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Register a new userRevisiting MOA 2013 BLG-220L: A Solar-type star with a Cold super-Jupiter Companion
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Aikaterini Vandorou
Publication date
2019
fields
Physics
and research's language is
English
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We present the analysis of high-resolution images of MOA-2013-BLG-220, taken with the Keck adaptive optics system 6 years after the initial observation, identifying the lens as a solar-type star hosting a super-Jupiter mass planet. The masses of planets and host-stars discovered by microlensing are often not determined from light curve data, while the star-planet mass-ratio and projected separation in units of Einstein ring radius are well measured. High-resolution follow-up observations after the lensing event is complete can resolve the source and lens. This allows direct measurements of flux, and the amplitude and direction of proper motion, giving strong constraints on the system parameters. Due to the high relative proper motion, $mu_{rm rel,Geo} = 12.62pm0.11$ mas/yr, the source and lens were resolved in 2019, with a separation of $77.1pm0.5$ mas. Thus, we constrain the lens flux to $K_{rm Keck,lens}= 17.92pm0.02$. By combining constraints from the model and Keck flux, we find the lens mass to be $M_L = 0.88pm0.05 M_odot$ at $D_L = 6.72pm0.59$ kpc. With a mass-ratio of $q=(3.00pm0.03)times10^{-3}$ the planets mass is determined to be $M_P = 2.74pm0.17 M_{J}$ at a separation of $r_perp = 3.03pm0.27$ AU. The lens mass is much higher than the prediction made by the Bayesian analysis that assumes all stars have an equal probability to host a planet of the measured mass ratio, and suggests that planets with mass ratios of a few 10$^{-3}$ are more common orbiting massive stars. This demonstrates the importance of high-resolution follow-up observations for testing theories like these.
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We report the discovery of MOA-2013-BLG-220Lb, which has a super-Jupiter mass ratio $q=3.01pm 0.02times 10^{-3}$ relative to its host. The proper motion, $mu=12.5pm 1, {rm mas},{rm yr}^{-1}$, is one of the highest for microlensing planets yet discovered, implying that it will be possible to separately resolve the host within $sim 7$ years. Two separate lines of evidence imply that the planet and host are in the Galactic disk. The planet could have been detected and characterized purely with follow-up data, which has important implications for microlensing surveys, both current and into the LSST era.
We present the discovery of the first Neptune analog exoplanet or super-Earth with Neptune-like orbit, MOA-2013-BLG-605Lb. This planet has a mass similar to that of Neptune or a super-Earth and it orbits at $9sim 14$ times the expected position of the snow-line, $a_{rm snow}$, which is similar to Neptunes separation of $ 11,a_{rm snow}$ from the Sun. The planet/host-star mass ratio is $q=(3.6pm0.7)times 10^{-4}$ and the projected separation normalized by the Einstein radius is $s=2.39pm0.05$. There are three degenerate physical solutions and two of these are due to a new type of degeneracy in the microlensing parallax parameters, which we designate the wide degeneracy. The three models have (i) a Neptune-mass planet with a mass of $M_{rm p}=21_{-7}^{+6} M_{Earth}$ orbiting a low-mass M-dwarf with a mass of $M_{rm h}=0.19_{-0.06}^{+0.05} M_odot$, (ii) a mini-Neptune with $M_{rm p}= 7.9_{-1.2}^{+1.8} M_{Earth}$ orbiting a brown dwarf host with $M_{rm h}=0.068_{-0.011}^{+0.019} M_odot$ and (iii) a super-Earth with $M_{rm p}= 3.2_{-0.3}^{+0.5} M_{Earth}$ orbiting a low-mass brown dwarf host with $M_{rm h}=0.025_{-0.004}^{+0.005} M_odot$ which is slightly favored. The 3-D planet-host separations are 4.6$_{-1.2}^{+4.7}$ AU, 2.1$_{-0.2}^{+1.0}$ AU and 0.94$_{-0.02}^{+0.67}$ AU, which are $8.9_{-1.4}^{+10.5}$, $12_{-1}^{+7}$ or $14_{-1}^{+11}$ times larger than $a_{rm snow}$ for these models, respectively. The Keck AO observation confirm that the lens is faint. This discovery suggests that low-mass planets with Neptune-like orbit are common. So processes similar to the one that formed Neptune in our own Solar System or cold super-Earth may be common in other solar systems.
We present a detailed analysis of survey and follow-up observations of microlensing event OGLE-2012-BLG-0406 based on data obtained from 10 different observatories. Intensive coverage of the lightcurve, especially the perturbation part, allowed us to accurately measure the parallax effect and lens orbital motion. Combining our measurement of the lens parallax with the angular Einstein radius determined from finite-source effects, we estimate the physical parameters of the lens system. We find that the event was caused by a $2.73pm 0.43 M_{rm J}$ planet orbiting a $0.44pm 0.07 M_{odot}$ early M-type star. The distance to the lens is $4.97pm 0.29$ kpc and the projected separation between the host star and its planet at the time of the event is $3.45pm 0.26$ AU. We find that the additional coverage provided by follow-up observations, especially during the planetary perturbation, leads to a more accurate determination of the physical parameters of the lens.
We present Keck/NIRC2 adaptive optics imaging of planetary microlensing event MOA-2007-BLG-400 that resolves the lens star system from the source. We find that the MOA-2007-BLG-400L planetary system consists of a $1.71pm 0.27 M_{rm Jup}$ planet orbiting a $0.69pm 0.04M_{odot}$ K-dwarf host star at a distance of $6.89pm 0.77,$kpc from the Sun. So, this planetary system probably resides in the Galactic bulge. The planet-host star projected separation is only weakly constrained due to the close-wide light curve degeneracy; the 2$sigma$ projected separation range is 0.6--$7.2,$AU. This host mass is at the top end of the range of masses predicted by a standard Bayesian analysis that assumes that all stars have an equal chance of hosting a star of the observed mass ratio. This and the similar result for event MOA-2013-BLG-220 suggests that more massive stars may be more likely to host planets with a mass ratio in the $0.002 < q < 0.004$ range that orbit beyond the snow line. These results also indicate the importance of host star mass measurements for exoplanets found by microlensing. The microlensing survey imaging data from NASAs Nancy Grace Roman Space Telescope (formerly WFIRST) mission will be doing mass measurements like this for a huge number of planetary events. This host lens is the highest contrast lens-source detected in microlensing mass measurement analysis (the lens being 10$times$ fainter than the source). We present an improved method of calculating photometry and astrometry uncertainties based on the Jackknife method, which produces more accurate errors that are $sim$$2.5 times$ larger than previous estimates.
We present an analysis of the anomalous microlensing event, MOA-2010-BLG-073, announced by the Microlensing Observations in Astrophysics survey on 2010-03-18. This event was remarkable because the source was previously known to be photometrically variable. Analyzing the pre-event source lightcurve, we demonstrate that it is an irregular variable over time scales >200d. Its dereddened color, $(V-I)_{S,0}$, is 1.221$pm$0.051mag and from our lens model we derive a source radius of 14.7$pm$1.3 $R_{odot}$, suggesting that it is a red giant star. We initially explored a number of purely microlensing models for the event but found a residual gradient in the data taken prior to and after the event. This is likely to be due to the variability of the source rather than part of the lensing event, so we incorporated a slope parameter in our model in order to derive the true parameters of the lensing system. We find that the lensing system has a mass ratio of q=0.0654$pm$0.0006. The Einstein crossing time of the event, $T_{rm{E}}=44.3$pm$0.1d, was sufficiently long that the lightcurve exhibited parallax effects. In addition, the source trajectory relative to the large caustic structure allowed the orbital motion of the lens system to be detected. Combining the parallax with the Einstein radius, we were able to derive the distance to the lens, $D_L$=2.8$pm$0.4kpc, and the masses of the lensing objects. The primary of the lens is an M-dwarf with $M_{L,p}$=0.16$pm0.03M_{odot}$ while the companion has $M_{L,s}$=11.0$pm2.0M_{rm{J}}$ putting it in the boundary zone between planets and brown dwarfs.
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