Showing papers by "James R. Graham published in 2006"

The Gemini Planet Imager

Abstract: The next major frontier in the study of extrasolar planets is direct imaging detection of the planets themselves. With high-order adaptive optics, careful system design, and advanced coronagraphy, it is possible for an AO system on a 8-m class telescope to achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets in the solar neighborhood. Such direct detection is sensitive to planets inaccessible to current radial-velocity surveys and allows spectral characterization of the planets, shedding light on planet formation and the structure of other solar systems. We have begun the construction of such a system for the Gemini Observatory. Dubbed the Gemini Planet Imager (GPI), this instrument should be deployed in 2010 on the Gemini South telescope. It combines a 2000-actuator MEMS-based AO system, an apodized-pupil Lyot coronagraph, a precision infrared interferometer for real-time wavefront calibration at the nanometer level, and a infrared integral field spectrograph for detection and characterization of the target planets. GPI will be able to achieve Strehl ratios > 0.9 at 1.65 microns and to observe a broad sample of science targets with I band magnitudes less than 8. In addition to planet detection, GPI will also be capable of polarimetric imaging of circumstellar dust disks, studies of evolved stars, and high-Strehl imaging spectroscopy of bright targets. We present here an overview of the GPI instrument design, an error budget highlighting key technological challenges, and models of the system performance.

The Signature of Primordial Grain Growth in the Polarized Light of the AU Mic Debris Disk

Abstract: We have used the Hubble Space Telescope/ACS coronagraph to make polarization maps of the AU Mic debris disk. The fractional linear polarization rises monotonically from about 0.05 to 0.4 between 20 and 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron sized grains in an optically thin disk. Disk models, which simultaneously fit the surface brightness and polarization, show that the inner disk (< 40-50 AU) is depleted of micron-sized dust by a factor of more than 300, which means that the disk is collision dominated. The grains have high maximum linear polarization and strong forward scattering. Spherical grains composed of conventional materials cannot reproduce these optical properties. A Mie/Maxwell-Garnett analysis implicates highly porous (91-94%) particles. In the inner Solar System, porous particles form in cometary dust, where the sublimation of ices leaves a "bird's nest" of refractory organic and silicate material. In AU Mic, the grain porosity may be primordial, because the dust "birth ring" lies beyond the ice sublimation point. The observed porosities span the range of values implied by laboratory studies of particle coagulation by ballistic cluster-cluster aggregation. To avoid compactification, the upper size limit for the parent bodies is in the decimeter range, in agreement with theoretical predictions based on collisional lifetime arguments. Consequently, AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects.

A photometric redshift of z = 6.39 ± 0.12 for GRB 050904

Abstract: Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified3. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.29^(+0.11)_(-0.12) (refs 5–7). Subsequently, it was measured spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.

Extreme Adaptive Optics for the Thirty Meter Telescope

Abstract: Direct detection of extrasolar Jovian planets is a major scientific motivation for the construction of future extremely large telescopes such as the Thirty Meter Telescope (TMT). Such detection will require dedicated high-contrast AO systems. Since the properties of Jovian planets and their parent stars vary enormously between different populations, the instrument must be designed to meet specific scientific needs rather than a simple metric such as maximum Strehl ratio. We present a design for such an instrument, the Planet Formation Imager (PFI) for TMT. It has four key science missions. The first is the study of newly-formed planets on 5-10 AU scales in regions such as Taurus and Ophiucus this requires very small inner working distances that are only possible with a 30m or larger telescope. The second is a robust census of extrasolar giant planets orbiting mature nearby stars. The third is detailed spectral characterization of the brightest extrasolar planets. The final targets are circumstellar dust disks, including Zodiacal light analogs in the inner parts of other solar systems. To achieve these, PFI combines advanced wavefront sensors, high-order MEMS deformable mirrors, a coronagraph optimized for a finely- segmented primary mirror, and an integral field spectrograph.

First scattered light images of debris disks around hd 53143 and hd 139664

Abstract: We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). With ages of 0.3-1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45° from edge-on, does not show radial variation in disk structure and has a width >55 AU. HD 139664 is seen close to edge-on and has a beltlike morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide-disk architecture with a characteristic width >50 AU.

Speckle Statistics in Adaptively Corrected Images

Abstract: Imaging observations are generally affected by a fluctuating background of speckles, a particular problem when detecting faint stellar companions at small angular separations. These speckles can be created by both short-lived atmospheric aberrations and slowly changing distortions in the optical system. Over the course of a long-exposure image, the combination of many independent realizations of speckle patterns forms a halo in the point-spread function (PSF) of characteristic scale Δθ ~ λ/r0 (where r0 is the coherence length in the pupil). While adaptive optics can increase the achievable image contrast, speckle noise remains a major source of random error, which decreases the sensitivity of companion detection observations near the diffraction limit. Knowing the distribution of the speckle intensities at a given location in the image plane is therefore important for understanding the noise limits of companion detection. The speckle noise limit in a long-exposure image is characterized by the intensity variance and the speckle lifetime. In this paper we address the former quantity through the distribution function of speckle intensity. Previous theoretical work has predicted a form for this distribution function at a single location in the image plane. We developed a fast readout mode to take short exposures of stellar images corrected by adaptive optics at the ground-based UCO/Lick Observatory, with integration times of 5 ms and a time between successive frames of 14.5 ms (λ = 2.2 μm). These observations temporally oversample and spatially Nyquist sample the observed speckle patterns. We show, for various locations in the image plane, that the observed distribution of speckle intensities is consistent with the predicted form. In addition, we demonstrate a method by which Ic and Is can be mapped over the image plane. As the quantity Ic is proportional to the PSF of the telescope free of random atmospheric aberrations, this method can be used for PSF calibration and reconstruction.

High Spectral Resolution Observations of the Massive Stars in the Galactic Center

Abstract: We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25'' of the Galactic center. We have identified a total of 21 emission-line stars, seven of which are new radial velocity detections, with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: (1) those with narrow 2.112, 2.113 μm He I doublet absorption lines, and (2) those with broad 2.058 μm He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 km s-1, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 μm emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree with previous stellar velocity studies that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multiepoch radial velocity measurements for IRS 16NE show a change in its velocity, presumably due to an unseen stellar companion.

The Challenges of Coronagraphic Astrometry

Abstract: A coronagraph in conjunction with adaptive optics provides an effective means to image faint companions of nearby stars from the ground The images from such a system are complex, however, and need to be fully characterized and understood before planets or disks can be detected against the glare from the host star Using data from the Lyot Project coronagraph, we investigate the difficulties of astrometric measurements in diffraction-limited coronagraphic images and consider the principal problem of determining the precise location of the occulted star We demonstrate how the image structure varies when the star is decentered from the optical axis and show how even small offsets (005λ/D or 5 mas) give rise to false sources in the image We consider methods of determining the star position from centroiding, instrument feedback, and analysis of point-spread function symmetry and conclude that internal metrology is the most effective technique

Constraining the Adaptive Optics Point-Spread Function in Crowded Fields: Measuring Photometric Aperture Corrections

Abstract: The point-spread function (PSF) of an adaptive optics (AO) system is often poorly known. This ignorance can lead to significant systematic errors. Since the degree of AO correction is sensitive to the observing conditions (seeing, wind speed, brightness of the wave front reference, etc.), it would be desirable to estimate the PSF from the data themselves rather than from observations of a PSF star at another time. We have developed a method to estimate the PSF delivered by an AO system in the case where the scene consists of a crowded star field. We model the modulation transfer function (MTF) of several key components of the imaging system (atmosphere filtered by an AO system, telescope pupil, and pixel array). The power spectrum of the image, even a dense star field, can be used to constrain our model, which in turn can be used to reconstruct the PSF. In the case of circularly symmetric PSFs, we demonstrate that the power spectrum of the source distribution function can be successfully removed from the measured MTF and that our fit successfully recovers input parameters from a model data set constructed from these parameters. We also show that the method yields reasonable fit parameters and a useful approximation to the PSF when applied to data from the laser guide star (LGS) AO system at the Keck Observatory. Comparison of Keck LGS AO data and Hubble Space Telescope observations with NICMOS show that photometric accuracy of a few percent can be achieved for data with Strehl ratios as low as 4%.

Discovery of an obscured broad line region in the high redshift radio

Abstract: This paper presents infrared spectra taken with the newly commissioned NIRSPEC spectrograph on the Keck II Telescope of the High Redshift Radio Galaxy MRC 2025-218 (z=2.63) These observations represent the deepest infrared spectra of a radio galaxy to date and have allowed for the detection of H�, (OIII) (4959/5007), (OI) (6300), H�, (NII) (6548/6583) and (SII) (6716/6713). The Hemission is very broad (FWHM = 9300 km/s) and luminous (2.6×10 44 ergs/s) and it is very comparable to the line widths and strengths of radio loud quasars at the same redshift. This strongly supports AGN unification models linking radio galaxies and quasars, although we discuss some of the outstanding differences. The (OIII) (5007) line is extremely strong and has extended emission with large relative velocities to the nucleus. We also derive that if the extended emission is due to star formation, each knot has a star formation rate comparable to a Lyman Break Galaxy at the same redshift. Subject headings: galaxies: active — galaxies: structure — galaxies: quasars — galaxies: kinematics and dynamics — infrared: galaxies

First scattered light images of debris disks around HD 53143 and HD 139664

Abstract: We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). With ages 0.3 - 1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in disk structure and has width >55 AU. HD 139664 is seen close to edge-on and has belt-like morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide disk architecture with characteristic width >50 AU.

GRB 050911: A black hole-neutron star merger or a naked GRB

Abstract: GRB 050911, discovered by the Swift Burst Alert Telescope, was not seen 4.6 hr later by the Swift X-ray Telescope, making it one of the very few X-ray nondetections of a gamma-ray burst (GRB) afterglow at early times. Theg-ray light curve shows at least three peaks, the first two of which ( ∼ and s, where T 0.8 T 0.2 0 0 is the trigger time) were short, each lasting 0.5 s. This was followed by later emission 10–20 s postburst. The T0 upper limit on the unabsorbed X-ray flux was ergs cm 2 s 1 (integrating 46 ks of data taken between 14 1.7# 10 September 11 and 18), indicating that the decay must have been rapid. All but one of the long bursts detected by Swift were above this limit at∼4.6 hr, whereas the afterglows of short bursts became undetectable more rapidly. Deep observations with Gemini also revealed no optical afterglow 12 hr after the burst, down to r p (5 j limit). We speculate that GRB 050911 may have been formed through a compact object (black hole– 24.0 neutron star) merger, with the later outbursts due to a longer disk lifetime linked to a large mass ratio between the merging objects. Alternatively, the burst may have occurred in a low-density environment, leading to a weak, or nonexistent, forward shock—the so-called “naked GRB” model. Subject heading: gamma rays: bursts

The Signature of Primordial Grain Growth in the Polarized Light of the AU Mic Debris Disk

Abstract: We have used the Hubble Space Telescope/ACS coronagraph to make polarization maps of the AU Mic debris disk. The fractional linear polarization rises monotonically from about 0.05 to 0.4 between 20 and 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron sized grains in an optically thin disk. Disk models, which simultaneously fit the surface brightness and polarization, show that the inner disk (< 40-50 AU) is depleted of micron-sized dust by a factor of more than 300, which means that the disk is collision dominated. The grains have high maximum linear polarization and strong forward scattering. Spherical grains composed of conventional materials cannot reproduce these optical properties. A Mie/Maxwell-Garnett analysis implicates highly porous (91-94%) particles. In the inner Solar System, porous particles form in cometary dust, where the sublimation of ices leaves a "bird's nest" of refractory organic and silicate material. In AU Mic, the grain porosity may be primordial, because the dust "birth ring" lies beyond the ice sublimation point. The observed porosities span the range of values implied by laboratory studies of particle coagulation by ballistic cluster-cluster aggregation. To avoid compactification, the upper size limit for the parent bodies is in the decimeter range, in agreement with theoretical predictions based on collisional lifetime arguments. Consequently, AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects.