Research
My research focuses on high contrast imaging of circumstellar material: disks, envelopes, jets, and substellar companions. The observational tools that I employ to understand these objects include adaptive optics, coronagraphy, differential polarimetry, mid-infrared imaging, and integral field spectroscopy.High contrast imaging of circumstellar disks
Advances in diffraction-limited imaging let us see with increasingly great detail into the close environments of young stars. There are two key observational challenges are twofold: First, the imaging system must have fine enough angular resolution to resolve the circumstellar environment on scales of a few
AU. The second, harder challenge is achieving sufficiently high contrast: scattered light from circumstellar dust is faint, usually less than a percent of the total flux, sometimes far less. Even a small amount of residual starlight can completely swamp the dust-scattered light and prevent detection.
To overcome this, I have used a variety of observational techniques, including AO polarimetry, coronagraphy, mid-infrared and millimeter imaging. Analysis and modeling of data for several disks is currently underway.
Another way to get around the contrast challenge is to study edge-on disks, where the disk itself blocks the star light, so that the disk's detailed structure is directly revealed as a central dust lane flanked by faint disk-reflected light.
The variation of its appearance with wavelength provides crucial information on the
disk's internal structure and the
properties of its constituent dust grains. Numerical scattered light modeling, using sophisticated computer codes running on large parallel clusters, can be used to derive the disk structure and dust
properties, yielding an increased understanding of grain properties
during protoplanetary disk evolution.
Collaborators: Karl Stapelfeldt, Deborah Padgett, Caer McCabe, Gaspard Duchene, Christophe Pinte, Paul Kalas, Francois Menard, and others.
Hubble Space Telescope NICMOS Coronagraphic Polarimetry of Disks
Planets form in circumstellar disks. We believe that at young ages, dust grains in disks grow and aggregate into planetesimals, followed by the accretion of gaseous atmospheres onto giant planets. The growth of dust grains in disks around young stars, from sub-micron ISM grains to macroscopic particles to centimeter-sized rocks, is thus a key part of planet formation processes. Multiwavelength imaging and polarimetry lets us probe the earliest stages of this process, by determining the scattering properties of grains, a key diagnostic for grain sizes and compositions.I am currently conducting high-contrast, high spatial resolution, polarimetric imaging of circumstellar disks around a sample of Herbig Ae stars, using the Hubble Space Telescope's NICMOS instrument. The resulting images will be analyzed using sophisticated 3D Monte Carlo radiative transfer models, to determine what physical disk parameters and dust properties give rise to the observed appearance and polarization. These proposed observations will provide a basis for comparison with ongoing HST polarimetry of T Tauri and debris disks, aiding our understanding of how the processes of grain growth scale with stellar mass and age.
Collaborators: Gaspard Duchene, Glenn Schneider, Dean Hines, Carol Grady, John Wisniewski, Christophe Pinte, and others.
Integral field spectroscopy of outflows from young stars
Outflows from young stars play a major role in star formation, both by removing angular momentum from circumstellar disks (and thus enabling accretion to continue), and by driving turbulence in molecular clouds (and thereby regulating the efficiency of star formation). But the precise physical mechanism powering these jets remains unclear, with the X-wind and disk-wind models offering different theories of magnetocentrifugal jet acceleration. High angular resolution observations can place indirect constraints on jet models by measuring the physical conditions, mass flux, and collimation of jets.
Studying the acceleration and collimation of jets requires high resolution observations of the immediate circumstellar environments. The necessary observations combine high angular resolution (< 0.1 arcsec ) with moderate spectral resolution (R = 2000 - 4000). Such observations can now be obtained using integral field spectrographs on large telescopes, such as OSIRIS on Keck and GMOS & NIFS on Gemini. Our ongoing observations will investigate the structure and properties of jets across a range of young stellar objects, helping to ascertain how stellar properties such as mass and age influence the resulting outflows.
Collaborators: James Graham, Ben ZuckermanDevelopment of the integral field spectrograph/polarimeter for the Gemini Planet Imager
We are on the verge of being able to directly image extrasolar planets
around nearby stars. With high-order adaptive optics and advanced
coronagraphy, an AO system on a 8-m telescope can achieve sufficient
contrast to detect warm self-luminous Jovian planets in the solar
neighborhood. A collaboration of institutions, in California, Canada,
and New York, has begun to build such a system, the Gemini Planet Imager (GPI).
GPI should begin observations in late 2010 on the Gemini South telescope. In addition to planet detection, GPI will also feature a polarimetric imaging mode for studies of circumstellar dust disks. As part of the GPI collaboration, I am working on the design of the polarimetry mode optics, the observing scenarios and calibration plans, and various other aspects of the science instrument design.
Collaborators: James Larkin, James Graham, Bruce Macintosh, Mike Fitzgerald, Christian Marois, and the rest of the GPI Project Team.
Other Projects
The Lyot Project, led by Dr. Ben R. Oppenheimer of the American Museum of Natural History in New York, developed the first optimized diffraction-limited coronagraph for a high order AO system, the Air Force's AEOS telescope on Maui. I developed the infrared science camera, Kermit, which is used with the Lyot Project coronagraph.This page last updated by Marshall on 2007-10-10.