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Register a new user$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf
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The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18pm1M_J$ or $20pm1M_J$. This is the lowest isolated-object mass measurement to date, only $sim$45% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9pm0.1$ kpc or $4.1pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.
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We analyze the binary microlensing event MOA-2016-BLG-231, which was observed from the ground and from Spitzer. The lens is composed of very low-mass brown dwarfs (BDs) with $M_1 = 21^{+12}_{-5} M_J$ and $M_2 = 9^{+5}_{-2} M_J$, and it is located in the Galactic disk $D_{rm L} = 2.85^{+0.88}_{-0.50} {rm kpc}$. This is the fifth binary brown dwarf discovered by microlensing, and the BD binary is moving counter to the orbital motion of disk stars. Constraints on the lens physical properties come from late time, non-caustic-crossing features of the Spitzer light curve. Thus, MOA-2016-BLG-231 shows how Spitzer plays a crucial role in resolving the nature of BDs in binary BD events with short timescale ($lesssim 10$ days).
We report the discovery and analysis of the planetary microlensing event OGLE-2017-BLG-0406, which was observed both from the ground and by the ${it Spitzer}$ satellite in a solar orbit. At high magnification, the anomaly in the light curve was densely observed by ground-based-survey and follow-up groups, and it was found to be explained by a planetary lens with a planet/host mass ratio of $q=7.0 times 10^{-4}$ from the light-curve modeling. The ground-only and ${it Spitzer}$-only data each provide very strong one-dimensional (1-D) constraints on the 2-D microlens parallax vector $bf{pi_{rm E}}$. When combined, these yield a precise measurement of $bf{pi_{rm E}}$, and so of the masses of the host $M_{rm host}=0.56pm0.07,M_odot$ and planet $M_{rm planet} = 0.41 pm 0.05,M_{rm Jup}$. The system lies at a distance $D_{rm L}=5.2 pm 0.5 {rm kpc}$ from the Sun toward the Galactic bulge, and the host is more likely to be a disk population star according to the kinematics of the lens. The projected separation of the planet from the host is $a_{perp} = 3.5 pm 0.3 {rm au}$, i.e., just over twice the snow line. The Galactic-disk kinematics are established in part from a precise measurement of the source proper motion based on OGLE-IV data. By contrast, the ${it Gaia}$ proper-motion measurement of the source suffers from a catastrophic $10,sigma$ error.
We report the discovery of a $Spitzer$ microlensing planet OGLE-2018-BLG-0596Lb, with preferred planet-host mass ratio $q sim 2times10^{-4}$. The planetary signal, which is characterized by a short $(sim 1~{rm day})$ bump on the rising side of the lensing light curve, was densely covered by ground-based surveys. We find that the signal can be explained by a bright source that fully envelops the planetary caustic, i.e., a Hollywood geometry. Combined with the source proper motion measured from $Gaia$, the $Spitzer$ satellite parallax measurement makes it possible to precisely constrain the lens physical parameters. The preferred solution, in which the planet perturbs the minor image due to lensing by the host, yields a Uranus-mass planet with a mass of $M_{rm p} = 13.9pm1.6~M_{oplus}$ orbiting a mid M-dwarf with a mass of $M_{rm h} = 0.23pm0.03~M_{odot}$. There is also a second possible solution that is substantially disfavored but cannot be ruled out, for which the planet perturbs the major image. The latter solution yields $M_{rm p} = 1.2pm0.2~M_{oplus}$ and $M_{rm h} = 0.15pm0.02~M_{odot}$. By combining the microlensing and $Gaia$ data together with a Galactic model, we find in either case that the lens lies on the near side of the Galactic bulge at a distance $D_{rm L} sim 6pm1~{rm kpc}$. Future adaptive optics observations may decisively resolve the major image/minor image degeneracy.
We report the analysis of planetary microlensing event OGLE-2018-BLG-1185, which was observed by a large number of ground-based telescopes and by the $Spitzer$ Space Telescope. The ground-based light curve indicates a low planet-host star mass ratio of $q = (6.9 pm 0.2) times 10^{-5}$, which is near the peak of the wide-orbit exoplanet mass-ratio distribution. We estimate the host star and planet masses with a Bayesian analysis using the measured angular Einstein radius under the assumption that stars of all masses have an equal probability to host this planet. The flux variation observed by $Spitzer$ was marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint that this flux variation should be $Delta f_{rm Spz} < 4$ instrumental flux units indicates a host mass of $M_{rm host} = 0.37^{+0.35}_{-0.21} M_odot$ and a planet mass of $m_{rm p} = 8.4^{+7.9}_{-4.7} M_oplus$. A Bayesian analysis including the full parallax constraint from $Spitzer$ suggests smaller host star and planet masses of $M_{rm host} = 0.091^{+0.064}_{-0.018} M_odot$ and $m_{rm p} = 2.1^{+1.5}_{-0.4} M_oplus$, respectively. Future high-resolution imaging observations with $HST$ or ELTs could distinguish between these two scenarios and help to reveal the planetary system properties in more detail.
We present microlensing planet OGLE-2017-BLG-0173Lb, with planet-host mass ratio either $qsimeq 2.5times 10^{-5}$ or $qsimeq 6.5times 10^{-5}$, the lowest or among the lowest ever detected. The planetary perturbation is strongly detected, $Deltachi^2sim 10,000$, because it arises from a bright (therefore, large) source passing over and enveloping the planetary caustic: a so-called Hollywood event. The factor $sim 2.5$ offset in $q$ arises because of a previously unrecognized discrete degeneracy between Hollywood events in which the caustic is fully enveloped and those in which only one flank is enveloped, which we dub Cannae and von Schlieffen, respectively. This degeneracy is accidental in that it arises from gaps in the data. Nevertheless, the fact that it appears in a $Deltachi^2=10,000$ planetary anomaly is striking. We present a simple formalism to estimate the sensitivity of other Hollywood events to planets and show that they can lead to detections close to, but perhaps not quite reaching, the Earth/Sun mass ratio of $3times 10^{-6}$. This formalism also enables an analytic understanding of the factor $sim 2.5$ offset in $q$ between the Cannae and von Schlieffen solutions. The Bayesian estimates for the host-mass, system distance, and planet-host projected separation are $M=0.39^{+0.40}_{-0.24},M_odot$, $D_L=4.8^{+1.5}_{-1.8},kpc$, and $a_perp=3.8pm 1.6,au$. The two estimates of the planet mass are $m_p=3.3^{+3.8}_{-2.1},M_oplus$ and $m_p=8^{+11}_{-6},M_oplus$. The measured lens-source relative proper motion $mu=6,masyr$ will permit imaging of the lens in about 15 years or at first light on adaptive-optics imagers on next-generation telescopes. These will allow to measure the host mass but probably cannot resolve the planet-host mass-ratio degeneracy.
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