January 2014

Surprising new class of “hypervelocity stars” discovered escaping the galaxy

This article was written by David Salisbury and published in Vanderbilt Research News. Link to original article.

Rogue Stars
Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. (Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory.)

An international team of astronomers has discovered a surprising new class of “hypervelocity stars” – solitary stars moving fast enough to escape the gravitational grasp of the Milky Way galaxy.

The discovery of this new set of “hypervelocity” stars was described at the annual meeting of the American Astronomical Society this week in Washington, D.C., and is published in the Jan. 1 issue of the Astrophysical Journal.

“These new hypervelocity stars are very different from the ones that have been discovered previously,” said Vanderbilt University graduate student Lauren Palladino, lead author on the study. “The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small – about the size of the sun – and the surprising part is that none of them appear to come from the galactic core.”

The discovery came as Palladino, working under the supervision of Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt, was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey, a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.

“It’s very hard to kick a star out of the galaxy,” said Holley-Bockelmann. “The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism.”

Kelly and Lauren
Lauren Palladino, right, and Kelly Holley-Bockelmann. (John Russell / Vanderbilt)

Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity. They also estimate that the Milky Way’s central black hole has a mass equivalent to four million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities. The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity. So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Palladino and her colleagues have discovered an additional 20 sun-sized stars that they characterize as possible hypervelocity stars. “One caveat concerns the known errors in measuring stellar motions,” she said. “To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does. We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real.”

The astronomers are following up with additional observations.

The new rogues appear to have the same composition as normal disk stars, so the astronomers do not think that their birthplace was in the galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the galaxy.

The big question is: what boosted these stars up to such extreme velocities? We are working on that now,” said Holley-Bockelmann.

Katharine Schlesinger from the Australian National University, Carlos Allende Prieto from the Universidad de La Laguna in Spain, Timothy Beers from the National Optical Astronomy Observatory in Tucson, Young Sun Lee from New Mexico State University and Donald Schneider from Pennsylvania State University also contributed to the discovery.

The research was supported by funds from the Graduate Assistance in Areas of National Need program, National Science Foundation grants AST 0847696, AST 0607482, Physics Frontier Center grants PHY 0216783, the Aspen Center for Physics, the Alfred P. Sloan Foundation and the U.S. Department of Energy Office of Science.

August 2013

A brighter method for measuring the surface gravity of distant stars

This article was written by David Salisbury and published in Vanderbilt Research News. Link to original article.

Astronomers have found a clever new way to slice and dice the flickering light from a distant star in a way that reveals the strength of gravity at its surface.

That is important because a star’s surface gravity is one of the key properties that astronomers use to calculate a star’s physical properties and assess its evolutionary state.

The new technique can also be used to significantly improve estimates of the sizes of the hundreds of exoplanets that have been discovered in the last 20 years. Current estimates have uncertainties ranging from 50 percent to 200 percent. Using the improved figures for the surface gravity of the host stars calculated by the new method should cut these uncertainties at least in half.

Keivan Stassun. (Daniel Dubois / Vanderbilt)
Keivan Stassun. (Daniel Dubois / Vanderbilt)

The technique was developed by a team of astronomers headed by Vanderbilt Professor of Physics and Astronomy Keivan Stassun and is described in the Aug. 22 issue of the journal Nature.

“Once you know a star’s surface gravity then you only need one other measurement, its temperature, which is pretty easy to obtain, to determine its mass, size and other important physical properties,” said Stassun.

“Measuring stellar surface gravities well has always been a difficult business,” added Gibor Basri, professor of astronomy at the University of California, Berkeley who contributed to the study. “So it is a very pleasant surprise to find that the subtle flickering of a star’s light provides a relatively easy way to do it.”

Measuring stellar gravity

There are three traditional methods for estimating a star’s surface gravity: photometric, spectroscopic and asteroseismic. The new flicker method is simpler than the older methods and more accurate than all but one of them.

Photometric methods look at how bright a star is in different colors. This distribution is linked to its surface gravity, temperature and chemical composition. It is a relatively easy observation to make and can be performed even on fairly faint stars, but does not produce a very accurate figure for surface gravity, having an uncertainty range of 90 to 150 percent.

Star granulation simulations
Simulations of granulation patterns on the surface of the Sun, sub-giant and giant stars. The scale of each simulation is proportional to the size of the blue image of earth next to it. (Courtesy of R. Trampedach, JILA/CU Boulder, CO)

The spectroscopic technique is more involved and is limited to relatively bright stars, but it has a lower uncertainty range of 25 to 50 percent. It works by closely examining the narrow spectral bands of light emitted by the elements in the star’s atmosphere. Generally speaking, high surface gravity widens the lines and lower surface gravity narrows them.

Asteroseismology is the gold standard, with accuracies of a few percent, but the measurements are even more difficult to make than spectroscopy and it is restricted to several hundred of the closest, brightest stars. The technique traces sound pulses that travel through the interior of a star at specific frequencies that are tied to its surface gravities. Small stars, like the sun, ring at a higher pitch while giant stars ring a lower pitch.

Much like asteroseismology, the new flicker method looks at variations in the star’s brightness, In this case it zeroes in on variations that last eight hours or less. These variations appear to be linked to granulation, the network of small cells that cover the surface of a star that are caused by columns of gas rising from the interior. On stars with high surface gravity, the granulation is finer and flickers at a higher frequency. On stars with low surface gravity, the granulation is coarser and they flicker at a lower frequency.

Exquisitely simple

The new method is remarkably simple – requiring only five lines of computer code to make the basic measurement – substantially reducing the cost and effort required to calculate the surface gravities of thousands of stars.

“The spectroscopic methods are like surgery. The analysis is meticulous and involved and very fine-grained,” said Stassun. “Flicker is more like ultrasound. You just run the probe around the surface and you see what you need to see. But its diagnostic power – at least for the purpose of measuring gravity – is as good if not better.”

To determine the accuracy of the flicker method, they used it to calculate the surface gravity of stars that have been analyzed using asteroseismology. They found that it has an uncertainty of less than 25 percent, which is better than both the photometric and spectroscopic methods. Its major limitation is that it requires extremely high quality data taken over long time periods. But this is precisely the type of observations made by Kepler while it was searching for periodic dips in light caused when exoplanets cross the face of a star. So the Flicker method can be applied to the tens of thousands of stars already being monitored by Kepler.

“The exquisite precision of the data from Kepler allows us to monitor the churning and waves on the surfaces of stars,” said team member Joshua Pepper, assistant professor of physics at Lehigh University. “This behavior causes subtle changes to a star’s brightness on the time scale of a few hours and tells us in great detail how far along these stars are in their evolutionary lifetimes.”

Playing with data yields discovery

Graduate student Fabienne Bastien was responsible for discovering that valuable information was embedded in starlight flicker. The discovery began when she was “playing around” with Kepler data using special data visualization software that Vanderbilt astronomers have developed for investigating large, multi-dimensional astronomy datasets. (The data visualization tool that enabled this discovery, called Filtergraph, is free to the public.)

Fabienne Bastien (Steve Green / Vanderbilt)
Fabienne Bastien (Steve Green / Vanderbilt)

“I was plotting various parameters looking for something that correlated with the strength of stars’ magnetic fields,” said Bastien. “I didn’t find it, but I did find an interesting correlation between certain flicker patterns and stellar gravity.”

When Bastien showed her discovery to Stassun, he was intrigued. So they performed the operation on the archived Kepler light curves of a few hundred sun-like stars.

When they plotted the overall variation in brightness of stars against their flicker intensity, they found an interesting pattern. As stars age, their overall variation falls gradually to a minimum. This is easily understood because the rate at which a star spins decreases gradually over time. As stars approach this minimum, their flicker begins to grow in complexity – a characteristic that the astronomers have labeled “crackle.” Once they reach this point, which they call the flicker floor, the stars appear to maintain this low level of variability for the rest of their lives, though it does appear to grow again as the stars approach the ends of their lives as red giant stars.

“This is an interesting new way to look at stellar evolution and a way to put our Sun’s future evolution into a grander perspective,” said Stassun.

When they ran their analysis on the sun’s light curve, for example, the researchers found that it is hovering just above the flicker floor, leading them to the prediction that the sun is approaching a time when it will undergo a fundamental transition to a state of minimum variability and, in the process, will lose its spots.

The research was funded by the Vanderbilt Initiative in Data-intensive Astrophysics (VIDA) and National Science Foundation grants AST-0849736 and AST-1009810.

This article was written by David Salisbury and published in Vanderbilt Research News. Link to original article.

May 2013

Vanderbilt’s role in new planet-finding space mission

This article was written by David Salisbury and published in Vanderbilt Research News. Link to original article.

exoplanet transit
Artist conception of exoplanet transiting the face of distant star. (Courtesy of MIT/Kavli Institute for Astrophysics and Space Research)

A team of Vanderbilt astronomers headed by Keivan Stassun will play a key role in the planet-seeking space telescope that NASA has just approved and scheduled for launch in 2017.

The $200 million spacecraft is called the Transiting Exoplanet Survey Satellite (TESS). When it is launched in 2017, TESS will perform the first space-borne all-sky survey for planets circling the brightest stars.

The project is headed by scientists at MIT’s Kavli Institute for Astrophysics and Space Research. Stassun is a co-principal investigator on the project and he and his team will be selecting the specific stars that the project will target in its search for subtle, periodic dips in brightness that occurs when a planet transits across a star’s face.

TESS
Artist concept of the TESS spacecraft. (Courtesy of MIT’s Kavli Institute for Astrophysics and Space Research)

“I got involved in 2011 during a sabbatical at MIT,” said Stassun, who is a professor of physics and astronomy at Vanderbilt. “At the time, one of the issues they were struggling with was how to select the most promising stars to search for planets.”

It was a process he had already worked through for KELT, the Kilodegree Extremely Little Telescope: a pair of small, land-based telescopes built and operated by Vanderbilt and Ohio State University that are designed specifically for identifying bright stars with planets. KELT, as will TESS, simultaneously records the light coming from millions of the brightest stars and so faces a similar selection problem. “I was able to tell them that we’ve already addressed the problem. They liked our approach so they invited us to join the mission,” he added.

When it is launched TESS will complement the observations being made by Kepler, NASA’s first mission capable of finding Earth-sized planets around other stars. Kepler was designed to observe much fainter stars than TESS but it is focused on a relatively small portion of the Milky Way galaxy. Three Vanderbilt astronomers, including Stassun, are also members of the Kepler science team. As of last January, Kepler had identified 2,740 candidate planets, which are steadily being confirmed by follow-up observations from other telescopes.

Most of the exoplanets that have been discovered to date are giant planets, like Jupiter. Kepler has discovered a number of smaller exoplanets, closer to the size of Earth. But Kepler has difficulty identifying smaller planets because the stars that it examines tend to be extremely faint, which makes it very difficult to confirm discoveries with ground-based telescopes. TESS, by contrast, is targeting the brightest stars specifically to make it easier to discover Earth-sized planets.

“We will be targeting mainly G-type stars similar to the Sun,” said Stassun. “Not only should concentrating on the brightest stars make it easier to find Earth-like planets, but, once we have found them, it should be possible to use large land-based telescopes to get spectra of the planet’s atmosphere.”

TESS target stars
Chart shows the stars that TESS will be targeting in red and those currently being targeted by the Kepler space telescope in blue. (Courtesy of MIT’s Kavli Institute for Astrophysics and Space Research.)

These spectra are important because they could provide information about the chemical make-up of the planetary atmospheres. If certain chemicals, like oxygen and methane, are detected it would be a strong indicator that a planet is capable of supporting life.

“We will also target a small number of red dwarf stars (such as Barnard’s star which was discovered by Vanderbilt’s first astronomer) because these are the stars nearest to us. If we are to imagine ever traveling to visit another world, it will be a world around one of these red dwarf neighbors,” Stassun said.

February 2013

Stassun on Producing Minority Ph.D. Recipients: How Vanderbilt Became the Nation’s Top Producer of Minority Ph.D. Recipients in Physics, Astronomy and Materials Science

This article was originally published in Vanderbilt News. Link to original article.

Keivan Stassun, physics and astronomy professor (DANIEL DUBOIS)
Keivan Stassun, physics and astronomy professor (DANIEL DUBOIS)

Not long after he arrived at Vanderbilt nine years ago, Keivan Stassun, professor of astronomy, began building on a newly forged alliance with Fisk University, a historically black college just two miles from the Vanderbilt campus, in an effort to increase the number of African Americans, Latinos, Native Americans and other minorities earning Ph.D. degrees in science. Since 2004 the Fisk–Vanderbilt Master’s-to-Ph.D. Bridge program, which he directs, has admitted more than 60 students. This year it will become the nation’s No. 1 producer of minority Ph.D. recipients in physics, astronomy and materials science. Students typically receive two years’ training in the master’s program at Fisk before entering Ph.D. programs at Vanderbilt or another institution.


How did the Fisk–Vanderbilt Bridge program come about?

When I was choosing among offers from several universities, I asked about opportunities and resources for doing world-class research. I also asked about opportunities and resources for really tackling what has been a century-long problem: the full inclusion of all Americans in science and engineering. Vanderbilt was the only place that gave what I considered a satisfactory answer—that they would back me as I worked to build something that would help put Vanderbilt in a leadership position nationally in tackling the challenge of diversity in higher education. That says a lot to energetic young scholars and has helped us attract the best junior faculty looking for a place to build a life centered on unique and visionary work.

Why did you opt for the Fisk partnership?

We knew other institutions had tried and failed to build partnerships that were merely handoffs, not true collaborations that would ensure students were adequately trained. One of the first questions we considered was in which areas of research we could legitimately build a student training program. Fisk has a world-class research program in a number of science and engineering areas, including nanophysics, the development of novel materials for space-based astrophysics, infrared spectroscopy of molecules, and synthesis of those molecules for materials science applications. That allowed us to build a collaboration wherein Vanderbilt astrophysicists like myself and Fisk nanophysicists such as bridge program co-director Arnold Burger could work together to develop cutting-edge detector technology for use in the next generation of space exploration. It gives us something specific around which to train students, and it allows us to compete for grants.

What makes the program attractive to students?

One of the most important things we have done for these students is to give them, in essence, the opportunity to audition in person. We know that, by and large, the students we are trying to attract often underperform on metrics like the Graduate Record Examination (GRE). A key insight for us was to get past the focus on standardized tests as the be-all and end-all. If the student has been working in your lab for two years, making discoveries alongside you, and taking hard graduate-level courses from you and publishing in peer-reviewed journals, then maybe we don’t need the GRE. We replaced a predictor of performance with actual performance. That has been one of the great innovations of our bridge program, and it’s the reason Columbia and MIT and Michigan are now emulating it. For decades the top-tier Ph.D.-granting institutions have wanted to make headway on the challenge of diversity at the Ph.D. level, and now they have our example to follow. It’s nice to be emulated.

Considering the amount of time this must require of faculty members, has it been difficult to enlist their involvement?

A serious commitment to training and mentoring certainly takes time, but it’s an investment that contributes to research productivity. These students contribute directly to discoveries in my lab. My faculty colleagues—especially David Ernst, Kelly Holley-Bockelmann, Donna Webb and David Cliffel—have found ways to make sure students are integrally connected with the mission of the university to discover and advance knowledge. If we want America to remain globally competitive, we must do a better job of tapping our own talent. And doing the right thing raises the bottom line across multiple measures of performance. We now have at least seven Vanderbilt faculty members who were able to be more competitive and ultimately win National Science Foundation CAREER Award grants—the top national award for junior science faculty—in part because they were able to demonstrate in their NSF proposals their involvement with this program.

Why is it so important to attract more minority students to science in particular?

Science, engineering and technology in the U.S. depend absolutely on the size and quality of the talent pool. Historically, we have not effectively drawn from what is now a third of our population. Fewer than half of our African American youth will graduate from high school, let alone be in a position to consider pursuing a Ph.D. In running a master’s-to-Ph.D. program that links Fisk and Vanderbilt, we have tried to articulate one very specific part of a larger system where we can exert leverage. I hope and I pray that the better angels among us continue to tackle those other very hard parts of the pipeline, because we must.

June 2012

Bridging the Gap in the Sciences

Vanderbilt is on track this year to become the number one producer of minority Ph.D. recipients in physics, astronomy and materials science, an area where minorities are grossly underrepresented. Watch the emotional journey of the latest doctoral graduates from the Fisk-Vanderbilt-Master’s-to-Ph.D. Bridge Program.

February 2010

Astronomer receives NSF award to study black hole evolution

This article was originally written and published by the Vanderbilt View staff. Link to original article.

Kelly Holley-Bockelmann
Kelly Holley-Bockelmann

Holley-Bockelmann will receive $1.1 million over five years

Kelly Holley-Bockelmann, assistant professor of physics and astronomy, has been awarded the National Science Foundation’s largest-ever Faculty Early Career Development grant in the field of astronomy. She will use the award to continue her studies of black holes while supporting Vanderbilt’s innovative program designed to make the university the top producer of underrepresented minorities with Ph.D.s in physics and astronomy.

Due to the availability of Recovery Act funds, Holley-Bockelmann will receive $1.1 million over five years. CAREER awards are considered NSF’s most prestigious honor for junior faculty members.

Holley-Bockelmann plans to address one of the fundamental mysteries that surrounds supermassive black holes, exotic objects weighing in at millions to billions of solar masses which astronomers have found lurking at the core of most galaxies, including the Milky Way.

Understanding how supermassive black holes form is important because they have played a major role in the evolution of the universe. Specifically, they appear to have had a major impact on the development of galaxies in ways such as affecting the rate at which they produce new stars.

Holley-Bockelmann also will use part of her grant to support the Fisk-Vanderbilt Master’s-to-Ph.D. Bridge Program, a partnership with historically black Fisk University designed to encourage underrepresented minorities and women to pursue careers in physics and other sciences. She is following in the footsteps of Vanderbilt Associate Professor of Physics and Astronomy Keivan Stassun, who received a CAREER Award in 2004 and used it to start the Bridge program.

“As a first-generation college student and a woman astronomer, it’s important for me to help students realize that they can be a scientist no matter where they come from or what they look like, as long as they love science enough to put in the hard work,” Holley-Bockelmann said.

Holley-Bockelmann, who is an adjunct professor at Fisk, will hire two Bridge graduates to assist in her black hole studies. Her grant is also providing “time release” for a Fisk instructor to finish up his doctoral degree. In addition, she is hiring a post-doctoral fellow to assist in her black hole studies and a graduate student to serve as a computational guru for the Bridge program.

This article was originally written and published by the Vanderbilt View staff. Link to original article.

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