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My research focusses on the field of star formation and utilizes primarily submillimeter and millimeter radio telescopes to study various facets of the early beginnings of stars in our galaxy. My work falls under several general categories:
Below, I describe briefly each of these areas of study and why they are of interest and importance in understanding star formation. For more detail about particular research projects, please go to my Current Projects page. You might also be interested in my publication page, or in seeing some recent preprints.
- debris disks around young main sequence stars
- magnetic field geometries and molecular cloud support
- chemical properties and kinematics of low-mass cores
- dust grain properties and populations.
Debris Disks around Main Sequence Stars
Debris, or "second generation", disks were first detected in the early 1980s by the InfraRed Astronomical Satellite (IRAS) during calibration observations on nearby main sequence (hydrogen burning) stars. IRAS detected significantly more emission from the star Vega than expected from the stellar photosphere. It was quickly realized that the source of the excess emission was circumstellar material around the star which was absorbing the star's light and re-emitting it in the infrared. Observations of other stars, such as beta Pictoris, led to the measure of similar excesses. Coronographic imaging (which uses a mask to block out the star's light to enable the observation of weaker emission features around the star similar to how a solar eclipse by the moon enables us to see our Sun's corona) around beta Pictoris immediately confirmed that the circumstellar material was in the form of a disk around the star. While circumstellar disks are common around stars during their formation, the observation of disks around main sequence stars of considerable age was unexpected.
During star formation, solid material is built up from smaller components into aggregates of larger sizes enabling the formation of planetesimals and ultimately, in some cases, planets. After a star has finished forming (is no longer accreting gas through a circumstellar disk), the gas-rich disk will be depleted by accretion onto planets or removed from the system by stellar winds. However, over time, the planetesimals (small rocky bodies), comets and asteroids remaining in the system will undergo collisions as their orbits are affected by larger bodies, such as planets. The collisions reduce these objects to smaller and smaller sizes over time, reducing them to scales of tens of microns (hence the term "debris" disks). Since the collisions are ongoing, the debris disk is continuously replenished by the grinding down of larger bodies.
Debris disks are of interest because they are thought to be harbingers of systems undergoing planet formation, when strong perturbations would cause frequent collisions. Hence, their frequency around main sequence stars is a measure of how many systems are likely to form planets. The detection of debris disks has in the past required the use of optical or near-infrared coronographs, because the starlight greatly dominates the emission from the disk at these wavelengths. However, we can also detect these disks in emission at longer wavelengths. In the submillimetre and millimetre regimes, the star's emission is negligible and the disk emission dominates, making detection with a sensitive submillimetre camera straightforward. The SCUBA camera (1997-2005) on the James Clerk Maxwell Telescope was the first such camera to detect a significant number of debris disks around main sequence stars, in some cases showing very different morphology in the disks that had already been detected at shorter wavelengths.
The Herschel Space Observatory, the largest far-IR telescope yet built to date, launched on 14 May 2009. Among the 42 key programs being executed on the telescope, there are three which target debris disks. I am the PI of the "DEBRIS" survey which stands for "Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre". DEBRIS is a statistical survey of 446 nearby A, F, G, K and M-type stars (classification of declining mass) for evidence of debris disks at 100, 160, 250, 350 and 500 micron. The primary wavelength is 100 micron, a wavelengths with maximized contrast between the disc emission and that of ths star. Details of the DEBRIS project can be found at our website: debris.astrosci.ca. Another survey, SUNSS (SCUBA-2 Unbiased Nearby Stars Survey) will use the successor instrument to SCUBA, SCUBA-2, to seach for excess submillimetre emission around 500 nearby main sequence stars. We will observe the 100 nearest A, F, G, K and M stars to the confusion limit of the James Clerk Maxwell Telescope at 850 micron to search for debris disk emission. This is a multinational survey among the JCMT partner countries. I am the Canadian co-ordinator of this survey.
Magnetic Field Geometries
Magnetic fields have long been thought to play a significant role in the structural support of giant molecular and dark clouds, which are large (10000 - 10 million solar mass) gaseous clouds within which star formation occurs. The interiors of these clouds are very cold (-260 to -250 degrees Celsius) and can become so dense in some places that gravitational collapse can occur, producing new stars. An ongoing puzzle in star formation has been the inefficiency of star formation in the galaxy, since the internal motions of the gas in molecular clouds appears universally insufficient to prevent entire clouds from collapsing under gravity. The most effective mechanism for additional support within molecular clouds is the presence of magnetic fields, which are coupled to the gas through the presence of charged ions. Since collisions between ions and neutral gas molecules are frequent when densities are high, the magnetic fields can also indirectly provide support to the neutral gas as well. Assessing observationally the contribution of magnetic fields to the energetics of cloud support has proven a real challenge, since we can only observe their impact on matter, not the magnetic fields themselves. Theoretical models argue strongly that magnetic fields provide support, but the geometry of the fields and their potential role in the regulation of star formation is still an active field of study.
Inside molecular clouds, molecules condense onto dust grains, which are often elongated and charged. The latter condition leads them to be aligned with one another when magnetic fields are present; the former condition means that once aligned, these grains produce polarized light (light which demonstrates a preferential electric field direction) either through absorption of light of wavelength comparable to the dust grains' size or through emission at longer wavelengths. Polarized emission from dust grains therefore provides information about the orientation of the magnetic field local to the emitting dust grains and is our only effective probe of magnetic field orientation (in the plane-of-the-sky) in regions of active star formation. The density of these clouds means that they are opaque to starlight, and therefore background stars (a requirement for absorption polarimetry) cannot be seen.
My doctoral thesis dealt with magnetic field geometries at length. The primary tool I use for observing polarized emission from dust is the SCUBA polarimeter at the James Clerk Mawell Telescope, located on Mauna Kea in Hawaii. For bright compact cores, one can also use the BIMA interferometer in Hat Creek, CA which provides much superior resolution but no information about large scale structures.
Dynamics and Chemical Properties of Low-Mass Cores
The process of star formation involves an inevitable collapse of gas under gravity to form a protostar. This process occurs in several phases: first, a core of overdense gas forms from the larger scale molecular cloud. This core has a typical scale of approximately 10000 AU. This core may be stable against gravity for some period of time, supported by internal kinetic motions in the gas, magnetic fields, and/or turbulence. The process by which some cores remain starless while others eventually undergo collapse at their centres has been one of extensive theoretical and observational study. For those which begin collapse, there are models which theorize that the collapse begins at the centre, while others suggest the cores may be squeezed by external pressure, beginning their collapse at the outermost radii.
When a central object, a protostar, forms and begins to accrete mass from the surrounding core, it is accompanied by a circumstellar disk, and interestingly, a bipolar outflow oriented perpendicular to the disk. The outer part of the core is called the envelope, and material is funneled from the envelope to the protostar via the disk. Measurements of the motions of the gas in starless cores and young stellar objects (YSOs, i.e., envelope/disk/outflow systems) are clearly vital to understand how the process of star formation occurs.
Because each atomic and molecular transition is associated with a very specific frequency, it is possible to determine relative motions of gas to an observer (this process is called spectroscopy), allowing us to determine the gas dynamics within YSOs and starless cores. The main source of information about molecular gas is obtained from rotational transitions in the wavelength range from the far-infrared (100 micron) to the millimeter. For instance, my collaborators and I have been studying the internal structure of the YSO Barnard 1c, an object undergoing collapse. This object has a prominent outflow (well traced by carbon monoxide and the ion HCO+), evidence of rotation within the core (with the dense gas tracer N2H+), and evidence of central heating from an isotope of carbon monoxide, C18O).
There is an additional challenge in the study of the interiors of these cores and YSOs which have varying temperatures (10-30K) and high density (from hundreds of thousands to millions of hydrogen molecules per cubic centimetre) compared to the ambient material of a molecular cloud (at densities of about 100 molecules per cubic centimetre). The challenge is that the various molecular tracers of kinematics undergo complex chemistry as conditions vary. Modeling potential chemical paths is a very difficult and complicated process. Rates of reactions rely on laboratory work, and slight changes in these rates can produce wildly different results for which species trace certain densities and temperatures.
Dust Grain Properties and Populations
Measurements of continuum radiation from molecular clouds over different wavelengths provides information about the global properties of the grains in the clouds (e.g. mean temperature and emissivity). However, measurements of spectral index (changes in flux over wavelength) cannot determine if a cloud is made up of different populations of grains (either physically mixed or differentiated spatially). In order to measure the properties of varying grain populations (sizes, temperatures, compositions), the additional tool of the polarization spectrum is needed.
The polarization spectrum is a measure of how much the polarization percentage (ratio of polarized to total flux) from a cloud changes with wavelength. In combination with the flux spectrum, this information can separate out different grain populations (see, for example, Vaillancourt 2002 for work on the OMC-1 core). Unfortunately, polarimeters are not usually part of the original designs on continuum instrumentation. This, coupled with the difficulty in obtaining polarimetry data where polarimeters exist leads to a paucity of polarization spectrum data. There are now a few objects for which multiple wavelength data exist, and I am part of an ongoing JCMT project to extend the spectrum of OMC-1 below 350 micron in order to better probe the dust grain populations there.
Last Updated May 2010