Abstract
We present the first measurement of the mass function of free-floating planets (FFPs), 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
t
E
< 0.5 days, and the shortest has
t
E
= 0.057 ± 0.016 days and an angular Einstein radius of
θ
E
= 0.90 ± 0.14
μ
as. We measure the detection efficiency depending on both
t
E
and
θ
E
with image-level simulations for the first time. These short events are well modeled by a power-law mass function,
dN
4
/
d
log
M
=
(
2.18
−
1.40
+
0.52
)
×
(
M
/
8
M
⊕
)
−
α
4
dex
−1
star
−1
with
α
4
=
0.96
−
0.27
+
0.47
for
M
/
M
⊙
< 0.02. This implies a total of
f
=
21
−
13
+
23
FFPs or very wide orbit planets of mass 0.33 <
M
/
M
⊕
< 6660 per star, with a total mass of
80
−
47
+
73
M
⊕
star
−1
. The number of FFPs is
19
−
13
+
23
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
α
∼ 0.9, which would imply a total mass of
171
−
52
+
80
M
⊕
star
−1
. This model predicts that Roman Space Telescope will detect
988
−
566
+
1848
FFPs with masses down to that of Mars (including
575
−
424
+
1733
with 0.1 ≤
M
/
M
⊕
≤ 1). The Sumi et al. large Jupiter-mass FFP population is excluded.
We report the analysis of OGLE-2019-BLG-0960, which contains the smallest mass-ratio microlensing planet found to date (q = 1.2–1.6 × 10^(−5) at 1σ). Although there is substantial uncertainty in the ...satellite parallax measured by Spitzer, the measurement of the annual parallax effect combined with the finite source effect allows us to determine the mass of the host star (M(L) = 0.3–0.6 Mꙩ), the mass of its planet (m(p) = 1.4–3.1 Mꚛ), the projected separation between the host and planet (a(⊥) = 1.2–2.3 au), and the distance to the lens system (D(L) = 0.6–1.2 kpc). The lens is plausibly the blend, which could be checked with adaptive optics observations. As the smallest planet clearly below the break in the mass-ratio function, it demonstrates that current experiments are powerful enough to robustly measure the slope of the mass-ratio function below that break. We find that the cross-section for detecting small planets is maximized for planets with separations just outside of the boundary for resonant caustics and that sensitivity to such planets can be maximized by intensively monitoring events whenever they are magnified by a factor A > 5. Finally, an empirical investigation demonstrates that most planets showing a degeneracy between (s > 1) and (s < 1) solutions are not in the regime (|log s| ≫ 0) for which the "close"/"wide" degeneracy was derived. This investigation suggests that there is a link between the "close"/"wide" and "inner/outer" degeneracies and also that the symmetry in the lens equation goes much deeper than symmetries uncovered for the limiting cases.
Abstract
We report the discoveries of low-mass free-floating planet (FFP) candidates from the analysis of 2006–2014 MOA-II Galactic bulge survey data. In this data set, we found 6111 microlensing ...candidates and identified a statistical sample consisting of 3535 high-quality single-lens events with Einstein radius crossing times in the range 0.057 <
t
E
/days < 757, including 13 events that show clear finite-source effects with angular Einstein radii of 0.90 <
θ
E
/
μ
as < 332.54. Two of the 12 events with
t
E
< 1 day have significant finite-source effects, and one event, MOA-9y-5919, with
t
E
= 0.057 ± 0.016 days and
θ
E
= 0.90 ± 0.14
μ
as, is the second terrestrial-mass FFP candidate to date. A Bayesian analysis indicates a lens mass of
0.75
−
0.46
+
1.23
M
⊕
for this event. The low detection efficiency for short-duration events implies a large population of low-mass FFPs. The microlensing detection efficiency for low-mass planet events depends on both the Einstein radius crossing times and the angular Einstein radii, so we have used image-level simulations to determine the detection efficiency dependence on both
t
E
and
θ
E
. This allows us to use a Galactic model to simulate the
t
E
and
θ
E
distribution of events produced by the known stellar populations and models of the FFP distribution that are fit to the data. Methods like this will be needed for the more precise FFP demographics determinations from Nancy Grace Roman Space Telescope data.
Aims.
The high-magnification microlensing event KMT-2021-BLG-1077 exhibits a subtle and complex anomaly pattern in the region around the peak. We analyze the lensing light curve of the event with the ...aim of revealing the nature of the anomaly.
Methods.
We test various models in combination with several interpretations: that the lens is a binary (2L1S), the source is a binary (1L2S), both the lens and source are binaries (2L2S), or the lens is a triple system (3L1S). We search for the best-fit models under the individual interpretations of the lens and source systems.
Results.
We find that the anomaly cannot be explained by the usual three-body (2L1S and 1L2S) models. The 2L2S model improves the fit compared to the three-body models, but it still leaves noticeable residuals. On the other hand, the 3L1S interpretation yields a model explaining all the major anomalous features in the lensing light curve. According to the 3L1S interpretation, the estimated mass ratios of the lens companions to the primary are ~1.56 × 10
−3
and ~1.75 × 10
−3
, which correspond to ~1.6 and ~1.8 times the Jupiter/Sun mass ratio, respectively, and therefore the lens is a multiplanetary system containing two giant planets. With the constraints of the event time-scale and angular Einstein radius, it is found that the host of the lens system is a low-mass star of mid-to-late M spectral type with amass of
M
h
= 0.14
−0.07
+0.19
M
Θ
, and it hosts two gas giant planets with masses of
M
p1
= 0.22
−0.12
+0.31
M
J
and
M
p2
= 0.25
−0.13
+0.35
. The planets lie beyond the snow line of the host with projected separations of
a
⊥,p1
= 1.26
−1.08
+1.41
AU and
a
⊥,p2
= 0.93
−0.80
+1.05
AU. The planetary system resides in the Galactic bulge at a distance of
D
L
= 8.24
−1.16
+1.02
kpc. The lens of the event is the fifth confirmed multiplanetary system detected by microlensing following OGLE-2006-BLG-109L, OGLE-2012-BLG-0026L, OGLE-2018-BLG-1011L, and OGLE-2019-BLG-0468L.
Aims.
The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very short anomaly relative to a single-lens single-source form. We investigate the light curve for the purpose of ...identifying the origin of the anomaly.
Methods.
We model the light curve under various interpretations. From this, we find four solutions, in which three solutions are found underthe assumption that the lens is composed of two masses (2L1S models), and the other solution is found under the assumption that the source is comprised of binary stars (1L2S model). The 1L2S model is ruled out based on the contradiction that the faint source companion is bigger than its primary, and one of the 2L1S solutions is excluded from the combination of the poorer fit, blending constraint, and lower overall probability, leaving two surviving solutions with the planet/host mass ratios of
q
~ 2.8 × 10
−5
and ~ 1.1 × 10
−5
. A subtle central deviation supports the possibility of a tertiary lens component, either a binary companion to the host with a very large or small separation, or a second planet lying near the Einstein ring, but it is difficult to claim a secure detection due to the marginal improvement of the fit, lack of consistency among different data sets, and difficulty in uniquely specifying the nature of the tertiary component.
Results.
With the observables of the event, it is estimated that the masses of the planet and host are ~ (6.9
M
⊕
, 0.75
M
⊙
) according to one solution and~(2.8
M
⊕
, 0.80
M
⊙
) according to the other, indicating that the planet is a super Earth around a K-type star, regardless of the solution. The fact that 16 (including the one reported in this work) out of 19 microlensing planets with
M
≲ 10
M
⊕
were detected during the last 6 yr nicely demonstrates the importance of high-cadence global surveys in detecting very low-mass planets.
Abstract
We present the analysis of three more planets from the KMTNet 2021 microlensing season. KMT-2021-BLG-0119Lb is a ∼6
M
Jup
planet orbiting an early M dwarf or a K dwarf, KMT-2021-BLG-0192Lb ...is a ∼2
M
Nep
planet orbiting an M dwarf, and KMT-2021-BLG-2294Lb is a ∼1.25
M
Nep
planet orbiting a very-low-mass M dwarf or a brown dwarf. These by-eye planet detections provide an important comparison sample to the sample selected with the AnomalyFinder algorithm, and in particular, KMT-2021-BLG-2294 is a case of a planet detected by eye but not by algorithm. KMT-2021-BLG-2294Lb is part of a population of microlensing planets around very-low-mass host stars that spans the full range of planet masses, in contrast to the planet population at ≲0.1 au, which shows a strong preference for small planets.
Abstract
We present an analysis of microlensing event OGLE-2019-BLG-0825. This event was identified as a planetary candidate by preliminary modeling. We find that significant residuals from the ...best-fit static binary-lens model exist and a xallarap effect can fit the residuals very well and significantly improves
χ
2
values. On the other hand, by including the xallarap effect in our models, we find that binary-lens parameters such as mass ratio,
q
, and separation,
s
, cannot be constrained well. However, we also find that the parameters for the source system such as the orbital period and semimajor axis are consistent between all the models we analyzed. We therefore constrain the properties of the source system better than the properties of the lens system. The source system comprises a G-type main-sequence star orbited by a brown dwarf with a period of
P
∼ 5 days. This analysis is the first to demonstrate that the xallarap effect does affect binary-lens parameters in planetary events. It would not be common for the presence or absence of the xallarap effect to affect lens parameters in events with long orbital periods of the source system or events with transits to caustics, but in other cases, such as this event, the xallarap effect can affect binary-lens parameters.
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 q=1.52+0.39-0.31x10-4 ...and separation s ≈ 1, the planet lies exactly at the break and likely peak of the exoplanet mass-ratio function derived by the Microlensing Observations in Astrophysics (MOA) Collaboration. 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 mplanet= 11.3 +19.2 -6.9M⨁ and a host star of mass Mhost=0.23+0.39-0.14M⨀, located at a distance 𝐃𝐋=7.9+1.0-1.0 kpc. With a prior that holds that planet occurrence scales in proportion to the host-star mass, the estimated lens system properties are mplanet=25+22 -15 M⨁, M Mhost=0.53+0.42 -0,32 M⨀, and DL=8.3+0.9-1.0.This planet qualifies for inclusion in the extended MOA-II exoplanet microlens sample.
Abstract
We analyze the MOA-2020-BLG-208 gravitational microlensing event and present the discovery and characterization of a new planet, MOA-2020-BLG-208Lb, with an estimated sub-Saturn mass. With a ...mass ratio
q
=
3.17
−
0.26
+
0.28
×
10
−
4
, the planet lies near the peak of the mass-ratio function derived by the MOA collaboration and near the edge of expected sample sensitivity. For these estimates we provide results using two mass-law priors: one assuming that all stars have an equal planet-hosting probability, and the other assuming that planets are more likely to orbit around more massive stars. In the first scenario, we estimate that the lens system is likely to be a planet of mass
m
planet
=
46
−
24
+
42
M
⊕
and a host star of mass
M
host
=
0.43
−
0.23
+
0.39
M
⊙
, located at a distance
D
L
=
7.49
−
1.13
+
0.99
kpc
. For the second scenario, we estimate
m
planet
=
69
−
34
+
37
M
⊕
,
M
host
=
0.66
−
0.32
+
0.35
M
⊙
, and
D
L
=
7.81
−
0.93
+
0.93
kpc
. The planet has a projected separation as a fraction of the Einstein ring radius
s
=
1.3807
−
0.0018
+
0.0018
. As a cool sub-Saturn-mass planet, this planet adds to a growing collection of evidence for revised planetary formation models.
The Microlensing Observations in Astrophysics (MOA-II) survey has performed high cadence, wide field observations of the Galactic Bulge from New Zealand since 2005. The hourly cadence of the survey ...during eight months of the year, across nearly 50 deg2 of sky, provides an opportunity to sample asteroid lightcurves in the broad MOA-R filter. We perform photometry of a subset of bright asteroids numbered observed by the survey. We obtain 26 asteroid rotation periods, including for two asteroids where no prior data exist, and present evidence for the possible non-principal axis rotation of (2011) Veteraniya. This archival search could be extended to several thousands of asteroids brighter than 22nd magnitude.
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