arXiv subjects

Wonyong Han

Publications and source records attributed to Wonyong Han.

At least 19 recordsLinked to original sources

arXiv2025

A Nearby Dark Molecular Cloud in the Local Bubble Revealed via H$_2$ Fluorescence

A longstanding prediction in interstellar theory posits that significant quantities of molecular gas, crucial for star formation, may be undetected due to being ``dark" in commonly used molecular gas tracers, such as carbon monoxide. We report the discovery of Eos, the closest dark molecular cloud, located just 94 parsecs from the Sun. This cloud is the first molecular cloud ever to be identified using H$_2$ far ultra-violet (FUV) fluorescent line emission, which traces molecular gas at the boundary layers of star-forming and supernova remnant regions. The cloud edge is outlined along the high-latitude side of the North Polar Spur, a prominent x-ray/radio structure. Our distance estimate utilizes 3D dust maps, the absorption of the soft X-ray background, and hot gas tracers such as O\,{\sc vi}; these place the cloud at a distance consistent with the Local Bubble's surface. Using high-latitude CO maps we note a small amount (M$_{\rm{H}_2}\approx$20-40\,M$_\odot$) of CO-bright cold molecular gas, in contrast with the much larger estimate of the cloud's true molecular mass (M$_{\rm{H}_2}\approx3.4\times 10^3$\,M$_\odot$), indicating most of the cloud is CO-dark. Combining observational data with novel analytical models and simulations, we predict this cloud will photoevaporate in 5.7 million years, placing key constraints on the role of stellar feedback in shaping the closest star-forming regions to the Sun.

astro-ph.GA↗

arXiv2020

Space Missions for Astronomy and Astrophysics in Korea: Past, Present, and Future

We review the history of space mission in Korea focusing on the field of astronomy and astrophysics. For each mission, scientific motivation and achievement are reviewed together with some technical details of the program including mission schedule. This review includes the ongoing and currently approved missions as well as some planned ones. Within the admitted limitations of authors' perspectives, some comments on the future direction of space program for astronomy and astrophysics in Korea are made at the end of this review.

astro-ph.IM↗

arXiv2019

Global distribution of far-ultraviolet emissions from highly ionized gas in the Milky Way

We present all-sky maps of two major FUV cooling lines, C IV and O VI, of highly ionized gas to investigate the nature of the transition-temperature gas. From the extinction-corrected line intensities of C IV and O VI, we calculated the gas temperature and the emission measure of the transition-temperature gas assuming isothermal plasma in the collisional ionization equilibrium. The gas temperature was found to be more or less uniform throughout the Galaxy with a value of (1.89 $\pm$ 0.06) $\times$ $10^5$ K. The emission measure of the transition-temperature gas is described well by a disk-like model in which the scale height of the electron density is $z_0=6_{-2}^{+3}$ kpc. The total mass of the transition-temperature gas is estimated to be approximately $6.4_{-2.8}^{+5.2}\times10^9 M_{\bigodot}$. We also calculated the volume-filling fraction of the transition-temperature gas, which was estimated to be $f=0.26\pm0.09$, and varies from $f\sim0.37$ in the inner Galaxy to $f\sim0.18$ in the outer Galaxy. The spatial distribution of C IV and O VI cannot be explained by a simple supernova remnant model or a three-phase model. The combined effects of supernova remnants and turbulent mixing layers can explain the intensity ratio of C IV and O VI. Thermal conduction front models and high-velocity cloud models are also consistent with our observation.

astro-ph.GA↗

arXiv2018

MIRIS Paα Galactic Plane Survey. I. Comparison with IPHAS Hα in l = 96°-116°

The Multipurpose InfraRed Imaging System (MIRIS) performed the MIRIS Paα Galactic Plane Survey (MIPAPS), which covers the entire Galactic plane within the latitude range of -3° < b < +3° at Paα (1.87 um). We present the first result of the MIPAPS data extracted from the longitude range of l = 96.5°-116.3°, and demonstrate the data quality and scientific potential of the data by comparing them with Hα maps obtained from the INT Photometric Hα Survey (IPHAS) data. We newly identify 90 H II region candidates in the WISE H II region catalog as definite H II regions by detecting the Paα and/or Hα recombination lines, out of which 53 H II regions are detected at Paα. We also report the detection of additional 29 extended and 18 point-like sources at Paα. We estimate the E(B-V) color excesses and the total Lyman continuum luminosities for H II regions by combining the MIPAPS Paα and IPHAS Hα fluxes. The E(B-V) values are found to be systematically lower than those estimated from point stars associated with H II regions. Utilizing the MIPAPS Paα and IPHAS Hα images, we obtain an E(B-V) map for the entire region of the H II region Sh2-131 with an angular size of ~2.5°. The E(B-V) map shows not only numerous high-extinction filamentary features but also negative E(B-V) regions, indicating Hα excess. The Hα excess and the systematic underestimation of E(B-V) are attributed to light scattered by dust.

astro-ph.GA↗

arXiv2017

Far-ultraviolet fluorescent molecular hydrogen emission map of the Milky Way Galaxy

We present the far-ultraviolet (FUV) fluorescent molecular hydrogen (H_2) emission map of the Milky Way Galaxy obtained with FIMS/SPEAR covering ~76% of the sky. The extinction-corrected intensity of the fluorescent H_2 emission has a strong linear correlation with the well-known tracers of the cold interstellar medium (ISM), including color excess E(B-V), neutral hydrogen column density N(H I), and H_alpha emission. The all-sky H_2 column density map was also obtained using a simple photodissociation region model and interstellar radiation fields derived from UV star catalogs. We estimated the fraction of H2 (f_H2) and the gas-to-dust ratio (GDR) of the diffuse ISM. The f_H2 gradually increases from <1% at optically thin regions where E(B-V) < 0.1 to ~50% for E(B-V) = 3. The estimated GDR is ~5.1 x 10^21 atoms cm^-2 mag^-1, in agreement with the standard value of 5.8 x 10^21 atoms cm^-2 mag^-1.

astro-ph.GA↗

arXiv2017

Long-term CCD Photometry and Physical Properties of the sdB+M Eclipsing System 2M 1533+3759

New CCD photometry of seven successive years from 2010 is presented for the HW Vir-type eclipsing binary 2M 1533+3759. Using the $VI$ light curves together with the radial-velocity data given by For et al. (2010), we determined the absolute parameters of each component to be $M_1$ = 0.442$\pm$0.012 M$_\odot$, $M_2$ = 0.124$\pm$0.005 M$_\odot$, $R_1$ = 0.172$\pm$0.002 R$_\odot$, $R_2$ = 0.157$\pm$0.002 R$_\odot$, $L_1$ = 19.4$\pm$1.4 L$_\odot$, and $L_2$ = 0.002$\pm$0.002 L$_\odot$. These indicate that 2M 1533+3759 is a detached system consisting of a normal sdB primary and an M7 dwarf companion. Detailed analyses of 377 minimum epochs, including our 111 timings, showed that the orbital period of the system remains constant during the past 12 yrs. Inspecting both types of minima, we found a delay of 3.9$\pm$1.0 s in the arrival times of the secondary eclipses relative to the primary eclipse times. This delay is in satisfactory agreement with the predicted Rømer delay of 2.7$\pm$1.4 s and the result is the second measurement in sdB+M eclipsing binaries. The time shift of the secondary eclipse can be explained by some combination of the Rømer delay and a non-zero eccentricity. Then, the binary star would have a very small eccentricity of $e \cos ω \simeq$ 0.0001.

astro-ph.SR↗

arXiv2016

Radiative transfer in disc galaxies $-$ V. The accuracy of the KB approximation

We investigate the accuracy of an approximate radiative transfer technique that was first proposed by Kylafis & Bahcall (hereafter the KB approximation) and has been popular in modelling dusty late-type galaxies. We compare realistic galaxy models calculated with the KB approximation with those of a three-dimensional Monte Carlo radiative transfer code SKIRT. The SKIRT code fully takes into account of the contribution of multiple scattering whereas the KB approximation calculates only single scattered intensity and multiple scattering components are approximated. We find that the KB approximation gives fairly accurate results if optically thin, face-on galaxies are considered. However, for highly inclined ($i \gtrsim 85^{\circ}$) and/or optically thick (central face-on optical depth $\gtrsim1$) galaxy models, the approximation can give rise to substantial errors, sometimes, up to $\gtrsim 40\%$. Moreover, it is also found that the KB approximation is not always physical, sometimes producing infinite intensities at lines of sight with high optical depth in edge-on galaxy models. There is no "simple recipe" to correct the errors of the KB approximation that is universally applicable to any galaxy models. Therefore, it is recommended that the full radiative transfer calculation be used, even though it's slower than the KB approximation.

astro-ph.GA↗

arXiv2015

Bright stars observed by FIMS/SPEAR

In this paper, we present a catalogue of the spectra of bright stars observed during the sky survey using the Far-Ultraviolet Imaging Spectrograph (FIMS), which was designed primarily to observe diffuse emissions. By carefully eliminating the contamination from the diffuse background, we obtain the spectra of 70 bright stars observed for the first time with a spectral resolution of 2--3 Å over the wavelength of 1370--1710 Å. The far-ultraviolet spectra of an additional 139 stars are also extracted with a better spectral resolution and/or higher reliability than those of the previous observations. The stellar spectral type of the stars presented in the catalogue spans from O9 to A3. The method of spectral extraction of the bright stars is validated by comparing the spectra of 323 stars with those of the International Ultraviolet Explorer (IUE) observations.

astro-ph.GA↗

arXiv2015

Photometric defocus observations of transiting extrasolar planets

We have carried out photometric follow-up observations of bright transiting extrasolar planets using the CbNUOJ 0.6m telescope. We have tested the possibility of obtaining high photometric precision by applying the telescope defocus technique allowing the use of several hundred seconds in exposure time for a single measurement. We demonstrate that this technique is capable of obtaining a root-mean-square scatter of order sub-millimagnitude over several hours for a V $\sim$ 10 host star typical for transiting planets detected from ground-based survey facilities. We compare our results with transit observations with the telescope operated in in-focus mode. High photometric precision is obtained due to the collection of a larger amount of photons resulting in a higher signal compared to other random and systematic noise sources. Accurate telescope tracking is likely to further contribute to lowering systematic noise by probing the same pixels on the CCD. Furthermore, a longer exposure time helps reducing the effect of scintillation noise which otherwise has a significant effect for small-aperture telescopes operated in in-focus mode. Finally we present the results of modelling four light-curves for which a root-mean-square scatter of 0.70 to 2.3 milli-magnitudes have been achieved.

astro-ph.SR↗

arXiv2014

The Pulsating sdB+M Eclipsing System NY Virginis and its Circumbinary Planets

We searched for circumbinary planets orbiting NY Vir in historical eclipse times including our long-term CCD data. Sixty-eight times of minimum light with accuracies better than 10 s were used for the ephemeris computations. The best fit to those timings indicated that the orbital period of NY Vir has varied due to a combination of two sinusoids with periods of $P_3$=8.2 yr and $P_4$=27.0 yr and semi-amplitudes of $K_3$=6.9 s and $K_4$=27.3 s, respectively. The periodic variations most likely arise from a pair of light-time effects due to the presence of third and fourth bodies that are gravitationally bound to the eclipsing pair. We have derived the orbital parameters and the minimum masses, $M_3 \sin i_3$ = 2.8 M$\rm_{Jup}$ and $M_4 \sin i_4$ = 4.5 M$\rm_{Jup}$, of both objects. A dynamical analysis suggests that the outer companion is less likely to orbit the binary on a circular orbit. Instead we show that future timing data might push its eccentricity to moderate values for which the system exhibits long-term stability. The results demonstrate that NY Vir is probably a star-planet system, which consists of a very close binary star and two giant planets. The period ratio $P_{3}/P_{4}$ suggests that a long-term gravitational interaction between them would result in capture into a nearly 3:10 mean motion resonance. When the presence of the circumbinary planets is verified and understood more comprehensively, the formation and evolution of this planetary system should be advanced greatly.

astro-ph.SR↗

arXiv2014

Global Far-Ultraviolet Properties of the Cygnus Loop

We present the C III λ977, O VI λλ1032, 1038 and N IV] λ1486 emission line maps of the Cygnus Loop, obtained with the newly processed data of Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR; also known as FIMS) mission. In addition, the Si IV+O IV] line complexes around 1400 Å are resolved into two separate emission lines, whose intensity demonstrates a relatively high Si IV region predicted in the previous study. The morphological similarity between the O VI and X-ray images, as well as a comparison of the O VI intensity with the value expected from the X-ray results, indicates that large portions of the observed O VI emissions could be produced from X-ray emitting gas. Comparisons of the far-ultraviolet (FUV) images with the optical and H I 21 cm images, reveal spatial variations of shock-velocity populations and high FUV extinction in the direction of a previously identified H I cloud. By calculating the FUV line ratios for several subregions of the Cygnus Loop, we investigate the spatial variation of the population of radiative shock velocities; and the effects of resonance scattering, X-ray emitting gas, and non-radiative shocks. The FUV and X-ray luminosity comparisons between the Cygnus Loop and the Vela supernova remnant suggest that the fraction of shocks in the early evolutionary stages is much larger in the Cygnus Loop.

astro-ph.GA↗

arXiv2012

Far-Ultraviolet Spectral Images of the Vela Supernova Remnant: Supplements and Comparisons with other Wavelength Images

We present the improved far-ultraviolet (FUV) emission-line images of the entire Vela supernova remnant (SNR) using newly processed SPEAR/FIMS data. The incomplete C III λ977 and O VI λλ1032, 1038 images presented in the previous study are updated to cover the whole region. The C IV λλ1548, 1551 image with a higher resolution and new images at Si IV λλ1394, 1403, O IV] λ1404, He II λ1640.5, and O III] λλ1661, 1666 are also shown. Comparison of emission line ratios for two enhanced FUV regions reveals that the FUV emissions of the east enhanced FUV region may be affected by nonradiative shocks of another very young SNR, the Vela Jr. SNR (RX J0852.0-4622, G266.6-1.2). This result is the first FUV detection that is likely associated with the Vela Jr. SNR, supporting previous arguments that the Vela Jr. SNR is close to us. The comparison of the improved FUV images with soft X-ray images shows that a FUV filamentary feature forms the boundary of the northeast-southwest asymmetrical sections of the X-ray shell. The southwest FUV features are characterized as the region where the Vela SNR is interacting with slightly denser ambient medium within the dim X-ray southwest section. From a comparison with the Hα image, we identify a ring-like Hα feature overlapped with an extended hot X-ray feature of similar size and two local peaks of C IV emission. Their morphologies are expected when the Hα ring is in direct contact with the near or far side of the Vela SNR.

astro-ph.GA↗

arXiv2011

Comparison of the Diffuse Halpha and FUV Continuum Backgrounds: On the Origins of the Diffuse Halpha Background

We compare the diffuse H$α$ map of our Galaxy with the FUV (1370-1710Å) continuum map. The H$α$ intensity correlates well with the FUV intensity. The H$α$/FUV intensity ratio increases in general with the H$α$ intensity and the FUV hardness ratio (1370-1520Å to 1560-1710Å), implying that late OB stars may be the main source of the H$α$ recombination line at high latitudes. The variation of the H$α$ intensity as a function of the Galactic latitude is also very similar to that of the FUV intensity. The results likely suggest that not only the original radiation sources of the H$α$ and FUV backgrounds but also the radiative transfer mechanisms responsible for the diffuse backgrounds are largely common. Therefore, we propose a scenario wherein the H$α$ background at high latitudes is mostly composed of two components, H$α$ photons produced by in-situ recombination at the ionized regions around late OB stars and dust-scattered light of the H$α$ photons originating from late OB stars.

astro-ph.GA↗

arXiv2011

Implications of the SPEAR FUV Maps on Our Understanding of the ISM

The distribution of a low-density transition temperature (10^4.5 - 10^5.5 K) gas in the interstellar medium conveys the character and evolution of diffuse matter in the Galaxy. This difficult to observe component of the ISM emits mainly in the far-ultraviolet (FUV) (912-1800 Å) band. We describe spectral maps of FUV emission lines from the highly ionized species CIV and OVI likely to be the dominant cooling mechanisms of transition temperature gas in the ISM. The maps were obtained using an orbital spectrometer, SPEAR, that was launched in 2003 and has observed the FUV sky with a spectral resolution of \sim 550 and an angular resolution of 10'. We compare distribution of flux in these maps with three basic models of the distribution of transition temperature gas. We find that the median distribution of CIV and OVI emission is consistent with the spatial distribution and line ratios expected from a McKee-Ostriker (MO) type model of evaporative interfaces. However, the intensities are a factor of three higher than would be expected at the MO preferred parameters. Some high intensity regions are clearly associated with supernova remnants and superbubble structures. Others may indicate regions where gas is cooling through the transition temperature.

astro-ph.GA↗

arXiv2009

A wide-field photometric survey for extratidal tails around five metal-poor globular clusters in the Galactic halo

Wide-field deep gri images obtained with the Megacam of the Canada-France-Hawaii Telescope (CFHT) are used to investigate the spatial configuration of stars around five metal-poor globular cluster M15, M30, M53, NGC 5053, and NGC 5466, in a field-of-view ~3 degree. Applying a mask filtering algorithm to the color-magnitude diagrams of the observed stars, we sorted cluster's member star candidates that are used to examine the characteristics of the spatial stellar distribution surrounding the target clusters. The smoothed surface density maps and the overlaid isodensity contours indicate that all of the five metal-poor globular clusters exhibit strong evidence of extratidal overdensity features over their tidal radii, in the form of extended tidal tails around the clusters. The orientations of the observed extratidal features show signatures of tidal tails tracing the clusters' orbits, inferred from their proper motions, and effects of dynamical interactions with the Galaxy. Our findings include detections of a tidal bridge-like feature and an envelope structure around the pair of globular clusters M53 and NGC 5053. The observed radial surface density profiles of target clusters have a deviation from theoretical King models, for which the profiles show a break at 0.5~0.7r_t, extending the overdensity features out to 1.5~2r_t. Both radial surface density profiles for different angular sections and azimuthal number density profiles confirm the overdensity features of tidal tails around the five metal-poor globular clusters. Our results add further observational evidence that the observed metal-poor halo globular clusters originate from an accreted satellite system, indicative of the merging scenario of the formation of the Galactic halo.

astro-ph.GA↗

arXiv2009

Long-term Photometric Behavior of the Eclipsing Binary GW Cephei

New CCD photometry during 4 successive years from 2005 is presented for the eclipsing binary GW Cep, together with reasonable explanations for the light and period variations. All historical light curves, obtained over a 30-year interval, display striking light changes, and are best modeled by the simultaneous existence of a cool spot and a hot spot on the more massive cool component star. The facts that the system is magnetically active and that the hot spot has consistently existed on the inner hemisphere of the star indicate that the two spots are formed by (1) magnetic dynamo-related activity on the cool star and (2) mass transfer from the primary to the secondary component. Based on 38 light-curve timings from the Wilson-Devinney code and all other minimum epochs, a period study of GW Cep reveals that the orbital period has experienced a sinusoidal variation with a period and semi-amplitude of 32.6 yrs and 0.009 d, respectively. In principle, these may be produced either by a light-travel-time effect due to a third body or by an active magnetic cycle of at least one component star. Because we failed to find any connection between luminosity variability and the period change, that change most likely arises from the existence of an unseen third companion star with a minimum mass of 0.22 $M_\odot$ gravitationally bound to the eclipsing pair.

astro-ph.SR↗

arXiv2007

Far-Ultraviolet Cooling Features of the Antlia Supernova Remnant

We present far-ultraviolet observations of the Antlia supernova remnant obtained with Far-ultraviolet IMaging Spectrograph (FIMS, also called SPEAR). The strongest lines observed are C IV 1548,1551 and C III 977. The C IV emission of this mixed-morphology supernova remnant shows a clumpy distribution, and the line intensity is nearly constant with radius. The C III 977 line, though too weak to be mapped over the whole remnant, is shown to vary radially. The line intensity peaks at about half the radius, and drops at the edge of the remnant. Both the clumpy distribution of C IV and the rise in the C IV to C III ratio towards the edge suggest that central emission is from evaporating cloudlets rather than thermal conduction in a more uniform, dense medium.

astro-ph↗

arXiv2006

Diffuse Far-ultraviolet Observation of the Lupus Loop Region

Diffuse far-ultraviolet (FUV) emissions from the Lupus Loop region (SNR 330.0+15.0) have been observed with Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR), also known as Far-ultraviolet IMaging Spectrograph (FIMS). We have detected several important ionic lines, including Si II*, C IV, and N IV], which characterize the warm, hot ionized gas in this region. The spatial variations in the line intensities of Si II* and C IV have also been studied in comparison with X-ray and dust observations. The result shows that they originate from the interface between the hot gas seen in the X-ray and the cooler H I shell with which dust is associated. The interface is rather diffuse, and the gases with different temperatures seem to co-exist in this region. A shock may exist upfront in the interface, but its velocity should be very small as no shock-related distinguished feature is seen in Ha.

astro-ph↗