Dust and Planet Formation in T Tauri Stars

One of the beauties of the Spitzer Space Telescope is that it can pinpoint the swirling dust disks around distant stars. Such dust, heated by the star, puts out an infrared signature that Spitzer can analyze to a degree hitherto unattainable. Now a team of astronomers has observed some 500 young T Tauri stars in the star-forming regions of the Orion nebula. They’ve been looking at how young stars spin, and the effects that dusty disks have on slowing their rotation.

T Tauri stars are ideal for this kind of work. They’re young objects (less than 10 million years old) that are still in the process of gravitational contraction. Such stars often show large accretion disks, but a variant called weak-lined T Tauri stars have little or no disk. Figuring out the various phases of T Tauri formation and how they relate to planets is thus a substantial challenge.

The answers Spitzer has provided are intriguing even if they leave many questions unanswered. Slow-spinning stars are five times more likely to have dust disks than fast-spinning ones. Which implies that the disks, the early construction zones for planets, have a role in slowing the star. But other factors also appear to be involved, including stellar winds. “We can now say that disks play some kind of role in slowing down stars in at least one region, but there could be a host of other factors operating in tandem. And stars might behave differently in different environments,” said Luisa Rebull (Spitzer Science Center, Pasadena).

T Tauri Stars and their Disks

Image: How does a disk put the brakes on its star? It is thought to yank on the star’s magnetic fields (green lines). When a star’s magnetic fields pass through a disk, they are thought to get bogged down like a spoon in molasses. This locks a star’s rotation to the slower-turning disk, so the star, while continuing to shrink, does not spin faster. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

What we’re aiming for is an understanding of how a star’s rotation rate factors into the formation of planetary systems around it. And things aren’t nearly as clearcut as they might seem — nobody is arguing that fast-spinning stars can’t develop planets. Indeed, Rebull says that a slow spinner may simply take more time than other stars to clear out its disk and begin planet formation.

The exoplanets we’ve learned about so far all circle slowly turning stars; our own Sun rotates once every 28 days, a relatively sedate pace. Finding planets around stars that rotate more quickly is thus key to understanding how rotation and planet formation are intertwined in young stars. That leaves plenty of work for the next generation of space and ground-based telescopes.

The paper is Rebull, Stauffer, Megeath et al., “A Correlation between Pre-Main-Sequence Stellar Rotation Rates and IRAC Excesses in Orion,” Astrophysical Journal 646 (20 July 2006), pp. 297 ff., available online.

Calculating the Distance to a Star

Centauri Dreams‘ fascination with the history of science occasionally yields to forgetfulness. Which is why this piece on German mathematician and astronomer Friedrich Wilhelm Bessel, runs two days after his birthday rather than on the occasion itself. Bessel (July 22, 1784 – March 17, 1846) would go on to perform remarkable work in the study of stellar distances that in many respects anticipated the work of today’s exoplanet hunters. He became, in fact, the first person to predict the existence of an unseen companion around another star.

Friedrich Wilhelm Bessel

That was quite an accomplishment in 1844, when Bessel announced the find around Sirius, based on minute deviations in the motion of the star. The discovery would be verified by Alvan Graham Clark in 1862 with the first observations of Sirius B. We now know that Sirius B orbits the primary at a distance of roughly 20 AU, not so far off the 23.7 AU mean separation between Centauri A and B, although there the resemblance stops — Sirius B is a white dwarf, at 12,000 kilometers in diameter much smaller than the A-class star it circles.

Bessel goes down as well for making the first measurement of the distance of a star other than the Sun. The method was parallax, observing the relative motion of the star 61 Cygni from both sides of the Earth’s orbit around Sol. His studies of 61 Cygni demonstrated that it had the greatest proper motion of all stars whose motion he could calculate. He worked out a distance of roughly 10 light years, not so far off the 11.1 light year distance now estimated for the star.

61 Cygni, a relatively dim object, made an interesting choice given far brighter and more nearby objects. At the Royal Observatory at the Cape of Good Hope in South Africa, a Scottish lawyer named Thomas Henderson began studying the Alpha Centauri system, the third brightest star after Sirius and Canopus. Henderson’s calculations of Alpha Centauri’s 41 trillion kilometer distance were actually complete before Bessel’s announcement about 61 Cygni was made but the latter’s work was circulated first. At the same time, Friedrich Georg Wilhelm Struve had measured the parallax of Vega, a converging of talent and interest at a propitious time for astronomy.

A final thought: The early work with parallax reminds us of the fact that while the method works for nearby stars (up to about 200 light years) it fails with more distant objects that show no apparent motion in the six months between observations. One virtue of a deep space probe like Claudio Maccone’s FOCAL platform would be that at 550 AU from the Sun, FOCAL could deliver a much longer baseline for calculating stellar distances. Extend that baseline to the Centauri system and you have a precision tool for mapping our part of the galaxy.

Exploration as Necessity

“The urge to explore, the quest of the part for the whole, has been the primary force in evolution since the first water creatures began to reconnoiter the land. We humans see this impulse as the drive to self-transcendence, the unfolding of self-awareness. The need to see the larger reality — from the mountaintop, the moon, or the Archimedean points of science — is the basic imperative of consciousness, the specialty of our species. If we insist that the human quest await the healing of every sore on the body politic, we condemn ourselves to stagnation. Living systems cannot remain static; they evolve or decline. They explore or expire. The inner experience of this imperative is curiosity and awe. The sense of wonder — the need to find our place in the whole — is not only the genesis of personal growth but the very mechanism of evolution, driving us to become more than we are. Exploration, evolution, and self-transcendence are but different perspectives on the same process.”

Wyn Wachhorst, The Dream of Spaceflight: Essays on the Near Edge of Infinity (New York: Basic Books, 2000), pp. 150-151.

A Dead Star Brightens

RS Ophiuchi, a binary system some 5000 light years from Earth, has given astronomers plenty to talk about since February, when it suddenly brightened. The phenomenon wasn’t unusual — RS Ophiuchi undergoes periodic outbursts — but this was the first since 1985, allowing powerful radio telescope arrays to study the results. By coordination between radio telescopes from South Africa to China, Hawaii to the UK, astronomers have pieced together the sequence of events that led to the explosion and have studied its aftermath.

The results are reported in the July 20 issue of Nature, revealing that a mere two weeks after first reports of the stellar eruption, an expanding blast wave extended to a distance of 10 AU. It was triggered by a nuclear explosion on the surface of a white dwarf that had been capturing gas from the nearby gas giant it orbits. Once enough gas collects on the white dwarf, a thermonuclear reaction begins, with the white dwarf’s energy output increasing to over 100,000 times that of the Sun.

Explosion in a binary system

Image: Artist’s impression of the binary system RS Ophiuchi. Hydrogen-rich gas is transferred from a red giant onto the surface of a white dwarf and has just exploded there. Credit: David A. Hardy & PPARC (www.astroart.org).

The gas, meanwhile, is ejected at several thousand kilometers per second, striking the atmosphere of the red giant and setting up blast waves that accelerate electrons close to the speed of light. The radio emissions of these electrons as they move through magnetic fields in the vicinity of the binaries are what the astronomers are examining.

The beauty of modern radio telescope work is that it can quickly become global in scope. With initial observations on the Very Long Baseline Array (VLBA), which extends from Hawaii to the Caribbean, the team continued to track the outburst using facilities in the USA (the Very Large Array) and the UK (MERLIN), as well as Europe’s VLBI Network, which includes telescopes in South Africa and China.

The work has already yielded its share of surprises, says Richard Porcas (Max Planck Institute for Radio Astronomy):

“A week after our first observations, we combined telescopes across Europe with two in China and another in South Africa and were surprised to find that the blast wave had become distorted. Over the next few months our observations have shown it turning from a ring into a cigar-like shape. It’s going to need a lot more work to understand exactly what causes this but either the explosion shoots jets of matter in opposite directions or somehow the atmosphere of the red giant is shaping the ejected material.”

Further explosions on RS Ophiuchi are inevitable as gas builds up again on the white dwarf. Are we seeing a supernova in the making as the white dwarf increases in mass, or is the tiny star shedding everything it captures from the red giant with each dramatic outburst? That’s one major question that future work on this distant binary should answer. The paper is O’Brien, Bode, Porcas et al., “An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi,” Nature 442 (20 July 2006), pp. 279-281 (abstract here).

Radar Views of Xanadu

Xanadu, that bright continent-sized aberration on the surface of Titan, begins to look somewhat familiar in new radar images from the Cassini orbiter. It’s surrounded by darker terrain, cut by rivers and filled with hills and valleys. Cassini could identify a crater, probably created by asteroid impact, and flatter areas into which the rivers flow — these are presumably lakes. Geologically and topographically (and ignoring for a moment the deep freeze), the place does look something like Earth, even if rain there falls in the form of methane.

Landscape in Xanadu

Image: A network of river channels is located atop Xanadu, the continent-sized region on Saturn’s moon Titan. This radar image was captured by the Cassini Radar Mapper on April 30, 2006. These winding, meandering river channels start from the top of the image and run like a fork in the road, splitting to the right and left of the image. At Titan’s chilly conditions, streams of methane and/or ethane might flow across parts of the region. Credit: NASA/JPL.

“This land is heavily tortured, convoluted and filled with hills and mountains,” said Steve Wall, the Cassini radar team’s deputy leader at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “There appear to be faults, deeply cut channels and valleys. Also, it appears to be the only vast area not covered by organic dirt. Xanadu has been washed clean. What is left underneath looks like very porous water ice, maybe filled with caverns.”

We’re learning more about Titan with every new Cassini pass, and we’ve got 29 more encounters in the next two years, twelve of which will use radar. A hearty well done, by the way, to Cassini scientist Jonathan Lunine (University of Arizona), who keeps the Coleridgean motif going, referring to Titan as “… a land where rivers flow down to a sunless sea.” Coleridge himself was mysterious about the vision that brought him his poem ‘Kubla Khan’ — supposedly the arrival of a visitor knocked him out of his reverie (perhaps an opium dream) leaving the poem forever incomplete — but it was of a place that could have been no more unusual and yet hauntingly familiar than distant Titan.