Transition discs in Ophiuchus and Taurus

July 7, 2015, Harvard-Smithsonian Center for Astrophysics
Transition discs in Ophiuchus and Taurus
A false-color image of the circumstellar transition disk around the star LkCa15, taken at submillimeter wavelengths. A new study finds that the most probable explanation for the inner gap in transition disks is the influence of one or more giant planets orbiting nearby. Credit: S. Andrews

A star is typically born with a disk of gas and dust encircling it, from which planets develop as dust grains in the disk collide, stick together and grow. These disks, warmed by the star to a range of temperatures above the cold, ambient interstellar material, can be detected at infrared or millimeter wavelengths, and their infrared color used to characterize their properties. Stars older than about five million years lack evidence for these disks, however, suggesting that by this age most of the disk material has either been converted into planets or smaller bodies, accreted onto the star, or dispersed from the system. Transition disks bridge this period in disk evolution: They have not yet been disbursed, but although they are present they emit only slightly in the infrared. Their emission shows characteristically cooler temperatures, and signs that the innermost (hottest) regions have already disappeared and left a gap (or cavity) in the ring.

CfA astronomer Sean Andrews and his colleagues have been studying transition disks in nearby located in constellations of Taurus and Ophiuchus. The astronomers note that the gaps in transition disks they see might have been caused by one or more of three processes: grain growth and planet formation that depleted the material, a giant planet in the vicinity that swept the region clean, or a stellar wind that blew away or evaporated the dust.

The astronomers set out to determine which of these processes was at work by comparing the properties of transition disks and normal disks. In particular, they compared the rates of mass accretion onto the star and the disk masses. They find, first of all, that these two quantities correlate well with the properties of the radiation, enabling them to control for other potential environmental variables. The transition disks tend to be associated with smaller accretion rates and larger masses than disks in the normal comparison stars, suggesting that the wind and grain growth scenarios are secondary. At least in these two star-forming regions, therefore, the team concludes that the most likely explanation for the gap is the presence of one or more giant .

Explore further: A head start for planet formation? Evidence of large dust grains in star-forming regions

More information: "Demographics of Transition Discs in Ophiuchus and Taurus," Joan R. Najita, Sean M. Andrews, and James Muzerolle, MNRAS 450, 3559, 2015.

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1 / 5 (3) Jul 07, 2015
What Herschel has demonstrated is that there are two separate phases for this process, and a rational person who is not overly attached to the textbook theories would realize that the first phase appears to correspond with the filamentation observed in high charge-density plasma. The stars form like beads along this current.

This is the inevitable solution to this problem, but I think we can all see by now that every other possible alternative will be considered before a classical electrodynamical solution based upon observations of laboratory plasmas.

This stubbornness was perhaps justified prior to Herschel, but we can now see the stars forming on these filaments. So, there are really no excuses at this point to taking another look at filamentation in laboratory plasmas.

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