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[en] We present the first measurement of the planet frequency beyond the 'snow line', for the planet-to-star mass-ratio interval -4.5 < log q < -2, corresponding to the range of ice giants to gas giants. We find (d2Npl)/(d log q d log s) = (0.36±0.15) dex-2 at the mean mass ratio q = 5 x 10-4 with no discernible deviation from a flat (Oepik's law) distribution in log-projected separation s. The determination is based on a sample of six planets detected from intensive follow-up observations of high-magnification (A>200) microlensing events during 2005-2008. The sampled host stars have a typical mass Mhost ∼ 0.5 M sun, and detection is sensitive to planets over a range of planet-star-projected separations (s -1max R E, smax R E), where R E ∼ 3.5 AU(Mhost/Msun)1/2 is the Einstein radius and s max ∼ (q/10-4.3)1/3. This corresponds to deprojected separations roughly three times the 'snow line'. We show that the observations of these events have the properties of a 'controlled experiment', which is what permits measurement of absolute planet frequency. High-magnification events are rare, but the survey-plus-follow-up high-magnification channel is very efficient: half of all high-mag events were successfully monitored and half of these yielded planet detections. The extremely high sensitivity of high-mag events leads to a policy of monitoring them as intensively as possible, independent of whether they show evidence of planets. This is what allows us to construct an unbiased sample. The planet frequency derived from microlensing is a factor 8 larger than the one derived from Doppler studies at factor ∼25 smaller star-planet separations (i.e., periods 2-2000 days). However, this difference is basically consistent with the gradient derived from Doppler studies (when extrapolated well beyond the separations from which it is measured). This suggests a universal separation distribution across 2 dex in planet-star separation, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all planetary systems were 'analogs' of the solar system, our sample would have yielded 18.2 planets (11.4 'Jupiters', 6.4 'Saturns', 0.3 'Uranuses', 0.2 'Neptunes') including 6.1 systems with two or more planet detections. This compares to six planets including one two-planet system in the actual sample, implying a first estimate of 1/6 for the frequency of solar-like systems.
[en] Of the approximately 350 extrasolar planets currently known, of order 10% orbit evolved stars with radii R* ∼> 2.5 Rsun. These planets are of particular interest because they tend to orbit more massive hosts, and have been subjected to variable stellar insolation over their recent histories as their primaries evolved off the main sequence. Unfortunately, we have limited information about the physical properties of these planets, as they were all detected by the radial velocity method and none have been observed to transit. Here, we evaluate the prospects for detecting transits of planetary companions to giant stars. We show that several of the known systems have a priori transit probabilities of ∼>10%, and about one transiting system is expected for the sample of host stars with R* ≥ 2.5 Rsun. Although the transits are expected to have very small amplitudes (∼few x 10-4) and long durations (∼>50 hr), we argue that the difficulty with detecting these signals in broadband light is one of systematic errors and practicality rather than photon noise, even for modest aperture (∼1 m) telescopes. We propose a novel method that may overcome these difficulties, which uses narrowband measurements to isolate the thin ring of chromospheric emission expected at the limb of giant stars. The transit signals in these narrow bands are expected to be larger in magnitude and briefer in duration than in broadband emission, and thus alleviating many of the difficulties with transit detection in broadband emission. Finally, we point out that it may be possible to discover planetary companions to giant stars using Kepler, provided that a sufficient number of such targets are monitored.
[en] We combine all available information to constrain the nature of OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first in a high-magnification event. These include photometric and astrometric measurements from the Hubble Space Telescope, as well as constraints from higher order effects extracted from the ground-based light curve, such as microlens parallax, planetary orbital motion, and finite-source effects. Our primary analysis leads to the conclusion that the host of Jovian planet OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M = 0.46 ± 0.04 Msun, distance Dl = 3.2 ± 0.4 kpc, and thick-disk kinematics vLSR ∼ 103 km s-1. From the best-fit model, the planet has mass Mp = 3.8 ± 0.4 MJupiter, lies at a projected separation rperpendicular = 3.6 ± 0.2AU from its host, and so has an equilibrium temperature of T ∼ 55 K, that is, similar to Neptune. A degenerate model gives similar planetary mass Mp = 3.4 ± 0.4 MJupiter with a smaller projected separation, rperpendicular = 2.1 ± 0.1AU, and higher equilibrium temperature, T ∼ 71 K. These results from the primary analysis suggest that OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that is hosted by an M dwarf. However, the formation of such high-mass planetary companions in the outer regions of M dwarf planetary systems is predicted to be unlikely within the core-accretion scenario. There are a number of caveats to this primary analysis, which assumes (based on real but limited evidence) that the unlensed light coincident with the source is actually due to the lens, that is, the planetary host. However, these caveats could mostly be resolved by a single astrometric measurement a few years after the event.
[en] The DEdicated MONitor of EXotransits (DEMONEX) was a 20-inch robotic and automated telescope to monitor bright stars hosting transiting exoplanets to discover new planets and improve constraints on the properties of known transiting planetary systems. We present results for the misaligned hot Jupiter XO-4b containing seven new transits from the DEMONEX telescope, including three full and four partial transits. We combine these data with archival light curves and archival radial velocity measurements to derive the host star mass and radius , the planet mass and radius , and a refined ephemeris of days and . We include archival Rossiter–McLaughlin measurements of XO-4 to infer the stellar spin–planetary orbit alignment of degrees. We test the effects of including various detrend parameters, theoretical and empirical mass–radius relations, and Rossiter–McLaughlin models. We infer that detrending against CCD position and time or airmass can improve data quality but can have significant effects on the inferred values of many parameters—most significantly and the observed central transit times TC. In the case of we find that the systematic uncertainty due to detrending can be three times that of the quoted statistical uncertainties. The choice of mass–radius relation has little effect on our inferred values of the system parameters. The choice of Rossiter–McLaughlin models can have significant effects on the inferred values of and the stellar spin–planet orbit angle λ.
[en] Parallax is the most fundamental technique for measuring distances to astronomical objects. Although terrestrial parallax was pioneered over 2000 years ago by Hipparchus (ca. 140 B.C.E.) to measure the distance to the Moon, the baseline of the Earth is so small that terrestrial parallax can generally only be applied to objects in the Solar System. However, there exists a class of extreme gravitational microlensing events in which the effects of terrestrial parallax can be readily detected and so permit the measurement of the distance, mass, and transverse velocity of the lens. Here we report observations of the first such extreme microlensing event OGLE-2007-BLG-224, from which we infer that the lens is a brown dwarf of mass M = 0.056 ± 0.004 M sun, with a distance of 525 ± 40 pc and a transverse velocity of 113 ± 21 km s-1. The velocity places the lens in the thick disk, making this the lowest-mass thick-disk brown dwarf detected so far. Follow-up observations may allow one to observe the light from the brown dwarf itself, thus serving as an important constraint for evolutionary models of these objects and potentially opening a new window on substellar objects. The low a priori probability of detecting a thick-disk brown dwarf in this event, when combined with additional evidence from other observations, suggests that old substellar objects may be more common than previously assumed.
[en] We present the results of a deep (15 ∼< r ∼< 23), 20 night survey for transiting planets in the intermediate-age open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. We do not detect any transiting planets among the ∼1450 observed cluster members. We do, however, identify a ∼1RJ candidate planet transiting a ∼0.8 M sun Galactic field star with a period of 0.77 days. The source is faint (V = 19.85 mag) and has an expected velocity semiamplitude of K ∼ 220 m s-1(M/MJ ). We conduct Monte Carlo transit injection and recovery simulations to calculate the 95% confidence upper limit on the fraction of cluster members and field stars with planets as a function of planetary radius and orbital period. Assuming a uniform logarithmic distribution in the orbital period, we find that <1.1%, <2.7%, and <8.3% of cluster members have 1.0RJ planets within extremely hot Jupiter (EHJ; 0.4 < P < 1.0 day), very hot Jupiter (VHJ; 1.0 < P < 3.0 day), and hot Jupiter (HJ; 3.0 < P < 5.0 day) period ranges, respectively. For 0.5RJ planets, the limits are less than 3.2% and less than 21% for EHJ and VHJ period ranges, respectively, while for 0.35RJ planets we can only place an upper limit of less than 25% on the EHJ period range. For a sample of 7814 Galactic field stars, consisting primarily of FGKM dwarfs, we place 95% upper limits of <0.3%, <0.8%, and <2.7% on the fraction of stars with a 1.0RJ EHJ, VHJ, and HJ, respectively, assuming that the candidate planet is not genuine. If the candidate is genuine, the frequency of ∼1.0RJ planets in the EHJ period range is 0.002% < f EHJ < 0.5% with 95% confidence. We place limits of <1.4%, <8.8%, and <47% for 0.5RJ planets, and a limit of <16% on 0.3RJ planets in the EHJ period range. This is the first transit survey to place limits on the fraction of stars with planets as small as Neptune.
[en] We develop a new photometry algorithm that is optimized for the Infrared Array Camera (IRAC) Spitzer time series in crowded fields and that is particularly adapted to faint or heavily blended targets. We apply this to the 170 targets from the 2015 Spitzer microlensing campaign and present the results of three variants of this algorithm in an online catalog. We present detailed accounts of the application of this algorithm to two difficult cases, one very faint and the other very crowded. Several of Spitzer's instrumental characteristics that drive the specific features of this algorithm are shared by Kepler and WFIRST, implying that these features may prove to be a useful starting point for algorithms designed for microlensing campaigns by these other missions
[en] We used Keck adaptive optics observations to identify the first planet discovered by microlensing to lie in or near the habitable zone, i.e., at projected separation r = 1.1 ± 0.1 AU from its ML = 0.86 ± 0.06 M ☉ host, being the highest microlensing mass definitely identified. The planet has a mass mp = 4.8 ± 0.3 M Jup, and could in principle have habitable moons. This is also the first planet to be identified as being in the Galactic bulge with good confidence: DL = 7.72 ± 0.44 kpc. The planet/host masses and distance were previously not known, but only estimated using Bayesian priors based on a Galactic model. These estimates had suggested that the planet might be a super-Jupiter orbiting an M dwarf, a very rare class of planets. We obtained high-resolution JHK images using Keck adaptive optics to detect the lens and so test this hypothesis. We clearly detect light from a G dwarf at the position of the event, and exclude all interpretations other than that this is the lens with high confidence (95%), using a new astrometric technique. The calibrated magnitude of the planet host star is HL = 19.16 ± 0.13. We infer the following probabilities for the three possible orbital configurations of the gas giant planet: 53% to be in the habitable zone, 35% to be near the habitable zone, and 12% to be beyond the snow line, depending on the atmospherical conditions and the uncertainties on the semimajor axis.
[en] We present a new analysis of the Jupiter+Saturn analog system, OGLE-2006-BLG-109Lb,c, which was the first double planet system discovered with the gravitational microlensing method. This is the only multi-planet system discovered by any method with measured masses for the star and both planets. In addition to the signatures of two planets, this event also exhibits a microlensing parallax signature and finite source effects that provide a direct measure of the masses of the star and planets, and the expected brightness of the host star is confirmed by Keck AO imaging, yielding masses of M*=0.51+0.05-0.04Msun, Mb = 231 ± 19 M+, and Mc = 86 ± 7 M+. The Saturn-analog planet in this system had a planetary light-curve deviation that lasted for 11 days, and as a result, the effects of the orbital motion are visible in the microlensing light curve. We find that four of the six orbital parameters are tightly constrained and that a fifth parameter, the orbital acceleration, is weakly constrained. No orbital information is available for the Jupiter-analog planet, but its presence helps to constrain the orbital motion of the Saturn-analog planet. Assuming co-planar orbits, we find an orbital eccentricity of ε=0.15+0.17-0.10 and an orbital inclination of i=640-70+40. The 95% confidence level lower limit on the inclination of i > 49 deg. implies that this planetary system can be detected and studied via radial velocity measurements using a telescope of ∼>30 m aperture.
[en] Gravitational microlensing events produced by lenses composed of binary masses are important because they provide a major channel for determining physical parameters of lenses. In this work, we analyze the light curves of two binary-lens events, OGLE-2006-BLG-277 and OGLE-2012-BLG-0031, for which the light curves exhibit strong deviations from standard models. From modeling considering various second-order effects, we find that the deviations are mostly explained by the effect of the lens orbital motion. We also find that lens parallax effects can mimic orbital effects to some extent. This implies that modeling light curves of binary-lens events not considering orbital effects can result in lens parallaxes that are substantially different from actual values and thus wrong determinations of physical lens parameters. This demonstrates the importance of routine consideration of orbital effects in interpreting light curves of binary-lens events. It is found that the lens of OGLE-2006-BLG-277 is a binary composed of a low-mass star and a brown dwarf companion.