Gravitational microlensing observations in the direction of the Galactic Bulge have come up with a surprising result: the discovery of ten previously unknown extrasolar planets that are not bound to host stars. These seemingly free-ranging Jupiter-mass objects could be in very distant orbits around host stars, but no hosts could be detected within a distance of 10 astronomical units from the free-floating planets. It seems possible, therefore, that planet scattering is a routine part of the planet formation process.

Unbound or distant planetary mass population detected by gravitational microlensing

Robust fast methods to classify variable light curves in large sky surveys are becoming increasingly important. While it is relatively straightforward to identify common periodic stars and particular transient events (supernovae, novae, microlensing events), there is no equivalent for non-periodic continuously varying sources (quasars, aperiodic stellar variability). In this paper, we present a fast method for modeling and classifying such sources. We demonstrate the method using ~86, 000 variable sources from the OGLE-II survey of the LMC and ~2700 mid-IR-selected quasar candidates from the OGLE-III survey of the LMC and SMC. We discuss the location of common variability classes in the parameter space of the model. In particular, we show that quasars occupy a distinct region of variability space, providing a simple quantitative approach to the variability selection of quasars.

Quantifying quasar variability as part of a general approach to classifying continuously varying sources

We present improved estimates of several global properties of the Milky Way, including its current star formation rate (SFR), the stellar mass contained in its disk and bulge+bar components, as well as its total stellar mass. We do so by combining previous measurements from the literature using a hierarchical Bayesian (HB) statistical method that allows us to account for the possibility that any value may be incorrect or have underestimated errors. We show that this method is robust to a wide variety of assumptions about the nature of problems in individual measurements or error estimates. Ultimately, our analysis yields a SFR for the Galaxy of $\dot{\mathrm{M}}_\star=1.65\pm0.19$ $\textrm{M}_\odot \textrm{yr}^{-1}$, assuming a Kroupa initial mass function (IMF). By combining HB methods with Monte Carlo simulations that incorporate the latest estimates of the Galactocentric radius of the Sun, $R_0$, the exponential scale length of the disk, $L_d$, and the local surface density of stellar mass, $\Sigma_\star(R_0)$, we show that the mass of the Galactic bulge+bar is $\textrm{M}_\star^B=0.91\pm0.07\times10^{10}$ $\textrm{M}_\odot$, the disk mass is $\textrm{M}_\star^D=5.17\pm1.11\times10^{10}$ $\textrm{M}_\odot$, and their combination yields a total stellar mass of $\textrm{M}_\star=6.08\pm1.14\times10^{10}$ $\textrm{M}_\odot$ (assuming a Kroupa IMF and an exponential disk profile). This analysis is based upon a new compilation of literature bulge mass estimates, normalized to common assumptions about the stellar initial mass function and Galactic disk properties, presented herein. We additionally find a bulge-to-total mass ratio for the Milky Way of $B/T=0.150^{+0.028}_{-0.019}$ and a specific star formation rate of $\dot{\mathrm{M}}_\star/\textrm{M}_\star=2.71\pm0.59\times10^{-11} \textrm{yr}^{-1}$.

http://iopscience.iop.org/article/10.1088/0004-637X/806/1/96/pdf

Improved Estimates of the Milky Way’s Stellar Mass and Star Formation Rate From Hierarchical Bayesian Meta-analysis

We report the detection of an extrasolar planet of mass ratio q~2×10-4 in microlensing event MOA-2007-BLG-192. The best-fit microlensing model shows both the microlensing parallax and finite source effects, and these can be combined to obtain the lens masses of M=0.060+0.028-0.021 Msolar for the primary and m=3.3+4.9-1.6 M? for the planet. However, the observational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimated without the benefit of a source star color measurement. As a result, the 2 ? limits on the mass ratio and finite source measurements are weak. Nevertheless, the microlensing parallax signal clearly favors a substellar mass planetary host, and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images taken with the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass. In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date, aside from the lowest mass pulsar planet.

http://iopscience.iop.org/article/10.1086/589940/pdf

A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192

We present the discovery of a Neptune-mass planet OGLE-2007-BLG-368Lb with a planet-star mass ratio of q = [9.5 ± 2.1] × 10^(-5) via gravitational microlensing. The planetary deviation was detected in real-time thanks to the high cadence of the Microlensing Observations in Astrophysics survey, real-time light-curve monitoring and intensive follow-up observations. A Bayesian analysis returns the stellar mass and distance at M_l = 0.64^(+0.21)_(–0.26) M_☉ and D_l = 5.9^(+0.9)_(–1.4) kpc, respectively, so the mass and separation of the planet are M_p = 20^(+7)_(–8) M_⊕ and a = 3.3^(+1.4)_(–0.8) AU, respectively. This discovery adds another cold Neptune-mass planet to the planetary sample discovered by microlensing, which now comprises four cold Neptune/super-Earths, five gas giant planets, and another sub-Saturn mass planet whose nature is unclear. The discovery of these 10 cold exoplanets by the microlensing method implies that the mass ratio function of cold exoplanets scales as dN_(pl)/d log q ∝ q^(–0.7±0.2) with a 95% confidence level upper limit of n < –0.35 (where dN_(pl)/d log q ∝ q^n). As microlensing is most sensitive to planets beyond the snow-line, this implies that Neptune-mass planets are at least three times more common than Jupiters in this region at the 95% confidence level.

/pdf/a-cold-neptune-mass-planet-ogle-2007-blg-368lb-cold-neptunes-4hlpdun330.pdf

A COLD NEPTUNE-MASS PLANET OGLE-2007-BLG-368Lb: COLD NEPTUNES ARE COMMON

We analyze the data of the gravitational microlensing survey carried out by the Microlensing Observations in Astrophysics (MOA) group during 2000 toward the Galactic bulge (GB). Our observations are designed to detect efficient high-magnification events with faint source stars and short-timescale events, by increasing the sampling rate up to ~6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg2. We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale tE,out in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth τ = 2.59 × 10-6 toward the GB for events with timescales 0.3 10). These events are useful for studies of extrasolar planets.

https://iopscience.iop.org/article/10.1086/375212/pdf

Microlensing Optical Depth toward the Galactic Bulge from Microlensing Observations in Astrophysics Group Observations during 2000 with Difference Image Analysis

S. Wilkinson

Papers

Microlensing Optical Depth toward the Galactic Bulge from Microlensing Observations in Astrophysics Group Observations during 2000 with Difference Image Analysis