM). Variations in turbulence regime and density-magnetic-field alignment across the warm (WNM), unstable (UNM), and cold (CNM) neutral media should imprint distinct signatures on the power spectra and $EE/BB$ power ratio, yet the relative contributions of these phases remain poorly constrained. Using high-resolution 3D magnetohydrodynamic simulations of a turbulent multiphase ISM coupled with synthetic dust polarization maps, we quantify phase-dependent turbulence, anisotropy, and alignment properties. We find that the trans-Alfvénic and transonic WNM and UNM are strongly anisotropic, exhibiting tight alignment of density and velocity structures with the local magnetic field. In contrast, the super-Alfvénic and supersonic CNM displays reduced anisotropy and weak alignment. These dynamical differences are reflected in the statistical scaling of fluctuations: the square root of the second-order velocity structure function exhibits a slope near $1/3$ in the WNM, near $1/2$ in the CNM, and intermediate in the UNM. Comparing our synthetic polarization power spectra with \textit{Planck} measurements, we find that polarization from UNM dust yields spectral slopes closest to the \textit{Planck}-inferred values, whereas WNM and CNM dust produce steeper and shallower spectra, respectively. The WNM yields $EE/BB>2$, the UNM gives $EE/BB\sim2$, and the CNM yields $EE/BB\approx1$. These results suggest that UNM dust may be an important contributor to the polarized foreground under typical high-latitude ISM conditions. We present predictions at 150 GHz to inform foreground modeling and separation.

astro-ph.GA↗

arXiv2026

3D B-fieLds in the InterStellar medium and Star-forming regions (3D-BLISS): I. Using Starlight Polarization in the Massive IRDC Filament G11.11-0.12

Measuring three-dimensional magnetic fields (3D B-fields) is essential to understand the formation and evolution of the interstellar medium and multi-scale star formation; however, the accurate measurement of 3D B-fields is still challenging. The dust polarization angles by magnetically aligned grains provide the projected B-fields onto the plane-of-sky, while the dust polarization degree provides the B-field's inclination angle with respect to the line-of-sight. Our previous theoretical studies proposed a new method of probing 3D B-fields using dust polarization combined with the Radiative Torque (RAT) alignment theory and demonstrated the accurate inference of B-field inclination angles using synthetic polarization data. In this paper, we report the first application of the new technique to study 3D B-fields and dust properties in the G11.11-0.12 filament (hereafter G11) from starlight polarization observations taken by ISRF/SIRPOL at $2.19\,\rmμm$. Using both observed starlight polarization and optical dust extinction curves from the Gaia mission, we constrained the maximum grain size of $0.25\,\rmμm$ and the grain elongation with an axial ratio of $s\gtrsim 1.4$ in the outer regions of G11. We calculated the alignment properties in G11 by using the \textsc{DustPOL\_py} code. The B-field's inclination angles in G11 are then inferred from the observed starlight polarization efficiency when the grain alignment is included, with a mean angle of $\sim 48$ degrees. From these inferred inclination angles, we found evidence of the local 3D arc-shaped B-field structure toward the sightline. These findings are important for understanding 3D B-field's roles in the formation and evolution of massive filamentary clouds.

astro-ph.GA↗

arXiv2026

SIGMA: A Physics-Based Benchmark for Gas Chimney Understanding in Seismic Images

Seismic images reconstruct subsurface reflectivity from field recordings, guiding exploration and reservoir monitoring. Gas chimneys are vertical anomalies caused by subsurface fluid migration. Understanding these phenomena is crucial for assessing hydrocarbon potential and avoiding drilling hazards. However, accurate detection is challenging due to strong seismic attenuation and scattering. Traditional physics-based methods are computationally expensive and sensitive to model errors, while deep learning offers efficient alternatives, yet lacks labeled datasets. In this work, we introduce \textbf{SIGMA}, a new physics-based dataset for gas chimney understanding in seismic images, featuring (i) pixel-level gas-chimney mask for detection and (ii) paired degraded and ground-truth image for enhancement. We employed physics-based methods that cover a wide range of geological settings and data acquisition conditions. Comprehensive experiments demonstrate that SIGMA serves as a challenging benchmark for gas chimney interpretation and benefits general seismic understanding.

cs.CV↗

arXiv2025

Grain alignment and dust evolution physics with polarisation (GRADE-POL). I. Dust polarisation modelling for isolated starless cores

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields, dust properties, and poses a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process needs to be studied thoroughly. The Magnetically enhanced Radiative Torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Our numerical modelling of dust polarisation using the MRAT theory demonstrated that the alignment efficiency of starlight polarisation ($p_{\rm ext}/A_{\rm V}$) and the degree of thermal dust polarisation ($p_{\rm em}$) first decrease slowly with increasing visual extinction ($A_{\rm V}$) and then falls steeply as $\propto A^{-1}_{\rm V}$ at large $A_{\rm V}$ due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope ($A^{\rm loss}_{\rm V}$) increases with the maximum grain size. By applying physical profiles suitable for a starless core 109 in the Pipe Nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation at R-band ($0.65\,μ$m) and H-band ($1.65\,μ$m), as well as emission polarisation at submillimetre ($870\,μ$m). Successful modelling of observational data requires perfect alignment of large grains as evidence of the MRAT mechanism, and larger maximum size with higher elongation at higher $A_{\rm V}$. The latter reveals the first evidence for the new model of anisotropic grain growth induced by magnetic grain alignment.

astro-ph.GA↗

arXiv2025

Probing 3D magnetic fields using starlight polarization and grain alignment theory

Polarization of starlight induced by dust grains aligned with the magnetic field (hereafter B-field) is widely used to measure the two-dimensional B-fields projected onto the plane-of-sky. Here, we introduce a new method to infer three-dimensional B-fields using starlight polarization. We show that the inclination angle or line-of-sight (LOS) component of B-fields can be constrained by the starlight polarization efficiency from observations, the alignment degree provided by the magnetically enhanced radiative torque (MRAT) alignment theory, and the effect of B-field tangling. We first perform synthetic observations of starlight polarization of magnetohydrodynamic (MHD) simulations of a filamentary cloud with our updated POLARIS code incorporating the modern MRAT theory. We test the new technique with synthetic observations and find that the B-field inclination angles can be accurately determined by the synthetic starlight polarization efficiency once the effects of grain alignment, dust properties, and B-field fluctuations are well characterized. The technique can provide an accurate constraint on B-field inclination angles using optical polarization in low-density regions $A_{\rm V}< 3$ with efficient MRAT alignment, whereas the technique can infer further to high-density regions with significant alignment loss at $A_{\rm V} \sim 8 - 30$ by using near-infrared polarization. Our new technique unlocks the full potential of tracing 3D B-fields and constraining dust properties and grain alignment physics on multiple scales of the diffuse interstellar medium and star-forming regions using multi-wavelength starlight polarization observations.

astro-ph.GA↗

arXiv2024

Numerical modeling of thermal dust polarization from aligned grains in the envelope of evolved stars with updated POLARIS

Magnetic fields are thought to influence the formation and evolution of circumstellar envelopes around evolved stars. Thermal dust polarization from aligned grains is a promising tool for probing magnetic fields and dust properties in these environments; however, a quantitative study on the dependence of thermal dust polarization on the physical properties of dust and magnetic fields for these circumstellar environments is still lacking. In this paper, we first perform the numerical modeling of thermal dust polarization in the IK Tau envelope using the magnetically enhanced radiative torque (MRAT) alignment mechanism implemented in our updated POLARIS code, accounting for the effect of grain drift relative to the gas. Despite experiencing grain drift and high gas density $n_{\rm gas} > 10^6\,\rm cm^{-3}$, the minimum grain size required for efficient MRAT alignment of silicate grains is $\sim 0.007 - 0.05\,\rmμm$ due to strong stellar radiation fields. Ordinary paramagnetic grains can achieve perfect alignment by MRAT in the inner envelope of $r < 500\,\rm au$ due to stronger magnetic fields of $B\sim10$ mG - 1G, producing the polarization degree of $\sim10\%$. The polarization degree can be enhanced to $\sim20-40\%$ for superparamagnetic grains with embedded iron inclusions. The magnetic field geometry affects the resulting polarization degree due to the projection effect. We investigate the effect of rotational disruption by RATs (RAT-D) and find that the RAT-D effect decreases the dust polarization degree due to the decrease in the maximum grain size. Our modeling results motivate further observational studies at far-infrared/sub-millimeter to constrain the properties of magnetic fields and dust in evolved star's envelopes.

astro-ph.SR↗

arXiv2024

Evidence of Grain Alignment by Magnetically Enhanced Radiative Torques from Multiwavelength Dust Polarization Modeling of HL Tau

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 $λ$ = 0.87, 1.3, and 3.1$\,$mm are constrained at $\sim$ 60, 80, and 90$\,μ$m, respectively.

astro-ph.SR↗

arXiv2024

Probing 3D magnetic fields using thermal dust polarization and grain alignment theory

Magnetic fields are ubiquitous in the universe and are thought to play an important role in various astrophysical processes. Polarization of thermal dust emission from dust grains aligned with the magnetic field is widely used to measure the two-dimensional magnetic field projected onto the plane of the sky (POS), but the component along the line of sight (LOS) is not yet reliably constrained with dust polarization. Here, we introduce a new method to infer three-dimensional (3D) magnetic fields using thermal dust polarization and grain alignment physics. We first develop a physical model of thermal dust polarization using the modern grain alignment theory based on the magnetically enhanced radiative torque (MRAT) alignment theory. We then test this model with synthetic observations of magnetohydrodynamic (MHD) simulations of a filamentary cloud with our updated POLARIS code. Combining the tested physical polarization model with synthetic polarization, we show that the B-field inclination angle can be accurately constrained by the polarization degree from synthetic observations. Compared to the true 3D magnetic fields, our method with grain alignment is more accurate than the previous methods that assume uniform grain alignment. This new technique paves the way for tracing 3D B-fields using thermal dust polarization and grain alignment theory and for constraining dust properties and grain alignment physics.

astro-ph.GA↗

arXiv2023

Effects of Barnett magnetic dipole-dipole interaction on grain growth and destruction

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.

astro-ph.GA↗

arXiv2022

Modeling extinction and reddening effects by circumstellar dust in the Betelgeuse envelope in the presence of radiative torque disruption

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 $α$ Orionis (Betelgeuse). Due to the RAT-D effects, large grains formed in the dust formation zone are disrupted into smaller species of size $a < 0.5\,\rmμm$. 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.

astro-ph.GA↗