Supersonic "Hail" Seeds Star Systems With Water

August 29, 2007

Evidence of water vapor "raining down" on a newly forming star system is offering the first direct look at how water likely gets incorporated into planets, NASA researchers announced.

(Related: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)

The water—enough to fill Earth's oceans five times over—falls at supersonic speeds in the form of a hail-like substance from the envelope of dust and gas that gave birth to the star.

The hail vaporizes when it smacks into the dusty disk around the embryonic star where planets are thought to take shape, according to models that best explain the observed data.

"This is the first time we've ever seen the process by which the surrounding envelope's material arrives at the disk," said Dan Watson, an astrophysicist at the University of Rochester in New York.

Watson is lead author of a paper describing the discovery in tomorrow's issue of the journal Nature.

"Since the disk is what's eventually going to give rise to the planetary system around the star, what we are seeing is the process by which that disk formed and therefore the initial conditions of planetary formation."

Charles Lada is an astrophysicist who studies the early evolution of stars and planets at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He was not involved in the latest research, but lauded the discovery.

"If the interpretation [of the data using models] holds up, these observations will represent a significant advance in the quest to understand the origin of protoplanetary disks," he said in an email.

Star Development

The new work is based on observations of an embryonic star system taken with NASA's Spitzer Space Telescope (see images of stellar nurseries captured by Spitzer).

Astronomers observe such protostar systems in the infrared spectrum, because visible light is easily absorbed by the systems' dusty environments, making them invisible to the naked eye.

Water vapor emits a distinctive spectrum in infrared light.

The protostar lies about a thousand light-years from Earth in a cloud gas and dust. The whole system is called NGC 1333-IRAS 4B, or IRAS 4B for short.

The star is a warm, dense blob of material at the core of the cloud. A disk of planet-forming material is believed to circle the blob.

The radius of the disk is just larger than the distance between Pluto and the sun: about 3.6 billion miles (5.8 billion kilometers).

Based on their data, Watson and his colleagues say that the surface of the disk is -153 degrees Fahrenheit (-103 degrees Celsius).

While this seems frigid by Earth standards, Watson explained, the properties of water are different at the atmospheric pressure of the protostar, which is about a billionth of the pressure at sea level on Earth.

In addition, material equal to 23 times the mass of Earth arrives at the disk each year, Watson said.

"That's the material that's heating on arrival and then gradually cooling as it joins the lower parts of the disk," he said.

"This is very wet stuff. The original state is very wet," he added. "There's plenty of water to make a solar system out of."

Right Angle

Of the 30 embryonic star systems observed with Spitzer, only IRAS 4B showed signs of water vapor.

According to Watson, this is most likely because the protostar's axis points almost directly at Earth.

"The other 29 could very well have just as much water emission as IRAS 4B, but they are turned the wrong way and you can't see them," he said.

The team has already identified hundreds more protostar systems like IRAS 4B and plans to observe them with the Spitzer telescope, including more stars that exhibit this rare orientation.

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