Is the Galactic bulge devoid of planets?

Considering a sample of 31 exoplanetary systems detected by gravitational microlensing, we investigate whether or not the estimated distances to these systems conform to the Galactic distribution of planets expected from models. We derive the expected distribution of distances and relative proper motions from a simulated microlensing survey, correcting for the dominant selection effects that affect the planet detection sensitivity as a function of distance, and compare it to the observed distribution using Anderson-Darling (AD) hypothesis testing. Taking the relative abundance of planets in the bulge to that in the disk, $f_{\rm bulge}$, as a model parameter, we find that our model is only consistent with the observed distribution for $f_{\rm bulge}<0.54$ (for a $p$-value threshold of 0.01) implying that the bulge may be devoid of planets relative to the disk. Allowing for a dependence of planet abundance on metallicity and host mass, or an additional dependence of planet sensitivity on event timescale does not restore consistency for $f_{\rm bulge}=1$. We examine the distance estimates of some events in detail, and conclude that some parallax-based distance estimates could be significantly in error. Only by combining the removal of one problematic event from our sample and the inclusion of strong dependences of planet abundance or detection sensitivity on host mass, metallicity and event timescale are we able to find consistency with the hypothesis that the bulge and disk have equal planet abundance.Comment: Revised following referee's report. 12 pages, 7 figures, 1 tabl

Optimal Survey Strategies and Predicted Planet Yields for the Korean Microlensing Telescope Network

The Korean Microlensing Telescope Network (KMTNet) will consist of three 1.6m telescopes each with a 4 deg^{2} field of view (FoV) and will be dedicated to monitoring the Galactic Bulge to detect exoplanets via gravitational microlensing. KMTNet's combination of aperture size, FoV, cadence, and longitudinal coverage will provide a unique opportunity to probe exoplanet demographics in an unbiased way. Here we present simulations that optimize the observing strategy for, and predict the planetary yields of, KMTNet. We find preferences for four target fields located in the central Bulge and an exposure time of t_{exp} = 120s, leading to the detection of ~2,200 microlensing events per year. We estimate the planet detection rates for planets with mass and separation across the ranges 0.1 <= M_{p}/M_{Earth} <= 1000 and 0.4 <= a/AU <= 16, respectively. Normalizing these rates to the cool-planet mass function of Cassan (2012), we predict KMTNet will be approximately uniformly sensitive to planets with mass 5 <= M_{p}/M_{Earth} <= 1000 and will detect ~20 planets per year per dex in mass across that range. For lower-mass planets with mass 0.1 <= M_{p}/M_{Earth} < 5, we predict KMTNet will detect ~10 planets per year. We also compute the yields KMTNet will obtain for free-floating planets (FFPs) and predict KMTNet will detect ~1 Earth-mass FFP per year, assuming an underlying population of one such planet per star in the Galaxy. Lastly, we investigate the dependence of these detection rates on the number of observatories, the photometric precision limit, and optimistic assumptions regarding seeing, throughput, and flux measurement uncertainties.Comment: 29 pages, 31 figures, submitted to ApJ. For a brief video explaining the key results of this paper, please visit: https://www.youtube.com/watch?v=e5rWVjiO26

'Auxiliary' Science with the WFIRST Microlensing Survey: Measurement of the Compact Object Mass Function over Ten Orders of Magnitude; Detection of ~10⁵ Transiting Planets; Astroseismology of ~10⁶ Bulge Giants; Detection of ~5x10³ Trans-Neptunian Objects; and Parallaxes and Proper Motions of ~6x10⁶ Bulge and Disk Stars

The Wide Field Infrared Survey Telescope (WFIRST) will monitor ∼2 deg² toward the Galactic bulge in a wide (∼1−2 μm) W149 filter at 15-minute cadence with exposure times of ∼50s for 6 seasons of 72 days each, for a total ∼41,000 exposures taken over ∼432 days, spread over the 5-year prime mission. This will be one of the deepest exposures of the sky ever taken, reaching a photon-noise photometric precision of 0.01 mag per exposure and collecting a total of ∼10⁹ photons over the course of the survey for a W149_(AB) ∼ 21 star. Of order 4×10⁷ stars will be monitored with W149_(AB) < 21, and 10⁸ stars with W145_(AB) < 23. The WFIRST microlensing survey will detect ∼54,000 microlensing events, of which roughly 1% (∼500) will be due to isolated black holes, and ∼3% (∼1600) will be due to isolated neutron stars. It will be sensitive to (effectively) isolated compact objects with masses as low as the mass of Pluto, thereby enabling a measurement of the compact object mass function over 10 orders of magnitude. Assuming photon-noise limited precision, it will detect ∼10⁵ transiting planets with sizes as small as ∼2 R⊕, perform asteroseismology of ∼10⁶ giant stars, measure the proper motions to ∼0.3% and parallaxes to ∼10% for the ∼6×10⁶ disk and bulge stars in the survey area, and directly detect ∼5×10³ Trans-Neptunian objects (TNOs) with diameters down to ∼10 km, as well as detect ∼10³ occulations of stars by TNOs during the survey. All of this science will completely serendipitous, i.e., it will not require modifications of the WFIRST optimal microlensing survey design. Allowing for some minor deviation from the optimal design, such as monitoring the Galactic center, would enable an even broader range of transformational science

KMT-2016-BLG-2052L: Microlensing Binary Composed of M Dwarfs Revealed from a Very Long Timescale Event

We present the analysis of a binary microlensing event, KMT-2016-BLG-2052, for which the lensing-induced brightening of the source star lasted for two seasons. We determine the lens mass from the combined measurements of the microlens parallax, π_E, and angular Einstein radius, θ_E. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of M_1 ~ 0.34 M⊙ and M_2 ~ 0.17 M⊙. The measured relative lens-source proper motion of μ ~ 3.9 mas yr^(−1) is smaller than ~5 mas yr−1 of typical Galactic lensing events, while the estimated angular Einstein radius of θ E ~ 1.2 mas is substantially greater than the typical value of ~0.5 mas. Therefore, it turns out that the long timescale of the event is caused by the combination of the slow μ and large θ_E rather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long timescales (t_E ≳ 100 days), we find that the probabilities that long timescale events are produced by lenses with masses ≥1.0 M⊙ and ≥3.0 M⊙ are ~19% and 2.6%, respectively, indicating that events produced by heavy lenses comprise a minor fraction of long timescale events. The results indicate that it is essential to determine lens masses by measuring both π_E and θ_E in order to firmly identify heavy stellar remnants, such as neutron stars and black holes

OGLE-2018-BLG-0022: First Prediction of an Astrometric Microlensing Signal from a Photometric Microlensing Event

In this work, we present the analysis of the binary microlensing event OGLE-2018-BLG-0022 that is detected toward the Galactic bulge field. The dense and continuous coverage with the high-quality photometry data from ground-based observations combined with the space-based Spitzer observations of this long timescale event enables us to uniquely determine the masses M_1 = 0.40 ± 0.05 M⊙ and M_2 = 0.13 ± 0.01 M⊙ of the individual lens components. Because the lens-source relative parallax and the vector lens-source relative proper motion are unambiguously determined, we can likewise unambiguously predict the astrometric offset between the light centroid of the magnified images (as observed by the Gaia satellite) and the true position of the source. This prediction can be tested when the individual-epoch Gaia astrometric measurements are released