* Astronomy

An odd array of 30 newly released images from the Hubble Space Telescope reveal planetary systems in the making.
The blobs and smudges, as astronomers described them, sit in the widely photographed Orion Nebula. Each object is known as a proplyds, or protoplanetary discs, and could be forming planets as you read this.
Among the images is one astronomers called a "space jellyfish." Its odd shape is created by shock waves that form when a wind of particles from a nearby massive star collides with the material of the proplyd.
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Title: A Spitzer Census of Transitional Protoplanetary Disks with AU-Scale Inner Holes
Authors: James Muzerolle, Lori E. Allen, S. Thomas Megeath, Jesus Hernandez, Robert A. Gutermuth

Protoplanetary disks with AU-scale inner clearings, often referred to as transitional disks, provide a unique sample for understanding disk dissipation mechanisms and possible connections to planet formation. Observations of young stellar clusters with the Spitzer Space Telescope have amassed mid-infrared spectral energy distributions for thousands of star-disk systems from which transition disks can be identified. From a sample of 8 relatively nearby young regions (d <= 400 pc), we have identified about 20 such objects, which we term "classical" transition disks, spanning a wide range of stellar age and mass. We also identified two additional categories representing more ambiguous cases: "warm excess" objects with transition-like spectral energy distributions but moderate excess at 5.8 microns, and "weak excess" objects with smaller 24 micron excess that may be optically thin or exhibit advanced dust grain growth and settling. From existing Halpha emission measurements, we find evidence for different accretion activity among the three categories, with a majority of the classical and warm excess transition objects still accreting gas through their inner holes and onto the central stars, while a smaller fraction of the weak transition objects are accreting at detectable rates. We find a possible age dependence to the frequency of classical transition objects, with fractions relative to the total population of disks in a given region of a few percent at 1-2 Myr rising to 10-20% at 3-10 Myr. The trend is even stronger if the weak and warm excess objects are included. Classical transition disks appear to be less common, and weak transition disks more common, around lower-mass stars (M <= 0.3 Msun).

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Dirty Stars Make Good Solar System Hosts
Some stars are lonely behemoths, with no surrounding planets or asteroids, while others sport a skirt of attendant planetary bodies. New research published this week in The Astrophysical Journal Letters explains why the composition of the stars often indicates whether their light shines into deep space, or whether a small fraction shines onto orbiting planets. When a star forms, collapsing from a dense cloud into a luminous ball, it and the disk of dust and gas orbiting it reflect the composition of that original cloud and the elements within it. While some clouds are poor in heavier elements, many have a wealth of these elements. These are the dirty stars that are good solar system hosts.
The new simulations by Mac Low and his colleagues Anders Johansen (Leiden Observatory in the Netherlands) and Andrew Youdin (Canadian Institute of Theoretical Astrophysics at the University of Toronto) compute just how planets and other bodies form as pebbles clump into mini-planets referred to as planetesimals. Their current work hinges on their previously published research (in Nature in 2007) that explains why rocks orbiting a star within the more slowly-revolving gas disk are not quickly dragged into the star itself because of the headwinds they feel. Like bicyclists drafting behind the leader in the Tour de France, the rocks draft behind each other, so that in orbits with more rocks, they feel less drag and drift towards the star more slowly. Rocks orbiting further out drift into those orbits, until there are so many that gravity can form them into mini-planets. This concentration of orbiting rocks in a gas disk is called a "streaming instability" and is the theoretical work of co-author Youdin.
The results of this paper will be presented on October 8, 2009 at a meeting of the Division of Planetary Sciences of the American Astronomical Society in Puerto Rico. Computer simulations were performed on the Huygens cluster in Amsterdam and the PIA cluster of the Max Planck Institute for Astronomy. Additional funding came from the NASA Origins of the Solar Systems Program and the NSF Cyberenabeled Discovery Initiative.

American Museum of Natural History

-- Edited by Blobrana on Wednesday 7th of October 2009 12:42:30 AM

Title: Formation of Water in the Warm Atmospheres of Protoplanetary Disks
Authors: A. E. Glassgold, R. Meijerink, J. R. Najita

The gas-phase chemistry of water in protoplanetary disks is analysed with a model based on X-ray heating and ionisation of the disk atmosphere. Several uncertain processes appear to play critical roles in generating the column densities of warm water that are detected from disks at infrared wavelengths. The dominant factors are the reactions that form molecular hydrogen, including formation on warm grains, and the ionisation and heating of the atmosphere. All of these can work together to produce a region of high water abundances in the molecular transition layer of the inner disk atmosphere, where atoms are transformed into molecules, the temperature drops from thousands to hundreds of Kelvins, and the ionisation begins to be dominated by the heavy elements. Grain formation of molecular hydrogen and mechanical heating of the atmosphere can play important roles in this region and directly affect the amount of warm water in protoplanetary disk atmospheres. Thus it may be possible to account for the existing measurements of water emission from Tauri disks without invoking transport of water from cooler to warmer regions. The hydroxyl radical OH is under-abundant in this model of disk atmospheres and requires consideration of additional production and excitation processes.

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