I'm looking at planetesimal formation within the Orion nebula. In HST images, several dozen large `dark disks' are seen - dark only because they are blocking the light from the ionized gas of the Orion nebula region, silhouette-style. These disks may well be young solar systems in-formation -- like ours, but younger, perhaps 106 years years old. The disks are composed of gas & dust: the dust is what blocks the light, and the gas is what supplies most of their mass.
The Orion region itself is visible predominantly because of the
Trapezium stars, OB stars which ionize the region nearby. Besides
lighting up the whole area like a flashlight, UV photons from the
Trapezium are tearing apart the disk like a cosmic leaf-blower
attacking a pile of sand. Models of planet formation that ignore
the effect of the OB stars may be skipping a good part of the physics!
Planets form by collisional growth of small particles. But, particles can be blown away from the system by the hot wind blown off from the disks. If particles grow large enough, fast enough, they're safe, but small particles can be entrained and lost by the gas. We are modeling the growth of small particles is the photoevaporated disks.
We are also looking at planetesimal formation by gravitational instability -- a collective force that brings many small dust grains together. Although this method is not new, interest in it has been revitalized recently. Historically, gravitational instability is usually difficult because gas motions inhibit graisn from coming together. We are looking at methods where this can be mitigated -- for instance, if photo-evaporation removes the gas first, then gravitational instability can work subsequently.
Processes in the disk are turbulent. The main source of the turbulence is escaping heat which convects through the disk's atmosphere. The heat is generated both from the initial collapse of the disk, and from turbulent viscosity generated by shear within the disk.
Our model tracks the size distribution n(r,z,R,t) for particles in the disk. The particle growth process is a) collisional coagulation, and the loss processes are b) UV photosputtering, and c) removal of particles by entrainment in photoevaporating gas. We use a variable-timestep integrator to track the state vector for t ~ 106 years.
We find that, for typical input parameters, the optical depth of the
disks is sharply terminated, at a distance R ~ 100 AU. Inward of this
distance, particle growth is fast enough that particles are large
enough (> 100 um) such that they are retained by the disk.
Outward of this distance, particle growth is slower, particles remain
small, and they are easily blown away by the escaping photoevaporative
wind. Furthermore, grains can quickly settle to the midplane; if this
happens and some gas is removed, then a gravitational instability in
the remaining dust can occur.
HST has been used to examine the disk images across the telescope's
entire wavelength range, 0.25 .. 2.2 um. In each case, the light
is generated behind the disk, photons pass through the disk,
and we observe the silhouette of the disk against the bright emission
nebula. The interaction between a photon and a particle is strongly
dependent on their relative sizes; very small particles (r << lambda)
cause blue skies & red sunsets.
In the case of the proplyds, we found the disks to be not blue, not
red, but completely grey across the wavelength range. This
implies that the typical particle sizes are at least several times the
wavelength, or
The presence of strong UV sources on a young disk appears to prevent the formation of large particles outward of 100 AU. Planets can't form there, because no planet-forming material there is safe from loss.
Formation of jovian-type planets may be affected in the inner nebula, depending on the growth speed of jovian cores. However, jovian-type planets may be formed if they're made quickly (e.g., Alan Boss's models of gravitational collapse), or in regions where they are sheltered from photo-evaporation (e.g., far from the Trapezium core).
The exact timescales are determined by a number of input parameters, some poorly determined: initial mass distributions, disk ages, projected disk distances, particle sticking probabilities, grain fractal properties, etc.
| 2008 | Icarus: Accretion of Jupiter's Atmosphere from a Supernova-Contaminated Star Cluster, Throop & Bally (submitted) |
| 2008 | AJ: Tail-End Bondi-Hoyle Accretion in Young Star Clusters: Implications for Disks, Planets, and Stars, Throop & Bally (PDF) |
| 2008 | AJ: Dynamics and Light Scattering in Saturn's Rings, Porco et al |
| 2008 | Space Science Reviews: Ralph: A Visible/Infrared Imager for the New Horizons Pluto/Kuiper Belt Mission, Reuter et al |
| 2007 | GRL: New Horizons Alice UV Observations of a Stellar Occultation by Jupiter's Atmosphere, Greathouse et al |
| 2007 | Science: Io’s Atmospheric Response to Eclipse: UV Aurorae Observations, Retherford et al (PDF) |
| 2007 | Science: New Horizons Mapping of Europa and Ganymede, Grundy et al (PDF) |
| 2007 | Science: Clump Detections and Limits on Moons in Jupiter’s Ring System, Showalter et al (PDF) |
| 2007 | Science: Jupiter’s Nightside Airglow and Aurora, Gladstone et al (PDF) |
| 2007 | Science: Polar Lightning and Decadal-Scale Cloud Variability on Jupiter, Baines et al (PDF) |
| 2005 | ASP Conf Proc: Evolution of UV-Irradiated Protoplanetary Disks, Bally et al |
| 2005 | ApJ: Phase Light Curves for Extrasolar Jupiters and Saturns, Dyudina et al |
| 2004 | Cambridge Jupiter book: Jupiter's ring-moon system, Burns et al |
| 2004 | Icarus: The Jovian rings: new results derived from Cassini, Galileo, Voyager, and Earth-based observations, Throop et al (PDF) |
| 2004 | Icarus: The size distribution of Jupiter's main ring from Galileo imaging and spectroscopy, Brooks et al |
| 2004 | ApJ: Can Photo-evaporation Trigger Planetesimal Formation?, Throop & Bally (astro-ph/0411647) |
| 2003 | Science: Cassini Imaging of Jupiter's Atmosphere, Satellites, and Rings, Porco et al |
| 2003 | ApJ: Evidence for Grain Growth in the Protostellar Disks of Orion, Shuping et al |
| 2001 | Science: Large Dust Grains in Young Circumstellar Disks, Throop et al (PDF) |
| 2000 | PhD thesis, Light Scattering and Evolution of Protoplanetary Disks and Planetary Rings (PDF, 2 MB) |
| 1997 | Icarus: G ring particle sizes derived from ring plane crossing observations |
I haven't listed everything here, or put up PDFs for them all. However, these are most of the interesting and orthogonal ones.
| October 2008 | Poster at Spitzer 'New Light on Circumstellar Disks' conference: Accretion onto Young Disks (PDF, 3 MB) |
| October 2008 | DPS talk: Accretion of Jupiter's Atmosphere from a Supernova-contaminated Star Cluster (PDF, 3 MB) |
| October 2008 | ITT-VIS about the New Horizons Geometry Visualizer, winner of IDL 2008 'Application of the Year' (PDF, 4 MB) |
| August 2008 | Mexican Star Formation Workshop, Mexico City |
| June 2008 | National Autonomous University of Mexico (UNAM), Mexico City: Environmental Hazards of Star Formation (PDF, 3 MB) |
| May 2008 | Mexican Astrobiology Society conference, Mexico City |
| April 2008 | DDA meeting, Boulder, CO (PDF, 2 MB) |
| March 2008 | NASA-Goddard, Greenbelt, MD |
| October 2007 | DPS talk: Accretion Onto Young Circumstellar Disks |
| October 2007 | Oklahoma-Texas Star Party talk #2: Pluto and New Horizons (PDF, 66 MB!) |
| October 2007 | Oklahoma-Texas Star Party talk #1: Star Formation (PDF, 8 MB) |
| September 2007 | University of Maryland, College Park |
| June 2007 | Poster at Extreme Solar Systems conference, Santorini, Greece (PDF, 3 MB) [NB: An excellent example of how not to do a poster!] |
| September 2006 | Physics of Circumstellar Disks conference, Porto, Portugal: Dense Star Clusters: Hazard or Haven? (PDF, 4 MB) |
| May 2006 | Little Thompson Observatory: The Orion Nebula (PDF, 9 MB) |
| September 2005 | New Mexico State University, Las Cruces (PDF, 8 MB) |
| June 2005 | Observatoire de Nice, France |
| June 2005 | Physics of Dusty Rings workshop, Berne, Switzerland: Cassini Observations of the Jovian Ring (PDF, 2 MB) |
| December 2004 | AGU Poster: Formation of Organic Molecules in Photo-evaporating Pre-planetary Disks |
| November 2004 | DPS talk Can Photo-evaporation Trigger Planetesimal Formation? (PDF) |
| December 2001 | DPS talk Azimuthal Asymmetry in the A Ring (PDF) |
| December 2001 | DPS talk Cassini Observations of Jupiter's Rings (PDF) |
| June 2001 | Boulder Jupiter meeting: Cassini observations of Jupiter's Rings (PDF) |
| April 2001 | Grinnell College: Dynamics of Planetary Rings |
| April 2001 | Grinnell College: The Frontiers of Star Formation |
| October 2000 | DPS talk (PDF) |
| October 2000 | Steward Observatory talk (PDF) |
| July 2000 | Ames talk (PDF) |
| May 2000 | PhD thesis talk (gzip'd postscript, 2.6 MB) |
| January 2000 | AAS talk: Large Grains in Young Circumstellar Disks (PDF) |
| October 1999 | DPS talk: Sandcastles in the Wind: Particle Growth in Externally Illuminated Young Circumstellar Disks (PS) |
| October 1996 | DPS talk: Particle Sizes in Saturn's G Ring |
| 2003 | Astronomy: Solar System Voyage (Serge Brunier) |
| 2003 | Astrobiology: Evolution of Planet Earth: The Impact of the Physical Environment (ed. Lister & Rothschild) |
| 2003 | Astronomy: The Measure of All Things - The Seven-Year Odyssey and Hidden Error that Transformed the World (Ken Adler) NB: A great book about the French surveyors who defined and then measured the length of the meter, measuring by hand the size of the Earth |
I am working with the instrument & science teams for Ralph, a visible-IR imaging spectrometer onboard the New Horizons mission to Pluto (and beyond!). The spacecraft launched in January 2006 and gets to Pluto in July 2015, after a Jupiter flyby in February 2007.
I wrote a slick web program (the New Horizons Geometry Visualizer, or just GV) which is being used by the science team to plan the encounter, and analyze a lot of the data from the Jupiter flyby. It is essentially a planetarium package, showing you what the sky looks like from New Horizons as it cruises through the Solar System. It can also be used for Cassini, Rosetta, Juno, and numerous other missions, as well as being very useful for planning observations from the Earth.
A detailed light scattering model for realistic, icy, non-spherical
particles, as applied to Voyager and HST data. We compared the
present-day ring (location, optical thickness, particle size, radial
profile) to end-results from a variety of origin scenarios, as
predicted by the Roche-zone evolutionary model of R. Canup.
We are also analyzing Galileo data of the Jovian rings, in
particular for particle size distributions, to be used to compare to
the output of long-term evolutionary models. We are currently working
on generating accurate phase curves and spectra of the main ring, based
on SSI and NIMS observations. Similar to our work with the G ring, we
are able to constrain the size distributions far better by using both
the spectra and phase curves than either one individually.
Size-dependent particle dynamics will be calculated using the models of
M. Horanyi, and formation histories constrained using the evolutionary
models of R. Canup.
Last modified 22-Oct-2008