* Astronomy

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The Life Cycles of Stars by Lina Awadallah

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Star-Forming Regions

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RE: Star Formation

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Title: Spiral arm triggering of star formation
Authors: Authors: Ian A. Bonnell

We present numerical simulations of the passage of clumpy gas through a galactic spiral shock, the subsequent formation of giant molecular clouds (GMCs) and the triggering of star formation. The spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. In addition to triggering the star formation process, the clumpy gas passing through the shock naturally generates the observed velocity dispersion size relation of molecular clouds. In this scenario, the internal motions of GMCs need not be turbulent in nature. The coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Globally unbound molecular clouds provides a simple explanation of the low efficiency of star formation. While dense regions in the shock become bound and collapse to form stars, the majority of the gas disperses as it leaves the spiral arm.

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Star-forming Regions

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RE: Star Formation

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New findings from a large survey of galaxies suggest that star formation is largely driven by the supply of raw materials, rather than by galactic mergers that triggers sudden bursts of star formation. Stars form when clouds of gas and dust collapse under the force of gravity, and the study supports a scenario in which exhaustion of a galaxy's gas supply leads to a gradual decline in the star-formation rate.

The results, presented this week at the American Astronomical Society (AAS) meeting in Washington, D.C., come from the Extended Groth Strip Survey, a collaborative effort using major ground-based and space-based telescopes to focus on one patch of sky that offers a clear view of the distant universe.

By analysing data from a combination of powerful instruments, researchers derived information on galaxy weights and star formation rates, as well as the numbers of stars already formed, for more than 3,500 galaxies. They found that the weight (or mass) of a galaxy is an important factor determining how fast it makes stars and how the star formation rate evolves over time.

These images show parts of the Extended Groth Strip, a sky region in the vicinity of the Big Dipper.
Credit: The DEEP2 Team, UC Berkeley and UC Santa Cruz, J. Lotz and K. Noeske, NASA Hubble Space Telescope.

"The picture we're getting is that heavy galaxies form stars early and rapidly, whereas smaller galaxies form their stars over longer timescales. What we see is consistent with mostly undisturbed galaxies using up their gas over time, like firewood burning down" - Kai Noeske, a postdoctoral researcher at the University of California, Santa Cruz, who presented the group's findings at the AAS meeting on Monday, January 9, 2006.

The study's findings shed light on ongoing debates over the physical mechanisms that activate star formation in galaxies--in particular, the importance of starbursts triggered by mergers of similar galaxies.
The Extended Groth Strip collaboration consists of astronomers from 16 institutions who have pooled their data and resources to create what is now one of the most intensely studied regions of the sky.
Light from distant galaxies takes billions of years to reach Earth, giving astronomers a window into the past. The galaxies included in this study cover a wide range of redshifts (a measure of distance) and corresponding "look back times," extending out to redshift 1.4 or as far back in time as 9 billion years, about two-thirds of the age of the universe. The study also encompassed galaxies with a wide range of masses.

"We have now been able to track star formation in galaxies out to modest distances, more than half the age of the universe, and we find that all galaxies, big or small, seem to be fading gradually so that they are less active today than they were further back in time" - David Koo, professor of astronomy and astrophysics at UCSC and a member of the team.

Astronomers have found from previous galaxy surveys that star formation activity becomes more intense as they probe farther back in time. One proposed explanation has been that galaxy mergers were more frequent in the past, triggering bursts of star formation due to compression of gas clouds during the merger process.

"We are finding that mergers do not appear to play the dominant role in star formation, because we see normal-looking, undisturbed galaxies that are undergoing large amounts of star formation. There probably are multiple mechanisms that can activate star formation. We are asking which is dominant. Mergers do drive star formation; they just don't seem to be the major driver" - David Koo.

Koo and Noeske are both members of the DEEP2 team, one of seven survey teams involved in the Extended Groth Strip Survey. DEEP (Deep Extragalactic Evolutionary Probe) began about 15 years ago, led by Koo and other UCSC astronomers using the twin 10-meter Keck Telescopes at the W. M. Keck Observatory in Hawaii and NASA's Hubble Space Telescope to conduct a large-scale survey of distant field galaxies. Phase 2 of the project, led by UCSC and UC Berkeley, began three years ago using the powerful DEIMOS spectrograph on the Keck II Telescope and has now gathered spectroscopic data from almost 40,000 distant galaxies.

DEEP2 has observed 13,000 galaxies in the Extended Groth Strip, one of four fields surveyed by the project. Joining the DEEP2 team in the Extended Groth Strip Survey is a broad consortium of other survey teams that are contributing data. Infrared data from NASA's Spitzer Space Telescope were especially important for Noeske's study, because they enable astronomers to see through the dust that obscures much of the star formation taking place in distant galaxies.

"Having the infrared data from Spitzer allows us to measure the star formation rates very accurately because we are no longer blinded by dust. This is an exceptional period of time for astronomy, because for the first time we are able to combine data from almost all of the important wavelengths" - David Koo.

The array of instruments trained on the Extended Groth Strip covers a tremendous range of wavelengths, including x-rays and radio waves, as well as infrared, visible, and ultraviolet light.

This work is linked to other projects that analyse Extended Groth Strip data. The same session at the AAS meeting includes a presentation by Kevin Bundy of the California Institute of Technology on how the termination of star formation is related to a galaxy's weight and environment. Projects led by Jennifer Lotz of the National Optical Astronomy Observatory (NOAO) and Lihwai Lin of National Taiwan University measure the frequency of galaxy mergers and their importance in the production of new stars over the past 8 billion years.

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Astrophysicists at the University of California, Berkeley, and Lawrence Livermore National Laboratory (LLNL) have exploded one of two competing theories about how stars form inside immense clouds of interstellar gas.

Using supercomputer simulations that take into account the turbulence within a cloud collapsing to form a star, the researchers conclude that the "competitive accretion" model cannot explain what astronomers observe of star-forming regions studied to date.

That model, which is less than 10 years old and is championed by some British astronomers, predicts that interstellar hydrogen clouds develop clumps in which several small cores - the seeds of future stars - form. These cores, less than a light year across, collapse under their own gravity and compete for gas in the surrounding clump, often gaining 10 to 100 times their original mass from the clump.

The alternative model, often termed the "gravitational collapse and fragmentation" theory, also presumes that clouds develop clumps in which proto-stellar cores form. But in this theory, the cores are large and, though they may fragment into smaller pieces to form binary or multiple star systems, contain nearly all the mass they ever will.

"In competitive accretion, the cores are seeds that grow to become stars; in our picture, the cores turn into the stars. The observations to date, which focus primarily on regions of low-mass star formation, like the sun, are consistent with our model and inconsistent with theirs" - Chris McKee, professor of physics and of astronomy at UC Berkeley.

"Competitive accretion is the big theory of star formation in Europe, and we now think it's a dead theory" - Richard Klein, an adjunct professor of astronomy at UC Berkeley and a researcher at LLNL.

Mark R. Krumholz, now a post-doctoral fellow at Princeton University, McKee and Klein report their findings in the Nov. 17 issue of Nature.
Both theories try to explain how stars form in cold clouds of molecular hydrogen, perhaps 100 light years across and containing 100,000 times the mass of our sun. Such clouds have been photographed in brilliant colour by the Hubble and Spitzer space telescopes, yet the dynamics of a cloud's collapse into one or many stars is far from clear. A theory of star formation is critical to understanding how galaxies and clusters of galaxies form.

"Star formation is a very rich problem, involving questions such as how stars like the sun formed, why a very large number of stars are in binary star systems, and how stars ten to a hundred times the mass of the sun form. The more massive stars are important because, when they explode in a supernova, they produce most of the heavy elements we see in the material around us" - Chris McKee.

The competitive accretion model was hatched in the late 1990s in response to problems with the gravitational collapse model, which seemed to have trouble explaining how large stars form. In particular, the theory couldn't explain why the intense radiation from a large protostar doesn't just blow off the star's outer layers and prevent it from growing larger, even though astronomers have discovered stars that are 100 times the mass of the sun.
While theorists, among them McKee, Klein and Krumholz, have advanced the gravitational collapse theory farther toward explaining this problem, the competitive accretion theory has come increasingly into conflict with observations. For example, the accretion theory predicts that brown dwarfs, which are failed stars, are thrown out of clumps and lose their encircling disks of gas and dust. In the past year, however, numerous brown dwarfs have been found with planetary disks.

"Competitive accretion theorists have ignored these observations. The ultimate test of any theory is how well it agrees with observation, and here the gravitational collapse theory appears to be the clear winner" - Richard Klein.

The model used by Krumholz, McKee and Klein is a supercomputer simulation of the complicated dynamics of gas inside a swirling, turbulent cloud of molecular hydrogen as it accretes onto a star. Theirs is the first study of the effects of turbulence on the rate at which a star accretes matter as it moves through a gas cloud, and it demolishes the "competitive accretion" theory.
Employing 256 parallel processors at the San Diego Supercomputer Centre at UC San Diego, they ran their model for nearly two weeks to show that it accurately represented star formation dynamics.

"For six months, we worked on very, very detailed, high-resolution simulations to develop that theory. Then, having that theory in hand, we applied it to star forming regions with the properties that one could glean from a star forming region" - Richard Klein.

The models, which also were run on supercomputers at Lawrence Berkeley National Laboratory and LLNL, showed that turbulence in the core and surrounding clump would prevent accretion from adding much mass to a protostar.

"We have shown that, because of turbulence, a star cannot efficiently accrete much more mass from the surrounding clump. In our theory, once a core collapses and fragments, that star basically has all the mass it is ever going to have. If it was born in a low-mass core, it will end up being a low-mass star. If it's born in a high mass core, it may become a high-mass star"- Richard Klein.

A slice through a 3-D simulation of a turbulent clump of molecular hydrogen, with the densest areas shown in red. The zoom-in shows a protostar accreting gas and creating a dense wake behind it. The simulation shows that a protostar, once formed, cannot accrete much more gas from the surrounding clump, contradicting the competitive accretion theory.
(Credit: Mark Krumholz)

McKee noted that the researchers' supercomputer simulation indicates competitive accretion may work well for small clouds with very little turbulence, but these rarely, if ever, occur and have not been observed to date. Real star formation regions have much more turbulence than assumed in the accretion model, and the turbulence does not quickly decay, as that model presumes. Some unknown processes, perhaps matter flowing out of protostars, keep the gases roiled up so that the core does not collapse quickly.

"Turbulence opposes gravity; without it, a molecular cloud would collapse far more rapidly than observed. Both theories assume turbulence is there. The key is (that) there are processes going on as stars begin to form that keep turbulence alive and prevent it from decaying. The competitive accretion model doesn't have any way to put this into the calculations, which means they're not modelling real star forming regions"- Richard Klein.

Klein, McKee and Krumholz continue to refine their model to explain how radiation from large protostars escapes without blowing away all the infalling gas. For example, they have shown that some of the radiation can escape through cavities created by the jets observed to come out the poles of many stars in formation. Many predictions of the theory may be answered by new and larger telescopes now under construction, in particular the sensitive, high-resolution ALMA telescope being constructed in Chile by a consortium of United States, European and Japanese astronomers, McKee said.
The work was supported by the National Aeronautics and Space Administration, the National Science Foundation and the Department of Energy.

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-- Edited by Blobrana at 17:50, 2005-11-17