Yong-Gi Kim

Orbital and spin variability of the intermediate polar BG CMi

Results of a CCD study of the variability of the cataclysmic variable BG CMi obtained at the Korean 1.8 m telescope in 2002-2005 are presented. The multi-comparison star method had been applied for better accuracy estimates. The linear ephemeris based on 19 mean maxima for 2002-2005 is HJD 2453 105.31448(6)+ 0.01057257716(198)(E-764707). The period differs from that predicted by the quadratic ephemeris by Pych et al. (1996, AcA, 46, 279) leading to a possible cycle miscount. The statistically optimal ephemeris is a fourth-order polynomial, as a quadratic or even a cubic ephemeris leads to unaceptably large residuals: Min.HJD = 2445 (120.28095(28) +0.0105729609(57)E - 1.58(32) x 10 -13 E 2 - 5.81(64) x 10 -19 E 3 + 4.92(41) x 10 -25 E 4 . Thus the rate of the spin-up of the white dwarf is decreasing. An alternative explanation is that the spin-up has been stopped during recent years. The deviations between the amplitudes of the spin variability in V and R, as well as between phases are not statistically significant. However, the orbital light curves exhibit distinct difference; the corresponding color index shows a nearly sinusoidal shape with a maximum at orbital phase ∼0.2. The variations of the amplitude of spin waves shows a short maximum at the phase of the orbital dip. The corrected ephemeris for orbital minima is Min.HJD = 2448 368.7225(12) + 0.13474841(6) . (E - 24849) with a narrow dip occuring 0.07P later. The rate of the spin period variation seems to be changed, justifying the necessity of regular observations of intermediate polars.

Comparison of the Two Follow-up Observation Strategies for Gravitational Microlensing Planet Searches

There are two different strategies of follow-up observations for the detection of planets by using microlensing. One is detecting the light-curve anomalies affected by the planetary caustic from continuous monitoring of all events detected by microlensing survey programs (type I strategy), and the other is detecting anomalies near the peak amplification affected by the central caustic from intensive monitoring of high-amplification events (type II strategy). It was shown by Griest & Safizadeh that the type II strategy yields high planet detection efficiency per event. However, it is not known whether the planet detection rate by this strategy can make up a substantial fraction of the total rate. In this paper, we estimate the relative planet detection rates expected under the two follow-up observation strategies. From this estimation, we find that the rate under the type II strategy is substantial and will comprise ~- of the total rate. We also find that compared to the type I strategy the type II strategy is more efficient in detecting planets located outside of the lensing zone. We determine the optimal monitoring frequency of the type II strategy to be ~20 times per night, which can be easily achieved by the current microlensing follow-up programs even with a single telescope.

TWO-COLOR VR CCD PHOTOMETRY OF THE INTERMEDIATE POLAR `RXS J062518.2+733433

Results of 7 nights of CCD VR photometry of the intermediate polar 1RXS J062518.2+733433 obtained at the Korean 1.8m telescope are reported. The corrected ephemeris for the orbital minimum is BJD (Orb.min)

CCD PHOTOMETRY USING MULTIPLE COMPARISON STARS

The accuracy of CCD observations obtained at the Korean 1.8m telescope has been studied. Seventeen comparison stars in the vicinity of the cataclysmic variable BG CMi have been measured. The `artificial` star has been used instead of the `control` star, what made possible to increase accuracy estimates by a factor of 1.3-2.1 times for `good` and `cloudy` nights, respectively. The algorithm of iterative determination of accuracy and weights of few comparison stars contributing to the artificial star, has been presented. The accuracy estimates for 13-mag stars are around mag for exposure times of 30 sec.

Nova-like cataclysmic variable TT Arietis - QPO behaviour coming back from positive superhumps

Aims. We study the variability of the nova-like cataclysmic variable TT Ari, on time-scales of between minutes and months. Methods. The observations in the filter R were obtained at the 40-cm telescope of the Chungbuk National University (Korea), 51 observational runs cover 226 h. The table of individual observations is available electronically. In our analysis, we applied several methods: periodogram, wavelet, and scalegram analysis. Results. TT Ari remained in a “negative superhump” state after its return from the “positive superhump” state, which lasted for 8 years. The ephemeris for 12 of the best pronounced minima is Tmin = BJD 2 453 747.0700(47) + 0.132322(53)E. The phases of minima may reach 0.2, which reflects the non-eclipse nature of these minima. The quasi-periodic oscillations (QPO) are present with a mean “period” of 21.6 min and mean semi-amplitude of 36 mmag. This value is consistent with the range 15−25 min reported for previous “negative superhump” states and does not support the hypothesis of secular decrease in the QPO period. Either the period, or the semi-amplitude show significant night-to-night variations. According to the position at the two-parameter diagrams (i.e. diagrams of pairs of parameters: time, mean brightness of the system, brightness of the source of QPO, amplitude, and timescale of the QPOs), the interval of observations was divided into 5 parts, showing different characteristics: 1) the “pre-outburst” stage; 2) the “rise to outburst”; 3) “top of the outbursts”; 4) “post-outburst QPO” state; and 5) “slow brightening”. The the QPO source was significantly brighter during the 10-day outburst, than during the preceding interval. However, after the outburst, the large brightness of the QPO source still existed for about 30 days, producing the stage “4”. The diagram for mQPO (¯ m) exhibits two groups in the brightness range 10. m 6−10. 8, which correspond to larger and smaller amplitudes of the QPO. For the group “5” only, statistically significant correlations were found, for which, with increasing mean brightness, the period, amplitude, and brightness of the of QPO source also increase. The mean brightness at the “negative superhump state” varies within 10. 3−11. m 2, so the system is brighter than at the “positive superhump” (11. m 3), therefore the “negative superhump” phenomenon may be interpreted by a larger accretion rate. The system is an excellent laboratory for studying processes resulting in variations on timescales of between seconds and decades and needs further monitoring at various states of activity.

The secondary star and distance of the polar V1309 Orionis

Context. The first phase-resolved JHK light curves of the eclipsing polar (AM Herculis binary) V1309 Ori are presented and interpreted. Aims. We separate the contributions from the secondary star and from other sources with the aim of determining a photometric distance. Methods. Simple model calculations show that the accretion stream and the cyclotron source on the accreting white dwarf are minor contributors to the infrared light, allowing an accurate determination of spectral type and absolute flux of the secondary star. Results. The unilluminated backside of the secondary star as seen in eclipse has spectral type dM0 to dM0+. Its dereddened magnitude is

Photometric Defocus Observations of Transiting Extrasolar Planets

K=13.58

Idling magnetic white dwarf in the synchronizing polar BY Cam. The Noah-2 project

at orbital phase

TWO-COLOR VR CCD PHOTOMETRY OF OLD NOVA V603 AQUILAE

\phi=0

Development of an AutoFlat program for the acquisition of effective flat images in the automated observation system

(eclipse). Using the calibrated surface brightness of M-stars and the published mass of the secondary,