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