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Search Results: 1 - 10 of 433118 matches for " J. D. Monnier "
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Asymmetric Beam Combination for Optical Interferometry
J. D. Monnier
Physics , 2001, DOI: 10.1086/320288
Abstract: Optical interferometers increasingly use single-mode fibers as spatial filters to convert varying wavefront distortion into intensity fluctuations which can be monitored for accurate calibration of fringe amplitudes. Here I propose using an asymmetric coupler to allow the photometric intensities of each telescope beam to be measured at the same time as the fringe visibility, but without the need for dedicated photometric outputs, which reduce the light throughput in the interferometric channels. In the read-noise limited case often encountered in the infrared, I show that a 53% improvement in signal-to-noise ratio for the visibility amplitude measurement is achievable, when compared to a balanced coupler setup with 50% photometric taps (e.g., the FLUOR experiment). In the Poisson-noise limit appropriate for visible light, the improvement is reduced to only ~8%. This scheme also reduces the cost and complexity of the beam combination since fewer components and detectors are required, and can be extended to more than two telescopes for "all-in-one" or pair-wise beam combination. Asymmetric beam combination can also be employed for monitoring scintillation and throughput variations in systems without spatial filtering.
On the Interferometric Sizes of Young Stellar Objects
J. D. Monnier,R. Millan-Gabet
Physics , 2002, DOI: 10.1086/342917
Abstract: Long-baseline optical interferometers can now detect and resolve hot dust emission thought to arise at the inner edge of circumstellar disks around young stellar objects (YSOs). We argue that the near-infrared sizes being measured are closely related to the radius at which dust is sublimated by the stellar radiation field. We consider how realistic dust optical properties and gas opacity dramatically affect the predicted location of this dust destruction radius, an exercise routinely done in other contexts but so far neglected in the analysis of near-infrared sizes of YSOs. We also present the accumulated literature of near-infrared YSO sizes in the form of a ``size-luminosity diagram'' and compare with theoretical expectations. We find evidence that large (>~ 1 micron) dust grains predominate in the inner disks of T Tauri and Herbig Ae/Be stars, under the assumption that the inner-most gaseous disks are optically-thin at visible wavelengths.
The inner regions of protoplanetary disks
C. P. Dullemond,J. D. Monnier
Physics , 2010, DOI: 10.1146/annurev-astro-081309-130932
Abstract: To understand how planetary systems form in the dusty disks around pre-main-sequence stars a detailed knowledge of the structure and evolution of these disks is required. While this is reasonably well understood for the regions of the disk beyond about 1 AU, the structure of these disks inward of 1 AU remains a puzzle. This is partly because it is very difficult to spatially resolve these regions with current telescopes. But it is also because the physics of this region, where the disk becomes so hot that the dust starts to evaporate, is poorly understood. With infrared interferometry it has become possible in recent years to directly spatially resolve the inner AU of protoplanetary disks, albeit in a somewhat limited way. These observations have partly confirmed current models of these regions, but also posed new questions and puzzles. Moreover, it has turned out that the numerical modeling of these regions is extremely challenging. In this review we give a rough overview of the history and recent developments in this exciting field of astrophysics.
A Dispersed Heterodyne Design for the Planet Formation Imager (PFI)
Michael J. Ireland,John D. Monnier
Physics , 2014, DOI: 10.1117/12.2057355
Abstract: The Planet Formation Imager (PFI) is a future world facility that will image the process of planetary formation. It will have an angular resolution and sensitivity sufficient to resolve sub-Hill sphere structures around newly formed giant planets orbiting solar-type stars in nearby star formation regions. We present one concept for this design consisting of twenty-seven or more 4m telescopes with kilometric baselines feeding a mid-infrared spectrograph where starlight is mixed with a frequency-comb laser. Fringe tracking will be undertaken in H-band using a fiber-fed direct detection interferometer, meaning that all beam transport is done by communications band fibers. Although heterodyne interferometry typically has lower signal-to-noise than direct detection interferometry, it has an advantage for imaging fields of view with many resolution elements, because the signal in direct detection has to be split many ways while the signal in heterodyne interferometry can be amplified prior to combining every baseline pair. We compare the performance and cost envelope of this design to a comparable direct-detection design.
Radio & Optical Interferometry: Basic Observing Techniques and Data Analysis
John D. Monnier,Ronald J. Allen
Physics , 2012, DOI: 10.1007/978-94-007-5618-2_7
Abstract: Astronomers usually need the highest angular resolution possible, but the blurring effect of diffraction imposes a fundamental limit on the image quality from any single telescope. Interferometry allows light collected at widely-separated telescopes to be combined in order to synthesize an aperture much larger than an individual telescope thereby improving angular resolution by orders of magnitude. Radio and millimeter wave astronomers depend on interferometry to achieve image quality on par with conventional visible and infrared telescopes. Interferometers at visible and infrared wavelengths extend angular resolution below the milli-arcsecond level to open up unique research areas in imaging stellar surfaces and circumstellar environments. In this chapter the basic principles of interferometry are reviewed with an emphasis on the common features for radio and optical observing. While many techniques are common to interferometers of all wavelengths, crucial differences are identified that will help new practitioners avoid unnecessary confusion and common pitfalls. Concepts essential for writing observing proposals and for planning observations are described, depending on the science wavelength, angular resolution, and field of view required. Atmospheric and ionospheric turbulence degrades the longest-baseline observations by significantly reducing the stability of interference fringes. Such instabilities represent a persistent challenge, and the basic techniques of phase-referencing and phase closure have been developed to deal with them. Synthesis imaging with large observing datasets has become a routine and straightforward process at radio observatories, but remains challenging for optical facilities. In this context the commonly-used image reconstruction algorithms CLEAN and MEM are presented. Lastly, a concise overview of current facilities is included as an appendix.
Discovery of a Near-Infrared Jet-Like Feature in the Z Canis Majoris System
R. Millan-Gabet,J. D. Monnier
Physics , 2002, DOI: 10.1086/345590
Abstract: We present near-infrared high resolution observations of the young binary system Z Canis Majoris using the adaptive optics system at the Keck-II telescope. Both components are unresolved at 1.25 and 1.65 microns, although high dynamic range images reveal a previously unknown jet-like feature in the circumstellar environment. We argue that this feature probably arises from light scattered off the walls of a jet-blown cavity, and proper motion studies of this feature can probe the dynamics of the bipolar outflow. Potentially, the morphology of the dust-laden cavity walls offers a new probe of the momentum profile and collimation of bipolar winds from young stellar objects. We also derive high precision binary parameters, which when combined with historical data have allowed the first detection of orbital motion. Lastly, our observations confirm the high degree of flux variability in the system; the North-West binary component is dominant at H-band, in contrast to all previous observations.
Spatially Resolved Mid-IR Imaging of the SR 21 Transition Disk
J. A. Eisner,J. D. Monnier,P. Tuthill,S. Lacour
Physics , 2009, DOI: 10.1088/0004-637X/698/2/L169
Abstract: We present mid-IR observations from Gemini/TReCS that spatially resolve the dust emission around SR 21. The protoplanetary disk around SR 21 is believed to have a cleared gap extending from stellocentric radii of ~0.5 AU to ~20 AU, based on modeling of the observed spectral energy distribution. Our new observations resolve the dust emission, and our data are inconsistent with the previous model. We require the disk to be completely cleared within ~10 AU, without the hot inner disk spanning ~0.25-0.5 AU posited previously. To fit the SED and mid-IR imaging data together, we propose a disk model with a large inner hole, but with a warm companion--possibly surrounded by circumstellar material of its own--residing near the outer edge of the cleared region. We also discuss a model with a narrow ring included in a large cleared inner disk region, and argue that it is difficult to reconcile with the data.
M. Zhao,J. D. Monnier,E. Pedretti,N. Thureau
Revista mexicana de astronomía y astrofísica , 2010,
Mid-infrared spectra of late-type stars: Long-term evolution
J. D. Monnier,T. R. Geballe,W. C. Danchi
Physics , 1999, DOI: 10.1086/307530
Abstract: Recent ground-based mid-infrared spectra of 29 late-type stars, most with substantial dust shells, are compared to ground-based spectra of these stars from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra of about half the stars show no detectable changes, implying that their distributions of circumstellar material and associated dust grain properties have changed little over this time interval. However, many of the stars with strong silicate features showed marked changes. In nearly all cases the silicate peak has strengthened with respect to the underlying continuum, although there is one case (VY~CMa) in which the silicate feature has almost completely disappeared. This suggests that, in general, an oxygen-rich star experiences long periods of gradual silicate feature strengthening, punctuated by relatively rare periods when the feature weakens. We discuss various mechanisms for producing the changes, favoring the slow evolution of the intrinsic dust properties (i.e., the chemical composition or grain structure). Although most IRAS spectra agree well with ground-based spectra, there are a number of cases where they fall well outside the expected range of uncertainty. In almost all such cases the slopes of the red and blue LRS spectra do not match in their region of overlap.
Pinwheel nebula around WR 98a
J. D. Monnier,P. G. Tuthill,W. C. Danchi
Physics , 1999, DOI: 10.1086/312352
Abstract: We present the first near-infrared images of dusty Wolf-Rayet star WR 98a. Aperture masking interferometry has been utilized to recover images at the diffraction-limit of the Keck-I telescope, ~<50 mas at 2.2 micron. Multi-epoch observations spanning about one year have resolved the dust shell into a ``pinwheel'' nebula, the second example of a new class of dust shell first discovered around WR 104 (Tuthill, Monnier, & Danchi 1999a). Interpreting the collimated dust outflow in terms of an interacting winds model, the binary orbital parameters and apparent wind speed are derived: a period of 565 +/- 50 days, a viewing angle of 35 +/- 6 degrees from the pole, and a wind speed of 99 +/- 23 mas/yr. This period is consistent with a possible ~588 day periodicity in the infrared light curve (Williams et al. 1995), linking the photometric variation to the binary orbit. Important implications for binary stellar evolution are discussed by identifying WR 104 and WR 98a as members of a class of massive, short-period binaries whose orbits were circularized during a previous red supergiant phase. The current component separation in each system is similar to the diameter of a red supergiant, indicating that the supergiant phase was likely terminated by Roche-lobe overflow, leading to the present Wolf-Rayet stage.
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