arXiv subjects

Thiem Hoang

Publications and source records attributed to Thiem Hoang.

At least 109 records · Page 6Linked to original sources

arXiv2020

Detectability of Thermal Emission from Sub-Relativistic Objects

We calculate the surface temperature and the resulting brightness of sub-relativistic objects moving through the Solar system due to collisional heating by gas and radiative heating by solar radiation. The thermal emission from objects of size $\gtrsim 100$ m and speed of $\gtrsim 0.1c$, can be detected by the upcoming {\it James Webb Space Telescope} out to a distance of $\sim 100$ au. Future surveys could therefore set interesting limits on the abundance of fast-moving interstellar objects or spacecraft.

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arXiv2020

Rotational Disruption of Astrophysical Dust and Ice: Theory and Applications

Dust is an essential component of the interstellar medium (ISM) and plays an important role in many different astrophysical processes and phenomena. Traditionally, dust grains are known to be destroyed by thermal sublimation, Coulomb explosions, sputtering, and shattering. The first two mechanisms arise from the interaction of dust with intense radiation fields and high-energy photons (extreme UV), which work in a limited astrophysical environment. The present review is focused on a new destruction mechanism present in the {\it dust-radiation interaction} that is effective in a wide range of radiation fields and has ubiquitous applications in astrophysics. We first describe this new mechanism of grain destruction, namely rotational disruption induced by Radiative Torques (RATs) or RAdiative Torque Disruption (RATD). We then discuss rotational disruption of nanoparticles by mechanical torques due to supersonic motion of grains relative to the ambient gas, which is termed MEchanical Torque Disruption (METD). These two new mechanisms modify properties of dust and ice (e.g., size distribution and mass), which affects observational properties, including dust extinction, thermal and nonthermal emission, and polarization. We present various applications of the RATD and METD mechanisms for different environments, including the ISM, star-forming regions, astrophysical transients, and surface astrochemistry.

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arXiv2020

Rotational disruption of dust grains by mechanical torques for high-velocity gas-grain collisions

Dust grains moving at hypersonic velocities of $v_{d}\gtrsim 100\rm km~s^{-1}$ through an ambient gas are known to be destroyed by nonthermal sputtering. Yet, previous studies of nonthermal sputtering disregarded the fact that dust grains can be spun-up to suprathermal rotation by stochastic mechanical torques from gas-grain collisions. In this paper, we show that such grain suprathermal rotation can disrupt a small grain into small fragments because induced centrifugal stress exceeds the maximum tensile strength of grain material, $S_{\rm max}$. We term this mechanism {\it MEchanical Torque Disruption} (METD). We find that METD is more efficient than nonthermal sputtering in destroying smallest grains ($a<10$ nm) of nonideal structures moving with velocities of $v_{d}<500 \rm km~s^{-1}$. The ratio of rotational disruption to sputtering time is $τ_{\rm disr}/τ_{\rm sp}\sim 0.7(S_{\rm max}/10^{9}\rm erg~cm^{-3})(\bar{A}_{\rm sp}/12)(Y_{\rm sp}/0.1)(a/0.01μm)^{3}(300\rm km~s^{-1}/v_{d})^{2}$ where $a$ is the radius of spherical grains, and $Y_{\rm sp}$ is sputtering yield. We also consider the high-energy regime and find that the rate of METD is reduced and becomes less efficient than sputtering for $v_{d}>500\rm km~s^{-1}$ because impinging particles only transfer part of their momentum to the grain. We finally discuss implications of the METD mechanism for the destruction of hypersonic grains accelerated by radiation pressure as well as grains in fast shocks. Our results suggest that the destruction of small grains by METD in fast shocks of supernova remnants may be more efficient than previously predicted by nonthermal sputtering, depending on grain internal structures.

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arXiv2020

Can Planet Nine Be Detected Gravitationally by a Sub-Relativistic Spacecraft?

Planet 9 was proposed as an explanation for the clustering of orbits for some trans-Neuptunian objects. Recently, the use of a sub-relativistic spacecraft was proposed to indirectly probe Planet 9's gravitational influence. Here we study the effects of the drag and electromagnetic forces exerted on a sub-relativistic spacecraft by the interstellar medium (ISM) and compare these forces with the gravitational force induced by Planet 9. We find that the resulting noise due to density and magnetic fluctuations would dominate over Planet 9's gravitational signal at sub-relativistic speeds, $v\gtrsim 0.001~c$. We then identify the parameter space required to overcome the drag and magnetic noise from the ISM turbulence and enable the detection of Planet 9's gravity. Finally, we discuss practical strategies to mitigate the effect of the drag and electromagnetic forces.

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arXiv2020

Effects of rotational disruption on the evolution of grain size distribution in galaxies

Interstellar dust grains can be spun up by radiative torques, and the resulting centrifugal force may be strong enough to disrupt large dust grains. We examine the effect of this rotational disruption on the evolution of grain size distribution in galaxies. To this goal, we modify our previous model by assuming that rotational disruption is the major small-grain production mechanism. We find that rotational disruption can have a large influence on the evolution of grain size distribution in the following two aspects especially for composites and grain mantles (with tensile strength $\sim 10^7$ erg cm$^{-3}$). First, because of the short time-scale of rotational disruption, the small-grain production occurs even in the early phase of galaxy evolution. Therefore, even though stars produce large grains, the abundance of small grains can be large enough to steepen the extinction curve. Secondly, rotational disruption is important in determining the maximum grain radius, which regulates the steepness of the extinction curve. For compact grains with tensile strength $\gtrsim 10^9$ erg cm$^{-3}$, the size evolution is significantly affected by rotational disruption only if the radiation field is as strong as (or the dust temperature is as high as) expected for starburst galaxies. For compact grains, rotational disruption predicts that the maximum grain radius becomes less than 0.2 $μ$m for galaxies with a dust temperature $\gtrsim 50$ K.

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arXiv2020

Rotational Desorption of Ice Mantles and Complex Molecules from Suprathermally Rotating Dust Grains around Young Stellar Objects

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 $a \lesssim 10$Å~ 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.

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arXiv2019

Gamma-ray Burst Afterglows: Time-Varying Extinction, Polarization, and Colors due to Rotational Disruption of Dust Grains

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 $>0.1 μ\rm m$) 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 $ λ_{\rm max}$ decreases rapidly from the standard value of $\sim 0.55 μ\rm m$ to $\sim 0.15 μ\rm m$ 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.

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arXiv2019

Physical Model of Dust Polarization by Radiative Torque Alignment and Disruption and Implications for Grain Internal Structures

Dust polarization depends on the physical and mechanical properties of dust, as well as the properties of local environments. To understand how dust polarization varies with grain mechanical properties and the local environment, in this paper, we model the wavelength-dependence polarization of starlight and polarized dust emission by aligned grains by simultaneously taking into account grain alignment and rotational disruption by radiative torques (RATs). We explore a wide range of the local radiation field and grain mechanical properties characterized by tensile strength. We find that the maximum polarization and the peak wavelength shift to shorter wavelengths as the radiation strength $U$ increases due to the enhanced alignment of small grains. Grain rotational disruption by RATs tends to decrease the optical-near infrared polarization but increases the ultraviolet polarization of starlight due to the conversion of large grains into smaller ones. In particular, we find that the submillimeter (submm) polarization degree at $850~μ\rm m$ ($P_{850}$) does not increase monotonically with the radiation strength or grain temperature ($T_{d}$), but it depends on the tensile strength of grain materials. Our physical model of dust polarization can be tested with observations toward star-forming regions or molecular clouds irradiated by a nearby star, which have higher radiation intensity than the average interstellar radiation field. Finally, we compare our predictions of the $P_{850}-T_{d}$ relationship with {\it Planck} data and find that the observed decrease of $P_{850}$ with $T_{d}$ can be explained when grain disruption by RATs is accounted for, suggesting that interstellar grains unlikely to have a compact structure but perhaps a composite one. The variation of the submm polarization with U (or $T_{d}$) can provide a valuable constraint on the internal structures of cosmic dust.

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arXiv2019

Chemistry on rotating grain surfaces: ro-thermal hopping and segregation of molecules in ice mantles

Grain surfaces play a central role in the formation and desorption of molecules in space. To form molecules on a grain surface, adsorbed species trapped in binding sites must be mobile and migrate to adjacent sites. Thermal hopping is a popular mechanism for the migration of adsorbed species when the grain surface is warmed up by stellar radiation. However, previous studies disregarded the fact that grains can be spun-up to suprathermal rotation by radiative torques (RATs) during grain heating process. To achieve an accurate model of surface astrochemistry, in this paper, we study the effect of grain suprathermal rotation by RATs on thermal hopping of adsorbed species on icy grain mantles. We find that centrifugal force due to grain suprathermal rotation can increase the mobility of radicals on/in the ice mantle compared to the prediction by thermal hopping, and we term this mechanism ro-thermal hopping. The rate of ro-thermal hopping depends both on the local radiation energy density (i.e., grain temperature) and gas density, whereas thermal hopping only depends on grain temperature. We calculate the decrease in grain temperature required by ro-thermal hopping to produce the same hopping rate as thermal hopping and find that it increases with increasing the diffusion energy and decreasing the gas density. We finally study the effect of grain suprathermal rotation on the segregation of ice mixtures and find that ro-thermal segregation of CO$_2$ from H$_2$O-CO$_2$ ices can occur at much lower temperatures than thermal segregation reported by experiments. Our results indicate that grain suprathermal rotation can enhance mobility, formation, desorption, and segregation of molecules in icy grain mantles.

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arXiv2019

Dust rotational dynamics in non-stationary shock: rotational disruption of nanoparticles by stochastic mechanical torques and spinning dust emission

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.

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arXiv2019

Chemistry on Rotating Grain Surface: Ro-Thermal Desorption of Molecules from Ice Mantles

It is widely believed that water and complex organic molecules (COMs) first form in the ice mantle of dust grains and are subsequently returned into the gas due to grain heating by intense radiation of protostars. Previous research on the desorption of molecules from the ice mantle assumed that grains are at rest which is contrary to the fact that grains are suprathermally rotating as a result of their interaction with an anisotropic radiation or gas flow. {To clearly understand how molecules are released in to the gas phase, the effect of grain suprathermal rotation on surface chemistry must be quantified}. In this paper, we study the effect of suprathermal rotation of dust grains spun-up by radiative torques on the desorption of molecules from icy grain mantles around protostars. We show that centrifugal potential energy due to grain rotation reduces the potential barrier of molecules and significantly enhances their desorption rate. We term this mechanism {\it rotational-thermal} or {\it ro-thermal} desorption. We apply the ro-thermal mechanism for studying the desorption of molecules from icy grains which are simultaneously heated to high temperatures and spun-up to suprathermal rotation by an intense radiation of protostars. We find that ro-thermal desorption is much more efficient than thermal desorption for molecules with high binding energy such as water and COMs. Our results have important implications for understanding the origin of COMs detected in star-forming regions and call for attention to the effect of suprathermal rotation of icy grains to use molecules as a tracer of physical conditions of star-forming regions.

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arXiv2019

Magnetic Properties of Dust Grains, Effect of Precession and Radiative Torque Alignment

Alignment of dust grains in astrophysical environments results in the polarization of starlight as well as the polarization of radiation emitted by dust. We demonstrate the advances in grain alignment theory allow the use of linear and circular polarization to probe not only the magnetic field, but also dust composition, the dust environment, etc. We revisit the process of grain alignment by Radiative Torques (RATs) and focus on constraining magnetic susceptibility of grains via observations. We discuss the possibility of observational testing of the magnetic properties of grains as the alignment changes from being in respect to the magnetic field to being in respect to the radiation direction. This opens both a possibility of constraining the uncertain parameters of the RATs theory and provides a new way of measuring magnetic fields in interstellar medium and circumstellar regions. We provide a detailed discussion of the precession induced both by the magnetic field and the anisotropic radiation and revisit a number of key processes related to magnetic response of the grains. We consider various effects that increase the rate of magnetic relaxation both in silicate and carbonaceous grains. In particular, we find a new relaxation process related to the change of the amplitude of internal magnetization within a wobbling triaxial grain and identify a range of grain sizes in which this effect can dominate the internal alignment of angular momentum within grain axes. We show that these relaxation processes significantly change the dynamics of grains in the presence of RATs. We apply our analysis for observed grain alignment in special environments to put constraints on the enhanced magnetic properties of dust grains in the cloud near supernovae, in cometary coma, and protoplanetary disks.

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arXiv2019

Dust Rotational Dynamics in C-shocks: Rotational Disruption of Nanoparticles by Stochastic Mechanical Torques and Spinning Dust Emission

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.

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arXiv2019

Time-Varying Extinction, Polarization, and Colors of Type Ia Supernovae due to Rotational Disruption of Dust Grains

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 ($λ_{\rm max}< 0.4 μm$). 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 $ λ_{\rm max}$ decreases rapidly from $\sim 0.55 μ$m to $\sim 0.15 μ$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. $λ_{\rm max}$ 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.

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arXiv2019

JCMT BISTRO Survey observations of the Ophiuchus Molecular Cloud: Dust grain alignment properties inferred using a Ricean noise model

The dependence of polarization fraction $p$ on total intensity $I$ in polarized submillimeter emission measurements is typically parameterized as $p\propto I^{-α}$ $(α\leq 1)$, and used to infer dust grain alignment efficiency in star-forming regions, with an index $α=1$ indicating near-total lack of alignment of grains with the magnetic field. In this work we demonstrate that the non-Gaussian noise characteristics of polarization fraction may produce apparent measurements of $α\sim 1$ even in data with significant signal-to-noise in Stokes $Q$, $U$ and $I$ emission, and so with robust measurements of polarization angle. We present a simple model demonstrating this behavior, and propose a criterion by which well-characterized measurements of polarization fraction may be identified. We demonstrate that where our model is applicable, $α$ can be recovered by fitting the $p-I$ relationship with the mean of the Rice distribution, without statistical debiasing of polarization fraction. We apply our model to JCMT BISTRO Survey POL-2 850$μ$m observations of three clumps in the Ophiuchus Molecular Cloud, finding that in the externally-illuminated Oph A region, $α\approx 0.34$, while in the more isolated Oph B and C, despite their differing star formation histories, $α\sim 0.6-0.7$. Our results thus suggest that dust grain alignment in dense gas is more strongly influenced by incident interstellar radiation field than by star formation history. We further find that grains may remain aligned with the magnetic field at significantly higher gas densities than has previously been believed, thus allowing investigation of magnetic field properties within star-forming clumps and cores.

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arXiv2019

Rotational Disruption of Dust Grains by Radiative Torques in Strong Radiation Fields

Massive stars, supernovae, and kilonovae are among the most luminous radiation sources in the universe. Observations usually show near- to mid-infrared (NIR--MIR, $λ\sim 1-5~μ$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 $a\lesssim 0.05~μ$m) relative to large grains ($a\gtrsim 0.1~μ$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$α$ photons from H\,{\sc ii} regions surrounding YMSCs.

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arXiv2019

The JCMT BISTRO Survey: The Magnetic Field of the Barnard 1 Star-Forming Region

We present the POL-2 850 $μ$m linear polarization map of the Barnard 1 clump in the Perseus molecular cloud complex from the B-fields In STar-forming Region Observations (BISTRO) survey at the James Clerk Maxwell Telescope. We find a trend of decreasing polarization fraction as a function of total intensity, which we link to depolarization effects towards higher density regions of the cloud. We then use the polarization data at 850 $μ$m to infer the plane-of-sky orientation of the large-scale magnetic field in Barnard 1. This magnetic field runs North-South across most of the cloud, with the exception of B1-c where it turns more East-West. From the dispersion of polarization angles, we calculate a turbulence correlation length of $5.0 \pm 2.5$ arcsec ($1500$ au), and a turbulent-to-total magnetic energy ratio of $0.5 \pm 0.3$ inside the cloud. We combine this turbulent-to-total magnetic energy ratio with observations of NH$_3$ molecular lines from the Green Bank Ammonia Survey (GAS) to estimate the strength of the plane-of-sky component of the magnetic field through the Davis-Chandrasekhar-Fermi method. With a plane-of-sky amplitude of $120 \pm 60$ $μ$G and a criticality criterion $λ_c = 3.0 \pm 1.5$, we find that Barnard 1 is a supercritical molecular cloud with a magnetic field nearly dominated by its turbulent component.

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arXiv2019

JCMT BISTRO survey: Magnetic Fields within the Hub-Filament Structure in IC 5146

We present the 850 $μ$m polarization observations toward the IC5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope (JCMT), as part of the B-fields In STar forming Regions Observations (BISTRO). This work is aimed at revealing the magnetic field morphology within a core-scale ($\lesssim 1.0$ pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether or not the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function (PDF) of the observed polarization fraction for low signal-to-noise ratio (SNR) data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in $A_V\sim$ 20--300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5$\pm$0.2 mG toward the central hub using the Davis-Chandrasekhar-Fermi method, corresponding to a mass-to-flux criticality of $\sim$ $1.3\pm0.4$ and an Alfvénic Mach number of $<$0.6. These results suggest that gravity and magnetic field is currently of comparable importance in the HFS, and turbulence is less important.

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