More Sun-like stars may have planetary systems than currently thought

Study of planetary disks around T Tauri stars

If David Weintraub and Jeff Bary are right, there may be a lot more planets circling stars like the Sun than current models of star and planet formation predict.
The associate professor of astronomy at Vanderbilt and his graduate student are taking a critical look at T Tauri stars. These are stellar adolescents, less than 10 million years old, which are destined to become stars similar to the Sun as they age.

Classical T Tauri stars – those less than 3 million years old – are invariably accompanied by a thick disk of dust and gas, which is often called a protoplanetary disk because it is a breeding ground for planet formation. Most older T Tauri stars show no signs of encircling disks. Because they are not old enough for planets to form, astronomers have concluded that most of these stars must loose their disk material before planetary systems can develop.

Weintraub and Bary are pursuing an alternative theory. They propose that most older T Tauri stars haven’t lost their disks at all: The disk material has simply changed into a form that is nearly invisible to Earth-based telescopes. They published a key observation supporting their hypothesis in the September 1 issue of the Astrophysical Journal Letter and the article was highlighted by the editors of Science magazine as particularly noteworthy. The two researchers currently are preparing to publish additional evidence in support of their hypothesis.

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. The second group of T Tauri stars that are somewhat older – between three to six billion years – and show no evidence of disks have been labeled as “naked” or “weak line” T Tauri stars.

Because there is no visible evidence that naked T Tauri stars possess protoplanetary disks. So astronomers have concluded 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. According to current theories, it takes about 10 million years to form a Jupiter-type planet and even longer to form a planet like Earth. If the models are correct and if most Sun-like stars loose their protoplanetary disks in the T Tauri stage, then very few stars like the Sun are likely to possess planetary systems.

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.

Current models do not take the evolution of protoplanetary disks into account. 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 knew 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.”

So Weintraub and Bary began searching for ways to determine if such “invisible protoplanetary disks” actually exist.

They decided that their best bet 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 fairly high temperature before it will give off infrared light.

The fact that T Tauri stars are also strong X-ray sources gave them an idea. Perhaps the X-rays coming from the star could act as an energy source capable of stimulating the molecular hydrogen. To produce enough light to be seen from earth, however, the molecular hydrogen could not b mixed with dust and had to be at an adequate density. Studying various theories of planet formation, they determined that the proper conditions should hold in a “flare region” near the outer edge of the protoplanetary disk.

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. As they argued in their Astrophysical Journal Letter article, 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. Although they do not completely answer the question, additional observations that the two are readying for publication provides additional support for their contention that DoAr21 contains a sizeable but invisible disk.

The new observations are the detection of the same molecular hydrogen emission line 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.

Weintraub and Bary admit that they have more work to do to in order to convince their colleagues to adopt their theory. They 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. They also intend to search for a second, fainter hydrogen emission line. If they find it, it will provide additional insights into the excitation process.

Currently, the number of naked T Tauri stars that have been discovered is much greater than the number of known classical T Tauri stars. If Weintraub and Bary are proven right, however, and a significant percentage of the naked T Tauri stars develop planetary systems, it means that solar systems similar to our own are a common sight in the universe.