arXiv subjects

Hee-Won Lee

Publications and source records attributed to Hee-Won Lee.

At least 37 records · Page 2Linked to original sources

arXiv2013

Asymmetric Absorption Profiles of Lyα and Lyβ in Damped Lyman Alpha Systems

Damped Lyα systems (DLAs) observed in the quasar spectra are characterized by a high neutral hydrogen column density $N_{HI}>2\times 10^{20}{\rm\ cm^{-2}}$. The absorption wing profiles are often fitted using the Voigt function due to the fact that the scattering cross section near resonant line center is approximately described by the Lorentzian function. Since a hydrogen atom has infinitely many $p$ states that participate in the electric dipole interaction, the cross section starts to deviate from the Lorentzian in an asymmetric way in the line wing regions. We investigate this asymmetry in the absorption line profiles around Lyα and Lyβ as a function of the neutral hydrogen column density $N_{HI}$. In terms of $Δλ\equivλ-λ_α$ we expand the Kramers-Heisenberg formula around Lyα to find $σ(λ)\simeq (0.5 f_{12})^2σ_{T}(Δλ/λ_α)^{-2}[1+3.792 (Δλ/λ_α)]$, where $f_{12}$ and $σ_T$ are the oscillator strength of Ly$α$ and the Thomson scattering cross section, respectively. In terms of $Δλ_2\equivλ-λ_β$ in the vicinity of Lyβ, the total scattering cross section, given as the sum of cross sections for Rayleigh and Raman scattering, is shown to be $σ(λ)\simeq σ_T(0.5f_{13})^2(1+R_0) (Δλ_2/λ_β)^{-2}[1-24.68(Δλ_2/λ_β)]$ with $f_{13}$ and the factor $R_0=0.1342$ being the oscillator strength for Lyβ and the ratio of Raman cross section to Rayleigh cross section, respectively. A redward asymmetry develops around Lyα whereas a blue asymmetry is obtained for Lyβ. The absorption center shifts are found to be almost proportional to the neutral hydrogen column density.

astro-ph.CO↗

arXiv2009

Raman Scattered HeII 6545 in the Young and Compact Planetary Nebula NGC 6790

We present the high resolution spectra of the youn and compact planerary nebula NGC 6790 obtained with the echelle spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman scattered He II 6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He II 1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profile with a simple geometric model, in which the neutral region is in the form of a cylindrical shell that is expanding from the central star. From our line profile analysis, the expansion velocity of the HI region lies in the range V_exp = 15- 19 Km/s. Less stringent constraints are put on the HI column density N_HI and covering factor C, where the total flux of Raman He II 6545 is consistent with the product of N_HI and C being CN_HI ~ 0.5 X 10^20 cm^-2. The Monte Carlo profiles from stationary emission models exhibit deficit in the wing parts. A much better fit is obtained when the He II emission region is assumed to take the form of a ring that slowly rotates with a rotation speed ~ 18Km/s. Brief discussions are presented regarding the mass loss processes and future observations.

astro-ph.SR↗

arXiv2007

Raman Scattered O VI $λ$ 6825 and the Accretion Disk Emission Model in the Symbiotic Stars V1016 Cygni and HM Sagittae

We present the high resolution spectra of the D type symbiotic stars V1016 Cygni and HM Sagittae obtained with the Bohyunsan Optical Echelle Spectrograph (BOES), and investigate the double-peaked asymmetric profiles of the Raman scattered O VI 6825. By adopting a wind accretion disk model, we assume that the O VI emission region is described by a Keplerian thin disk. The Raman scattering occurs in a neutral region near the giant, taking in the form of a slow stellar wind, part of which is ionized by the strong UV radiation from the hot white dwarf. Using a Monte Carlo technique, we compute the line profiles that are modulated by the slow spherical stellar wind from the giant component with the ionization front approximated by a hyperboloid. In order to account for the asymmetry and the existence of a central dip in the profiles, we add an O VI resonance scattering region between the hot white dwarf and the giant star which hinders the incidence of slightly blue O VI photons upon the H I region. Overall good fits to the observed data are obtained from our model, which lends support to the accretion disk emission model in these objects. The best fitting parameters for V1016 Cyg are $v_o=30{\rm km s^{-1}}$, $v_\infty=11{\rm km s^{-1}}$, and $v_{c}=10{\rm km s^{-1}}$, where $v_o$, $v_\infty$ and $v_{c}$ are the velocity of the outer disk rim, the terminal velocity of the giant wind, and the velocity component of the resonance scattering O VI region along the binary axis, respectively. Similar fitting parameters $v_o=27{\rm km s^{-1}}$, $v_\infty=10{\rm km s^{-1}}$ and $v_{c}=9{\rm km s^{-1}}$ are obtained for HM Sge. We also investigate the effect of a hot spot in a disk that is well known in accretion disks in cataclysmic variables.

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arXiv2004

Centre Shift of the Raman Scattered He II $λ$ 4850 in the Symbiotic Star V1016 Cygni

We present our spectroscopic data around H$β$ of the symbiotic star V1016 Cyg obtained with the Bohyunsan Echelle Spectrograph, in order to secure the broad emission feature at around 4850Å, which is formed through Raman scattering of He II $λ$ 972. The total cross section around Ly$γ$ is approximately given by $σ(λ)\simeq 1.7\times 10^{-28}[λ_{Lyγ}/(λ-λ_{Lyγ})]^2{\rm cm^2}$, with $λ_{Lyγ}$ being the line centre wavelength of Ly$γ$. We find a centre shift redward by an amount $Δλ= +0.64{\rm Å}$ in the Raman scattered He II $λ$ 4850. This redward centre shift is exactly analogous to the effect for the Raman scattered He II $λ$ 6545 blueward of H$α$ discussed in our previous study. We compute the branching ratios of Raman scattering into the level $2s$ and levels $3s,$ and 3d, which are subsequently incorporated in our Monte Carlo code. Assuming that He II $λ$ 972 emission is characterized by a single Gaussian profile, the redward peak shift observed in the Raman scattered He II 4850 feature corresponds to the neutral column density $N_{HI}=1.2\times 10^{21}{\rm\ cm^{-2}}$. Assuming that the covering factor $\sim 0.1$ of the scattering region with respect to the He II emission region and adopting a simple spherical stellar wind model, we may place an upper bound $\dot M \le 3.6\times 10^{-7}{\rm M_\odot yr^{-1}}$ for the mass loss rate of the giant component of V1016 Cyg. Our estimate can be severely affected by the inematics of the scattering and He II emission regions and the exact atomic physics, about which brief discussions are presented.

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arXiv2004

Rayleigh Scattering Cross Section Redward of Ly$α$ by Atomic Hydrogen

We present a low energy expansion of the Kramers-Heisenberg formula for atomic hydrogen in terms of $(ω/ω_l)$, where $ω_l$ and $ω$ are the angular frequencies corresponding to the Lyman limit and the incident radiation, respectively. The leading term is proportional to $(ω/ω_l)^4$, which admits a well-known classical interpretation. With higher order terms we achieve accuracy with errors less than 4 % of the scattering cross sections in the region $ω/ω_l\le 0.6$. In the neighboring region around Ly$α$ ($ω/ω_l >0.6$), we also present an explicit expansion of the Kramers-Heisenberg formula in terms of $Δω\equiv (ω-ω_{Lyα})/ω_{Lyα}$. The accuracy with errors less than 4 % can be attained for $ω/ω_l \ge 0.6$ with the expansion up to the fifth order of $Δω$. We expect that these formulae will be usefully applied to the radiative transfer in high neutral column density regions, including the Gunn-Peterson absorption troughs and Rayleigh scattering in the atmospheres of giants.

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arXiv2004

On the Center Shifts of Raman Scattered He II and H$α$ Wings in Symbiotic Stars

Using a Monte Carlo technique, we investigate the center shifts that are expected to occur for the broad H$α$ wings and the He II $λ$ 6545 feature as a function of the neutral hydrogen column density in symbiotic stars.These two features are proposed to be formed via Raman scattering of UV continuum around Ly$β$ and He II $λ$ 1025 emission line by neutral hydrogen. The branching ratio of Raman scattering to Rayleigh scattering for UV radiation around Ly$β$ is a non-linearly increasing function of the wavelength, which results in enhanced Raman optical fluxes redward of the H$α$ line center as $N_{HI}$ increases. However, we find that the amount of wing center shift is quite small about $20{\rm km s^{-1}}$ as $N_{HI}$ increases from $10^{20}{\rm cm^{-2}}$ to $10^{21}{\rm cm^{-2}}$. Assuming that He II $λ$~1025 emission is characterized by a Gaussian profile, the Raman scattered He II 6545 feature exhibits near Gaussian profiles with the peak shifted redward for $N_{HI}<10^{22}{\rm cm^{-2}}$. The redward center shift amounts to $1{\rm Å}$ for $N_{HI}\sim 10^{20}{\rm cm^{-2}}$ and decreases as $N_{HI}$ increases up to $N_{HI}\sim 10^{22}{\rm cm^{-2}}$, above which no center shift is observed. The redward peak shift is due to the fact that the incident emission profile is symmetric with respect to He II $λ$~1025 line center whereas the Raman conversion rate is increasing towards the line center of Ly$β$. We emphasize that the determination of $N_{HI}$ by locating the exact peak position of the He II 6545 feature will lift the degeneracy to allow a more accurate estimate of the covering factor of the neutral region, providing strong constraints on the mass loss process occurring in symbiotic stars.

astro-ph↗

arXiv2003

Asymmetric Deviation of the Scattering Cross Section around Ly$α$ by Atomic Hydrogen

We investigate the asymmetry of the scattering cross section of radiation around Ly$α$ by atomic hydrogen, which may be applied to analyses of scattering media with high column neutral hydrogen densities including damped Ly$α$ absorption systems of quasars. The exact scattering cross section is given by the Kramers-Heisenberg formula obtained from the fully quantum mechanical second-order time dependent theory, where, in the case of hydrogen, each matrix element is given in a closed analytical form. The asymmetric deviation of the scattering cross section from the Lorentzian near the line center is computed by expanding the Kramers-Heisenberg formula in terms of $Δω/ω_{Lyα}$,where $ω_{Lyα}$ is the angular frequency of the Ly$α$ transition and $Δω$ is the deviation of incident radiation from $ω_{Lyα}$. To the first order of $Δω/ω_{Lyα}$, we obtain $σ(ω) = σ_T (0.5 f_{12} ω_{Lyα}/Δω)^2 (1-1.79Δω/ ω_{Lyα})$, where $σ_T$ is the Thomson scattering cross section and $f_{12}=0.4162$ is the oscillator strength for the Ly$α$ transition. With this deviation, the line center of the damped wing profile apparently shifts blueward of the true Ly$α$ line center. In the case of a dampedLy$α$ system with a H I column density $5\times 10^{21}{\rm cm^{-2}}$, the apparent line center shift relative to the true center amounts to $0.2{\rm Å}$ resulting in an underestimation of redshift by $Δz\sim 10^{-4}$. A measurable underestimation by an amount of $Δz\sim 10^{-3}$ is expected for absorbing systems with $N_{HI}\ge 4\times 10^{22}{\rm cm^{-2}}$.

astro-ph↗

arXiv2003

Raman Scattered He II $λ$ 6545 Line in the Symbiotic Star V1016 Cygni

We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m Canada-France-Hawaii Telescope, in order to illustrate a method to measure the covering factor of the neutral scattering region around the giant component with respect to the hot emission region around the white dwarf component. In the spectrum, we find broad wings around H$α$ and a broad emission feature around 6545${\rm Å}$ that is blended with the [N II]$ λ$ 6548 line. These two features are proposed to be formed by Raman scattering by atomic hydrogen, where the incident radiation is proposed to be UV continuum radiation around Ly$β$ in the former case and He II $λ$ 1025 emission line arising from $n=6\to n=2$ transitions for the latter feature. We remove the H$α$ wings by a template Raman scattering wing profile and subtract the [N II] $λ$ 6548 line using the 3 times stronger [N II] $λ$ 6583 feature in order to isolate the He II Raman scattered 6545 Åline. We obtain the flux ratio $F_{6545}/F_{6560}=0.24$ of the He II $λ$ 6560 emission line and the 6545 Åfeature for V1016 Cyg. Under the assumption that the He II emission from this object is isotropic, this ratio is converted to the ratio $Φ_{6545}/Φ_{1025}=0.17$ of the number of the incident photons and that of the scattered photons. This implies that the scattering region with H I column density $N_{HI}\ge 10^{20}{\rm cm^{-2}}$ covers 17 per cent of the emission region. By combining the presumed binary period $\sim 100$ yrs of this system we infer that a significant fraction of the slow stellar wind from the Mira component is ionized and that the scattering region around the Mira extends a few tens of AU, which is closely associated with the mass loss process of the Mira component.

astro-ph↗

arXiv2002

Polarisation of the Broad H alpha Wing in Symbiotic Stars

In many symbiotic stars there appear broad wings around H alpha, of which the formation mechanism proposed thus far includes a fast outflow, the inner accretion disc motion, electron scattering and Raman scattering of Ly beta. We adopt a Monte Carlo technique to simulate the Raman scattering of UV photons that are converted into optical photons around H alpha forming broad wings, and compute its polarisation. Noting that many symbiotic stars exhibit a bipolar nebular morphology and polarisation flip in the red wing part of the Raman scattered O {\tiny{VI}} features, we assume that the neutral scattering region is composed of the two components. The first component is a static cylindrical shell with finite thickness and the other component is a finite slab that is moving away with velocity v_p = 100 km s^{-1} along the symmetry axis of the first component. The cylindrical shell component yields polarisation in the direction parallel to the cylinder axis. The polarisation near the line-centre is weaker than in the far wing regions because of the large Rayleigh scattering numbers due to the large scattering cross sections near the line centre. The receding polar scattering component produces strong polarisation in the direction perpendicular to the cylinder axis. When the both scattering components coexist, the polarisation is characterised by weak parallel polarisation near the line-centre and strong perpendicular polarisation in the red part. We discuss the observational implications of our computation.

astro-ph↗

arXiv2002

Profiles of the Resonance Doublets Formed in Bipolar Winds in Symbiotic Stars

We compute the profiles of resonance doublet lines formed in bipolar winds with velocity greater than the doublet separation in symbiotic stars. Particular attention has been paid on the doublet line ratio, where the conversion of the short wavelength component arising from the $2S_{1/2}-2P_{3/2}$ transition into the long wavelengh component for the transition $2S_{1/2}-2P_{1/2}$ plays an essential role in determining the flux line ratio of the doublet. We adopted a Monte Carlo technique and the Sobolev approximation. Our bipolar winds take the form of a cone and are characterised by the terminal wind velocity, mass loss rate and the opening angle of the cone. When the observer is in the polar direction and the Sobolev optical depth is moderate $τ_{Sob}\simeq 1$, then we mainly obtain profiles with inverted flux line ratios, where the short wavelength component is weaker than the long wavelength component. When the observer is in the equatorial direction, we find that the profiles are characterised by two broad components, where the long wavelength component is the broader and stronger of the two. We conclude that the profiles obtained in our model provide qualitative understanding of broad and inverted intensity ratio of the doublets in symbiotic stars and that the bipolarity of the stellar winds can be inferred from the broader and stronger long wavelength components in the doublet.

astro-ph↗

arXiv2002

Lya Polarisation in Thick H I Media

We have investigated the linear polarisation of Lya transferred in a static, dustless, and thick plane-parallel slab that is uniformly filled with neutral hydrogen. Because the scattering phase function is characterised by the dipole type for wing scatterings of Lya, which is in high contrast with more isotropic scattering phase function associated with resonant scatterings, the polarisation of the emergent Lya is dominantly controlled by the number of wing scatterings before escape from the slab. When the scattering medium is extremely thick, photons emergent in the grazing direction are polarised in the direction perpendicular to the slab normal up to 12%, which coincides with the limit attained by the Thomson scattered radiation in a semi-infinite thick medium investigated by Chandrasekhar in 1960. The number of wing scatterings before escape is sensitively dependent on the product of the Voigt parameter $a$ and the line-centre optical depth $τ_0$, with which the variation of the degree of polarisation can also be explained in comparison with the Thomson scattering case. The polarisation direction is parallel to the slab normal when $aτ_0<10^3$, while it is perpendicular when $aτ_0>10^3$. Our simulated spectropolarimetry of Lya from a highly thick medium is characterised by negligible polarisation around the line centre, parallel polarisation in the near wing parts and perpendicular polarisation in the far wing parts, where the polarisation direction is measured with respect to the slab normal. The polarisation flip naturally reflects the diffusive nature of the Lya transfer process in both real space and frequency space. We also discuss the beaming effect that is associated with the parallel polarisation in the near wing parts of the emission.

astro-ph↗

arXiv2002

P-Cygni Type Lya from Starburst Galaxies

P-Cygni type Lya profiles exhibited in nearly half of starburst galaxies, both nearby and high-z, are believed to be formed by an expanding supershell surrounding a star-forming region. We apply the Monte Carlo code which was developed previously for static and plane-parallel medium to calculate the Lya line transfer in a supershell of neutral hydrogen which are expanding radially in a spherical bulk flow. We consider typical cases that the supershell has the Lya line-centre optical depth of $τ_0=10^5-10^7$, a radial expansion velocity of $V_exp = 300 km/s, and the turbulence of b=40 km/s. We find that there appear a few emission peaks at the frequencies corresponding to (2N-1) V_exp, where the order of back scatterings N > 1. As V_exp -> b, the emergent profiles become similar to those for the static medium and the peaks are less prominent. We also investigate the effects of column density of the supershell on the emergent Lya profiles. We find that the number and the flux ratios of emission peaks are determined by interplay of $τ_0$ and V_exp of the supershell. We discuss the effects of dust extinction and the implication of our works in relation to recent spectroscopic observations of starburst galaxies.

astro-ph↗

arXiv2001

Lyman alpha line formation in starbursting galaxies II. Extremely Thick, Dustless, and Static HI Media

The Lya line transfer in an extremely thick medium of neutral hydrogen is investigated by adopting an accelerating scheme in our Monte Carlo code to skip a large number of core or resonant scatterings. This scheme reduces computing time significantly with no sacrifice in the accuracy of the results. We applied this numerical method to the Lya transfer in a static, uniform, dustless, and plane-parallel medium. Two types of photon sources have been considered, the midplane source and the uniformly distributed sources. The emergent profiles show double peaks and absorption trough at the line-center. We compared our results with the analytic solutions derived by previous researchers, and confirmed that both solutions are in good agreement with each other. We investigated the directionality of the emergent Lya photons and found that limb brightening is observed in slightly thick media while limb darkening appears in extremely thick media. The behavior of the directionality is noted to follow that of the Thomson scattered radiation in electron clouds, because both Lya wing scattering and Thomson scattering share the same Rayleigh scattering phase function. The mean number of wing scatterings just before escape is in exact agreement with the prediction of the diffusion approximation. The Lya photons constituting the inner part of the emergent profiles follow the relationship derived from the diffusion approximation. We present a brief discussion on the application of our results to the formation of Lya broad absorption troughs and P-Cygni type Lya profiles seen in the UV spectra of starburst galaxies.

astro-ph↗

arXiv2001

The Kramers-Heisenberg Formula and the Gunn-Peterson Trough from the First Objects in the Universe

Recent cosmological observations indicate that the reionized universe may have started at around z=6, where a significant suppression around Ly alpha has been observed from the neutral intergalactic medium. The associated neutral hydrogen column density is expected to exceed 10^{21}cm^{-2}},where it is very important to use the accurate scattering cross section known as the Kramers-Heisenberg formula that is obtained from the fully quantum mechanical time-dependent second order perturbation theory. We present the Kramers-Heisenberg formula and compare it with the formula introduced in a heuristic way by Peebles (1993) treating the hydrogen atom as a two-level atom, from which we find a devitaion by a factor of two in the red wing region far from the line center. Adopting simple cosmological models, we compute the Gunn-Peterson optical depths and the trough profiles. Our results are compared with the works performed by Madau & Rees (2000), who adopted the cross section introduced by Peebles. We find deviations up to 5 per cent in the Gunn-Peterson transmission coefficient for an accelerated expanding universe in the red off-resonance wing part with the rest wavelength Delta_lambda~10{AA}.

astro-ph↗

arXiv2001

Compact Objects and Accretion Disks

Recent developments in the spectropolarimetric study of compact objects, specifically black holes (stellar and massive) and neutron stars are reviewed. The lectures are organized around five topics: disks, jets, outflows, neutron stars and black holes. They emphasize physical mechanisms and are intended to bridge the gap between the fundamentals of polarimetry and the phenomenology of observed cosmic sources of polarized radiation, as covered by the other lecturers. There has been considerable recent progress in spectropolarimetry from radio through optical frequencies and this is producing some unique diagnostics of the physical conditions around compact objects. It is argued that there is a great need to develop a correspondingly sensitive polarimetric capability at ultraviolet through gamma-ray energies.

astro-ph↗

arXiv2001

Raman Scattered He II Line in the Planetary Nebula M2-9 and the Symbiotic Stars RR Telescopii and He 2-106

In this Letter, we report the detection of an emission feature around 6545 Åin the spectra of the bipolar planetary nebula M2-9 and the symbiotic stars RR Telescopii and He 2-106 and propose to identify it as the He II Raman scattered feature. This feature was predicted by Nussbaumer, Schmid & Vogel (1989), who suggested that it is formed through Raman scattering by atomic hydrogen of He II $n=6\to n=2$ photons with slightly shorter wavelength than that of Ly$β$. The scattering cross section $\sim 10^{-20}{\rm cm^2}$ for this process implies the existence of a neutral hydrogen component with a column density $N_{HI}\sim 10^{20}{\rm cm^{-2}}$ around the He II emission regions in these objects, which are believed to be associated with the mass loss process in the late stage of stellar evolution. Brief discussions on the astrophysical implications of Raman scattering in these objects are presented.

astro-ph↗

arXiv2001

Lyman alpha line formation in starbursting galaxies I. Moderately thick, dustless, and static HI media

We investigate the Lyman alpha line transfer in nearby and high redshift starbursting galaxies, where the effect of high optical depths and the role of dust in the scattering medium are expected to be conspicuous and should be treated in a very careful manner. We present our first results in dustless, static, and uniform HI media with moderate Lyman alpha line center optical depths τ_0=10^{3-6}. We assume that the temperatures of media to be T=10^{1-4}K, and use a Monte Carlo technique. We investigate the basic processes of the line transfer and confirm the criterion of aτ_0>10^3 for the validity of diffusion approximation suggested by Neufeld in 1990, where $a$ is the Voigt parameter. Adopting the model suggested by Tenorio-Tagle et al., we performed calculations on the Lyman alpha line formation for each evolutionary stage of an expanding supershell. The emergent Lyman alpha profiles are characterized by the double peaks and the absorption trough at the line center. We found that the absorption troughs expected in most of the evolutionary stages are not wide enough to be observed with current instruments. However, the absorption trough in the Lyman alpha emission profile from an expanding recombining supershell can be marginally detected.

astro-ph↗

arXiv2000

Raman-Scattering Wings of H alpha in Symbiotic Stars

Nussbaumer et al. (1989) proposed that broad H$α$ wings can be formed through Raman scattering of Ly$β$ photons, and in this Letter we argue that the H$α$ wings prevalently seen in symbiotic stars may be indeed formed in this way. Assuming a flat incident UV radiation around Ly$β$, we generate template wing profiles around H$α$ that are formed through Raman scattering in a plane-parallel H~I region. We perform profile fitting analyses to show that the template wing profiles are in excellent agreement with the observed ones that are provided by van Winckel et al. (1993) and Ivison et al. (1994). The wing flux is determined by the scattering H~I column density and the incident Ly$β$ flux strength and profile. From our profile analysis it is proposed that the Raman scattering component may be identified with the neutral envelope with a column density ranging $10^{18-20} {\rm cm^{-2}}$ that surrounds the binary system. We briefly discuss alternative candidates for the wing formation mechanism and observational implications of Raman scattering in symbiotic stars and in other astronomical objects including planetary nebulae, post AGB stars and active galactic nuclei.

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