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Light 'Echos' Used to Measure Size of Baby Star's Crib

By studying reflected light from a young star system, astronomers have precisely measured the size of the innermost gap between the star and where planets will eventually form.

Often, stars are too distant for us to make any sense of their surroundings. But in the case of baby stars, surrounded by protoplanetary disks, there's an ingenious trick astronomers can use to examine the structure of their dusty birthplaces.

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A star may form from a molecular cloud of gas that, should the conditions be just right, collapses under a mutual gravity. This collapse will form a knot of dense material that may fuse to produce the core of a young star. Over time, material will collect around this protostar, forming a swirling disk. Eventually, planets will coalesce from this protoplanetary disk. For us to better understand how the planets in the solar system formed, astronomers are very curious about studying the disks around other stars.

"Understanding protoplanetary disks can help us understand some of the mysteries about exoplanets, the planets in solar systems outside our own," said postdoctoral research associate Huan Meng, of the University of Arizona, Tucson. "We want to know how planets form and why we find large planets called ‘hot Jupiters' close to their stars."

Unless young star systems are on our interstellar doorstep, it can be hard for the structure of these disks to be seen.

However, by studying the fluctuations of brightness of a star called YLW 16B, approximately 400 light-years from Earth, Meng and his collaborators were able to detect reflected starlight from the innermost boundary of the star's protoplanetary disk, making extremely precise measurements of its location and structure.

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This particular star is of approximately the same mass as our sun, but it is only 1 million years old (compared to our 4.6 billion year-old sun, this star isn't much more than a stellar embryo). This makes it an ideal candidate to better understand the physics of our solar system before any planets began to form around the young sun.

Using data from NASA's Spitzer space telescope, which observes the cosmos in infrared light, and from ground-based observatories, the astronomers applied a technique called "photo-reverberation" to study the starlight bouncing off the protoplanetary disk's inner edge.

It just so happens that YLW 16B has a variable and unpredictable fluctuation in emissions, so the astronomers measured these emission fluctuations and waited for the reflected light to bounce off the disk. The variations in star brightness could then be matched with the light echo, which arrived shortly after. The time lag could then be used to derive the distance of the star from the protoplanetary disk's inner edge.

For this star system, the gap between star and inner disk is around 0.08 AU - where 1 AU, or astronomical unit, is the average distance between the sun and Earth's orbit. As a better comparison, the inner edge is approximately one quarter of the distance that Mercury orbits the sun.

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These observations were also able to deduce that the disk was thick, providing an interesting additional clue as to how much material the disk may contain.

Young stars are bright and possess powerful stellar winds that "blow-out" the inner protoplanetary disk, leaving a gap (as illustrated by the image, top). Understanding how big this gap is and its location from the star will help us improve models of baby star systems, ultimately teaching us a little about how our solar system may have formed 4.6 billion years ago.

Source: NASA/JPL-Caltech

This illustration shows a young star, erupting with magnetism, surrounded by a protoplanetary disk.

Astronomers using the Hubble Space Telescope recently completed the largest and most sensitive survey of dust surrounding young star systems. The survey zoomed-in on stars that are between 10 million to 1 billion years old and the source of the dust is thought to be the left-over debris from planet, asteroid and comet collisions after systems of planets have formed.

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The research is akin to looking far back into the history of our solar system, seeing the inevitable dusty mess left over after the Earth and other planets evolved. "It's like looking back in time to see the kinds of destructive events that once routinely happened in our solar system after the planets formed," said Glenn Schneider, of the University of Arizona's Steward Observatory and lead scientist on the survey team.

Read on to see some of the beautiful variety of circumstellar disks observed by Hubble.

One of the major findings to come from this survey is the stunning diversity of dust surrounding these young stars. Traditionally, circumstellar dust is thought to settle into an orderly disk-like shape -- but it turns out that the opposite is true.

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"We find that the systems are not simply flat with uniform surfaces," said Schneider. "These are actually pretty complicated three-dimensional debris systems, often with embedded smaller structures. Some of the substructures could be signposts of unseen planets."

One stunning observation of the star HD 181327 exhibits a bright ring of dust containing irregularities, potential evidence of a massive collision that has scattered debris far and wide. "This spray of material is fairly distant from its host star — roughly twice the distance that Pluto is from the Sun," said Christopher Stark of NASA's Goddard Space Flight Center, Greenbelt, Md., and co-investigator in the survey team. "Catastrophically destroying an object that massive at such a large distance is difficult to explain, and it should be very rare. If we are in fact seeing the recent aftermath of a massive collision, the unseen planetary system may be quite chaotic."

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Another interpretation for the irregularities could be some kind of interaction with unseen interstellar material. "Our team is currently analyzing follow-up observations that will help reveal the true cause of the irregularity," added Schneider.

Like the diversity of exoplanetary systems astronomers have discovered, it appears the accompanying dust disks also share this characteristic, possibly indicative of gravitational interactions with planets orbiting the stars surveyed by Hubble.

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"How are the planets affecting the disks, and how are the disks affecting the planets? There is some sort of interdependence between a planet and the accompanying debris that might affect the evolution of these exoplanetary debris systems," said Schneider.

Since 1995, thousands of exoplanets have been discovered orbiting stars in our galaxy. Over the same period, however, only a couple of dozen circumstellar disks have been imaged directly. This is down to the fact that the scattered light off these disks is extremely faint (around 100,000 times fainter than the parent star's light). The technology and techniques are only recently becoming available for scientists to not only block the star's blinding light, but to also boost the sensitivity of observations to pick out this scattered light that would otherwise be obscured from view. Fortunately, Hubble's high-contrast imaging has been key in making this survey a success.

By studying these disks of dust and their surprising variety of morphologies may help astronomers better understand how the Earth-moon and Pluto-Charon systems formed. Through planetary collisions, the debris from the early solar system may have coalesced to create many of the natural satellites we see today, 4.6 billion years later. The results of this survey have been published in The Astrophysical Journal.

For more information about this Hubble survey and high-resolution images, browse the HubbleSite.org news release.