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29 March 2024
 
  » arxiv » astro-ph/0211473

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Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars
I. Bains ; R.J. Cohen ; A. Louridas ; A.M.S. Richards ; D. Rosa-Gonzalez ; J.A.Yates . ;
Date 21 Nov 2002
Journal Mon.Not.Roy.Astron.Soc. 342 (2003) 8
Subject astro-ph
Affiliation1,2), R.J. Cohen , A. Louridas (3,2), A.M.S. Richards , D. Rosa-Gonzalez (4,2) and J.A.Yates (5,1). ( University of Hertfordshire, UK, Jodrell Bank Observatory, UK, University of Kent, UK, INAOE, Mexico, UCL, UK.
AbstractWe present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O masers are found in thick expanding shells with inner radii ~ 6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly-filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4 au and filling factors of only ~ 0.01 with respect to the whole H2O maser shells. The CSE velocity structure gives additional evidence the maser clouds are density bounded. Masing clouds can be identified over a similar timescale to their sound crossing time (~2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.
Source arXiv, astro-ph/0211473
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