Herschel detection explains the origin of water in a carbon star
1 September 2010ESA's Herschel Space Observatory has detected water vapour in a location previously thought to be impossible - in the atmosphere of an ageing, red giant carbon star. The rich and detailed data provided by Herschel can be explained within a new framework in which ultraviolet photons play a key role. These results are reported in the 2 September issue of Nature.
By studying the various phases in the life cycle of a star, astronomers can piece together the processes at play in the evolution of stars and in their interactions with their environment. Even a relatively unremarkable low-mass star (one with a mass less than about 8-9 times that of our Sun) can turn up some surprises, and that is what CW Leonis, also known as IRC +10216, an ageing, red giant carbon star has done.
The life cycle of a low-mass star ends more with a whimper than with a bang: after the hydrogen reservoir in its core has been consumed, it starts to burn the next available nuclear fuel, helium, converting it into carbon. When the helium is exhausted, however, the star cannot reach the extremely high temperatures required for the nuclear fusion of heavier elements. At this stage, the star will have inflated (with the stellar radius increasing by a factor of several hundred to a thousand) and become a red giant. It now experiences a phase of heavy mass loss, during which it will eject its outer layers, forming a circumstellar shell containing dust and molecules - this period is referred to as the Asymptotic Giant Branch (AGB) phase, the name referring to the location of the star on the Hertzsprung-Russell diagram. At the same time, a small remnant of the star, rich in carbon and oxygen, continues to contract and evolves to a tiny, extremely hot white dwarf.
The gas expelled into interstellar space by the very strong stellar winds of these AGB stars is rich in heavy elements, especially carbon and oxygen, with the relative abundances of the two varying from star to star. CW Leonis, is the brightest (in infrared light) and closest red giant star, and has an envelope dominated by carbon. Such a carbon-rich environment is expected to drive a series of organic chemistry reactions, with almost all of the oxygen present being tied up in molecules of carbon monoxide (CO) and silicon monoxide (SiO).
It was therefore a complete surprise when, in 2001, observations of CW Leonis conducted with the Submillimetre Wave Astronomy Satellite (SWAS) revealed the presence of water vapour (H2O) in the envelope of the star. "Water is an extremely important molecule, central to the very existence of life on Earth, hence its detection in an unexpected cosmic environment triggered a number of investigations to try and figure out its origin," says Leen Decin from the University of Leuven, who led the investigation of CW Leonis (using data from ESA's Herschel Space Observatory) that is reported in the journal Nature.
The unequivocal signature from the SWAS observations was a spectral line corresponding to the ground-state of the water molecule and with a temperature of only 61 Kelvin; this placed the water in the outer, cool and diffuse envelope of the star. Astronomers explained this baffling discovery by assuming that the water arose from the vaporisation of a cloud of icy bodies, such as comets or dwarf planets, surrounding the star. However, other mechanisms were also invoked to explain the presence of water vapour in the envelope of CW Leonis and the detection of a single line was not sufficient to allow very strong constraints to be placed.
The availability of a sensitive new probe - in the form of the spectrometers on the Herschel Space Observatory - has dramatically changed this situation. In November 2009, Herschel observed CW Leonis with both the SPIRE and PACS spectrometers, covering the wavelength range between 55 and 670 microns. "Thanks to Herschel's superb sensitivity and spectral resolution, we were able to identify more than 60 lines of water, corresponding to a whole series of energetic levels of the molecule," explains Decin.
The detection of a plethora of lines emitted by the same molecule provides crucial information: since each line corresponds to a specific energy and hence to a specific temperature, the multitude of lines helps to trace the source of the water throughout the circumstellar envelope, with lines corresponding to higher temperatures placing the molecule closer and closer to the star's surface. The high-precision spectra obtained with the Herschel spectrometers exhibit lines that indicate temperatures up to 1000 Kelvin, implying that water exists not only in the outer envelope - as indicated with the SWAS data - but also in the intermediate and inner envelope of CW Leonis; this calls for a revised mechanism to explain the new observational evidence.
In order to produce water in such a carbon-rich environment, atomic oxygen has to be released from the molecules where it is locked up (mostly CO and SiO) before it can combine with hydrogen. Energetic radiation, in this case from ultraviolet (UV) photons, is required to dissociate these oxygen-carrier molecules.
Circumstellar envelopes have been shown to be clumpy in structure. "It is their patchy structure that allows UV photons from interstellar space to penetrate deep enough through the envelope," explains Decin. "Well within the envelope, UV photons trigger a set of reactions that can produce the observed distribution of water, as well as other, very interesting molecules, such as ammonia (NH3)."
These Herschel data challenge our current knowledge of the chemistry that takes place in the envelopes surrounding ageing stars, and draws attention to the importance of photochemistry induced by UV photons in such environments. A similar process can, for example, explain also the opposite situation, namely the presence of carbon-rich molecules observed in AGB stars with envelopes that are dominated by oxygen.
"This very interesting result highlights the ability of Herschel to broaden our horizons by virtue of its new unique observing capabilities. Here they are providing us with new important insights on the origin of water and other molecules in a particular cosmic environment," comments Göran Pilbratt, Herschel Project Scientist.
In the upcoming months, Herschel will probe other carbon stars in order to test the validity of this mechanism over a larger sample of observations.
Notes for editors:
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia, with important participation from NASA.
PACS is an imaging photometer and integral field line spectrometer covering wavelengths between 57 and 210 µm. PACS was built by a consortium of institutes and university departments from across Europe, and is led by Albrecht Poglitsch of the Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany. Consortium members are: UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAPD, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).
The SPIRE instrument comprises an imaging photometer (camera) and an imaging spectrometer. The camera operates in three wavelength bands centred on 250, 350 and 500 μm, and so can make images of the sky simultaneously in three submillimetre "colours". The spectrometer covers the range 200 – 670 μm, allowing the spectral features of atoms and molecules to be measured. SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). The consortium is led by Professor Matt Griffin of Cardiff University.
The observations with SPIRE and PACS of CW Leonis were performed as part of the MESS - Mass-loss of Evolved StarS Herschel Key Programme, led by M. Groenewegen, Royal Observatory of Belgium. CW Leonis has also been observed with the HIFI instrument, as part of the HIFISTARS Herschel Key Programme, led by Valentin Bujarrabal, Observatorio Astronomico Nacional, Spain.
Decin, L., Agúndez, M., Barlow, M. J., et al., "Warm water vapour in the sooty outflow from a luminous carbon star", Nature, 467, 2010. DOI: 10.1038/nature09344
Katholieke Universiteit Leuven
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