ALMA observations of the variable ¹²CO/¹³CO ratio around the asymptotic giant branch star R Sculptoris^⋆,⋆⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: wouter.vlemmings@chalmers.se
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

Abstract

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.