Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have made the first complete millimeter-wavelength image of the ring of dusty debris surrounding the young star Fomalhaut, and found that the ice content of colliding exocomets within it is similar to comets in our own Solar System.
Composite image of the Fomalhaut star system. The ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. The central dot is the unresolved emission from the star, which is about twice the mass of our Sun. Optical data from the NASA/ESA Hubble Space Telescope is in blue; the dark region is a coronagraphic mask, which filtered out the otherwise overwhelming light of the central star. Image credit: ALMA / ESO / NAOJ / NRAO / M. MacGregor / NASA / ESA / Hubble / P. Kalas / B. Saxton / AUI / NSF.
Fomalhaut is a young star located in the constellation of Piscis Austrinus, approximately 25 light-years from Earth.
Also known as alpha Piscis Austrini and HD 216956, Fomalhaut is 440 million years old, or about one-tenth the age of the Solar System, and is one of only about twenty systems in which planets have been imaged directly.
Earlier ALMA observations of the star — taken in 2012 when the telescope was still under construction — revealed only about one half of the debris disk. Though that image was merely a test of ALMA’s initial capabilities, it nonetheless provided tantalizing hints about the nature and possible origin of the disk.
The new ALMA observations offer a stunningly complete view of this glowing band of debris and also suggest that there are chemical similarities between its icy contents and comets in the Solar System.
“ALMA has given us this staggeringly clear image of a fully formed debris disk,” said Dr. Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations.
“We can finally see the well-defined shape of the disk, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance.”
As revealed in the new ALMA image, a brilliant band of icy dust about 2 billion km wide has formed approximately 20 billion km from the star.
“Using the new ALMA data and detailed computer modeling, we were able to calculate the precise location, width, and geometry of the disk. These parameters confirm that such a narrow ring is likely produced through the gravitational influence of planets in the system,” said Dr. MacGregor, lead author on the team’s first paper.
The new observations are also the first to definitively show ‘apocenter glow,’ a phenomenon predicted in a 2016 paper by MIT researcher Margaret Pan, who is also a co-author on the new ALMA papers.
Like all objects with elongated orbits, the dusty material in the Fomalhaut disk travels more slowly when it is farthest from the star. As the dust slows down, it piles up, forming denser concentrations in the more distant portions of the disk. These dense regions can be seen by ALMA as brighter millimeter-wavelength emission.
Using the same ALMA dataset, but focusing on distinct millimeter-wavelength signals naturally emitted by molecules in space, the team also detected vast stores of carbon monoxide gas in precisely the same location as the debris disk.
“These data allowed us to determine that the relative abundance of carbon monoxide plus carbon dioxide around Fomalhaut is about the same as found in comets in our own Solar System,” said Dr. Luca Matrà, a researcher at the University of Cambridge and lead author on the team’s second paper.
“This chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own.”
The researchers believe this gas is either released from continuous comet collisions or the result of a single, large impact between supercomets hundreds of times more massive than Hale-Bopp.
The presence of this well-defined debris disk around Fomalhaut, along with its curiously familiar chemistry, may indicate that this system is undergoing its own version of the Late Heavy Bombardment, a period approximately 4 billion years ago when the Earth and other planets were routinely struck by swarms of asteroids and comets left over from the formation of our Solar System.
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Meredith A. MacGregor et al. 2017. A Complete ALMA Map of the Fomalhaut Debris Disk. ApJ, accepted for publication; arXiv: 1705.05867
L. Matrà et al. 2017. Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets. ApJ, accepted for publication; arXiv: 1705.05868
