Anand Sivaramakrishnan

A New High Contrast Imaging Program at Palomar Observatory

We describe a new instrument that forms the core of a long-term high contrast imaging program at nthe 200 inch (5 m) Hale Telescope at Palomar Observatory. The primary scientific thrust is to obtain images and low-resolution nspectroscopy of brown dwarfs and young exoplanets of several Jupiter masses in the vicinity of stars nwithin 50 pc of the Sun. The instrument is a microlens-based integral field spectrograph integrated with a diffraction-limited, napodized-pupil Lyot coronagraph. The entire combination is mounted behind the Palomar adaptive optics n(AO) system. The spectrograph obtains imaging in 23 channels across the J and H bands (1.06–1.78 μm). The nimage plane of our spectrograph is subdivided by a 200 × 200 element microlens array with a plate scale of 19.2 mas nper microlens, critically sampling the diffraction-limited point-spread function at 1.06 μm. In addition to obtaining nspectra, this wavelength resolution allows suppression of the chromatically dependent speckle noise, which we ndescribe. In addition, we have recently installed a novel internal wave front calibration system that will provide ncontinuous updates to the AO system every 0.5–1.0 minutes by sensing the wave front within the coronagraph. The nPalomar AO system is undergoing an upgrade to a much higher order AO system (PALM-3000): a 3388-actuator ntweeter deformable mirror working together with the existing 241-actuator mirror. This system, the highest-resolution nAO corrector of its kind, will allow correction with subapertures as small as 8.1 cm at the telescope pupil using nnatural guide stars. The coronagraph alone has achieved an initial dynamic range in the H band of 2 × 10^(-4) at 1″, nwithout speckle noise suppression. We demonstrate that spectral speckle suppression provides a factor of 10–20 nimprovement over this, bringing our current contrast at 1″ to ~2 × 10^(-5). This system is the first of a new generation nof apodized-pupil coronagraphs combined with high-order adaptive optics and integral field spectrographs (e.g., nGPI, SPHERE, HiCIAO), and we anticipate that this instrument will make a lasting contribution to high-contrast nimaging in the Northern Hemisphere for years.

Temporal Evolution of Coronagraphic Dynamic Range and Constraints on Companions to Vega

The major obstacle to the direct detection of companions to nearby stars is the overwhelming brightness of the host star. Current instruments employing the combination of adaptive optics (AO) and coronagraphy can typically detect objects within 2 of the star that are ~104-105 times fainter. Correlated speckle noise is one of the biggest obstacles limiting such high-contrast imaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphically occulted H-band images of the star Vega at the 3.63 m AEOS telescope located on Haleakala, Hawaii. This data set is unique for studying the temporal behavior of speckle noise and represents the first time such a study on highly corrected coronagraphic AO images has been carried out in a quantitative way. We find the speckle pattern to be highly stable in both position and time in our data. This is due to the fact that the AO system corrects disturbances to the stellar wave front at the level where the instrumental wave front errors dominate. Because of this, we find that our detection limit is not significantly improved simply with increased exposure time alone. However, we are able to improve our dynamic range by 1.5-2 mag through subtraction of static/quasi-static speckles in two rotating frames: the telescope pupil frame and the deformable mirror frame. The highly stable nature of speckles will exist for any program using coronagraphy and high-order AO. Furthermore, from our data, we are able to constrain the mass of any purported companion to Vega to be less than ~45MJ at 8 AU and less than ~30MJ at 16 AU, radii not previously probed at these sensitivities.

SPECKLE SUPPRESSION THROUGH DUAL IMAGING POLARIMETRY, AND A GROUND-BASED IMAGE OF THE HR 4796A CIRCUMSTELLAR DISK

We demonstrate the versatility of a dual imaging polarimeter working in tandem with a Lyot coronagraph and adaptive optics to suppress the highly static speckle noise pattern—the greatest hindrance to ground-based direct imaging of planets and disks around nearby stars. Using a double difference technique with the polarimetric data, we quantify the level of speckle suppression, and hence improved sensitivity, by placing an ensemble of artificial faint companions into real data, with given total brightness and polarization. For highly polarized sources within 0. 5, we show that we achieve 3 to 4 mag greater sensitivity through polarimetric speckle suppression than simply using a coronagraph coupled to a high-order adaptive optics system. Using such a polarimeter with a classical Lyot coronagraph at the 3.63 m Advanced Electro-Optical System telescope, we have obtained a 6.5! detection in the H band of the 76 AU diameter circumstellar debris disk around the star HR 4796A. Our data represent the first definitive ground-based near-IR polarimetric image of the HR 4796A debris disk and clearly show the two outer ansae of the disk, evident in Hubble Space Telescope NICMOS/STIS imaging. Comparing our peak linearly polarized flux with the total intensity in the lobes as observed by NICMOS, we derive a lower limit to the fractional linear polarization of >29% caused by dust grains in the disk. In addition, we fit simple morphological models of optically thin disks to our data allowing us to constrain the dust disk scale height (2.5 +5.0 # 1.3 AU) and scattering asymmetry parameter (g =

APODIZED PUPIL LYOT CORONAGRAPHS FOR ARBITRARY APERTURES. II. THEORETICAL PROPERTIES AND APPLICATION TO EXTREMELY LARGE TELESCOPES

cos %= 0.20 +.07 # .10 ). These values are consistent with several lines of evidence suggesting that the HR 4796A disk is dominated by a micron-sized dust population, and are indeed typical of disks in transition between those surrounding the Herbig Ae stars to those associated with Vega-like stars.

PARALLACTIC MOTION FOR COMPANION DISCOVERY: AN M-DWARF ORBITING ALCOR

We study the application of Lyot coronagraphy to future Extremely Large Telescopes (ELTs), showing that Apodized Pupil Lyot Coronagraphs enable high-contrast imaging for exoplanet detection and characterization with ELTs. We discuss the properties of the optimal pupil apodizers for this application (generalized prolate spheroidal functions). The case of a circular aperture telescope with a central obstruction is considered in detail, and we discuss the effects of primary mirror segmentation and secondary mirror support structures as a function of the occulting mask size. In most cases where inner working distance is critical, e.g., for exoplanet detection, these additional features do not alter the solutions derived with just the central obstruction, although certain applications such as quasar-host galaxy coronagraphic observations could benefit from designs that explicitly accomodate ELT spider geometries. We illustrate coronagraphic designs for several ELT geometries including ESO/OWL, the Thirty Mirror Telescope, the Giant Magellan Telescope, and describe numerical methods for generating these designs.

DISCOVERY AND CHARACTERIZATION OF A FAINT STELLAR COMPANION TO THE A3V STAR ζ VIRGINIS

The A5V star Alcor has an M3-M4 dwarf companion, as evidenced by a novel astrometric technique. Imaging spectroscopy combined with adaptive optics coronagraphy allowed for the detection and spectrophotometric characterization of the point source at a contrast of ~6 J- and H-band magnitudes and separation of 1 from the primary star. The use of an astrometric pupil plane grid allowed us to determine the projected separations between the companion and the coronagraphically occulted primary star to ≤3 mas precision at two observation epochs. Our measurements demonstrate common parallactic and proper motion over the course of 103 days, significantly shorter than the period of time needed for most companion confirmations through proper motion measurements alone. This common parallax method is potentially more rigorous than common proper motion, ensuring that the neighboring bodies lie at the same distance, rather than relying on the statistical improbability that two objects in close proximity to each other on the sky move in the same direction. The discovery of a low-mass (~0.25 M_☉) companion around a bright (V = 4.0 mag), nearby (d= 25 pc) star highlights a region of binary star parameter space that to date has not been fully probed.

Sensing Phase Aberrations behind Lyot Coronagraphs

Through the combination of high-order adaptive optics and coronagraphy, we report the discovery of a faint stellar companion to the A3V star ζ Virginis. This companion is ~7 mag fainter than its host star in the H band, and infrared imaging spanning 4.75 years over five epochs indicates this companion has common proper motion with its host star. Using evolutionary models, we estimate its mass to be 0.168^(+0.012) _(–0.016) M_☉, giving a mass ratio for this system q = 0.082^(+0.007)_(–0.008). Assuming the two objects are coeval, this mass suggests an M4V-M7V spectral type for the companion, which is confirmed through integral field spectroscopic measurements. We see clear evidence for orbital motion from this companion and are able to constrain the semimajor axis to be ≳24.9 AU, the period ≳124 yr, and eccentricity ≳0.16. Multiplicity studies of higher mass stars are relatively rare, and binary companions such as this one at the extreme low end of the mass ratio distribution are useful additions to surveys incomplete at such a low mass ratio. Moreover, the frequency of binary companions can help to discriminate between binary formation scenarios that predict an abundance of low-mass companions forming from the early fragmentation of a massive circumstellar disk. A system such as this may provide insight into the anomalous X-ray emission from A stars, hypothesized to be from unseen late-type stellar companions. Indeed, we calculate that the presence of this M-dwarf companion easily accounts for the X-ray emission from this star detected by ROSAT.

Gemini Planet Imager Coronagraph Testbed Results

Direct detection of young extrasolar planets orbiting nearby stars can be accomplished from the ground with extreme adaptive optics and coronagraphy in the near-infrared, as long as this combination can provide an image with a dynamic range of 107 after the data are processed. Slowly varying speckles due to residual phase aberrations that are not measured by the primary wave-front sensor are the primary obstacle to achieving such a dynamic range. In particular, non-common optical path aberrations occurring between the wave-front sensor and the coronagraphic occulting spot degrade performance the most. We analyze the passage of both low and high spatial frequency phase ripples, as well as low-order Zernike aberrations, through an apodized pupil Lyot coronagraph in order to demonstrate the way coronagraphic filtering affects various aberrations. We derive the coronagraphically induced cutoff frequency of the filtering and estimate coronagraphic contrast losses due to low-order Zernike aberrations: tilt, astigmatism, defocus, coma, and spherical aberration. Such slowly varying path errors can be measured behind a coronagraph and corrected by a slowly updated optical path delay precompensation or offset asserted on the wave front by the adaptive optics (AO) system. We suggest ways of measuring and correcting all but the lowest spatial frequency aberrations using Lyot plane wave-front data, in spite of the complex interaction between the coronagraph and those mid-spatial frequency aberrations that cause image plane speckles near the coronagraphic focal plane mask occulters edge. This investigation provides guidance for next-generation coronagraphic instruments currently under construction.

TESTING THE APODIZED PUPIL LYOT CORONAGRAPH ON THE LABORATORY FOR ADAPTIVE OPTICS EXTREME ADAPTIVE OPTICS TESTBED

The Gemini Planet Imager (GPI) is an extreme AO coronagraphic integral field unit YJHK spectrograph destined for first light on the 8m Gemini South telescope in 2011. GPI fields a 1500 channel AO system feeding an apodized pupil Lyot coronagraph, and a nIR non-common-path slow wavefront sensor. It targets detection and characterizion of relatively young (<2GYr), self luminous planets up to 10 million times as faint as their primary star. We present the coronagraph subsystems in-lab performance, and describe the studies required to specify and fabricate the coronagraph. Coronagraphic pupil apodization is implemented with metallic half-tone screens on glass, and the focal plane occulters are deep reactive ion etched holes in optically polished silicon mirrors. Our JH testbed achieves H-band contrast below a million at separations above 5 resolution elements, without using an AO system. We present an overview of the coronagraphic masks and our testbed coronagraphic data. We also demonstrate the performance of an astrometric and photometric grid that enables coronagraphic astrometry relative to the primary star in every exposure, a proven technique that has yielded on-sky precision of the order of a milliarsecond.

ACTIVE GALACTIC NUCLEUS AND QUASAR SCIENCE WITH APERTURE MASKING INTERFEROMETRY ON THE JAMES WEBB SPACE TELESCOPE

We present testbed results of the Apodized Pupil Lyot Coronagraph (APLC) at the Laboratory for Adaptive Optics (LAO). These results are part of the validation and tests of the coronagraph and of the Extreme Adaptive Optics (ExAO) for the Gemini Planet Imager (GPI). The apodizer component is manufactured with a halftone technique using black chrome microdots on glass. Testing this APLC (like any other coronagraph) requires extremely good wavefront correction, which is obtained to the 1 nm rms level using the microelectricalmechanical systems (MEMS) technology, on the ExAO visible testbed of the LAO at the University of Santa Cruz. We used an APLC coronagraph without central obstruction, both with a reference super-polished flat mirror and with the MEMS to obtain one of the first images of a dark zone in a coronagraphic image with classical adaptive optics using a MEMS deformable mirror (without involving dark hole algorithms). This was done as a complementary test to the GPI coronagraph testbed at American Museum of Natural History, which studied the coronagraph itself without wavefront correction. Because we needed a full aperture, the coronagraph design is very different from the GPI design. We also tested a coronagraph with central obstruction similar to thatmorexa0» of GPI. We investigated the performance of the APLC coronagraph and more particularly the effect of the apodizer profile accuracy on the contrast. Finally, we compared the resulting contrast to predictions made with a wavefront propagation model of the testbed to understand the effects of phase and amplitude errors on the final contrast.«xa0less