| 1. |
- Danilovich, Taissa, 1987, et al.
(författare)
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Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 569, s. Art. no. A76-
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Tidskriftsartikel (refereegranskat)abstract
- Context. S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. Aims. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. Methods. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. Results. We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0 x 10(-6) M-circle dot yr(-1) based on the (CO)-C-12 lines. The estimated (CO)-C-12/(CO)-C-13 ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5 x 10(-5), which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3 x 10(-6), and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6 x 10(-6), and distribution of CN in W Aql's circumstellar envelope using ground-based data. We find an SiO abundance of 3 x 10(-6), and an NH3 abundance of 1.7 x 10(-5), confined to a small envelope. If we include uncertainties in the adopted circumstellar model - in the adopted abundance distributions, etc. - the errors in the abundances are of the order of factors of a few. The data also suggest that, in terms of HCN, S-type and M-type AGB stars are similar, and in terms of H2O, S-type AGB stars are more like C-type than M-type AGB stars. We detect excess blue-shifted emission in several molecular lines, possibly due to an asymmetric outflow. Conclusions. The estimated abundances of circumstellar HCN, SiO and H2O place W Aql in between M-and C-type AGB stars, i.e., the abundances are consistent with an S-type classification.
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| 2. |
- Maercker, Matthias, 1979, et al.
(författare)
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The detached dust shells around the carbon AGB stars R Sculptoris and V644 Scorpii
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570, s. A101-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The morphology of the circumstellar envelopes (CSE) around asymptotic giant branch (AGB) stars gives information on the mass-loss process from the star, its evolution and wind, and on the effect of binary interaction. However, determining the distribution of dust in the circumstellar envelopes is difficult. Observations of polarised, dust-scattered stellar light in the optical have produced images with high-spatial resolution of the envelopes around evolved stars. For sources with detached shells in particular, this method has proven extremely successful. Detached shells are believed to be created during a thermal pulse, and studying them can constrain the time scales and physical properties of one of the main drivers of late stellar evolution. Aims. We aim at determining the morphology of the detached shells around the carbon AGB stars R Scl and V644 Sco. In particular, we attempt to constrain the radii and widths of the detached dust shells around the stars and compare them to observations of the detached gas shells. Methods. We observed the polarised, dust-scattered stellar light around the carbon AGB stars R Scl and V644 Sco using the PolCor instrument mounted on the ESO 3.6 m telescope. Observations were done with a coronographic mask to block out the direct stellar light. The polarised images clearly show the detached shells around R Scl and V644 Sco. Using a dust radiative transfer code to model the dust-scattered polarised light, we constrained the radii and widths of the shells. Results. We determine radii of 19 ''.5 and 9 ''.4 for the detached dust shells around R Scl and V644 Sco, respectively. Both shells have an overall spherical symmetry and widths of approximate to 2 ''. For R Scl, we can compare the observed dust emission directly with high spatialresolution maps of CO(3-2) emission from the shell observed with ALMA. We find that the dust and gas coincide almost exactly, indicating a common evolution. The data presented here for R Scl are the most detailed observations of the entire dusty detached shell to date. For V644 Sco, these are the first direct measurements of the detached shell. Also here we find that the dust most likely coincides with the gas shell. Conclusions. The observations are consistent with a scenario where the detached shells are created during a thermal pulse. The determined radii and widths will constrain hydrodynamical models describing the pre-pulse mass loss, the thermal pulse, and postpulse evolution of the star.
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| 3. |
- Olofsson, Hans, 1952, et al.
(författare)
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A Possible Solution to the Mass-Loss Problem in M-type AGB Stars
- 2011
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Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. ; 445, s. 365-366
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 4. |
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| 5. |
- Ramstedt, Sofia, et al.
(författare)
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The (CO)-C-12/(CO)-C-13 ratio in AGB stars of different chemical type Connection to the C-12/C-13 ratio and the evolution along the AGB
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 566, s. A145-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this paper is to investigate the evolution of the C-12/C-13 ratio along the AGB through the circumstellar (CO)-C-12/(CO)-C-13 abundance ratio. This is the first time a sample including a significant number of M-and S-type stars is analysed together with a carbon-star sample of equal size, making it possible to investigate trends among the different types and establish evolutionary effects. Methods. The circumstellar (CO)-C-12/(CO)-C-13 abundance ratios are estimated through a detailed radiative transfer analysis of single-dish radio line emission observations. Several different transitions have been observed for each source to ensure that a large extent of the circumstellar envelope is probed and the radiative transfer model is well constrained. The radiative transfer model is based on the Monte Carlo method and has been benchmarked against a set of similar codes. It assumes that the radiation field is non-local and solves the statistical equilibrium equations in full non-local thermodynamic equilibrium. The energy balance equation, determining the gas temperature distribution, is solved self-consistently, and the effects of thermal dust radiation (as estimated from the spectral energy distribution) are taken into account. First, the (CO)-C-12 radiative transfer is solved, assuming an abundance (dependent on the chemical type of the star), to give the physical parameters of the gas, i.e. mass-loss rate. M. gas expansion velocity, v(e), and gas temperature distribution. Then, the (CO)-C-13 radiative transfer is solved using the results of the (CO)-C-12 model giving the (CO)-C-13 abundance. Finally, the (CO)-C-12/(CO)-C-13 abundance ratio is calculated. Results. The circumstellar (CO)-C-12/(CO)-C-13 abundance ratio differs between the three spectral types. This is consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence; however, this is the first time this has been shown observationally for a relatively large sample covering all three spectral types. The median value of the (CO)-C-13 abundance in the inner circumstellar envelope is 1.6 x 10(-5), 2.3 x 10(-5), and 3.0 x 10(-5) for the M-type. S-type, and carbon stars of the sample, respectively, corresponding to (CO)-C-12/(CO)-C-13 abundance ratios of 13, 26, and 34, respectively. The spread in the (CO)-C-13 abundance, quantified by the ratio between the 90th and 10th percentile, is 4, 3, and 15 for the M-type. S-type, and carbon stars, respectively. Interestingly, the abundance ratio spread of the carbon stars is much larger than for the M-and S-type stars, even when excluding J-type carbon stars, in line with what could be expected from evolution on the AGB. We find no correlation between the isotopologue ratio and the mass-loss rate, as would be expected if both increase as the star evolves.
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| 6. |
- Ramstedt, Sofia, et al.
(författare)
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The Mass-Loss Rates and Molecular Abundances of S-type AGB Stars
- 2011
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Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. - 9781583817704 ; 445, s. 263-268
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Ramstedt, Sofia, et al.
(författare)
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The wonderful complexity of the Mira AB system
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570, s. Art. no. L14-
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Tidskriftsartikel (refereegranskat)abstract
- We have mapped the (CO)-C-12(3-2) line emission around the Mira AB system at 0 ''.5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0 ''.487 +/- 0 ''.006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5 ''. In the blue wing of the line emission, offset from the main line, several large (similar to 5-10 '') opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.
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| 8. |
- Sacuto, Stéphane, et al.
(författare)
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The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 551, s. A72-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 mu m. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 mu m visibility measurements for all baselines. Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 mu m spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.
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| 9. |
- Schöier, Fredrik, 1969, et al.
(författare)
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AGB Mass-Loss Characteristics: Rates, Temporal Variation, and Geometries
- 2011
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Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. - 9781583817704 ; 445, s. 253-262
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 10. |
- Schöier, Fredrik, 1969, et al.
(författare)
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The abundance of HCN in the circumstellar envelopes of AGB stars different chemical type
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 550
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Tidskriftsartikel (refereegranskat)abstract
- Aims. A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. Methods. In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. Results. The median values for the derived abundances of HCN (with respect to H-2) are 3 x 10(-5), 7 x 10(-7) and 10(-7) for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. Conclusions. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.
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| 11. |
- Ueta, T., et al.
(författare)
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The Herschel Planetary Nebula Survey (HerPlaNS) I. Data overview and analysis demonstration with NGC 6781
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565, s. A36-
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Tidskriftsartikel (refereegranskat)abstract
- Context. This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNe) under the Herschel Space Observatory open time 1 program, Herschel Planetary Nebula Survey (HerPlaNS). Aims. Using the HerPlaNS data set, we look into the PN energetics and variations of the physical conditions within the target nebulae. In the present work, we provide an overview of the survey, data acquisition and processing, and resulting data products. Methods. We performed (1) PACS/SPIRE broadband imaging to determine the spatial distribution of the cold dust component in the target PNe and (2) PACS/SPIRE spectral-energy-distribution and line spectroscopy to determine the spatial distribution of the gas component in the target PNe. Results. For the case of NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the amorphous carbon-rich dust shell and the surrounding halo having temperatures of 26-40 K. The PACS/SPIRE multiposition spectra show spatial variations of far-.IR lines that reflect the physical stratification of the nebula. We demonstrate that spatially resolved far-IR line diagnostics yield the (T-e, n(e)) profiles, from which distributions of ionized, atomic, and molecular gases can be determined. Direct comparison of the dust and gas column mass maps constrained by the HerPlaNS data allows to construct an empirical gas-to-dust mass ratio map, which shows a range of ratios with the median of 195 +/- 110. The present analysis yields estimates of the total mass of the shell to be 0.86 M-circle dot, consisting of 0.54 M-circle dot of ionized gas, 0.12 M-circle dot of atomic gas, 0.2 M-circle dot of molecular gas, and 4 x 10(-3) M-circle dot of dust grains. These estimates' also suggest that the central star of about 1.5 M-circle dot initial mass is terminating its PN evolution onto the white dwarf cooling track. Conclusions. The HerPlaNS data provide various diagnostics for both the dust and gas components in a spatially resolved manner. In the forthcoming papers of the HerPlaNS series we will explore the HerPlaNS data set fully for the entire sample of 11 PNe.
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| 12. |
- Vlemmings, W. H. T., et al.
(författare)
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ALMA observations of the variable (CO)-C-12/(CO)-C-13 ratio around the asymptotic giant branch star R Sculptoris
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. L1-
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Tidskriftsartikel (refereegranskat)abstract
- The (CO)-C-12/(CO)-C-13 ratio is often used as a measure of the C-12/C-13 ratio in the circumstellar environment, carrying important information about the stellar nucleosynthesis. External processes can change the (CO)-C-12 and (CO)-C-13 abundances, and spatially resolved studies of the (CO)-C-12/(CO)-C-13 ratio are needed to quantify the effect of these processes on the globally determined values. Additionally, such studies provide important information on the conditions in the circumstellar environment. The detached-shell source R Scl, displaying CO emission from recent mass loss, in a binary-induced spiral structure as well as in a clumpy shell produced during a thermal pulse, provides a unique laboratory for studying the differences in CO isotope abundances throughout its recent evolution. We observed both the (CO)-C-12(J = 3 -> 2) and the (CO)-C-13(J = 3 -> 2) line using ALMA. We find significant variations in the (CO)-C-12/(CO)-C-13 intensity ratios and consequently in the abundance ratios. The average CO isotope abundance ratio is at least a factor three lower in the shell (similar to 19) than that in the present-day (less than or similar to 300 years) mass loss (>60). Additionally, variations in the ratio of more than an order of magnitude are found in the shell itself. We attribute these variations to the competition between selective dissociation and isotope fractionation in the shell, of which large parts cannot be warmer than similar to 35 K. However, we also find that the (CO)-C-12/(CO)-C-13 ratio in the present-day mass loss is significantly higher than the C-12/C-13 ratio determined in the stellar photosphere from molecular tracers (similar to 19). The origin of this discrepancy is still unclear, but we speculate that it is due to an embedded source of UV-radiation that is primarily photo-dissociating (CO)-C-13. This radiation source could be the hitherto hidden companion. Alternatively, the UV-radiation could originate from an active chromosphere of R Scl itself. Our results indicate that caution should be taken when directly relating the (CO)-C-12/(CO)-C-13 intensity and C-12/C-13 abundance ratios for specific asymptotic giant branch stars, in particular binaries or stars that display signs of chromospheric stellar activity.
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