Ground-based Adaptive Optics Research Articles

Physical Properties of Binary Brown Dwarfs

Astrometric observations of binary brown dwarfs yield dynamical masses of the components independently of theoretical models. We give an update on our long-term high-resolution spectroscopic and photometric monitoring programme of spatially resolved binary brown dwarfs using ground-based adaptive optics and the Hubble Space Telescope. We present current orbital fits, including refined dynamical mass estimate of the Kelu-1 AB system. The results seem to support the previously reported trend that evolutionary and atmospheric models might underestimate the mass of very-low-mass stars and brown dwarfs.

58 RADIO SOURCES NEAR BRIGHT NATURAL GUIDE STARS,

We present a preliminary survey of 58 radio sources within the isoplanatic patches (r < 25'') of bright (11 < R < 12) stars suitable for use as natural guide stars with high-order adaptive optics (AO). An optical and near-infrared imaging survey was conducted utilizing tip-tilt corrections in the optical and AO in the near-infrared. Spectral energy distributions were fit to the multi-band data for the purpose of obtaining photometric redshifts using the Hyperz code. Several of these photometric redshifts were confirmed with spectroscopy, a result that gives more confidence to the redshift distribution for the whole sample. Additional long-wavelength data from Spitzer, SCUBA, SHARC2, and VLA supplement the optical and near-infrared data. We find the sample generally follows and extends the magnitude–redshift relation found for more powerful local radio galaxies. The survey has identified several reasonably bright (H = 19 − 20) objects at significant redshifts (z > 1) that are now within the capabilities of the current generation of AO-fed integral-field spectrographs. These objects constitute a unique sample that can be used for detailed ground-based AO studies of galactic structure, evolution, and active galactic nucleus formation at high redshift.

DeepL'‐ andM‐band Imaging for Planets around Vega and ε Eridani

We have obtained deep adaptive optics (AO) images of Vega and Eri to search for planetary mass companions. We observed at the MMT in the L' (3.8 μm) and M (4.8 μm) bands using Clio, a recently commissioned imager optimized for these wavelengths. Observing at these long wavelengths represents a departure from the H band (1.65 μm) more commonly used for AO imaging searches for extrasolar planets. The long wavelengths offer better predicted planet/star flux ratios and cleaner (higher Strehl) AO images at the cost of lower diffraction-limited resolution and higher sky background. We have not detected any planets or planet candidates around Vega or Eri. We report the sensitivities obtained around both stars, which correspond to upper limits on any planetary companions which may exist. The sensitivities of our L'- and M-band observations are comparable to those of the best H-regime observations of these stars. For Eri, our M-band observations deliver considerably better sensitivity to close-in planets than any previously published results, and we show that the M band is by far the best wavelength choice for attempts at ground-based AO imaging of the known planet Eri b. The Clio camera itself, with MMTAO, may be capable of detecting Eri b at its 2010 apastron, given a multinight observing campaign. Clio appears to be the only currently existing AO imager that has a realistic possibility of detecting Eri b.

High resolution science with high redshift galaxies

We summarize the high-resolution science that has been done on high redshift galaxies with Adaptive Optics (AO) on the world’s largest ground-based facilities and with the Hubble Space Telescope (HST). These facilities complement each other. Ground-based AO provides better light gathering power and in principle better resolution than HST, giving it the edge in high spatial resolution imaging and high resolution spectroscopy. HST produces higher quality, more stable PSF’s over larger field-of-views in a much darker sky-background than ground-based AO, and yields deeper wide-field images and low-resolution spectra than the ground. Faint galaxies have steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting the hierarchical formation of galaxies over cosmic time. HST has imaged this process in great structural detail to z ≲ 6, and ground-based AO and spectroscopy has provided measurements of their masses and other physical properties with cosmic time. Last, we review how the 6.5 m James Webb Space Telescope (JWST) will measure First Light, reionization, and galaxy assembly in the near–mid-IR after 2013.

Very high contrast integral field spectroscopy of AB Doradus C: 9-mag contrast at 0.2 arcsec without a coronagraph using spectral deconvolution†

We present an extension of the spectral deconvolution (SD) method to achieve very high contrast at small inner working radii. We apply the method to the specific case of ground-based adaptive optics fed integral field spectroscopy (without a coronagraph). Utilizing the wavelength dependence of the Airy and speckle patterns, we make an accurate estimate of the point spread function that can be scaled and subtracted from the data cube. The residual noise in the resulting spectra is very close to the photon noise from the starlight halo. We utilize the technique to extract a very high signal-to-noise ratio H- and K-band spectrum of AB Doradus (AB Dor) C, the low-mass companion to AB Dor A. By effectively eliminating all contamination from AB Dor A, the extracted spectrum retains both continuum and spectral features. The achieved 1σ contrast is 9 mag at 0.2 arcsec, 11 mag at 0.5 arcsec, in 20 min exposure time, at an effective spectral bandwidth of 5.5 nm, proving that the method is applicable even in low-Strehl regimes. The SD method clearly demonstrates the efficacy of image slicer based integral field units in achieving very high contrast imaging spectroscopy at small angular separations, validating their use as high-contrast spectrographs/imagers for extreme adaptive optics systems.

Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions

Accurate astrometry and photometry of saturated and coronagraphic point spread functions (PSFs) are fundamental to both ground- and space-based high contrast imaging projects. For ground-based adaptive optics imaging, differential atmospheric refraction and flexure introduce a small drift of the PSF with time, and seeing and sky transmission variations modify the PSF flux distribution. For space-based imaging, vibrations, thermal fluctuations and pointing jitters can modify the PSF core position and flux. These effects need to be corrected to properly combine the images and obtain optimal signal-to-noise ratios, accurate relative astrometry and photometry of detected objects as well as precise detection limits. Usually, one can easily correct for these effects by using the PSF core, but this is impossible when high dynamic range observing techniques are used, like coronagrahy with a non-transmissive occulting mask, or if the stellar PSF core is saturated. We present a new technique that can solve these issues by using off-axis satellite PSFs produced by a periodic amplitude or phase mask conjugated to a pupil plane. It will be shown that these satellite PSFs track precisely the PSF position, its Strehl ratio and its intensity and can thus be used to register and to flux normalize the PSF. A laboratory experiment is also presented to validate the theory. This approach can be easily implemented in existing adaptive optics instruments and should be considered for future extreme adaptive optics coronagraph instruments and in high-contrast imaging space observatories.

The Star Formation Histories of the Bulge and Disk of M31 from Resolved Stars in the Near-Infrared

We discuss H and K observations of three fields in the bulge and disk of M31 obtained with the ALTAIR adaptive optics system and NIRI instrument on Gemini North. These are the highest resolution and deepest near-infrared observations obtained to date of the inner regions of M31 and demonstrate the promise of ground-based adaptive optics for studying the crowded regions of nearby galaxies. We have combined our observations with previously published Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer observations of nine M31 fields and have derived the coarse star formation histories of M31's bulge and inner disk. From fits to the MK luminosity functions, we find the stellar population mix to be dominated by old, nearly solar-metallicity stars. The old populations, which we define as having age 6 Gyr, indeed dominate the star formation histories at all radii independent of the relative contributions of bulge and disk stars. Although all of our fields contain some bulge contribution, our results suggest that there is no age difference between the bulge and disk to the limit of our precision.

An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near‐Infrared Color‐Magnitude Diagrams

We present an empirical method to derive photometric metallicity, reddening, and distance to old stellar populations by using a few major features of the red giant branch (RGB) in near-IR color-magnitude diagrams. We combine the observed RGB features with a set of equations linking the global metallicity [M/H] to suitable RGB parameters (colors, magnitudes, and slope), as calibrated from a homogeneous sample of Galactic globular clusters with different metallicities. This technique can be applied to efficiently derive the main population parameters of old stellar systems, with the goal of using ground-based adaptive optics and space facilities to probe the stellar content of remote galaxies.

Discovery of a Light Echo from SN 2003gd

Archival HST/ACS data reveal details of a light echo around SN 2003gd in the galaxy M74, only the fifth supernova around which resolved echoes have been reported. An echo is detected 03 from the supernova between position angles of 250° and 360°, with a fainter signal present at a few other position angles. This material lies ~180 pc in front of the supernova, with a thickness of 60-120 pc, and may delineate the disk of M74. This structure has a gas density of 1-2 cm-3, typical of the interstellar medium; however, the dust grains are smaller than those found in our Galaxy, with maximum grain sizes around 0.25 μm. Since only one epoch of data exists, in two wave bands and with a low signal-to-noise ratio, deeper annual visits should be made with HST and ground-based adaptive optics telescopes.

Limits of Adaptive Optics for High‐Contrast Imaging

The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10-4 to 10-5 in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10-6 and 10-7 can be achieved within 1'' on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.

Low‐Order Aberrations in Band‐limited Lyot Coronagraphs

We study the way Lyot coronagraphs with unapodized entrance pupils respond to small, low-order phase aberrations. This study is applicable to ground-based adaptive optics coronagraphs operating at 90% and higher Strehl ratios, as well as to some space-based coronagraphs with intrinsically higher Strehl ratio imaging. We utilize a second-order expansion of the monochromatic point-spread function (written as a power spectrum of a power series in the phase aberration over clear aperture) to derive analytical expressions for the response of a Lyot coronagraph (BLC) to small, low-order, phase aberrations. The BLC possesses a focal plane mask with an occulting spot whose opacity profile is a spatially band-limited function rather than a hard-edged, opaque disk. The BLC is, to first order, insensitive to tilt and astigmatism. Undersizing the stop in the reimaged pupil plane (the Lyot plane) following the focal plane mask can alleviate second-order effects of astigmatism, at the expense of system throughput and angular resolution. The optimal degree of such undersizing depends on individual instrument designs and goals. Our analytical work engenders physical insight and complements existing numerical work on this subject. Our methods can be extended to treat the passage of higher order aberrations through band-limited Lyot coronagraphs by using our polynomial decomposition or an analogous Fourier approach.

The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared

High spatial resolution observations in the 1 to 3.5 micron region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the H, Kp (2.12/0.4 micron) and L bands, NICMOS data in filters between 1.1 and 2.2 micron, and integral field spectroscopic data from BEAR, an Imaging FTS, in the HeI 2.06 micron and the Br$\gamma$ line regions. Analysis of all these data provides a completely new picture of the main component, IRS 13E, which appears as a cluster of seven individual stars within a projected diameter of ~0.5'' (0.02 pc). The brightest sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are all massive stars, 13E1 a blue object, with no detected emission line while 13E2 and 13E4 are high-mass emission line stars. 13E2 is at the WR stage and 13E4 a massive O-type star. 13E3A and B are extremely red objects, proposed as other examples of dusty WR stars. All these sources have a common westward proper motion. 13E5, is a red source similar to 13E3A/B. This concentration of comoving massive hot stars, IRS 13E, is proposed as the remaining core of a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) of ~1300 M_sol. This detection plays in favor of a scenario in which the helium stars and the other hot stars in the central pc originate from the stripping of a massive cluster formed several tens of pc from the center. The detection of a discrete X-ray emission (Baganoff et al. 2003) at the IRS~13 position is examined in this context.

Exoplanet Recognition Using a Wavelet Analysis Technique

The recognition of a planet orbiting around a star (and in general of a faint companion in a binary system) in direct imaging observations is limited at distances smaller than around 1'' by the speckle and photon noise. There are also other sources of noises as artifacts produced by the adaptive optics (AO) systems that make the identification of a planet even more difficult. Techniques to recognize a planet in an automatic (and objective) way are therefore quite useful. In this Letter, we test the ability of a wavelet analysis technique for recognizing exoplanets in deep images obtained with ground-based telescopes and AO facilities. We present the automatic procedure developed for the detection of exoplanets, and we validate it on a deep image of a young, nearby star, which has been one of the targets of a planet search survey done with NACO/VLT. Preliminary tests show that with the proposed algorithm, it is indeed possible to recognize, in an automatic way, a planet of 12MJ at a distance of 05, a planet of 5MJ at a distance of 07, and a planet of 3MJ at a distance of 1'' from the star. We also show that a star/planet systems with a maximum ΔM = 4.7 mag contrast in the K band at 02 can be recognized automatically. This is a typical contrast for a 5MJ-10MJ planet and a late-type star (late M spectral type).

Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip

We present new empirical calibrations of the red giant branch (RGB) bump and tip based on a homogeneous near-infrared data base of 24 Galactic globular clusters. The luminosities of the RGB bump and tip in the J, H and K bands and their dependence on the cluster metallicity have been studied, yielding empirical relationships. By using recent transformations between the observational and theoretical planes, we also derived similar calibrations in terms of bolometric luminosity. Direct comparisons between updated theoretical models and observations show an excellent agreement. The empirical calibration of the RGB tip luminosity in the near-infrared passbands presented here is a fundamental tool to derive distances to galaxies far beyond the Local Group, in view of using the new ground-based adaptive optics facilities and, in the near future, the James Webb Space Telescope.

Magnetocentrifugal Launching of Jets from Accretion Disks. II. Inner Disk‐driven Winds

We numerically follow the time evolution of axisymmetric outflows magnetocentrifugally driven from the inner portion of accretion disks from their launching surface to large observable distances. Special attention is paid to the collimation of part of the outflow into a dense, narrow jet around the rotation axis after a steady state has been reached. For parameters typical of T Tauri stars, we define a fiducial jet as that outlined by the contour of constant density at 104 cm-3. We find that the jet, so defined, appears nearly cylindrical well above the disk, in agreement with previous asymptotic analyses. Closer to the equatorial plane, the density contour can either bulge outward or pinch inward, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jetlike feature is always formed around the axis, there is no guarantee that the high-density axial jet would dominate the more tenuous, wide-angle part of the wind. Specifically, on the 100 AU scale, resolvable by the Hubble Space Telescope and ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind mass flux enclosed by the fiducial jet can vary substantially, again depending on the launching conditions. We show two examples in which the fraction is ~20% and ~45%. These dependences may provide a way to constrain the conditions at the launching surface, which are poorly known at present.

Adaptive optics observations of asteroid (216) Kleopatra

The large main-belt asteroid (216) Kleopatra has been for long suspected to be a binary object, mainly due to its large lightcurve amplitude. However, recent observations suggest that it is a single bone-shaped or bi-lobated body (Ostro et al. 2000; Tanga et al. 2001). We present results obtained from ground-based adaptive optics observations, and in agreement with the radar raw-observations, the images show two prominent lobes. Making use of the MISTRAL deconvolution technique, the restored images yield a well-separated binary object. Nevertheless, the spatial resolution of the 3.6m ESO telescope is limited and a dumbbell-shaped body could yield similar features. Further simulations show that adaptive optics observations with an 8-meter class telescope analyzed with the powerful MISTRAL deconvolution technique could overcome this limitation.

Hubble Space Telescope/WFPC2 Images of the GG Tauri Circumbinary Disk

We present the first visible wavelength images of the GG Tau circumbinary ring obtained with the Hubble Space Telescope's (HST) Wide Field and Planetary Camera 2. Scattered light from the ring is detected in both V- and I-band images. The images show that the ring is smooth, except for a small gap that could be a shadow caused by material between the stars and the ring. The spokes seen extending from the stars to the ring in ground-based adaptive optics images are not seen in our data, which suggests that they may be image artifacts. The nearside/farside surface brightness ratio is 6.9 in the I band, consistent with forward scattering by small dust grains. The azimuth of the peak ring surface brightness appears offset by 13° from the azimuth closest to us, as seen in previous near-IR HST observations. This may indicate that the ring is warped or somehow shadowed by the circumstellar disks. The color of the ring is redder than the combined light from the stars as observed by HST, confirming previous measurements that indicate that circumstellar disks may introduce extinction of light illuminating the ring. We detect a bright, compact arc of material 03 from the secondary star at an azimuth opposite the primary. It appears to be too large to be a circumstellar disk and is not at the expected location for dust trapped at a Lagrange point.

Accretion Disks around Young Stars: An Observational Perspective

Accretion disks are pivotal elements in the formation and early evolution of solar-like stars. On top of supplying the raw material, their internal conditions also regulate the formation of planets. Their study therefore holds the key to solve this long standing mystery: how did our Solar System form? This chapter focuses on observational studies of the circumstellar environment, and in particular of circumstellar disks, associated with pre-main sequence solar-like stars. The direct measurement of disk parameters poses an obvious challenge: at the distance of the typical star forming regions ( e.g. ~140 pc for Taurus), a planetary system like ours (with diameter ≅50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35'' . Yet its surface brightness is low in comparison to the bright central star and high angular and high contrast imaging techniques are required if one hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or better and high contrast have been available for just about 10 years now. They are covering a large part of the electromagnetic spectrum, from the UV/Optical with HST and the near-infrared from ground-based adaptive optics systems, to the millimetric range with long-baseline radio interferometers. It is therefore not surprising that our knowledge of the structure of the disks surrounding low-mass stars has made a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historical perpective, the road that led to the idea that most solar-like stars are surrounded by an accretion disk at one point in their early life and how, nowadays, their structural and physical parameters can be estimated from direct observations. We will follow by a short discussion of a few of the constraints available regarding the evolution and dissipation of these disks. This last topic is particularly relevant today to understand the mechanism leading to the formation of planets.

Optical and Near-Infrared Imaging of Young Binary Star Environments

We review recent imaging results on the circumstellar matter of young binaries obtained using the Hubble Space Telescope (HST) and by PUEO, a ground-based adaptive optics (AO) system operating at CFHT. In the area of circumbinary disks, new results for GG Tau help form a more complete picture of this prototype system. However, HST images of the UY Aur ring indicate a more complex system than first thought. A new example of a circumstellar disk in a multiple star system has been found in HV Tau, joining HK Tau in this category. Examples of envelopes, outflow cavities, and jets are shown for several young binaries. About half of the nearby young binaries in our survey possess some kind of local nebulosity at optical or near-IR wavelengths.

Timescales of Disk Evolution and Planet Formation: [ITAL]HST[/ITAL], Adaptive Optics, and [ITAL]ISO[/ITAL] Observations of Weak-Line and Post–T Tauri Stars

We present high spatial resolution HST and ground-based adaptive optics observations and high-sensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post? (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ?30 AU from the primary stars. No such objects were found within 300 AU of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTSs and PTTSs in ScoCen reveal infrared excesses which are clearly above photospheric levels and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTSs in ScoCen compared with the younger classical and weak-line TTSs in Cha can be attributed to a deficiency of smaller size (0.1 to 1 ?m) dust grains relative to larger size (?5 ?m) dust grains in the disks of the PTTSs. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to become dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.