Showing papers by "Chung-Uk Lee published in 2008"

Microlensing Detections of Planets in Binary Stellar Systems

Abstract: We demonstrate that microlensing can be used for detecting planets in binary stellar systems. This is possible because in the geometry of planetary binary systems, in which the planet orbits one of the binary components and the other binary star is located at a large distance, both planet and secondary companion produce perturbations in a common region around the planet-hosting binary star, and thus the signatures of both planet and binary companion can be detected in the light curves of high-magnification lensing events. We find that identifying planets in binary systems is optimized when the secondary is located within a certain range that depends on the type of the planet. The proposed method can detect planets with masses down to one-tenth of the Jupiter mass in binaries with separations 100 AU. These ranges of planet masses and binary separations are not covered by other methods, and thus microlensing would be able to make the planetary binary sample richer.

A Photometric Study of the Short-Period Close Binary V432 Persei and its Implications for the Star's Evolution

Abstract: We present new multicolor charge-coupled device (CCD) photometry for the short-period close binary V432 Per, made on six nights between 2006 February and December. A period study of the system, based on all published and newly observed times of minimum light, reveals that the orbital period has varied as a beat effect due to the combination of a secular period increase and a sinusoidal variation, with a period of 35.3 yr and semi-amplitude of 0.0104 d. The continuous period increase can be interpreted as mass transfer from the less massive secondary star to the more massive primary component in the system with a rate of about 1.16 × 10–7 M ☉ yr–1. The sinusoidal component of the period variability could, in principle, be produced by either a light-time effect due to a third body or a magnetic modulation seated in at least one component star, but light variations do not conform to a prediction of the second mechanism. New light curves, showing a total eclipse at secondary minimum and a variable O'Connell effect, are best modeled by including a third light source and a cool spot on the primary star. The model also represents Agerer's light curves. Our results indicate V432 Per, at present, to be in a broken or a marginal contact stage, investing a hotter, more massive primary star with a spectral type of G4 and a cooler, less massive secondary star with a spectral type of G8–G9. We suggest that the most likely explanation of the sinusoidal variation is the existence of a gravitationally bound M-type tertiary companion. When all of this is verified and understood more comprehensively, the formation and evolution of this binary system should be greatly advanced.

A Long-term Photometric Study of the Near-contact Binary RU Ursae Minoris

Abstract: New CCD photometric observations of the near-contact binary RU UMi were obtained from1999 December to 2000 April and later during 2003 March. All available times of minimum light, including our own measurements, have been examined and indicate that the orbital period of the system has experienced a continuous period decrease with a rate of (dP/dt)obs = -1.6 × 10-8 d yr-1. This period decrease cannot be explained trivially by mass transfer from the more massive primary to the less massive secondary, because our light-curve synthesis indicates that RU UMi is in a semidetached configuration with the primary star filling 95% of its inner Roche lobe and the secondary filling its limiting lobe completely. We present the case that the observed period change may be produced by a combination of AML due to magnetic braking with a rate of (dP/dt)mb = -1.0 × 10-7 d yr-1 and of mass transfer from the secondary to the primary at a rate of 2 = -6.0 × 10-8 M⊙ yr-1. Historical light curves of RU UMi over a 32-yr interval display variably asymmetrical secondary minima. From analyses of velocity and light curves and of the light curves alone, the light variations are best represented by a variable cool spot that has consistently existed on the inner hemisphere of the secondary component. There is a weak indication that the hot star is no longer a main sequence one.

Photometric Properties of the Near-contact Binary GW Geminorum

Abstract: New multiband CCD photometry is presented for the eclipsing binary GW Gem; the $RI$ light curves are the first ever compiled. Four new minimum timings have been determined. Our analysis of eclipse timings observed during the past 79 years indicates a continuous period increase at a fractional rate of +(1.2$\pm$0.1)$\times10^{-10}$, in excellent agreement with the value $+1.1\times10^{-10}$ calculated from the Wilson-Devinney binary code. The new light curves display an inverse O'Connell effect increasing toward longer wavelengths. Hot and cool spot models are developed to describe these variations but we prefer a cool spot on the secondary star. Our light-curve synthesis reveals that GW Gem is in a semi-detached, but near-contact, configuration. It appears to consist of a near-main-sequence primary star with a spectral type of about A7 and an evolved early K-type secondary star that completely fills its inner Roche lobe. Mass transfer from the secondary to the primary component is responsible for the observed secular period change.

The sdB+M Eclipsing System HW Virginis and its Circumbinary Planets

Abstract: For the very short-period sdB eclipsing binary HW Vir, we present new CCD photometry made from 2000 through 2008. In order to obtain consistency of the binary parameters, our new light curves were analyzed simultaneously with previously published radial-velocity data. The secondary star parameters of $M_2$=0.14 M$_\odot$, $R_2$=0.18 R$_\odot$, and $T_2$=3,084 K are consistent with those of an M6-7 main sequence star. More than 250 times of minimum light, including our 41 timings and spanning more than 24 yrs, were used for a period study. From a detailed analysis of the $O$--$C$ diagram, it emerged that the orbital period of HW Vir has varied as a combination of a downward-opening parabola and two sinusoidal variations, with cycle lengths of $P_3$=15.8 yr and $P_4$=9.1 yr and semi-amplitudes of $K_3$=77 s and $K_4$=23 s, respectively. The continuous period decrease with a rate of $-8.28\times10^{-9}$ d yr$^{-1}$ may be produced by angular momentum loss due to magnetic stellar wind braking but not by gravitational radiation. Of the possible causes of the cyclical components of the period change, apsidal motion and magnetic period modulation can be ruled out. The most reasonable explanation of both cyclical variations is a pair of light-travel-time effects driven by the presence of two substellar companions with projected masses of $M_3 \sin i_3$=19.2 M$\rm_{Jup}$ and $M_4 \sin i_4$=8.5 M$\rm_{Jup}$. The two objects are the first circumbinary planets known to have been formed in a protoplanetary disk as well the first ones discovered by using the eclipse-timing method. The detection implies that planets could be common around binary stars just as are planets around single stars and demonstrates that planetary systems formed in a circumbinary disk can survive over long time scales.

Limits of Binaries that Can Be Characterized by Gravitational Microlensing

Abstract: 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 by-product. 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 separations of ~60 times 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 ~20 times the Einstein radius.

Limits of Binaries That Can Be Characterized by Gravitational Microlensing

Abstract: 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.