Artist’s impression of the disc around the young star TW Hydrae
Artist’s impression of the disc around the young star TW Hydrae
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Artist’s impression of the disc around the young star TW Hydrae
Methanol around the young star TW Hydrae
The organic molecule methyl alcohol
(methanol) has been found by the Atacama Large Millimeter/Submillimeter
Array (ALMA) in the TW Hydrae protoplanetary disc. This is the first
such detection of the compound in a young planet-forming disc. Methanol
is the only complex organic molecule as yet detected in discs that
unambiguously derives from an icy form. Its detection helps astronomers
understand the chemical processes that occur during the formation of
planetary systems and that ultimately lead to the creation of the
ingredients for life.
The protoplanetary disc around the young star TW Hydrae
is the closest known example to Earth, at a distance of only about 170
light-years. As such it is an ideal target for astronomers to study
discs. This system closely resembles what astronomers think the Solar
System looked like during its formation more than four billion years
ago.
The Atacama Large Millimeter/Submillimeter Array (ALMA)
is the most powerful observatory in existence for mapping the chemical
composition and the distribution of cold gas in nearby discs. These
unique capabilities have now been exploited by a group of astronomers
led by Catherine Walsh (Leiden Observatory, the Netherlands) to
investigate the chemistry of the TW Hydrae protoplanetary disc.
The ALMA observations have revealed the fingerprint of gaseous methyl alcohol, or methanol (CH3OH),
in a protoplanetary disc for the first time. Methanol, a derivative of
methane, is one of the largest complex organic molecules detected in
discs to date. Identifying its presence in pre-planetary objects
represents a milestone for understanding how organic molecules are
incorporated into nascent planets.
Furthermore, methanol is itself a building block for more complex species of fundamental prebiotic importance, like amino acid compounds. As a result, methanol plays a vital role in the creation of the rich organic chemistry needed for life.
Catherine Walsh, lead author of the study, explains: “Finding
methanol in a protoplanetary disc shows the unique capability of ALMA to
probe the complex organic ice reservoir in discs and so, for the first
time, allows us to look back in time to the origin of chemical
complexity in a planet nursery around a young Sun-like star.”
Gaseous methanol in a protoplanetary disc has a unique importance in
astrochemistry. While other species detected in space are formed by
gas-phase chemistry alone, or by a combination of both gas and
solid-phase generation, methanol is a complex organic compound which is
formed solely in the ice phase via surface reactions on dust grains.
The sharp vision of ALMA has also allowed astronomers to map the
gaseous methanol across the TW Hydrae disc. They discovered a ring-like
pattern in addition to significant emission from close to the central
star [1].
The observation of methanol in the gas phase, combined with
information about its distribution, implies that methanol formed on the
disc’s icy grains, and was subsequently released in gaseous form. This
first observation helps to clarify the puzzle of the methanol ice–gas
transition [2], and more generally the chemical processes in astrophysical environments [3].
Ryan A. Loomis, a co-author of the study, adds: “Methanol in
gaseous form in the disc is an unambiguous indicator of rich organic
chemical processes at an early stage of star and planet formation. This
result has an impact on our understanding of how organic matter
accumulates in very young planetary systems.”
This successful first detection of cold gas-phase methanol in a
protoplanetary disc means that the production of ice chemistry can now
be explored in discs, paving the way to future studies of complex
organic chemistry in planetary birthplaces. In the hunt for
life-sustaining exoplanets, astronomers now have access to a powerful
new tool.
Notes
[1] A ring of methanol between 30 and 100 astronomical units
(au) reproduces the pattern of the observed methanol data from ALMA.
The identified structure supports the hypothesis that the bulk of the
disc ice reservoir is hosted primarily on the larger (up to
millimetre-sized) dust grains, residing in the inner 50 au, which have
become decoupled from the gas, and drifted radially inwards towards the
star.
[2] In this study, rather than thermal desorption
(with methanol released at temperatures higher than its sublimation
temperature), other mechanisms are supported and discussed by the team,
including photodesorption by ultraviolet photons and reactive
desorption. More detailed ALMA observations would help to definitely
favour one scenario among the others.
[3] Radial variation of chemical species in the disc midplane composition, and specifically the locations of snowlines,
are crucial for understanding the chemistry of nascent planets.The
snowlines mark the boundary beyond which a particular volatile chemical
species is frozen out onto dust grains. The detection of methanol also
in the colder outer regions of the disc shows that it is able to escape
off the grains at temperatures much lower than its sublimation
temperature, necessary to trigger thermal desorption.
More Information
This research was presented in a paper entitled “First detection of
gas-phase methanol in a protoplanetary disk”, by Catherine Walsh et al.,
published in Astrophysical Journal, Volume 823, Number 1.
The team is composed of Catherine Walsh (Leiden Observatory, Leiden
University, Leiden, The Netherlands), Ryan A. Loomis
(Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts,
USA), Karin I. Öberg (Harvard-Smithsonian Center for Astrophysics,
Cambridge, Massachusetts, USA), Mihkel Kama (Leiden Observatory, Leiden
University, Leiden, The Netherlands), Merel L. R. van't Hoff (Leiden
Observatory, Leiden University, Leiden, The Netherlands), Tom J. Millar
(School of Mathematics and Physics, Queen’s University Belfast, Belfast,
UK), Yuri Aikawa (Center for Computational Sciences, University of
Tsukuba, Tsukuba, Japan), Eric Herbst (Departments of Chemistry and
Astronomy, University of Virginia, Charlottesville, Virginia, USA),
Susanna L. Widicus Weaver (Department of Chemistry, Emory University,
Atlanta, Georgia, USA) and Hideko Nomura (Department of Earth and
Planetary Science, Tokyo Institute of Technology, Tokyo, Japan).
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of 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.
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 16 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the
United Kingdom, along with the host state of Chile. ESO carries out an
ambitious programme focused on the design, construction and operation of
powerful ground-based observing facilities enabling astronomers to make
important scientific discoveries. ESO also plays a leading role in
promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in Chile: La Silla,
Paranal and Chajnantor. At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
observatory and two survey telescopes. VISTA works in the infrared and
is the world’s largest survey telescope and the VLT Survey Telescope is
the largest telescope designed to exclusively survey the skies in
visible light. ESO is a major partner in ALMA, the largest astronomical
project in existence. And on Cerro Armazones, close to Paranal, ESO is
building the 39-metre European Extremely Large Telescope, the E-ELT,
which will become “the world’s biggest eye on the sky”.
Links
- Research paper
- Earlier ALMA observations of organic compounds in discs
- Photos of ALMA
- Other press releases featuring ALMA
Contacts
Catherine Walsh
Leiden Observatory
Leiden University, The Netherlands
Tel: +31 71527 ext 6287
Email: cwalsh@strw.leidenuniv.nl
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
