arXiv subjects

Hee-Won Lee

Publications and source records attributed to Hee-Won Lee.

44 records · Page 3Linked to original sources

arXiv2000

Lyman alpha Transfer in a thick, dusty, and static medium

We developed a Monte Carlo code that describes the resonant Lyman alpha line transfer in an optically thick, dusty, and static medium. The code was tested against the analytic formula derived by Neufeld (1990). We explain the line transfer mechanism for a wide range of line center optical depths by tracing histories of photons in the medium. We find that photons escape from the medium by a series of wing scatterings, during which polarization may develop. We applied our code to examine the amount of dust extinction around the Lyman alpha in primeval galaxies. Brief discussions on the astrophysical application of our work are presented.

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arXiv1999

Broad H$α$ Wing Formation in the Planetary Nebula IC 4997

The young and compact planetary nebula IC 4997 is known to exhibit very broad wings with a width exceeding $5000 {\rm km s^{-1}}$ around H$α$. We propose that the broad wings are formed through Rayleigh-Raman scattering involving atomic hydrogen, by which Ly$β$ photons with a velocity width of a few $10^2 {\rm km s^{-1}}$ are converted to optical photons and fill the H$α$ broad wing region. The conversion efficiency reaches 0.6 near the line center where the scattering optical depth is much larger than 1 and rapidly decreases in the far wings.Assuming that close to the central star there exists an unresolved inner compact core of high density, $n_H\sim 10^{9-10} {\rm cm^{-3}}$, we use the photoionization code `CLOUDY' to show that sufficient Ly$β$ photons for scattering are produced. Using a top-hat incident profile for the Ly$β$ flux and a scattering region with a H~I column density $N_{HI}=2\times 10^{20} {\rm cm^{-2}}$ and a substantial covering factor, we perform a profile fitting analysis to obtain a satisfactory fit to the observed flux. We briefly discuss the astrophysical implications of the Rayleigh-Raman processes in planetary nebulae and other emission objects.

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arXiv1999

Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Tel

In this paper, we argue that in the symbiotic star RR Tel the existence of an accretion disk around the hot companion is strongly implied by the characteristic features exhibited by the Raman-scattered O VI lines around 6830 Åand 7088 Å. High degrees of polarization and double-peaked profiles in the Raman-scattered lines and single-peak profiles for other emission lines are interpreted as line-of-sight effects, where the H I scatterers near the giant see an incident double-peaked profile and an observer with a low inclination sees single-peak profiles. It is predicted that different mass concentrations around the accretion disk formed by a dusty wind may lead to the disparate ratios of the blue peak strength to the red counterpart observed in the 6830 and 7088 features. We discuss the evolutionary links between symbiotic stars and bipolar protoplanetary nebulae and conclude that the Raman scattering processes may play an important role in investigation of the physical properties of these objects.

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arXiv1998

Polarization Structures in the Thomson-Scattered Emission Lines in Active Galactic Nuclei

A line photon incident in an electron-scattering medium is transferred in a diffusive way both in real space and in frequency space, and the mean number of scatterings changes as the wavelength shifts from the line center. This leads to the profile broadening and polarization dependence on the wavelength shift as a function of the Thomson optical depth $τ_T$. We find that the polarization of the Thomson-scattered emission lines has a dip around the line center when $τ_T$ does not exceed a few. Various structures such as the polarization flip are also seen. An application to an ionized halo component surrounding the broad emission line region in active galactic nuclei is considered and it is found that the polarization structures may still persist. Brief discussions on observational implications are given.

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arXiv1998

On the Polarization of H-alpha Lines Scattered by Neutral Hydrogen in Active Galactic Nuclei

Raman scattering by atomic hydrogen converts the UV continuum around Ly$β$ into optical continuum around H$α$, and the basic atomic physics has been discussed in several works on symbiotic stars. We propose that the same process may operate in active galactic nuclei (AGN) and calculate the linear polarization of the broad emission lines Raman-scattered by a high column neutral hydrogen compnent. The conversion efficiency of the Raman scattering process is discussed and the expected scattered flux is computed using the spectral energy distribution of an AGN given by a typical power law. The high column H {\sc i} component in AGN is suggested by many observations encompassing radio through UV and X-ray ranges. When the neutral hydrogen component with a column density $\sim 10^{22} cm^{-2}$ is present around the active nucleus, it is found that the scattered H$α$ is characterized by a very broad width $\sim 20,000 km/s$ and that the strength of the polarized flux is comparable to that of the electron-scattered flux expected from a conventional unified model of narrow line AGN. The width of the scattered flux is mainly determined by the column density of the neutral scatterers where the total scattering optical depth becomes of order unity. The asymmetry in the Raman scattering cross section around Ly$β$ introduces red asymmetric polarized profiles around H$α$. The effects of the blended Ly$β$ and O {\sc vi} 1034 doublet are also investigated. We briefly discuss the spectropolarimetric observations performed on the Seyfert galaxy IRAS 110548-1131 and the narrow line radio galaxy Cyg A.

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arXiv1998

Polarization of the Lyman alpha line from an anisotropically expanding H~I shell in primeval galaxies

We compute the polarization of the Lyman alpha line photons emerging from an anisotropically expanding and optically thick medium, which is expected to operate in many Lyman alpha emitting objects including the primeval galaxy DLA~2233+131 and Lyman break galaxies. In the case of a highly optically thick medium, the escape of resonance line photons is achieved by a large number of resonant local scatterings followed by a small number of scatterings in the damping wing. We show that some polarization can develop because the wing scatterings are coupled with strong spatial diffusion which depends on the scattering geometry and kinematics. The case of a slab with a finite scattering optical depth and expansion velocity of ~ 100 kms^{-1} is investigated and it is found that Lyman alpha photons are emergent with the linear degree of polarization up to 10% when the typical scattering optical depth tau>=10^5. We subsequently investigate the polarization of Lyman alpha photons emerging from a spherical shell obscured partially by an opaque component and we obtain ~ 5 % of polarization. It is proposed that a positive detection of polarized Lyman alpha with with P-Cygni type profile from cosmological objects can be a strong test of the expanding shell structure obscured by a disk-like component.

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arXiv1998

Lyman alpha line formation in an expanding H I superbubble in the primeval galaxy DLA 2233+131

Various types of galaxies observed in the cosmological scales show P-Cygni type profiles in the Lyman alpha emission lines. A Monte Carlo code is developed to investigate the Lyman alpha line transfer in an optically thick and moving medium with a careful consideration of the scattering in the damping wing. The main features in emergent line profiles include a primary emission peak in the red part and a much weaker secondary peak in the blue part. The primary peak recedes to the red and the width of the feature increases as $N_{HI}$ increases. The P-Cygni type profile in the Lyman alpha emission line of DLA 2233+131 ($z=3.15$) is noted to be similar to those found in the nearby galaxies and distant ($3<z<3.5$) H II galaxies. Our numerical results are applied to show that the DLA system may possess an expanding H I supershell with bulk flow of $\sim 200 km/s$ and that the H I column density $N_{HI}$ is approximately $10^{20} cm^{-2}$. From the observed Lyman alpha flux and adopting a typical size of the emission region $\sim 1 \kpc$, we estimate the electron density of the H II region to be $\sim 1 \cm^{-3} and the mass of H II region $\sim 10^8 \msun$. We also conclude that it requires $\sim 10^3$ O5 stars for photoionization, which is comparable to first-ranked H II regions found in nearby spiral and irregular galaxies.

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arXiv1997

On the Profiles and Polarization of Raman Scattered Emission Lines in Symbiotic Stars:II. Numerical Simulations

A Monte Carlo method is used to calculate the profiles and the polarization of the Raman scattered O VI lines(lambda lambda 6827,7088) in symbiotic stars. A point-like isotropic UV radiation source is assumed and a simple spherical wind model is adopted for the kinematics of the scattering material from the cool giant. We first investigate the case where the incident line photons are described by a Gaussian profile having a width of 10^4 K. We subsequently investigate the effects of the extended ionized region and non-spherical wind models including a disk-type wind and a bipolar wind. The cases where the emissin source is described by non-Gaussian profiles are briefly studied. Finally, as an additional component for the kinematics of symbiotic stars the orbital motion of the hot component around the cool giant is included and the effect on the spectropolarimetry is investigated. In this case the polarization direction changes around the red part of the Raman-scattered emission lines, when the observer's line of sight is perpendicular to the orbital plane, and no such effect is seen when the line of sight lies in the orbital plane. Furthermore, complex peak structures are seen in the degree of polarization and polarized flux, which have often been observed in several symbiotic systems including RR Tel. Brief observational consequences and preditions are discussed in relation to the present and future spectropolarimetry for symbiotic stars. It is concluded that spectropolarimetry may provide a powerful diagnostic of the physical conditions of symbiotic stars.

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