arXiv subjects

Thiem Hoang

Publications and source records attributed to Thiem Hoang.

At least 145 records · Page 8Linked to original sources

arXiv2017

Effect of alignment on polarized infrared emission from polycyclic aromatic hydrocarbons

Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentially provides a new way to test basic physics of the alignment of ultrasmall grains. In this paper, we present a new model of polarized PAH emission that takes into account the effect of PAH alignment with the magnetic field. We first generate a large sample of the grain angular momentum $\bf J$ by simulating the alignment of PAHs due to resonance paramagnetic relaxation that accounts for various interaction processes. We then calculate the polarization level of PAH emission features, for the different phases of the ISM, including the cold neutral medium (CNM), reflection nebulae (RN), and photodissociation regions (PDRs). We find that a moderate degree of PAH alignment can significantly enhance the polarization degree of PAH emission compared to the previous results obtained with randomly oriented $\bf J$. In particular, we find that smallest, negatively charged PAHs in RN can be excited to slightly suprathermal rotation due to enhanced ion collisional excitation, resulting in an increase of the polarization with the ionization fraction. Our results suggest that RN is the most favorable environment to observe polarized PAH emission and to test alignment physics of nanoparticles. Finally, we present an explicit relationship between the polarization level of PAH emission and the degree of external alignment for the CNM and RN. The obtained relationship will be particularly useful for testing alignment physics of PAHs by future observations.

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arXiv2017

Properties and alignment of interstellar dust grains toward Type Ia Supernovae with anomalous polarization curves

Recent photometric and polarimetric observations of type Ia supernovae (SNe Ia) show unusually low total-to-selective extinction ratio ($R_{V}<2$) and wavelength of maximum polarization ($λ_{max}<0.4μm$) for several SNe Ia, which indicates peculiar properties of interstellar (IS) dust in the SN hosted galaxies and/or the presence of circumstellar (CS) dust. In this paper, we use inversion technique to infer best-fit grain size distribution and alignment function of interstellar grains along the lines of sight toward four SNe Ia with anomalous extinction and polarization data (SNe 1986G, 2006X, 2008fp, and 2014J). We find that to reproduce low values of $R_{V}$, a significant enhancement in the mass of small grains of radius $a< 0.1μm$ is required. For SN 2014J, a simultaneous fit to observed extinction and polarization data is unsuccessful if the entire data is attributed to IS dust (model 1), but a good fit is obtained when accounting for the contribution of CS dust (model 2). For SN 2008fp, our fitting results for model 1 show that, to reproduce an extreme value of $λ_{\max}\sim 0.15μm$, very small silicate grains must be aligned as efficiently as big grains. We suggest that tiny grains in the intervening molecular cloud can be aligned efficiently by radiative torques (RATs) from the SNe Ia. The resulting time dependence polarization from this RAT alignment model can be tested by observing at ultraviolet wavelengths. Our results are in favor of the existence of CS dust in SN 2014J, but its presence in SN 2008fp remains uncertain.

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arXiv2016

The interaction of relativistic spacecrafts with the interstellar medium

The Breakthrough Starshot initiative aims to launch a gram-scale spacecraft to a speed of $v\sim 0.2$c, capable of reaching the nearest star system, $α$ Centauri, in about 20 years. However, a critical challenge for the initiative is the damage to the spacecraft by interstellar gas and dust during the journey. In this paper, we quantify the interaction of a relativistic spacecraft with gas and dust in the interstellar medium. For gas bombardment, we find that damage by track formation due to heavy elements is an important effect. We find that gas bombardment can potentially damage the surface of the spacecraft to a depth of $\sim 0.1$ mm for quartz material after traversing a gas column of $N_{\rm H}\sim 2\times 10^{18}\rm cm^{-2}$ along the path to $α$ Centauri, whereas the effect is much weaker for graphite material. The effect of dust bombardment erodes the spacecraft surface and produces numerous craters due to explosive evaporation of surface atoms. For a spacecraft speed $v=0.2c$, we find that dust bombardment can erode a surface layer of $\sim 0.5$ mm thickness after the spacecraft has swept a column density of $N_{\rm H}\sim 3\times 10^{17}\rm cm^{-2}$, assuming the standard gas-to-dust ratio of the interstellar medium. Dust bombardment also damages the spacecraft surface by modifying the material structure through melting. We calculate the equilibrium surface temperature due to collisional heating by gas atoms as well as the temperature profile as a function of depth into the spacecraft. Our quantitative results suggest methods for damage control, and we highlight possibilities for shielding strategies and protection of the spacecraft.

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arXiv2016

A unified model of grain alignment: radiative alignment of interstellar grains with magnetic inclusions

The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by earlier studies. The alignment of such grains depends on the so-called RAT parameter $q^{\max}$ that is determined by the grain shape. In this paper, we elaborate our model of RAT alignment for grains with enhanced magnetic susceptibility due to iron inclusions, such that RAT alignment is magnetically enhanced for which we term MRAT mechanism. Such grains can get aligned with high angular momentum at the so-called high-J attractor points, achieving a high degree of alignment. Using our analytical model of RATs we derive the critical value of the magnetic relaxation parameter $δ_{m}$ to produce high-J attractor points as functions of $q^{\max}$ and the anisotropic radiation angle relative to the magnetic field $ψ$. We find that if about $10\%$ of total iron abundance present in silicate grains are forming iron clusters, it is sufficient to produce high-J attractor points for all reasonable values of $q^{\max}$. To calculate the degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations from gas collisions and magnetic fluctuations. We show that large grains can achieve perfect alignment when the high-J attractor point is present, regardless of the values of $q^{\max}$. Our obtained results pave the way for physical modeling of polarized thermal dust emission as well as magnetic dipole emission. We also find that millimeter-sized grains in accretion disks may be aligned with the magnetic field if they are incorporated with iron nanoparticles.

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arXiv2016

Grain Alignment: Role of Radiative Torques and Paramagnetic Relaxation

Polarization arising from aligned dust grains presents a unique opportunity to study magnetic fields in the diffuse interstellar medium and molecular clouds. Polarization from circumstellar regions, accretion disks and comet atmospheres can also be related to aligned dust.To reliably trace magnetic fields quantitative theory of grain alignment is required. Formulating the theory that would correspond to observations was one of the longstanding problems in astrophysics. Lately this problem has been successfully addressed, and in this review we summarize some of the most important theoretical advances in the theory of grain alignment by radiative torques (RATs) that act on realistic irregular dust grains. We discuss an analytical model of RATs and the ways to make RAT alignment more efficient, e.g. through paramagnetic relaxation when grains have inclusions with strong magnetic response. For very small grains for which RAT alignment is inefficient, we also discuss paramagnetic relaxation and a process termed resonance relaxation. We provide an extensive analysis of the observational tests of grain alignment theory.

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arXiv2016

Spinning dust emission from ultrasmall silicates: emissivity and polarization spectrum

Anomalous microwave emission (AME) is an important Galactic foreground of Cosmic Microwave Background (CMB) radiation. It is believed that the AME arises from rotational emission by spinning polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). In this paper, we assume that a population of ultrasmall silicate grains may exist in the ISM, and quantify rotational emissivity from these tiny particles and its polarization spectrum. We found that spinning silicate nanoparticles can produce strong rotational emission when those small grains follow a log-normal size distribution. The polarization fraction of spinning dust emission from tiny silicates increases with decreasing the dipole moment per atom ($β$) and can reach $P\sim 20\%$ for $β\sim 0.1$D at grain temperature of 60 K. We identify a parameter space $(β,Y_{Si})$ for silicate nanoparticles in which its rotational emission can adequately reproduce both the observed AME and the polarization of the AME, without violating the observational constraints by the ultraviolet extinction and polarization of starlight. Our results reveal that rotational emission from spinning silicate may be an important source of the AME.

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arXiv2015

Anomalous Microwave Emission from Spinning Dust and its Polarization Spectrum

Nearly twenty years after the discovery of anomalous microwave emission (AME) that contaminates to the cosmic microwave background (CMB) radiation, its origin remains inconclusive. Observational results from numerous experiments have revealed that AME is most consistent with spinning dust emission from rapidly spinning ultrasmall interstellar grains. In this paper, I will first review our improved model of spinning dust, which treats realistic dynamics of wobbling non-spherical grains, impulsive interactions of grains with ions in the ambient plasma, and some other important effects. I will then discuss recent progress in quantifying the polarization of spinning dust emission from polycyclic aromatic hydrocarbons. I will finish with a brief discussion on remaining issues about the origins of AME.

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arXiv2015

Polarization of Magnetic Dipole Emission and Spinning Dust Emission from Magnetic Nanoparticles

Magnetic dipole emission (MDE) from interstellar magnetic nanoparticles is an important Galactic foreground in the microwave frequencies, and its polarization level may pose great challenges for achieving reliable measurements of cosmic microwave background (CMB) B-mode signal. To obtain theoretical constraints on the polarization of MDE, we first compute the degree of alignment of big silicate grains incorporated with magnetic inclusions. We find that, in realistic conditions of the interstellar medium, thermally rotating big grains with magnetic inclusions are weakly aligned and achieve {\it alignment saturation} when the magnetic alignment rate becomes much faster than the rotational damping rate. We then compute the degree of alignment for free-flying magnetic nanoparticles, taking into account various interaction processes of grains with the ambient gas and radiation field, including neutral collisions, ion collisions, and infrared emission. We find that the rotational damping by infrared emission can significantly decrease the degree of alignment of small particles from the saturation level, whereas the excitation by ion collisions can enhance the alignment of ultrasmall particles. Using the computed degrees of alignment, we predict the polarization level of MDE from free-flying magnetic nanoparticles to be rather low. Such a polarization level is within the upper limits measured for anomalous microwave emission (AME), which indicates that MDE from free-flying iron particles may not be ruled out as a source of AME. We also quantify spinning dust emission from free-flying iron nanoparticles with permanent magnetic moments and find that its emissivity is one order of magnitude lower than that from spinning polycyclic aromatic hydrocarbons (PAHs). Finally, we compute the polarization spectra of spinning dust emission from PAHs for the different interstellar magnetic fields.

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arXiv2015

On origin and destruction of relativistic dust and its implication for ultrahigh energy cosmic rays

Dust grains may be accelerated to relativistic speeds by radiation pressure, diffusive shocks, and other acceleration mechanisms. Such relativistic grains have been suggested as primary particles of ultrahigh energy cosmic rays (UHECRs). In this paper, we first revisit the problem of acceleration by radiation pressure and calculate maximum grain velocities achieved. We find that grains can be accelerated to relativistic speeds with Lorentz factor $γ< 2$ by powerful radiation sources, which is lower than earlier estimates showing that $γ$ could reach $\sim 10$. We then investigate different destruction mechanisms for relativistic grains traversing a variety of environments. In solar radiation, we find that the destruction by thermal sublimation and Coulomb explosions is important. We also quantify grain destruction due to electronic sputtering by ions and grain-grain collisions. Electronic sputtering is found to be rather inefficient, whereas the evaporation following grain-grain collisions is shown to be an important mechanism for which the $a \sim 0.01- 1μ$m grains would be destroyed after sweeping a gas column $N_{Coul}\sim 5\times 10^{19}-5\times 10^{20}cm^{-2}$. Relativistic dust in the interstellar medium and intergalactic medium (IGM) would be disrupted by Coulomb explosions due to collisional charging after traversing a gas column $N_{Coul} \sim 10^{17}cm^{-2}$ unless grain material is very strong. We show that photoelectric emission by optical and ultraviolet background radiation is also significant for the destruction of relativistic dust in the IGM. The obtained results indicate that relativistic dust from galaxies would be destroyed before reaching the Earth's atmosphere and unlikely to account for UHECRs.

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arXiv2015

Modeling Grain Alignment by Radiative Torques and Hydrogen Formation Torques in Reflection Nebula

Reflection nebulae--dense cores--illuminated by surrounding stars offer a unique opportunity to directly test our quantitative model of grain alignment based on radiative torques (RATs) and to explore new effects arising from additional torques. In this paper, we first perform detailed modeling of grain alignment by RATs for the IC 63 reflection nebula illuminated both by a nearby $γ$ Cas star and the diffuse interstellar radiation field. We calculate linear polarization $p_λ$ of background stars by radiatively aligned grains and explore the variation of fractional polarization ($p_λ/A_V$) with visual extinction $A_{V}$ across the cloud. Our results show that the variation of $p_{V}/A_{V}$ versus $A_{V}$ from the dayside of IC 63 to its center can be represented by a power-law ($p_{V}/A_{V}\propto A_{V}^η$) with different slopes depending on $A_{V}$. We find a shallow slope $η\sim- 0.1$ for $A_{V}< 3$ and a very steep slope $η\sim -2$ for $A_{V}> 4$. We then consider the effects of additional torques due to H$_{2}$ formation and model grain alignment by joint action of RATs and H$_2$ torques. We find that $p_{V}/A_{V}$ tends to increase with an increasing magnitude of H$_{2}$ torques. In particular, the theoretical predictions obtained for $p_{V}/A_{V}$ and peak wavelength $λ_{\max}$ in this case show an improved agreement with the observational data. Our results reinforce the predictive power of the RAT alignment mechanism in a broad range of environmental conditions and show the effect of pinwheel torques in environments with efficient H$_2$ formation. Physical parameters involved in H$_2$ formation may be constrained using detailed modeling of grain alignment combined with observational data. In addition, we discuss implications of our modeling for interpreting latest observational data by {\it Planck} and other ground-based instruments.

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arXiv2014

Grain Alignment by Radiative Torques in Special Conditions and Implications

Grain alignment by radiative torques (RATs) has been extensively studied for various environment conditions, including interstellar medium, dense molecular clouds, and accretion disks, thanks to significant progress in observational, theoretical and numerical studies. In this paper, we explore the alignment by RATs and provide quantitative predictions of dust polarization for a set of astrophysical environments that can be tested observationally. We first consider the alignment of grains in the local interstellar medium and compare predictions for linear polarization by aligned grains with recent observational data for nearby stars. We then revisit the problem of grain alignment in accretions disks by taking into account the dependence of RAT alignment efficiency on the anisotropic direction of radiation fields relative to magnetic fields. Moreover, we study the grain alignment in interplanetary medium, including diffuse Zodiacal cloud and cometary comae, and calculate the degree of circular polarization (CP) of scattered light arising from single scattering by aligned grains. We also discuss a new type of grain alignment, namely the alignment with respect to the ambient electric field instead of the alignment with the magnetic field. We show that this type of alignment can allow us to reproduce the systematic features of CP observed across a cometary coma. Our findings suggest that polarized Zodiacal dust emission may be an important polarized foreground component, which should be treated carefully in cosmic microwave background experiments.

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arXiv2014

Paramagnetic alignment of small grains: a novel method for measuring interstellar magnetic fields

We present a novel method to measure the strength of interstellar magnetic fields based on ultraviolet (UV) polarization of starlight, which is in part produced by weakly aligned, small interstellar grains. We begin with calculating degrees of alignment of small (size $a\sim 0.01μ$m) and very small ($a\sim 0.001μ$m) grains in the interstellar magnetic field due to the Davis-Greenstein paramagnetic relaxation and resonance paramagnetic relaxation. We compute the degrees of paramagnetic alignment with the ambient magnetic field $B$ using Langevin equations. In this paper, we take into account various processes essential for the dynamics of small grains, including infrared (IR) emission, electric dipole emission, plasma drag and collisions with neutral and ionized species. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and IR wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model requires a higher degree of alignment of small grains for the case with the peak wavelength of polarization $λ_{\max}<0.55μ$m, which exhibits an excess UV polarization relative to the Serkowski law, compared to the typical case $λ_{\max}=0.55μ$m. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e. of $p(6μm^{-1})/p_{\max}$) with $λ_{\max}^{-1}$ by appealing to the higher degree of alignment of small grains. We identify paramagnetic relaxation as the cause of the alignment of small grains and utilize the dependence of the degree of alignment on the magnetic field strength $B$ to suggest a new way to measure $B$ using the observable parameters $λ_{\max}$ and $p(6μm^{-1})/p_{\max}$.[Abridged]

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arXiv2013

Constraint on the Polarization of Electric Dipole Emission from Spinning Dust

Planck results have revealed that the electric dipole emission from polycyclic aromatic hydrocarbons (PAHs) is the most reliable explanation for anomalous microwave emission that interferes with cosmic microwave background (CMB) radiation experiments. The emerging question is to what extent this emission component contaminates to the polarized CMB radiation. We present constraints on polarized dust emission for the model of grain size distribution and grain alignment that best fits to observed extinction and polarization curves. Two stars with a prominent polarization feature at wavelength 2175 Angstrom, HD 197770 and HD 147933-4, are chosen for our study. For HD 197770, we find that the model with aligned silicate grains plus weakly aligned PAHs can successfully reproduce the 2175 Angstrom polarization feature; whereas, for HD 147933-4, we find that the alignment of only silicate grains can account for that feature. The alignment function of PAHs for the best-fit model to the HD 197770 data is employed to constrain polarized spinning dust emission. We find that the degree of polarization of spinning dust emission is about 1.6 percent at frequency ~ 3 GHz and declines to below 0.9 percent for frequency above 20 GHz. We also predict the degree of polarization of thermal dust emission at 353 GHz to be ~ 11 percent and 14 percent for the lines of sight to the HD 197770 and HD 147933-4 stars, respectively.

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arXiv2012

Spinning Dust Emission from Wobbling Grains: Important Physical Effects and Implications

We review major progress on the modeling of electric dipole emission from rapidly spinning tiny dust grains, including polycyclic aromatic hydrocarbons (PAHs). We begin by summarizing the original model of spinning dust proposed by Draine and Lazarian and recent theoretical results improving the Draine and Lazarian model. The review is focused on important physical effects that were disregarded in earlier studies for the sake of simplicity and recently accounted for by us, including grain wobbling due to internal relaxation, impulsive excitation by single-ion collisions, the triaxiality of grain shape, charge fluctuations, and the turbulent nature of astrophysical environments. Implications of the spinning dust emission for constraining physical properties of tiny dust grains and environment conditions are discussed. We discuss the alignment of tiny dust grains and possibility of polarized spinning dust emission. Suggestions for constraining the alignment of tiny grains and polarization of spinning dust emission are also discussed.

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arXiv2011

Acceleration of Small Dust Grains due to Charge Fluctuations

We consider the acceleration of very small dust grains including Polycyclic Aromatic Hydrocarbons (PAHs) arising from the electrostatic interactions of dust grains that have charge fluctuations in time due to charging events. We simulate the charge fluctuations of very small grains due to their sticking collisions with electrons and ions in plasma, and the emission of photoelectrons by UV photons using Monte Carlo method. We identify the acceleration induced by the charge fluctuations as the dominant acceleration mechanism of very small grains in the diffuse interstellar medium (ISM). We show that this acceleration mechanism is efficient for environments with low degree of ionization (i.e., large Debye length), where the charge fluctuations are slow but have a large amplitude. We also discuss the implications of the present mechanism for grain coagulation and shattering in the diffuse ISM, molecular clouds, and protoplanetary disks.

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arXiv2011

Revisiting Acceleration of Charged Grains in Magnetohydrodynamic Turbulence

We study the acceleration of charged grains by magnetohydrodynamics (MHD) turbulence in the interstellar medium (ISM). We begin with revisiting gyroresonance acceleration by taking into account the fluctuations of grain guiding center along a uniform magnetic field (i.e. nonlinear theory--NLT). We calculate grain velocities due to gyroresonance by fast MHD modes using the NLT for different phases of the ISM, and compare with results obtained using quasi-linear theory (QLT). We find for the parameters applicable to the typical ISM phases that the fluctuations of grain guiding center reduce grain velocities by less than 15 percent, but they can be important for more special circumstances. We confirm that large grains can be accelerated to super-Alfvenic velocities through gyroresonance. For such super-Alfvenic grains, we investigate the effect of further acceleration via transit time damping (TTD) by fast modes. We find that due to the broadening of resonance condition in the NLT, the TTD acceleration is not only important for the cosines of grain pitch angle relative to the magnetic field mu>V_{A}/v, but also for mu<V_{A}/v where v is the grain velocity and V_{A} is the Alfven speed. We show that the TTD acceleration is dominant over the gyroresonance for large grains, and can increase substantially grain velocities induced by gyroresonance acceleration. We quantify another stochastic acceleration mechanism arising from low frequency Alfven waves. We discuss the range of applicability of the mechanisms and their implications.

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arXiv2011

Spinning Dust Emission: Effects of irregular grain shape, transient heating and comparison with WMAP results

Planck is expected to answer crucial questions on the early Universe, but it also provides further understanding on anomalous microwave emission. Electric dipole emission from spinning dust grains continues to be the favored interpretation of anomalous microwave emission. In this paper, we present a method to calculate the rotational emission from small grains of irregular shape with moments of inertia $I_{1}> I_{2}> I_{3}$. We show that a torque-free rotating irregular grain with a given angular momentum radiates at multiple frequency modes. The resulting spinning dust spectrum has peak frequency and emissivity increasing with the degree of grain shape irregularity, which is defined by $I_{1}:I_{2}:I_{3}$. We discuss how the orientation of dipole moment $\bmu$ in body coordinates affects the spinning dust spectrum for different regimes of internal thermal fluctuations. We show that the spinning dust emissivity for the case of strong thermal fluctuations is less sensitive to the orientation of $\bmu$ than in the case of weak thermal fluctuations. We calculate spinning dust spectra for a range of gas density and dipole moment. The effect of compressible turbulence on spinning dust emission intensity is investigated. We show that the emission intensity in a turbulent medium increases by a factor from 1.2-1.4 relative to that in a uniform medium, as sonic Mach number $M_{s}$ increases from 2-7. Finally, spinning dust parameters are constrained by fitting our improved model to five-year {\it Wilkinson Microwave Anisotropy Probe} cross-correlation foreground spectra, for both the H$α$-correlated and 100 $μ$m-correlated emission spectra.

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arXiv2010

Acceleration of small astrophysical grains due to charge fluctuations

We discuss a novel mechanism of dust acceleration which may dominate for particles smaller than $\sim0.1~μ$m. The acceleration is caused by their direct electrostatic interactions arising from fluctuations of grain charges. The energy source for the acceleration are the irreversible plasma processes occurring on the grain surfaces. We show that this mechanism of charge-fluctuation-induced acceleration likely affects the rate of grain coagulation and shattering of the population of small grains.

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