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Register a new userSpectroscopic characterisation of microlensing events Towards a new interpretation of OGLE-2011-BLG-0417
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The microlensing event OGLE-2011-BLG-0417 is an exceptionally bright lens binary that was predicted to present radial velocity variation at the level of several km/s. Pioneer radial velocity follow-up observations with the UVES spectrograph at the ESO - VLT of this system clearly ruled out the large radial velocity variation, leaving a discrepancy between the observation and the prediction. In this paper, we further characterise the microlensing system by analysing its spectral energy distribution (SED) derived using the UVES spectrum and new observations with the ARCoIRIS (CTIO) near-infrared spectrograph and the Keck adaptive optics instrument NIRC2 in the J, H, and Ks bands. We determine the mass and distance of the stars independently from the microlensing modelling. We find that the SED is compatible with a giant star in the Galactic bulge and a foreground star with a mass of 0.94+/-0.09Msun at a distance of 1.07+/-0.24kpc. We find that this foreground star is likely the lens. Its parameters are not compatible with the ones previously reported in the literature (0.52+/-0.04Msun at 0.95+/-0.06kpc), based on the microlensing light curve. A thoughtful re-analysis of the microlensing event is mandatory to fully understand the reason of this new discrepancy. More importantly, this paper demonstrates that spectroscopic follow-up observations of microlensing events are possible and provide independent constraints on the parameters of the lens and source stars, hence breaking some degeneracies in the analysis. UV-to-NIR low-resolution spectrographs like X-SHOOTER (ESO - VLT) could substantially contribute to this follow-up efforts, with magnitude limits above all microlensing events detected so far.
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We report the discovery of a Jupiter-mass planet orbiting an M-dwarf star that gave rise to the microlensing event OGLE-2011-BLG-0265. Such a system is very rare among known planetary systems and thus the discovery is important for theoretical studies of planetary formation and evolution. High-cadence temporal coverage of the planetary signal combined with extended observations throughout the event allows us to accurately model the observed light curve. The final microlensing solution remains, however, degenerate yielding two possible configurations of the planet and the host star. In the case of the preferred solution, the mass of the planet is $M_{rm p} = 0.9pm 0.3 M_{rm J}$, and the planet is orbiting a star with a mass $M = 0.22pm 0.06 M_odot$. The second possible configuration (2$sigma$ away) consists of a planet with $M_{rm p}=0.6pm 0.3 M_{rm J}$ and host star with $M=0.14pm 0.06 M_odot$. The system is located in the Galactic disk 3 -- 4 kpc towards the Galactic bulge. In both cases, with an orbit size of 1.5 -- 2.0 AU, the planet is a cold Jupiter -- located well beyond the snow line of the host star. Currently available data make the secure selection of the correct solution difficult, but there are prospects for lifting the degeneracy with additional follow-up observations in the future, when the lens and source star separate.
We present the analysis of the gravitational microlensing event OGLE-2011-BLG-0251. This anomalous event was observed by several survey and follow-up collaborations conducting microlensing observations towards the Galactic Bulge. Based on detailed modelling of the observed light curve, we find that the lens is composed of two masses with a mass ratio q=1.9 x 10^-3. Thanks to our detection of higher-order effects on the light curve due to the Earths orbital motion and the finite size of source, we are able to measure the mass and distance to the lens unambiguously. We find that the lens is made up of a planet of mass 0.53 +- 0.21,M_Jup orbiting an M dwarf host star with a mass of 0.26 +- 0.11 M_Sun. The planetary system is located at a distance of 2.57 +- 0.61 kpc towards the Galactic Centre. The projected separation of the planet from its host star is d=1.408 +- 0.019, in units of the Einstein radius, which corresponds to 2.72 +- 0.75 AU in physical units. We also identified a competitive model with similar planet and host star masses, but with a smaller orbital radius of 1.50 +- 0.50 AU. The planet is therefore located beyond the snow line of its host star, which we estimate to be around 1-1.5 AU.
We present an analysis of microlensing event OGLE-2016-BLG-0693, based on the survey-only microlensing observations by the OGLE and KMTNet groups. In order to analyze the light curve, we consider the effects of parallax, orbital motion, and baseline slope, and also refine the result using a Galactic model prior. From the microlensing analysis, we find that the event is a binary composed of a low-mass brown dwarf 49+-20 M_J companion and a K- or G-dwarf host, which lies at a distance 5.0+-0.6 kpc toward the Galactic bulge. The projected separation between the brown dwarf and its host star is less than 5 AU, and thus it is likely that the brown dwarf companion is located in the brown dwarf desert.
Global second-generation microlensing surveys aim to discover and characterize extrasolar planets and their frequency, by means of round-the-clock high-cadence monitoring of a large area of the Galactic bulge, in a controlled experiment. We report the discovery of a giant planet in microlensing event MOA-2011-BLG-322. This moderate-magnification event, which displays a clear anomaly induced by a second lensing mass, was inside the footprint of our second-generation microlensing survey, involving MOA, OGLE and the Wise Observatory. The event was observed by the survey groups, without prompting alerts that could have led to dedicated follow-up observations. Fitting a microlensing model to the data, we find that the timescale of the event was t_E=23.2 +/-0.8 days, and the mass ratio between the lens star and its companion is q=0.028 +/-0.001. Finite-source effects are marginally detected, and upper limits on them help break some of the degeneracy in the system parameters. Using a Bayesian analysis that incorporates a Galactic structure model, we estimate the mass of the lens at 0.39 +0.45/-0.19 M_sun, at a distance of 7.56 +/-0.91 kpc. Thus, the companion is likely a planet of mass 11.6 +13.4/-5.6 M_J, at a projected separation of 4.3 +1.5/-1.2 AU, rather far beyond the snow line. This is the first pure-survey planet reported from a second-generation microlensing survey, and shows that survey data alone can be sufficient to characterize a planetary model. With the detection of additional survey-only planets, we will be able to constrain the frequency of extrasolar planets near their systems snow lines.
We analyze the combined Spitzer and ground-based data for OGLE-2017-BLG-1140 and show that the event was generated by a Jupiter-class $(m_psimeq 1.6,M_{rm jup})$ planet orbiting a mid-late M dwarf $(Msimeq 0.2,M_odot)$ that lies $D_{LS}simeq 1.0,mathrm{kpc}$ in the foreground of the microlensed, Galactic-bar, source star. The planet-host projected separation is $a_perp simeq 1.0,mathrm{au}$, i.e., well-beyond the snow line. By measuring the source proper motion ${mathbf{mu}}_s$ from ongoing, long-term OGLE imaging, and combining this with the lens-source relative proper motion ${mathbf{mu}}_mathrm{rel}$ derived from the microlensing solution, we show that the lens proper motion ${mathbf{mu}}_l={mathbf{mu}}_mathrm{rel} + {mathbf{mu}}_s$ is consistent with the lens lying in the Galactic disk, although a bulge lens is not ruled out. We show that while the Spitzer and ground-based data are comparably well fitted by planetary (i.e., binary-lens, 2L1S) models and by binary-source (1L2S) models, the combination of Spitzer and ground-based data decisively favor the planetary model. This is a new channel to resolve the 2L1S/1L2S degeneracy, which can be difficult to break in some cases.
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