CIRCUMSTELLAR¹²C/¹³C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS

Abstract

A survey of the 12 C/ 13 C ratio toward circumstellar envelopes has been conducted at millimeter wavelengths using the facilities of the Arizona Radio Observatory (ARO). The ratios were obtained for a sample of local C-and O-rich asymptotic giant branch and supergiant stars from observations of the 12 C and 13 C isotopologues of CO and CN, respectively. The J = 1 → 0 transitions of both molecules were observed at λ = 3 mm using the ARO 12 m telescope, while the J = 2 → 1 lines of the two species were measured using the ARO Sub-Millimeter Telescope (SMT) at λ = 1 mm. The 12 C/ 13 C ratios were determined from the CO data by modeling both transitions simultaneously with a circumstellar radiative transfer code, which can account for the high opacities present in the emission from this species. In the case of CN, the hyperfine structure was used to evaluate opacity effects. Ratios obtained independently from CO and CN are in good agreement. For the C-rich envelopes, the ratios fall in the range 12 C/ 13 C ∼ 25-90, while the O-rich shells have values of 10-35. Ratios of 12 C/ 13 C ∼ 3-14 are found for the supergiant stars, with the exception of VY CMa, where the values lie in the range 25-46. All ratios obtained in this study are 89, the solar value, suggesting that substantial carbon-13 enrichment may be currently occurring in the local interstellar medium. A qualitative model was constructed based on first and third dredge-up convective mixing that can reproduce the observed ratios. Substantial mixing of H-burning products must occur to explain the ratios in the O-rich objects, while a wide range of 12 C/ 13 C values can be generated by only a few percent mixing of He-burning ashes in the C-rich case. The 12 C/ 13 C ratios obtained in this study should help improve stellar yield models and contribute to the understanding of Galactic chemical evolution.

A chemical inventory of the S-type AGB starχ Cygni based onHerschel/HIFI observations of circumstellar line emission

“…This suggests a 12 C/ 13 C isotopic ratio that is higher than what is found in M-type AGB stars (Knapp & Chang 1985;Milam et al 2009) and more in line with those derived for carbon stars (Knapp & Chang 1985;Milam et al 2009). However, we note here that Hinkle et al (1976) derive a 12 CO/ 13 CO-ratio of 25 ± 7 from CO vibrational lines (i.e., from lines originating in the stellar atmosphere) and that Milam et al (2009) Recently, Menten et al (2010) presented HIFI observations of the ortho-NH 3 J K = 1 0 → 0 0 transition for a sample of four oxygen-rich evolved stars. They report relatively high NH 3 abundances in the range from 2 × 10 −7 to 3 × 10 −6 .…”

Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles

“…The spread on 12 C/ 13 C for the other stars in the sample is quite large and we do not see any clear differences in the first-order estimates of 12 C/ 13 C when considering the different types of stars in the sample. Milam et al (2009) found 12 C/ 13 C ∼ 13 for NML Cyg and 12 C/ 13 C ∼ 14 for IRC +10420, while for VY CMa they derived a much higher value between 25 and 46. We find similar ratios with a first order estimate for VY CMa of 14.8, and values of 8.7 and 8.1 for NML Cyg and IRC +10420, respectively.…”

“…They analysed the multiple components for 12 C/ 13 C separately, resulting in three different values for VY CMa (46, 33 and 25) and 2 values for V Hya (71 and 72). Milam et al (2009) claim a ±30% uncertainty on their 12 C/ 13 C-values. Olofsson et al (1993) and Oudmaijer et al (2009) suggest that the acceleration of winds around C-rich stars is more efficient than that of winds around O-rich stars because of differences in dust absorption properties.…”

“…In this respect, OH/IR stars -typified by high luminosities, large infrared excesses and high mass-loss rates -are the best candidates for having very low 12 C/ 13 C isotope ratios (García-Hernández et al 2007, and references therein). Observational 12 C/ 13 C-values for AGB stars range from as low as 1 (Ohnaka & Tsuji 1999) up to about 90 (Milam et al 2009). Lebzelter et al (2008, and references therein) mention some clustering of empirical values of 12 C/ 13 C between 50 and 70 for galactic C-rich stars, while their evolutionary models even predict values higher than 100.…”

“…We find similar ratios with a first order estimate for VY CMa of 14.8, and values of 8.7 and 8.1 for NML Cyg and IRC +10420, respectively. We note that Milam et al (2009) deconvolved the line profiles for VY CMa and V Hya into separate velocity components, since the different structures probed could have discrepant chemical properties (see also Ziurys et al 2007, for VY CMa). They analysed the multiple components for 12 C/ 13 C separately, resulting in three different values for VY CMa (46, 33 and 25) and 2 values for V Hya (71 and 72).…”