• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.66.1-66.1
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• 2021

Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward Far-Ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size a < 0.1 ㎛ in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to Mid-Infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size a > 0.1 ㎛ are significantly disrupted to smaller sizes by RATD up to dRATD > 100 pc in the polar direction and dRATD ~ 10 pc in the torus region. Consequently, optical-MIR extinction decreases, whereas FUV-near-Ultraviolet extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to selective visual extinction ratio thus significantly drops to RV < 3.1 with decreasing distances to AGN center due to the enhancement of small grains. The dependence of RV with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms that are responsible for destroying very small grains of a <0.05 ㎛ is the key for explaining the dichotomy observed "SMC" and "gray" extinction curve toward many AGN.

• Atmosphere
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• v.20 no.4
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• pp.483-494
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• 2010

The vertical profiles of the extinction coefficient, the backscatter coefficient, and the lidar ratio (i.e., extinction-to-backscattering ratio) for Asian dust and pollution aerosols are determined from Raman (inelastic) and elastic backscatter signals. The values of lidar ratios during two polluted days is found between 52 and 82 sr (July 22, 2009) and 40~60 sr (July 31, 2009) at 52 nm, with relatively low value of particle depolarization ratio (<5%) and high value of sun photometer-derived Angstrom exponent (> 1.2). However, lidar ratios between 25 and 40 sr are found during two Asian dust periods (October 20, 2009 and March 15, 2010), with 10~20% of particle depolarization ratio and the relatively low value of sun photometer-derived Angstrom exponent (< 0.39). The lidar ratio, particle depolarization ratio and color ratio are useful optical parameter to distinguish non-spherical coarse dust and spherical fine pollution aerosols. The comparison of aerosol extinction profiles determined from inelastic-backscatter signals by the Raman method and from elastic-backscatter signals by using the Fernald method with constant value of lidar ratio (50 sr) have shown that reliable aerosol extinction coefficients cannot be determined from elastic-backscatter signals alone, because the lidar ratio varies with aerosol types. A combined Raman and elastic backscatter lidar system can provide reliable information about the aerosol extinction profile and the aerosol lidar ratio.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.35.2-35.2
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• 2010

It is crucial to understand the dust properties in the early universe since they provide important clues about how the early cosmic star formation should be interpreted in the presence of dust extinction. GRB 071025 is an unusually red GRB that occured at high redshift, offering an unique opportunity to study the dust properties in the early universe. We investigate the extinction properties of GRB 071025 through the analysis of RIJHK data obtained with the 1-m telescope at Mt. Lemmon Optical Astronomy Observatory (LOAO) and Simultaneous Quad Infrared Imaging Device (SQIID) on the Kitt-Peak Mayall 4-m telescope. Our dataset is independent from that in a previous work (Perley et al. 2010) where a small systematic photometric errors could complicate the interpretation. After determining the temporal power law exponent with five I-band frames from LOAO, we construct a multi-band monochromatic SED of the GRB afterglow. By using various extinction laws, we find that the SED is best fitted with models that incorporate SNe II dust and derive a photometric redshift of 4.99(+0.12/-0.03). Our results strongly support the prior claim that dusts in GRB 071025 originate mainly from supernovae, implying SNe II predominantly contributed to the dust enrichment in the early universe (z ~ 5).

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.35.1-35.1
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• 2015

We present the results of extinction measurements toward the main ejecta shell of the Cassiopeia A supernova (SN) remnant using the flux ratios between the two near-infrared (NIR) [Fe II] lines at 1.26 and $1.64{\mu}m$. We find a clear correlation between the NIR extinction (E(J-H)) and the radial velocity of ejecta knots, showing that redshifted knots are systematically more obscured than blueshifted ones. This internal "self-extinction" strongly indicates that a large amount of SN dust resides inside and around the main ejecta shell. At one location in the southern part of the shell, we measure E(J-H) by the SN dust of $0.23{\pm}0.05mag$. By analyzing the spectral energy distribution of thermal dust emission at that location, we show that there are warm (~100K) and cool (~40K) SN dust components and that the latter is responsible for the observed E(J-H). We investigate the possible grain species and size of each component and find that the warm SN dust needs to be silicate grains such as $MgSiO_3$, $Mg_2SiO_4$, and $SiO_2$, whereas the cool dust could be either small (${\leq}0.01{\mu}m$) Fe or large (${\geq}0.01{\mu}m$) Si grains. We suggest that the warm and cool dust components in Cassiopeia A represent grain species produced in diffuse SN ejecta and in dense ejecta clumps, respectively.

• Asian Journal of Atmospheric Environment
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• v.9 no.1
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• pp.91-99
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• 2015

The Multi-wavelength Raman LIDAR (MRL) system was developed to enable a better understanding of the complex properties of aerosols in the atmosphere. In this study, the microphysical, optical, and radiative properties of mixed aerosols were retrieved using the discrete aerosol observation products from the MRL. The dust mixing ratio, which is the proportion of dust particles to the total mixed, was derived using the particle depolarization ratio. It was employed in the retrieval of backscattering and extinction coefficient profiles for dust and non-dust particles. The vertical profiles of aerosol optical properties were then used as input parameters in the inversion algorithm for the retrieval of microphysical parameters including the effective radius, refractive index, and the single scattering albedo (SSA). Those products were successfully applied to an analysis of radiative flux using a radiative transfer model. The relationship between the MRL derived extinction and aerosol radiative forcing (ARF) in short-wavelength was assessed over Gwangju, Korea. The results clearly demonstrate that the MRL-derived extinction profiles are a good surrogate for use in the estimation of optical, microphysical, and radiative properties of aerosols. It is considered that the analytical results shown in this study can be used to provide a better understanding of air quality and the variation of local radiative effects due to aerosols.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.36.2-36.2
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• 2018

Radiative transfer models were developed to understand the optical polarizations in edge-on galaxies, which are observed to occur even outside the geometrically thin dust disk, with a scale height of ~0.2 kpc. In order to reproduce the vertically extended polarization structure, we find it is essential to include a geometrically thick dust layer in the radiative transfer model, in addition to the commonly-known thin dust layer. The models include polarizations due to both dust scattering and dichroic extinction which is responsible for the observed interstellar polarization in the Milky Way. It is found that the magnetic fields in edge-on galaxies are in general vertical (or poloidal) except the central part, where the magnetic fields are mainly toroidal. We also find that the polarization level is enhanced if the clumpiness of the interstellar medium, and the dichroic extinction by vertical magnetic fields in the outer regions of the dust lane are included in the radiative transfer model. The predicted degree of polarization outside the dust lane was found to be consistent with that (ranging from 1% to 4%) observed in NGC 891.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.44.3-44.3
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• 2019

Massive stars, supernovae, and kilonovae are among the most luminous radiation sources in the universe. Observations usually show near- to mid-infrared (NIR-MIR, 1-5~micron) emission excess from H II regions around young massive star clusters (YMSCs) and anomalous dust extinction and polarization towards Type Ia supernova (SNe Ia). The popular explanation for such NIR-MIR excess and unusual dust properties is the predominance of small grains (size a<0.05micron) relative to large grains (a>0.1micron) 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, namely the RAdiative Torque Disruption (RATD) mechanism, which can resolve this question. We find that RATD can destroy large grains located within a distance of ~ 1 pc from a massive star of luminosity L~ 10^4L_sun and a supernova. This increases the abundance of small grains relative to large grains and successfully reproduces the observed NIR-MIR excess and anomalous dust extinction/polarization. We show that small grains produced by RATD can also explain the steep far-UV rise in extinction curves toward starburst and high redshift galaxies, as well as the decrease of the escape fraction of Ly-alpha photons observed from HII regions surrounding YMSCs.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.40.2-41
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• 2016

To understand the observed kinematics in the narrow-line region (NLR) of type 2 AGNs, we construct a model of 3-D biconical outflow combined with a thin dust plane. The model consists of two identical cones whose apex is located at the nucleus, and the cones are axisymmetric with respect to the bicone axis. After we define the properties of the bicone and the dust plane, we calculate a spatially integrated velocity and velocity dispersion along the line-of-sight using various physical parameters. As we test the effect of model parameters, we find three key parameters determining the integrated kinematics: intrinsic outflow velocity, bicone inclination, and the amount of dust extinction. The velocity dispersion increases as the intrinsic outflow velocity or the bicone inclination increases, while the velocity shift increases as the amount of dust extinction increases. We confirm that the integrated velocity dispersion can be a good indicator of the intrinsic outflow velocity unless dust extinction is not very strong (>~80%), while the effect of dust extinction can be alleviated by combining the integrated velocity and the velocity dispersion. Based on the simulated velocity distributions using the 3-D models, the variety of the observed [O $_{III}$] line profiles of type 2 AGNs can be well reproduced. In addition, we perform Monte Carlo simulations based on the different sets of model parameters. By comparing the model results with the observed [O $_{III}$] kinematics of ~39,000 SDSS type 2 AGNs (Woo et al. 2016), we find that the observed [O $_{III}$] velocity-velocity dispersion distribution is well reproduced by the biconical outflow model, enabling us to constrain the intrinsic physical parameters of outflows.

• Journal of Astronomy and Space Sciences
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• v.20 no.3
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• pp.175-184
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• 2003

To investigate the SEDs of galaxies considering the dust extinction processes in the galactic disks, we present the population synthesis models for normal galaxies with dusty disks. We use PEGASE (Fioc & Rocca-Volmerange 1997) to model them with standard input parameters for stars and new dust parameters. We find that the model results are strongly dependent on the dust parameters as well as other parameters (e.g. star formation history). We compare the model results with the observations and discuss about the possible explanations. We find that the dust opacity functions derived from studies of asymptotic giant branch stars are useful for modeling a galaxy with a dusty disk.

• Atmosphere
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• v.18 no.4
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• pp.287-297
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• 2008

The vertical profiles and optical properties of Asian dust are investigated using ground-based measurements from 1998 to 2002. Vertical profiles of aerosol extinction coefficient are evaluated using MPL (Micro Pulse Lidar) data. Optical parameters such as aerosol optical thickness ($\tau$), ${\AA}ngstr\ddot{o}m$ exponent ($\alpha$), single scattering albedo ($\omega$), refractive index, and volume size distribution are analyzed with sun/sky radiometer data for the same period. We can separate aerosol vertical profiles into three categories. First category named as 'Asian dust case', which aerosol extinction coefficient is larger than $0.15km^{-1}$ and dust layer exists from surface up to 3-4km. Second category named as 'Elevated aerosol case', which aerosol layer exists between 2 and 6km with 1-2.5km thickness, and extinction coefficient is smaller than $0.15km^{-1}$. Third category named as 'Clear sky case', which aerosol extinction coefficient appears smaller than $0.15km^{-1}$. and shows that diurnal variation of background aerosol in urban area. While optical parameters for first category indicate that $\tau$ and $\alpha$ are $0.63{\pm}0.14$, $0.48{\pm}0.19$, respectively. Also, aerosol volume concentration is increased for range of 1 and $4{\mu}m$, in coarse mode. Optical parameters for second category can be separated into two different types. Optical properties of first type are very close to Asian dust cases. Also, dust reports of source region and backward trajectory analyses assure that these type is much related with Asian dust event. However, optical properties of the other type are similar to those of urban aerosol. For clear sky case, $\tau$ is relatively smaller and $\alpha$ is larger compare with other cases. Each case shows distinct characteristics in aerosol optical parameters.