Circumstellar environment of the M-type AGB star R Doradus

Abstract

Context. Our current insights into the circumstellar chemistry of asymptotic giant branch (AGB) stars are largely based on studies of carbon-rich stars and stars with high mass-loss rates. Aims. In order to expand the current molecular inventory of evolved stars we present a spectral scan of the nearby, oxygen-rich star R Dor, a star with a low mass-loss rate (∼ 2 × 10 −7 M ⊙ yr −1 ). Methods. We carried out a spectral scan in the frequency ranges 159.0 − 321.5 GHz and 338.5 − 368.5 GHz (wavelength range 0.8 − 1.9 mm) using the SEPIA/Band-5 and SHeFI instruments on the APEX telescope and we compare it to previous surveys, including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss rate (∼ 5 × 10 −6 M ⊙ yr −1 ). Results. The spectrum of R Dor is dominated by emission lines of SO 2 and the different isotopologues of SiO. We also detect CO, H 2 O, HCN, CN, PO, PN, SO, and tentatively TiO 2 , AlO, and NaCl. Sixteen out of approximately 320 spectral features remain unidentified. Among these is a strong but previously unknown maser at 354.2 GHz, which we suggest could pertain to H 2 SiO, silanone.With the exception of one, none of these unidentified lines are found in a similarly sensitive survey of IK Tau performed with the IRAM 30 m telescope. We present radiative transfer models for five isotopologues of SiO ( 28 SiO, 29 SiO, 30 SiO, Si 17 O, Si 18 O), providing constraints on their fractional abundance and radial extent. We derive isotopic ratios for C, O, Si, and S and estimate that, based on our results for 17 O/ 18 O, R Dor likely had an initial mass in the range 1.3− 1.6 M ⊙ , in agreement with earlier findings based on models of H 2 O line emission. From the presence of spectral features recurring in many of the measured thermal and maser emission lines we tentatively identify up to five kinematical components in the outflow of R Dor, indicating deviations from a smooth, spherical wind. Key words. Stars: AGB and post-AGB -stars: individual: R Dor -stars: mass loss -astrochemistry 1 http://www.iram.fr/IRAMFR/GILDAS/ 2 http://www.apex-telescope.org/backends/ffts/ Article number, page 2 of 66 E. De Beck and H. Olofsson: Circumstellar environment of the M-type AGB star R Dor CN CO H 2 O HCN SiO PO SO SO 2 u/t Flux contribution Line contribution 50 % 50 %

The nascent wind of AGB star R doradus: evidence for a recent episode of enhanced mass loss

“…Projected distances between ∼20 and ∼100 au host a slow radial wind with Doppler velocities reaching up to ∼6 km s −1 ; it is characterized by strong inhomogeneity, both in direction and radially, as had been noted earlier by De Beck & Olofsson (2018). The former takes the form of separate cores, together covering very large solid angles, and the latter suggests the occurrence of an episode of enhanced mass loss at the scale of a century ago.…”

“…In the present paper, we study instead the slower wind, exclusive of higher Doppler velocities. Earlier studies by Khouri et al (2016), Danilovich et al (2016), De Beck & Olofsson (2018), Van de Sande et al (2018), Decin et al (2018), Homan et al (2018) and Vlemmings et al (2018) have contributed a considerable amount of detailed information of relevance to the physicochemistry and dynamics of both dust and gas in this slower wind, however without attempting a detailed description of the morpho-kinematics, which is the aim of the present paper.…”

(Sub)mm-Wavelength Observations of Pre-Planetary Nebulae and Young Planetary Nebulae

“…To date, most studies on circumstellar chemistry have focused on AGB stars, even though still, at present, full spectral surveys, covering instantaneously a wide range of frequencies and multiple transitions for many species, have been performed only for a few objects. Some examples are the C-rich AGB star IRC+10216 [41,42], probably the best studied AGB star, and the O-rich AGB stars IK Tau and R Dor [43,44]. The chemistries of a number of pPNe are also relatively well characterized, particularly the C-rich pPNe CRL 618 [45] and CRL 2688 [46] and the O-rich source OH 231.8+4.2 [47,48].…”

Observation of High Doppler Velocity Wings in the Nascent Wind of R Doradus

“…The circumstellar envelope of R Dor has been probed by ALMA with a resolution of ∼4 au up to some 60 au from the star [7,10,11] and the dust has been observed at the VLT with a resolution of 1.2 au [23]. In addition, below ∼15 au, the analyses of Danilovich et al [24], De Beck & Olofsson [25] and Van de Sande et al [26] have contributed a considerable amount of detailed information of relevance to the physico-chemistry and dynamics of both dust and gas. At larger distances from the star, an analysis of ALMA observations of SO(J K = 6 5 − 5 4 ) emission [27], probing distances between 20 and 100 au, gives evidence for the wind to host a radial outflow covering large solid angles and displaying strong inhomogeneity both in direction and radially: the former takes the form of multiple cores and the latter displays a radial dependence suggesting an episode of enhanced mass loss having occurred a century or so ago.…”