ABSTRACT We report the results of the statistical analysis of planetary signals discovered in MOA-II microlensing survey alert system events from 2007 to 2012. We determine the survey sensitivity as ...a function of planet-star mass ratio, q, and projected planet-star separation, s, in Einstein radius units. We find that the mass-ratio function is not a single power law, but has a change in slope at q ∼ 10−4, corresponding to ∼20 M⊕ for the median host-star mass of ∼0.6 . We find significant planetary signals in 23 of the 1474 alert events that are well-characterized by the MOA-II survey data alone. Data from other groups are used only to characterize planetary signals that have been identified in the MOA data alone. The distribution of mass ratios and separations of the planets found in our sample are well fit by a broken power-law model of the form for q > qbr and for q < qbr, where qbr is the mass ratio of the break. We also combine this analysis with the previous analyses of Gould et al. and Cassan et al., bringing the total sample to 30 planets. This combined analysis yields , n = −0.93 0.13, , and for qbr 1.7 × 10−4. The unbroken power-law model is disfavored with a p-value of 0.0022, which corresponds to a Bayes factor of 27 favoring the broken power-law model. These results imply that cold Neptunes are likely to be the most common type of planets beyond the snow line.
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.
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.
We present a detailed elemental abundance study of 90 F and G dwarf, turn-off, and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing ...events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. Four main findings are presented: (1) a wide metallicity distribution with distinct peaks at Fe/H = -1.09, -0.63, -0.20, + 0.12, + 0.41; (2) a highfraction of intermediate-age to young stars where at Fe/H > 0 more than 35% are younger than 8 Gyr, and for Fe/H ≲-0.5 most stars are 10 Gyr or older; (3) several episodes of significant star formation in the bulge has been identified: 3, 6, 8, and 11 Gyr ago; (4) tentatively the "knee" in the α-element abundance trends of the sub-solar metallicity bulge is located at a slightly higher Fe/H than in the local thick disk. These findings show that the Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. There are also additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy. Additionally, our analysis of this enlarged sample suggests that the (V-I)0 colour of the bulge red clump should be revised to 1.09.
We present measurements of the microlensing optical depth and event rate toward the Galactic Bulge (GB) based on two years of the MOA-II survey. Our event rate and optical depth analysis uses 474 ...events with well-defined microlensing parameters. We find that the event rate is maximized at low latitudes and a longitude of l approximately 1degrees. The main difference is the lack of long duration events in the RCG sample due to a known selection effect. Our results are consistent with previous optical depth measurements, but they are somewhat lower than previous all-source measurements, and slightly higher than previous RCG optical depth measurements. This suggests that the previously observed difference in optical depth measurements between all-source and RCG samples may largely be due to statistical fluctuations. These event rate measurements toward the central GB are necessary to predict the microlensing event rate and to optimize the survey fields in future space missions such as Wide Field Infrared Space Telescope.
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.
We present the analysis of four candidate short-duration binary microlensing events from the 2006-2007 MOA Project short-event analysis. Three of these events are determined to be microlensing ...events, while the fourth is most likely caused by stellar variability. One of these events, MOA-bin-1, is due to a planet, and it is the first example of a planetary event in which the stellar host is only detected through binary microlensing effects. This is one of the most massive and widest separation planets found by microlensing. The scarcity of such wide-separation planets also has implications for interpretation of the isolated planetary mass objects found by this analysis. In particular, if the entire isolated planet sample found by Sumi et al. consists of planets bound in wide orbits around stars, we find that it is likely that the median orbital semimajor axis is <30 AU.
We report the discovery of a super-Earth-mass planet in the microlensing event MOA-2012-BLG-505. This event has the second shortest event timescale of tE = 10 1 days where the observed data show ...evidence of a planetary companion. Our 15 minute high cadence survey observation schedule revealed the short subtle planetary signature. The system shows the well known close/wide degeneracy. The planet/host-star mass ratio is q = 2.1 × 10−4 and the projected separation normalized by the Einstein radius is s = 1.1 or 0.9 for the wide and close solutions, respectively. We estimate the physical parameters of the system by using a Bayesian analysis and find that the lens consists of a super-Earth with a mass of orbiting around a brown dwarf or late-M-dwarf host with a mass of with a projected star-planet separation of . The system is at a distance of 7.2 1.1 kpc, i.e., it is likely to be in the Galactic bulge. The small angular Einstein radius (θE = 0.12 0.02 mas) and short event timescale are typical for a low-mass lens in the Galactic bulge. Such low-mass planetary systems in the Bulge are rare because the detection efficiency of planets in short microlensing events is relatively low. This discovery may suggest that such low-mass planetary systems are abundant in the Bulge and currently on-going high cadence survey programs will detect more such events and may reveal an abundance of such planetary systems.
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.
We present the MOA Collaboration light-curve data for the planetary microlensing event OGLE-2015-BLG-0954, which was previously announced in a paper by the KMTNet and OGLE Collaborations. The MOA ...data cover the caustic exit, which was not covered by the KMTNet or Optical Gravitational Lensing Experiment (OGLE) data, and they provide a more reliable measurement of the finite source effect. The MOA data also provide a new source color measurement that reveals a lens-source relative proper motion of rel = 11.8 0.8 mas yr−1, which compares to the value of rel = 18.4 1.7 mas yr−1 reported in the KMTNet-OGLE paper. This new MOA value for rel has an a priori probability that is a factor of 100 times larger than the previous value, and it does not require a lens system distance of DL < 1 kpc. Based on the corrected source color, we find that the lens system consists of a planet of mass orbiting a star at an orbital separation of and a distance of .
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