From NASA Chandra: “Chandra May Have First Evidence of a Young Star Devouring a Planet”

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From NASA Chandra

July 18, 2018
Media contacts:
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu

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Credit: Illustration: NASA/CXC/M.Weiss; X-ray spectrum: NASA/CXC/MIT/H.M.Günther
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Chandra data indicates that a young star has likely destroyed and consumed an infant planet.

If confirmed, this would be the first time that astronomers have witnessed such an event.

The star, known as RW Aur A, is a few million years old and is located about 450 light years from Earth.

Studying this may help astronomers gain insight into the processes affecting the early stages of planet development.

This artist’s illustration depicts the destruction of a young planet or planets, which scientists may have witnessed for the first time using data from NASA’s Chandra X-ray Observatory, as described in our latest press release. If this discovery is confirmed, it would give insight into the processes affecting the survival of infant planets.

RW Aur A is a star about 450 light years from Earth, making it relatively nearby. Since the 1930s, astronomers have studied RW Aur A and been curious about why the optical light from this star changes over time. In recent years, scientists have observed that this variability has increased, with the star dimming even more and for longer periods of time.

To investigate this mystery, a team of astronomers used Chandra to get information in the X-ray band of the electromagnetic spectrum. X-rays are generally emitted by more energetic and hotter phenomena than their optical light counterparts and can reveal information about different elements, including iron.

The Chandra data suggest that the most recent dimming event from RW Aur A was caused by the collision of two “planetesimals” (that is, infant planetary bodies still in the process of formation), including at least one object large enough to be a planet.

Because the X-rays come from the hot outer atmosphere of the star, changes in the X-ray spectrum — the intensity of X-rays measured at different energies — over these three observations were used to probe the density and composition of the absorbing material around the star.

The team found that the dips in both optical and X-ray light are caused by dense gas obscuring the star’s light. Also, a Chandra observation in 2017 showed strong emission from iron atoms, indicating that the disk contained at least 10 times more iron than in the 2013 observation during a bright period.

The Chandra spectra from the 2013 and 2017 observations are shown in an inset in the graphic. The sharp peak on the right side of the 2017 spectrum is a signature of a large amount of iron.

The researchers propose the excess iron was created when the two planetesimals collided. If one or both planetary bodies are made partly of iron, their smash-up could release a large amount of iron into the star’s disk and temporarily obscure its light as the material falls into the star.

Since 1937, astronomers have puzzled over the curious variability of a young star named RW Aur A, located about 450 light years from Earth. Every few decades, the star’s optical light has faded briefly before brightening again. In recent years, astronomers have observed the star dimming more frequently, and for longer periods.

Using Chandra, a team of scientists may have uncovered what caused the star’s most recent dimming event: a collision of two infant planetary bodies, including at least one object large enough to be a planet. As the resulting planetary debris fell into the star, it would generate a thick veil of dust and gas, temporarily obscuring the star’s light.

“Computer simulations have long predicted that planets can fall into a young star, but we have never before observed that,” says Hans Moritz Guenther, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research who led the study. “If our interpretation of the data is correct, this would be the first time that we directly observe a young star devouring a planet or planets.”

The star’s previous dimming events may have been caused by similar smash-ups, of either two planetary bodies or large remnants of past collisions that met head-on and broke apart again.

RW Aur A is located in the Taurus-Auriga Dark Clouds, which host stellar nurseries containing thousands of infant stars. Very young stars, unlike our relatively mature sun, are still surrounded by a rotating disk of gas and clumps of material ranging in size from small dust grains to pebbles, and possibly fledgling planets. These disks last for about 5 million to 10 million years.

RW Aur A is estimated to be several million years old, and is still surrounded by a disk of dust and gas. This star and its binary companion star, RW Aur B, are both about the same mass as the sun.

The noticeable dips in the optical brightness of RW Aur A that occurred every few decades each lasted for about a month. Then, in 2011, the behavior changed. The star dimmed again, this time for about six months. The star eventually brightened, only to fade again in mid-2014. In November 2016, the star returned to its full brightness, and then in January 2017 it dimmed again.

Chandra was used to observe the star during an optically bright period in 2013, and then dim periods in 2015 and 2017, when a decrease in X-rays was also observed.

Because the X-rays come from the hot outer atmosphere of the star, changes in the X-ray spectrum — the intensity of X-rays measured at different wavelengths — over these three observations were used to probe the density and composition of the absorbing material around the star.

The team found that the dips in both optical and X-ray light are caused by dense gas obscuring the star’s light. The observation in 2017 showed strong emission from iron atoms, indicating that the disk contained at least 10 times more iron than in the 2013 observation during a bright period.

Guenther and colleagues suggest the excess iron was created when two planetesimals, or infant planetary bodies, collided. If one or both planetary bodies are made partly of iron, their smash-up could release a large amount of iron into the star’s disk and temporarily obscure its light as the material falls into the star.

A less favored explanation is that small grains or particles such as iron can become trapped in parts of a disk. If the disk’s structure changes suddenly, such as when the star’s partner star passes close by, the resulting tidal forces might release the trapped particles, creating an excess of iron that can fall into the star.

The scientists hope to make more observations of the star in the future, to see whether the amount of iron surrounding it has changed — a measure that could help researchers determine the size of the iron’s source. For example, if about the same amount of iron appears in a year or two that may indicate it comes from a relatively massive source.

“Much effort currently goes into learning about exoplanets and how they form, so it is obviously very important to see how young planets could be destroyed in interactions with their host stars and other young planets, and what factors determine if they survive,” A paper describing these results led by Hans Moritz Guenther (MIT’s Kavli Institute for Astrophysics) appears in the most recent issue of The Astrophysical Journal and is available online. The other authors on the paper are Til Birnstiel (Ludwig-Maximillians-University Munich), David Huenemoerder (MIT), David Principe (MIT), Christian Schneider (Hamburger Sternwarte), Scott Wolk (Harvard-Smithsonian Center for Astrophysics), Franky Dubois (Astrolab IRIS), Ludwig Logiie (Astrolab IRIS), Steve Rau (Astrolab IRIS), and Siegfried Vanaverbeke (Astrolab IRIS).

See the full article here .


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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.