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ALMA Observations of the Variable 12CO/13CO Ratio Around the Asymptotic Giant Branch Star R Sculptoris,

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ALMA Observations of the Variable 12CO/13CO Ratio Around the Asymptotic Giant Branch Star R Sculptoris�, A&A 556, L1 (2013) Astronomy DOI: 10.1051/0004-6361/201321821 & c ESO 2013 Astrophysics Letter to the Editor ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris, W. H. T. Vlemmings1, M. Maercker2,3, M. Lindqvist1, S. Mohamed4,H.Olofsson1, S. Ramstedt5, M. Brunner6, M. A. T. Groenewegen7, F. Kerschbaum6, and M. Wittkowski3 1 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden e-mail: [email protected] 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 stud- ies 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 1. Introduction stellar atmosphere models to photospheric lines. There have also been several papers in which the carbon isotope ratio is deter- The study of different isotope ratios provides information on the mined in the circumstellar envelope itself, mainly by observa- enrichment history of the interstellar medium. In particular the tions of the 12CO/13CO ratio (e.g. Groenewegen et al. 1996; 12 13 C/ C ratio carries an imprint of stellar evolution and nucle- Greaves & Holland 1997; Schöier & Olofsson 2000; Milam et al. 12 osynthesis, as C is directly produced in the triple-α process, 2009). One of the main conclusions from Schöier & Olofsson 13 12 while C is created from C as an intermediate product of the (2000) is that the 12CO/13CO abundance ratio is a good proxy for carbon-nitrogen-oxygen (CNO) cycle. As the importance of the determining the 12C/13C ratio provided accurate radiative trans- 12 13 CNO cycle increases for more massive stars, the C/ C ratio fer modeling is performed. can be used to trace the past star formation rate and stellar mass However, the aforementioned CO observations were all per- function (e.g. Prantzos et al. 1996; Greaves & Holland 1997). formed with single-dish telescopes, which are insensitive to spa- Several studies have determined the photospheric 12C/13Cra- tial variations in the 12CO/13CO intensity ratio. The effect of, for tio in evolved asymptotic giant branch (AGB) stars (e.g. Lambert example, an inhomogeneous circumstellar environment or (hid- et al. 1986; Ohnaka & Tsuji 1996; Abia & Isern 1997), by fitting den) companion on the 12CO/13CO ratio throughout the circum- stellar envelope is thus poorly known. Previous submillimeter Appendices are available in electronic form at interferometer instruments were not sensitive enough to map the 13 http://www.aanda.org often weak CO emission at sufficient angular resolution. This Data cubes of maps (FITS) are only available at the CDS via has now changed with the construction of ALMA, which for the anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)orvia first time provides sufficient angular resolution and sensitivity to http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/556/L1 map the circumstellar 13CO emission of AGB stars in detail. Article published by EDP Sciences L1, page 1 of 6 A&A 556, L1 (2013) -32 32 15 −35 km/s−32 km/s −29 km/s −26 km/s 30 45 33 00 -32 32 15 −23 km/s −20 km/s −17 km/s −14 km/s 30 45 Declination (J2000) 33 00 -32 32 15 −11 km/s −8 km/s −5 km/s −2 km/s 13 30 Fig. 1. CO(J = 3 → 2) intensity contours. The channels are averaged over three km s−1. 45 The panels are labeled with their VLSR and the beam is indicated in the top left panel. The 13CO contour levels are drawn at −4σ 33 00 01 27 00 26 59 58 57 01 27 00 26 59 58 57 (dashed) and 4, 8, 12, and 16σ, with σ = Right ascension (J2000) 15 mJy beam−1. Here we present 13CO(J = 3 → 2) ALMA observations of the for the time and frequency dependence of the system tempera- carbon-rich AGB star R Scl located at ∼290 pc (Knapp et al. tures, we improved the antenna positions and performed a man- 2003). This star is one of about a dozen detached-shell sources, ual delay calibration. Bandpass calibration was performed on the where the shell is thought to be created due to mass-loss mod- quasar 3C 454.3 (2.2Jybeam−1). The primary flux calibration ulation during a He-shell flash (i.e., a thermal pulse) (Olofsson was made using Neptune and bootstrapping to the gain calibra- et al. 1990). The 12CO(J = 3 → 2) ALMA observations, taken tor J0137-245 (0.49 Jy beam−1). Based on the calibrator fluxes, together with the observations presented here, revealed in addi- the absolute flux calibration has an uncertainty of ∼10%. tion to the shell an unexpected spiral connecting the shell to the Imaging was then done using the CASA clean algo- present-day (∼<300 yr) mass loss (Maercker et al. 2012, hereafter rithm, smoothing the data to 3 km s−1 to detect the weak M+12). This spiral pattern is thought to be caused by the motion 13CO(J = 3 → 2) emission. Similar smoothing was performed of a previously unknown companion at ∼60 AU from R Scl. on the 12CO(J = 3 → 2) line for a direct comparison. The rms in the emission line channels was ∼15 mJy beam−1 and ∼ −1 13 = → 12 = → 2. ALMA observations and results 25 mJy beam for the CO(J 3 2) and CO(J 3 2) lines, respectively. The increase in the rms noise for the The 12CO(J = 3 → 2) and 13CO(J = 3 → 2) emission lines of 12CO(J = 3 → 2) line is due to the complex structure of the emis- R Scl were observed using ALMA Band 7 (275-373 GHz) on sion (M+12). Because of the lack of short spacings, not all October 3–4 and 18–19 2011 during the ALMA cycle 0 ob- CO flux is recovered. Emission at scales larger than ∼15 is re- serving program. The Band 7 data contained four spectral win- solved out. Based on single-dish observations of 12CO(J = 3 → dows of 1.875 GHz and 3840 channels each that were tuned 2) and 13CO(J = 3 → 2) with the APEX telescope, we estimate at 345.1 GHz, 343.3 GHz, 331.1 GHz, and 333.0 GHz. This ∼25% of the emission is recovered for both transitions (see allowed us to simultaneously cover the 12CO(J = 3 → 2) and Appendix A). Our conclusions on the intensity ratio are thus not 13CO(J = 3 → 2) at 345.795 GHz and 330.587 GHz, respec- affected. tively. The results of the 12CO(J = 3 → 2) observations were The 13CO(J = 3 → 2) emission (hereafter denoted with only presented in M+12 and here we focus specifically on the 13CO) detected around R Scl is shown in Fig. 1.Wefindemis- 13CO(J = 3 → 2) line. The channel spacing of 0.488 MHz re- sion in the detached shell, for which the 12CO(J = 3 → 2) (here- sulted after Hanning smoothing, in a maximum spectral reso- after 12CO) maps are presented in M+12.
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