Universe's Most Massive Stars Can Form in Near Isolation, New Study Finds

This is the most detailed observational study to date of massive stars that appear (from the ground) to be alone. The scientists used the Hubble Space Telescope to zoom in on eight of these giants, which range from 20 to 150 times as massive as the Sun. The stars they looked at are in the Small Magellanic Cloud, a dwarf galaxy that's one of the Milky Way's nearest neighbors.

Their results, published in the Dec. 20 edition of The Astrophysical Journal, show that five of the stars had no neighbors large enough for Hubble to discern. The remaining three appeared to be in tiny clusters of ten or fewer stars.

Doctoral student Joel Lamb and associate professor Sally Oey, both in the Department of Astronomy, explained the significance of their findings.
"My dad used to fish in a tiny pond on his grandma's farm," Lamb said. "One day he pulled out a giant largemouth bass. This was the biggest fish he's caught, and he's fished in a lot of big lakes. What we're looking at is analogous to this. We're asking: 'Can a small pond produce a giant fish? Does the size of the lake determine how big the fish is?' The lake in this case would be the cluster of stars.

"Our results show that you can, in fact, form big stars in small ponds."
The most massive stars direct the evolution of their galaxies. Their winds and radiation shape interstellar gas and promote the birth of new stars. Their violent supernovae explosions create all the heavy elements that are essential for life and the Earth. That's why astronomers want to understand how and where these giant stars form. There is currently a big debate about their origins, Oey said.

One theory is that the mass of a star depends on the size of the cluster in which it is born, and only a large star cluster could provide a dense enough source of gas and dust to bring about one of these massive stars. The opposing theory, and the one that this research supports, is that these monstrous stars can and do form more randomly across the universe -- including in isolation and in very small clusters.

"Our findings don't support the scenario that the maximum mass of a star in a cluster has to correlate with the size of the cluster," Oey said.

The researchers acknowledge the possibility that all of the stars they studied might not still be located in the neighborhood they were born in. Two of the stars they examined are known to be runaways that have been kicked out of their birth clusters. But in several cases, the astronomers found wisps of leftover gas nearby, strengthening the possibility that the stars are still in the isolated places where they formed.

The research is funded by NASA and the National Science Foundation.

Left: Star 302, as viewed from the ground. Right: Star 302 as viewed through the Hubble Space Telescope, which can zoom in roughly 40 times closer. From the ground, everything within the circle appears to be one star. (Credit: Courtesy of Joel Lamb)

Trace Amounts of Water Created Oceans on Earth and Other Terrestrial Planets, Study Suggests.

One question that has baffled planetary scientists is how oceans formed on the surface of terrestrial planets like Earth -- rocky planets made of silicate and metals. It's believed that in addition to Earth, the terrestrial planets Mars and Venus may have had oceans soon after their formation. There is ample evidence to suggest that these planets formed from rocky clumps called planetesimals that later combined in high-energy collisions and left their surfaces covered in molten rock, or magma. It didn't take long for these magma oceans to cool, and many researchers contend that oceans of water were created later on, when icy objects like comets and asteroids deposited water on the rocky planets.

But a recent study by an MIT planetary scientist suggests that the planetesimals themselves provided the water that created oceans. As Lindy Elkins-Tanton, the Mitsui Career Development Assistant Professor of Geology in MIT's

Department of Earth, Atmospheric and Planetary Sciences, reports in a recent paper in Astrophysics and Space Science, these planetesimals contained trace amounts of water -- at least .01 to .001 percent of their total mass (scientists don't know the precise size of planetesimals, but they estimate that those that created Earth were between hundreds and thousands of kilometers in diameter). In the paper, Elkins-Tanton says it is likely that even tiny amounts of water in the planetesimals could create steam atmospheres that later cooled and condensed into liquid oceans on terrestrial planets.

"These little bits of water get processed into planets in ways we can predict," says Elkins-Tanton, who created new models to detail the chemistry and physics of planet solidification. By suggesting that the majority of rocky planets formed water oceans early in their history, her analysis could help determine which planets outside the solar system, or exoplanets, might have or have had water and would therefore be possible candidates for hosting -- or having hosted -- life. This only applies to rocky exoplanets because most of the more than 500 exoplanets discovered to date are thought to be too hot and gaseous to host life.

Cooling planets
Samples of meteorites that originated from planetesimals indicate that the rocky bodies contained tiny amounts of water. To determine what happened to the water inside the planetesimals, Elkins-Tanton examined every step of the solidification process for rocky planets in the solar system (she didn't consider gas giants like Jupiter because the physics of how these planets form is entirely different). While this process had been modeled previously, no one had investigated whether water in planetesimals could produce oceans.

Elkins-Tanton first modeled how magma crystallizes into minerals on a theoretical rocky planet. This allowed her to calculate how much water from the planetesimals would be captured inside those minerals, and how much would remain in the magma as it cooled. She then incorporated details about the saturation level of magma into the models and observed that any water that doesn't dissolve in the magma would form bubbles. The models revealed that as the planet cools and forms a solid mantle, the bubbles in its magma oceans would rise to form a thick, steam atmosphere covering the planet. That steam would eventually collapse to create liquid oceans.

The idea that trace amounts of water in planetesimals could give rise to vast oceans may seem far-fetched until one considers how small an ocean can be relative to the size and mass of a planet. Earth's current oceans, for instance, make up just .02 percent of the planet's mass, excluding its metal core. Thus, if the majority of the small amounts of water in a planetesimal reaches a planetary surface as its magma solidifies, this would be enough to form oceans that are similar to Earth's.

For Earth, Elkins-Tanton estimates that this process occurred within tens of millions of years after the planetesimals crashed together, meaning that the planet could have been habitable pretty soon after it formed. She predicts that the same process could take up to hundreds of millions of years for super-Earths, or exoplanets that are at least twice as big as Earth and are just now being discovered. Because the research suggests that rocky super-Earths should have grown oceans soon after they formed, and because water is required for life as we know it, it's possible that these planets may have hosted -- or even still host -- life.

The life of oceans
"The study gives us a very important starting point for understanding the evolution and history of planets," says Pin Chen, a research scientist at NASA's Jet Propulsion Laboratory, who studies planetary atmospheres. He is confident that the research can be used to make predictions about oceans on exoplanets because "it is so well-grounded in fundamental principles of physics, chemistry and thermal dynamics."

Although the analysis suggests that oceans are expected to be prevalent in the early history of a rocky planet, it doesn't provide details about how long these oceans would last, which Chen says is critical for figuring out what happened to the oceans that may have covered Mars and Venus. Because atmospheres are responsible for releasing water from oceans into space, he suggests additional modeling of the interactions between the atmosphere and mantle of a young rocky planet.

In future work, Elkins-Tanton plans to model the chemistry of these atmospheres to figure out what kinds of atmospheres could be created by the solidification process, such as an oxidizing atmosphere (contains oxygen) or a reducing atmosphere (contains hydrogen). She's also interested in determining what conditions other than a liquid ocean might help initiate life on a terrestrial planet.

'Greener' Climate Prediction Shows Plants Slow Warming

The cooling effect would be -0.3 degrees Celsius (C) (-0.5 Fahrenheit (F)) globally and -0.6 degrees C (-1.1 F) over land, compared to simulations where the feedback was not included, said Lahouari Bounoua, of Goddard Space Flight Center, Greenbelt, Md. Bounoua is lead author on a paper detailing the results published Dec. 7 in the journal Geophysical Research Letters.

Without the negative feedback included, the model found a warming of 1.94 degrees C globally when carbon dioxide was doubled.

Bounoua stressed that while the model's results showed a negative feedback, it is not a strong enough response to alter the global warming trend that is expected. In fact, the present work is an example of how, over time, scientists will create more sophisticated models that will chip away at the uncertainty range of climate change and allow more accurate projections of future climate.
"This feedback slows but does not alleviate the projected warming," Bounoua said.

To date, only some models that predict how the planet would respond to a doubling of carbon dioxide have allowed for vegetation to grow as a response to higher carbon dioxide levels and associated increases in temperatures and precipitation.
Of those that have attempted to model this feedback, this new effort differs in that it incorporates a specific response in plants to higher atmospheric carbon dioxide levels. When there is more carbon dioxide available, plants are able to use less water yet maintain previous levels of photosynthesis. The process is called "down-regulation." This more efficient use of water and nutrients has been observed in experimental studies and can ultimately lead to increased leaf growth. The ability to increase leaf growth due to changes in photosynthetic activity was also included in the model. The authors postulate that the greater leaf growth would increase evapotranspiration on a global scale and create an additional cooling effect.

"This is what is completely new," said Bounoua, referring to the incorporation of down-regulation and changed leaf growth into the model. "What we did is improve plants' physiological response in the model by including down-regulation. The end result is a stronger feedback than previously thought."
The modeling approach also investigated how stimulation of plant growth in a world with doubled carbon dioxide levels would be fueled by warmer temperatures, increased precipitation in some regions and plants' more efficient use of water due to carbon dioxide being more readily available in the atmosphere. Previous climate models have included these aspects but not down-regulation. The models without down-regulation projected little to no cooling from vegetative growth.

Scientists agree that in a world where carbon dioxide has doubled -- a standard basis for many global warming modeling simulations -- temperature would increase from 2 to 4.5 degrees C (3.5 to 8.0 F). (The model used in this study found warming -- without incorporating the plant feedback -- on the low end of this range.) The uncertainty in that range is mostly due to uncertainty about "feedbacks" -- how different aspects of the Earth system will react to a warming world, and then how those changes will either amplify (positive feedback) or dampen (negative feedback) the overall warming.
An example of a positive feedback would be if warming temperatures caused forests to grow in the place of Arctic tundra. The darker surface of a forest canopy would absorb more solar radiation than the snowy tundra, which reflects more solar radiation. The greater absorption would amplify warming. The vegetative feedback modeled in this research, in which increased plant growth would exert a cooling effect, is an example of a negative feedback. The feedback quantified in this study is a result of an interaction between all these aspects: carbon dioxide enrichment, a warming and moistening climate, plants' more efficient use of water, down-regulation and the ability for leaf growth.
This new paper is one of many steps toward gradually improving overall future climate projections, a process that involves better modeling of both warming and cooling feedbacks.

"As we learn more about how these systems react, we can learn more about how the climate will change," said co-author Forrest Hall, of the University of Maryland-Baltimore County and Goddard Space Flight Center. "Each year we get better and better. It's important to get these things right just as it's important to get the track of a hurricane right. We've got to get these models right, and improve our projections, so we'll know where to most effectively concentrate mitigation efforts."

The results presented here indicate that changes in the state of vegetation may already be playing a role in the continental water, energy and carbon budgets as atmospheric carbon dioxide increases, said Piers Sellers, a co-author from NASA's Johnson Space Center, Houston, Texas.
"We're learning more and more about how our planet really works," Sellers said. "We have suspected for some time that the connection between vegetation photosynthesis and the surface energy balance could be a significant player in future climate. This study gives us an indication of the strength and sign of one of these biosphere-atmosphere feedbacks."

A new NASA modeling effort found that in a doubled-carbon dioxide world plant growth could lessen global warming by about 0.3 degrees C globally. The same model found that the world would warm by 1.94 degrees C without this cooling feedback factored in. Image: Great Smoky Mountains National Park. (Credit: National Park Service)