Sasha Hinkley

THE SOLAR-SYSTEM-SCALE DISK AROUND AB AURIGAE

The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust-dominated structures. The inner disk, which has not been studied previously at sufficient resolution and imaging dynamic range, seems to contain very little gas inside a radius of least 130 AU from the star. Using adaptive optics coronagraphy and polarimetry, we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of ~100 AU. This structure seems consistent with crude models of mean motion resonances or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the objects mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.

High-Contrast Observations in Optical and Infrared Astronomy

High-contrast observations in optical and infrared astronomy are defined as any observation requiring a technique to reveal a celestial object of interest that is in such close angular proximity to another source brighter by a factor of at least 10 5 that optical effects hinder or prevent the collection of photons directly from the target of observation. This is a relatively new type of observation that enables research on previously obscured parts of the Universe. In particular, it is most applicable to comparative planetary science, a field that directly attacks such questions as “how common are planetary systems? What types of planets exist, and are there planets other than Earth that are capable of supporting life as we know it?” We survey the scientific motivations for high-contrast observations, provide an overview of the techniques currently being used or developed, and discuss some ideas and studies for future prospects.

The challenges of coronagraphic astrometry

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 (0.05λ/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.

CATACLYSMIC AND CLOSE BINARIES IN STAR CLUSTERS. V. ERUPTING DWARF NOVAE, FAINT BLUE STARS, X-RAY SOURCES, AND THE CLASSICAL NOVA IN THE CORE OF M80

Large populations of cataclysmic variables (CVs) in globular clusters have long been predicted, but the number of absolutely certain cluster CVs known in globular clusters is still less than 10. HST and Chandra observers have recently found dozens of CV candidates in several populous globular clusters. Confirmation and characterization of these candidates are extremely difficult; thus, identification of unambiguous CVs remains important. We have searched all archival HST images of the dense globular cluster M80 for erupting dwarf novae (DNe) and to check the outburst behaviors of two very blue objects first identified a decade ago. Two new erupting DNe were found in eight searched epochs, making M80 a record holder for erupting DNe. The quiescent classical nova in M80 varies by no more than a few tenths of a magnitude on timescales of minutes to years, and a similar faint, blue object varies by a similar amount. Simulations and completeness tests indicate that there are at most three erupting DNe like SS Cyg and at most nine U Gem-like DNe in M80. Either this very dense cluster contains about an order of magnitude fewer CVs than theory predicts, or most M80 CVs are extremely faint and/or erupt very infrequently like WZ Sge. We have detected a sequence of 54 objects running parallel to the main sequence and several tenths of a magnitude blueward of it. These blue objects are significantly more centrally concentrated than the main-sequence stars, but not as centrally concentrated as the blue stragglers. We suggest that these objects are white dwarf-red dwarf binaries and that some are the faint CV population of M80.

THE LUMINOUS ERUPTING DWARF NOVA CV 1 IN THE DENSE GLOBULAR CLUSTER M15

Despite decades-old predictions of the expected presence of dozens of cataclysmic variables in the cores of globular clusters, the number of irrefutable, outbursting candidates is still barely a handful. Using multiwavelength, multiepoch Hubble Space Telescope images, we have produced outburst and quiescence light curves for the recently discovered large-amplitude variable CV 1 in the core of the post–core-collapse globular cluster M15. The light curves and blue colors show that the object is a bona fide dwarf nova, with absolute magnitude at maximum light rivaling that of the most luminous known dwarf novae.

ERUPTING DWARF NOVAE IN THE LARGE MAGELLANIC CLOUD

We report the first likely detections of erupting dwarf novae (DNe) in an external galaxy: the Large Magellanic Cloud. Six candidates were isolated from approximately a million stars observed every second night over 11 nights with the Cerro Tololo Inter-American Observatory 8K × 8K Mosaic2 CCD imager. Artificial dwarf nova and completeness tests suggest that we are seeing only the brightest of the LMC DNe, probably SS Cygni–like cataclysmic variables (CVs), but possibly SU UMa type cataclysmics undergoing superoutbursts. We derive crude but useful limits on the LMC DN surface density and on the number of DNe in the LMC. Many thousands of CVs in the Magellanic Clouds can be discovered and characterized with 8 m class telescopes.