In a star's final days, astronomers hunt 'signal of impending doom'

ScienceDaily (Nov. 30, 2011) — An otherwise nondescript binary star system in the Whirlpool Galaxy has brought astronomers tantalizingly close to their goal of observing a star just before it goes supernova.

The study, submitted in a paper to the Astrophysical Journal, provides the latest result from an Ohio State University galaxy survey underway with the Large Binocular Telescope, located in Arizona.

In the first survey of its kind, the researchers have been scanning 25 nearby galaxies for stars that brighten and dim in unusual ways, in order to catch a few that are about to meet their end. In the three years since the study began, this particular unnamed binary system in the Whirlpool Galaxy was the first among the stars they've cataloged to produce a supernova.

The astronomers were trying to find out if there are patterns of brightening or dimming that herald the end of a star's life. Instead, they saw one star in this binary system dim noticeably before the other one exploded in a supernova during the summer of 2011.

Though they're still sorting through the data, it's likely that they didn't get any direct observations of the star that exploded -- only its much brighter partner.

Yet, principal investigator Christopher Kochanek, professor of astronomy at Ohio State and the Ohio Eminent Scholar in Observational Cosmology, does not regard this first result as a disappointment. Rather, it's a proof of concept.

"Our underlying goal is to look for any kind of signature behavior that will enable us to identify stars before they explode," he said. "It's a speculative goal at this point, but at least now we know that it's possible."

"Maybe stars give off a clear signal of impending doom, maybe they don't," said study co-author Krzystof Stanek, professor of astronomy at Ohio State, "But we'll learn something new about dying stars no matter the outcome."

Postdoctoral researcher Dorota Szczygiel, who led the study of this supernova, explained why the galaxy survey is important.

"The odds are extremely low that we would just happen to be observing a star for several years before it went supernova. We would have to be extremely lucky," she said.

"With this galaxy survey, we're making our own luck. We're studying all the variable stars in 25 galaxies, so that when one of them happens go supernova, we've already compiled data on it." The supernova, labeled 2011dh, was first detected on May 31 and is still visible in telescopes. It originated from a binary star system in the Whirlpool Galaxy -- also known as M51, one of the galaxies that the Ohio State astronomers have been observing for three years.

The system is believed to have contained one very bright blue star and one even brighter red star. From what the astronomers can tell, it's likely that the red star is the one that dimmed over the three years, before the blue star initiated the supernova.

When the Ohio State researchers reviewed the Large Binocular Telescope data as well as Hubble Space Telescope images of M51, they saw that the red star had dimmed by about 10 percent over three years, at a pace of three percent per year.

Szczygiel believes that the red star likely survived its partner's supernova.

"After the light from the explosion fades away, we should be able to see the companion that did not explode," she said.

As astronomers gather data from more supernovae -- Kochanek speculates that as many as one per year could emerge from their data set -- they could assemble a kind of litmus test to predict whether a particular star is near death. Whether it's going to spawn a supernova or shrink into a black hole, there may be particular signals visible on the surface, and this study has shown that those signals are detectable.

The team won't be watching our sun for any changes, however. At less than 10 percent of the mass of the star in supernova 2011dh, our star will most likely meet a very boring end.

"There'll be no supernova for our sun -- it'll just fizzle out," Kochanek said. "But that's okay -- you don't want to live around an exciting star."

This research was supported by the National Science Foundation.

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Astronomers look to neighboring galaxy for star formation insight

ScienceDaily (Nov. 30, 2011) — An international team of astronomers has mapped in detail the star-birthing regions of the nearest star-forming galaxy to our own, a step toward understanding the conditions surrounding star creation.

Led by University of Illinois astronomy professor Tony Wong, the researchers published their findings in the December issue of the Astrophysical Journal Supplement Series.

The Large Magellanic Cloud (LMC) is a popular galaxy among astronomers both for its nearness to our Milky Way and for the spectacular view it provides, a big-picture vista impossible to capture of our own galaxy.

"If you imagine a galaxy being a disc, the LMC is tilted almost face-on so we can look down on it, which gives us a very clear view of what's going on inside," Wong said.

Although astronomers have a working theory of how individual stars form, they know very little about what triggers the process or the environmental conditions that are optimal for star birth. Wong's team focused on areas called molecular clouds, which are dense patches of gas -- primarily molecular hydrogen -- where stars are born. By studying these molecular clouds and their relationship to new stars in the galaxy, the team hopes to learn more about the metamorphosis of gas clouds into stars.

"When we study star formation, an important question is, what is the environment doing? How does the location of star formation reflect the conditions of that environment? There's no better place to study the wider environment than the LMC."

Using a 22-meter-diameter radio telescope in Australia, the astronomers mapped more than 100 molecular clouds in the LMC and estimated their sizes and masses, identifying regions with ample material for making stars. This seemingly simple task engendered a surprising find.

Conventional wisdom states that most of the molecular gas mass in a galaxy is apportioned to a few large clouds. However, Wong's team found many more low-mass clouds than they expected -- so many, in fact, that a majority of the dense gas may be sprinkled across the galaxy in these small molecular clouds, rather than clumped together in a few large blobs.

"We thought that the big clouds hog most of the mass," Wong said, "but we found that in this galaxy, it appears that the playing field is more level. The low-mass clouds are quite numerous and they actually contribute a significant amount of the mass. This provides the first evidence that the common wisdom about molecular clouds may not apply here."

The large numbers of these relatively low-mass clouds means that star-forming conditions in the LMC may be relatively widespread and easy to achieve. The findings raise some interesting questions about why some galaxies stopped their star formation while others have continued it.

To better understand the connection between molecular clouds and star formation, the team compared their molecular cloud maps to maps of infrared radiation, which reveal where young stars are heating cosmic dust.

For the comparison, they exploited a carefully selected sample of newborn heavy stars compiled by U. of I. astronomy professor You-Hua Chu and resident scientist Robert Gruendl, who also were co-authors of the paper. These stars are so young that they are still deeply embedded in cocoons of gas and dust.

"It turns out that there's actually very nice correspondence between these young massive stars and molecular clouds," Wong said. "That's not entirely surprising, but it's reassuring. We assume that these stars have to form in molecular clouds, and it tells us that the molecular clouds do hang around long enough for us to see them associated with these massive young stars."

Wong hopes to continue to study the relationship between molecular clouds and star formation in greater detail. If researchers can determine the relative ages of young stars, they can correlate these against molecular clouds to figure out which clouds have star formation, how long the clouds live and what eventually leads to their destruction. They also plan to use a newly constructed array of telescopes in Chile to see the cloud environment in higher resolution, pinpointing exactly where inside the molecular cloud star formation will occur.

"This study provides us with our most detailed view of an entire population of clouds in another galaxy," Wong said. "We can say with great confidence that these clouds are where the stars form, but we are still trying to figure out why they have the properties they do."

The National Science Foundation and NASA supported this work.

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The above story is reprinted from materials provided by University of Illinois at Urbana-Champaign.

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Journal Reference:

Tony Wong, Annie Hughes, Jürgen Ott, Erik Muller, Jorge L. Pineda, Jean-Philippe Bernard, You-Hua Chu, Yasuo Fukui, Robert A. Gruendl, Christian Henkel, Akiko Kawamura, Ulrich Klein, Leslie W. Looney, Sarah Maddison, Yoji Mizuno, Deborah Paradis, Jonathan Seale. The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud. The Astrophysical Journal Supplement Series, 2011; 197 (2): 16 DOI: 10.1088/0067-0049/197/2/16

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Astronomers discover new exoplanet similar in size to Earth; Planet Kepler-21b found using space and ground-based observations

ScienceDaily (Nov. 30, 2011) — The NASA Kepler Mission is designed to survey a portion of our region of the Milky Way Galaxy to discover Earth-size planets in or near the "habitable zone," the region in a planetary system where liquid water can exist, and determine how many of the billions of stars in our galaxy have such planets. It now has another planet to add to its growing list.

A research team led by Steve Howell, of NASA's Ames Research Center, has shown that one of the brightest stars in the Kepler star field has a planet with a radius only 1.6 that of Earth's radius and a mass no greater that 10 Earth masses, circling its parent star with a 2.8-day period. With such a short period, and such a bright star, the team of over 65 astronomers -- which included David Silva, Ken Mighell and Mark Everett of the National Optical Astronomy Observatory (NOAO) -- needed multiple telescopes on the ground to support and confirm their Kepler observations. These included the 4-meter Mayall telescope and the WIYN telescope at Kitt Peak National Observatory.

With a period of only 2.8 days, this planet, designated Kepler-21b, is only about 6 million kilometers away from its parent star. By comparison Mercury, the closest planet to the sun, has a period of 88 days and a distance from the sun almost ten times greater, or 57 million km. So Kepler 21b is far hotter than any place humans could venture. The team calculates that the temperature at the surface of the planet is about 1900 K, or 2960 F. While this temperature is nowhere near the habitable zone in which liquid water might be found, the planet's size is approaching that of Earth.

The parent star, HD 179070, is quite similar to our sun: its mass is 1.3 solar masses, its radius is 1.9 solar radii, and its age, based on stellar models, is 2.84 billion years (or a bit younger than the sun's 4.6 billion years). HD 179070 is spectral type F6 IV, a little hotter and brighter than the sun. By astronomical standards, HD 179070 is fairly close, at a distance from the sun of 352 light years. While it cannot be seen by the unaided eye, a small telescope can easily pick it out.

Part of the difficulty in detecting this planet is the realization, from the Kepler mission, that many stars show short period brightness oscillations. The effect of these must be removed from the stellar light in order to uncover the regular, but very small, dimming caused by the planet passing in front of the star. The Kepler mission observed this field for over 15 months, and the team combined the observations to enable them to detect this tiny, periodic signal. They also relied on spectroscopic and imaging data from a number of ground based telescopes.

The results of this work have been accepted for publication in the Astrophysical Journal.

NOAO is operated by Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

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Astronomers pin down galaxy collision rates by comparing Hubble images to supercomputer simulations

ScienceDaily (Oct. 27, 2011) — A new analysis of images from the Hubble Space Telescope combined with supercomputer simulations of galaxy collisions has cleared up years of confusion about the rate at which smaller galaxies merge to form bigger ones. This paper, led by Jennifer Lotz of Space Telescope Science Institute, is about to be published in The Astrophysical Journal.

Galaxies grow mostly by acquiring small amounts of matter from their surroundings. But occasionally galaxies merge with other galaxies large or small. Collisions between big galaxies can change rotating disk galaxies like the Milky Way into featureless elliptical galaxies, in which the stars are moving every which way.

In order to understand how galaxies have grown, it is essential to measure the rate at which galaxies merge. In the past, astronomers have used two principal techniques: counting the number of close pairs of galaxies about to collide and by counting the number of galaxies that appear to be disturbed in various ways. The two techniques are analogous to trying to estimate the number of automobile accidents by counting the number of cars on a collision course versus counting the number of wrecked cars seen by the side of the road.

However, these studies have often led to discrepant results. "These different techniques probe mergers at different 'snapshots' in time along the merger process," Lotz says. "Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers (5%) than those that searched for galaxies with disturbed shapes, evidence that they're in smashups (25%)."

In the new work, all the previous observations were reanalyzed using a key new ingredient: highly accurate computer simulations of galaxy collisions. These simulations, which include the effects of stellar evolution and dust, show the lengths of time over which close galaxy pairs and various types of galaxy disturbances are likely to be visible. Lotz's team accounted for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed the effects of different orbits for the galaxies, possible collision impacts, and how the galaxies were oriented to each other.

The simulations were done by T. J. Cox (now at Carnegie Observatories in Pasadena), Patrik Jonsson (now at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts), and Joel Primack (at the University of California, Santa Cruz -- UCSC), using small supercomputers at UCSC and the large Columbia supercomputer at NASA Ames Research Center. These simulations were "observed" as if through Hubble Space Telescope by Jennifer Lotz in a series of papers with Cox, Jonsson, and Primack that were published over the past three years. A key part of the analysis was a new way of measuring galaxy disturbances that was developed by Lotz, Primack, and Piero Madau in 2004. All this work was begun when Lotz was a postdoc with Primack, and Cox and Jonsson were his graduate students.

"Viewing the simulations was akin to watching a slow-motion car crash," Lotz says. "Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies."

"The new paper led by Jennifer Lotz for the first time makes sense of all the previous observations, and shows that they are consistent with theoretical expectations," says Primack. "This is a great example of how new astronomical knowledge is now emerging from a combination of observations, theory, and supercomputer simulations." Primack now heads the University of California High-Performance AstroComputing Center (UC-HiPACC), headquartered at the University of California, Santa Cruz.

This research was funded by grants from NASA and NSF, and Hubble Space Telescope and Spitzer Space Telescope Theory Grants

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Lotz, Jennifer M.; Jonsson, Patrik; Cox, T. J.; Croton, Darren; Primack, Joel R.; Somerville, Rachel S.; Stewart, Kyle. The Major and Minor Galaxy Merger Rates at z < 1.5. The Astrophysical Journal, 2011 [link]

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Astronomers pin down galaxy collision rate with Hubble data

ScienceDaily (Oct. 27, 2011) — A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.

The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA's Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.

The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time. Lotz's team found that galaxies gained quite a bit of mass through collisions with other galaxies. Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz's team found these mergers happened three times more often than encounters between two hefty galaxies.

"Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies," Lotz explains.

The team's results are accepted for publication appeared in The Astrophysical Journal.

The problem with previous Hubble estimates is that astronomers used different methods to count the mergers.

"These different techniques probe mergers at different 'snapshots' in time along the merger process," Lotz says. "It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them. If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they're in smashups."

To figure out how many encounters happen over time, Lotz needed to understand how long merging galaxies would look like "wrecks" before they settle down and begin to look like normal galaxies again.

That's why Lotz and her team turned to highly detailed computer simulations to help make sense of the Hubble photographs. The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.

Creating the computer models was a time-consuming process. Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. "Viewing the simulations was akin to watching a slow-motion car crash," Lotz says.

The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.

"Our simulations offer a realistic picture of mergers between galaxies," Lotz says.

In addition to studying the smashups between giant galaxies, the team also analyzed encounters among puny galaxies. Spotting collisions with small galaxies are difficult because the objects are so dim relative to their larger companions.

"Dwarf galaxies are the most common galaxy in the universe," Lotz says. "They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time."

Lotz compared her simulation images with pictures of thousands of galaxies taken from some of Hubble's largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. She and other groups had identified about a thousand merger candidates from these surveys but initially found very different merger rates.

"When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results," Lotz says.

Her next goal is to analyze galaxies that were interacting around 11 billion years ago, when star formation across the universe peaked, to see if the merger rate rises along with the star formation rate. A link between the two would mean galaxy encounters incite rapid star birth.

In addition to Lotz, the coauthors of the paper include Patrik Jonsson of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass; T. J. Cox of Carnegie Observatories in Pasadena, Calif.; Darren Croton of the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Hawthorn, Australia; Joel R. Primack of the University of California, Santa Cruz; Rachel S. Somerville of the Space Telescope Science Institute and The Johns Hopkins University in Baltimore, Md.; and Kyle Stewart of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

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