Host Star Mass Research Articles

Radiation Hydrodynamics Simulations of Protoplanetary Disks: Stellar Mass Dependence of the Disk Photoevaporation Rate

Abstract Recent multiwavelength observations suggest that inner parts of protoplanetary disks (PPDs) have shorter lifetimes for heavier host stars. Since PPDs around high-mass stars are irradiated by strong ultraviolet radiation, photoevaporation may provide an explanation for the observed trend. We perform radiation hydrodynamics simulations of photoevaporation of PPDs for a wide range of host star mass of M * = 0.5–7.0 M ⊙. We derive disk mass-loss rate M ̇ , which has strong stellar dependence as M ̇ ≈ 7.30 × 10 − 9 ( M * / M ⊙ ) 2 M ⊙ yr − 1 . The absolute value of M ̇ scales with the adopted far-ultraviolet and X-ray luminosities. We derive the surface mass-loss rates and provide polynomial function fits to them. We also develop a semianalytic model that well reproduces the derived mass-loss rates. The estimated inner-disk lifetime decreases as the host star mass increases, in agreement with the observational trend. We thus argue that photoevaporation is a major physical mechanism for PPD dispersal for a wide range of the stellar mass and can account for the observed stellar mass dependence of the inner-disk lifetime.

MOA-2009-BLG-319Lb: A Sub-Saturn Planet inside the Predicted Mass Desert

Abstract We present an adaptive optics (AO) analysis of images from the Keck II telescope NIRC2 instrument of the planetary microlensing event MOA-2009-BLG-319. The ∼10 yr baseline between the event and the Keck observations allows the planetary host star to be detected at a separation of 66.5 ± 1.7 mas from the source star, consistent with the light-curve model prediction. The combination of the host star brightness and light-curve parameters yields host star and planet masses of M host = 0.524 ± 0.048M ⊙ and m p = 67.3 ± 6.2M ⊕ at a distance of D L = 7.1 ± 0.7 kpc. The star−planet projected separation is 2.03 ± 0.21 au. The planet-to-star mass ratio of this system, q = (3.857 ± 0.029) × 10−4, places it in the predicted “planet desert” at 10−4 < q < 4 × 10−4 according to the runaway gas accretion scenario of the core accretion theory. Seven of the 30 planets in the Suzuki et al. sample fall in this mass ratio range, and this is the third with a measured host mass. All three of these host stars have masses of 0.5 ≤ M host/M ⊙ ≤ 0.7, which implies that this predicted mass ratio gap is filled with planets that have host stars within a factor of two of 1M ⊙. This suggests that runaway gas accretion does not play a major role in determining giant planet masses for stars somewhat less massive than the Sun. Our analysis has been accomplished with a modified DAOPHOT code that has been designed to measure the brightness and positions of closely blended stars. This will aid in the development of the primary method that the Nancy Grace Roman Space Telescope mission will use to determine the masses of microlens planets and their hosts.

HATS-34b and HATS-46b: re-characterization using TESS and Gaia

ABSTRACT We present a revised characterization of the previously discovered transiting planet systems HATS-34 and HATS-46. We make use of the newly available space-based light curves from the NASA TESS mission and high-precision parallax and absolute photometry measurements from the ESA Gaia mission to determine the mass and radius of the planets and host stars with dramatically increased precision and accuracy compared to published values, with the uncertainties in some parameters reduced by as much as a factor of 7. Using an isochrone-based fit, for HATS-34 we measure a revised host star mass and radius of $0.952_{-0.020}^{+0.040}$ $\, \mathrm{M}_\odot$ and 0.9381 ± 0.0080 $\, \mathrm{R}_\odot$, respectively, and a revised mass and radius for the transiting planet of 0.951 ± 0.050 MJ and 1.282 ± 0.064 RJ, respectively. Similarly, for HATS-46 we measure a revised mass and radius for the host star of 0.869 ± 0.023 $\, \mathrm{M}_\odot$ and 0.894 ± 0.010 $\, \mathrm{R}_\odot$, respectively, and a revised mass and radius for the planet of 0.158 ± 0.042 MJ and 0.951 ± 0.029 RJ, respectively. The uncertainties that we determine on the stellar and planetary masses and radii are also substantially lower than re-determinations that incorporate the Gaia results without performing a full re-analysis of the light curves and other observational data. We argue that, in light of Gaia and TESS, a full re-analysis of previously discovered transiting planets is warranted.

Giants eating giants: mass loss and giant planets modifying the luminosity of the tip of the giant branch

During the red giant phase, stars loose mass at the highest rate since birth. The mass-loss rate is not fixed, but varies from star-to-star by up to 5%, resulting in variations of the star's luminosity at the tip of the red giant branch (TRGB) . Also, most stars, during this phase, engulf part of their planetary system, including their gas giant planets and possibly brown drafs. Gas giant planet masses range between 0.1 to 2% of the host star mass. The engulfing of their gas giants planets can modify their luminosity at the TRGB, i.e. the point at which the He-core degeneracy is removed. We show that the increase in mass of the star by the engulfing of the gas giant planets only modifies the luminosity of a star at the TRGB by less than 0.1%, while metallicity can modify the luminosity of a star at the TRGB by up to 0.5%. However, the increase in turbulence of the convective envelope of the star, i.e., modification of the mixing length, has a more dramatic effect, on the star's luminosity, which we estimate could be as large as 5%. The effect is always in the direction to increase the turbulence and thus the mixing length which turns into a systematic decrease of the luminosity of the star at the TRGB . We find that the star-to-star variation of the mass-loss rate will dominate the variations in the luminosity of the TRGB with a contribution at the 5% level. If the star-to-star variation is driven by environmental effects —as it is reasonable to assume –-, the same effects can potentially create an environmentally-driven mean effect on the luminosity of the tip of the red giant branch of a galaxy. Engulfment of a brown dwarf will have a more dramatic effect. Finally, we touch upon how to infer the frequency, and identify the engulfment, of exoplanets in low-metallicity RGB stars through high resolution spectroscopy as well as how to quantify mass loss rate distributions from the morphology of the horizontal branch.

A Gas Giant Planet in the OGLE-2006-BLG-284L Stellar Binary System

Abstract We present the analysis of microlensing event OGLE-2006-BLG-284, which has a lens system that consists of two stars and a gas giant planet with a mass ratio of q p = (1.26 ± 0.19) × 10−3 to the primary. The mass ratio of the two stars is q s = 0.289 ± 0.011, and their projected separation is s s = 2.1 ± 0.7 au, while the projected separation of the planet from the primary is s p = 2.2 ± 0.8 au. For this lens system to have stable orbits, the three-dimensional separation of either the primary and secondary stars or the planet and primary star must be much larger than the projected separations. Since we do not know which is the case, the system could include either a circumbinary or a circumstellar planet. Because there is no measurement of the microlensing parallax effect or lens system brightness, we can only make a rough Bayesian estimate of the lens system masses and brightness. We find host star and planet masses of , , and , and the K-band magnitude of the combined brightness of the host stars is . The separation between the lens and source system will be ∼90 mas in mid-2020, so it should be possible to detect the host system with follow-up adaptive optics or Hubble Space Telescope observations.

California-Kepler Survey. IX. Revisiting the Minimum-mass Extrasolar Nebula with Precise Stellar Parameters

Abstract We investigate a possible correlation between the solid surface density Σ of the minimum-mass extrasolar nebula (MMEN) and the host star mass M ⋆ and metallicity [Fe/H]. Leveraging on the precise host star properties from the California-Kepler Survey (CKS), we found that (a/1 au)−1.75±0.07 (M ⋆/M ⊙)1.04±0.22 100.22±0.05[Fe/H] for Kepler-like systems (1–4R ⊕; a < 1 au). The strong M ⋆ dependence is reminiscent of previous dust continuum results that the solid disk mass scales with M ⋆. The weaker [Fe/H] dependence shows that sub-Neptune planets, unlike giant planets, form readily in lower metallicity environment. The innermost region (a < 0.1 au) of an MMEN maintains a smooth profile despite a steep decline of planet occurrence rate: a result that favors the truncation of disks by corotating magnetospheres with a range of rotation periods, rather than the sublimation of dust. The Σ of Kepler multitransiting systems shows a much stronger correlation with M ⋆ and [Fe/H] than singles. This suggests that the dynamically hot evolution that produced single systems also partially removed the memory of formation in disks. Radial-velocity planets yielded a MMEN very similar to CKS planets; transit-timing-variation planets’ postulated convergent migration history is supported by their poorly constrained MMEN. We found that lower mass stars have a higher efficiency of forming/retaining planets: for Sun-like stars, about 20% of the solid mass within ∼1 au are converted/preserved as sub-Neptunes, compared to 70% for late-K to early-M stars. This may be due to the lower binary fraction, lower giant-planet occurrence, or the longer disk lifetime of lower mass stars.

Massive discs around low-mass stars

ABSTRACT We use a suite of smoothed particle hydrodynamic simulations to investigate the susceptibility of protoplanetary discs to the effects of self-gravity as a function of star–disc properties. We also include passive irradiation from the host star using different models for the stellar luminosities. The critical disc-to-star mass ratio for axisymmetry (for which we produce criteria) increases significantly for low-mass stars. This could have important consequences for increasing the potential mass reservoir in a proto Trappist-1 system, since even the efficient Ormel et al. formation model will be influenced by processes like external photoevaporation, which can rapidly and dramatically deplete the dust reservoir. The aforementioned scaling of the critical Md/M* for axisymmetry occurs in part because the Toomre Q parameter has a linear dependence on surface density (which promotes instability) and only an $M_*^{1/2}$ dependence on shear (which reduces instability), but also occurs because, for a given Md/M*, the thermal evolution depends on the host star mass. The early phase stellar irradiation of the disc (for which the luminosity is much higher than at the zero age main sequence, particularly at low stellar masses) can also play a key role in significantly reducing the role of self-gravity, meaning that even solar mass stars could support axisymmetric discs a factor two higher in mass than usually considered possible. We apply our criteria to the DSHARP discs with spirals, finding that self-gravity can explain the observed spirals so long as the discs are optically thick to the host star irradiation.

Fragmentation favoured in discs around higher mass stars

ABSTRACT We investigate how a protoplanetary disc’s susceptibility to gravitational instabilities and fragmentation depends on the mass of its host star. We use 1D disc models in conjunction with 3D smoothed particle hydrodynamics simulations to determine the critical disc-to-star mass ratios at which discs become unstable against fragmentation, finding that discs become increasingly prone to the effects of self-gravity as we increase the host star mass. The actual limit for stability is sensitive to the disc temperature, so if the disc is optically thin stellar irradiation can dramatically stabilize discs against gravitational instability. However, even when this is the case we find that discs around 2 M⊙ stars are prone to fragmentation, which will act to produce wide-orbit giant planets and brown dwarfs. The consequences of this work are twofold: that low-mass stars could in principle support high disc-to-star mass ratios, and that higher mass stars have discs that are more prone to fragmentation, which is qualitatively consistent with observations that favour high-mass wide-orbit planets around higher mass stars. We also find that the initial masses of these planets depends on the temperature in the disc at large radii, which itself depends on the level of stellar irradiation.

MOA-bin-29b: A Microlensing Gas-giant Planet Orbiting a Low-mass Host Star

Abstract We report the discovery of a gas-giant planet orbiting a low-mass host star in the microlensing event MOA-bin-29 that occurred in 2006. We find five degenerate solutions with the planet/host-star mass ratio of q ∼ 10−2. The Einstein radius crossing time of all models are relatively short (∼4–7 days), which indicates that the mass of host star is likely low. The measured lens-source proper motion is 5–9 mas yr−1 depending on the models. Since only finite source effects are detected, we conduct a Bayesian analysis in order to obtain the posterior probability distribution of the lens physical properties. As a result, we find the lens system is likely to be a gas-giant orbiting a brown dwarf or a very late M-dwarf in the Galactic bulge. The probability distributions of the physical parameters for the five degenerate models are consistent within the range of error. By combining these probability distributions, we conclude that the lens system is a gas giant with a mass of orbiting a brown dwarf with a mass of at a projected star–planet separation of . The lens distance is , i.e., likely within the Galactic bulge.

Planetary Nebulae and How to Find Them: Color Identification in Big Broadband Surveys

Planetary nebulae (PNe) provide tests of stellar evolution, can serve as tracers of chemical evolution in the Milky Way and other galaxies, and are also used as a calibrator of the cosmological distance ladder. Current and upcoming large scale photometric surveys have the potential to complete the census of PNe in our galaxy and beyond, but it is a challenge to disambiguate partially or fully unresolved PNe from the myriad other sources observed in these surveys. Here we carry out synthetic observations of nebular models to determine $ugrizy$ color-color spaces that can successfully identify PNe among billions of other sources. As a primary result we present a grid of synthetic absolute magnitudes for PNe at various stages of their evolution, and we make comparisons with real PNe colors from the Sloan Digital Sky Survey. We find that the $r-i$ versus $g-r$, and the $r-i$ versus $u-g$, color-color diagrams show the greatest promise for cleanly separating PNe from stars, background galaxies, and quasars. Finally, we consider the potential harvest of PNe from upcoming large surveys. For example, for typical progenitor host star masses of $\sim$3 M$_\odot$, we find that the Large Synoptic Survey Telescope (LSST) should be sensitive to virtually all PNe in the Magellanic Clouds with extinction up to $A_{\rm V}$ of $\sim$5 mag; out to the distance of Andromeda, LSST would be sensitive to the youngest PNe (age less than $\sim$6800 yr) and with $A_{\rm V}$ up to 1 mag.

The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10 to 100 au

Abstract We present a statistical analysis of the first 300 stars observed by the Gemini Planet Imager Exoplanet Survey. This subsample includes six detected planets and three brown dwarfs; from these detections and our contrast curves we infer the underlying distributions of substellar companions with respect to their mass, semimajor axis, and host stellar mass. We uncover a strong correlation between planet occurrence rate and host star mass, with stars M * > 1.5 M ⊙ more likely to host planets with masses between 2 and 13M Jup and semimajor axes of 3–100 au at 99.92% confidence. We fit a double power-law model in planet mass (m) and semimajor axis (a) for planet populations around high-mass stars (M * > 1.5 M ⊙) of the form , finding α = −2.4 ± 0.8 and β = −2.0 ± 0.5, and an integrated occurrence rate of % between 5–13M Jup and 10–100 au. A significantly lower occurrence rate is obtained for brown dwarfs around all stars, with % of stars hosting a brown dwarf companion between 13–80M Jup and 10–100 au. Brown dwarfs also appear to be distributed differently in mass and semimajor axis compared to giant planets; whereas giant planets follow a bottom-heavy mass distribution and favor smaller semimajor axes, brown dwarfs exhibit just the opposite behaviors. Comparing to studies of short-period giant planets from the radial velocity method, our results are consistent with a peak in occurrence of giant planets between ∼1 and 10 au. We discuss how these trends, including the preference of giant planets for high-mass host stars, point to formation of giant planets by core/pebble accretion, and formation of brown dwarfs by gravitational instability.

The EBLM Project

Measurements of the physical properties of stars at the lower end of the main sequence are scarce. In this context we report masses, radii and surface gravities of ten very-low-mass stars in eclipsing binary systems, with orbital periods of the order of several days. The objects probe the stellar mass-radius relation in the fully convective regime, M⋆ ≲ 0.35 M⊙, down to the hydrogen burning mass-limit, MHB ∼ 0.07 M⊙. The stars were detected by the WASP survey for transiting extra-solar planets, as low-mass, eclipsing companions orbiting more massive, F- and G-type host stars. We use eclipse observations of the host stars, performed with the TRAPPIST, Leonhard Euler and SPECULOOS telescopes, and radial velocities of the host stars obtained with the CORALIE spectrograph, to determine the physical properties of the low-mass companions. Surface gravities of the low-mass companions are derived from the eclipse and orbital parameters of each system. Spectroscopic measurements of the host star effective temperature and metallicity are used to infer the host star mass and age from stellar evolution models for solar-type stars. Masses and radii of the low-mass companions are then derived from the eclipse and orbital parameters of the binary systems. The objects are compared to stellar evolution models for low-mass stars, to test for an effect of the stellar metallicity and orbital period on the radius of low-mass stars in close binary systems. Measurements are found to be in good agreement with stellar evolution models; a systematic inflation of the radius of low-mass stars with respect to model predictions is limited to 1.6 ± 1.2%, in the fully convective low-mass regime. The sample of ten objects indicates a scaling of the radius of low-mass stars with the host star metallicity. No correlation between stellar radii and the orbital periods of the binary systems is determined. A combined analysis with thirteen comparable objects from the literature is consistent with this result.

The Sub-Neptune Desert and Its Dependence on Stellar Type: Controlled by Lifetime X-Ray Irradiation

Abstract Short-period sub-Neptunes with substantial volatile envelopes are among the most common type of known exoplanets. However, recent studies of the Kepler population have suggested a dearth of sub-Neptunes on highly irradiated orbits, where they are vulnerable to atmospheric photoevaporation. Physically, we expect this “photoevaporation desert” to depend on the total lifetime X-ray and extreme ultraviolet flux, the main drivers of atmospheric escape. In this work, we study the demographics of sub-Neptunes as a function of lifetime exposure to high-energy radiation and host-star mass. We find that for a given present-day insolation, planets orbiting a 0.3 M ⊙ star experience ∼100× more X-ray flux over their lifetimes versus a 1.2 M ⊙ star. Defining the photoevaporation desert as a region consistent with zero occurrence at 2σ, the onset of the desert happens for integrated X-ray fluxes greater than 1.43 × 1022 to 8.23 × 1020 as a function of planetary radii for 1.8–4 R ⊕. We also compare the location of the photoevaporation desert for different stellar types. We find much greater variability in the desert onset in the bolometric flux space compared to the integrated X-ray flux space, suggestive of photoevaporation driven by steady-state stellar X-ray emissions as the dominant control on desert location. Finally, we report tentative evidence for the sub-Neptune valley, first seen around Sun-like stars, for M&K dwarfs. The discovery of additional planets around low-mass stars from surveys such as the Transiting Exoplanet Survey Satellite mission will enable detailed exploration of these trends.

A Model-independent Mass and Moderate Eccentricity for β Pic b

Abstract We use a cross-calibration of Hipparcos and Gaia second data release astrometry for β Pic to measure the mass of the giant planet β Pic b (13 ± 3 ) in a comprehensive joint orbit analysis that includes published relative astrometry and radial velocities. Our mass uncertainty is somewhat higher than previous work because our astrometry from the Hipparcos–Gaia Catalog of Accelerations accounts for the error inflation and systematic terms that are required to bring the two data sets onto a common astrometric reference frame, and because we fit freely for the host-star mass (1.84 ± 0.05 M ⊙). This first model-independent mass for a directly imaged planet is inconsistent with cold-start models given the age of the β Pic moving group (22 ± 6 Myr) but consistent with hot- and warm-start models, concordant with past work. We find a higher eccentricity (0.24 ± 0.06) for β Pic b compared to previous orbital fits. If confirmed by future observations, this eccentricity may help explain inner edge, scale height, and brightness asymmetry of β Pic’s disk. It could also potentially signal that β Pic b has migrated inward to its current location, acquiring its eccentricity from interaction with the 3:1 outer Lindblad resonance in the disk.

Microlensing Results Challenge the Core Accretion Runaway Growth Scenario for Gas Giants

Abstract We compare the planet-to-star mass-ratio distribution measured by gravitational microlensing to core accretion theory predictions from population synthesis models. The core accretion theory’s runaway gas accretion process predicts a dearth of intermediate-mass giant planets that is not seen in the microlensing results. In particular, the models predict ∼10 × fewer planets at mass ratios of than inferred from microlensing observations. This tension implies that gas giant formation may involve processes that have hitherto been overlooked by existing core accretion models or that the planet-forming environment varies considerably as a function of host-star mass. Variation from the usual assumptions for the protoplanetary disk viscosity and thickness could reduce this discrepancy, but such changes might conflict with microlensing results at larger or smaller mass ratios, or with other observations. The resolution of this discrepancy may have important implications for planetary habitability because it has been suggested that the runaway gas accretion process may have triggered the delivery of water to our inner solar system. So, an understanding of giant planet formation may help us to determine the occurrence rate of habitable planets.

The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap

Abstract The distribution of planet sizes encodes details of planet formation and evolution. We present the most precise planet size distribution to date based on Gaia parallaxes, Kepler photometry, and spectroscopic temperatures from the California-Kepler Survey. Previously, we measured stellar radii to 11% precision using high-resolution spectroscopy; by adding Gaia astrometry, the errors are now 3%. Planet radius measurements are, in turn, improved to 5% precision. With a catalog of ∼1000 planets with precise properties, we probed in fine detail the gap in the planet size distribution that separates two classes of small planets, rocky super-Earths and gas-dominated sub-Neptunes. Our previous study and others suggested that the gap may be observationally under-resolved and inherently flat-bottomed, with a band of forbidden planet sizes. Analysis based on our new catalog refutes this; the gap is partially filled in. Two other important factors that sculpt the distribution are a planet’s orbital distance and its host-star mass, both of which are related to a planet’s X-ray/UV irradiation history. For lower-mass stars, the bimodal planet distribution shifts to smaller sizes, consistent with smaller stars producing smaller planet cores. Details of the size distribution including the extent of the “sub-Neptune desert” and the width and slope of the gap support the view that photoevaporation of low-density atmospheres is the dominant evolutionary determinant of the planet size distribution.

A Planetary Microlensing Event with an Unusually Red Source Star: MOA-2011-BLG-291

Abstract We present the analysis of the planetary microlensing event MOA-2011-BLG-291, which has a mass ratio of q = (3.8 ± 0.7) × 10−4 and a source star that is redder (or brighter) than the bulge main sequence. This event is located at a low Galactic latitude in the survey area that is currently planned for NASA’s Wide Field Infrared Survey Telescope (WFIRST) exoplanet microlensing survey. This unusual color for a microlensed source star implies that we cannot assume that the source star is in the Galactic bulge. The favored interpretation is that the source star is a lower main-sequence star at a distance of D S = 4.9 ± 1.3 kpc in the Galactic disk. However, the source could also be a turn-off star on the far side of the bulge or a subgiant in the far side of the Galactic disk if it experiences significantly more reddening than the bulge red clump stars. However, these possibilities have only a small effect on our mass estimates for the host star and planet. We find host star and planet masses of and from a Bayesian analysis with a standard Galactic model, under the assumption that the planet hosting probability does not depend on the host mass or distance. However, if we attempt to measure the host and planet masses with host star brightness measurements from high angular resolution follow-up imaging, the implied masses will be sensitive to the host star distance. The WFIRST exoplanet microlensing survey is expected to use this method to determine the masses for many of the planetary systems that it discovers, so this issue has important design implications for the WFIRST exoplanet microlensing survey.

Host Star Dependence of Small Planet Mass–Radius Distributions

Abstract The planet formation environment around M dwarf stars is different than around G dwarf stars. The longer hot protostellar phase, activity levels and lower protoplanetary disk mass of M dwarfs all may leave imprints on the composition distribution of planets. We use hierarchical Bayesian modeling conditioned on the sample of transiting planets with radial velocity mass measurements to explore small planet mass–radius distributions that depend on host star mass. We find that the current mass–radius data set is consistent with no host star mass dependence. These models are then applied to the Kepler planet radius distribution to calculate the mass distribution of close-orbiting planets and how it varies with host star mass. We find that the average heavy element mass per star at short orbits is higher for M dwarfs compared to FGK dwarfs, in agreement with previous studies. This work will facilitate comparisons between microlensing planet surveys and Kepler, and will provide an analysis framework that can readily be updated as more M dwarf planets are discovered by ongoing and future surveys such as K2 and the Transiting Exoplanet Survey Satellite.

Evidence of an Upper Bound on the Masses of Planets and Its Implications for Giant Planet Formation

Abstract Celestial bodies with a mass of have been found orbiting nearby stars. It is unknown whether these objects formed like gas-giant planets through core accretion or like stars through gravitational instability. I show that objects with orbit metal-rich solar-type dwarf stars, a property associated with core accretion. Objects with do not share this property. This transition is coincident with a minimum in the occurrence rate of such objects, suggesting that the maximum mass of a celestial body formed through core accretion like a planet is less than . Consequently, objects with orbiting solar-type dwarf stars likely formed through gravitational instability and should not be thought of as planets. Theoretical models of giant planet formation in scaled minimum-mass solar nebula Shakura–Sunyaev disks with standard parameters tuned to produce giant planets predict a maximum mass nearly an order of magnitude larger. To prevent newly formed giant planets from growing larger than , protoplanetary disks must therefore be significantly less viscous or of lower mass than typically assumed during the runaway gas accretion stage of giant planet formation. Either effect would act to slow the Type I/II migration of planetary embryos/giant planets and promote their survival. These inferences are insensitive to the host star mass, planet formation location, or characteristic disk dissipation time.

The Star Blended with the MOA-2008-BLG-310 Source Is Not the Exoplanet Host Star

Abstract High-resolution Hubble Space Telescope (HST) image analysis of the MOA-2008-BLG-310 microlens system indicates that the excess flux at the location of the source found in the discovery paper cannot primarily be due to the lens star because it does not match the lens–source relative proper motion, , predicted by the microlens models. This excess flux is most likely to be due to an unrelated star that happens to be located in close proximity to the source star. Two epochs of HST observations indicate proper motion for this blend star that is typical of a random bulge star but is not consistent with a companion to the source or lens stars if the flux is dominated by only one star, aside from the lens. We consider models in which the excess flux is due to a combination of an unrelated star and the lens star, and this yields a 95% confidence level upper limit on the lens star brightness of and . A Bayesian analysis using a standard Galactic model and these magnitude limits yields a host star mass of and a planet mass of at a projected separation of au. This result illustrates that excess flux in a high-resolution image of a microlens–source system need not be due to the lens. It is important to check that the lens–source relative proper motion is consistent with the microlensing prediction. The high-resolution image analysis techniques developed in this paper can be used to verify the WFIRST exoplanet microlensing survey mass measurements.