Young Woon Kang

CHEMICAL INHOMOGENEITY IN RED GIANT BRANCH STARS AND RR LYRAE VARIABLES IN NGC 1851: TWO SUBPOPULATIONS IN RED GIANT BRANCH*

We investigate the red giant branch (RGB) subpopulations of NGC 1851 using Ca uvby photometry. Our color-magnitude diagrams show that the RGB stars have two subpopulations and the RGB stars in NGC 1851 appear to have distinct elemental abundance patterns with the [Ca/H] abundance. We discuss that the elemental abundance patterns can be explained by the contributions from the asymptotic giant stars, confirming the previous studies by others. The RR Lyrae variables in NGC 1851 appear to have a large metallicity spread and, perhaps, a bimodal metallicity distribution. Our period shift analysis of the RR Lyrae variables shows that the helium enhancement appears to be insignificant in the NGC 1851 RR Lyrae population. However, the helium enrichment scenario in the blue or the red parts of horizontal branch (HB) cannot be completely ruled out. Our results show that about 18% of the bright RGB stars have enhanced CNO abundances, sharply in contrast to previous estimates by others (38%), making it difficult to explain the double subgiant, RGB sequences, and the bimodal HB distribution by their number ratios.

The Chromospherically Active Contact Binary CE Leonis

We present complete VRI light curves of the contact binary CE Leo and provide detailed photometric solutions, as well as an analysis of its period variation. A total of 73 times of minima over 50 years, including our observations, were used for the period study. The complex period variation can be sorted into a linear period improvement, a period of P ¼ 0:30342771 days, a secular period increase at the rate of ˙ P=P ¼þ 3:05 ;10 � 7 days yr � 1 ,a nd a2 2:6 � 0:5 yr periodic component. The � 22.6 yr periodic variation in the OC residuals most likely arises from the light-travel time effect from a low-mass (m3 � 0:3Msini) dM tertiary companion moving in an eccentric (e 0 ¼ 0:61 � 0:04) orbit. However, it is also possible that this periodic variation arises from the effects of a magnetic activity cycle, known as the Applegate mechanism. The light curves show asymmetries in the two different maxima. The 1998 light curves show that primary maximunm was 0.042, 0.038, and 0.038 mag brighter than secondary maximum in V, R ,a ndI, respectively. The durations of totality for three epochs were 18.3 minutes in 1989, 19.7 minutes in 1998, and 18.8 minutes in 2001. The relative depth of primary minimum was found to vary between 1.20 and 1.32 mag. We have analyzed the light curves from the three epochs using the 1993 version of the Wilson-Devinney differential corrections computer code to find a unique solution for CE Leo. The corresponding spectral type of the secondary star from the colors and effective temperature isdK2. The asymmetric light curves can be explained by the effects of starspots. We find that the light curves are best fitted by employing a cool spot on the cooler, larger component of the system. The spot-effect parameter SE ¼ 0:05 found for CE Leo is relatively large for W UMa binaries.

PROBING THE SPATIAL DISTRIBUTION OF EXTRASOLAR PLANETS WITH GRAVITATIONAL MICROLENSING

To search for extrasolar planets, current microlensing follow-up experiments are monitoring events caused by stellar-mass lenses, hoping to detect the planets signature of the short-duration perturbation in the smooth lensing light curve of the primary. According to this strategy, however, it is possible to detect only planets located within a narrow region of separations from central stars. As a result, even if a large sample of planets are detected and the separations from their central stars are determined, it will be difficult to draw meaningful result about the spatial distribution of planets. An additional channel of microlensing planet detection is provided if the monitoring frequency of survey experiments is dramatically increased. From high-frequency monitoring experiments, such as the recently proposed Galactic Exoplanet Survey Telescope mission, one can detect two additional populations of planets, which are free-floating planets and bound planets with wide orbits around central stars. In this paper, we investigate the lensing properties of events caused by wide-orbit planets and find that the light curves of a significant fraction of these events will exhibit signatures of central stars, enabling one to distinguish them from events caused by free-floating planets. Because of the large primary/planet mass ratio, the effect of the central star endures to considerable separations. We find that for a Jupiter-mass planet the signatures of the central star can be detected with fractional deviations of ?5% from the best-fitting single-lens light curves for 80% of events caused by bound planets with separations 10 AU, and the probability is still substantial for planets with separations up to ~20 AU. Therefore, detecting a large sample of these events will provide useful information about the distribution of extrasolar planets around their central stars. Proper estimation of the probability of distinguishing wide-orbit and free-floating planets will also be important for the correct determination of the frequency of free-floating planets, whose microlensing sample will be contaminated by wide-orbit planets.

Raman-scattered He II λ6545 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 Hα and a broad emission feature around 6545 A that is blended with the [N II] λ6548 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 Lyβ in the former case and the He II λ1025 emission line arising from n = 6 → n = 2 transitions for the latter feature. We remove the Hα wings by a template Raman scattering wing profile and subtract the [N II] λ6548 line using the 3 times stronger [N II] λ6583 feature in order to isolate the He II Raman-scattered 6545 A line. We obtain the flux ratio F6545/F6560 = 0.24 of the He II λ6560 emission line and the 6545 A feature for V1016 Cyg. Under the assumption that the He II emission from this object is isotropic, this ratio is converted to the ratio Φ6545/Φ1025 = 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 ≥ 1020 cm-2 covers 17% of the emission region. By combining the presumed binary period of ~100 yr for 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. It is argued that the Raman-scattered He II λ6545 line is an important and useful tool to investigate the mass-loss process occurring in the late stage of stellar evolution.

Radioactive elements in stellar atmospheres

The identification of lines of radioactive elements (Tc, Pm and elements with 83<Z<100) in the spectra of chemically peculiar stars HD101065, HR465, HD965 is made. Three possible explanations are proposed: natural radioactive decay of Th and U in the upper levels of stellar atmospheres, contamination of stellar atmosphere by recent SN explosion, and spallation reactions.

A Photometric Study of EB-type eclipsing binary GO CYG

Two colour (B and V) photoelectric observations of EB-type eclipsing binary GO Cyg were carried out for 15 nights during October and December 1996. Three new times of minimum lights have been derived. With these and previously published times of minimum lights, a continuous period increase of dP/dt = 1.51 × 10-7 d yr-1 was estimated from the quadratic light element. B and V light curves have been analyzed by the method of the Wilson and Devinney Differential Correction. GO Cyg is confirmed to belong to the group of near contact systems based on light curve analysis and its absolute dimensions.

Photoelectric observations of long-period RS CVn binary CQ aurigae

Long-period RS CVn-type binary CQ Aur was observed in 1987 and 1988.B andV light curves for the 1987 and 1988 epochs have been analyzed for the photometric solution by the modified Wilson and Devinney computing code with adjustable spot parameters. One-spot fit is good enough for the 1987 epoch while a wider distribution of spots in longitude is required for the 1988 epoch. The spot covers approximately 3% of total surface area of the cooler component. Absolute dimensions were deduced by combining Poppers spectroscopic results. The volume of the cooler component is approximately 100 times larger than that of the hotter component. Both components are subgiants but the cooler component evolved more than the hotter component.

Astrometric Detection of Double Gravitational Microlensing Events

If a gravitational microlensing event is caused by a widely separated binary lens and the source approaches both lens components, the source flux is successively magnified by the individual lenses, resulting in double microlensing events. If events are observed astrometrically, double lensing events are expected to occur with an increased frequency due to the long-range astrometric effect of the companion. We find that although the trajectory of the source star image centroid shifts of an astrometric double lensing event has a distorted shape from both of the elliptical ones induced by the individual single-lens components, event duplication can be identified readily by the characteristic loop in the trajectory formed during the sources passage close to the companion. We determine and compare the probabilities of detecting double lensing events from both photometric and astrometric lensing observations by deriving analytic expressions for the relations between binary lensing parameters to become double lensing events. From this determination, we find that for a given set of the binary separation and the mass ratio, the astrometric probability is roughly an order higher than the photometric probability. Therefore, we predict that a significant fraction of events that will be followed up by using future high-precision interferometeric instruments will be identified as double lensing events.

Evidence of Chromospheric Activity on HR 1099

We present an observational evidence of the chromospheric activity on HR1099. The IUE spectra of HR 1099 were used for the ultraviolet photometry and for the intensity variation of chromospheric emission line. Ultraviolet light curves were made from the low and high dispersion IUE spectra and were compared with a optical light curve. We have analyzed the ultraviolet and optical light curves by the Wilson and Devinney computing code including dark spots. The intensity variation of Mg II emission line depends on the orbital phase. The maximum intensity of Mg II emission line occurs at the phase of light minimum where dark spot visibility is maximum due to strong chromospheric activity.

Simultaneous solutions for photometric and spectroscopic observations of TX UMa

B andV light curves for one epoch and radial velocity curves of three different epochs have been analyzed to revise the solution of TX UMa. The solution has been adjusted simultaneously in the light curves and radial velocity curves by the method of Wilson and Devinneys differential correction. The primary stars surface rotation rate to synchronous rate is determined as 1.768 from one of the radial velocity curves. The absolute dimension of the system has been deduced based on the simultaneous solution. The primary star is well fitted to the evolutionary track for a single star while the secondary star, while filling its Roche lobe, is fitted to the evolutionary track for a close binary system.