arXiv subjects

W. Brandner

Publications and source records attributed to W. Brandner.

At least 163 records · Page 9Linked to original sources

arXiv2014

GRAVITY: the Calibration Unit

We present in this paper the design and characterisation of a new sub-system of the VLTI 2nd generation instrument GRAVITY: the Calibration Unit. The Calibration Unit provides all functions to test and calibrate the beam combiner instrument: it creates two artificial stars on four beams, and dispose of four delay lines with an internal metrology. It also includes artificial stars for the tip-tilt and pupil guiding systems, as well as four metrology pick-up diodes, for tests and calibration of the corresponding sub-systems. The calibration unit also hosts the reference targets to align GRAVITY to the VLTI, and the safety shutters to avoid the metrology light to propagate in the VLTI-lab. We present the results of the characterisation and validtion of these differrent sub-units.

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arXiv2014

The GRAVITY metrology system: modeling a metrology in optical fibers

GRAVITY is the second generation VLT Interferometer (VLTI) instrument for high-precision narrow-angle astrometry and phase-referenced interferometric imaging. The laser metrology system of GRAVITY is at the heart of its astrometric mode, which must measure the distance of 2 stars with a precision of 10 micro-arcseconds. This means the metrology has to measure the optical path difference between the two beam combiners of GRAVITY to a level of 5 nm. The metrology design presents some non-common paths that have consequently to be stable at a level of 1 nm. Otherwise they would impact the performance of GRAVITY. The various tests we made in the past on the prototype give us hints on the components responsible for this error, and on their respective contribution to the total error. It is however difficult to assess their exact origin from only OPD measurements, and therefore, to propose a solution to this problem. In this paper, we present the results of a semi-empirical modeling of the fibered metrology system, relying on theoretical basis, as well as on characterisations of key components. The modeling of the metrology system regarding various effects, e.g., temperature, waveguide heating or mechanical stress, will help us to understand how the metrology behave. The goals of this modeling are to 1) model the test set-ups and reproduce the measurements (as a validation of the modeling), 2) determine the origin of the non-common path errors, and 3) propose modifications to the current metrology design to reach the required 1nm stability.

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arXiv2014

The interferometric baselines and GRAVITY astrometric error budget

GRAVITY is a new generation beam combination instrument for the VLTI. Its goal is to achieve microarsecond astrometric accuracy between objects separated by a few arcsec. This $10^6$ accuracy on astrometric measurements is the most important challenge of the instrument, and careful error budget have been paramount during the technical design of the instrument. In this poster, we will focus on baselines induced errors, which is part of a larger error budget.

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arXiv2014

GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age

The stellar population in the central parsec of the Galaxy is dominated by an old (several Gyr) population, but young, massive stars dominate the luminosity function. We have studied the most luminous of these stars, GCIRS 7, in order to constrain the age of the recent star formation event in the Galactic Centre and to characterise it as an interferometric reference for observations of the Galactic Centre with the instrument GRAVITY, which will equip the Very Large Telescope Interferometer in the near future. We present the first H-band interferometric observations of GCIRS 7, obtained using the PIONIER visitor instrument on the VLTI using the four 8.2-m unit telescopes. In addition, we present unpublished K-band VLTI/AMBER data, build JHKL light-curves based on data spanning 4 decades, and measured the star's effective temperature using SINFONI spectroscopy. GCIRS 7 is marginally resolved at H-band (in 2013: uniform-disk diameter=1.076+/-0.093mas, R=960+/-92Rsun at 8.33+/-0.35kpc). We detect a significant circumstellar contribution at K-band. The star and its environment are variable in brightness and in size. The photospheric H-band variations are well modelled with two periods: P0~470+/-10 days (amplitude ~0.64mag) and long secondary period LSP~2700-2850 days (~1.1mag). As measured from CO equivalent width, =3600+/-195K. The size, periods, luminosity ( =-8.44+/-0.22) and effective temperature are consistent with an M1 supergiant with an initial mass of 22.5+/-2.5Msun and an age of 6.5-10Myr (depending on rotation). This age is in remarkable agreement with most estimates for the recent star formation event in the central parsec. Caution should be taken when using this star as an interferometric reference as it is variable in size, is surrounded by a variable circumstellar environment and large convection cells may form on its photosphere.

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arXiv2014

The Moving Group Targets of the SEEDS High-Contrast Imaging Survey of Exoplanets and Disks: Results and Observations from the First Three Years

We present results from the first three years of observations of moving group targets in the SEEDS high-contrast imaging survey of exoplanets and disks using the Subaru telescope. We achieve typical contrasts of ~10^5 at 1" and ~10^6 beyond 2" around 63 proposed members of nearby kinematic moving groups. We review each of the kinematic associations to which our targets belong, concluding that five, βPictoris (~20 Myr), AB Doradus (~100 Myr), Columba (~30 Myr), Tucana-Horogium (~30 Myr), TW Hydrae (~10 Myr), are sufficiently well-defined to constrain the ages of individual targets. Somewhat less than half of our targets are high-probability members of one of these moving groups. For all of our targets, we combine proposed moving group membership with other age indicators where available, including Ca II HK emission, X-ray activity, and rotation period, to produce a posterior probability distribution of age. SEEDS observations discovered a substellar companion to one of our targets, κAnd, a late B star. We do not detect any other substellar companions, but do find seven new close binary systems, of which one still needs to be confirmed. A detailed analysis of the statistics of this sample, and of the companion mass constraints given our age probability distributions and exoplanet cooling models, will be presented in a forthcoming paper.

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arXiv2014

A global cloud map of the nearest known brown dwarf

Brown dwarfs -- substellar bodies more massive than planets but not massive enough to initiate the sustained hydrogen fusion that powers self-luminous stars -- are born hot and slowly cool as they age. As they cool below about 2,300 K, liquid or crystalline particles composed of calcium aluminates, silicates and iron condense into atmospheric 'dust', which disappears at still cooler temperatures (around 1,300 K). Models to explain this dust dispersal include both an abrupt sinking of the entire cloud deck into the deep, unob- servable atmosphere and breakup of the cloud into scattered patches (as seen on Jupiter and Saturn). Thus far, observations of brown dwarfs have been limited to globally integrated measurements, which can reveal surface inhomogeneities but cannot unambiguously resolve surface features. Here we report a two-dimensional map of a brown dwarf's surface that allows identification of large-scale bright and dark features, indicative of patchy clouds.

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arXiv2014

Characterization of the Benchmark Binary NLTT 33370

We report the confirmation of the binary nature of the nearby, very low-mass system NLTT 33370 with adaptive optics imaging and present resolved near-infrared photometry and integrated light optical and near-infrared spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show significant orbital motion between 2013 February and 2013 April. Optical spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708 Angstrom absorption that indicate the system is younger than field age. VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and spectral morphology that is consistent with other young, very low-mass dwarfs. We combine the constraints from all age diagnostics to estimate a system age of ~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and B, respectively. The observed spectra, derived temperatures, and estimated age combine to constrain the component spectral types to the range M6-M8. Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from the estimated luminosities of the components. KPNO-Phoenix spectra allow us to estimate the systemic radial velocity of the binary. The Galactic kinematics of NLTT 33370AB are broadly consistent with other young stars in the Solar neighborhood. However, definitive membership in a young, kinematic group cannot be assigned at this time and further follow-up observations are necessary to fully constrain the system's kinematics. The proximity, age, and late-spectral type of this binary make it very novel and an ideal target for rapid, complete orbit determination. The system is one of only a few model calibration benchmarks at young ages and very low-masses.

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arXiv2013

Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation

Embedded clusters like W3 Main are complex and dynamically evolving systems that represent an important phase of the star formation process. We aim at the characterization of the entire stellar content of W3 Main in a statistical sense to identify possible differences in evolutionary phase of the stellar populations and find clues about the formation mechanism of this massive embedded cluster. Methods. Deep JHKs imaging is used to derive the disk fraction, Ks-band luminosity functions and mass functions for several subregions in W3 Main. A two dimensional completeness analysis using artificial star experiments is applied as a crucial ingredient to assess realistic completeness limits for our photometry. We find an overall disk fraction of 7.7 $\pm$ 2.3%, radially varying from 9.4 $\pm$ 3.0 % in the central 1 pc to 5.6 $\pm$ 2.2 % in the outer parts of W3 Main. The mass functions derived for three subregions are consistent with a Kroupa and Chabrier mass function. The mass function of IRSN3 is complete down to 0.14 Msun and shows a break at M $\sim$ 0.5 Msun. We interpret the higher disk fraction in the center as evidence for a younger age of the cluster center. We find that the evolutionary sequence observed in the low-mass stellar population is consistent with the observed age spread among the massive stars. An analysis of the mass function variations does not show evidence for mass segregation. W3 Main is currently still actively forming stars, showing that the ionizing feedback of OB stars is confined to small areas ($\sim$ 0.5 pc). The FUV feedback might be influencing large regions of the cluster as suggested by the low overall disk fraction.

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arXiv2013

Direct Imaging Detection of Methane in the Atmosphere of GJ 504 b

Most exoplanets detected by direct imaging so far have been characterized by relatively hot (> ~1000 K) and cloudy atmospheres. A surprising feature in some of their atmospheres has been a distinct lack of methane, possibly implying non-equilibrium chemistry. Recently, we reported the discovery of a planetary companion to the Sun-like star GJ 504 using Subaru/HiCIAO within the SEEDS survey. The planet is substantially colder (<600 K) than previously imaged planets, and has indications of fewer clouds, which implies that it represents a new class of planetary atmospheres with expected similarities to late T-type brown dwarfs in the same temperature range. If so, one might also expect the presence of significant methane absorption, which is characteristic of such objects. Here, we report the detection of deep methane absorption in the atmosphere of GJ 504 b, using the Spectral Differential Imaging mode of HiCIAO to distinguish the absorption feature around 1.6 um. We also report updated JHK photometry based on new Ks-band data and a re-analysis of the existing data. The results support the notion that GJ 504 b has atmospheric properties distinct from other imaged exoplanets, and will become a useful reference object for future planets in the same temperature range.

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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

Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504

Several exoplanets have recently been imaged at wide separations of >10 AU from their parent stars. These span a limited range of ages (<50 Myr) and atmospheric properties, with temperatures of 800--1800 K and very red colors (J - H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly-imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0] Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted for the core accretion mechanism. GJ 504 b is also significantly cooler (510 [+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets, as well as their atmospheric properties.

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arXiv2013

Proper motions of molecular hydrogen outflows in the Rho Ophiuchi molecular cloud

We aim to take a census of molecular hydrogen emission line objects (MHOs) in the Rho Ophiuchi molecular cloud and to make the first systematic proper motion measurements of these objects in this region. Deep H2 near-infrared imaging is performed to search for molecular hydrogen emission line objects. Multi-epoch data are used to derive the proper motions of the features of these objects, and the lengths and opening angles of the molecular hydrogen outflows. Our imaging covers an area of about 0.11 deg2 toward the L1688 core in the Rho Ophiuchi molecular cloud. In total, six new MHOs are discovered, 32 previously known MHOs are detected, and the proper motions for 86 features of the MHOs are measured. The proper motions lie in the range of 14 to 247 mas/yr, corresponding to transversal velocities of 8 to 140 km s-1 with a median velocity of about 35 km s-1. Based on morphology and proper motion measurements, 27 MHOs are ascribed to 21 driving sources. The molecular hydrogen outflows have a median length of about 0.04 pc and random orientations. We find no obvious correlation between H2 jet length, jet opening angle, and the evolutionary stage of the driving sources as defined by their spectral indices. We find that the fraction of protostars (23%) that drive molecular hydrogen outflows is similar to the one for Class II sources (15%). For most molecular hydrogen outflows, no obvious velocity variation along the outflow has been found. In Ophiuchus the frequency of occurrence of molecular hydrogen outflows has no strong dependency on the evolutionary stage of the driving source during the evolution from the protostellar stage to the classical T Tauri stage.

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arXiv2013

New Techniques for High-Contrast Imaging with ADI: the ACORNS-ADI SEEDS Data Reduction Pipeline

We describe Algorithms for Calibration, Optimized Registration, and Nulling the Star in Angular Differential Imaging (ACORNS-ADI), a new, parallelized software package to reduce high-contrast imaging data, and its application to data from the SEEDS survey. We implement several new algorithms, including a method to register saturated images, a trimmed mean for combining an image sequence that reduces noise by up to ~20%, and a robust and computationally fast method to compute the sensitivity of a high-contrast observation everywhere on the field-of-view without introducing artificial sources. We also include a description of image processing steps to remove electronic artifacts specific to Hawaii2-RG detectors like the one used for SEEDS, and a detailed analysis of the Locally Optimized Combination of Images (LOCI) algorithm commonly used to reduce high-contrast imaging data. ACORNS-ADI is written in python. It is efficient and open-source, and includes several optional features which may improve performance on data from other instruments. ACORNS-ADI requires minimal modification to reduce data from instruments other than HiCIAO. It is freely available for download at www.github.com/t-brandt/acorns-adi under a BSD license.

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arXiv2013

Imaging Discovery of the Debris Disk Around HIP 79977

We present Subaru/HiCIAO H-band high-contrast images of the debris disk around HIP 79977, whose pres- ence was recently inferred from an infrared excess. Our images resolve the disk for the first time, allowing characterization of its shape, size, and dust grain properties. We use angular differential imaging (ADI) to reveal the disk geometry in unpolarized light out to a radius of ~2", as well as polarized differential imaging (PDI) to measure the degree of scattering polarization out to ~1.5". In order to strike a favorable balance between suppression of the stellar halo and conservation of disk flux, we explore the application of principal component analysis (PCA) to both ADI and reference star subtraction. This allows accurate forward modeling of the effects of data reduction on simulated disk images, and thus direct comparison with the imaged disk. The resulting best-fit values and well-fitting intervals for the model parameters are a surface brightness power-law slope of S_out = -3.2 [-3.6,-2.9], an inclination of i = 84° [81°,86°], a high Henyey-Greenstein forward-scattering parameter of g = 0.45 [0.35, 0.60], and a non-significant disk-star offset of u = 3.0 [-1.5, 7.5] AU = 24 [-13, 61] mas along the line of nodes. Furthermore, the tangential linear polarization along the disk rises from ~10% at 0.5" to ~45% at 1.5". These measurements paint a consistent picture of a disk of dust grains produced by collisional cascades and blown out to larger radii by stellar radiation pressure.

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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

The Structure of Pre-transitional Protoplanetary Disks I: Radiative Transfer Modeling of the Disk+Cavity in the PDS 70 system

Through detailed radiative transfer modeling, we present a disk+cavity model to simultaneously explain both the SED and Subaru H-band polarized light imaging for the pre-transitional protoplanetary disk PDS 70. Particularly, we are able to match not only the radial dependence, but also the absolute scale, of the surface brightness of the scattered light. Our disk model has a cavity 65 AU in radius, which is heavily depleted of sub-micron-sized dust grains, and a small residual inner disk which produces a weak but still optically thick NIR excess in the SED. To explain the contrast of the cavity edge in the Subaru image, a factor of ~1000 depletion for the sub-micron-sized dust inside the cavity is required. The total dust mass of the disk may be on the order of 1e-4 M_sun, only weakly constrained due to the lack of long wavelength observations and the uncertainties in the dust model. The scale height of the sub-micron-sized dust is ~6 AU at the cavity edge, and the cavity wall is optically thick in the vertical direction at H-band. PDS 70 is not a member of the class of (pre-)transitional disks identified by Dong et al. (2012), whose members only show evidence of the cavity in the millimeter-sized dust but not the sub-micron-sized dust in resolved images. The two classes of (pre-)transitional disks may form through different mechanisms, or they may just be at different evolution stages in the disk clearing process.

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