arXiv subjects

Motohide Tamura

Publications and source records attributed to Motohide Tamura.

At least 217 records · Page 12Linked to original sources

arXiv2018

Fiber mode scrambler for the Subaru infrared Doppler instrument (IRD)

We report the results of fiber mode scrambler experiments for the Infra-Red Doppler instrument (IRD) on the Subaru 8.2-m telescope. IRD is a fiber-fed, high precision radial velocity (RV) instrument to search for exoplanets around nearby M dwarfs at near-infrared wavelengths. It is a high-resolution spectrograph with an Echelle grating. The expected RV measurement precision is 1 m s-1 with a state of the art laser frequency comb for the wavelength calibration. In IRD observations, one of the most significant instrumental noise is a change of intensity distribution of multi-mode fiber exit, which degrades RV measurement precision. To stabilize the intensity distribution of fiber exit an introduction of fiber mode scrambler is mandatory. Several kinds of mode scramblers have been suggested in previous research, though it is necessary to determine the most appropriate mode scrambler system for IRD. Thus, we conducted systematic measurements of performance for a variety of mode scramblers, both static and dynamic. We tested various length multi-mode fibers, an octagonal fiber, a double fiber scrambler, and two kinds of dynamic scramblers, and their combinations. We report the performances of these mode scramblers and propose candidate mode scrambler systems for IRD.

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arXiv2018

Distortion of Magnetic Fields in a Starless Core III: Polarization--Extinction Relationship in FeSt 1-457

The relationship between dust polarization and extinction was determined for the cold dense starless molecular cloud core FeSt 1-457 based on the background star polarimetry of dichroic extinction at near-infrared wavelengths. Owing to the known (three-dimensional) magnetic field structure, the observed polarizations from the core were corrected by considering (a) the subtraction of the ambient polarization component, (b) the depolarization effect of inclined distorted magnetic fields, and (c) the magnetic inclination angle of the core. After these corrections, a linear relationship between polarization and extinction was obtained for the core in the range up to $A_V \approx 20$ mag. The initial polarization vs. extinction diagram changed dramatically after the corrections of (a) to (c), with the correlation coefficient being refined from 0.71 to 0.79. These corrections should affect the theoretical interpretation of the observational data. The slope of the finally obtained polarization--extinction relationship is $P_H / E_{H-K_s} = 11.00 \pm 0.72$ $\%$ ${\rm mag}^{-1}$, which is close to the statistically estimated upper limit of the interstellar polarization efficiency (Jones 1989). This consistency suggests that the upper limit of interstellar polarization efficiency might be determined by the observational viewing angle toward polarized astronomical objects.

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arXiv2018

Three Small Planets Transiting a Hyades Star

We present the discovery of three small planets transiting K2-136 (LP 358 348, EPIC 247589423), a late K dwarf in the Hyades. The planets have orbital periods of $7.9757 \pm 0.0011$, $17.30681^{+0.00034}_{-0.00036}$, and $25.5715^{+0.0038}_{-0.0040}$ days, and radii of $1.05 \pm 0.16$, $3.14 \pm 0.36$, and $1.55^{+0.24}_{-0.21}$ $R_\oplus$, respectively. With an age of 600-800 Myr, these planets are some of the smallest and youngest transiting planets known. Due to the relatively bright (J=9.1) host star, the planets are compelling targets for future characterization via radial velocity mass measurements and transmission spectroscopy. As the first known star with multiple transiting planets in a cluster, the system should be helpful for testing theories of planet formation and migration.

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arXiv2018

Life Beyond the Solar System: Remotely Detectable Biosignatures

For the first time in human history, we will soon be able to apply the scientific method to the question "Are We Alone?" The rapid advance of exoplanet discovery, planetary systems science, and telescope technology will soon allow scientists to search for life beyond our Solar System through direct observation of extrasolar planets. This endeavor will occur alongside searches for habitable environments and signs of life within our Solar System. While the searches are thematically related and will inform each other, they will require separate observational techniques. The search for life on exoplanets holds potential through the great diversity of worlds to be explored beyond our Solar System. However, there are also unique challenges related to the relatively limited data this search will obtain on any individual world. This white paper reviews the scientific community's ability to use data from future telescopes to search for life on exoplanets. This material summarizes products from the Exoplanet Biosignatures Workshop Without Walls (EBWWW). The EBWWW was constituted by a series of online and in person activities, with participation from the international exoplanet and astrobiology communities, to assess state of the science and future research needs for the remote detection of life on planets outside our Solar System.

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arXiv2018

Exoplanets around Low-mass Stars Unveiled by K2

We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96-33 days. For one of the planets (K2-151b) we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius $R_p$ on stellar insolation and metallicity [Fe/H]. We confirm that for periods $P\lesssim 2$ days, planets with a radius $R_p\gtrsim 2\,R_\oplus$ are less common than planets with a radius between 1-2$\,R_\oplus$. We also see a hint of the "radius valley" between 1.5 and 2$\,R_\oplus$ that has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources, or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: those few planets larger than about 3 $R_\oplus$ are found around the most metal-rich M dwarfs.

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arXiv2017

An H-band Vector Vortex Coronagraph for the Subaru Coronagraphic Extreme-Adaptive Optics System

The vector vortex is a coronagraphic imaging mode of the recently commissioned Subaru Coronagraphic Extreme-Adaptive Optics (SCExAO) platform on the 8-m Subaru Telescope. This multi-purpose high-contrast visible and near-infrared (R- to K-band) instrument is not only intended to serve as a VLT-class "planet-imager" instrument in the Northern hemisphere, but also to operate as a technology demonstration testbed ahead of the ELTs-era, with a particular emphasis on small inner-working angle (IWA) coronagraphic capabilities. The given priority to small-IWA imaging led to the early design choice to incorporate focal-plane phase-mask coronagraphs. In this context, a test H-band vector vortex liquid crystal polymer waveplate was provided to SCExAO, to allow a one-to-one comparison of different small-IWA techniques on the same telescope instrument, before considering further steps. Here we present a detailed overview of the vector vortex coronagraph, from its installation and performances on the SCExAO optical bench, to the on-sky results in the extreme AO regime, as of late 2016/early 2017. To this purpose, we also provide a few recent on-sky imaging examples, notably high-contrast ADI detection of the planetary-mass companion κAndromedae b, with a signal-to-noise ratio above 100 reached in less than 10 mn exposure time.

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arXiv2017

Data Reduction Pipeline for the CHARIS Integral-Field Spectrograph I: Detector Readout Calibration and Data Cube Extraction

We present the data reduction pipeline for CHARIS, a high-contrast integral-field spectrograph for the Subaru Telescope. The pipeline constructs a ramp from the raw reads using the measured nonlinear pixel response, and reconstructs the data cube using one of three extraction algorithms: aperture photometry, optimal extraction, or $χ^2$ fitting. We measure and apply both a detector flatfield and a lenslet flatfield and reconstruct the wavelength- and position-dependent lenslet point-spread function (PSF) from images taken with a tunable laser. We use these measured PSFs to implement a $χ^2$-based extraction of the data cube, with typical residuals of ~5% due to imperfect models of the undersampled lenslet PSFs. The full two-dimensional residual of the $χ^2$ extraction allows us to model and remove correlated read noise, dramatically improving CHARIS' performance. The $χ^2$ extraction produces a data cube that has been deconvolved with the line-spread function, and never performs any interpolations of either the data or the individual lenslet spectra. The extracted data cube also includes uncertainties for each spatial and spectral measurement. CHARIS' software is parallelized, written in Python and Cython, and freely available on github with a separate documentation page. Astrometric and spectrophotometric calibrations of the data cubes and PSF subtraction will be treated in a forthcoming paper.

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arXiv2017

Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457

Three dimensional (3D) magnetic field information on molecular clouds and cores is important for revealing their kinematical stability (magnetic support) against gravity which is fundamental for studying the initial conditions of star formation. In the present study, the 3D magnetic field structure of the dense starless core FeSt 1-457 is determined based on the near-infrared polarimetric observations of the dichroic polarization of background stars and simple 3D modeling. With an obtained angle of line-of-sight magnetic inclination axis $θ_{\rm inc}$ of $45^{\circ}\pm10^{\circ}$ and previously determined plane-of-sky magnetic field strength $B_{\rm pol}$ of $23.8\pm12.1$ $μ{\rm G}$, the total magnetic field strength for FeSt 1-457 is derived to be $33.7\pm18.0$ $μ{\rm G}$. The critical mass of FeSt 1-457, evaluated using both magnetic and thermal/turbulent support is ${M}_{\rm cr} = 3.70\pm0.92$ ${\rm M}_{\odot}$, which is identical to the observed core mass, $M_{\rm core}=3.55\pm0.75$ ${\rm M}_{\odot}$. We thus conclude that the stability of FeSt 1-457 is in a condition close to the critical state. Without infalling gas motion and no associated young stars, the core is regarded to be in the earliest stage of star formation, i.e., the stage just before the onset of dynamical collapse following the attainment of a supercritical condition. These properties would make FeSt 1-457 one of the best starless cores for future studies of the initial conditions of star formation.

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arXiv2017

Radial Velocity Measurements of an Orbiting Star Around Sgr A*

During the next closest approach of the orbiting star S2/S0-2 to the Galactic supermassive black hole (SMBH), it is estimated that RV uncertainties of ~ 10 km/s allow us to detect post-Newtonian effects throughout 2018. To evaluate an achievable uncertainty in RV and its stability, we have carried out near-infrared, high resolution (R ~ 20,000) spectroscopic monitoring observations of S2 using the Subaru telescope and the near-infrared spectrograph IRCS from 2014 to 2016. The Br-gamma absorption lines are used to determine the RVs of S2. The RVs we obtained are 497 km/s, 877 km/s, and 1108 km/s in 2014, 2015, and 2016, respectively. The statistical uncertainties are derived using the jackknife analysis. The wavelength calibrations in our three-year monitoring are stable: short-term (hours to days) uncertainties in RVs are < 0.5 km/s, and a long-term (three years) uncertainty is 1.2 km/s. The uncertainties from different smoothing parameter, and from the partial exclusion of the spectra, are found to be a few km/s. The final results using the Br-gamma line are 497 +- 17 (stat.) +- 3 (sys.) km/s in 2014, 877 +- 15 (stat.) +- 4 (sys.) km/s in 2015, and 1108 +- 12 (stat.) +- 4 (sys.) km/s in 2016. When we use two He I lines at 2.113\mum in addition to Br-gamma, the mean RVs are 513 km/s and 1114 km/s for 2014 and 2016, respectively. The standard errors of the mean are 16.2 km/s (2014) and 5.4 km/s (2016), confirming the reliability of our measurements. The difference between the RVs estimated by Newtonian mechanics and general relativity will reach about 200 km/s near the next pericenter passage in 2018. Therefore our RV uncertainties of 13 - 17 km/s with Subaru enable us to detect the general relativistic effects in the RV measurements with more than 10 sigma in 2018.

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arXiv2017

Red-edge position of habitable exoplanets around M-dwarfs

One of the possible signs of life on distant habitable exoplanets is the red-edge, which is a rise in the reflectivity of planets between visible and near-infrared (NIR) wavelengths. Previous studies suggested the possibility that the red-edge position for habitable exoplanets around M-dwarfs may be shifted to a longer wavelength than that for Earth. We investigated plausible red-edge position in terms of the light environment during the course of the evolution of phototrophs. We show that phototrophs on M-dwarf habitable exoplanets may use visible light when they first evolve in the ocean and when they first colonize the land. The adaptive evolution of oxygenic photosynthesis may eventually also use NIR radiation, by one of two photochemical reaction centers, with the other center continuing to use visible light. These "two-color" reaction centers can absorb more photons, but they will encounter difficulty in adapting to drastically changing light conditions at the boundary between land and water. NIR photosynthesis can be more productive on land, though its evolution would be preceded by the Earth-type vegetation. Thus, the red-edge position caused by photosynthetic organisms on habitable M-dwarf exoplanets could initially be similar to that on Earth and later move to a longer wavelength.

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arXiv2017

The Fundamental Stellar Parameters of FGK Stars in the SEEDS Survey

Large exoplanet surveys have successfully detected thousands of exoplanets to-date. Utilizing these detections and non-detections to constrain our understanding of the formation and evolution of planetary systems also requires a detailed understanding of the basic properties of their host stars. We have determined the basic stellar properties of F, K, and G stars in the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS) survey from echelle spectra taken at the Apache Point Observatory's 3.5m telescope. Using ROBOSPECT to extract line equivalent widths and TGVIT to calculate the fundamental parameters, we have computed Teff, log(g), vt, [Fe/H], chromospheric activity, and the age for our sample. Our methodology was calibrated against previously published results for a portion of our sample. The distribution of [Fe/H] in our sample is consistent with that typical of the Solar neighborhood. Additionally, we find the ages of most of our sample are $< 500 Myrs$, but note that we cannot determine robust ages from significantly older stars via chromospheric activity age indicators. The future meta-analysis of the frequency of wide stellar and sub-stellar companions imaged via the SEEDS survey will utilize our results to constrain the occurrence of detected co-moving companions with the properties of their host stars.

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arXiv2017

Constraining accretion signatures of exoplanets in the TW Hya transitional disk

We present a near-infrared direct imaging search for accretion signatures of possible protoplanets around the young stellar object (YSO) TW Hya, a multi-ring disk exhibiting evidence of planet formation. The Pa$β$ line (1.282 $μ$m) is an indication of accretion onto a protoplanet, and its intensity is much higher than that of blackbody radiation from the protoplanet. We focused on the Pa$β$ line and performed Keck/OSIRIS spectroscopic observations. Although spectral differential imaging (SDI) reduction detected no accretion signatures, the results of the present study allowed us to set 5$σ$ detection limits for Pa$β$ emission of $5.8\times10^{-18}$ and $1.5\times10^{-18}$ erg/s/cm$^2$ at 0\farcs4 and 1\farcs6, respectively. We considered the mass of potential planets using theoretical simulations of circumplanetary disks and hydrogen emission. The resulting masses were $1.45\pm 0.04$ M$_{\rm J}$ and $2.29 ^{+0.03}_{-0.04}$ M$_{\rm J}$ at 25 and 95 AU, respectively, which agree with the detection limits obtained from previous broadband imaging. The detection limits should allow the identification of protoplanets as small as $\sim$1 M$_{\rm J}$, which may assist in direct imaging searches around faint YSOs for which extreme adaptive optics instruments are unavailable.

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arXiv2017

Distortion of Magnetic Fields in a Starless Core: Near-Infrared Polarimetry of FeSt 1-457

Magnetic fields are believed to play an important role in controlling the stability and contraction of dense condensations of gas and dust leading to the formation of stars and planetary systems. In the present study, the magnetic field of FeSt 1-457, a cold starless molecular cloud core, was mapped on the basis of the polarized near-infrared light from 185 background stars after being dichroically absorbed by dust aligned with the magnetic field in the core. A distinct "hourglass-shaped" magnetic field was identified in the region of the core, which was interpreted as the first evidence of a magnetic field structure distorted by mass condensation in a starless core. The steep curvature of the magnetic field lines obtained in the present study indicates that the distortion was mainly created during the formation phase of the dense core. The derived mass-to-magnetic flux ratio indicates that the core is in a magnetically supercritical state. However, the stability of the core can be considered to be in a nearly critical state if the additional contributions from the thermal and turbulent support are included. Further diffusion of the magnetic field and/or turbulent dissipation would cause the onset of dynamical collapse of the core. The geometrical relationship between the direction of the magnetic field lines and the elongation of the core was found to be in good agreement with the theoretical predictions for the formation of Sun-like stars under the influence of a magnetic field.

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arXiv2017

First results from BISTRO -- a SCUBA-2 polarimeter survey of the Gould Belt

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC 1 region. We see that the magnetic field lies approximately perpendicular to the famous 'integral filament' in the densest regions of that filament. Furthermore, we see an 'hour-glass' magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density north-eastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

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arXiv2017

The shadow knows: using shadows to investigate the structure of the pretransitional disk of HD 100453

We present GPI polarized intensity imagery of HD 100453 in Y-, J-, and K1 bands which reveals an inner gap ($9 - 18$ au), an outer disk ($18-39$ au) with two prominent spiral arms, and two azimuthally-localized dark features also present in SPHERE total intensity images (Wagner 2015). SED fitting further suggests the radial gap extends to $1$ au. The narrow, wedge-like shape of the dark features appears similar to predictions of shadows cast by a inner disk which is misaligned with respect to the outer disk. Using the Monte Carlo radiative transfer code HOCHUNCK3D (Whitney 2013), we construct a model of the disk which allows us to determine its physical properties in more detail. From the angular separation of the features we measure the difference in inclination between the disks 45$^{\circ}$, and their major axes, PA = 140$^{\circ}$ east of north for the outer disk and 100$^{\circ}$for the inner disk. We find an outer disk inclination of $25 \pm 10^{\circ}$ from face-on in broad agreement with the Wagner 2015 measurement of 34$^{\circ}$. SPHERE data in J- and H-bands indicate a reddish disk which points to HD 100453 evolving into a young debris disk.

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arXiv2017

A Curved Magnetic field in the ring-like shell of bubble N4

We report the detection of a curved magnetic field in the ring-like shell of the bubble N4, derived from near-infrared polarization of reddened diskless stars located behind this bubble. The magnetic field in the shell is curved and parallel to the ring-like shell, and its strength is estimated to be $\sim120\,μ$G in the plane of the sky. The magnetic field strength in the shell is significantly enhanced compared to the local field strength. We calculate the mass-to-flux ratio for the submillimeter clumps in the shell and find that they are all magnetically subcritical. Our results demonstrate that the magnetic field strengthens as the interstellar medium is compressed into a shell, and suggest that the magnetic field has the potential to hinder star formation triggered by \ion{H}{2} region expansion.

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arXiv2017

Near-Infrared Polarimetric Study of N159/N160 Star-Forming Complex in the Large Magellanic Cloud

We present near-infrared polarimetric results for N159/N160 star-forming complex, in the Large Magellanic Cloud (LMC) with SIRPOL, the polarimeter of the Infrared Survey Facility (IRSF). We separated foreground sources using their visual extinction derived from near-infrared photometric data. The 45 young stellar candidates and 2 high excitation blobs were matched with our sources, and 12 of them showed the high polarization that was not originated from the interstellar dust. We made a polarimetric catalog of 252, 277, and 89 sources at $J$, $H$, and $K_s$ bands, respectively. Based on the ratios of polarization degree among these bands, we verify that the origin of these polarized sources is the dichroic extinction from the aligned interstellar dust by magnetic field and that the ratios follow a power-law dependence of $P_λ$ $\sim$ $λ^{-0.9}$. The linear polarization vectors projected on the H$α$ image of the complex turned out to follow local magnetic field structure. The vector map overlaid on dust and gas emissions shows the close correlation between magnetic field structure and surrounding interstellar medium. We suggest that the derived magnetic field structure supports the sequential formation scenario of the complex.

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arXiv2017

The Sizes and Depletions of the Dust and Gas Cavities in the Transitional Disk J160421.7-213028

We report ALMA Cycle 2 observations of 230 GHz (1.3 mm) dust continuum emission, and $^{12}$CO, $^{13}$CO, and C$^{18}$O J = 2-1 line emission, from the Upper Scorpius transitional disk [PZ99] J160421.7-213028, with an angular resolution of ~0".25 (35 AU). Armed with these data and existing H-band scattered light observations, we measure the size and depth of the disk's central cavity, and the sharpness of its outer edge, in three components: sub-$μ$m-sized "small" dust traced by scattered light, millimeter-sized "big" dust traced by the millimeter continuum, and gas traced by line emission. Both dust populations feature a cavity of radius $\sim$70 AU that is depleted by factors of at least 1000 relative to the dust density just outside. The millimeter continuum data are well explained by a cavity with a sharp edge. Scattered light observations can be fitted with a cavity in small dust that has either a sharp edge at 60 AU, or an edge that transitions smoothly over an annular width of 10 AU near 60 AU. In gas, the data are consistent with a cavity that is smaller, about 15 AU in radius, and whose surface density at 15 AU is $10^{3\pm1}$ times smaller than the surface density at 70 AU; the gas density grades smoothly between these two radii. The CO isotopologue observations rule out a sharp drop in gas surface density at 30 AU or a double-drop model as found by previous modeling. Future observations are needed to assess the nature of these gas and dust cavities, e.g., whether they are opened by multiple as-yet-unseen planets or photoevaporation.

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