| 1. |
- Arroyo-Torres, B., et al.
(författare)
-
VLTI/AMBER Studies of the Atmospheric Structure and Fundamental Parameters of Red Giant and Supergiant Stars
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; , s. 91-96
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present recent near-IR interferometric studies of red giant and super giant stars, which are aimed at obtaining information on the structure of the atmospheric layers and constraining the fundamental parameters of these objects. The observed visibilities of six red supergiants (RSGs), and also of one of the five red giants observed, indicate large extensions of the molecular layers, as previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RED) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable Rosseland angular radii for our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.
|
|
| 2. |
- Arroyo-Torres, B., et al.
(författare)
-
VLTI/AMBER studies of the atmospheric structure and fundamental parameters of red giant and supergiant stars
- 2015
-
Ingår i: Why Galaxies Care about AGB Stars III: A Closer Look in Space and Time. Proceedings of ASP Conference Series. - 9781583818794 ; 497, s. 91-96
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present recent near-IR interferometric studies of red giant and supergiant stars, which are aimed at obtaining information on the structure of the atmospheric layers and at constraining the fundamental parameters of these objects. The observed visibilities of the red supergiants (RSGs) and also of one red giant indicate large extensions of the molecular layers, as those previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RHD) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable angular Rosseland radii of our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.
|
|
| 3. |
- Brunner, M., et al.
(författare)
-
A Spiral Goes Viral: Observations and Modeling of the Circumstellar Envelope around R Sculptoris
- 2015
-
Ingår i: Astronomical Society of the Pacific Conference Series; Why galaxies care about AGB stars III: A closer look in space and time; Vienna, Austria; Jul 28–Aug 1, 2015. - 9781583818794 ; 497, s. 215-217
-
Konferensbidrag (refereegranskat)abstract
- The complex circumstellar envelope (CSE) around the carbon-rich asymptotic giant branch (AGB) star R Sculptoris has been observed with ALMA, revealing a remarkable and up to now unique geometry, consisting of a thin, spherically symmetric detached shell enclosing a spiral structure. By comparing ALMA data in three different spectral bands - covering different excitation states of the CO molecule - the gas structure and mass-loss evolution of this object can be analyzed. A morphological comparison between molecular line data (ALMA) and dust images (Herschel/PACS) gives hints to the spatial distribution of both constituents of the circumstellar envelope, as well as interaction processes between the different media. Dust continuum radiative transfer modeling of the detached shell around R Scl allows us to constrain the dust parameters and delivers spectral energy distributions comparable to observations.
|
|
| 4. |
- Chiavassa, A., et al.
(författare)
-
3D Hydrodynamical Simulations of Evolved Stars and Observations of Stellar Surfaces
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 - 9781583818787 ; , s. 11-21
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code (COBOLD)-B-5 and how the accurate observables are computed with the post-processing radiative transfer code OPTIM3D. The synergy between observations and theoretical work is supported by a proper and quantitative analysis using these simulations, and by strong constraints from the observational side.
|
|
| 5. |
- Cruzalebes, P., et al.
(författare)
-
Brightness Asymmetries of Cool Giants and Supergiants Measured with VLTI-AMBER
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; , s. 109-110
-
Konferensbidrag (refereegranskat)abstract
- We report on a search for asymmetries on the photospheres of 16 giants and supergiants. We found that all targets have surface-brightness asymmetries, but the most prominent ones are found along the upper AGB.
|
|
| 6. |
- Danilovich, Taissa, 1987, et al.
(författare)
-
Detailed Modelling of the Circumstellar Envelope of the S-type AGB Star W Aquilae
- 2015
-
Ingår i: Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna JUL 28-AUG 01, 2014. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; 497, s. 219-220
-
Konferensbidrag (refereegranskat)abstract
- We present new Herschel(1) HIFI (de Graauw et al. 2010) and PACS (Poglitsch et al. 2010) sub-millimeter 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.
|
|
| 7. |
- De Beck, Elvire, 1985, et al.
(författare)
-
SMA Spectral Line Imaging Survey at 279-355 GHz of the Oxygen-rich AGB Star IK Tau
- 2015
-
Ingår i: Astronomical Society of the Pacific Conference Series; Why galaxies care about AGB stars III: A closer look in space and time; Vienna, Austria; Jul 28–Aug 1, 2015. - 9781583818794 ; 497, s. 73-78
-
Konferensbidrag (refereegranskat)abstract
- Dedicated, unbiased spectral scans of asymptotic giant branch stars have so far been published only for a few carbon-rich stars, with a strong focus on the nearby and bright IRC +10216. We present results from a spectral survey of the circumstellar envelope of the oxygen-rich AGB star IK Tau obtained with the Submillimeter Array (SMA) at similar to 0 ''.9 angular resolution in the frequency range 279 - 355 GHz, expanding the molecular inventory for M-type evolved stars and filling an observational gap. The survey shows over 140 emission lines, belonging to more than 30 species. The emission of AlO and of several vibrationally excited species traces the acceleration of the wind. Isotopic ratios for carbon, silicon, and sulfur will be derived from the observed emission of isotopologues of CO, SiO, SiS, HCN, SO, and SO2. This will allow us to constrain the AGB nucleosynthesis of IK Tau. We highlight the first detection of PO and PN around an oxygen-rich AGB star, detected at unexpectedly high abundances, and emphasise the importance of unbiased spectral surveys of AGB stars and the need for updated chemical models.
|
|
| 8. |
- De Nutte, R., et al.
(författare)
-
Nucleosynthesis in AGB Stars Traced by Oxygen Isotopic Ratios
- 2015
-
Ingår i: Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna, 28- July - 1 Aug. 2014: (Astronomical Society of the Pacific Conference Series). - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; 497, s. 289-294, s. 289-294
-
Konferensbidrag (refereegranskat)abstract
- Isotopic ratios are by far the best diagnostic tracers of the stellar origin of elements, as they are very sensitive to the precise conditions in the nuclear burning regions. They allow us to give direct constraints on stellar evolution models and on the progenitor mass. However, up to now different isotopic ratios have been well constrained for only a handful of Asymptotic Giant Branch (AGB) stars. We present new data on isotopologue lines of a well-selected sample of AGB stars, covering the three spectral classes of C-, S- and M-type stars. We report on the first efforts made in determining accurate isotopologue fractions, focusing on oxygen isotopes which are a crucial tracer of the poorly constrained extra mixing processes in stellar atmospheres.
|
|
| 9. |
- Eriksson, Kjell, et al.
(författare)
-
A New Library of Synthetic Spectra and Photometry for Evolved C Stars
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; , s. 111-112
-
Konferensbidrag (refereegranskat)abstract
- We present a library of synthetic spectra and photometry calculated on the basis of a grid of atmosphere and wind models for carbon-rich, pulsating AGB stars. The spectra cover the range between 0.35 and 25 mu m with a spectral resolution of R=200. The corresponding photometric variations during several pulsation periods were computed for standard broadband filters in the visual and near-infrared.
|
|
| 10. |
- Freytag, Bernd
(författare)
-
Studying the Generation of Shock Waves in AGB Stars with 3-Dimensional Radiation-Hydrodynamics Simulations
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 - 9781583818787 ; , s. 23-29
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the Sun, low-amplitude small-scale acoustic waves are just detectable in the photosphere and start to become dynamically relevant only in the lower chromosphere. The generation of these waves by non-stationary convective flows can be studied in detail by local 3-D radiation-hydrodynamics simulations. Using this technique for global models of AGB stars reveals roughly similar phenomena but on a larger scale and with much higher amplitude. Convection cells spanning a significant fraction of the entire surface produce strong waves that cause a network of smaller shocks in the inner photosphere and occasional global shocks, travelling outward in large arcs. Material falling back interacts with the surface convection cells. A new generation of 3-D RHD simulations of these layers with CO5BOLD is presented and analyzed with particular attention given to acoustic waves and shock fronts.
|
|
