Well-corrected Subaperture Research Articles

A DIM CANDIDATE COMPANION TO ϵ CEPHEI

Using a vector vortex coronagraph behind the 1.5-m well-corrected subaperture (WCS) at Palomar, we detected a second object very close to \epsilon Cephei, a \delta Scuti F0 IV star. The candidate companion, ~50 times fainter than \epsilon Cephei, if physically associated, is a late-type K or early M star, and lies at an angular separation of 330 mas, or 1.1 \lambda/D for the WCS, making it the smallest angle detection ever realized with a coronagraph in terms of \lambda/D units. The projected separation of the putative companion is ~8.6 AU, most likely on a highly eccentric orbit. The recently detected near-infrared excess is thus likely not due to hot dust. Moreover, we also show that the previously reported IRAS 60 \mu m excess was due to source confusion on the galactic plane.

Phase mask coronagraphy at JPL and Palomar

For the imaging of faint companions, phase mask coronagraphy has the dual advantages of a small inner working angle and high throughput. This paper summarizes our recent work in developing phase masks and in demonstrating their capabilities at JPL. Four-quadrant phase masks have been manufactured at JPL by means of both evaporation and etching, and we have been developing liquid crystal vortex phase masks in partnership with a commercial vendor. Both types of mask have been used with our extreme adaptive optics well-corrected subaperture at Palomar to detect known brown dwarf companions as close as ~ 2.5 λ/D to stars. Moreover, our recent vortex masks perform very well in laboratory tests, with a demonstrated infrared contrast of about 10^(−6) at 3 λ/D, and contrasts of a few 10^(−7) with an initial optical wavelength device. The demonstrated performance already meets the needs of ground-based extreme adaptive optics coronagraphy, and further planned improvements are aimed at reaching the 10^(−10) contrast needed for terrestrial exoplanet detection with a space-based coronagraph.

Imaging faint companions very close to stars

AbstractA vortex coronagraph on our extreme adaptive optics “well-corrected subaperture” on the Hale telescope has recently allowed the imaging of the triple-planet HR8799 system with a 1.5 m subaperture. Moreover, a faint, low-mass companion to a second star was imaged only one diffraction beam width away from the primary. These results illustrate the potential of the vortex coronagraph, which can enable exoplanet imaging and characterization with smaller telescopes than previously thought.

IMAGING THE DEBRIS DISK OF HD 32297 WITH A PHASE-MASK CORONAGRAPH AT HIGH STREHL RATIO

We present the first Ks-band image of the scattered light from HD 32297's debris disk. HD 32297 is an A5 star known to possess a nearly edge-on disk with a high fractional luminosity Ld/L⋆ ≈ 0.003 in the infrared. Our image was obtained using phase-mask coronagraphy on the 1.6 m well-corrected subaperture on the 5 m Palomar Hale telescope, and reaches an inner working angle of 400 mas. We confirm the previously reported disk asymmetry and central hole. The gray J − Ks disk color appears similar on the two sides of the disk, which would demand a minimum grain size of at least a few microns, larger than thermal arguments suggest. One possible explanation is a planetesimal clump in a resonant orbit with an outward migrating giant planet, located at larger radii than the warm dust emitting at longer wavelengths. Our observations also clearly demonstrate the benefits of operating in the extreme adaptive optics regime with a coronagraph able to reach a very small inner working angle.

IMAGING FAINT BROWN DWARF COMPANIONS CLOSE TO BRIGHT STARS WITH A SMALL, WELL-CORRECTED TELESCOPE APERTURE

We have used our 1.6 m diameter off-axis well-corrected sub-aperture (WCS) on the Palomar Hale telescope in concert with a small inner-working-angle (IWA) phase-mask coronagraph to image the immediate environs of a small number of nearby stars. Test cases included three stars (HD 130948, HD 49197 and HR7672) with known brown dwarf companions at small separations, all of which were detected. We also present the initial detection of a new object close to the nearby young G0V star HD171488. Follow up observations are needed to determine if this object is a bona fide companion, but its flux is consistent with the flux of a young brown dwarf or low mass M star at the same distance as the primary. Interestingly, at small angles our WCS coronagraph demonstrates a limiting detectable contrast comparable to that of extant Lyot coronagraphs on much larger telescopes corrected with current-generation AO systems. This suggests that small apertures corrected to extreme adaptive optics (ExAO) levels can be used to carry out initial surveys for close brown dwarf and stellar companions, leaving followup observations for larger telescopes.

Extreme Adaptive Optics Imaging with a Clear and Well‐Corrected Off‐Axis Telescope Subaperture

Rather than using an adaptive optics (AO) system to correct a telescope's entire pupil, it can instead be used to more finely correct a smaller subaperture. Indeed, existing AO systems can be used to correct a subaperture 1/3 to 1/2 the size of a 5-10 m telescope to extreme adaptive optics (ExAO) levels. We discuss the potential performance of a clear off-axis well-corrected subaperture (WCS), and describe our initial imaging results with a 1.5 m diameter WCS on the Palomar Observatory's Hale Telescope. These include measured Strehl ratios of 0.92-0.94 in the infrared (2.17 μm) and ≈0.12 in the B band, the latter allowing a binary of separation 0.34 to be easily resolved in the blue. Such performance levels enable a variety of novel observational modes, such as infrared ExAO, visible-wavelength AO, and high-contrast coronagraphy. One specific application suggested by the high Strehl ratio stability obtained (1%) is the measurement of planetary transits and eclipses. Also described is a simple dark hole experiment carried out on a binary star, in which a comatic phase term was applied directly to the deformable mirror, in order to shift the diffraction rings to one side of the point-spread function.