Carbon isotopic abundance ratios in S-type stars^⋆

¹ Astronomy DepartmentUniversity of Washington, Seattle, WA 98195-1580, USA
e-mail: walleg@uw.edu; balick@uw.edu
² Observatorio Astronómico Nacional, Apartado 1143, Alcalá de Henares, 28800 Madrid, Spain
e-mail: alcolea@oan.es; v.bujarrabal@oan.es
³ Physical Sciences Dept., Everett Community College, Everett, WA, USA
e-mail: avanture@everettcc.edu

Received: 24 June 2010
Accepted: 26 September 2011

Abstract

We have used the IRAM 30 m telescope on Pico de Veleta to observe 11 S-type stars in the J = 1–0 and J = 2–1 transitions of both ¹²C¹⁶O and ¹³C¹⁶O in order to derive the ¹²CO/¹³CO ratio in their circumstellar envelopes. Detections of high signal to noise were achieved for 7 stars. A mean isotope ratio of 35.3 ± 6.2 was found. This ratio fits between the ratio for late M stars of about 12  ±  3 and that of carbon stars of 58 ± 17. Hence, we confirm the often proposed evolutionary sequence from late M through type S to carbon stars. Mass loss rates of 5 × 10^-8 to 3 × 10^-6   M_⊙ y^-1 are derived from the present CO data using a simple uniform outflow model with line-dependent opacities, in very good agreement with previous mass-loss estimates.