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Register a new userMapping circumstellar magnetic fields of late-type evolved stars with the Goldreich-Kylafis effect: CARMA observations at $lambda 1.3$ mm of R Crt and R Leo
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Mapping magnetic fields is the key to resolving what remains an unclear physical picture of circumstellar magnetic fields in late-type evolved stars. Observations of linearly polarized emission from thermal molecular line transitions due to the Goldreich-Kylafis (G-K) effect provides valuable insight into the magnetic field geometry in these sources that is complementary to other key studies. In this paper, we present the detection of spectral-line polarization from both the thermal $J=2-1$ CO line and the $v=1, J=5-4$ SiO maser line toward two thermal-pulsating (TP-) AGB stars, R Crt and R Leo. The observed fractional linear polarization in the CO emission is measured as $m_lsim 3.1%$ and $m_lsim9.7%$ for R Crt and R Leo respectively. A circumstellar envelope (CSE) model profile and the associated parameters are estimated and used as input to a more detailed modeling of the predicted linear polarization expected from the G-K effect. The observed thermal line polarization level is consistent with the predicted results from the G-K model for R Crt; additional effects need to be considered for R Leo.
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We present new interferometer molecular observations of R Leo taken at 1.2 mm with the Atacama Large Millimeter Array with an angular resolution up to ~0.026 arcsec. These observations permit us to resolve the innermost envelope of this star revealing the existence of a complex structure that involves extended continuum emission and molecular emission showing a non-radial gas velocity distribution. This molecular emission displays prominent red-shifted absorptions located right in front to the star typical of material infall and lateral gas motions compatible with the presence of a torus-like structure.
The CARMA 1.3 mm polarization system consists of dual-polarization receivers that are sensitive to right- (R) and left-circular (L) polarization, and a spectral-line correlator that measures all four cross polarizations (RR, LL, LR, RL) on each of the 105 baselines connecting the 15 telescopes. Each receiver comprises a single feed horn, a waveguide circular polarizer, an orthomode transducer (OMT), two heterodyne mixers, and two low-noise amplifiers (LNAs), all mounted in a cryogenically cooled dewar. Here we review the basics of polarization observations, describe the construction and performance of key receiver components (circular polarizer, OMT, and mixers -- but not the correlator), and discuss in detail the calibration of the system, particularly the calibration of the R-L phase offsets and the polarization leakage corrections. The absolute accuracy of polarization position angle measurements was checked by mapping the radial polarization pattern across the disk of Mars. Transferring the Mars calibration to the well known polarization calibrator 3C286, we find a polarization position angle of $chi = 39.2 pm 1^{circ}$ for 3C286 at 225 GHz, consistent with other observations at millimeter wavelengths. Finally, we consider what limitations in accuracy are expected due to the signal-to-noise ratio, dynamic range, and primary beam polarization.
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 ($sim2times10^{-7}M_{odot}$/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 ($sim5times10^{-6}M_{odot}$/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.
We report and analyse FIR observations of two Herbig Be stars, R Mon and PDS 27, obtained with Herschels instruments PACS and SPIRE. We construct SEDs and derive the infrared excess. We extract line fluxes from the PACS and SPIRE spectra and construct rotational diagrams in order to estimate the excitation temperature of the gas. We derive CO, [OI] and [CI] luminosities to determine physical conditions of the gas, as well as the dominant cooling mechanism. We confirm that the Herbig Be stars are surrounded by remnants from their parental clouds, with an IR excess that mainly originates in a disc. In R Mon we detect [OI], [CI], [CII], CO (26 transitions), water and OH, while in PDS 27 we only detect [CI] and CO (8 transitions). We attribute the absence of OH and water in PDS 27 to UV photo-dissociation and photo-evaporation. From the rotational diagrams, we find several components for CO: we derive $T_{rot}$ 949$pm$90 K, 358$pm$20 K & 77$pm$12 K for R Mon, 96$pm$12 K & 31$pm$4 K for PDS 27 and 25$pm$8 K & 27$pm$6 K for their respective compact neighbours. The forsterite feature at 69$mu$m was not detected in either of the sources, probably due to the lack of (warm) crystalline dust in a flat disc. We find that cooling by molecules is dominant in the Herbig Be stars, while this is not the case in Herbig Ae stars where cooling by [OI] dominates. Moreover, we show that in the Herbig Be star R Mon, outflow shocks are the dominant gas heating mechanism, while in Herbig Ae stars this is stellar. The outflow of R Mon contributes to the observed line emission by heating the gas, both in the central spaxel/beam covering the disc and the immediate surroundings, as well as in those spaxels/beams covering the parabolic shell around it. PDS 27, a B2 star, has dispersed a large part of its gas content and/or destroyed molecules; this is likely given its intense UV field.
The circumstellar ammonia (NH$_3$) chemistry in evolved stars is poorly understood. Previous observations and modelling showed that NH$_3$ abundance in oxygen-rich stars is several orders of magnitude above that predicted by equilibrium chemistry. In this article, we characterise the spatial distribution and excitation of NH$_3$ in the O-rich circumstellar envelopes (CSEs) of four diverse targets: IK Tau, VY CMa, OH 231.8+4.2, and IRC +10420 with multi-wavelength observations. We observed the 1.3-cm inversion line emission with the Very Large Array (VLA) and submillimetre rotational line emission with the Heterodyne Instrument for the Far-Infrared (HIFI) aboard Herschel from all four targets. For IK Tau and VY CMa, we observed the rovibrational absorption lines in the $ u_2$ band near 10.5 $mu$m with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). We also attempted to search for the rotational transition within the $v_2=1$ state near 2 mm with the IRAM 30m Telescope towards IK Tau. Non-LTE radiative transfer modelling, including radiative pumping to the vibrational state, was carried out to derive the radial distribution of NH$_3$ in these CSEs. Our modelling shows that the NH$_3$ abundance relative to molecular hydrogen is generally of the order of $10^{-7}$, which is a few times lower than previous estimates that were made without considering radiative pumping and is at least 10 times higher than that in the C-rich CSE of IRC +10216. Incidentally, we also derived a new period of IK Tau from its $V$-band light curve. NH$_3$ is again detected in very high abundance in O-rich CSEs. Its emission mainly arises from localised spatial-kinematic structures that are probably denser than the ambient gas. Circumstellar shocks in the accelerated wind may contribute to the production of NH$_3$. (Abridged abstract)
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