arXiv subjects

Sang-Hyeon Ahn

Publications and source records attributed to Sang-Hyeon Ahn.

18 recordsLinked to original sources

arXiv2023

A superconducting tensor detector for mid-frequency gravitational waves: its multi-channel nature and main astrophysical targets

Mid-frequency band gravitational-wave detectors will be complementary for the existing Earth-based detectors (sensitive above 10 Hz or so) and the future space-based detectors such as LISA, which will be sensitive below around 10 mHz. A ground-based superconducting omnidirectional gravitational radiation observatory (SOGRO) has recently been proposed along with several design variations for the frequency band of 0.1 to 10 Hz. For three conceptual designs of SOGRO (e.g., pSOGRO, SOGRO and aSOGRO), we examine their multi-channel natures, sensitivities and science cases. One of the key characteristics of the SOGRO concept is its six detection channels. The response functions of each channel are calculated for all possible gravitational wave polarizations including scalar and vector modes. Combining these response functions, we also confirm the omnidirectional nature of SOGRO. Hence, even a single SOGRO detector will be able to determine the position of a source and polarizations of gravitational waves, if detected. Taking into account SOGRO's sensitivity and technical requirements, two main targets are most plausible: gravitational waves from compact binaries and stochastic backgrounds. Based on assumptions we consider in this work, detection rates for intermediate-mass binary black holes (in the mass range of hundreds up to $10^{4}$ $M_\odot$) are expected to be $0.0014-2.5 \,\, {\rm yr}^{-1}$. In order to detect stochastic gravitational wave background, multiple detectors are required. Two aSOGRO detector networks may be able to put limits on the stochastic background beyond the indirect limit from cosmological observations.

gr-qc↗

arXiv2019

Economic Power, Population, and the Size of Astronomical Community

The number of astronomers for a country registered to the IAU is known to have a correlation with the GDP. However, the robustness of this relationship can be doubted, because the fraction of astronomers joining the IAU differs from country to country. Here we revisit this correlation by using the recent data updated as of 2017, and then we find a similar correlation by using the total enumeration of astronomers and astrophysicists with PhD degrees and working in each country, instead of adopting the number of IAU members. We confirm the existence of two subgroup in the correlation. One group consists of European advanced countries having long history of modern astronomy, while the other group consists of countries having experienced recent rapid economic development. In order to find causation in the correlation, we obtain the long-term variations of the number of astronomers, population, and the GDP for a number of countries to find that the number of astronomers per citizen for recently developing countries has increased more rapidly as GDP per capita increased, than that for fully developed countries. We collect a demographic data of the Korean astronomical community. From these findings we estimate the proper size of the Korean astronomical community by considering the society's economic power and population. The current number of PhD astronomers working in Korea is approximately 310, but it should be 550 that is large enough to be comparable and competitive to the sizes of Spainish, Canadian, and Japanese astronomical communities. We discuss on the way how to overcome the vulnerability of the Korean astronomical community, based on the statistics of national R&D expenditure structure comparing with that of other major advanced countries.

cs.DL↗

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.

astro-ph.GA↗

arXiv2018

Broad Wings around H$α$ and H$β$ in the Two S-Type Symbiotic Stars Z Andromedae and AG Draconis

Symbiotic stars often exhibit broad wings around Balmer emission lines, whose origin is still controversial. We present the high resolution spectra of the S type symbiotic stars Z Andromedae and AG Draconis obtained with the ESPaDOnS and the 3.6 m Canada France Hawaii Telescope to investigate the broad wings around H$α$ and H$β$. When H$α$ and H$β$ lines are overplotted in the Doppler space, it is noted that H$α$ profiles are overall broader than H$β$ in these two objects. Adopting a Monte Carlo approach, we consider the formation of broad wings of H$α$ and H$β$ through Raman scattering of far UV radiation around Ly$β$ and Ly$γ$ and Thomson scattering by free electrons. Raman scattering wings are simulated by choosing an H I region with a neutral hydrogen column density $N_{HI}$ and a covering factor $CF$. For Thomson wings, the ionized scattering region is assumed to cover fully the Balmer emission nebula and is characterized by the electron temperature $T_e$ and the electron column density $N_e$. Thomson wings of H$α$ and H$β$ have the same width that is proportional to $T_e^{1/2}$. However, Raman wings of H$α$ are overall three times wider than H$β$ counterparts, which is attributed to different cross section for Ly$β$ and Ly$γ$. Normalized to have the same peak values and presented in the Doppler factor space. H$α$ wings of Z And and AG Dra are observed to be significantly wider than H$β$ counterpart, favoring the Raman scattering origin of broad Balmer wings.

astro-ph.SR↗

arXiv2018

Escape of Resonantly Scattered Ly$β$ and H$α$ from Hot and Optically Thick Media

We investigate the escape of Ly$β$ from emission nebulae with a significant population of excited hydrogen atoms in the level $n=2$, rendering them optically thick in H$α$. The transfer of Ly$β$ line photons in these optically thick regions is complicated by the presence of another scattering channel leading to re-emission of H$α$, alternating their identities between Ly$β$ and H$α$. In this work, we develop a Monte Carlo code to simulate the transfer of Ly$β$ line photons incorporating the scattering channel into H$α$. Both H$α$ and Ly$β$ lines are formed through diffusion in frequency space, where a line photon enters the wing regime after a fairly large number of resonance scatterings with hydrogen atoms. Various line profiles of H$α$ and Ly$β$ emergent from our model nebulae are presented. It is argued that the electron temperature is a critical parameter which controls the flux ratio of emergent Ly$β$ and H$α$. Specifically for $T=3 \times 10^4{\rm\ K}$ and H$α$ line center optical depth $τ_α=10$, the number flux ratio of emergent Ly$β$ and H$α$ is $\sim 49$ percent, which is quite significant. We propose that the leaking Ly$β$ can be an interesting source for the formation of H$α$ wings observed in many symbiotic stars and active galactic nuclei. Similar broad H$α$ wings are also expected in Ly$α$ emitting halos found in the early universe, which can be potentially probed by the {\it James Webb Telescope} in the future.

astro-ph.HE↗

arXiv2017

Proto-type installation of a double-station system for the optical-video-detection and orbital characterisation of a meteor/fireball in South Korea

We give a detailed description of the installation and operation of a double-station meteor detection system which formed part of a research & education project between Korea Astronomy Space Science Institute and Daejeon Science Highschool. A total of six light-sensitive CCD cameras were installed with three cameras at SOAO and three cameras at BOAO observatory. A double-station observation of a meteor event enables the determination of the three-dimensional orbit in space. This project was initiated in response to the Jinju fireball event in March 2014. The cameras were installed in October/November 2014. The two stations are identical in hardware as well as software. Each station employes sensitive Watec-902H2 cameras in combination with relatively fast f/1.2 lenses. Various fields of views were used for measuring differences in detection rates of meteor events. We employed the SonotaCo UFO software suite for meteor detection and their subsequent analysis. The system setup as well as installation/operation experience is described and first results are presented. We also give a brief overview of historic as well as recent meteor (fall) detections in South Korea. For more information please consult http://meteor.kasi.re.kr .

astro-ph.IM↗

arXiv2003

Singly-Peaked P-Cygni type Ly$α$ from starburst galaxies

We present results of Monte Carlo calculations for the Lya line transfer in an expanding dusty supershell, where Lya source is a well-localized star cluster in a starburst galaxy.The escape of Lya photons from such system is achieved by a number of back-scattering, and so a series of emission peaks are formed redward of the systemic redshift by back-scattering. However, majority of observed Lya emission from starbursts show singly-peaked asymmetric profiles. We find in this paper that, in order to form a singly-peaked Lya emission, dust should be distributed in the ionized bubble, as well as within the supershell of neutral hydrogen. We also find that the overall escape fraction of Lya photons is determined by the HI column density of the supershell, the expansion velocity of the supershell, and the spatial distribution of dust. However, the kinematic information of the expanding supershell is preserved in the profile of Lya emission even when the supershell is dusty. Our results are potentially useful to fit the P-Cygni type Lya line profiles from starburst galaxies, either nearby galaxies or high-z Lyman break galaxies (LBGs).

astro-ph↗

arXiv2003

Meteors And Showers A Millennium Ago

Meteors can be classified into either sporadic meteors or showery meteors. We compile the meteor records in the astronomical archives in the Chronicle of the Koryo dynasty (918-1392), and investigate the spatial distribution of meteor streams along the orbit of the Earth from the 10th century to the 14th century. We see that meteors from meteor streams signalize themselves over noisy sporadic meteors, and that the seasonal activity of sporadic meteors was apparently regular. We discover the presence of a few meteor streams by analysing about 700 meteors in the Koryo period. We also compile the records of meteor showers and storms in the chronicles of Korea, Japan, China, Arab, and Europe, and compare their appearance dates with those of showers obtained above, as well as with the modern observations. We confirm that the three sets of data are in agreement with each other. The representative meteor showers are the Perseids, the Leonids, and the $η$-Aquarids/Orionids pair formed by Halley's comet. The other weak or relic meteor streams are also observable but uncertain. Hence, we witness the regularity of meteor activity, which is seen to persist for a millennium.

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↗

arXiv2002

GRB Explosion in a Galactic Supershell : GRB 971214

Among a number of gamma-ray bursts whose host galaxies are known, GRB971214 stands out for its high redshift(z=3.25) and the Lya emission line having a P-Cygni type profile. which is interpreted to be a direct consequence of the expanding supershell. From a simple Voigt-fitting analysis, we estimate the expansion velocity of the supershell and the neutral column density. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius $R=10E^{1/2}_{53}$pc, its age t=4.7\times 10^3 E^{1/2}_{53} yrs, and the density of the ambient medium $n_1=5.4E^{-1/2}_{53} cm^{-3}$, where $E_{53}=E/10^{53}$ ergs. I estimate the kinetic energy of the supershell to be $E_k=7.3\times 10^{52} E_{53}$ ergs. I provide an observational evidence that the gamma-ray burst can occur in a giant H II region whose environment is similar to that in star-forming 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

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

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.

astro-ph↗

arXiv1999

Environment of The Gamma-Ray Burst GRB971214 : A Giant H II Region surrounded by A Galactic Supershell

Among a number of gamma ray bursts whose host galaxies are known, GRB971214 stands out for its high redshift $z\ge 3$ and the Ly$α$ emission line having a P-Cygni type profile, which is interpreted to be a direct consequence of the expanding supershell. From a profile fitting analysis we estimate the expansion velocity of the supershell $v_{exp} = 1500\kms$ and the neutral column density $N_{HI}=10^{20}\cm^{-2}$. The redshift $z=3.418$ of the host galaxy proposed by Kulkarni et al. (1998) has been revised to be $z=3.425$ from our profile analysis. The observed Ly$α$ profile is fitted well by a Gaussian curve, which yields the Ly$α$ luminosity $L_{Lyα}=(1.8\pm0.8)\times10^{42}\ergs \s^{-1}$. Assuming that the photon source is a giant H II region, we deduce the electron number density in the H II region $n_e=(40\pm10) ({R \over {100 \pc}})^{-1.5}\cm^{-3}$, which corresponds to the illumination by about $10^4$ O5 stars. We estimate the star-formation rate to be $R_{SF} = (7\pm3){\rm M}_\odot\yr^{-1}$ with the internal and the Galactic extinction corrected. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius $R = 18 E_{53}^{1/2} \pc$, its age $t = 4.7\times10^3\ E_{53}^{1/2} \yrs$, and the density of the ambient medium $n_1 = 5.4\ E_{53}^{-1/2}\cm^{-3}$, where $E_{53}= E/10^{53}\ergs$.And we estimate the kinetic energy of the supershell to be $E_k=7.3\times10^{52} E_{53} \ergs$. These values are consistent with the hypothesis that the supershell is the remnant of a gamma ray burst.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗