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Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

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Kim S.-L. , Chum M.-Y. , Park B.-G. , Kim S. C. , Lee S. H. , Lee M. G. , Ann H. B. , Sung H. , Jeon Y.-B. , Yuk I.-S.

Acta Astronomica

2001

tom Vol. 51, No. 1

49--63

We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry of NGC 2506, we obtain interstellar reddening of E(B-V)=0.04±0.03 mag, distance modulus (V-M_V)0=12.5±0.1 mag, and age log(t/yr)=9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs in stars younger than log(t/yr)=8.4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: ≥20% for NGC 2506 and ≈50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters.

OGLE-2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy

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Skowron J. , Ryu Y.-H. , Hwang K.-H. , Udalski A. , Mróz P. , Kozłowski S. , Soszyński I. , Pietrukowicz P. , Szymański M. K. , Poleski R. , Ulaczyk K. , Pawlak M. , Rybicki K. , Iwanek P. , Albrow M. D. , Chung S.-J. , Gould A. , Han C. , Jung Y. K. , Shin I.-G. , Shvartzvald Y. , Yee J. C. , Zang W. , Zhu W. , Cha S.-M. , Kim D.-J. , Kim H.-W. , Kim S.-L. , Lee C.-U. , Lee D.-J. , Lee Y. , Park B.-G. , Pogge R. W.

Acta Astronomica

2018

tom Vol. 68, No. 1

43--61

We report the discovery of microlensing planet OGLE-2017-BLG-0373Lb. We show that while the planet-host system has an unambiguous microlens topology, there are two geometries within this topology that fit the data equally well, which leads to a factor 2.5 difference in planet-host mass ratio, i.e., q=1.5×10-3 vs. q=0.6×10-3. We show that this is an "accidental degeneracy" in the sense that it is due to a gap in the data. We dub it "the caustic-chirality degeneracy". We trace the mathematical origins of this degeneracy, which should enable similar degenerate solutions to be easily located in the future. A Bayesian estimate, based on a Galactic model, yields a host mass M=0.25+0.30 -0.15 M⊙ at a distance DL=5.9+1.3 -1.95 kpc. The lens-source relative proper motion is relatively fast, μ=9 mas/yr, which implies that the host mass and distance can be determined by high-resolution imaging after about 10 years. The same observations could in principle resolve the discrete degeneracy in q, but this will be more challenging.

OGLE-2017-BLG-1434Lb: Eighth q<1×10-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

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Udalski A. , Ryu Y.-H. , Sajadian S. , Gould A. , Mróz P. , Poleski R. , Szymański M. K. , Skowron J. , Soszyński I. , Kozłowski S. , Pietrukowicz P. , Ulaczyk K. , Pawlak M. , Rybicki K. , Iwanek P. , Albrow M. D. , Chung S. J. , Han C. , Hwang K.-H. , Jung Y. K. , Shin I.-G. , Shvartzvald Y. , Yee J. C. , Zang W. , Zhu W. , Cha S.-M. , Kim D.-J. , Kim H.-W. , Kim S.-L. , Lee C.-U. , Lee D.-J. , Lee Y. , Park B.-G. , Pogge R. W. , Bozza W. , Dominik M. , Helling C. , Hundertmark M. , Jørgensen U. G. , Longa-Peña P. , Lowry S. , Burgdorf M. , Campbell-White J. , Ciceri S. , Evans D. , Jaimes R. F. , Fujii Y. I. , Haikala L. K. , Henning T. , Hinse T. C. , Mancini L. , Peixinho N. , Rahvar S. , Rabus M. , Skottfelt J. , Snodgrass C. , Southworth J. , von Essen C.

Acta Astronomica

2018

tom Vol. 68, No. 1

1--42

We report the discovery of a cold Super-Earth planet (mp=4.4±0.5 M⊕) orbiting a low-mass (M=0.23±0.03 M⊙) M dwarf at projected separation a⊥=1.18±0.10 a.u., i.e., about 1.9 times the distance the snow line. The system is quite nearby for a microlensing planet, DL=0.86±0.09 kpc. Indeed, it was the large lens-source relative parallax πrel=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax" πE∝(πrel/M)1/2 that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q<1×10-4. We apply a new planet-detection sensitivity method, which is a variant of "V/Vmax", to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d lnq ∝ qp, with p=1.05+0.78 -0.68, which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n=-0.93±0.13 at higher mass ratios q≳2×10-4. We combine our result with that of Suzuki et al. to obtain p=0.73+0.42 -0.34.