As an alien sun blazes through its death throes, it is apparently
vaporizing a surrounding swarm of comets, releasing a huge cloud of
water vapor. The discovery, reported in an article to be published
tomorrow in the journal Nature, is the result of observations with the
Submillimeter Wave Astronomy Satellite (SWAS), a small radio observatory
NASA launched into space in December 1998.
The new SWAS observations provide the first evidence that extra-solar
planetary systems contain water, a molecule that is an essential
ingredient for known forms of life. “Over the past two years, SWAS has
detected water vapor from a wide variety of astronomical sources,” said
Dr. Gary Melnick of the Harvard-Smithsonian Center for Astrophysics,
Cambridge, MA, Principal Investigator on the SWAS mission. “What makes
the results we are reporting today so unusual is that we have found a
cloud of water vapor around a star where we would not ordinarily have
expected to find water.”
The star in question is an aging giant star designated by astronomers as
IRC+10216, also known as CW Leonis, located 500 light-years (almost
3,000 trillion miles) from Earth in the direction of the constellation
Leo.
“IRC+10216 is a carbon-rich star in which the concentration of carbon
exceeds that of oxygen,” Melnick said. “In such stars, we expect all the
oxygen atoms to be bound up in the form of carbon monoxide (an oxygen
atom and a carbon atom bound together), with almost nothing left over to
form water (one oxygen atom bound to two hydrogen atoms). Yet we see
substantial concentrations of water vapor around this star; the most
plausible explanation for this water vapor is that it is being vaporized
from the surfaces of orbiting comets, ‘dirty snowballs’ that are
composed primarily of water ice.”
From its vantage point in orbit above the absorbing effects of water in
Earth’s atmosphere, SWAS is capable of detecting the distinctive
radiation emitted by water vapor in space. The observations of water
vapor around IRC+10216 suggest that other stars may be surrounded by
planetary systems similar to our own. Over the past decade, more than 50
stars have been shown to have large planets in orbit around them, but
little is known about the composition of those planets.
In order to explain the water vapor concentration that SWAS has
detected, several hundred billion comets would be needed at distances
from the star between 75 and 300 times the distance of the Earth from
the Sun.
“That sounds like a lot,” said Saavik Ford, a graduate student at Johns
Hopkins University in Baltimore who is a co-author of the article
reporting the discovery. “But the total mass required of this swarm of
orbiting comets is similar to the original mass of the Kuiper Belt, a
collection of comets that orbits our own Sun beyond the orbit of
Neptune. In our own solar system, these comets orbit the Sun quietly for
the most part; occasionally a comet comes in close to the Sun, starts to
vaporize, and displays the characteristic coma and tail that we are
familiar with. But IRC+10216 is so much more luminous than the Sun that
comets start to vaporize even at the distance of the Kuiper Belt. So one
has several hundred billion comets all vaporizing at once.”
The SWAS observations of IRC+10216 paint a picture of the future of our
solar system. “We think we are witnessing the type of apocalypse that
will ultimately befall our own planetary system,” said SWAS team member
Dr. David Neufeld, a Johns Hopkins professor of physics and astronomy.
“Several billion years from now, the Sun will become a giant star and
its power output will increase five thousand fold. As the luminosity of
the Sun increases, a wave of water vaporization will spread outwards
through the solar system, starting with Earth’s oceans and extending
well beyond the orbit of Neptune. Icy bodies as large as Pluto will be
mostly vaporized, leaving a cinder of hot rock.”
SWAS was built and operated by NASA with support from the German
government and the participation of the Harvard-Smithsonian Center for
Astrophysics; the University of Massachusetts at Amherst; Cornell
University, Ithaca, NY; the Johns Hopkins University; the University of
Cologne in Germany; Ball Aerospace, Boulder, CO; and Millitech (now
Telaxis Communication Corp.), South Deerfield, MA.
In addition to Melnick, Neufeld and Ford, the other co-authors are Dr.
David Hollenbach of NASA’s Ames Research Center, Moffett Field, CA, and
Dr. Matthew Ashby of the Harvard-Smithsonian Center for Astrophysics.
Images and additional information on SWAS can be found on the Internet
at:
http://cfa-www.harvard.edu/cfa/oir/Research/swas.html
Related links
orbiting another star, Nature (subscription required)
Authors: Gary J. Melnick (CfA), David A. Neufeld (JHU), K.E. Saavik Ford (JHU), David J. Hollenbach (NASA-Ames), Matthew L.N. Ashby (CfA)
Comments: 15 pages, to appear in Nature
Since 1995, astronomers have discovered planets with masses comparable to that of Jupiter (318 times Earth’s mass) in orbit around approximately 60 stars. Although unseen directly, the presence of these planets is inferred by the small reflex motions that they gravitationally induce on the star they orbit; these result in small periodic wavelength shifts in the stellar spectrum. Since this method favors the detection of massive objects orbiting in close proximity to the star, the question of whether these systems also contain analogs of the smaller constituents of our Solar System has remained unanswered. Using an alternative approach, we report here observations of an aging carbon-star, IRC+10216, that reveal the presence of circumstellar water vapor, a molecule not expected in measurable abundances around such a star and thus a distinctive signature of an orbiting cometary system. The only plausible explanation for this water vapor is that the recent evolution of IRC+10216 – which is accompanied by a prodigious increase in its luminosity – is now causing the vaporization of a collection of orbiting icy bodies, a process first considered in a previous theoretical study.
Authors: K.E. Saavik Ford, David A. Neufeld (JHU)
Comments: 14 pages of text (AASTeX), plus 4 ps figures. Accepted for publication in the Astrophysical Journal Letters
We argue that the presence of water vapor in the circumstellar outflow of a carbon-rich AGB star is potentially a distinctive signature of extra-solar cometary systems. Detailed models show that at suitable distances from the star, water ice can survive well into the carbon-rich AGB phase; water vapor abundances as large as 10^-6 could result from the vaporization of a collection of orbiting icy bodies with a total mass comparable to what
might have been originally present in the solar system’s Kuiper Belt. In particular, the recently-reported detection by the Submillimeter Wave
Astronomy Satellite of water vapor in the circumstellar outflow of IRC+10216 can be explained if ~10 Earth masses of ice is present at a distance
~300 AU from that carbon-rich star. Future observations with the Herschel Space Observatory (HSO, formerly known as FIRST) will facilitate
sensitive multi-transition observations of water, yielding line ratios that can establish the radial distribution of water vapor in IRC+10216. The greater
sensitivity of HSO will also allow searches for water vapor to be carried out in a much larger sample of carbon-rich AGB stars.
