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The mystery of the disappearing planetary disks

Vanderbilt University

When stars like the sun are adolescents - less than 10 million years old - many of them appear to loose the circumstellar disks of dust and gas that they were born with in less time than it takes the swirling material to consolidate into planets. This has led astronomers to conclude that solar systems around sun-like stars may be fairly rare.

But two Vanderbilt astronomers - Jeff Bary and David Weintraub - think something else may be going on. Rather than loosing their disks, these stars may actually keep the disks while the disks become harder and harder to detect from Earth as direct result of the planet-building process.

Speaking on May 27th at the annual meeting of the American Astronomical Society in Nashville, graduate student Jeff Bary presented this alternative theory and the evidence that he and Weintraub, an associate professor of astronomy, have gathered to support it.

When mid-sized stars like the sun are young, they are known as T Tauri stars. Classical T Tauri stars - those less than 3 million years old - are invariably accompanied by highly visible disks that provide the material for planet formation. Most older T Tauri stars (called "naked" or "weak line" T Tauri stars) show no signs of encircling disks, however.

According to current theories, it takes more than 10 million years for planets to form. That has led astronomers to conclude that most sun-like stars must loose their disk material before planetary systems can develop. They hypothesize that the material must have been absorbed by the star or blown out into interplanetary space or pulled away by the gravitational attraction of a nearby star in the first few million years of the star's existence.

Because they are more likely to emit X-rays, far more naked T Tauri stars have been discovered than their younger, classical cousins. This suggests that the mechanism, or mechanisms, that destroys T Tauri disks is relatively commonplace.

This picture doesn't sit well with Weintraub, however. "Approaching it from a planetary evolution point of view, I have not been comfortable with some of the underlying assumptions," he says.

That is because current models do not take the evolution of protoplanetary disks into account. The dense disks of dust and gas surrounding classical T Tauri stars are easily visible because dust glows brightly in the infrared region of the spectrum. Although infrared light is invisible to the naked eye, it is readily detectable with specially equipped telescopes.

Over time, the disk material should begin agglomerating into solid objects called planetesimals. As the planetesimals grow, an increasing amount of the mass in the disk becomes trapped inside these solid objects where it cannot emit light directly into space. The constituents of the disk that astronomers know how to detect - small grains of dust and carbon monoxide molecules - should quickly disappear during the first steps of planet building.

"Rather than the disk material dissipating," says Bary, "It may simply become invisible to our instruments." To test their hypothesis, Weintraub and Bary began searching for ways to determine if such "invisible protoplanetary disks" actually exist.

They decided that the best experimental approach was to search for evidence of molecular hydrogen, the main constituent of the protoplanetary disk, which should persist much longer than the dust grains and carbon monoxide. Unfortunately, molecular hydrogen is notoriously difficult to stimulate into emitting light: It must be heated to a high temperature before it will give off infrared light.

The fact that T Tauri stars are also strong X-ray sources gave Weintraub and Bary an idea. Perhaps the X-rays coming from the star could act as an energy source capable of stimulating molecular hydrogen. To produce enough light to be seen from earth, however, the molecular hydrogen can not be bound to dust grains and has to be at an adequate density. Studying various theories of planet formation, they convinced themselves that the proper conditions may exist in the thick region of the disk at about the same distance that Jupiter and Saturn orbit the sun.

The next step was to get observation time on a big telescope to put their out-of-the-mainstream theory to the test. After repeated rejections, they were finally allocated viewing time on the four-meter telescope at the National Optical Astronomical Observatory in Kitt Peak, Arizona. When they finally took control of the telescope and pointed it toward one of their prime targets - a naked, apparently diskless T Tauri star named DoAr21 - they found the faint signal for which they were searching.

"We found evidence for hydrogen molecules where no hydrogen molecules were thought to exist," says Weintraub. When Bary calculated the amount of hydrogen involved in producing this signal, however, he came up with about a billionth of the mass of the Sun, not even enough to make the Moon. They believe that they have detected only the proverbial tip of the iceberg, since most of the hydrogen gas will not radiate in the infrared. But the calculation raises the question of whether the molecular hydrogen that they detected is part of a complete protoplanetary disk or just its shadowy remains.

Following this initial observation, the astronomers found the same emission lines around three classical T Tauri stars with visible protoplanetary disks. The strength of the hydrogen emission lines in the three is comparable to that measured at DoAr21. In addition, they have calculated the ratio between the mass of hydrogen molecules that are producing the infrared emissions and the mass of the entire disk in the three systems. For all three they calculate that the ratio is about one in 100 million.

"If the ratio between the amount of hydrogen emitting in the infrared and the total amount of hydrogen in the disk is about the same in the two types of T Tauri stars, which is not an unreasonable assumption, this suggests the naked T Tauri star has a sizable but hard-to-detect disk," says Bary.

In a recent set of observations, they have discovered hydrogen emission lines around eight more T Tauris - three weak and five classical. They are in the process of analyzing the signals from these stars.

All told, they have now found 12 T Tauris that possess this distinctive feature. That is a significant fraction of the T Tauris stars that the astronomers have surveyed thus far. Weintraub and Bary have been allocated time on a larger telescope, the eight-meter Gemini South in Chile, and plan to survey 50 more naked T Tauri stars to see how many of them produce the same molecular hydrogen emissions. If a large number of them do, it will indicate that they have discovered a general mechanism involved in the planetary formation process.

If Weintraub and Bary are proven right and a significant percentage of the naked T Tauri stars have not lost their protoplanetary disks, it means that solar systems similar to our own may be a common sight in the universe.

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