Microlensing Event Moa-2007-BLG-400: Exhuming the Buried Signature of a Cool, Jovian-Mass Planet

Abstract

We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb. The planet was detected in a high-magnification microlensing event (with peak magnification A max = 628) in which the primary lens transited the source, resulting in a dramatic smoothing of the peak of the event. The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source, and as a result the planetary signature is also smoothed out by the finite source size. Thus the deviation from a single-lens fit is broad and relatively weak (∼ few percent). Nevertheless, we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals, and detailed analysis yields a fairly precise planet/star mass ratio of q = 2.6 ± 0.4 × 10 −3 , in accord with the large significance (∆χ 2 = 1070) of the detection. The planet/star projected separation is subject to a strong close/wide degeneracy, leading to two indistinguishable solutions that differ in separation by a factor of ∼ 8.5. Upper limits on flux from the lens constrain its mass to be M < 0.75 M ⊙ (assuming it is a main-sequence star). A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of ∼ 0.2 − 0.5M ⊙ and thus a planet mass of ∼ 0.5 − 1.3M Jup . The separation and equilibrium temperature are ∼ 0.6 −1.1 AU (∼ 5.3 −9.7 AU) and ∼ 103 K (∼ 34 K) for the close (wide) solution. If the primary is a mainsequence star, follow-up observations would enable the detection of its light and so a measurement of its mass and distance.

OGLE 2008–BLG–290: an accurate measurement of the limb darkening of a galactic bulge K Giant spatially resolved by microlensing

“…The event peaked on that same night (June 15). Because such events are reasonably sensitive to Jupiter-mass planets, as recently shown by Dong et al (2009), amateur telescopes from the μFUN network were alerted, resulting in an excellent coverage of the peak region from 9 different telescopes.…”

“…Our exploration of the parameter space uses the q w α parameterization introduced and justified by Dong et al (2009). q is the mass ratio between the two components of the lens system (q < 1), w is the width of the central caustic as defined by Chung et al (2005) (given there by their Eq.…”

“…Han & Kim (2009) argue that a 5% deviation can be detected even when the central caustic width w is larger than a quarter of the source diameter (0.5 ρ * ). Indeed, with adequate photometric accuracy, Dong et al (2009) We therefore use a grid with w between 10 −3 and 10 −2 and q between 10 −4 and 10 −2 . We find a marginal improvement of 55 in χ 2 in a region centred around log q = −2.5, log w = −2.5 and α = −0.459 rad, corresponding to w = 0.14 ρ * .…”

“…In addition, there are a comparable number of events that required including limb darkening in their analysis (such as MACHO Alert 95−30), but for which a sufficiently accurate limb-darkening measurement was not (and often could not be) performed. These include two other single-lens events (Yee et al 2009;Batista et al 2009), as well as all ten published planetary microlensing events, in which a star with a planet acted as the lens (Bond et al 2004;Udalski et al 2005;Beaulieu et al 2006;Gould et al 2006;Gaudi et al 2008;Bennett et al 2008;Dong et al 2009;Sumi et al 2010;Janczak et al 2010).…”

Mass measurement of a single unseen star and planetary detection efficiency for OGLE 2007-BLG-050

“…In these specific cases, for which the impact parameter can be very small, finite-source effects might strongly affect and diminish a possible planetary signal (e.g., Dong et al 2009b;Bennett et Rhie 1996). In the absence of any deviation from a finite-source single point-lens model, one can still compute the planet detection efficiency in order to derive upper limits on the probability that the lens harbors a planet (Gaudi & Sackett 2000).…”