M. J. Lehner
A5067226472- Stellar, planetary, and galactic studies
- Astro and Planetary Science
- Astronomy and Astrophysical Research
- Astronomical Observations and Instrumentation
- Planetary Science and Exploration
- Astrophysics and Star Formation Studies
- Dark Matter and Cosmic Phenomena
- Adaptive optics and wavefront sensing
- Gamma-ray bursts and supernovae
- Atomic and Subatomic Physics Research
- Galaxies: Formation, Evolution, Phenomena
- Geomagnetism and Paleomagnetism Studies
- Astrophysical Phenomena and Observations
- Particle physics theoretical and experimental studies
- Particle Detector Development and Performance
- Isotope Analysis in Ecology
- Geophysics and Gravity Measurements
- Space Exploration and Technology
- CCD and CMOS Imaging Sensors
- Radiation Detection and Scintillator Technologies
- Cosmology and Gravitation Theories
- Geology and Paleoclimatology Research
- Nuclear Physics and Applications
- Scientific Research and Discoveries
- History and Developments in Astronomy
Institute of Astronomy and Astrophysics, Academia Sinica
2014-2024
University of Pennsylvania
2013-2023
Center for Astrophysics Harvard & Smithsonian
2012-2022
National Taiwan University
2013-2017
California University of Pennsylvania
2009-2016
Academia Sinica
2014-2016
Las Cumbres Observatory Global Telescope Network
2016
Harvard University Press
2008-2010
Campbell Collaboration
1995-2009
Kavli Institute for Particle Astrophysics and Cosmology
1996-2007
We report on our search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7 years photometry 11.9 million stars in LMC reveals 13 - 17 events. This is significantly more than $\sim$ 2 to 4 events expected from lensing by known stellar populations. The timescales ($\that$) range 34 230 days. estimate optical depth with $2 < \that 400$ days be 1.2 ^{+0.4}_ {-0.3} \ten{-7}$, an additional 20% 30% systematic error. spatial distribution mildly inconsistent LMC/LMC disk...
The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring millions stars Large Magellanic Cloud (LMC), Small (SMC), and Galactic bulge used to gravitational microlensing events caused by these otherwise invisible objects. Analysis first 2.1 yr photometry 8.5 million LMC reveals eight candidate events. This substantially more than number expected (~1.1) from lensing known stellar populations. timescales (t) range 34 145 days. We...
The EROS and MACHO collaborations have each published upper limits on the amount of planetary mass dark matter in Galactic Halo obtained from gravitational microlensing searches. In this paper two are combined to give a much stronger constraint abundance low MACHOs.
We present the first detection of parallax effects in a gravitational microlensing event. Parallax event observed only from Earth appears as distortion (otherwise symmetrical) light curve arising motion around Sun. This can be detected if duration is not much less than year and projected velocity lens larger orbital about The presented here has (or Einstein diameter crossing time) = 220 days clearly shows resulting Earth's motion. find that 75 ± 5 km s-1 at an angle 28° 4° direction...
We present new limits on the allowed masses of a dark matter (DM) halo consisting primordial black holes (PBH) (or any other massive compact object). analyze two years data from Kepler satellite, searching for short-duration bumps caused by gravitational microlensing. After removing background events variable stars, flare events, and comets or asteroids moving through field, we find no microlensing candidates. measure efficiency our selection criteria adding millions simulated lensing into...
We present our analysis on new limits of the dark matter (DM) halo consisting primordial black holes (PBHs) or massive compact objects. a search first two yr publicly available Kepler mission data for potential signatures gravitational microlensing caused by these objects as well an extensive astrophysical sources background error. These include variable stars, flare events, and comets asteroids that are moving through field. discuss detecting using light curves, presenting measurements...
Abstract The Outer Solar System Origins Survey (OSSOS), a wide-field imaging program in 2013–2017 with the Canada–France–Hawaii Telescope, surveyed 155 deg 2 of sky to depths m r = 24.1–25.2. We present 838 outer solar system discoveries that are entirely free ephemeris bias. This increases inventory trans-Neptunian objects (TNOs) accurately known orbits by nearly 50%. Each minor planet has 20–60 Gaia /Pan-STARRS-calibrated astrometric measurements made over 2–5 oppositions, which allows...
The recent discovery by Pan-STARRS1 of 1I/2017 U1 (`Oumuamua), on an unbound and hyperbolic orbit, offers a rare opportunity to explore the planetary formation processes other stars, effect interstellar environment planetesimal surface. 1I/`Oumuamua's close encounter with inner Solar System in 2017 October was unique chance make observations matching those used characterize small-body populations our own System. We present near-simultaneous g$^\prime$, r$^\prime$, J photometry colors...
Quaoar is a classical Trans-Neptunian Object (TNO) with an area equivalent diameter of 1,100 km and orbital semi-major axis 43.3 astronomical units. Based on stellar occultations observed between 2018 2021, inhomogeneous ring (Q1R, Quaoar's first ring) was detected around this body. Aims. A new occultation by August 9th, 2022 aiming to improve shape models the physical parameters Q1R while searching for additional material Methods. The provided nine effective chords across Quaoar, pinning...
We report the detection of 45 candidate microlensing events in fields toward Galactic bulge. These come from analysis 24 containing 12.6 million stars observed for 190 days 1993. Many these are extremely high signal-to-noise ratio and remarkable examples gravitational microlensing. The distribution peak magnifications is shown to be consistent with interpretation events. Using a subsample 1.3 "clump giant" whose distance efficiency well known, we find 13 estimate optical depth bulge as...
We report on a search for long duration microlensing events towards the Large Magellanic Cloud. find none, and therefore put limits contribution of high mass objects to Galactic dark matter. At 95% confidence level we exclude in range 0.3 solar masses 30.0 from contributing more than 4 times 10^11 halo. Combined with earlier results, this means that under 30 cannot make up entire matter halo if is typical size. For halo, 10 contribute less 40%
We present the microlensing optical depth toward Galactic bulge based on detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses 3 yr data, covering ~17 million stars ~4 deg2, to a source-star baseline magnitude limit V = 23. The DIA technique improves quality photometry crowded fields, and allows us detect more with faint source stars. find that this method increases number by 85% compared standard technique. light curves are presented, fit...
view Abstract Citations (206) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The MACHO Project First-Year Large Magellanic Cloud Results: Microlensing Rate and the Nature of Galactic Dark Halo Alcock, C. ; Allsman, R. A. Axelrod, T. S. Bennett, D. P. Cook, K. H. Freeman, Griest, Guern, J. Lehner, M. Marshall, L. Park, -S. Perlmutter, Peterson, B. Pratt, Quinn, Rodgers, W. Stubbs, Sutherland, Since July 1992, project has been carrying out...
We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from standard point-source model. 95-30 was observed in real time by Global Microlensing Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout event. interpret light-curve "fine structure" as indicating transit lens across extended face source star. This signifies resolution star several...
We have monitored 8.6 million stars in the Large Magellanic Cloud for 1.1 years and found 3 events consistent with gravitational microlensing. place strong constraints on Galactic halo content form of compact lensing objects mass range $10^{-4} \msun$ to $10^{-1} \msun$. Three is fewer than expected a standard spherical this range, but appears exceed number from known populations. Fitting naive model our data yields MACHO fraction $f = 0.20^{+0.33}_{-0.14}$, which implies total (inside 50...
Using 7 years of MACHO survey data, we present a new determination the optical depth to microlensing towards Galactic bulge. We select sample 62 events (60 unique) on clump giant sources and perform detailed efficiency analysis. use only because these are bright bulge stars not as strongly affected by blending other events. subsample 42 concentrated in just 4.5 square degrees, find tau = 2.17^{+0.47}_{-0.38} x 10^{-6} at (l,b) (1.50, -2.68), somewhat smaller than found most previous studies,...
ABSTRACT Although the majority of Centaurs are thought to have originated in scattered disk, with high-inclination members coming from Oort cloud, origin component trans-Neptunian objects (TNOs) remains uncertain. We report discovery a retrograde TNO, which we nickname “Niku,” detected by Pan-STARRS 1 Outer Solar System Survey. Our numerical integrations show that orbital dynamics Niku very similar 2008 KV 42 (Drac), half-life ∼500 Myr. Comparing TNOs and ( q > 10 au, < 100 i 60°),...
We report the discovery, tracking and detection circumstances for 85 trans-Neptunian objects (TNOs) from first 42 deg$^{2}$ of Outer Solar System Origins Survey (OSSOS). This ongoing $r$-band survey uses 0.9 field-of-view MegaPrime camera on 3.6 m Canada-France-Hawaii Telescope. Our orbital elements these TNOs are precise to a fractional semi-major axis uncertainty $<0.1\%$. achieve this precision in just two oppositions, as compared normal 3-5 via dense observing cadence innovative...
The MACHO Project is a microlensing survey that monitors the brightnesses of ∼60 million stars in Large Magellanic Cloud (LMC), Small Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, significant fraction all astronomical photometry. We describe calibration two‐color photometry transformation to standard Kron‐Cousins V R system. Calibrated may be properly compared with other observations on system, enhancing astrophysical value these data....
We present an analysis of the longest timescale microlensing events discovered by MACHO Collaboration during a 7 year survey Galactic bulge. find 6 that exhibit very strong parallax signals due, in part, to accurate photometric data from GMAN and MPS collaborations. The fit parameters are used likelihood analysis, which is able estimate distance masses lens objects based upon standard model velocity distribution. This indicates most likely 5 lenses > 1 Msun, suggests substantial fraction...