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CW Leo
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Title: An independent distance estimate to CW Leo
Authors: M. A. T. Groenewegen, M. J. Barlow, J. A. D. L. Blommaert, J. Cernicharo, L. Decin, H. L. Gomez, P. C. Hargrave, F. Kerschbaum, D. Ladjal, T. L. Lim, M. Matsuura, G. Olofsson, B. Sibthorpe, B. M. Swinyard, T. Ueta, J. Yates

CW Leo has been observed six times between October 2009 and June 2012 with the SPIRE instrument on board the Herschel satellite. Variability has been detected in the flux emitted by the central star with a period of 639 ±4 days, in good agreement with determinations in the literature. Variability is also detected in the bow shock around CW Leo that had previously been detected in the ultraviolet and Herschel PACS/SPIRE data. Although difficult to prove directly, our working hypothesis is that this variability is directly related to that of the central star. In this case, fitting a sine curve with the period fixed to 639 days results in a time-lag in the variability between bow shock and the central star of 402 ±37 days. The orientation of the bow shock relative to the plane of the sky is unknown. For an inclination angle of zero degrees, the observed time-lag translates into a distance to CW Leo of 130 ±13 pc, and for non-zero inclination angles the distance is smaller. Fitting the shape of the bow shock with an analytical model (Wilkin 1996), the effect of the inclination angle on the distance may be estimated. Making the additional assumption that the relative peculiar velocity between the interstellar medium (ISM) and CW Leo is determined entirely by the star space velocity with respect to the local standard of rest (i.e. a stationary ISM), the inclination angle is found to be (-33.3 ±0.8) degrees based on the observed proper motion and radial velocity. Using the Wilkin model, our current best estimate of the distance to CW Leo is 123 ±14 pc. For a distance of 123 pc, we derive a mean luminosity of 7790 ±150 Lsol (internal error).

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Giant star comes with ancient tree rings

The sun, bless its life-giving heart, is boring. Sure, it has the occasional outburst hurling charged particles into space that sometimes land on Earth with troublesome consequences - but for the most part it goes about the workaday business of burning nuclear fuel without incident.
But our star may not always be so staid. One day, it could start throwing off clumps of stardust that would act like cosmic tree rings. The clues to this arboreal future come from a more evolved star called CW Leo, which is already doing just that.

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IRC+10216
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Giant star expels multiple dust shells, researchers find

An international team led by Leen Decin, a K.U.Leuven astronomer, has discovered a series of dust shells in the vicinity of CW Leonis, a dying giant star. The star expelled the shells in the course of its long life: the most distant shell was expelled 16,000 years ago and, in that time, has drifted more than 7,000 billion kilometres from the star.

"Until recently, it was thought that giant star's surroundings were homogenous: evenly distributed matter without any exceptionally large clumps, but there are more and more indicators suggesting that this is not a reliable picture. New images from the Herschel satellite confirm this in a spectacular way: We discovered more than a dozen shells expelled throughout the star's life as a giant. The weakest shell we found is 7,000 billion kilometres from the star" -  Leen Decin.

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Herschel probes the dusty history of a giant star

About 5 thousand million years from now, our Sun will expand into a red giant, swelling to such a size that it may swallow the Earth. It will then begin to shed huge amounts of dust, surrounding itself with an expanding circumstellar envelope (CSE) that ultimately will become a planetary nebula. New insights into this process have been revealed by ESA's Herschel Space Observatory, which is providing unprecedented images of the complex, outer structure of a nearby CSE.
As part of a long term programme to study aging stars, known as the Mass loss of Evolved StarS (MESS) survey, Herschel's Photodetector Array Camera and Spectrometer (PACS) instrument has been used to observe a nearby, carbon-rich star known as IRC+10216, or CW Leonis.

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Position (J2000):     R.A. 09 47 57.406  |  Dec. +13° 16' 43.56''



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01/09/2010

Herschel has discovered that ultraviolet starlight is a key ingredient for making water in the atmosphere of some stars. It is the only explanation for why a dying star is surrounded by a gigantic cloud of hot water vapour. 
Every recipe needs a secret ingredient. When astronomers discovered an unexpected cloud of water vapour around the old star CW Leonis in 2001, they immediately began searching for the source. Water is known to be present around several types of stars, but CW Leonis is a "carbon star" and therefore thought not to produce water. Initially they suspected the star's heat must be evaporating comets or even dwarf planets to produce the water.
Now, Herschel's PACS and SPIRE instruments have revealed that the secret ingredient is ultraviolet light, because the water vapour is too hot to have come from the destruction of icy celestial bodies and is distributed throughout the stellar wind, including deep down near the surface of the star itself.  This suggests that the water is being created by a previously unsuspected chemical process where ultraviolet radiation from interstellar space is breaking up the carbon monoxide and releasing oxygen atoms that can then react with hydrogen to form water molecules.

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IRC+10216
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Title: Herschel/HIFI observations of IRC+10216: water vapor in the inner envelope of a carbon-rich AGB star
Authors: David A. Neufeld (JHU), Eduardo González-Alfonso (Alcala de Henares), Gary J. Melnick (CfA), Miroslaw Schmidt, Ryszard Szczerba (N. Copernicus Astro. Cntr.), Leen Decin (K.U. Leuven), Alex de Koter (Amsterdam & Utrecht), Fredrik Schöier (Onsala), José Cernicharo (CAB)

We report the results of observations of ten rotational transitions of water vapor toward the carbon-rich AGB (asymptotic giant branch) star IRC+10216 (CW Leonis), carried out with Herschel's HIFI instrument. Each transition was securely detected by means of observations using the dual beam switch mode of HIFI. The measured line ratios imply that water vapor is present in the inner outflow at small distances (few x 1.E+14 cm) from the star, confirming recent results reported by Decin et al. from observations with Herschel's PACS and SPIRE instruments. This finding definitively rules out the hypothesis that the observed water results from the vaporization of small icy objects in circular orbits. The origin of water within the dense C-rich envelope of IRC+10216 remains poorly understood. We derive upper limits on the H2-17O/H2-16O and H2-18O/H2-16O isotopic abundance ratios of ~ 5.E-3 (3 sigma), providing additional constraints on models for the origin of the water vapour in IRC+10216.

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Herschel Finds Water in a Cosmic Desert

The Herschel infrared space observatory has discovered that ultraviolet starlight is the key ingredient for making water in space. It is the only explanation for why a dying star is surrounded by a gigantic cloud of hot water vapour. Herschel is a European Space Agency mission with important participation from NASA.
Every recipe needs a secret ingredient. When astronomers discovered an unexpected cloud of water vapour around the old star IRC+10216 using NASA's Submillimeter Wave Astronomy Satellite in 2001, they immediately began searching for the source. Stars like IRC+10216 are known as carbon stars and are thought not to make much water. Initially they suspected the star's heat must be evaporating comets or even dwarf planets to produce the water.

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IRC +10216 or CW Leonis is the best studied carbon star, but also a very peculiar one with the central star being embedded in a thick dust envelope. Therefore, its energy is emitted mostly at infrared wavelengths: in fact, IRC +10216 is the brightest object in the sky at a wavelength of 10 m. Recent speckle observations (Weigelt et al. 1998 A&A,333,51, Haniff and Buscher 1998 A&A,334,5) are beginning to show the complex structure of the dust envelope.
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