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Poindexter, Shawn DavidSharpening The Tools of Gravitational Microlensing
Doctor of Philosophy, The Ohio State University, 2009, Astronomy

We attempt to identify all microlensing parallax events for which the parallax fit improves ΔΧ2 > 100 relative to a standard microlensing model. We outline a procedure to identify three types of discrete degeneracies and find many new degenerate solutions in 16 previously published and 6 unpublished events. The lens of event OGLE-2003-BLG-84 may be a Jupiter-mass free-floating planet candidate based on a weak 3σ detection of finite-source effects. These events were examined for xallarap, which can mimic parallax. We find that 23% of these events are strongly affected by xallarap.

The mid-IR flux ratios of the two images of the gravitationally lensed quasar HE 1104-1805 show no wavelength dependence to within 3% across 3.6-8.0 microns, no time dependence over 6 months and agree with the broad emission line flux ratios. This indicates that the mid-IR emission likely comes from scales large enough to be little affected by microlensing and that there is little differential extinction between the images. We measure a revised time-delay between these two images. We also observed uncorrelated variations of ~ 0.05 mag/yr-1 which we attribute to microlensing of the optical emission from the accretion disk. The optical colors have also changed significantly in the sense that image A is now redder than image B, rather than bluer as it was in 1993.

Based on the microlensing variability of the two-image gravitational lens HE 1104-1805 observed between 0.4 and 8 microns, we have measured the size and wavelength-dependent structure of the quasar accretion disk. Modeled as a power law in temperature, we measure a B-band half-light radius of R = 6.7(+6.2)(-3.2)x10^15 cm (68% CL) and a logarithmic slope of 0.61(+0.21)(-0.17) (68% CL) for our standard model with a logarithmic prior on the disk size. Both the scale and the slope are consistent with simple thin disk models. The observed fluxes favor a slightly shallower temperature profile.

Using 11-years of OGLE V-band photometry of Q2237+0305, we measure the transverse velocity of the lens galaxy and the mean mass of its stars. We measure the inclination i of the accretion disk to be cos i > 0.63 at 68% confidence. Very edge on (cos i < 0.34) solutions are ruled out at 95% confidence. For the first time, we fully include the random motions of the stars in the lens galaxy in the analysis of the light curves. We find the best fit transverse velocity of the lens galaxy is ~ 420 km/s to the Northeast. There is no strong correlation between the direction of motion of the lens galaxy and the orientation of the disk. The mean stellar mass is 0.37(+1.07)(-0.26) solar masses after including a well-defined velocity prior. We measure the V-band radius of the accretion disk. We also show for the first time that analyzing subsets of a microlensing light curve, in this case the first and second halves of the OGLE V-band light curve, give mutually consistent physical results.

Committee:

Christopher Kochanek, PhD (Advisor); Andrew Gould, PhD (Committee Member); Bradley Peterson, PhD (Committee Member)

Subjects:

Astronomy

Keywords:

gravitational lensing; microlensing; parallax; xallarap; quasar microlensing; accretion disk

Yee, Jennifer Chun MingExploring the Extremes of Exoplanet Detection and Characterization in High-Magnification Microlensing Events
Doctor of Philosophy, The Ohio State University, Astronomy
The field of microlensing planet searches is about to enter a new phase in which wide-field surveys will be the dominant mode of planet detection. In addition, there are now plans to execute microlensing surveys from space allowing the technique to reach smaller planets and resolve some of difficulties of ground-based microlensing where the resolution is poor. This new phase of observations also requires a new mode of analysis in which events are analyzed en masse rather than as individuals. Until now, there has not been any investigation into the detection threshold for planets in real data. Some people have suggested that the threshold for detecting planets may be as small as Delta chi^2 of 160, and that is frequently used in microlensing simulations of planet yields. However, no planets have been published with signals that small. I have done the first empirical investigation of the detection threshold for planets in high-magnification microlensing events. I found that MOA-2008-BLG-310 (Delta chi^2=880), MOA-2011-BLG-293 (Delta chi^2=500 without followup data), and MOA-2010-BLG-311 (Delta chi^2=80) form a sequence that spans from detected with high confidence (mb310) to marginally detected (mb293) to something too small to claim with confidence (mb311). This suggests that the detection threshold for planets in high-magnification events is 500 <= Delta chi^2<880. I have also analyzed OGLE-2008-BLG-279 to determine the range of planets that are detectable for this event given the excellent data quality and the high-magnification. This event illustrates that high-magnification events will still be important in the era of surveys because each event is much more sensitive to planets than any individual low-magnification event. Because they probe the central caustics, high-magnification events are sensitive to planets at any angle, meaning that they place more stringent limits on the presence of planets. For this event, Jupiter-mass companions can be ruled out from 0.5-20 AU. As the field extends to new modes of observations, it is worth considering how we can maximize the information we can obtain for each microlensing event, particularly given the limitation that microlensing is primarily sensitive to mass ratio rather than planet mass. I propose a means to take advantage of the excellent light curves that will be available from space and combine them with ground-based observations to measure microlens parallax for a large fraction of the microlensing events that will be seen by a space-based microlensing survey. This measurement will enable the measurement of the planet masses for these events.

Committee:

Andrew Gould (Advisor); B. Scott Gaudi (Committee Member); Richard Pogge (Committee Member)

Subjects:

Astronomy; Astrophysics

Keywords:

gravitational lensing - micro; planets and satellites - detection; microlensing; planets

Clanton, Christian DwainThe Demographics of Exoplanetary Companions to M Dwarfs: Synthesizing Results from Microlensing, Radial Velocity, and Direct Imaging Surveys
Doctor of Philosophy, The Ohio State University, 2016, Astronomy

Over the past 20 years, we have learned that exoplanets are ubiquitous throughout our Galaxy and show a diverse set of demographics, yet there is much work to be done to understand this diversity. Determining the distributions of the fundamental properties of exoplanets will provide vital clues regarding their formation and evolution. This is a difficult task, as exoplanet surveys are not uniformly sensitive to the full range of planet parameter space. Various observational biases and selection effects intrinsic to each of the different discovery techniques constrain the types of planets to which they are sensitive. Herein, I record a collection of the first studies to develop and apply the methodology of synthesizing results from multiple detection techniques to construct a statistically-complete census of planetary companions to M dwarfs that samples a wide region of their parameter space.

I present a robust comparison of exoplanet discoveries from microlensing and radial velocity (RV) surveys of M dwarfs which infer giant planet frequencies that differ by more than an order of magnitude and are, prima facie, in direct conflict. I demonstrate that current, state-of-the-art RV surveys are capable of detecting only the high-mass tail of the population of planets beyond the ice line inferred by microlensing studies, engendering a large, apparent difference in giant planet frequency. This comparison further establishes that results from these types of surveys are, in fact, consistent over the region of parameter space wherein their sensitivities overlap. A synthesis of results from microlensing and RV surveys yields planet occurrence rates for M dwarfs that span several orders of magnitude in mass and orbital period. On average, each M dwarf hosts about two planets, and while Jupiter and super-Jupiter companions are relatively rare (~3%), gas giants, in general, are quite common (~15%). These occurrence rates are significantly lower than those inferred around FGK stars and are thus, at least qualitatively, consistent with the predictions of core accretion theory.

Finally, I present a synthesis of results from microlensing, RV, and direct imaging surveys that improve constraints on the demographics of long-period, massive planetary companions to M dwarfs. I demonstrate that the results of five different surveys for exoplanets employing these three independent techniques are consistent with a single population of planets described by a simple, joint power-law distribution function in mass and semimajor axis, and provide constraints on the parameters of such a population. The final result is the most statistically-complete census of exoplanets that has hitherto been constructed for a given type of host star, spanning a mass range of 1-104 M and an orbital period range of 1-105 days. This work represents an important benchmark for all future exoplanet population studies, and the methodologies developed herein are applicable to new and larger data sets of forthcoming “next-generation” surveys.

Committee:

B. Scott Gaudi (Advisor); Todd A. Thompson (Committee Member); Marc H. Pinsonneault (Committee Member)

Subjects:

Astronomy

Keywords:

exoplanets; demographics; planet occurrence rates; planet frequency; M dwarfs; low-mass stars; microlensing; radial velocity; direct imaging; planet formation

AN, JIN HYEOKAstrophysics from binary-lens microlensing
Doctor of Philosophy, The Ohio State University, 2002, Astronomy
Microlensing events, especially ones due to a lens composed of a binary system can provide new channels to approach some of old questions in astronomy. Here, by modeling lightcurves of three binary-lens microlensing events observed by PLANET, I illustrate specific applications of binary-lens microlensing to real astrophysical problems. The lightcurve of a prototypical caustics-crossing binary-lens microlensing event OGLE-1999-BUL-23, which has been especially densely covered by PLANET during its second caustic crossing, enables me to measure the linear limb-darkening coefficients of the source star in I and V bands. The results are more or less consistent with theoretical predictions based on stellar atmosphere models, although the nonlinearity of the actual stellar surface brightness profile may have complicated the interpretation, especially for I band. Next, I find that the model for the lightcurve of EROS BLG-2000-5, another caustics-crossing binary-lens microlensing event, but exhibiting an unusually long second caustic crossing as well as a very prominent third peak due to a close approach to a cusp, requires incorporation of the microlens parallax and the binary orbital motion. Its projected Einstein radius is derived from the measured microlens parallax, and its angular Einstein radius is inferred from the finite source effect on the lightcurve, combined with an estimate of the source angular size given by the source position on the color-magnitude diagram. The lens mass is found by combining the above two quantities. This event marks the first case that the parallax effects are detected for a caustic-crossing event and the first time that the lens mass degeneracy has been broken. The analysis of the lightcurve of MACHO 97-BLG-47, an almost normal event with a well-covered short-duration anomaly near the peak, shows that it is caused by an extreme-separation binary-lens. Although lightcurve anomalies that are similar to what was observed for MACHO 97-BLG-47 may result from either a planet around the lens or an extreme-separation binary, this result demonstrates that the two classes of events can be distinguished in practice.

Committee:

ANDREW GOULD (Advisor)

Subjects:

Physics, Astronomy and Astrophysics

Keywords:

gravitational microlensing; binary stars: general; stars: fundamental parameters

Henderson, Calen BarnettThe Korean Microlensing Telescope Network: Expectations for a Cold Exoplanet Census through a Global Microlensing Survey
Doctor of Philosophy, The Ohio State University, 2015, Astronomy
The Korean Microlensing Telescope Network (KMTNet) consists of three 1.6m telescopes each with a 4 deg2 field of view (FoV) and is 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. My dissertation focuses on the results of simulations I have written and analyses I have performed that together provide estimates of and facilitate intuition about the number and variety of systmes KMTNet will detect and how best to maximize their scientific yield. First I present my simulations that optimize the observing strategy for, and predict the planetary yields of, KMTNet. I estimate the planet detection rates for planets with mass and separation across the ranges 0.1 ≤ Mp/M ≤ 1000 and 0.4 ≤ a/AU ≤ 16, respectively, and also for free-floating planets. I furthermore 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. Next I explore several possible avenues for constraining the flux of the lens for these predicted KMTNet detections. I examine the potential to obtain lens flux measurements by 1) imaging the lens once it is spatially resolved from the source, 2) measuring the elongation of the point spread function of the microlensing target (lens+source) when the lens and source are still unresolved, and 3) taking prompt follow-up photometry. In each case I simulate observing programs for a representative example of current ground-based adaptive optics (AO) facilities, future ground-based AO facilities, and future space telescopes. Lastly, I provide a list of microlensing events toward the Galactic bulge with high relative lens-source proper motion that are therefore good candidates for constraining the lens mass with future high-resolution imaging. I investigate all events from 2004 – 2013 that display detectable finite-source effects, a feature that facilitates event characterization. In total, I present 20 events. I present the first analysis for 6 new events, including OGLE-2004-BLG-368, MOA-2005-BLG-36, OGLE-2012-BLG-0211, OGLE-2012-BLG-0456, MOA-2012-BLG-532, and MOA-2013-BLG-029, and extract the analyses for 14 others from the literature. These events comprise an important testbed for refining the methodology of the high-resolution photometric techniques, particularly given the expected yield from KMTNet.

Committee:

Scott Gaudi (Advisor); Andrew Gould (Committee Member); Richard Pogge (Committee Member); Christopher Orban (Other)

Subjects:

Astronomy; Astrophysics

Keywords:

astronomy; astrophysics; microlensing; gravitational lensing; exoplanets