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Alcalá de Henares, Spain

Bujarrabal V.,Observatorio Astronomico Nacional. Ap 112 | Mikolajewska J.,pernicus Astronomical Center | Alcolea J.,Observatorio Astronomico Nacional IGN | Quintana-Lacaci G.,Institute Radioastronomia Milimetrica IRAM
Astronomy and Astrophysics | Year: 2010

Aims. We have studied the molecular content of the circumstellar environs of symbiotic stellar systems, in particular of the well know objects R Aqr and CH Cyg. The study of molecules in these stars will help for understanding the properties of the very inner shells around the cool stellar component from which molecular emission is expected to come. Methods. We performed mm-wave observations with the IRAM 30 m telescope of the 12CO J = 1-0 and J = 2-1, 13CO J = 1-0 and J = 2-1, and SiO J = 5-4 transitions in the symbiotic stars R Aqr, CH Cyg, and HM Sge. The data were analyzed by means of a simple analytical description of the general properties of molecular emission from the inner shells around the cool star. Numerical calculations of the expected line profiles were also performed that took the level population and radiative transfer under such conditions into account. Results. Weak emission of 12CO J = 1-0 and J = 2-1 was detected in R Aqr and CH Cyg and a good line profile of 12CO J = 2-1 in R Aqr was obtained. The intensities and profile shapes of the detected lines are compatible with emission coming from a very small shell around the Mira-type star, with a radius comparable to or slightly smaller than the distance to the hot dwarf companion, 10 14-2×1014 cm. We argue that other possible explanations are improbable. This region probably shows properties similar to those characteristic of the inner shells around standard AGB stars: outwards expansion at about 5-25 km s-1, with a significant acceleration of the gas, temperatures decreasing with radius between about 1000 and 500 K, and densities ~109-3×108 cm-3. Our model calculations are able to explain the asymmetric line shape observed in 12CO J = 2-1 from R Aqr, in which the relatively weaker blue part of the profile would result from selfabsorption by the outer layers (in the presence of a velocity increase and a temperature decrease with radius). The mass-loss rates are somewhat higher than in standard AGB stars, as often happens for symbiotic systems. In R Aqr, we find that the total mass of the CO emitting region is ∼2-3×10-5 M⊙, corresponding to Ṁ ∼ 5×10-6-10-5 M⊙ yr-1 and compatible with results obtained from dust emission. Considering other existing data on molecular emission, we suggest that the limited extent of the molecule-rich gas in symbiotic systems is mainly due to molecule photodissociation by the radiation of the hot dwarf star. © ESO 2010.

Kamohara R.,Japan National Astronomical Observatory | Bujarrabal V.,Observatorio Astronomico Nacional. Ap 112 | Honma M.,Japan National Astronomical Observatory | Nakagawa A.,Kagoshima University | And 8 more authors.
Astronomy and Astrophysics | Year: 2010

Aims: We aim to study the spatial distribution of the v = 1 and v = 2 J = 1-0 SiO maser emission from R Aqr, a well known stellar symbiotic system. In particular, we intend to determine the annual parallax and proper motion of the source by means of measurements of the absolute coordinates of the maser spots. Methods: We performed VLBI observations of the v = 1 and v = 2 J = 1-0 maser emission, at 7 mm wavelength, using VERA. We present observations made in 11 epochs, between December 2004 and October 2006, and observations by other authors are also discussed. VERA provides very high spatial resolution and accurate astrometric measurements, thanks to its double-beam observing system. From fitting ring-like structures to the maser spot distributions, we determine absolute J2000 coordinates of the central star. (SiO maser emission is known to be distributed around the star forming spot rings at a few stellar radii.) Results: Maps with accurate absolute coordinates were obtained in 8 epochs. From the coordinates determined for the central star, we measured parallax and linear proper motion. We obtain π = 4.7 ± 0.8 mas, compatible with, but much more accurate than, the Hipparcos value (5.07 ± 3.15 mas), and deduce a distance of R Aqr D = 214-32 +45 pc. Our accurate astrometry also yields a reliable comparison between the spot distributions of both v = 1 and v = 2 J = 1-0 lines. We find that both masers come from similar regions, i.e. at similar distances from the star and defining common spot clusters, confirming previous results. But the coincidences between spots of both lines are very rare, within the spatial and spectral resolution of our experiments. This result is found for eight epochs, spanning more than one pulsation cycle (of the cool Mira-type component). We argue that explaining the finer details of the v = 1, 2 J = 1-0 distributions found here would require new theoretical efforts. Finally, our observations also allow the study of the structure and dynamics of the close circumstellar shells in R Aqr. We do not confirm the previous suggestion that these shells are rotating; instead, we conclude that the observed kinematics is very probably caused by pulsations and random movements. The spatial distribution of the maser spots is found to be variable, but to show a stable axial symmetry. © 2010 ESO.

Justtanont K.,Chalmers University of Technology | Decin L.,Catholic University of Leuven | Decin L.,University of Amsterdam | Schoier F.L.,Chalmers University of Technology | And 29 more authors.
Astronomy and Astrophysics | Year: 2010

Aims. A set of new, sensitive, and spectrally resolved, sub-millimeter line observations are used to probe the warm circumstellar gas around the S-type AGB star χ Cyg. The observed lines involve high rotational quantum numbers, which, combined with previously obtained lower-frequency data, make it possible to study in detail the chemical and physical properties of, essentially, the entire circumstellar envelope of χ Cyg. Methods. The data were obtained using the HIFI instrument aboard Herschel, whose high spectral resolution provides valuable information about the line profiles. Detailed, non-LTE, radiative transfer modelling, including dust radiative transfer coupled with a dynamical model, has been performed to derive the temperature, density, and velocity structure of the circumstellar envelope. Results. We report the first detection of circumstellar H2O rotational emission lines in an S-star. Using the high-J CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho-and para-H2O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H2O abundance (relative to H 2) is (1.1±0.2) × 10-5, much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1±0.6 is close to the high-temperature equilibrium limit, consistent with H2O being formed in the photosphere. © 2010 ESO.

Justtanont K.,Chalmers University of Technology | Khouri T.,University of Amsterdam | Maercker M.,University of Bonn | Maercker M.,European Southern Observatory | And 21 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form part of the guaranteed time key program HIFISTARS are presented. The aim of this program is to study the dynamical structure, mass-loss driving mechanism, and chemistry of the outflows from AGB stars as a function of chemical composition and initial mass. Methods. We used the HIFI instrument to observe nine AGB stars, mainly in the H 2O and high rotational CO lines. We investigate the correlation between line luminosity, line ratio and mass-loss rate, line width and excitation energy. Results. A total of nine different molecules, along with some of their isotopologues have been identified, covering a wide range of excitation temperature. Maser emission is detected in both the ortho-and para-H 2O molecules. The line luminosities of ground state lines of ortho-and para-H 2O, the high-J CO and NH 3 lines show a clear correlation with mass-loss rate. The line ratios of H 2O and NH 3 relative to CO J = 6-5 correlate with the mass-loss rate while ratios of higher CO lines to the 6-5 is independent of it. In most cases, the expansion velocity derived from the observed line width of highly excited transitions formed relatively close to the stellar photosphere is lower than that of lower excitation transitions, formed farther out, pointing to an accelerated outflow. In some objects, the vibrationally excited H 2O and SiO which probe the acceleration zone suggests the wind reaches its terminal velocity already in the innermost part of the envelope, i.e., the acceleration is rapid. Interestingly, for R Dor we find indications of a deceleration of the outflow in the region where the material has already escaped from the star. © 2012 ESO.

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