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  • 1. Hjalmarson, Å.
    et al.
    Frisk, U.
    Olberg, M.
    Bergman, P.
    Bernath, P.
    Biver, N.
    Black, J. H.
    Booth, R. S.
    Buat, V.
    Crovisier, J.
    Curry, C. L.
    Dahlgren, M.
    Encrenaz, P. J.
    Falgarone, E.
    Feldman, P. A.
    Fich, M.
    Florén, H. G.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Fredrixon, M.
    Gerin, M.
    Gregersen, E. M.
    Hagström, M.
    Harju, J.
    Hasegawa, T.
    Horellou, C.
    Johansson, L. E. B.
    Kyrölä, E.
    Kwok, S.
    Larsson, B.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lecacheux, A.
    Liljeström, T.
    Lindqvist, M.
    Liseau, R.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Llewellyn, E. J.
    Mattila, K.
    Mégie, G.
    Mitchell, G. F.
    Murtagh, D.
    Nyman, L.-Å.
    Nordh, H. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, A. O. H.
    Olofsson, G.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pagani, L.
    Persson, G.
    Plume, R.
    Rickman, H.
    Ristorcelli, I.
    Rydbeck, G.
    Sandqvist, Aa.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    von Schéele, F.
    Serra, G.
    Torchinsky, S.
    Tothill, N. F.
    Volk, K.
    Wiklind, T.
    Wilson, C. D.
    Winnberg, A.
    Witt, G.
    Highlights from the first year of Odin observations2003Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 402, s. L39-L46Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Key Odin operational and instrumental features and highlights from our sub-millimetre and millimetre wave observations of H2O, H218O, NH3, 15NH3 and O2 are presented, with some insights into accompanying Odin Letters in this A&A issue. We focus on new results where Odin's high angular resolution, high frequency resolution, large spectrometer bandwidths, high sensitivity or/and frequency tuning capability are crucial: H2O mapping of the Orion KL, W3, DR21, S140 regions, and four comets; H2O observations of Galactic Centre sources, of shock enhanced H2O towards the SNR IC443, and of the candidate infall source IRAS 16293-2422; H218O detections in Orion KL and in comet Ikeya-Zhang; sub-mm detections of NH3 in Orion KL (outflow, ambient cloud and bar) and ρ Oph, and very recently, of 15NH3 in~Orion KL. Simultaneous sensitive searches for the 119 GHz line of O2 have resulted in very low abundance limits, which are difficult to accomodate in chemical models. We also demonstrate, by means of a quantitative comparison of Orion KL H2O results, that the Odin and SWAS observational data sets are very consistently calibrated. Odin is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes), and the Centre National d'études Spatiales (CNES, France). The Swedish Space Corporation (SSC) has been the prime industrial contractor, and is also responsible for the satellite operation from its Odin Mission Control Centre at SSC in Solna and its Odin Control Centre at ESRANGE near Kiruna in northern Sweden. See also the SNSB Odin web page: http://www.snsb.se/eng_odin_intro.shtml

  • 2.
    Justtanont, K.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Bergman, P.
    Onsala Space Observatory.
    Larsson, B.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, F. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Frisk, U.
    Hasegawa, T.
    Hjalmarson, Å.
    Kwok, S.
    Olberg, M.
    Sandqvist, Aa.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Volk, K.
    Elitzur, M.
    W Hya through the eye of Odin. Satellite observations of circumstellar submillimetre H2O line emission2005Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 439, s. 627-633Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H{2}O, 1{10}-101, at 557 GHz. The line is clearly of circumstellar origin. Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of (2.5±0.5)×10-7 Mȯ yr-1, and a circumstellar H{2}O abundance of (2.0±1.0)×10-3. The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission, and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow. The very high water abundance, higher than the cosmic oxygen abundance, can be explained by invoking an injection of excess water from evaporating icy bodies in the system. The required extra mass of water is quite small, on the order of 0.1 Moplus.

  • 3.
    Larsson, Bengt
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Liseau, Rene
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pagani, Laurent
    Bergman, Per
    Bernath, Peter
    Biver, Nicolas
    Black, John
    Booth, Roy
    Buat, Veronique
    Crovisier, Jacques
    Curry, Charles
    Dahlgren, Magnus
    Encrenaz, Pierre
    Falgarone, Edith
    Feldman, Paul
    Fish, Michel
    Florén, Hans-Gustav
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Fredrixon,
    Frisk, Urban
    Gahm, Gösta
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Gerin, Maryvonne
    Hagström, Magne
    Harju, Jorma
    Hasegawa, Tatsuhiko
    Hjalmarsson, Åke
    Johansson, Lars
    Justtanout, Kay
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Klotz, Alain
    Kytölä, Erikii
    Kwok, Sun
    Lecacheux, Alain
    Liljeström, Tarja
    Llewellyn, Edward
    Lundin, Stefan
    Mégie, Gérard
    Mitchell, Gary
    Murtagh, Donal
    Nordh, Lennart
    Nyman, Lars-Åke
    Olberg, Michael
    Olofsson, Henrik
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Persson, Glen
    Plume, Rene
    Rickman, Hans
    Ristorcelli, Isabelle
    Rydbeck, Gustaf
    Sandqvist, Aage
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    von Scheele, Fredrik
    Serra, Guy
    Torchinsky, Steve
    Tothill, Nick
    Volk, Kevin
    Wiklind, Tommy
    Wilson, Christine
    Winnberg, Anders
    Witt, George
    Meteorologiska institutionen (MISU).
    Molecular oxygen in the rho Ophiuchi cloud2007Inngår i: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 466, nr 3, s. 5-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context: Molecular oxygen, O2, has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission.

    Aims: The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods: The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs.

    Results: We detect a spectral line at v_LSR =+3.5 km s-1 with Δ v_FWHM=1.5 km s-1, parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O2 (NJ = 11 - 1_0) transition at 118 750.343 MHz.

    Conclusions: The abundance of molecular oxygen, relative to H{2} , is 5 × 10-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

    Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Étude Spatiale (CNES). The Swedish Space Corporation has been the industrial prime contractor and also is operating the satellite. Appendix A is only available in electronic form at http://www.aanda.org

  • 4.
    Maercker, Matthias
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, F.L.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Onsala Space Observatory, Sweden.
    Bergman, P.
    Frisk, U.
    Hjalmarson, Å.
    Justannont, K.
    Kwok, S.
    Larsson, Bengt
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olberg, M.
    Sandqvist, Aage
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Circumstellar water vapour in M-type AGB stars: constraints from H2O(1,10-1,01) lines obtained with odin2009Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 494, s. 243-252Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. A detailed radiative transfer code has been previously used to model circumstellar ortho-H2O line emission towards six M-type asymptotic giant branch stars using Infrared Space Observatory Long Wavelength Spectrometer data. Collisional and radiative excitation, including the v(2) = 1 state, was considered. Aims. Spectrally resolved circumstellar H2O(1(10)-1(01)) lines have been obtained towards three M-type AGB stars using the Odin satellite. This provides additional strong constraints on the properties of circumstellar H2O, in particular on the chemistry in the stellar atmosphere, and the photodissociation in the outer envelope. Methods. Infrared Space Observatory and Odin satellite H2O line data are used as constraints for radiative transfer models. Special consideration is taken to the spectrally resolved Odin line profiles, and the effect of excitation to the first excited vibrational states of the stretching modes (v(1) = 1 and v(3) = 1) on the derived abundances is estimated. A non-local, radiative transfer code based on the accelerated lambda iteration formalism is used. A statistical analysis is performed to determine the best-fit models. Results. The H2O abundance estimates are in agreement with previous estimates. The inclusion of the Odin data sets stronger constraints on the size of the H2O envelope. The H2O(1(10)-1(01)) line profiles require a significant reduction in expansion velocity compared to the terminal gas expansion velocity determined in models of CO radio line emission, indicating that the H2O emission lines probe a region where the wind is still being accelerated. Including the v(3) = 1 state significantly lowers the estimated abundances for the low-mass-loss-rate objects. This shows the importance of detailed modelling, in particular the details of the infrared spectrum in the range 3 to 6 mu m, to estimate accurate circumstellar H2O abundances. Conclusions. Spectrally resolved circumstellar H2O emission lines are important probes of the physics and chemistry in the inner regions of circumstellar envelopes around asymptotic giant branch stars. Predictions for H2O emission lines in the spectral range of the upcoming Herschel/HIFI mission indicate that these observations will be very important in this context.

  • 5.
    Maercker, Matthias
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, Fredrik
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Circumstellar H2O in M-type AGB stars2008Inngår i: Organic Matter in Space, 2008, s. 163-164Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type AGB star W Hya. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains.

  • 6.
    Maercker, Matthias
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, Fredrik
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Bergman, Per
    Ramstedt, Sofia
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI2008Inngår i: Astronomy and Astrophysics, Vol. 479, nr 1, s. 779-791Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context: Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type asymptotic giant branch star W Hya. This has lead to the speculation that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains. Aims: By performing a detailed radiative transfer analysis, we determine fractional abundances of circumstellar H2O in the envelopes around six M-type asymptotic giant branch stars. The models are also used to predict H2O spectral line emission for the upcoming Herschel/HIFI mission. Methods: We use Infrared Space Observatory Long Wavelength Spectrometer spectra to constrain the circumstellar fractional abundance distribution of ortho-H2O, using a non-local thermal equilibrium, and non-local, radiative transfer code based on the accelerated lambda iteration formalism. The mass-loss rates and kinetic temperature structures for the sample stars are determined through radiative transfer modelling of CO line emission based on the Monte-Carlo method. The density and temperature profiles of the circumstellar dust grains are determined through spectral energy distribution modelling using the publicly available code Dusty. Results: The determined ortho-H2O abundances lie between 2×10-4 and 1.5×10-3 relative to H2, with the exception of WX Psc, which has a much lower estimated ortho-H2O abundance of only 2×10-6, possibly indicating H2O adsorption onto dust grains or recent mass-loss-rate modulations. The estimated abundances are uncertain by, at best, a factor of a few. Conclusions: The high water abundance found for the majority of the sources suggests that either the “normal” chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H2O (e.g. from icy bodies like Kuiper belt objects) play a role. However, more detailed information on the physical structure and the velocity field of the region where the water vapour lines are formed is required to improve abundance estimates. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI. These spectrally resolved lines cover a wide range of excitation conditions and will provide valuable additional information on the physical and chemical properties of the inner stellar wind where H2O is abundant.

  • 7. Minier, V.
    et al.
    André, Ph.
    Bergman, P.
    Motte, F.
    Wyrowski, F.
    Le Pennec, J.
    Rodriguez, L.
    Boulade, O.
    Doumayrou, E.
    Dubreuil, D.
    Gallais, P.
    Hamon, G.
    Lagage, P.-O.
    Lortholary, M.
    Martignac, J.
    Revéret, V.
    Roussel, H.
    Talvard, M.
    Willmann, G.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArTéMiS2009Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 501, s. L1-L4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aims: Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 μ m with the CEA P-ArTéMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArTéMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 μm. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.

  • 8. Olofsson, A. O. H.
    et al.
    Olofsson, G.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Hjalmarson, Å.
    Bergman, P.
    Black, J. H.
    Booth, R. S.
    Buat, V.
    Curry, C. L.
    Encrenaz, P. J.
    Falgarone, E.
    Feldman, P.
    Fich, M.
    Florén, H. G.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Frisk, U.
    Gerin, M.
    Gregersen, E. M.
    Harju, J.
    Hasegawa, T.
    Johansson, L. E. B.
    Kwok, S.
    Larsson, B.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lecacheux, A.
    Liljeström, T.
    Liseau, R.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Mattila, K.
    Mitchell, G. F.
    Nordh, H. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olberg, M.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pagani, L.
    Plume, R.
    Ristorcelli, I.
    Rydbeck, G.
    Sandqvist, Aa.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    von Schéele, F.
    Serra, G.
    Tothill, N. F.
    Volk, K.
    Wilson, C. D.
    Odin water mapping in the Orion KL region2003Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 402, s. L47-L54Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    New results from water mapping observations of the Orion KL region using the submm/mm wave satellite Odin (2.1\arcmin beam size at 557 GHz), are presented. The ortho-H2O \jkktrans{1}{1}{0}{1}{0}{1} ground state transition was observed in a 7arcminx 7arcmin rectangular grid with a spacing of 1\arcmin, while the same line of H218O was measured in two positions, Orion KL itself and 2\arcmin south of Orion KL. In the main water species, the KL molecular outflow is largely resolved from the ambient cloud and it is found to have an extension of 60\arcsec-110\arcsec. The H2O outflow profile exhibits a rather striking absorption-like asymmetry at the line centre. Self-absorption in the near (or ``blue'') part of the outflow (and possibly in foreground quiescent halo gas) is tentatively suggested to play a role here. We argue that the dominant part of the KL H218O outflow emission emanates from the compact (size ~ 15\arcsec) low-velocity flow and here estimate an H2O abundance of circa 10-5 compared to all H2 in the flow - an order of magnitude below earlier estimates of the H2O abundance in the shocked gas of the high-velocity flow. The narrow ambient cloud lines show weak velocity trends, both in the N-S and E-W directions. H218O is detected for the first time in the southern position at a level of ~ 0.15 K and we here estimate an H2O abundance of (1-8) x 10-8. Odin is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes), and the Centre National d'Études Spatiales (CNES, France). The Swedish Space Corporation (SSC) was the industrial prime contractor and is also responsible for the satellite operation.

  • 9.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Molecular Abundances in AGB Circumstellar Envelopes2005Inngår i: Proceedings of the International Astronomical Union, ISSN 1743-9213, Vol. 1, nr S231, s. 499-508Artikkel, forskningsoversikt (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    n this review the present status of molecules in circumstellar envelopes of AGB stars is presented. Emphasis is put on the determination of abundances, and estimates of their uncertainties, from an observational point of view. Despite an impressive number of circumstellar species detected, about 60, there remains much work before general conclusions can be drawn. In particular, sophisticated radiative transfer modelling of circumstellar line emission must be done. This requires a detailed knowledge of the stellar and circumstellar properties, as well as basic molecular physics/chemistry data.

  • 10.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Molecules in the envelopes of late-type stars2006Inngår i: Astrochemistry: Recent Successes and Current Challenges, 2006, s. 499-508Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    n this review the present status of molecules in circumstellar envelopes of AGB stars is presented. Emphasis is put on the determination of abundances, and estimates of their uncertainties, from an observational point of view. Despite an impressive number of circumstellar species detected, about 60, there remains much work before general conclusions can be drawn. In particular, sophisticated radiative transfer modelling of circumstellar line emission must be done. This requires a detailed knowledge of the stellar and circumstellar properties, as well as basic molecular physics data.

  • 11.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The circumstellar environment of asymptotic giant branch stars2006Inngår i: The Many Facets of the Universe - Revelations by New Instruments, 2006, s. 75-100Konferansepaper (Annet vitenskapelig)
  • 12.
    Olofsson, Hans
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Maercker, Matthias
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Eriksson, K.
    Gustafsson, B.
    Schöier, F.
    High-resolution HST/ACS images of detached shells around carbon stars  2010Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 515, s. A27-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. Overall spherically symmetric, geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch (AGB) carbon stars. Their dynamical ages lie in the range of 103 to 104 years. A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash.

    Aims. The detached shells carry information on their formation process, as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion.

    Methods. The youngest detached shells, those around the carbon stars R Scl and U Cam, are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope. Quantitative results are derived assuming optically thin dust scattering.

    Results. The detached dust shells around R Scl and U Cam are found to be consistent with an overall spherical symmetry. They have radii of 192 (corresponding to a linear size of 8 × 1016 cm) and 77 (5 × 1016 cm), widths of 12 (5 × 1015 cm) and 06 (4 × 1015 cm), and dust masses of 3 × 10-6 and 3 × 10-7 , respectively. The dynamical ages of the R Scl and U Cam shells are estimated to be 1700 and 700 yr, respectively, and the shell widths correspond to time scales of 100 and 50 yr, respectively. Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the R Scl shell. Average clump dust masses are estimated to be about 2 × 10-9 . Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (1´´ for U Cam and 2´´ for R Scl).

    Conclusions. The results are consistent with the interpretation of geometrically thin gas and dust shells formed by a mass-loss eruption during a He-shell flash, and where interaction with a previous wind plays a role as well. The mass loss responsible for the shells must have been remarkably isotropic, and, if wind interaction plays a role, this also applies to the mass loss prior to the eruption. Clumpy structure is present in the R Scl shell, possibly as a consequence of the mass loss itself, but more likely as a consequence of instabilities in the expanding shell.

  • 13.
    Ramstedt, Sofia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, Fredrik L.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The physics and chemistry of circumstellar envelopes of S-stars on the AGB2008Inngår i: Proceedings of the International Astronomical Union, ISSN 1743-9213, E-ISSN 1743-9221, Vol. 4, nr S251, s. 167-168Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Presented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.

  • 14.
    Ramstedt, Sofia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, Fredrik L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lundgren, Adreas A.
    On the reliability of mass-loss-rate estimates for AGB stars2008Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 487, nr 2, s. 645-657Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a mass-loss-rate estimator for AGB stars. Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate-to high-mass-loss-rate AGB stars with different photospheric chemistries. Methods. Mass-loss rates for 10 intermediate-to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles. Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of similar to 3 for intermediate- to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H(2)-abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.

  • 15.
    Ramstedt, Sofia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Schöier, Fredrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lundgren, Andreas
    Mass-loss properties of S-stars on the AGB2006Inngår i: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 454, nr 2, s. L103-L106Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have used a detailed non-LTE radiative transfer code to model new APEX CO(J = 3 → 2) data, and existing CO radio line data, on a sample of 40 AGB S-stars. The derived mass-loss-rate distribution has a median value of 2 × 10-7~Mȯ yr-1, and resembles values obtained for similar samples of M-stars and carbon stars. Possibly, there is a scarcity of high-mass-loss-rate (≥10-5~Mȯ yr-1) S-stars. The distribution of envelope gas expansion velocities is similar to that of the M-stars, the median is 7.5 km s-1, while the carbon stars, in general, have higher gas expansion velocities. The mass-loss rate correlates well with the gas expansion velocity, in accordance with results for M-stars and carbon stars.

  • 16.
    Schöier, Fredrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Bast, Jeanette
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lindqvist, Michael
    The abundance of SiS in circumstellar envelopes around AGB stars2007Inngår i: Astronomy and Astrophysics, Vol. 473, nr 3, s. 871-882Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aims.Given their photospheric origin and refractive nature, SiS molecules can provide major constraints on the relative roles of dust condensation and non-equilibrium processes in regulating the chemistry in circumstellar envelopes around evolved stars.

    Methods: New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including assessment of the effect of thermal dust grains in the excitation analysis.

    Results: We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O > 1) have a mean fractional abundance of 3.1 × 10-6, about one order of magnitude higher than that found for the M-type AGB stars (C/O < 1) where the mean value is 2.7 × 10-7. These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similarly to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.

  • 17.
    Schöier, Fredrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Fong, David
    Olofsson, Hans
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Zhang, Qizhou
    Patel, Nimesh
    The Distribution of SiO in the Circumstellar Envelope around IRC +102162006Inngår i: The Astrophysical Journal, ISSN 0004-637X, Vol. 649, nr 2, s. 965-972Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    New interferometric observations of SiO J=5-->4 circumstellar line emission around the carbon star IRC +10216 using the Submillimeter Array are presented. Complemented by multitransition single-dish observations, including infrared observations of rovibrational transitions, detailed radiative transfer modeling suggests that the fractional abundance of SiO in the inner part of the envelope, between ~3 and 8 stellar radii, is as high as ~1.5×10-6. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (re~2.4×1016 cm) low fractional abundance (f0~1.7×10-7) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind, while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2-5 on an angular scale corresponding to a timescale of about 200 yr. In addition, constraints on the velocity structure of the wind are obtained.

  • 18. Winnberg, A.
    et al.
    Deguchi, S.
    Reid, M. J.
    Nakashima, J.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Habing, H. J.
    CO in OH/IR stars close to the Galactic centre2009Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 497, s. 177-181Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aims: A pilot project has been carried out to measure circumstellar CO emission from three OH/IR stars close to the Galactic centre. The intention was to find out whether it would be possible to conduct a large-scale survey for mass-loss rates using, for example, the Atacama large millimeter array (ALMA). Such a survey would increase our understanding of the evolution of the Galactic bulge. Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is necessary as a “spatial filter” in this region of space because of the confusion with interstellar CO emission. Results: Towards two of the stars, CO emission was detected with positions and radial velocities coinciding within the statistical errors with the corresponding data of the associated OH sources. However, for one of the stars the line profile is not what one expects for an unresolved expanding circumstellar envelope. We believe that this CO envelope is partially resolved and that this star therefore is a foreground star not belonging to the bulge. Conclusions: The results of the observations have shown that it is possible to detect line profiles of circumstellar CO from late-type stars both within and in the direction of the Galactic bulge. ALMA will be able to detect CO emission in short integrations with sensitivity sufficient to estimate mass-loss rates from a large number of such stars.

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