New Method Used to Detect Exomoon Around 4 Jupiter Mass Free Floating Gas Giant

NEW METHOD TO MEASURE PROPER MOTIONS OF MICROLENSED SOURCES: APPLICATION TO CANDIDATE FREE-FLOATING-PLANET EVENT MOA-2011-BLG-262

Authors:

Skowron et al

Abstract:

We develop a new method to measure source proper motions in microlensing events, which can partially overcome problems due to blending. It takes advantage of the fact that the source position is known precisely from the microlensing event itself. We apply this method to the event MOA-2011-BLG-262, which has a short timescale t E = 3.8 day, a companion mass ratio q = 4.7 × 10–3, and a very high or high lens-source relative proper motion μrel = 20 mas yr–1 or 12 mas yr–1 (for two possible models). These three characteristics imply that the lens could be a brown dwarf or a massive planet with a roughly Earth-mass "moon." The probability of such an interpretation would be greatly increased if it could be shown that the high lens-source relative proper motion was primarily due to the lens rather than the source. Based on the long-term monitoring data of the Galactic bulge from the Optical Gravitational Lensing Experiment, we measure the source proper motion that is small, ${\boldsymbol {\mu }}_s = (-2.3, -0.9)\pm (2.8,2.6)\,{\rm mas\:yr^{-1}}$ in a (north, east) Galactic coordinate frame. These values are then important input into a Bayesian analysis of the event presented in a companion paper by Bennett et al.

MOA-2011-BLG-262Lb: a SubEarth Mass ExoMoon Orbiting a Gas Giant?

MOA-2011-BLG-262Lb: A SUB-EARTH-MASS MOON ORBITING A GAS GIANT PRIMARY OR A HIGH VELOCITY PLANETARY SYSTEM IN THE GALACTIC BULGE

Authors:

Bennett et al

Abstract:

We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product ML πrel where ML is the lens system mass and πrel is the lens-source relative parallax. If the lens system is nearby (large πrel), then ML is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, μrel = 19.6 ± 1.6 mas yr–1, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, there is an alternate solution that has a lower proper motion and fits the data almost as well. This solution is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of $M_{\rm host} = 0.12^{+0.19}_{ -0.06}\,M_\odot$ and $m_{\rm comp} = 18^{+28}_{ -10}\,{M_\oplus }$, at a projected separation of $a_\perp = 0.84^{+0.25}_{ -0.14}$ AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.