Circumstellar environment of the M-type AGB star R Dor. APEX spectral
  scan at $159.0-368.5$ GHz
  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. 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 ($\sim2\times10^{-7}M_{\odot}$/yr). We
carried out a spectral scan in the frequency ranges 159.0-321.5GHz and
338.5-368.5 GHz (wavelength range 0.8-1.9mm) 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 ($\sim5\times10^{-6}M_{\odot}$/yr). 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 30m 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 R Dor
likely had an initial mass in the range 1.3-1.6$M_{\odot}$, 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.
