Journal ArticleDOI

# OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: Two Microlensing Planets through the Planetary-caustic Channel

01 Jun 2021-The Astronomical Journal (American Astronomical Society)-Vol. 161, Iss: 6, pp 293

Topics:

### 1. Introduction

• The signature of a microlensing planet is almost always a short-lasting anomaly in the smooth and symmetric lensing light curve produced by the host of the planet.
• The bias toward central anomalies is mostly attributed to the limitation of early lensing surveys.
• The observational cadence of the lensing surveys in this phase has greatly increased with the employment of largeformat cameras yielding very wide fields of view.
• Planets are detected through the planetary-caustic channel as anomalies produced by the source’s approach close to the planetary caustic, which denotes one of the two sets of planet-induced caustics lying away from the host.

### 2. Observation

• The two planetary events were observed by the two lensing surveys conducted by the OGLE and KMTNet groups.
• In both surveys, observations were mainly conducted in the I band, and a fraction of the images were taken in the V band to determine the color of the microlensed source stars.
• The OGLE cadence for this direction is 3–10 times per night.
• The KMTNet collaboration also observed the event, with designation KMT-2018-BLG-2071, located in their two overlapping fields (BLG41 and BLG22).

### 3. Light Curve Analysis

• The next three (s, q, α) describe the binary-lens geometry: the projected binary separation (scaled to θE), the binary mass ratio (q=M2/M1), and the angle between the source trajectory and the binary axis as measured in a clockwise sense, respectively.
• From the configurations, it is found that the anomalies of both events are produced by the source crossing over the planetary caustic of the lens system.

### 4. Physical Parameters

• The results in Table 1 show that for both events, the normalized source radii are precisely measured.
• For the disk VD, the authors adopt Gaussian forms of f (vy, vz)= f (vy)f (vz) from Han & Gould (1995), which they then modify to consider the change in the matter distribution.
• The estimated lens properties for the individual events are listed in Table 4.
• This implies that the effective cross-section of these caustics for a planet-induced perturbation and their lensing behavior are similar to those of resonant caustics.

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OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: Two Microlensing Planets
through the Planetary-caustic Channel
Youn Kil Jung
1,2,15
, Cheongho Han
3,15
, Andrzej Udalski
4,16
, Andrew Gould
1,5,6,15
, Jennifer C. Yee
7,15
and
Michael D. Albrow
8
, Sun-Ju Chung
1,2
, Kyu-Ha Hwang
1
, Yoon-Hyun Ryu
1
, In-Gu Shin
1
, Yossi Shvartzvald
9
,
Wei Zhu
10
, Weicheng Zang
11
, Sang-Mok Cha
1,12
, Dong-Jin Kim
1
, Hyoun-Woo Kim
1
, Seung-Lee Kim
1,2
,
Chung-Uk Lee
1,2
, Dong-Joo Lee
1
, Yongseok Lee
1,12
, Byeong-Gon Park
1,2
, Richard W. Pogge
5
(The KMTNet Collaboration),
and
Przemek Mróz
4,13
, Michał K. Szymański
4
, Jan Skowron
4
4,5
, Igor Soszyński
4
, Paweł Pietrukowicz
4
,
Szymon Kozłowski
4
, Krzystof Ulaczyk
14
, Krzysztof A. Rybicki
4
, Patryk Iwanek
4
, and Marcin Wrona
4
(The OGLE Collaboration)
1
Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
2
University of Science and Technology, Korea, 217 Gajeong-ro Yuseong-gu, Daejeon 34113, Korea
3
Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
4
Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
5
Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
6
Max-Planck-Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
7
Center for Astrophysics|Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
8
University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
9
Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
10
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St., Toronto, ON M5S 3H8, Canada
11
Department of Astronomy, Tsinghua University, Beijing 100084, Peoples Republic of China
12
School of Space Research, Kyung Hee University, Yongin 17104, Republic of Korea
13
Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
14
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
Received 2021 January 31; revised 2021 March 23; accepted 2021 April 14; published 2021 June 3
Abstract
We present the analyses of two microlensing events, OGLE-2018-BLG-0567 and OGLE-2018-BLG-0962. In both
events, the short-lasting anomalies were densely and continuously covered by two high-cadence surveys. The light-
curve modeling indicates that the anomalies are generated by source crossings over the planetary caustics induced by
planetary companions to the hosts. The estimated planet/host separation (scaled to the angular Einstein radius θ
E
) and
mass ratio are (s, q × 10
3
) = (1.81 ± 0.02, 1.24 ± 0.07) and (s, q × 10
3
) = (1.25 ± 0.03, 2.38 ± 0.08), respectively.
From Bayesian analyses, we estimate the host and planet masses as
=
-
+
-
+
MM M M, 0.25 , 0.32
hp
0.13
0.27
0.17
0.34
J
(
)( )
and
=
-
+
-
+
MM M M, 0.54 , 1.34
hp
0.28
0.33
0.70
0.82
J
(
)( )
, respectively. These planetary systems are located at a distance of
-
+
7
.06 kpc
1.15
0.93
for OGLE-2018-BLG-0567 and
-
+
6.50 kpc
1.75
1.06
for OGLE-2018-BLG-0962, suggesting that they are likely
to be near the Galactic bulge. The two events prove the capability of current high-cadence surveys for nding planets
through the planetary-caustic channel. We nd that most published planetary-caustic planets are found in Hollywood
events in which the source size strongly contributes to the anomaly cross-section relative to the size of the caustic.
Unied Astronomy Thesaurus concepts: Gravitational microlensing (672); Gravitational microlensing exoplanet
detection (2147)
Supporting material: data behind gures
1. Introduction
The signature of a microlensing planet is almost always a
short-lasting anomaly in the smooth and symmetric lensing
light curve produced by the host of the planet. In principle, the
signature can appear at any position of the lensing light curve
(Gaudi 2012). In reality, however, the signatures of planets
detected in the earlier phase of lensing experiments appeared
mainly near the peak of lensing light curves.
The bias toward central anomalies is mostly attributed to the
limitation of early lensing surveys. With a roughly 1-day
cadence of the rst-generation survey experiments, e.g., the
Massive Compact Halo Objects (MACHOs; Alcock et al. 1995),
the rst phase of Optical Gravitational Lensing Experiment
(OGLE-I; Udalski et al. 1992),andtherst phase of
Microlensing Observations in Astrophysics (MOA-I; Bond
et al. 2001) surveys, it was difcult to detect planetary signals
lasting of an order of 1 day or less by the survey experiments. To
meet the cadence requirement for planet detections, Gould &
Loeb (1992) proposed an observational mode, in which wide-
eld surveys with a low cadence monitor a large area of the sky
mainly to detect lensing events, and follow-up experiments
conduct high-cadence observations for a small number of
lensing events detected by the surveys using a network of
The Astronomical Journal, 161:293 (12pp), 2021 June https://doi.org/10.3847/1538-3881/abf8bd
15
The KMTNet Collaboration.
16
The OGLE Collaboration.
1

multiple narrow-eld telescopes. However, this mode of
observations had the drawback that only a handful of lensing
events could be monitored by follow-up observations. Combined
with the low probability of planetary perturbations, this implied a
low planet detection rate for this phase of the experiments. In
fact, for the rst several years, there were no securely detected
planets using this mode, although there was one tentative
detection in the event MACHO 98-BLG-35 (Rhie et al. 2000).
The rst three microlensing detections were found using the
survey+follow-up strategy. In the rst event, OGLE 2003-
BLG-235/MOA 2003-BLG-53 (Bond et al. 2004), the planet
was found by the surveys, but the MOA survey carried out
additional follow-up observations in response to the planetary
anomaly. The next two planets, OGLE-2005-BLG-071Lb
(Udalski et al. 2005) and OGLE-2005-BLG-390Lb (Beaulieu
et al. 2006), were both found through extensive follow-up
observations of known microlensing events that were initiated
before the planetary anomaly began. The discovery of OGLE-
2005-BLG-071Lb provided a practical lesson in the value of
high-magnication events (Griest & Sazadeh 1998) for
detecting planets through follow-up observations.
In the following years, the planet detection rate using the
survey+follow-up mode was substantially increased by focus-
ing on events with very high magnications. Several factors
contributed to the increase of the detection rate. First, the planet
detection efciency for high-magnication events is high. This
is because a planet located in the lensing zone of its host always
induces a small central caustic near the position of the host, and
during a high-magnication event, the source passes close to
the central caustic. This yields a high probability that a planet
will produce a perturbation and also connes that perturbation
to a short duration of time while the event is highly magnied,
not throughout the whole event. As a result, the time of the
planetary signal, i.e., the peak of the light curve, can be
predicted in advance and enable one to efciently use resources
for follow-up observations. By contrast, predicting the time of a
planetary signal through other channels is difcult. Finally,
highly magnied source stars are bright enough to be observed
with small-aperture telescopes, down to submeter amateur-class
telescopes, and this enables one to maximize available
telescopes for follow-up observations, e.g., OGLE-2005-
BLG-071 (Udalski et al.
2005; Dong et al. 2009). Thus, the
planets detected from the survey+follow-up experiments were
detected mainly through the high-magni cation channel, and
this led to the bias toward central-caustic perturbations.
The current planetary lensing experiments are in the second
phase, in which lensing events are observed by high-cadence
surveys. The observational cadence of the lensing surveys in
this phase has greatly increased with the employment of large-
format cameras yielding very wide elds of view. The MOA
experiment entered a new phase (MOA-II) by upgrading its
instrument with a new wide-eld camera composed of ten
2k × 4k chips yielding a 2.2 deg
2
eld of view (Sumi et al.
2013). The OGLE survey is in its fourth phase (OGLE-IV)
using a 1.4 deg
2
camera composed of 32 2k × 4k chips
(Udalski et al. 2015). The Korea Microlensing Telescope
Network (KMTNet) survey, which commenced its full
operation in 2016 (Kim et al. 2016), utilizes three globally
distributed telescopes, each of which has a camera with a
4.0 deg
2
eld of view. Being able to cover a large area of sky
from a single exposure, the observational cadence of the
current survey experiments now reaches Γ 4hr
1
toward the
dense bulge elds. This enables planet detections without
With the operation of the high-cadence surveys, the
detection rate of planets is rapidly increasing. One important
reason for the rapid increase of the detection rate is that planets
can be detected not only through the central-caustic channel but
also through the additional planetary-caustic channel. Planets
are detected through the planetary-caustic channel as anomalies
produced by the sources approach close to the planetary
caustic, which denotes one of the two sets of planet-induced
caustics lying away from the host. The planetary caustic lies at
a position with a separation from the host of s 1/s, and thus
planetary signals produced by this caustic can appear at any
part of the lensing light curve depending on the planethost
separation s (normalized to the angular Einstein radius θ
E
). The
planetary caustic is substantially larger than the central caustic,
and thus the probability of a planetary perturbation is higher.
Another importance of detecting planets through the planetary-
caustic channel is that interpreting the planetary signal is
usually not subject to the closewide degeneracy (Griest &
Sazadeh 1998; Dominik 1999), which causes ambiguity in
estimating the planethost separations for most planets detected
through the central-caustic channel.
In this paper, we present the analysis of two planetary
microlensing events, OGLE-2018-BLG-0567 and OGLE-
2018-BLG-0962, for which planets are both detected through
a planetary-caustic channel. For both events, the signatures of
the planets were densely and continuously covered by two
ously interpret the planetary signals.
2. Observation
The two planetary events were observed by the two lensing
surveys conducted by the OGLE and KMTNet groups. The
OGLE survey uses the 1.3 m telescope that is located at the Las
Campanas Observatory in Chile. The KMTNet survey utilizes
three 1.6 m telescopes that are located at the Siding Spring
Observatory in Australia (KMTA), the Cerro Tololo Inter-
american Observatory in Chile (KMTC), and the South African
Astronomical Observatory in South Africa (KMTS). The global
distribution of the KMTNet telescopes makes it possible to
continuously monitor the events. In both surveys, observations
were mainly conducted in the I band, and a fraction of the
images were taken in the V band to determine the color of the
microlensed source stars.
OGLE-2018-BLG-0567,
=
R.A ., decl.
J2000
(
)
(17:56:04.42,
27:59:13.6),or(l, b) = (1°.99, 1°.49) in Galactic coordi-
nates, was discovered on 2018 April 14 by the OGLE Early
Warning System (EWS; Udalski 2003). The event was
independently found by the KMTNet survey as KMT-2018-
BLG-0890 from its event-nding algorithm (Kim et al. 2018).
The observational cadence for the event is Γ = 1hr
1
for
OGLE and Γ = 2hr
1
for KMTNet.
OGLE-2018-BLG-0962 was discovered by the OGLE EWS
on 2018 June 2. It is located at
=R.A ., decl.
J2000
()
(17:52:41.95,
32:18:33.3) or Galactic coordinates of (l, b) = ( 2°.11,
3°.04). The OGLE cadence for this direction is 310 times
per night. The KMTNet collaboration also observed the event,
with designation KMT-2018-BLG-2071, located in their two
overlapping elds (BLG41 and BLG22). In combination, the
KMTNet observations have a frequency of Γ = 3hr
1
for
KMTC and Γ = 2.25 hr
1
for KMTS and KMTA.
2
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.

For both events, the data sets were reduced based on the
image-subtraction methodology (Tomaney & Crotts 1996;
Alard & Lupton 1998), specically Albrow et al. (2009) for
KMTNet and Woźniak (2000) for OGLE. The photometric
error bars were then readjusted following the prescription
presented in Yee et al. (2012). We note that for the source
color measurement, we additionally carried out pyDIA (Albrow
2017) reductions for a subset of the KMTNet data, which
simultaneously returns the light curve and eld-star photometry
on the same system.
3. Light Curve Analysis
Figures 1 and 2 show the light curves of OGLE-2018-BLG-
0567 and OGLE-2018-BLG-0962, respectively. It is found that
the two events share various characteristics in common. First,
the apparent peak magnications of the baseline single-lens
single-source (1L1S) light curves are not high: A
peak
1.6 for
OGLE-2018-BLG-0567 and A
peak
4.9 for OGLE-2018-BLG-
0962. Second, the light curves of both events exhibit strong
short-term positive anomalies from the baseline 1L1S curves.
Third, the anomalies appear when the 1L1S-model lensing
magnications are low. All these characteristics strongly
suggest that the anomalies are produced by source crossings
over the planetary caustics induced by planetary companions to
(2L1S) modeling of the events under the interpretation that the
events were produced by lenses composed of two masses, M
1
and M
2
.
The standard 2L1S modeling requires one to include seven
tting parameters to describe an observed light curve. The rst
three are the Paczyński (1986) parameters (t
0
, u
0
, t
E
), which are
respectively the time of closest source approach to the lens, the
impact parameter (scaled to θ
E
), and the event timescale. The
next three (s, q, α) describe the binary-lens geometry: the
projected binary separation (scaled to θ
E
), the binary mass ratio
(q = M
2
/M
1
), and the angle between the source trajectory and
the binary axis as measured in a clockwise sense, respectively.
The last describes the source radius ρ = θ
*
/θ
E
, where θ
*
is the
angular source radius. See Figure 6 of Jung et al. (2015) for a
graphical presentation of the parameters.
Figure 1. Light curve of OGLE-2018-BLG-0567. The black solid curve on the data is the best-t 2L1S solution. The upper panel shows the enlarged view of the
planet-induced anomaly centered on
¢~
H
JD 8270
. The second and fourth panels show the residuals from the solution. The lensing parameters of the solution are
listed in Table 1 and the caustic geometry is shown in Figure 3. Note that we use the V-band data only for the source color measurement.
(The data used to create this gure are available.)
3
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.

The modeling is conducted following the procedure
described in Jung et al. (2015). In the rst step, we carry out
grid searches for the binary parameters (s, q, α), with
100 × 100 × 21 different grid points. The ranges of the grid
parameters are
-
s1 log 1
,
-
q5 log 0
, and 0
α 2π. At each grid, we x (s, q) and nd the remaining
parameters using Markov Chain Monte Carlo (MCMC) χ
2
minimization. In this modeling, the initial values of the
parameters (t
0
, u
0
, t
E
) are given as the values estimated from
a 1L1S t for the data excluding the anomaly. The initial value
of the normalized source radius is estimated from the caustic-
crossing timescale, which is related to the event timescale by
t
*
= ρt
E
. Here we use inverse ray shooting (Kayser et al. 1986;
Schneider & Weiss 1987) to compute the nite-source lensing
magnications. The ux values from the source, f
S,i
, and blend,
f
B,i
, for the data set obtained from the ith observatory are
estimated by f
i
(t) = f
S,i
A(t) + f
B,i
, where f
i
is the observed ux.
Once local solutions are found from the rst-round modeling,
we rene the individual locals by releasing all tting
parameters and allowing them to be free parameters in
an MCMC.
From the modeling, it is found that the observed lensing light
curves of both events are well described by unique 2L1S
models, in which the mass ratios between M
1
and M
2
are in the
planetary regime. In addition, the planetary perturbations in
both light curves are not subject to the closewide degeneracy.
The estimated binary parameters are (s, q) = (1.81,
1.24 × 10
3
) for OGLE-2018-BLG-0567 and (s, q) = (1.25,
2.38 × 10
3
) for OGLE-2018-BLG-0962. The full lensing
parameters and their uncertainties are presented in Table 1. The
model curves of the solutions are drawn over the data points in
Figures 1 and 2 for OGLE-2018-BLG-0567 and OGLE-2018-
BLG-BLG-0962, respectively.
In Figures 3 and 4, we present the lens-system congurations
of the individual events, showing the source trajectory with
respect to the lens components and resulting caustics. From the
congurations, it is found that the anomalies of both events are
produced by the source crossing over the planetary caustic of the
lens system. For OGLE-2018-BLG-0567, the source size is
comparable to the caustic size, and thus the detailed caustic-
crossing features, two caustic spikes and a U-shape trough
region between the spikes, were smeared out by nite-source
Figure 2. Light curve of OGLE-2018-BLG-0962. The upper panels show the close-up views of the regions around
¢~
H
JD 8271.5
(left) and
¢~
JD 8273.8
(right)
when the planet-induced perturbations occur. The lensing parameters of the 2L1S solution are listed in Table 1 and the caustic geometry is shown in Figure 4.
(The data used to create this gure are available.)
4
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.

effects. For OGLE-2018-BLG-0962, on the other hand, the
caustic is much bigger than the source size, and thus the detailed
caustic-crossing feature of the anomaly is well delineated. It is
found that the rst part of the anomaly, centered at
¢= - ~
H
JD HJD 2,450,000 days 8271.5()
, was produced by
the source passing over the two caustic segments that ank the
inner cusp (on the binary axis) of the planetary caustic, and
the second part, centered at
¢~
H
JD 8273.8
, was generated by
the source passage over the adjacent (off-axis) cusp.
We investigate the possibility of other interpretations of the
events. Especially for OGLE-2018-BLG-0567, the short-term
perturbation might, in principle, be produced by a second
source (1L2S) with a large ux ratio (Gaudi 1998). Hence, we
conduct an additional modeling of the event with the 1L2S
interpretation (Jung et al. 2017). We search for the solution
with eight tting parameters: 2 × (t
0
, u
0
, ρ) for the two sources,
I-band ux ratio q
F,I
, and a shared timescale t
E
. The results are
listed in Table 2.Wend that the 1L2S solution is disfavored
by Δχ
2
> 500. In addition, the 1L2S model not only provides a
worse t to the peak of the perturbation, but also fails to
recover the decrease in magnitude seen before and after the
peak. See Figure 5.
We check the feasibility of constraining the microlens
parallax, π
E
, by conducting an additional modeling of the light
curve. In this modeling, we simultaneously consider the
microlens-parallax and lens-orbital effects (Gould 1992;
Dominik 1998), because the light curve deviations induced
by the parallax effect can be correlated with the deviations
induced by the lens-orbital motion (Batista et al. 2011). For
OGLE-2018-BLG-0567, we nd that it is difcult to accurately
determine π
E
not only because the improvement of the t,
Δχ
2
9, is very minor, but also because the parallax vector
Figure 3. Caustic geometry of OGLE-2018-BLG-0567. The line with an arrow is the source trajectory relative to the binary-lens axis. The open circles (scaled by the
normalized source radius ρ) on the trajectory are the source positions at the times of observations. The two orange circles are the positions of binary-lens masses (M
1
and M
2
). In each panel, the cuspy closed curve drawn in black represents the caustic. The upper panel shows the enlarged view of the planetary caustic. Lengths are
scaled to the angular Einstein radius of the lens system.
Table 1
Lensing Parameters
Parameters OGLE-2018-BLG-0567 OGLE-2018-BLG-0962
c
tot
2
/dof 8677.3/9252 6892.1/6833
t
0
(
¢H
JD
) 8244.845 ± 0.025 8262.494 ± 0.053
u
0
0.733 ± 0.026 0.207 ± 0.032
t
E
(days) 24.641 ± 1.064 28.739 ± 0.298
s 1.806 ± 0.019 1.246 ± 0.027
q (10
3
) 1.240 ± 0.068 2.375 ± 0.076
α (rad) 0.623 ± 0.045 0.590 ± 0.044
ρ (10
3
) 17.675 ± 0.831 1.137 ± 0.048
f
S
0.842 ± 0.035 0.039 ± 0.007
f
B
1.026 ± 0.035 0.274 ± 0.007
5
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.

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Abstract: We present the analysis of the microlensing event KMT-2018-BLG-1743. The light curve of the event, with a peak magnification $A_{\rm peak}\sim 800$, exhibits two anomaly features, one around the peak and the other on the falling side of the light curve. An interpretation with a binary lens and a single source (2L1S) cannot describe the anomalies. By conducting additional modeling that includes an extra lens (3L1S) or an extra source (2L2S) relative to a 2L1S interpretation, we find that 2L2S interpretations with a planetary lens system and a binary source best explain the observed light curve with $\Delta\chi^2\sim 188$ and $\sim 91$ over the 2L1S and 3L1S solutions, respectively. Assuming that these $\Delta\chi^2$ values are adequate for distinguishing the models, the event is the fourth 2L2S event and the second 2L2S planetary event. The 2L2S interpretations are subject to a degeneracy, resulting in two solutions with $s>1.0$ (wide solution) and $s<1.0$ (close solution). The masses of the lens components and the distance to the lens are $(M_{\rm host}/M_\odot, M_{\rm planet}/M_{\rm J}, D_{\rm L}/{\rm kpc}) \sim (0.19^{+0.27}_{-0.111}, 0.25^{+0.34}_{-0.14}, 6.48^{+0.94}_{-1.03})$ and $\sim (0.42^{+0.34}_{-0.25}, 1.61^{+1.30}_{-0.97}, 6.04^{+0.93}_{-1.27})$ according to the wide and close solutions, respectively. The source is a binary composed of an early G dwarf and a mid M dwarf. The values of the relative lens-source proper motion expected from the two degenerate solutions, $\mu_{\rm wide}\sim 2.3$mas yr$^{-1}$ and $\mu_{\rm close} \sim 4.1$mas yr$^{-1}$, are substantially different, and thus the degeneracy can be broken by resolving the lens and source from future high-resolution imaging observations.

Journal ArticleDOI
Abstract: KMT-2016-BLG-2605, with planet-host mass ratio $q=0.012\pm 0.001$, has the shortest Einstein timescale, $t_\e = 3.41\pm 0.13\,$days, of any planetary microlensing event to date. This prompts us to examine the full sample of 7 short ($t_\e<7\,$day) planetary events with good $q$ measurements. We find that six have clustered Einstein radii $\theta_\e = 115\pm 20\,\muas$ and lens-source relative proper motions $\mu_\rel\simeq 9.5\pm 2.5\,\masyr$. For the seventh, these two quantities could not be measured. These distributions are consistent with a Galactic-bulge population of very low-mass (VLM) hosts near the hydrogen-burning limit. This conjecture could be verified by imaging at first adaptive-optics light on next-generation (30m) telescopes. Based on a preliminary assessment of the sample, "planetary" companions (i.e., below the deuterium-burning limit) are divided into "genuine planets", formed in their disks by core accretion, and very low-mass brown dwarfs, which form like stars. We discuss techniques for expanding the sample, which include taking account of the peculiar "anomaly dominated" morphology of the KMT-2016-BLG-2605 light curve.

Journal ArticleDOI
Abstract: KMT-2016-BLG-2605, with planet-host mass ratio $q=0.012\pm 0.001$, has the shortest Einstein timescale, $t_\e = 3.41\pm 0.13\,$days, of any planetary microlensing event to date. This prompts us to examine the full sample of 7 short ($t_\e<7\,$day) planetary events with good $q$ measurements. We find that six have clustered Einstein radii $\theta_\e = 115\pm 20\,\muas$ and lens-source relative proper motions $\mu_\rel\simeq 9.5\pm 2.5\,\masyr$. For the seventh, these two quantities could not be measured. These distributions are consistent with a Galactic-bulge population of very low-mass (VLM) hosts near the hydrogen-burning limit. This conjecture could be verified by imaging at first adaptive-optics light on next-generation (30m) telescopes. Based on a preliminary assessment of the sample, "planetary" companions (i.e., below the deuterium-burning limit) are divided into "genuine planets", formed in their disks by core accretion, and very low-mass brown dwarfs, which form like stars. We discuss techniques for expanding the sample, which include taking account of the peculiar "anomaly dominated" morphology of the KMT-2016-BLG-2605 light curve.

Journal ArticleDOI

Abstract: We present the analysis of the microlensing event KMT-2018-BLG-1743. The light curve of the event, with a peak magnification $A_{\rm peak}\sim 800$, exhibits two anomaly features, one around the peak and the other on the falling side of the light curve. An interpretation with a binary lens and a single source (2L1S) cannot describe the anomalies. By conducting additional modeling that includes an extra lens (3L1S) or an extra source (2L2S) relative to a 2L1S interpretation, we find that 2L2S interpretations with a planetary lens system and a binary source best explain the observed light curve with $\Delta\chi^2\sim 188$ and $\sim 91$ over the 2L1S and 3L1S solutions, respectively. Assuming that these $\Delta\chi^2$ values are adequate for distinguishing the models, the event is the fourth 2L2S event and the second 2L2S planetary event. The 2L2S interpretations are subject to a degeneracy, resulting in two solutions with $s>1.0$ (wide solution) and $s<1.0$ (close solution). The masses of the lens components and the distance to the lens are $(M_{\rm host}/M_\odot, M_{\rm planet}/M_{\rm J}, D_{\rm L}/{\rm kpc}) \sim (0.19^{+0.27}_{-0.111}, 0.25^{+0.34}_{-0.14}, 6.48^{+0.94}_{-1.03})$ and $\sim (0.42^{+0.34}_{-0.25}, 1.61^{+1.30}_{-0.97}, 6.04^{+0.93}_{-1.27})$ according to the wide and close solutions, respectively. The source is a binary composed of an early G dwarf and a mid M dwarf. The values of the relative lens-source proper motion expected from the two degenerate solutions, $\mu_{\rm wide}\sim 2.3$mas yr$^{-1}$ and $\mu_{\rm close} \sim 4.1$mas yr$^{-1}$, are substantially different, and thus the degeneracy can be broken by resolving the lens and source from future high-resolution imaging observations.

##### References
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Journal ArticleDOI
Abstract: Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims: A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods: The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results: Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330-680 nm) and GRP (630-1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions: Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

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Abstract: Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.

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Abstract: Since the Hipparcos mission and recent large scale surveys in the optical and the near-infrared, new constraints have been obtained on the structure and evolution history of the Milky Way. The population synthesis approach is a useful tool to interpret such data sets and to test scenarios of evolution of the Galaxy. We present here new constraints on evolution parameters obtained from the Besancon model of population synthesis and analysis of optical and near-infrared star counts. The Galactic potential is computed self-consistently, in agreement with Hipparcos results and the observed rotation curve. Constraints are posed on the outer bulge structure, the warped and flared disc, the thick disc and the spheroid populations. The model is tuned to produce reliable predictions in the visible and the near-infrared in wide photometric bands from U to K. Finally, we describe applications such as photometric and astrometric simulations and a new classification tool based on a Bayesian probability estimator, which could be used in the framework of Virtual Observatories. As examples, samples of simulated star counts at different wavelengths and directions are also given.

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Abstract: The relations between colors of the JHKL systems of several observatories are examined, and linear relations are derived for transformation between the (J-K), (J-H), (H-K), and (K-L) colors in the different systems. A homogenized system is proposed, based on the systems of Glass (1984) and Johnson et al. (1966). The homogenized data sets are used to derive intrinsic colors for a number of giants and dwarfs. The passbands of several IR systems are estimated and the synthetic colors of the systems are compared using blackbody and stellar fluxes. The passbands were adjusted in wavelength to produce agreement with observed relations between different systems, making it possible to estimate the effective wavelengths of the different natural systems.

2,145 citations

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