New tools reveal astronomical mysteries

Artist's conception of dusty disk around young star TW Hydrae.
Credit: Bill Saxton, NRAO/AUI/NSF

By NRAO, Socorro, New Mexico

Date: February 17, 2012

Two new and powerful research tools are helping astronomers gain key insights needed to transform our understanding of important processes across the breadth of astrophysics. The Atacama Large Millimeter/submillimeter Array (ALMA), and the newly expanded Karl G. Jansky Very Large Array (VLA) offer scientists vastly improved and unprecedented capabilities for frontier research.

The cutting-edge research enabled by these powerful telescope systems extends from unlocking the mysteries of star- and planet-formation processes in the Milky Way and nearby galaxies to probing the emergence of the first stars and galaxies at the universe’s “cosmic dawn,” and along the way helping scientists figure out where Earth’s water came from.

A trio of scientists outlined recent accomplishments of ALMA and the Jansky VLA, both of which are in the “early science” phase of their development, as construction progresses toward their completion.

One exciting area where the two facilities are expected to unlock long-standing mysteries is the study of how new stars and planets form in our Milky Way Galaxy and in its nearby neighbors.

“These new ‘eyes’ will allow us to study, at unprecedented scales, the motion of gas and dust in the disks surrounding young stars, and put our theories of planet formation to the test,” said David Wilner from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. In addition, he added, the new telescopes will help show the first stages of planet formation — the growth of dust grains and pebbles in the disks — as well as show the gravitational interactions between the disks and new planets embedded within them.

“The power of ALMA and the expanded VLA also will allow us to study many more young stars and solar systems — probably thousands — than we could before. This will help us understand the processes that produce the huge diversity we already see in extrasolar planetary systems,” Wilner said.

One set of early ALMA observations of a disk around a young star nearly 170 light-years from Earth promises to shed light on a much closer question — the origin of Earth’s oceans. Scientists think much of our planet’s water came from comets bombarding the young Earth, but aren’t sure just how much.

The key clue has been the fact that our seawater contains a higher percentage of deuterium — a heavy isotope of hydrogen — than is found in the gas between stars in our galaxy. Scientists think this enrichment of deuterium is caused by low-temperature chemical reactions in the cold outer regions of the disk surrounding the young Sun — the region from which comets arise. The new ALMA observations, however, show that in a disk surrounding the young star TW Hydrae deuterium also is found in the warmer region closer to the star.

“With further studies like this, we are on the path to more precisely measuring the percentage of Earth’s ocean water that might have come from comets,” Wilner said.

Looking beyond the Milky Way, Christine Wilson from McMaster University in Ontario, Canada, points out that ALMA and the expanded VLA will give astronomers the ability to carefully study star formation in widely different types of galaxies, from the very faint to the extremely luminous and active ones.

“This will help us understand what regulates the rate at which stars form in galaxies,” Wilson said. One result from the VLA, however, seems to add to the mystery about this. John Cannon of Macalester College in St. Paul, Minnesota, and his colleagues studied a small star-forming galaxy and found that its mass is largely dark matter rather than the gas usually thought of as the fuel for star formation. “Their sample of small, but star-forming, galaxies has low amounts of gas, and this is puzzling,” Wilson said.

The two new telescopes also will help extend the study of galaxy evolution and star formation back to the universe’s youth, 10 or 12 billion years ago.

“The Jansky VLA and ALMA are ideally suited to reveal important new facts about very distant galaxies, which we see as they were when the universe was a fraction of its current age,” said Kartik Sheth of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia. “The new capabilities of these two facilities will show us the details of dust and gas in galaxies of this early epoch, thus helping us learn how such galaxies evolved into the types we see in the current universe.”

Already, Sheth said, both instruments have provided tantalizing glimpses of both atomic and molecular gas in galaxies as distant as 12 billion light-years.

“The huge range of ages in galaxies that we will be able to observe with these facilities represents a big step in piecing together the full history of how galaxies formed, evolved, and made stars over the vast span of cosmic time,” Sheth said.

“The early research results from ALMA and the Jansky VLA show the tremendous potential of these facilities for studies of galaxies and their history,” said Fred K. Y. Lo from the NRAO. “However, this is just one area of research in which these telescopes will make landmark contributions to our understanding of astronomical processes. ALMA and the Jansky VLA are leading tools for answering the most important questions of 21st-century astrophysics.”

Giant Eruption from Eta Carinae

These images reveal light from a massive stellar outburst in the Carina Nebula reflecting off dust clouds surrounding a behemoth double-star system. The color image at left shows the Carina Nebula, a star-forming region located 7,500 light-years from Earth. The massive double-star system Eta Carinae resides near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. The images were taken over an eight-year span by the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the CTIO.

By Carnegie Institution for Science, Washington, D.C.

Date: February 15, 2012

Eta Carinae, one of the most massive stars in our Milky Way Galaxy, unexpectedly increased in brightness in the 19th century. For 10 years in the mid-1800s, it was the second-brightest star in the sky — now it is not even in the top 100. The increase in luminosity was so great that it earned the rare title of Great Eruption. New research from a team, including Carnegie’s Jose Prieto, now at Princeton University in New Jersey, has used a “light echo” technique to demonstrate that this eruption was much different than previously thought.

Eta Carinae is a Luminous Blue Variable (LBV), meaning it has periods of dimness followed by periods of brightness. The variations in brightness of an LBV are caused by increased instability and loss of mass. The Great Eruption was an extreme and unique event in which the star, which is more than 100 times the mass of the Sun, lost several times the mass of our star. Scientists have believed that this rare type of eruption was caused by a stellar wind.

The team of scientists, led by Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland, used images of Eta Carinae over eight years to study light echoes of the Great Eruption. For the first time, they observed light from the eruption that bounced, or echoed, off interstellar dust tens of light-years from the star. Those extra light-years mean that the light is reaching Earth now rather than in the 1800s when people on Earth observed the light that traveled here directly.

They then used the Magellan and du Pont telescopes at Las Campanas Observatories in Chile to obtain spectra of the echoes of light. The spectra allow them to precisely separate the light into its constituents, much like a drop of rain naturally acts as a prism and separates sunlight into the colors of the rainbow. These observations give important information about the chemical composition, temperature, and velocity of the material ejected during the 19th century Great Eruption.

Most surprisingly, their observations show that the Great Eruption is different from “supernova impostors,” events in nearby galaxies that are thought to be eruptions from LBVs. For example, the Great Eruption was significantly cooler than allowed by simple stellar-wind models used to explain supernova impostors.

“This star’s Giant Eruption has been considered a prototype for all supernova imposters in external galaxies,” Prieto said. “But this research indicates that it is actually a rather unique event.”

Scientists still don’t know what phenomenon caused Eta Carinae to erupt and lose such a quantity of mass without being destroyed. Further research is necessary to determine whether other proposed models could have triggered this activity instead.

Venus is spinning slower than before

Figure : Venus Express
Research done By ESA, Noordwijk, Netherlands

Published: February 10, 2012

The European Space Agency’s (ESA) Venus Express spacecraft has discovered that our cloud-covered neighbor spins a little slower than previously measured. Peering through the dense atmosphere in the infrared, the orbiter found surface features were not quite where they should be.

Using the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover, scientists studied surface features and discovered that some were displaced by up to 12 miles (20 kilometers) from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s.

These detailed measurements from orbit are helping scientists determine whether Venus has a solid or liquid core, which will help our understanding of the planet’s creation and how it evolved.

If Venus has a solid core, its mass must be more concentrated towards the center. In this case, the planet’s rotation would react less to external forces.

The most important of those forces is due to the dense atmosphere — more than 90 times the pressure of Earth’s, and high-speed weather systems, which are believed to change the planet’s rotation rate through friction with the surface.

Earth experiences a similar effect, where it is largely caused by wind and tides. The length of an Earth day can change by roughly a millisecond, and it depends seasonally upon wind patterns and temperatures over the course of a year.

In the 1980s and 1990s, the Venera and Magellan orbiters made radar maps of the surface of Venus, long shrouded in mystery as well as a dense, crushing, and poisonous atmosphere. These maps gave us our first detailed global view of this unique and hostile world.

Over its four-year mission, Magellan was able to watch features rotate under the spacecraft, allowing scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days.

However, surface features seen by Venus Express some 16 years later could only be lined up with those observed by Magellan if the length of the Venus day is on average 6.5 minutes longer than Magellan measured.

This also agrees with the most recent long-duration radar measurements from Earth.

“When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of,” said Nils Müller from the DLR German Aerospace Center.

Scientists, including Özgur Karatekin from the Royal Observatory of Belgium, looked at the possibility of short-term random variations in the length of a Venus day, but concluded these should average themselves out over longer timescales.

On the other hand, other recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period. Other effects could also be at work, including exchanges of angular momentum between Venus and Earth when the two planets are relatively close to each other.

“An accurate value for Venus’ rotation rate will help in planning future missions because precise information will be needed to select potential landing sites,” said Håkan Svedhem from ESA.

While further study is needed, it’s clear that Venus Express is penetrating far deeper into the mysteries of this enigmatic planet then anyone dreamed.