Stellar Archaeology Traces Milky Way's History

Structure of Milky Way's Halo (Artist's Illustration)

Credit: NASA, ESA, and A. Feild (STScI)

Signature of a White Dwarf

Credit: NASA,ESA, and A. Feild and J. Kalirai (STScI)

Unfortunately, stars don't have birth certificates. So, astronomers have a tough time figuring out their ages. Knowing a star's age is critical for understanding how our Milky Way galaxy built itself up over billions of years from smaller galaxies.

Jason Kalirai of the Space Telescope Science Institute and The Johns Hopkins University's Center for Astrophysical Sciences, both in Baltimore, Md., has found the next best thing to a star's birth certificate. Using a new technique, Kalirai probed the burned-out relics of Sun-like stars, called white dwarfs, in the inner region of our Milky Way galaxy's halo. The halo is a spherical cloud of stars surrounding our galaxy's disk.

Those stars, his study reveals, are 11.5 billion years old, younger than the first generation of Milky Way stars. They formed more than 2 billion years after the birth of the universe 13.7 billion years ago. Previous age estimates, based on analyzing normal stars in the inner halo, ranged from 10 billion to 14 billion years.

Kalirai's study reinforces the emerging view that our galaxy's halo is composed of a layer-cake structure that formed in stages over billions of years.

"One of the biggest questions in astronomy is, when did the different parts of the Milky Way form?" Kalirai said. "Sun-like stars live for billions of years and are bright, so they are excellent tracers, offering clues to how our galaxy evolved over time. However, the biggest hindrance we have in inferring galactic formation processes in the Milky Way is our inability to measure accurate ages of Sun-like stars. In this study, I chose a different path: I studied stars at the end of their lives to determine their masses and then connected those masses to the ages of their progenitors. Given the nature of these dead stars, their masses are easier to measure than Sun-like stars."

Kalirai targeted white dwarfs in the galaxy's halo because those stars are believed to be among the galaxy's first homesteaders. Some of them are almost as old as the universe itself. These ancient stars provide a fossil record of our Milky Way's infancy, possessing information about our galaxy's birth and growth. "The Milky Way's halo represents the premier hunting ground in which to unravel the archaeology of when and how the galaxy's assembly processes occurred," Kalirai explained.

His results will appear online May 30 in a letter to the journal Nature.

White dwarfs divulge their properties so freely because they have a distinct spectral signature. Kalirai analyzed their signatures using archival spectroscopic data from the European Southern Observatory's Very Large Telescope at the Paranal Observatory in Chile. The spectroscopic data are part of the SN Ia Progenitor Survey (SPY), a census of white dwarf stars in the Milky Way. Spectroscopy divides light into its constituent colors, yielding information about a star's characteristics, including its mass and temperature. In his study, Kalirai first analyzed the spectra of several newly minted white dwarfs in the galaxy's inner halo to measure their masses. "The hottest white dwarfs are the descendants of Sun-like stars that have just extinguished their hydrogen fuel," he explained. "The masses of these white dwarfs are proportional to the masses of their progenitors, and we can use that mass to establish the age of the parent stars."

To measure the halo's age, Kalirai compared the masses of the halo stars with those of six newly formed white dwarfs in the ancient globular star cluster M4. Fortunately, the cluster is one of Hubble's favorite targets, and astronomers have a reliable age for when it formed, 12.5 billion years ago. Kalirai found these dead cluster stars in archival visible-light images of nearly 2,000 white dwarfs taken by the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope.

He applied the same techniques that he used on the halo white dwarfs to these cluster white dwarfs. The spectroscopic observations for these stellar remnants came from the W.M. Keck Observatory in Hawaii. His measurements revealed that the halo white dwarfs are heavier than those in M4, indicating the progenitor stars that are evolving into white dwarfs today are also heavier. Therefore, these stars are younger than the M4 stars. More massive stars consume their hydrogen fuel at a faster rate and therefore end their lives more quickly than lighter-weight stars.

Although Kalirai's result is based on a small sample of stars, it does support recent work proposing that the halo is composed of two different populations of stars.

According to the research, the Milky Way's construction schedule began with the oldest globular star clusters and dwarf galaxies, which formed a few hundred million years after the big bang, settling into what is now the galaxy's halo. These populations merged over billions of years to form the structure of our Milky Way. Stars in the inner halo were born during the assembly process. Over time, the Milky Way gobbled up older dwarf galaxies that formed less than 2 billion years after the big bang. Their ancient stars settled into the outskirts of the halo, creating the outer halo.

"In the previous work, the inner population was shown to be different from the outer population in terms of the velocities and chemical abundances of the stars," Kalirai said. "There were no constraints, however, on whether there was an age difference between the two populations. Now, our work suggests an age for the inner halo stars.

"We know some of the remote globular clusters in the outer halo are much older than the inner halo stars, perhaps around 13.5 billion years old," Kalirai contined. "So, our prediction is that if you find white dwarfs in the outer halo, they would have formed from older generations of Sun-like stars. The present day masses of stars in the generation that are now forming white dwarfs would be lower, and therefore the white dwarf masses — which we can measure — will also be lower."

Kalirai hopes to apply his new technique on more halo white dwarfs in his quest to help uncover our galaxy's history.

"One of the interesting questions about the inner halo stars is, did all of them form at the same time, or did they form over a span of time?" Kalirai said. "A sample of 20 to 30 white dwarfs would allow us to see if the inferred ages from the white dwarf masses span from 11 billion to 13 billion years. That could tell us that the accretion events that helped build up the Milky Way kept happening for several billion years, as opposed to all predominantly happening at one epoch."

CONTACT

Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
410-338-4493
dweaver@stsci.edu

Jason Kalirai
Space Telescope Science Institute, Baltimore, Md.
410-338-4747
jkalirai@stsci.edu

More about → Stellar Archaeology Traces Milky Way's History

One Supernova Type, Two Different Sources

The Tycho supernova remnant is the result of a Type Ia supernova explosion. The explosion was observed by Danish astronomer Tycho Brahe in 1572. More than 400 years later, the ejecta from that explosion has expanded to fill a bubble 55 light-years across. In this image, low-energy X-rays (red) show expanding debris from the supernova explosion and high energy X-rays (blue) show the blast wave - a shell of extremely energetic electrons. Credit: X-ray: NASA/CXC/Rutgers/K.Eriksen et al.; Optical: DSS. Low Resolution Image (jpg)

Cambridge, MA - The exploding stars known as Type Ia supernovae serve an important role in measuring the universe, and were used to discover the existence of dark energy. They're bright enough to see across large distances, and similar enough to act as a "standard candle" - an object of known luminosity. The 2011 Nobel Prize in Physics was awarded for the discovery of the accelerating universe using Type Ia supernovae. However, an embarrassing fact is that astronomers still don't know what star systems make Type Ia supernovae.

Two very different models explain the possible origin of Type Ia supernovae, and different studies support each model. New evidence shows that both models are correct - some of these supernovae are created one way and some the other.

"Previous studies have produced conflicting results. The conflict disappears if both types of explosion are happening," explained Smithsonian astronomer and Clay Fellow Ryan Foley (Harvard-Smithsonian Center for Astrophysics).

Type Ia supernovae are known to originate from white dwarfs - the dense cores of dead stars. White dwarfs are also called degenerate stars because they're supported by quantum degeneracy pressure.

In the single-degenerate model for a supernova, a white dwarf gathers material from a companion star until it reaches a tipping point where a runaway nuclear reaction begins and the star explodes. In the double-degenerate model, two white dwarfs merge and explode. Single-degenerate systems should have gas from the companion star around the supernova, while the double-degenerate systems will lack that gas.

"Just like mineral water can be with or without gas, so can supernovae," said Robert Kirshner, Clowes Professor of Astronomy at Harvard University and a co-author on the study.

Foley and his colleagues studied 23 Type Ia supernovae to look for signatures of gas around the supernovae, which should be present only in single-degenerate systems. They found that the more powerful explosions tended to come from "gassy" systems, or systems with outflows of gas. However, only a fraction of supernovae show evidence for outflows. The remainder seem to come from double-degenerate systems.

"There are definitely two kinds of environments - with and without outflows of gas. Both are found around Type Ia supernovae," Foley said.

This finding has important implications for measurements of dark energy and the expanding universe. If two different mechanisms are at work in Type Ia supernovae, then the two types must be considered separately when calculating cosmic distances and expansion rates.

"It's like measuring the universe with a mix of yardsticks and meter sticks - you'll get about the same answer, but not quite. To get an accurate answer, you need to separate the yardsticks from the meter sticks," Foley explained.

This study raises an interesting question - if two different mechanisms create Type Ia supernovae, why are they homogeneous enough to serve as standard candles?

"How can supernovae coming from different systems look so similar? I don't have the answer for that," said Foley.

The paper describing this research will appear in the Astrophysical Journal and is available online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

More about → One Supernova Type, Two Different Sources

Four white dwarf stars caught in the act of consuming 'earth-like' exoplanets

Rocky material in orbit around a white dwarf star (centre). Collisions turn larger material into dust, some of which then rains down on to the white dwarf. Credit: © Mark A. Garlick / space-art.co.uk / University of Warwick

University of Warwick astrophysicists have pinpointed four white dwarf stars surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth. The scientists publish their results in a paper in the journal Monthly Notices of the Royal Astronomical Society.

White dwarfs are the final stage of life of stars like our Sun, the residual cores of material left behind after their available fuel for nuclear reactions has been exhausted. Using the Hubble Space Telescope to carry out the biggest survey to date of the chemical composition of the atmospheres of white dwarf stars, the researchers found that the most frequently occurring elements in the dust around these four white dwarfs were oxygen, magnesium, iron and silicon – the four elements that make up roughly 93 per cent of the Earth.

However an even more significant observation was that this material also contained an extremely low proportion of carbon, which matched very closely that of the Earth and the other rocky planets orbiting closest to our own Sun.

This is the first time that such low proportions of carbon have been measured in the atmospheres of white dwarf stars polluted by debris. Not only is this clear evidence that these stars once had at least one rocky exoplanet which they have now destroyed, the observations must also pinpoint the last phase of the death of these worlds.

The atmosphere of a white dwarf is made up of hydrogen and/or helium, so any heavy elements that come into their atmosphere are dragged downwards to their core and out of sight within a matter of days by the dwarf’s high gravity. Given this, the astronomers must literally be observing the final phase of the death of these worlds as the material rains down on the stars at rates of up to 1 million kilograms every second.

Not only is this clear evidence that these stars once had rocky exoplanetary bodies which have now been destroyed, the observations of one particular white dwarf, PG0843+516, may also tell the story of the destruction of these worlds.

This star stood out from the rest owing to the relative overabundance of the elements iron, nickel and sulphur in the dust found in its atmosphere. Iron and nickel are found in the cores of terrestrial planets, as they sink to the centre owing to the pull of gravity during planetary formation, and so does sulphur thanks to its chemical affinity to iron.

Therefore, researchers believe they are observing White Dwarf PG0843+516 in the very act of swallowing up material from the core of a rocky planet that was large enough to undergo differentiation, similar to the process that separated the core and the mantle of the Earth.

Professor Boris Gänsicke of the Department of Physics at the University of Warwick, who led the study, said the destructive process which caused the discs of dust around these distant white dwarfs is likely to one day play out in our own solar system.

“What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth. As stars like our Sun reach the end of their life, they expand to become red giants when the nuclear fuel in their cores is depleted.

‘When this happens in our own solar system, billions of years from now, the Sun will engulf the inner planets Mercury and Venus. It’s unclear whether the Earth will also be swallowed up by the Sun in its red giant phase - but even if it survives, its surface will be roasted.

‘During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our solar systems.

‘This may even shatter entire terrestrial planets, forming large amounts of asteroids, some of which will have chemical compositions similar to those of the planetary core. In our solar system, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf.

‘It is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet.”

The University of Warwick led team surveyed more than 80 white dwarfs within a few hundred light years of the Sun, using the Cosmic Origin Spectrograph onboard the Hubble Space Telescope.


Images and captions

The following high resolution artist impressions are available. They were all created for the University of Warwick by the space artist Mark A. Garlick and are free for use by media or the University but are otherwise copyright as follows: “© Mark A. Garlick / space-art.co.uk / University of Warwick”

First Artist’s impression by Mark A. Garlick

http://bit.ly/KornIK The inner region of an exo-planetary system where four terrestrial planets orbit a solar-like star.

Second artist’s impression by Mark A. Garlick

http://bit.ly/ItlgIP The host star is running out of hydrogen in the core, swells up, and its surface becomes cooler. It is also losing mass, which causes the planets to move further out. The perturbation of the orbits may lead to collisions that will generate large amounts of rocky debris.

Third artist’s impression by Mark A. Garlick

http://bit.ly/IEhrxJ This depicts what the researchers are now observing. A white dwarf sits in the centre of the remnant of a planetary system. Asteroid sized debris is scattered inwards by interaction with the remaining planets and is tidally disrupted as it approaches the white dwarf forming a disc of dust some of which is raining down onto the star. The researchers have found that the composition of the debris that has just fallen onto the four white dwarfs matches the composition of Earth-like rocky worlds.

Image that brings together all three artist’s impressions by Mark A. Garlick together in one sequence http://bit.ly/K02jev

Science contact

Professor Boris Gänsicke, Department of Physics University of Warwick
Tel: +44 (0)2476 574741
Email: boris.gaensicke@warwick.ac.uk


Media contacts

Anna Blackaby
University of Warwick Science Press Officer
Tel: +44 (0)2476 575910
Mob: +44 (0) 7785 433155
Email: a.blackaby@warwick.ac.uk

Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: rm@ras.org.uk


Further information

The new work is published in “The chemical diversity of exo-terrestrial planetary debris around white dwarfs”, B. T. Gänsicke, D. Koester, J. Farihi, J. Girven, S.G.Parsons, E. Breedt, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper is available at http://arxiv.org/abs/1205.0167

Notes for editors


The Royal Astronomical Society

The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Follow the RAS on Twitter via @royalastrosoc

More about → Four white dwarf stars caught in the act of consuming 'earth-like' exoplanets

Closest Type Ia Supernova in Decades Solves a Cosmic Mystery

Early close-ups of a Type Ia supernova allow Berkeley Lab scientists and their colleagues to picture its progenitor and infer how it exploded

The Palomar Transient Factory caught SN 2011fe in the Pinwheel Galaxy in the vicinity of the Big Dipper on 24 August, 2011. Found just hours after it exploded and only 21 million light years away, the discovery triggered the closest-ever look at a young Type Ia supernova. (Image by B. J. Fulton, Las Cumbres Observatory Global Telescope Network. Click here for better resolution.)

Type Ia supernovae (SN Ia’s) are the extraordinarily bright and remarkably similar “standard candles” astronomers use to measure cosmic growth, a technique that in 1998 led to the discovery of dark energy – and 13 years later to a Nobel Prize, “for the discovery of the accelerating expansion of the universe.” The light from thousands of SN Ia’s has been studied, but until now their physics – how they detonate and what the star systems that produce them actually look like before they explode – has been educated guesswork.

Peter Nugent of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) heads the Computational Cosmology Center in the Lab’s Computational Research Division and also leads the Lab’s collaboration in the multi-institutional Palomar Transient Factory (PTF). On August 24 of this year, searching data as it poured into DOE’s National Energy Research Scientific Computing Center (NERSC) from an automated telescope on Palomar Mountain in California, Nugent spotted a remarkable object. It was shortly confirmed as a Type Ia supernova in the Pinwheel Galaxy, some 21 million light-years distant. That’s unusually close by cosmic standards, and the nearest SN Ia since 1986; it was subsequently given the official name SN 2011fe.

Nugent says, “We caught the supernova just 11 hours after it exploded, so soon that we were later able to calculate the actual moment of the explosion to within 20 minutes. Our early observations confirmed some assumptions about the physics of Type Ia supernovae, and we ruled out a number of possible models. But with this close-up look, we also found things nobody had dreamed of.”

“When we saw SN2011fe, I fell off my chair,” says PTF team member Mansi Kasliwal of the Carnegie Institution for Science and the California Institute of Technology. “Its brightness was too faint to be a supernova and too bright to be nova. Only follow-up observations in the next few hours revealed that this was actually an exceptionally young Type Ia supernova.”

Because they could closely study the supernova during its first few days, the team was able to gather the first direct evidence for what at least one SN Ia looked like before it exploded, and what happened next. Their results are reported in the 15 December, 2011, issue of the journal Nature.

Confirming a carbon-oxygen white dwarf

Scientists long ago developed models of Type Ia supernovae based on their evolving brightness and spectra. The models assume the progenitor is a binary system – about half of all stars are in binary systems – in which a very dense, very small white-dwarf star made of carbon and oxygen orbits a companion, from which it sweeps up additional matter. There’s a specific limit to how massive the white dwarf can grow, equal to about 1.4 times the mass of our sun, before it can no longer support itself against gravitational collapse.

“As it approaches the limit, conditions are met in the center so that the white dwarf detonates in a colossal thermonuclear explosion, which converts the carbon and oxygen to heavier elements including nickel,” says Nugent. “A shock wave rips through it and ejects the material in a bright expanding photosphere. Much of the brightness comes from the heat of the radioactive nickel as it decays to cobalt. Light also comes from ejecta being heated by the shock wave, and if this runs into the companion star it can be reheated, adding to the luminosity.”

By examining how SN 2011fe’s brightness evolved – its so-called early-time light curve – and the features of its early-time spectra, members of the PTF team were able to constrain how big the exploding star was, when it exploded, what might have happened during the explosion, and what kind of binary star system was involved.

The first observations of SN 2011fe were carried out at the Liverpool Telescope at La Palma in the Canary Islands, followed within hours by the Shane Telescope at Lick Observatory in California and the Keck I Telescope on Mauna Kea in Hawaii. These were shortly followed by NASA’s orbiting Swift Observatory.

Says Nugent, “We made an absurdly conservative assumption that the earliest luminosity was due entirely to the explosion itself and would increase over time in proportion to the size of the expanding fireball, which set an upper limit on the radius of the progenitor.”

Daniel Kasen, an assistant professor of astronomy and physics at the University of California at Berkeley and a faculty scientist in Berkeley Lab’s Nuclear Science Division, explains that “it only takes a few seconds for the shock wave to tear apart the star, but the debris heated in the explosion will continue to glow for several hours. The bigger the star, the brighter this afterglow. Because we caught this supernova so early, and with such sensitive observations, we were able to directly constrain the size of the progenitor.”

“Sure enough, it could only have been a white dwarf,” says Nugent. “The spectra gave us the carbon and oxygen, so we knew we had the first direct evidence that a Type Ia supernova does indeed start with a carbon-oxygen white dwarf.”

The expected and the unexpected

“The early-time light curve also constrained the radius of the binary system,” says Nugent, “so we got rid of a whole bunch of models,” ranging from old red giant stars to other white dwarfs in a so-called “double-degenerate” system.

Kasen explains that “if there was a giant companion star orbiting nearby, we should have seen some fireworks when the debris from the supernova crashed into it.” A red giant would have made the supernova brighter by several orders of magnitude early on. “Because we didn’t observe any bright flashes like that, we determined that the companion star could not have been much bigger than our sun.”

Nor was there much chance the companion was another white dwarf in a double-degenerate system, unless it had somehow avoided being torn apart and littering the surroundings with debris. A shock wave plowing through that kind of rubble would have produced a burst of early light the observers couldn’t have missed. So unless the companion was positioned almost exactly between the exploding star and the observers on Earth, closer to it than a 10th the diameter of our sun – an unlikely set of circumstances – the white dwarf’s companion had to be a main-sequence star.

While these observations pointed to a “normal” SN Ia, the way the white dwarf exploded held surprises. Typical of what would be expected, early spectra obtained by the Lick three-meter telescope showed many intermediate-mass elements spewing out of the expanding fireball, including ionized oxygen, magnesium, silicon, calcium, and iron, traveling 16,000 kilometers a second – more than five percent of the speed of light. Yet some oxygen was traveling much faster, at over 20,000 kilometers a second.

“The high-velocity oxygen shows that the oxygen wasn’t evenly distributed when the white dwarf blew up,” Nugent says, “indicating unusual clumpiness in the way it was dispersed.” But more interesting, he says, is that “whatever the mechanism of the explosion, it showed a tremendous amount of mixing, with some radioactive nickel mixed all the way to the photosphere. So the brightness followed the expanding surface almost exactly. This is not something any of us would have expected.”

PTF team member Mark Sullivan of the University of Oxford says, “Understanding how these giant explosions create and mix materials is important because supernovae are where we get most of the elements that make up the Earth and even our own bodies – for instance, these supernovae are a major source of iron in the universe. So we are all made of bits of exploding stars.”

“It is rare that you have eureka moments in science, but it happened four times on this supernova,” says Andy Howell, coleader of PTF’s SN Ia team: “The super-early discovery; the crazy first spectrum; when we figured out it had to be a white dwarf; and then, the Holy Grail, when we figured out details of the second star.”

Howell adds, “We’re like Captain Ahab … except our white whale is a white dwarf. We’re obsessed with proving they cause supernovae, but the evidence has been eluding us for decades.” This time, he says, “We got our whale … and we lived.”

“This first close SN Ia in the era of modern instrumentation will undoubtedly become the best-studied thermonuclear supernova in history,” the PTF team notes in their Nature paper, and “will form the new foundation upon which our knowledge of more distant Type Ia supernovae is built.”

Two decades after the Berkeley-Lab-based Supernova Cosmology Project, led by 2011 Nobel Prize-winner in Physics Saul Perlmutter, proved that Type Ia supernovae could be used to measure the expansion history of the universe, Berkeley Lab astrophysicists and computer scientists have finally gotten a close-up look at what these remarkable cosmic mileposts really look like.

###

“Supernova 2011fe from an exploding carbon-oxygen white dwarf star,” by Peter E. Nugent, Mark Sullivan, S. Bradley Cenko, Rollin C. Thomas, Daniel Kasen, D. Andrew Howell, David Bersier, Joshua S. Bloom, S. R. Kulkarni, Michael T. Kandrashoff, Alexei V. Filippenko, Jeffrey M. Silverman, Geoffrey W. Marcy, Andrew W. Howard, Howard T. Isaacson, Kate Maguire, Nao Suzuki, James E. Tarlton, Yen-Chen Pan, Lars Bildsten, Benjamin J. Fulton, Jerod T. Parrent, David Sand, Philipp Podsiadlowski, Federica B. Bianco, Benjamin Dilday, Melissa L. Graham, Joe Lyman, Phil James, Mansi M. Kasliwal, Nicholas M. Law, Robert M. Quimby, Isobel M. Hook, Emma S. Walker, Paolo Mazzali, Elena Pian, Eran O. Ofek, Avishay Gal-Yam and Dovi Poznanski, appears in the 15 December, 2011, issue of Nature. Berkeley Lab authors in addition to Peter Nugent include Rollin Thomas, Daniel Kasen, Nao Suzuki, and Dovi Poznanski.

The Palomar Transient Factory is an international collaboration of scientists and engineers from the California Institute of Technology, DOE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, NASA’s Infrared Processing and Analysis Center, the University of California at Berkeley, Las Cumbres Observatory Global Telescope Network, the University of Oxford, Columbia University, the Weizmann Institute of Science in Israel, and Pennsylvania State University. The Principal Investigator of the PTF is Caltech’s Professor S. R. Kulkarni. The High Performance Wireless Research and Education Network (HPWREN) of the University of California at San Diego’s Applied Network Research provides Palomar Observatory’s high-speed data connection. Visit the PTF website at http://www.astro.caltech.edu/ptf/

The National Energy Research Scientific Computing Center (NERSC), located at Lawrence Berkeley National Laboratory, is the primary high-performance computing facility for scientific research sponsored by the U.S. Department of Energy’s Office of Science. Visit their website at http://www.nersc.gov/

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the Unites States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at http://science.energy.gov/

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for DOE’s Office of Science. For more, visit http://www.lbl.gov

More about → Closest Type Ia Supernova in Decades Solves a Cosmic Mystery

Evolved Stars Locked in Fatalistic Dance

Two white dwarfs have been discovered on the brink of a merger. In just 900,000 years, material will start to stream from one star to the other (as shown in this artist's conception), beginning the process that may end with a spectacular supernova explosion. Watching these stars fall in will allow astronomers to test Einstein's general theory of relativity as well as the origin of a special class of supernovae. Credit: David A. Aguilar (CfA). High Resolution Image (jpg)

Cambridge, MA - White dwarfs are the burned-out cores of stars like our Sun. Astronomers have discovered a pair of white dwarfs spiraling into one another at breakneck speeds. Today, these white dwarfs are so near they make a complete orbit in just 13 minutes, but they are gradually slipping closer together. About 900,000 years from now - a blink of an eye in astronomical time - they will merge and possibly explode as a supernova. By watching the stars converge, scientists will test both Einstein's general theory of relativity and the origin of some peculiar supernovae.

The two white dwarfs are circling at a bracing speed of 370 miles per second (600 km/s), or 180 times faster than the fastest jet on Earth.

"I nearly fell out of my chair at the telescope when I saw one star change its speed by a staggering 750 miles per second in just a few minutes," said Smithsonian astronomer Warren Brown, lead author of the paper reporting the find.

The brighter white dwarf contains about a quarter of the Sun's mass compacted into a Neptune-sized ball, while its companion has more than half the mass of the Sun and is Earth-sized. A penny made of this white dwarf's material would weigh about 1,000 pounds on Earth.

Their mutual gravitational pull is so strong that it deforms the lower-mass star by three percent. If the Earth bulged by the same amount, we would have tides 120 miles high.

The discovery team has been hunting for pairs of white dwarfs using the MMT telescope at the Whipple Observatory on Mt. Hopkins, Arizona. These star pairs are too close together to distinguish photographically. By looking at the spectra, however, Brown and his team were able to differentiate the two stars and measure their relative motions. These stars are also oriented such that they eclipse each other every 6 minutes.

"If there were aliens living on a planet around this star system, they would see one of their two suns disappear every 6 minutes - a fantastic light show." said Smithsonian astronomer and co-author Mukremin Kilic.

These eclipses provide a very accurate clock, which is extremely useful for measuring any changes in the system.

General relativity predicts that moving objects will create ripples in the fabric of space-time, called gravitational waves. These waves carry away energy, causing the stars to inch closer together and orbit each other faster and faster.

"Though we have not yet directly measured gravitational waves with modern instruments, we can test their existence by measuring the change in the separation of these two stars," said co-author J. J. Hermes, a graduate student at the University of Texas at Austin. "Because they don't seem to be exchanging mass, this system is an exceptionally clean laboratory to perform such a test."

The team expects to conduct this test in a few months, when the star pair emerges from behind the Sun as seen from Earth.

Some models predict merging white dwarf pairs such as these are the source of a rare class of unusually faint stellar explosions called underluminous supernovae.

"If these systems are responsible for underluminous supernovae, we will detect these binary white dwarf systems with the same frequency that we see the supernovae. Our survey isn't complete, but so far, the numbers agree," said Brown.

This work will provide an important observational test on theories of white dwarf mergers, which are thought to produce many kinds of supernovae, not just the underluminous type.

This research appears in a paper accepted for publication by The Astrophysical Journal Letters. Brown's co-authors are Mukremin Kilic (Harvard-Smithsonian Center for Astrophysics/CfA), J. J. Hermes (University of Texas at Austin), Carlos Allende Prieto (Instituto de Astrofisica de Canarias, Spain), Scott J. Kenyon (CfA) and D. E. Winget (University of Texas at Austin).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source: HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS

More about → Evolved Stars Locked in Fatalistic Dance