297 research outputs found
RoboTAP: Target priorities for robotic microlensing observations
Context. The ability to automatically select scientifically-important transient events from an alert stream of many such events, and to conduct follow-up observations in response, will become increasingly important in astronomy. With wide-angle time domain surveys pushing to fainter limiting magnitudes, the capability to follow-up on transient alerts far exceeds our follow-up telescope resources, and effective target prioritization becomes essential. The RoboNet-II microlensing program is a pathfinder project, which has developed an automated target selection process (RoboTAP) for gravitational microlensing events, which are observed in real time using the Las Cumbres Observatory telescope network.
Aims. Follow-up telescopes typically have a much smaller field of view compared to surveys, therefore the most promising microlensing events must be automatically selected at any given time from an annual sample exceeding 2000 events. The main challenge is to select between events with a high planet detection sensitivity, with the aim of detecting many planets and characterizing planetary anomalies.
Methods. Our target selection algorithm is a hybrid system based on estimates of the planet detection zones around a microlens. It follows automatic anomaly alerts and respects the expected survey coverage of specific events.
Results. We introduce the RoboTAP algorithm, whose purpose is to select and prioritize microlensing events with high sensitivity to planetary companions. In this work, we determine the planet sensitivity of the RoboNet follow-up program and provide a working example of how a broker can be designed for a real-life transient science program conducting follow-up observations in response to alerts; we explore the issues that will confront similar programs being developed for the Large Synoptic Survey Telescope (LSST) and other time domain surveys
Precise mass measurement of OGLE-2013-BLG-0132/MOA-2013-BLG-148: a Saturn mass planet orbiting an M-dwarf
We revisit the planetary microlensing event
OGLE-2013-BLG-0132/MOA-2013-BLG-148 using Keck adaptive optics imaging in 2013
with NIRC2 and in 2020, 7.4 years after the event, with OSIRIS. The 2020
observations yield a source and lens separation of mas, which
provides us with a precise measurement of the heliocentric proper motion of the
event mas . We measured the
magnitude of the lens in K-band as . Using these
constraints, we refit the microlensing light curve and undertake a full
reanalysis of the event parameters including the microlensing parallax
and the distance to the source D. We confirm the results obtained
in the initial study by \cite{Mroz_2017} and improve significantly upon the
accuracy of the physical parameters. The system is an M dwarf of orbited by a cold, Saturn-mass planet of
at projected separation = 3.14 0.28 AU. This work
confirms that the planetary system is at a distance of 3.48 0.36 kpc,
which places it in the Galactic disk and not the Galactic bulge.Comment: 16 pages, 7 figures. Resubmitted to AJ after minor revision
A Planetary Microlensing Event with an Unusually Red Source Star: MOA-2011-BLG-291
We present the analysis of planetary microlensing event MOA-2011-BLG-291,
which has a mass ratio of and a source star that
is redder (or brighter) than the bulge main sequence. This event is located at
a low Galactic latitude in the survey area that is currently planned for NASA's
WFIRST exoplanet microlensing survey. This unusual color for a microlensed
source star implies that we cannot assume that the source star is in the
Galactic bulge. The favored interpretation is that the source star is a lower
main sequence star at a distance of kpc in the Galactic disk.
However, the source could also be a turn-off star on the far side of the bulge
or a sub-giant in the far side of the Galactic disk if it experiences
significantly more reddening than the bulge red clump stars. However, these
possibilities have only a small effect on our mass estimates for the host star
and planet. We find host star and planet masses of and from a Bayesian
analysis with a standard Galactic model under the assumption that the planet
hosting probability does not depend on the host mass or distance. However, if
we attempt to measure the host and planet masses with host star brightness
measurements from high angular resolution follow-up imaging, the implied masses
will be sensitive to the host star distance. The WFIRST exoplanet microlensing
survey is expected to use this method to determine the masses for many of the
planetary systems that it discovers, so this issue has important design
implications for the WFIRST exoplanet microlensing survey
Adaptive Optics Imaging Breaks the Central Caustic Cusp Approach Degeneracy in High Magnification Microlensing Events
We report new results for the gravitational microlensing target
OGLE-2011-BLG-0950 from adaptive optics (AO) images using the Keck observatory.
The original analysis by Choi et al. 2012 reports degenerate solutions between
planetary and stellar binary lens systems. This is due to a degeneracy in high
magnification events where the shape of the light curve peak can be explained
by a source approach to two different cusp geometries with different source
radius crossing times. This particular case is the most important type of
degeneracy for exoplanet demographics, because the distinction between a
planetary mass or stellar binary companion has direct consequences for
microlensing exoplanet statistics. The 8 and 10-year baselines between the
event and the Keck observations allow us to directly measure a relative proper
motion of mas/yr, which confirms the detection of the lens star
system and directly rules out the planetary companion models that predict a
smaller relative proper motion. The combination of the lens
brightness and close stellar binary light curve parameters yield primary and
secondary star masses of and at a distance of kpc, and a primary-secondary projected separation of
AU. Since this degeneracy is likely to be common, the
high resolution imaging method described here will be used to disentangle the
central caustic cusp approach degeneracy for events observed by the
\textit{Roman} exoplanet microlensing survey using the \textit{Roman} images
taken near the beginning or end of the survey.Comment: Revised version, 19 pages, 8 figures. AJ, 164, 21
Keck Observations Confirm a Super-Jupiter Planet Orbiting M Dwarf OGLE-2005-BLG-071L
We present adaptive optics imaging from the NIRC2 instrument on the Keck II telescope that resolves the exoplanet host (and lens) star as it separates from the brighter source star. These observations yield the K-band brightness of the lens and planetary host star, as well as the lens-source relative proper motion, µ_(rel,H), in the heliocentric reference frame. The µ_(rel,H) measurement allows for the determination of the microlensing parallax vector, π_E, which had only a single component determined by the microlensing light curve. The combined measurements of µ_(rel,H) and K L provide the masses of the host star, M_(host) = 0.426 ± 0.037 M⊙, and planet, m_p = 3.27 ± 0.32M_(Jupiter) with a projected separation of 3.4 ± 0.5 au. This confirms the tentative conclusion of a previous paper that this super-Jupiter mass planet, OGLE-2005-BLG-071Lb, orbits an M dwarf. Such planets are predicted to be rare by the core accretion theory and have been difficult to find with other methods, but there are two such planets with firm mass measurements from microlensing, and an additional 11 planetary microlens events with host mass estimates <0.
0.5M⊙ and planet mass estimates >2 Jupiter masses that could be confirmed by high angular follow-up observations. We also point out that OGLE-2005-BLG-071L has separated far enough from its host star that it should be possible to measure the host-star metallicity with spectra from a high angular resolution telescope such as Keck, the Very Large Telescope, the Hubble Space Telescope, or the James Webb Space Telescope
OGLE-2014-BLG-0289: Precise Characterization of a Quintuple-peak Gravitational Microlensing Event
We present the analysis of the binary-microlensing event OGLE-2014-BLG-0289. The event light curve exhibits five very unusual peaks, four of which were produced by caustic crossings and the other by a cusp approach. It is found that the quintuple-peak features of the light curve provide tight constraints on the source trajectory, enabling us to precisely and accurately measure the microlensing parallax πE. Furthermore, the three resolved caustics allow us to measure the angular Einstein radius θE. From the combination of πE and θE, the physical lens parameters are uniquely determined. It is found that the lens is a binary composed of two M dwarfs with masses M1 = 0.52 ± 0.04 M⊙ and M2 = 0.42 ± 0.03 M⊙ separated in projection by a⊥ = 6.4 ± 0.5 au. The lens is located in the disk with a distance of DL = 3.3 ± 0.3 kpc. The reason for the absence of a lensing signal in the Spitzer data is that the time of observation corresponds to the flat region of the light curve
<i>Spitzer</i> microlens measurement of a massive remnant in a well-separated binary
We report the detection and mass measurement of a binary lens OGLE-2015-BLG-1285La,b, with the more massive component having M1 > 1.35 M⊙ (80% probability). A main-sequence star in this mass range is ruled out by limits on blue light, meaning that a primary in this mass range must be a neutron star (NS) or black hole (BH). The system has a projected separation r⊥ = 6.1 ± 0.4 AU and lies in the Galactic bulge. These measurements are based on the "microlens parallax" effect, i.e., comparing the microlensing light curve as seen from Spitzer, which lay at 1.25 AU projected from Earth, to the light curves from four ground-based surveys, three in the optical and one in the near-infrared. Future adaptive optics imaging of the companion by 30 m class telescopes will yield a much more accurate measurement of the primary mass. This discovery both opens the path and defines the challenges to detecting and characterizing BHs and NSs in wide binaries, with either dark or luminous companions. In particular, we discuss lessons that can be applied to future Spitzer and Kepler K2 microlensing parallax observations
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