arXiv subjects

Heon-Young Chang

Publications and source records attributed to Heon-Young Chang.

At least 19 recordsLinked to original sources

arXiv2026

A Search for Exoplanets around Northern Circumpolar Stars X. The origin of radial velocity variations in the evolved star HD 216595

Detecting planetary companions around evolved stars, particularly asymptotic giant branch (AGB) stars, is challenging due to intrinsic stellar variability such as surface convection, pulsations, and mass loss, which can produce radial velocity (RV) signals that mimic Keplerian motion. We investigate the origin of long-period, low-amplitude RV variations observed in the AGB star HD 216595 based on high-resolution spectroscopic data spanning approximately 16 years obtained with the Bohyunsan Optical Astronomy Observatory Echelle Spectrograph (BOES) and the Las Cumbres Observatory Network of Robotic Echelle Spectrographs (NRES) instruments. The RV measurements reveal a statistically significant periodic signal at 567 days that can be described by a Keplerian model consistent with a substellar companion. However, no strong correlations are found between the RV variations and stellar activity indicators, including line bisectors, chromospheric activity, and photometric variability, although weak signals at similar timescales are present in some diagnostics. Given the stellar properties of HD 216595 and similarities to previously reported cases, the observed RV variations are likely related to intrinsic stellar processes, although a companion-induced origin cannot be definitively ruled out. Further progress will require improved diagnostics and more sophisticated modeling to disentangle stellar variability from genuine orbital signals.

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arXiv2023

Long-period radial velocity variations of nine M red giants: The detection of sub-stellar companions around HD 6860 and HD 112300

Context. Certain periodic variations of radial velocities (RV) of wobbling giants originate from exoplanets. Indeed, a number of exoplanets have been discovered around giant stars. Aims. The purpose of our study is to find low-amplitude and long-period RV variations around bright M (super) giants in the RGB (or AGB) stage, which are long-period variables (LPVs) or high-proper-motion (HPM) stars. Methods. High-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO) was used to record numerous spectra of nine giants. The observation period for the targets spans 16 years, from 2005 to 2022. Results. We found from the precise RV observations of nine M giants two sub-stellar companions, one with a 28.26$^{+2.05}_{-2.17}$ $M_{J}$ orbiting period of 663.87$^{+4.61}_{-4.31}$ days at a distance of 2.03$^{+0.01}_{-0.01}$ AU (HD 6860) and the other, with a 15.83$^{+2.33}_{-2.74}$ $M_{J}$ orbiting period of 466.63 $^{+1.47}_{-1.28}$ days at a distance of 1.33 $^{+0.08}_{-0.11}$ AU (HD 112300). Our estimate of the stellar parameters for HD 6860 makes it currently the largest star with a sub-stellar companion. We also found RV variations mimicking a planetary companion in HD 18884 and confirmed LPVs in two stars, HD 39801 and HD 42995. The RV variations of some stars seem to be associated with stellar activities rather than reflex orbital motion due to their companions. Such variations are also detected even for HD 6860 and HD 112300, hosting sub-stellar companions.

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arXiv2016

Classifying Gamma-Ray Bursts with Gaussian Mixture Model

Using Gaussian Mixture Model (GMM) and Expectation Maximization Algorithm, we perform an analysis of time duration ($T_{90}$) for \textit{CGRO}/BATSE, \textit{Swift}/BAT and \textit{Fermi}/GBM Gamma-Ray Bursts. The $T_{90}$ distributions of 298 redshift-known \textit{Swift}/BAT GRBs have also been studied in both observer and rest frames. Bayesian Information Criterion has been used to compare between different GMM models. We find that two Gaussian components are better to describe the \textit{CGRO}/BATSE and \textit{Fermi}/GBM GRBs in the observer frame. Also, we caution that two groups are expected for the \textit{Swift}/BAT bursts in the rest frame, which is consistent with some previous results. However, \textit{Swift} GRBs in the observer frame seem to show a trimodal distribution, of which the superficial intermediate class may result from the selection effect of \textit{Swift}/BAT.

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arXiv2015

Radio Afterglows and Host Galaxies of Gamma-Ray Bursts

Considering the contribution of the emission from the host galaxies of gamma-ray bursts (GRBs) to the radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of events, e.g. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of host galaxy to the total radio peak emission and the observational frequency. Especially, toward lower frequencies, the contribution from the host increases significantly. The correlation can be used to get a useful estimate for the radio brightness of those host galaxies which only have very limited radio afterglow data. Using this prediction, we re-considered the theoretical radio afterglow light curves for four kinds of events, i.e. high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from the host galaxies and aiming at exploring the detectability of these events by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of $z=1$, most of the events can be detected by FAST easily. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them.

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arXiv2013

Microlensing by a wide-separation planet: detectability and boundness

We explore the detection condition of a wide-separation planet through the perturbation induced by the planetary caustic for various microlensing parameters, especially for the size of the source stars. By constructing the fractional deviation maps at various positions in the space of microlensing parameters, we find that the pattern of the fractional deviation depends on the ratio of the source radius to the caustic size, and the ratio satisfying the observational threshold varies with the star-planet separation. We have also obtained the upper limits of the source size that allows the detection of the signature of the host star as a function of the separation for given observational threshold. It is shown that this relation further leads one to a simple analytic condition for the star-planet separation to detect the boundness of wide-separation planets as a function of the mass ratio and the source radius. For example, when 5% of the detection threshold is assumed, for a source star with the radius of ~1 R_sun, an Earth-mass planet and a Jupiter-mass planet can be recognized of its boundness when it is within the separation range of ~10 AU and ~30 AU, respectively. We also compare the separation ranges of detection by the planetary caustic with those by the central caustic. It is found that when the microlensing light curve caused by the planetary caustic happens to be analyzed, one may afford to support the boundness of the wide-separation planet farther than when that caused by the central caustic is analyzed. Finally, we conclude by briefly discussing the implication of our findings on the next-generation microlensing experiments.

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arXiv2011

Bimodal Distribution of Area-Weighted Latitude of Sunspots And Solar North-South Asymmetry

We study the latitudinal distribution of sunspots observed from 1874 to 2009 using the center-of-latitude (COL). We calculate COL by taking the area-weighted mean latitude of sunspots for each calendar month. We then form the latitudinal distribution of COL for the sunspots appearing in the northern and southern hemispheres separately, and in both hemispheres with unsigned and signed latitudes, respectively. We repeat the analysis with subsets which are divided based on the criterion of which hemisphere is dominant for a given solar cycle. Our primary findings are as follows: (1) COL is not monotonically decreasing with time in each cycle. Small humps can be seen (or short plateaus) around every solar maxima. (2) The distribution of COL resulting from each hemisphere is bimodal, which can well be represented by the double Gaussian function. (3) As far as the primary component of the double Gaussian function is concerned, for a given data subset, the distributions due to the sunspots appearing in two different hemispheres are alike. Regardless of which hemisphere is magnetically dominant, the primary component of the double Gaussian function seems relatively unchanged. (4) When the northern (southern) hemisphere is dominant the width of the secondary component of the double Gaussian function in the northern (southern) hemisphere case is about twice as wide as that in the southern (northern) hemisphere. (5) For the distribution of the COL averaged with signed latitude, whose distribution is basically described by a single Gaussian function, it is shifted to the positive (negative) side when the northern (southern) hemisphere is dominant. Finally, we conclude by briefly discussing the implications of these findings on the variations in the solar activity.

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arXiv2011

Changes in Sea-Level Pressure over South Korea Associated with High-Speed Solar Wind Events

We explore a possibility that the daily sea-level pressure (SLP) over South Korea responds to the high-speed solar wind event. This is of interest in two aspects: First, if there is a statistical association this can be another piece of evidence showing that various meteorological observables indeed respond to variations in the interplanetary environment. Second, this can be a very crucial observational constraint since most models proposed so far are expected to preferentially work in higher latitude regions than the low latitude region studied here. We have examined daily solar wind speed ${\rm V}$, daily SLP difference ${\rm ΔSLP}$, and daily ${\rm \log(BV^{2})}$ using the superposed epoch analysis in which the key date is set such that the daily solar wind speed exceeds 800 ${\rm kms^{-1}}$. We find that the daily ${\rm ΔSLP}$ averaged out of 12 events reaches its peak at day +1 and gradually decreases back to its normal level. The amount of positive deviation of ${\rm ΔSLP}$ is +2.5 hPa. The duration of deviation is a few days. We also find that ${\rm ΔSLP}$ is well correlated with both the speed of solar wind and ${\rm \log(BV^{2})}$. The obtained linear correlation coefficients and chance probabilities with one-day lag for two cases are $r \simeq 0.81$ with $P> 99.9%$, and $r \simeq 0.84$ with $P> 99.9%$, respectively. We conclude by briefly discussing future direction to pursue.

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arXiv2011

Dependence of GCRs influx on the Solar North-South Asymmetry

We investigate the dependence of the amount of the observed galactic cosmic ray (GCR) influx on the solar North-South asymmetry using the neutron count rates obtained from four stations and sunspot data in archives spanning six solar cycles from 1953 to 2008. We find that the observed GCR influxes at Moscow, Kiel, Climax and Huancayo stations are more suppressed when the solar activity in the southern hemisphere is dominant compared with when the solar activity in the northern hemisphere is dominant. Its reduction rates at four stations are all larger than those of the suppression due to other factors including the solar polarity effect on the GCR influx. We perform the student's t-test to see how significant these suppressions are. It is found that suppressions due to the solar North-South asymmetry as well as the solar polarity are significant and yet the suppressions associated with the former are larger and more significant.

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arXiv2010

Detection Probability of a Low-Mass Planet for Triple Lens Events: Implication of Properties of Binary-Lens Superposition

In view of the assumption that any planetary system is likely to be composed of more than one planet, and the multiple planet system with a large mass planet has more chance of detailed follow-up observations, the multiple planet system may be an efficient way to search for sub-Jovian planets. We compare the magnification pattern of the triple lens system with that of a best-fitted binary system composed of a star and a Jovian mass planet, and check the probability in detecting the low-mass secondary planet whose signature will be superposed on that of the primary Jovian mass planet. Detection probabilities of the low-mass planet in the triple lens system are quite similar to the probability in detecting such a low-mass planet in a binary system with a star and only a low-mass planet, which shows that the signature of a low-mass planet can be effectively detected even when it is concurrent with the signature of the more massive planet, implying that the binary superposition approximation works over a relatively broad range of planet mass ratio and separations, and the inaccuracies thereof do not significantly affect the detection probability of the lower mass secondary planet. Since the signature of the Jovian mass planet will be larger and lasts longer, thereby warranting more intensive follow-up observations, the actual detection rate of the low-mass planet in a triple system with a Jovian mass can be significantly higher than that in a binary system with a low-mass planet only. We conclude that it may be worthwhile to develop an efficient algorithm to search for `super-Earth' planets in the paradigm of the triple lens model for high-magnification microlensing events.

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arXiv2008

Microlensing Optical Depth Revisited with Recent Star Counts

More reliable constraints on the microlensing optical depth comes from a better understanding of the Galactic model. Based on well-constrained Galactic bulge and disk models constructed from survey observations, such as, HST, 2MASS, and SDSS, we calculate the microlensing optical depths toward the Galactic bulge fields, and compare them with recent results of microlensing surveys. We test chi^2 statistics of microlensing optical depths expected from those models, as well as previously proposed models, using two types of data: optical depth map in (l, b) and averaged optical depth over the Galactic longitude l as a function of the latitude b. From this analysis, we find that the Galactic bulge models of 2MASS, Han & Gould (2003), and G2 of Stanek et al. (1997) show a good agreement with the microlensing optical depth profiles for all the microlensing observations, compared with E2 of Stanek et al. (1997). We find, on the other hand, that models involving an SDSS disk model produce relatively higher chi^2 values. It should be noted that modeled microlensing optical depths diverge in the low Galactic latitude, |b| < 2 arcdeg. Therefore, we suggest the microlensing observation toward much closer to central regions of the Galaxy to further test the proposed Galactic models, if it is more technically feasible than waiting for large data set of microlensing events.

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arXiv2008

Limits of Binaries That Can Be Characterized by Gravitational Microlensing

Due to the high efficiency of planet detections, current microlensing planet searches focus on high-magnification events. High-magnification events are sensitive to remote binary companions as well and thus a sample of wide-separation binaries are expected to be collected as a byproduct. In this paper, we show that characterizing binaries for a portion of this sample will be difficult due to the degeneracy of the binary-lensing parameters. This degeneracy arises because the perturbation induced by the binary companion is well approximated by the Chang-Refsdal lensing for binaries with separations greater than a certain limit. For binaries composed of equal mass lenses, we find that the lens binarity can be noticed up to the separations of $\sim 60$ times of the Einstein radius corresponding to the mass of each lens. Among these binaries, however, we find that the lensing parameters can be determined only for a portion of binaries with separations less than $\sim 20$ times of the Einstein radius.

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arXiv2006

Determination of Stellar Ellipticities in Future Microlensing Surveys

We propose a method that can determine the ellipticities of source stars of microlensing events produced by binary lenses. The method is based on the fact that the products of the caustic-crossing timescale, $Δt$, and the cosine of the caustic incidence angle of the source trajectory, $κ$, of the individual caustic crossings are different for events involving an elliptical source, while the products are the same for events associated with a circular source. The product $Δt_\perp =Δt \cosκ$ corresponds to the caustic-crossing timescale when the incidence angle of the source trajectory is $κ=0$. For the unique determination of the source ellipticity, resolutions of at least three caustic crossings are required. Although this requirement is difficult to achieve under the current observational setup based on alert/follow-up mode, it will be possible with the advent of future lensing experiments that will survey wide fields continuously at high cadence. For typical Galactic bulge events, the difference in $Δt_\perp$ between caustic crossings is of the order of minutes depending on the source orientations and ellipticities. Considering the monitoring frequency of the future lensing surveys of $\sim 6$ times/hr and the improved photometry especially of the proposed space-based survey, we predict that ellipticity determinations by the proposed method will be possible for a significant fraction of multiple caustic-crossing binary lens events involving source stars having non-negligible ellipticities.

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arXiv2003

Radio Variability of the Sagittarius $ {\rm A^*}$ due to an Orbiting Star

Recently, unprecedentedly accurate data on the orbital motion of stars in the vicinity of the Sagittarius $ {\rm A^*}$ have been available. Such information can be used not only to constrain the mass of the supermassive black hole (SMBH) in the Galactic center but also to study the source of the radio emission. Two major competing explanations of the radio spectrum of the Sagittarius $ {\rm A^*}$ are based on two different models, that is, hot accretion disk and jet. Hence, independent observational constraints are required to resolve related issues. We explore the possibility of using the observational data of the star S2, currently closest to the Galactic center, to distinguish physical models for the radio emission of the Sagittarius $ {\rm A^*}$, by applying the stellar cooling model to the Sagittarius $ {\rm A^*}$ with the orbital parameters derived from the observation. The relative difference in the electron temperature due to the stellar cooling from the star S2 at the pericenter is a few parts of a thousand and the consequent relative radio luminosity difference is order of $10^{-4}$. Therefore, one could possibly expect to observe the radio flux variation with a periodic or quasi-periodic modulation in the frequency range at $ν\la {\rm 100 MHz}$ if the radiatively inefficient hot accretion flows are indeed responsible for the radio emission, contrary to the case of a jet. According to our findings, even though no periodic radio flux variations have been reported up to date a radiatively inefficient hot accretion disk model cannot be conclusively ruled out.

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arXiv2003

Probing Structures of Distant Extrasolar Planets with Microlensing

Planetary companions to the source stars of a caustic-crossing binary microlensing events can be detected via the deviation from the parent light curves created when the caustic magnifies the star light reflecting off the atmosphere or surface of the planets. The magnitude of the deviation is delta_p e_p rho_p^{-1/2}, where e_p is the fraction of starlight reflected by the planet and rho_p is the angular radius of the planet in units of angular Einstein ring radius. Due to the extraordinarily high resolution achieved during the caustic crossing, the detailed shapes of these perturbations are sensitive to fine structures on and around the planets. We consider the signatures of rings, satellites, and atmospheric features on caustic-crossing microlensing light curves. We find that, for reasonable assumptions, rings produce deviations of order 10% delta_p, whereas satellites, spots, and zonal bands produce deviations of order 1% delta_p. We consider the detectability of these features using current and future telescopes, and find that, with very large apertures (>30m), ring systems may be detectable, whereas spots, satellites, and zonal bands will generally be difficult to detect. We also present a short discussion of the stability of rings around close-in planets, noting that rings are likely to be lost to Poynting-Robertson drag on a timescale of order 10^5 years, unless they are composed of large (>>1 cm) particles, or are stabilized by satellites.

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arXiv2002

Direct Lens Imaging of Galactic Bulge Microlensing Events

Recently, from the Hubble Space Telescope (HST) images of one of the Large Magellanic Cloud (LMC) events taken 6.3 years after the original lensing measurement, Alcock et al. were able to directly image the lens. Although the first resolved lens was identified for an LMC event, much more numerous lenses are expected to be resolved for Galactic bulge events. In this paper, we estimate the fraction of Galactic bulge events whose lenses can be directly imaged under the assumption that all bulge events are caused by normal stars. For this determination, we compute the distribution of lens proper motions of the currently detected Galactic bulge events based on standard models of the geometrical and kinematical distributions of lenses and their mass function. We then apply realistic criteria for lens resolution, and the result is presented as a function of the time elapsed after an original lensing measurement, $Δt$. If followup observations are performed by using an instrument with a resolving power of θ_{PSF}=0.1'', which corresponds to that of HST equipped with the new Advanced Camera for Surveys, we estimate that lenses can be resolved for ~ 3% and 22% of disk-bulge events and for ~0.3% and 6% of bulge self-lensing events after $Δt=10$ and 20 years, respectively. The fraction increases substantially with the increase of the resolving power. If the instrument has a resolution of θ_{PSF}=0.05'', which can be achieved by the {\it Next Generation Space Telescope}, we estimate that lenses can be resolved for $\sim 22%$ and 45% of disk-bulge events and for ~6% and 23% of bulge self-lensing events after Δt=10 and 20 years, respectively.

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arXiv2002

Radio/X-Ray Luminosity Relation for X-Ray Bright Galactic Nuclei: Implications on Weighing Supermassive Black Hole

Optically thin and geometrically thick accretion flows are known to be responsible for the observed radio/X-ray luminosity relation of the X-ray bright galactic nuclei. It has also been suggested that supermassive black hole masses can be estimated from measurements of the core radio luminosity and the X-ray luminosity by using the advection-dominated accretion flow (ADAF) model. In this study we increase the number of data available by compiling the radio/X-ray fluxes and the mass in published literatures, and compare the observed ratio of the luminosities with predictions from various models of optically thin accretion flows. Semi-analytically derived relations of the luminosities are presented in cases of the standard ADAF model and modified ADAF models, in which a truncation of inner parts of the flows and winds causing a reduction of the infalling matter are included. We show that the observed relation can be used indeed to estimate the supermassive black hole mass, provided that properties of such accretion flows are known. Having investigated sensitivities of the method on modifications of the 'standard' ADAF model, we find that a general trend of model predictions from the 'standard' ADAF, the truncated ADAF and the 'windy' ADAF are somewhat indistinguishable. We also find, however, that the extreme case of the windy model is inconsistent with currently available observational data, unless microphysics parameters are to be substantially changed. High resolution radio observations, however, are required to avoid the contamination of non-disk components, such as, a jet component, which, otherwise, results in the over-estimated SMBH mass.

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arXiv2002

Spectral Behavior of Solar Oscillations Modulated by Magnetic Variation

The principal aim of observational helioseismology is to determine mode parameters of the solar oscillations as accurately as possible. Yet estimates of the mode parameters are subject to many sources of noise, including inevitable gaps in data, stochastic nature of excitation processes, interferences among modes. Another uncertainty in frequency determination results from a modulation of the oscillation frequency in itself. It is well known that variation in the mean strength of the solar magnetic field modulates the frequency. We consider effects of the solar magnetic field variation on the spectral behavior of the power spectrum using a simple model. We show that the solar magnetic field variation can cause unwanted sidelobes beside the main peak in the power spectrum, and consequently show that the effect of the frequency modulation due to the solar magnetic field variation can lead a bias in frequency determinations. This effect should be considered seriously particularly when the $ l \ne 0$ $p$-mode multiplets are analyzed to measure the rotational splitting since the separations could be comparable. In addition to a bias in frequency estimates, the line width is also likely to be overestimated due to a line broadening. We therefore suggest that determination of the mode parameters should be derived with due care considering the magnetic field variation when a long data set is analyzed. The frequency modulation should be taken into account in the analysis to make a solid conclusion on any subtle dependence of the mode parameters, such as, that on the solar cycle. We conclude by pointing out that a new method is required to accommodate the stochastic force and the phase variation.

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arXiv2002

Long-Term X-Ray Variabilities of the Seyfert Galaxy MCG-2-58-22 : Secular Flux Decrease and Flares

We have studied the long-term X-ray light curve (2$-$10 keV) of the luminous Seyfert 1 galaxy MCG-2-58-22 by compiling data, from various X-ray satellites, which together cover more than 20 years. We have found two distinct types of time variations in the light curve. One is a gradual and secular decrease of the X-ray flux, and the other is the episodic increase of X-ray flux (or flare) by a factor of 2$-$4 compared with the level expected from the secular variation. We detected 3 such flares in total; a representative duration for the flares is $\sim $2 years, with intervening quiescent intervals lasting $\sim 6-8$ years. We discuss a few possible origins for these variabilities. Though a standard disk instability theory may explain the displayed time variability in the X-ray light curve, the subsequent accretions of stellar debris, from a tidal disruption event caused by a supermassive black hole in MCG-2-58-22, cannot be ruled out as an alternative explanation.

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