Astronomers identify signature of Earth-eating stars

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

Some Sun-like stars are ‘Earth-eaters.’ During their development they ingest large amounts of the rocky material from which ‘terrestrial’ planets like Earth, Mars and Venus are made.

Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a model that estimates the effect that such a diet has on a star’s chemical composition and has used it to analyze a pair of twin stars that both have their own planets.

The results of the study were published online May 7 in the Astrophysical Journal.

“Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets,” said Vanderbilt Professor of Astronomy Keivan Stassun, who supervised the study. “After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail, element by element.”

This ability will add substantially to astronomers’ understanding of the process of planet formation as well as assist in the ongoing search for Earth-like exoplanets, according to the astronomers.

First, some background: Stars consist of more than 98 percent hydrogen and helium. All the other elements make up less than 2 percent of their mass. Astronomers have arbitrarily defined all the elements heavier than hydrogen and helium as metals and have coined the term “metallicity” to refer to the ratio of the relative abundance of iron to hydrogen in a star’s chemical makeup.

Since the mid-1990’s, when astronomers developed the capability to detect extrasolar planets in large numbers, there have been several studies that attempt to link star metallicity with planet formation. In one such study, researchers at Los Alamos National Laboratory argued that stars with high metallicity are more likely to develop planetary systems than those with low metallicity. Another study concluded that hot Jupiter-sized planets are found predominantly circling stars with high metallicity while smaller planets are found circling stars with a wide range of metal content.

Trey Mack. (Steve Green - Vanderbilt)
Trey Mack. (Steve Green – Vanderbilt)

Building on the work of coauthor Simon Schuler of the University of Tampa, who expanded the examination of stars’ chemical composition beyond their iron content, Mack took this type of analysis a step further by looking at the abundance of 15 specific elements relative to that of the Sun. He was particularly interested in elements like aluminum, silicon, calcium and iron that have melting points higher than 1,200 degrees Fahrenheit (600 degrees Celsius) because these are the refractory materials that serve as building blocks for Earth-like planets.

Mack, Schuler and Stassun decided to apply this technique to the planet-hosting binary pair designated HD 20781 and HD 20782. Both stars should have condensed out of the same cloud of dust and gas and so both should have started with the same chemical compositions. This particular binary pair is the first one discovered where both stars have planets of their own.

Both of the stars in the binary pair are G-class dwarf stars similar to the Sun. One star is orbited closely by two Neptune-size planets. The other possesses a single Jupiter-size planet that follows a highly eccentric orbit. The difference in their planetary systems make the two stars ideal for studying the connection between exoplanets and the chemical composition of their stellar hosts.

When they analyzed the spectrum of the two stars the astronomers found that the relative abundance of the refractory elements was significantly higher than that of the Sun. They also found that the higher the melting temperature of a particular element, the higher was its abundance, a trend that serves as a compelling signature of the ingestion of Earth-like rocky material. They calculated that each of the twins would have had to consume an additional 10-20 Earth-masses of rocky material to produce the chemical signatures. Specifically, the star with the Jupiter-sized planet appears to have swallowed an extra ten Earth masses while the star with the two Neptune-sized planets scarfed down an additional 20.

The results support the proposition that a star’s chemical composition and the nature of its planetary system are linked.

Solar Spectrum
What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. (N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF)

“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter,” said Mack. “The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets.

“With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect.”

Following this logic, it is unlikely that either of the binary twins possesses terrestrial planets. At one twin, the two Neptune-sized planets are orbiting the star quite closely, at one-third the distance between the Earth and the Sun. At the other twin, the Jupiter-sized planet spends a lot of time in the outer reaches of the planetary system but it’s eccentric orbit also brings It in extremely close to the star. The astronomers speculate that the reason the star with the two Neptune-size planets ingested more terrestrial material than its twin was because the two planets were more efficient at pushing material into their star than the single Jupiter-sized planet was at pushing material into its star.

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

If the chemical signature of G-class stars that swallow rocky planets proves to be universal, “when we find stars with similar chemical signatures, we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets,” said Mack. “And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own.”

Added Stassun: “This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask. The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?”

The research was supported by National Science Foundation grants AAG AST-1009810 and PAARE AST-0849736.

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

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.

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.

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.

Bridging the Gap in the Sciences

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