53 research outputs found
Measurement of Dependence of Microlensing Planet Frequency on the Host Star Mass and Galactocentric Distance by Using a Galactic Model
Nunota K., Koshimoto N., Suzuki D., et al. Measurement of Dependence of Microlensing Planet Frequency on the Host Star Mass and Galactocentric Distance by Using a Galactic Model. Astrophysical Journal 967, 77 (2024); https://doi.org/10.3847/1538-4357/ad3cdc.We measure the dependence of planet frequency on host star mass, M L, and distance from the Galactic center, R L, using a sample of planets discovered by gravitational microlensing. We compare the two-dimensional distribution of the lens-source proper motion, ÎŒ rel, and the Einstein radius crossing time, t E, measured for 22 planetary events from Suzuki et al. with the distribution expected from Galactic model. Assuming that the planet-hosting probability of a star is proportional to M L m R L r , we calculate the likelihood distribution of (m,r). We estimate that r = 0.10 â 0.37 + 0.51 and m = 0.50 â 0.70 + 0.90 under the assumption that the planet-hosting probability is independent of the mass ratio. We also divide the planet sample into subsamples based on their mass ratio, q, and estimate that m = â 0.08 â 0.65 + 0.95 for q 10â3. Although uncertainties are still large, this result implies a possibility that, in orbits beyond the snowline, massive planets are more likely to exist around more massive stars whereas low-mass planets exist regardless of their host star mass
Confirmation of Color Dependent Centroid Shift Measured After 1.8 years with HST
We measured precise masses of the host and planet in OGLE-2003-BLG-235
system, when the lens and source were resolving, with 2018 Keck high resolution
images. This measurement is in agreement with the observation taken in 2005
with the Hubble Space Telescope (HST). In 2005 data, the lens and sources were
not resolved and the measurement was made using color-dependent centroid shift
only. Nancy Grace Roman Space Telescope will measure masses using data
typically taken within 3-4 years of the peak of the event which is much shorter
baseline compared to most of the mass measurements to date. Hence, color
dependent centroid shift will be one of the primary method of mass measurements
for Roman. Yet, mass measurements of only two events (OGLE-2003-BLG-235 and
OGLE-2005-BLG-071) are done using the color dependent centroid shift method so
far. The accuracy of the measurements using this method are neither completely
known nor well studied. The agreement of Keck and HST results, shown in this
paper, is very important since this agreement confirms the accuracy of the mass
measurements determined at a small lens-source separation using the color
dependent centroid shift method. This also shows that with >100 high resolution
images, Roman telescope will be able to use color dependent centroid shift at
3-4 years time baseline and produce mass measurements. We find that
OGLE-2003-BLG-235 is a planetary system consists of a 2.34 +- 0.43M_Jup planet
orbiting a 0.56 +- 0.06M_Sun K-dwarf host star at a distance of 5.26 +- 0.71
kpc from the Sun.Comment: Submitted to AJ, under review. arXiv admin note: substantial text
overlap with arXiv:2009.0232
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
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, ”_(rel,H), in the heliocentric reference frame. The ”_(rel,H) measurement allows for the determination of the microlensing parallax vector, Ï_E, which had only a single component determined by the microlensing light curve. The combined measurements of ”_(rel,H) and K L provide the masses of the host star, M_(host) = 0.426 ± 0.037 Mâ, 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.
0.5Mâ 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
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
Free-Floating planet Mass Function from MOA-II 9-year survey towards the Galactic Bulge
We present the first measurement of the mass function of free-floating
planets (FFP) or very wide orbit planets down to an Earth mass, from the MOA-II
microlensing survey in 2006-2014. Six events are likely to be due to planets
with Einstein radius crossing times, days, and the shortest has
days and an angular Einstein radius of as. We measure the detection efficiency depending on both
and with image level simulations for the first
time. These short events are well modeled by a power-law mass function,
dexstar with for . This implies a total of FFP or very wide orbit
planets of mass per star, with a total mass of
per star. The number of FFPs is
times the number of planets in wide orbits (beyond the snow line), while the
total masses are of the same order. This suggests that the FFPs have been
ejected from bound planetary systems that may have had an initial mass function
with a power-law index of , which would imply a total mass of
star. This model predicts that Roman Space
Telescope will detect FFPs with masses down to that of
Mars (including with ). The
Sumi(2011) large Jupiter-mass FFP population is excluded.Comment: 17 pages, 7 figures, accepted for publication in A
The Galactic Center with Roman
We advocate for a Galactic center (GC) field to be added to the Galactic
Bulge Time Domain Survey (GBTDS). The new field would yield high-cadence
photometric and astrometric measurements of an unprecedented 3.3
million stars toward the GC. This would enable a wide range of science cases,
such as finding star-compact object binaries that may ultimately merge as
LISA-detectable gravitational wave sources, constraining the mass function of
stars and compact objects in different environments, detecting populations of
microlensing and transiting exoplanets, studying stellar flares and variability
in young and old stars, and monitoring accretion onto the central supermassive
black hole. In addition, high-precision proper motions and parallaxes would
open a new window into the large-scale dynamics of stellar populations at the
GC, yielding insights into the formation and evolution of galactic nuclei and
their co-evolution with the growth of the supermassive black hole. We discuss
the possible trade-offs between the notional GBTDS and the addition of a GC
field with either an optimal or minimal cadence. Ultimately, the addition of a
GC field to the GBTDS would dramatically increase the science return of Roman
and provide a legacy dataset to study the mid-plane and innermost regions of
our Galaxy.Comment: 19 pages, 3 figures. Submitted to the NASA Roman Core Community
Surveys White Paper Cal
MOA-2020-BLG-135Lb: A New Neptune-class Planet for the Extended MOA-II Exoplanet Microlens Statistical Analysis
We report the light-curve analysis for the event MOA-2020-BLG-135, which
leads to the discovery of a new Neptune-class planet, MOA-2020-BLG-135Lb. With
a derived mass ratio of and separation
, the planet lies exactly at the break and likely peak of the
exoplanet mass-ratio function derived by the MOA collaboration (Suzuki et al.
2016). We estimate the properties of the lens system based on a Galactic model
and considering two different Bayesian priors: one assuming that all stars have
an equal planet-hosting probability and the other that planets are more likely
to orbit more massive stars. With a uniform host mass prior, we predict that
the lens system is likely to be a planet of mass
and a host star of mass
, located at a distance
. With a prior that holds that planet
occurrence scales in proportion to the host star mass, the estimated lens
system properties are ,
, and . This planet qualifies for inclusion in the extended MOA-II
exoplanet microlens sample.Comment: 22 pages, 6 figures, 4 tables, submitted to the AAS Journal
OGLE-2014-BLG-0221Lb: A Jupiter Mass Ratio Companion Orbiting Either a Late-type Star or a Stellar Remnant
Kirikawa R., Sumi T., Bennett D.P., et al. OGLE-2014-BLG-0221Lb: A Jupiter Mass Ratio Companion Orbiting Either a Late-type Star or a Stellar Remnant. Astronomical Journal 167, 154 (2024); https://doi.org/10.3847/1538-3881/ad2703.We present the analysis of the microlensing event OGLE-2014-BLG-0221, a planetary candidate event discovered in 2014. The photometric light curve is best described by a binary-lens single-source model. Our light-curve modeling finds two degenerate models, with event timescales of t E ⌠70 days and âŒ110 days. These timescales are relatively long, indicating that the discovered system would possess a substantial mass. The two models are similar in their planetary parameters with a Jupiter mass ratio of q ⌠10â3 and a separation of s ⌠1.1. Bayesian inference is used to estimate the physical parameters of the lens, revealing that the shorter timescale model predicts 65% and 25% probabilities of a late-type star and white dwarf host, respectively, while the longer timescale model favors a black hole host with a probability ranging from 60% to 95%, under the assumption that stars and stellar remnants have equal probabilities of hosting companions with planetary mass ratios. If the lens is a remnant, this would be the second planet found by microlensing around a stellar remnant. The current separation between the source and lens stars is 41-139 mas depending on the models. This indicates the event is now ready for high-angular-resolution follow-up observations to rule out either of the models. If precise astrometric measurements are conducted in multiple bands, the centroid shift due to the color difference between the source and lens would be detected in the luminous lens scenario
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