Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Dor: Non-LTE abundance analysis of CO, SiO, and HCN

(abridged) Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. We have analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with literature data. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We have performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the outflow probed by our molecular data. For SiO, we find that the initial abundance lies between $5.5 \times 10^{-5}$ and $6.0 \times 10^{-5}$ w.r.t. H$_2$. The abundance profile is constant up to $60\ \pm 10\ R_*$, after which it declines following a Gaussian profile with an $e$-folding radius of $3.5 \pm 0.5 \times 10^{13}$ cm. For HCN, we find an initial abundance of $5.0 \times 10^{-7}$ w.r.t. H$_2$. The Gaussian profile that describes the decline starts at the stellar surface and has an $e$-folding radius $r_e$ of $1.85 \pm 0.05 \times 10^{15}$ cm. We cannot to unambiguously identify the mechanism by which SiO is destroyed at $60\ \pm 10\ R_*$. The initial abundances found are larger than previously determined (except for one previous study on SiO), which might be due to the inclusion of higher-$J$ transitions. The difference in abundance for SiO and HCN compared to high mass-loss rate Mira star IK Tau might be due to different pulsation characteristics of the central star and/or a difference in dust condensation physics.status: publishe

Nucleosynthesis in AGB stars traced by oxygen isotopic ratios. I - Determining the stellar initial mass by means of the 17O/18O ratio

© ESO 2017. Aims. We seek to investigate the 17O/18O ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period. Methods. Circumstellar 13C16O, 12C17O, and 12C18O line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface 17O/18O abundance ratio. Results. Stellar evolution models predict the 17O/18O ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 MȮ. This makes the measured ratio a probe of the initial stellar mass. Conclusions. Observed 17O/18O ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the 17O/18O ratio to the stellar pulsation period.Accepted for publication in Astronomy and Astrophysics. Revised: implemented comments from refereestatus: publishe

Determining the stellar initial mass by means of the 17O/18O ratio on the AGB

<p>This poster presents newly obtained circumstellar ¹²C¹⁷O and ¹²C¹⁸O line observations, from which the line intensity are then related directly to the ¹⁷O/¹⁸O surface abundance ratio for a sample of nine AGB stars covering the three spectral types (). These ratios are evaluated in relation to a fundamental stellar evolution parameters: the stellar initial mass. The ¹⁷O/¹⁸O ratio is shown to function as an effective method of determining the initial stellar mass. Through comparison with predictions by stellar evolution models, accurate initial mass estimates are calculated for all nine sources. </p

The discovery of a planetary candidate around the evolved low-mass Kepler giant star HD 175370

We report on the discovery of a planetary companion candidate with a minimum mass M sin i = 4.6 ± 1.0 MJupiter orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our programme to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coudé echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph High Efficiency and Resolution Mercator Echelle Spectrograph of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 ± 4.5 d) variation with a semi-amplitude K = 133 ± 25 m s- 1, superimposed on a long-term trend. A low-mass stellar companion with an orbital period of ˜88 yr in a highly eccentric orbit and a planet in a Keplerian orbit with an eccentricity e = 0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry, we find that HD 175370 is most likely a low-mass red giant branch or asymptotic giant branch star.12 pages, 10 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review. The version of record DOI=10.1093/mnras/stw2379 is available online at: http://mnras.oxfordjournals.org/cgi/reprint/stw2379?ijkey=TaJttgdCtBDXV36&keytype=refstatus: publishe