arXiv subjects

M. Fukagawa

Publications and source records attributed to M. Fukagawa.

At least 19 recordsLinked to original sources

arXiv2019

Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star

We report the analysis of additional multiband photometry and spectroscopy and new adaptive optics (AO) imaging of the nearby planetary microlensing event TCP J05074264+2447555 (Kojima-1), which was discovered toward the Galactic anticenter in 2017 (Nucita et al.). We confirm the planetary nature of the light-curve anomaly around the peak while finding no additional planetary feature in this event. We also confirm the presence of apparent blending flux and the absence of significant parallax signal reported in the literature. The AO image reveals no contaminating sources, making it most likely that the blending flux comes from the lens star. The measured multiband lens flux, combined with a constraint from the microlensing model, allows us to narrow down the previously unresolved mass and distance of the lens system. We find that the primary lens is a dwarf on the K/M boundary (0.581 \pm 0.033 M_sun) located at 505 \pm 47 pc and the companion (Kojima-1Lb) is a Neptune-mass planet (20.0 \pm 2.0 M_earth) with a semi-major axis of 1.08 ^{+0.62}_{-0.18} au. This orbit is a few times smaller than those of typical microlensing planets and is comparable to the snow-line location at young ages. We calculate that the a priori detection probability of Kojima-1Lb is only \sim 35%, which may imply that Neptunes are common around the snow line, as recently suggested by the transit and radial velocity techniques. The host star is the brightest among the microlensing planetary systems (Ks = 13.7), offering a great opportunity to spectroscopically characterize this system, even with current facilities.

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arXiv2019

ALMA survey of Class II protoplanetary disks in Corona Australis: a young region with low disk masses

In recent years, the disk populations in a number of young star-forming regions have been surveyed with ALMA. Understanding the disk properties and their correlation with those of the central star is critical to understand planet formation. In particular, a decrease of the average measured disk dust mass with the age of the region has been observed. We conducted high-sensitivity continuum ALMA observations of 43 Class II young stellar objects in CrA at 1.3 mm (230 GHz). The typical spatial resolution is 0.3". The continuum fluxes are used to estimate the dust masses of the disks, and a survival analysis is performed to estimate the average dust mass. We also obtained new VLT/X-Shooter spectra for 12 of the objects in our sample. 24 disks are detected, and stringent limits have been put on the average dust mass of the non-detections. Accounting for the upper limits, the average disk mass in CrA is $6\pm3\,\rm M_\oplus$, significantly lower than that of disks in other young (1-3 Myr) star forming regions (e.g. Lupus) and appears consistent with the 5-10 Myr old Upper Sco. The position of the stars in our sample on the HR diagram, however, seems to confirm that that CrA has age similar to Lupus. Neither external photoevaporation nor a lower than usual stellar mass distribution can explain the low disk masses. On the other hand, a low-mass disk population could be explained if the disks are small, which could happen if the parent cloud has a low temperature or intrinsic angular momentum, or if the the angular momentum of the cloud is removed by some physical mechanism such as magnetic braking. In order to fully explain and understand the dust mass distribution of protoplanetary disks and their evolution, it may also be necessary to take into consideration the initial conditions of star and disk formation process, which may vary from region to region, and affect planet formation.

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arXiv2019

Spatial segregation of dust grains in transition disks. SPHERE observations of 2MASS J16083070-3828268 and RXJ1852.3-3700

Context. The mechanisms governing the opening of cavities in transition disks are not fully understood. Several processes have been proposed but their occurrence rate is still unknown. Aims. We present spatially resolved observations of two transition disks and aim at constraining their vertical and radial structure using multiwavelength observations. Methods. We have obtained near-IR scattered light observations with VLT/SPHERE of the transition disks J1608 and J1852. We complement our datasets with ALMA observations and with unresolved photometric observations covering a wide range of wavelengths. We performed radiative transfer modeling to analyze the morphology of the disks and compare the results with a sample of 20 other transition disks observed with both SPHERE and ALMA. Results. The scattered light image of J1608 reveals a very inclined disk, with two bright lobes and a large cavity. J1852 shows an inner ring extending beyond the coronagraphic radius up to 15au, a gap and a second ring at 42au. Our radiative transfer model of J1608 indicates that the millimeter-sized grains are less extended vertically and radially than the micron-sized grains, indicating advanced settling and radial drift. We find good agreement with the observations of J1852 with a similar model, but due to the low inclination of the system, the model remains partly degenerate. The analysis of 22 transition disks shows that, in general, the cavities observed in scattered light are smaller than the ones detected at millimeter wavelengths. Conclusions. The analysis of a sample of transition disks indicates that the small grains can flow inward of the region where millimeter grains are trapped. While 15 out of the 22 cavities in our sample could be explained by a planet of less than 13 Jupiter masses, the others either require the presence of a more massive companion or of several low-mass planets.

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arXiv2016

Water ice at the surface of HD 100546 disk

We made near infrared multicolor imaging observations of a disk around Herbig Be star HD100546 using Gemini/NICI. K (2.2\,$μ$m), H$_2$O ice (3.06\,$μ$m), and L'(3.8\,$μ$m) disk images were obtained and we found the 3.1\,$μ$m absorption feature in the scattered light spectrum, likely due to water ice grains at the disk surface. We compared the observed depth of the ice absorption feature with the disk model based on \cite{Oka2012} including water ice photodesorption effect by stellar UV photons. The observed absorption depth can be explained by the both disk models with/without photodesorption effect within the measurement accuracy, but slightly favors the model with photodesorption effects, implying that the UV photons play an important role on the survival/destruction of ice grains at the Herbig Ae/Be disk surface. Further improvement on the accuracy of the observations of the water ice absorption depth is needed to constrain the disk models.

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arXiv2014

OGLE-2008-BLG-355Lb: A Massive Planet around A Late type Star

We report the discovery of a massive planet OGLE-2008-BLG-355Lb. The light curve analysis indicates a planet:host mass ratio of q = 0.0118 +/- 0.0006 at a separation of 0.877 +/- 0.010 Einstein radii. We do not measure a significant microlensing parallax signal and do not have high angular resolution images that could detect the planetary host star. Therefore, we do not have a direct measurement of the host star mass. A Bayesian analysis, assuming that all host stars have equal probability to host a planet with the measured mass ratio implies a host star mass of M_h = 0.37_{-0.17}^{+0.30} M_Sun and a companion of mass M_P = 4.6^{+3.7}_{-2.2} M_Jup, at a projected separation of r_proj = 1.70^{+0.29}_{-0.30} AU. The implied distance to the planetary system is D_L = 6.8 +/- 1.1 kpc. A planetary system with the properties preferred by the Bayesian analysis would be a challenge to the core-accretion model of planet formation, as the core-accretion model predicts that massive planets are far more likely to form around more massive host stars. This core accretion model prediction is not consistent with our Bayesian prior of an equal probability of host stars of all masses to host a planet with the measured mass ratio. So, if the core accretion model prediction is right, we should expect that follow-up high angular resolution observations will detect a host star with a mass in the upper part of the range allowed by the Bayesian analysis. That is, the host would probably be a K or G dwarf.

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arXiv2013

MOA-2008-BLG-379Lb: A Massive Planet from a High Magnification Event with a Faint Source

We report analysis of high microlensing event MOA-2008-BLG-379, which has a strong microlensing anomaly at its peak, due to a massive planet with a mass ratio of q = 6.9 x 10^{-3}. Because the faint source star crosses the large resonant caustic, the planetary signal dominates the light curve. This is unusual for planetary microlensing events, and as a result, the planetary nature of this light curve was not immediately noticed. The planetary nature of the event was found when the MOA Collaboration conducted a systematic study of binary microlensing events previously identified by the MOA alert system. We have conducted a Bayesian analysis based on a standard Galactic model to estimate the physical parameters of the lens system. This yields a host star mass of M_L = 0.66_{-0.33}^{+0.29} M_Sun orbited by a planet of mass m_P = 4.8_{-2.4}^{+2.1} M_Jup at an orbital separation of a = 4.1_{-1.5}^{+1.9} AU at a distance of D_L = 3.6 +/- 1.3 kpc. The faint source magnitude of I_S = 21.30 and relatively high lens-source relative proper motion of mu_rel = 7.6 +/- 1.6 mas/yr implies that high angular resolution adaptive optics or Hubble Space Telescope observations are likely to be able to detect the source star, which would determine the masses and distance of the planet and its host star.

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arXiv2013

Characterization of the gaseous companion κ Andromedae b: New Keck and LBTI high-contrast observations

We previously reported the direct detection of a low mass companion at a projected separation of 55+-2 AU around the B9 type star κ Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. We present new angular differential imaging (ADI) images of the Kappa Andromedae system at 2.146 (Ks), 3.776 (L'), 4.052 (NB 4.05) and 4.78 μm (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We derive a more accurate J = 15.86 +- 0.21, H = 14.95 +- 0.13, Ks = 14.32 +- 0.09 mag for κ And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at Ks and L' band. We derive NB 4.05 = 13.0 +- 0.2 and M' = 13.3 +- 0.3 mag and estimate Log10(L/Lsun) = -3.76 +- 0.06. We build the 1-5 microns spectral energy distribution of the companion and compare it to seven PHOENIX-based atmospheric models in order to derive Teff = 1900+100-200 K. Models do not set constrains on the surface gravity. ``Hot-start" evolutionary models predict masses of 14+25-2 MJup based on the luminosity and temperature estimates, and considering a conservative age range for the system (30+120-10 Myr). ``warm-start" evolutionary tracks constrain the mass to M >= 11 MJup. Therefore, the mass of κ Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of κ And b.

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arXiv2013

DUst Around NEarby Stars. The survey observational results

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. We used {\it Herschel}/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 $μ$m were obtained, complemented in some cases with observations at 70 $μ$m, and at 250, 350 and 500 $μ$m using SPIRE. The observing strategy was to integrate as deep as possible at 100 $μ$m to detect the stellar photosphere. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of $\sim$ 12.1% $\pm$ 5% before \emph{Herschel} to $\sim$ 20.2% $\pm$ 2%. A significant fraction ($\sim$ 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160$μ$m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.

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arXiv2012

Spiral Arms in the Asymmetrically Illuminated Disk of MWC 758 and Constraints on Giant Planets

We present the first near-IR scattered light detection of the transitional disk associated with the Herbig Ae star MWC 758 using data obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru, and 1.1 micron HST/NICMOS data. While sub-millimeter studies suggested there is a dust-depleted cavity with r=0.35, we find scattered light as close as 0.1 (20-28 AU) from the star, with no visible cavity at H, K', or Ks. We find two small-scaled spiral structures which asymmetrically shadow the outer disk. We model one of the spirals using spiral density wave theory, and derive a disk aspect ratio of h ~ 0.18, indicating a dynamically warm disk. If the spiral pattern is excited by a perturber, we estimate its mass to be 5+3,-4 Mj, in the range where planet filtration models predict accretion continuing onto the star. Using a combination of non-redundant aperture masking data at L' and angular differential imaging with Locally Optimized Combination of Images at K' and Ks, we exclude stellar or massive brown dwarf companions within 300 mas of the Herbig Ae star, and all but planetary mass companions exterior to 0.5. We reach 5-sigma contrasts limiting companions to planetary masses, 3-4 MJ at 1.0 and 2 MJ at 1.55 using the COND models. Collectively, these data strengthen the case for MWC 758 already being a young planetary system.

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arXiv2012

Direct Imaging Discovery of a `Super-Jupiter' Around the late B-Type Star Kappa And

We present the direct imaging discovery of an extrasolar planet, or possible low-mass brown dwarf, at a projected separation of 55 +/- 2 AU (1.058 +/- 0.007 arcsec) from the B9-type star Kappa And. The planet was detected with Subaru/HiCIAO during the SEEDS survey, and confirmed as a bound companion via common proper motion measurements. Observed near-infrared magnitudes of J = 16.3 +/- 0.3, H = 15.2 +/- 0.2, Ks = 14.6 +/- 0.4, and L' = 13.12 +/- 0.09 indicate a temperature of ~1700 K. The galactic kinematics of the host star are consistent with membership in the Columba association, implying a corresponding age of 30 +20 -10 Myr. The system age, combined with the companion photometry, points to a model-dependent companion mass ~12.8 MJup. The host star's estimated mass of 2.4-2.5 Msun places it among the most massive stars ever known to harbor an extrasolar planet or low-mass brown dwarf. While the mass of the companion is close to the deuterium burning limit, its mass ratio, orbital separation, and likely planet-like formation scenario imply that it may be best defined as a `Super-Jupiter' with properties similar to other recently discovered companions to massive stars.

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arXiv2012

Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius

We report high-resolution (0.07 arcsec) near-infrared polarized intensity images of the circumstellar disk around the star 2MASS J16042165-2130284 obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our $H$-band data, which clearly exhibits a resolved, face-on disk with a large inner hole for the first time at infrared wavelengths. We detect the centrosymmetric polarization pattern in the circumstellar material as has been observed in other disks. Elliptical fitting gives the semimajor axis, semiminor axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14 $^{\circ}$, respectively. The disk is asymmetric, with one dip located at P.A.s of $\sim85^{\circ}$. Our observed disk size agrees well with a previous study of dust and CO emission at submillimeter wavelength with Submillimeter Array. Hence, the near-infrared light is interpreted as scattered light reflected from the inner edge of the disk. Our observations also detect an elongated arc (50 AU) extending over the disk inner hole. It emanates at the inner edge of the western side of the disk, extending inward first, then curving to the northeast. We discuss the possibility that the inner hole, the dip, and the arc that we have observed may be related to the existence of unseen bodies within the disk.

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arXiv2012

High-Resolution Near-Infrared Polarimetry of a Circumstellar Disk around UX Tau A

We present H-band polarimetric imagery of UX Tau A taken with HiCIAO/AO188 on the Subaru Telescope. UX Tau A has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0.15 (21 AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UX Tau A which extends to 120 AU, at a spatial resolution of 0.1 (14 AU). It is inclined by 46 \pm 2 degree as the west side is nearest. Although SED modeling and sub-millimeter imagery suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25 - 30 AU, we detect no evidence of a gap at the limit of our inner working angle (23 AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66 %) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh scattering nor Mie scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with the radii of 30 micron meter is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations and have grown in the circumstellar disk of UX Tau A.

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arXiv2012

High-Contrast NIR Polarization Imaging of MWC480

One of the key predictions of modeling from the IR excess of Herbig Ae stars is that for protoplanetary disks, where significant grain growth and settling has occurred, the dust disk has flattened to the point that it can be partially or largely shadowed by the innermost material at or near the dust sublimation radius. When the self-shadowing has already started, the outer disk is expected to be detected in scattered light only in the exceptional cases that the scale height of the dust disk at the sublimation radius is smaller than usual. High-contrast imaging combined with the IR spectral energy distribution allow us to measure the degree of flattening of the disk, as well as to determine the properties of the outer disk. We present polarimetric differential imaging in $H$ band obtained with Subaru/HiCIAO of one such system, MWC 480. The HiCIAO data were obtained at a historic minimum of the NIR excess. The disk is detected in scattered light from 0\farcs2-1\farcs0 (27.4-137AU). Together with the marginal detection of the disk from 1998 February 24 by HST/NICMOS, our data constrain the opening half angle for the disk to lie between 1.3$\leqθ\leq 2.2^\circ$. When compared with similar measures in CO for the gas disk from the literature, the dust disk subtends only $\sim$30% of the gas disk scale height (H/R$\sim$0.03). Such a dust disk is a factor of 5-7 flatter than transitional disks, which have structural signatures that giant planets have formed.

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arXiv2012

Mid-IR imaging of the transitional disk of HD169142: Measuring the size of the gap

The disk around the Herbig Ae star HD\,169142 was imaged and resolved at 18.8 and 24.5\,$μ$m using Subaru/COMICS. We interpret the observations using a 2D radiative transfer model and find evidence for the presence of a large gap. The MIR images trace dust that emits at the onset of the strong rise in the spectral energy distribution (SED) at 20\,$μ$m, therefore are very sensitive to the location and characteristics of the inner wall of the outer disk and its dust. We determine the location of the wall to be 23$^{+3}_{-5}$\,AU from the star. An extra component of hot dust must exist close to the star. We find that a hydrostatic optically thick inner disk does not produce enough flux in the NIR and an optically thin geometrically thick component is our solution to fit the SED. Considering the recent findings of gaps and holes in a number of Herbig Ae/Be group I disks, we suggest that such disk structures may be common in group I sources. Classification as group I should be considered a support for classification as a transitional disk, though improved imaging surveys are needed to support this speculation.

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arXiv2012

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory

We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r~46AU, our observations reveal the presence of scattered light components as close as 0.2" (~28AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0.5" (~70AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h~0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

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arXiv2011

The SPICA coronagraphic instrument (SCI) for the study of exoplanets

We present the SPICA Coronagraphic Instrument (SCI), which has been designed for a concentrated study of extra-solar planets (exoplanets). SPICA mission provides us with a unique opportunity to make high contrast observations because of its large telescope aperture, the simple pupil shape, and the capability for making infrared observations from space. The primary objectives for the SCI are the direct coronagraphic detection and spectroscopy of Jovian exoplanets in infrared, while the monitoring of transiting planets is another important target. The specification and an overview of the design of the instrument are shown. In the SCI, coronagraphic and non-coronagraphic modes are applicable for both an imaging and a spectroscopy. The core wavelength range and the goal contrast of the coronagraphic mode are 3.5--27$μ$m, and 10$^{-6}$, respectively. Two complemental designs of binary shaped pupil mask coronagraph are presented. The SCI has capability of simultaneous observations of one target using two channels, a short channel with an InSb detector and a long wavelength channel with a Si:As detector. We also give a report on the current progress in the development of key technologies for the SCI.

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arXiv2011

Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae

We report high-resolution 1.6 $\micron$ polarized intensity ($PI$) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU ($0."15$) up to 554 AU (3.$"$85), which have been obtained by the high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed complicated and asymmetrical structures in the inner part ($\lesssim$140 AU) of the disk, while confirming the previously reported outer ($r$ $\gtrsim$200 AU) spiral structure. We have imaged a double ring structure at $\sim$40 and $\sim$100 AU and a ring-like gap between the two. We found a significant discrepancy of inclination angles between two rings, which may indicate that the disk of AB Aur is warped. Furthermore, we found seven dips (the typical size is $\sim$45 AU or less) within two rings as well as three prominent $PI$ peaks at $\sim$40 AU. The observed structures, including a bumpy double ring, a ring-like gap, and a warped disk in the innermost regions, provide essential information for understanding the formation mechanism of recently detected wide-orbit ($r$ $>$20 AU) planets.

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arXiv2010

Pre-transitional disk nature of the AB Aur disk

The disk around AB Aur was imaged and resolved at 24.6\,$μ$m using the Cooled Mid-Infrared Camera and Spectrometer on the 8.2m Subaru Telescope. The gaussian full-width at half-maximum of the source size is estimated to be 90 $\pm$ 6 AU, indicating that the disk extends further out at 24.6\,$μ$m than at shorter wavelengths. In order to interpret the extended 24.6\,$μ$m image, we consider a disk with a reduced surface density within a boundary radius $R_c$, which is motivated by radio observations that suggest a reduced inner region within about 100 AU from the star. Introducing the surface density reduction factor $f_c$ for the inner disk, we determine that the best match with the observed radial intensity profile at 24.6\,$μ$m is achieved with $R_c$=88 AU and $f_c$=0.01. We suggest that the extended emission at 24.6\,$μ$m is due to the enhanced emission from a wall-like structure at the boundary radius (the inner edge of the outer disk), which is caused by a jump in the surface density at $R_c$. Such reduced inner disk and geometrically thick outer disk structure can also explain the more point-like nature at shorter wavelengths. We also note that this disk geometry is qualitatively similar to a pre-transitional disk, suggesting that the AB Aur disk is in a pre-transitional disk phase.

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