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 the Sun. Optical data from the Hubble Space Telescope
is in blue; the dark region is a coronagraphic mask, which filtered out
the otherwise overwhelming light of the central star. Credit: ALMA
(ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton
(NRAO/AUI/NSF) | Download image
ALMA image of the
debris disk in the Fomalhaut star system. The ring is approximately 20
billion kilometers from the central star and it is about 2 billion
kilometers wide. The central dot is the unresolved emission from the
star, which is about twice the mass of the Sun. Credit: ALMA
(ESO/NAOJ/NRAO); M. MacGregor | Download image
An international team of astronomers using the Atacama Large
Millimeter/submillimeter Array (ALMA) has made the first complete
millimeter-wavelength image of the ring of dusty debris surrounding the
young star Fomalhaut. This remarkably well-defined band of rubble and
gas is likely the result of exocomets smashing together near the outer
edges of a planetary system 25 light-years from Earth. Observations
Suggest Chemical Kinship to Comets in Our Own Solar System.
Earlier ALMA observations of Fomalhaut
— taken in 2012 when the telescope was still under construction –
revealed only about one half of the debris disk. Though this first 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 suggest that there are chemical similarities
between its icy contents and comets in our own solar system.
ALMA has given us this staggeringly clear image of a fully formed debris disk,” said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA, 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.”
Fomalhaut is a relatively nearby star system and one of only about 20
in which planets have been imaged directly. The entire system is
approximately 440 million years old, or about one-tenth the age of our
solar system.
As revealed in the new ALMA image, a brilliant band of icy dust about
2 billion kilometers wide has formed approximately 20 billion
kilometers from the star.
Debris disks are common features around young stars and represent a
very dynamic and chaotic period in the history of a solar system.
Astronomers believe they are formed by the ongoing collisions of comets
and other planetesimals in the outer reaches of a recently formed
planetary system. The leftover debris from these collisions absorbs
light from its central star and reradiates that energy as a faint
millimeter-wavelength glow that can be studied with ALMA.
Using the new ALMA data and detailed computer modeling, the
researchers could 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,
noted MacGregor.
The new ALMA observations are also the first to definitively show
“apocenter glow,” a phenomenon predicted in a 2016 paper by lead author
Margaret Pan, a scientist at the Massachusetts Institute of Technology
in Cambridge and 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 researchers 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 Luca Matrà with the University of Cambridge, UK, 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.” Matrà and his colleagues 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 the Solar System.
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 the Solar System.
“Twenty years ago, the best millimeter-wavelength telescopes gave the first fuzzy maps of sand grains orbiting Fomalhaut. Now with ALMA’s full capabilities the entire ring of material has been imaged,” concluded Paul Kalas, an astronomer at the University of California at Berkeley and principal investigator on these observations. “One day we hope to detect the planets that influence the orbits of these grains.”
Additional information
This research is presented in a paper titled “A complete ALMA map of
the Fomalhaut debris disk,” M. MacGregor, et al., appearing in the Astrophysical Journal,
and “Detection of exocometary CO within the 440MYR-old Fomalhaut belt: A
similar CO+CO2 ice abundance in exocomets and solar system comets,” L.
Matrà et al., appearing in the Astrophysical Journal.
This work benefited from: NASA’s Nexus for Exoplanet System Science
(NExSS) research coordination network sponsored by NASA’s Science
Mission Directorate, NASA grants NNX15AC89G, NNX15AD95G, NSF grant
AST-1518332, NSF Graduate Research Fellowship DGE1144152, and from NRAO
Student Observing Support. This work has also been possible thanks to an
STFC postgraduate studentship and the European Union through ERC grant
number 279973.
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO),
the U.S. National Science Foundation (NSF) and the National Institutes
of Natural Sciences (NINS) of Japan in cooperation with the Republic of
Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in
cooperation with the National Research Council of Canada (NRC) and the
National Science Council of Taiwan (NSC) and by NINS in cooperation with
the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space
Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its
Member States; by the National Radio Astronomy Observatory (NRAO),
managed by Associated Universities, Inc. (AUI), on behalf of North
America; and by the National Astronomical Observatory of Japan (NAOJ) on
behalf of East Asia. The Joint ALMA Observatory (JAO) provides the
unified leadership and management of the construction, commissioning and
operation of ALMA.
Contacts
Nicolás Lira T.
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Email: nicolas.lira@alma.cl
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Charlottesville, Virginia, USA
Tel: +1 434 296 0314
Cell: +1 202 236 6324
E-mail: cblue@nrao.edu
Masaaki Hiramatsu
Education and Public Outreach Officer, NAOJ Chile
Observatory Tokyo, Japan
Tel: +81 422 34 3630
E-mail: hiramatsu.masaaki@nao.ac.jp
Richard Hook
Public Information Officer, ESO
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Contacts
Nicolás Lira T.
Press Coordinator
Joint ALMA Observatory
Santiago, Chile
Tel: +56 2 24 67 65 19
Cell: +56 9 94 45 77 26
Email: nicolas.lira@alma.cl
Charles E. Blue
Public Information Officer
National Radio Astronomy Observatory
Charlottesville, Virginia, USA
Tel: +1 434 296 0314
Cell: +1 202 236 6324
E-mail: cblue@nrao.edu
Masaaki Hiramatsu
Education and Public Outreach Officer, NAOJ Chile
Observatory Tokyo, Japan
Tel: +81 422 34 3630
E-mail: hiramatsu.masaaki@nao.ac.jp
Richard Hook
Public Information Officer, ESO
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
