Volume 556, August 2013
|Number of page(s)||6|
|Published online||24 July 2013|
ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris⋆,⋆⋆
Department of Earth and Space SciencesChalmers University of Technology,
Onsala Space Observatory,
2 Argelander Institute für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
3 European Southern Observatory, Karl Schwarzschild Str. 2, Garching bei München, Germany
4 South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, Western Cape, South Africa
5 Department of Physics and Astronomy, Division of Astronomy & Space Physics, Uppsala University, PO Box 516, 751 20 Uppsala, Sweden
6 University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
7 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
Received: 2 May 2013
Accepted: 30 June 2013
The 12CO/13CO ratio is often used as a measure of the 12C/13C ratio in the circumstellar environment, carrying important information about the stellar nucleosynthesis. External processes can change the 12CO and 13CO abundances, and spatially resolved studies of the 12CO/13CO ratio are needed to quantify the effect of these processes on the globally determined values. Additionally, such studies provide important information on the conditions in the circumstellar environment. The detached-shell source R Scl, displaying CO emission from recent mass loss, in a binary-induced spiral structure as well as in a clumpy shell produced during a thermal pulse, provides a unique laboratory for studying the differences in CO isotope abundances throughout its recent evolution. We observed both the 12CO(J = 3 → 2) and the 13CO(J = 3 → 2) line using ALMA. We find significant variations in the 12CO/13CO intensity ratios and consequently in the abundance ratios. The average CO isotope abundance ratio is at least a factor three lower in the shell (~19) than that in the present-day (≲300 years) mass loss (>60). Additionally, variations in the ratio of more than an order of magnitude are found in the shell itself. We attribute these variations to the competition between selective dissociation and isotope fractionation in the shell, of which large parts cannot be warmer than ~35 K. However, we also find that the 12CO/13CO ratio in the present-day mass loss is significantly higher than the 12C/13C ratio determined in the stellar photosphere from molecular tracers (~19). The origin of this discrepancy is still unclear, but we speculate that it is due to an embedded source of UV-radiation that is primarily photo-dissociating 13CO. This radiation source could be the hitherto hidden companion. Alternatively, the UV-radiation could originate from an active chromosphere of R Scl itself. Our results indicate that caution should be taken when directly relating the 12CO/13CO intensity and 12C/13C abundance ratios for specific asymptotic giant branch stars, in particular binaries or stars that display signs of chromospheric stellar activity.
Key words: stars: abundances / stars: AGB and post-AGB / stars: carbon / circumstellar matter
Appendices are available in electronic form at http://www.aanda.org
Data cubes of maps (FITS) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/556/L1
© ESO, 2013
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