Two telescopes spy two dust rings

via Astronomy Now.

The twin 10 metre Keck telescopes have been used to resolve a question about the dust disc surrounding the star 51 Ophiuchi. Many stars have discs of small particles of dust around them, most notably stars where planet formation is going on. The remains of the dust disc around our own star are sufficiently dense to reflect enough sunlight to be occasionally be seen as an eerie cone of Zodiacal Light shining along the ecliptic (in the zodiac). Discs around other stars are therefore known as exozodiacal clouds.

The disc in the case of the star in question seemed to be a bit confusing. Direct observations seemed to show a compact disc, hugging its host star tightly. Spectroscopic measurements of the velocity of the disc through Doppler shifting of its light, however, suggested the disc was large. The twin Keck telescopes were called on to work together as one, a set-up known as the Keck Interferometer Nuller, where Interferometer describes the process of adding together light from multiple telescopes. Nuller refers to the process of blocking out light from the central star.

KIN allows a far better resolution than the single telescopes together and blocking out the central star allowed a better than usual look at the fainter parts of the disc. The result was to see that there are effectively two discs circling the star. One extends four times the Earth-Sun distance from the surface of the star, the other goes from this distance to ten times the distance between the Sun and the Kuiper belt.

The difference in the two disks is due to their composition. Large, heavy particles are drawn toward stars by gravity. Smaller, lighter particles are more easily blown back by the stellar winds, producing two discs, an inner one of larger particles and an outer one of smaller particles. The particles are produced by the collisions of asteroids and other planetary debris as well as particles coming together under gravity. The inner disc, containing particles of ten micrometers or larger, is therefore where the particles are likely to be generated and the outer disc, containing particles one hundred times smaller, similar in size to particles of smoke, is the result of material excavated by the stellar winds and radiation pressure.

Credit: NASA/GSFC/Marc Kuchner and Francis Reddy

Credit: NASA/GSFC/Marc Kuchner and Francis Reddy

51 Ophiuchi is 260 times as luminous as the Sun, suggesting the radiation pressure from the star is far greater, making the process of separation more efficient.


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