| 1. |
- Ahmad, A., et al.
(författare)
-
Properties of self-excited pulsations in 3D simulations of AGB stars and red supergiants
- 2023
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 669
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The characteristic variability of cool giants and supergiants is attributed to a combination of stellar pulsation and large-scale convective flows. Full 3D radiation-hydrodynamical modelling is an essential tool for understanding the nature of these dynamical processes. The parameter space in our 3D model grid of red giants has expanded in recent years. These models can provide many insights on the nature and properties of the pulsations, including the interplay between convection and pulsations.Aims. We treat 3D dynamical models of asymptotic giant branch (AGB) stars and red supergiants (with current masses 1 M-circle dot <= M-* <= 12 M-circle dot) similar to observational data. We aim to explore the relation between stellar parameters and the properties of the self-excited pulsations.Methods. Output from global 'star-in-a-box' models computed with the CO5BOLD radiation-hydrodynamics code were analysed, particularly in regards to the pulsation properties, to find possible correlations with input and emergent stellar parameters. The fast Fourier transform was applied to spherically averaged mass flows to identify possible radial pulsation periods beneath the photosphere of the modelled stars. Stellar parameters were investigated for correlations with the extracted pulsation periods.Results. We find that the pulsation periods varied with the stellar parameters in good agreement with the current expectations. The pulsation periods follow Ritter's period-mean density relation well and our AGB models agree with period-luminosity relations derived from observations. A mass estimate formula was derived from the 3D models, relating the stellar mass to the fundamental mode pulsation period and the stellar radius.Conclusions. While the non-linearity of the interplay between the self-excited pulsations and the self-consistent convection complicates analyses, the resulting correlations are in good agreement with respect to current theoretical and observational understandings.
|
|
| 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. - : 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.
|
|
| 3. |
- Arroyo-Torres, B., et al.
(författare)
-
What causes the large extensions of red supergiant atmospheres? : Comparisons of interferometric observations with 1D hydrostatic, 3D convection, and 1D pulsating model atmospheres
- 2015
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 575
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. This research has two main goals. First, we present the atmospheric structure and the fundamental parameters of three red supergiants (RSGs), increasing the sample of RSGs observed by near-infrared spectro-interferometry. Additionally, we test possible mechanisms that may explain the large observed atmospheric extensions of RSGs. Methods. We carried out spectro-interferometric observations of the.RSGs V602 Car, EID 95687, and EID 183589 in the near-infrared K-band (1.92-2.47 mu m) with the VLTI/AMBER instrument at medium spectral resolution (R similar to 1500). To categorize and comprehend the extended atmospheres, we compared our observational results to predictions by available hydrostatic PHOENIX, available 3D convection, and new 1D self-excited pulsation models of RSGs. Results. Our near-infrared flux spectra of V602 Car, HD 95687, and HD 183589 are well reproduced by the PHOENIX model atmospheres. The continuum visibility values are consistent with a limb-darkened disk as predicted by the PHOENIX models, allowing us to determine the angular diameter and the fundamental parameters of our sources. Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model visibilities do not predict the large observed extensions of molecular layers, most remarkably in the CO bands. Likewise, the 3D convection models and the ID pulsation models with typical parameters of RSGs lead to compact atmospheric structures as well, which are similar to the structure of the hydrostatic PHOENIX models. They can also not explain the observed decreases in the visibilities and thus the large atmospheric molecular extensions. The full sample of our RSGs indicates increasing observed atmospheric extensions with increasing luminosity and decreasing surface gravity, and no correlation with effective temperature or variability amplitude. Conclusions. The location of our RSG sources in the Hertzsprung-Russell diagram is contirm.ed to be consistent with the red limits of recent evolutionary tracks. The observed extensions of the atmospheric layers of our sample of RSGs are comparable to those of Mira stars. This phenomenon is not predicted by any of the considered model atmospheres including as 311) convection and new 1D pulsation models of.RSGs. This confirms that neither convection nor pulsation alone can levitate the molecular atmospheres of.RSGs. Our observed correlation of atmospheric extension with luminosity supports a scenario of radiative acceleration on Doppler-shifted molecular lines.
|
|
| 4. |
- Battino, U., et al.
(författare)
-
Application Of A Theory And Simulation-Based Convective Boundary Mixing Model For AGB Star Evolution And Nucleosynthesis
- 2016
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 827:1
-
Tidskriftsartikel (refereegranskat)abstract
- The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the C-13 pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 andM = 3M(circle dot), and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of C-12 and O-16 are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the Ne-22(alpha, n)Mg-25 activation and the s-process efficiency in the C-13-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the C-13-pocket with a mass of approximate to 10(-4) M-circle dot. The final s-process abundances are characterized by 0.36<[s/Fe] < 0.78 and the heavy-to-light s-process ratio is -0.23< [hs/ls] < 0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work.
|
|
| 5. |
- Bonifacio, P., et al.
(författare)
-
Using CO5BOLD models to predict the effects of granulation on colours .
- 2017
-
Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract.In order to investigate the effects of granulation on fluxes and colours, we computedthe emerging fluxes from the models in theCO5BOLDgrid with metallicities [M/H]=0.0,–1.0,–2.0 and –3.0. These fluxes have been used to compute colours in different photometric systems.We explain here how our computations have been performed and provide some results.Key words.Convection – Hydrodynamics - Stars: atmosphere
|
|
| 6. |
- Bonifacio, P., et al.
(författare)
-
Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours : I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS, Gaia, and SDSS systems
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 611:9
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities.Aim. We aim to study, in a quantitative way, the impact of granulation on colours.Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a “3D correction” that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models.Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (G − Hp), (BP − RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic.Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.
|
|
| 7. |
- Caffau, E., et al.
(författare)
-
The photospheric solar oxygen project - I. Abundance analysis of atomic lines and influence of atmospheric models
- 2008
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 488:3, s. 1031-1046
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code. Aims. We investigate the photospheric oxygen abundance considering lines from atomic transitions. We also consider the relationship between the solar model used and the resulting solar oxygen abundance, to understand whether the downward abundance revision is specifically related to 3D hydrodynamical effects. Methods. We performed a new determination of the solar photospheric oxygen abundance by analysing different high-resolution high signal-to-noise ratio atlases of the solar flux and disc-centre intensity, making use of the latest generation of CO5BOLD 3D solar model atmospheres. Results. We find 8.73 <= log (N-O/N-H) + 12 <= 8.79. The lower and upper values represent extreme assumptions on the role of collisional excitation and ionisation by neutral hydrogen for the NLTE level populations of neutral oxygen. The error of our analysis is +/- (0.04 +/- 0.03) dex, the last being related to NLTE corrections, the first error to any other effect. The 3D "granulation effects" do not play a decisive role in lowering the oxygen abundance. Conclusions. Our recommended value is log (N-O/N-H) = 8.76 +/- 0.07, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen. The reasons for lower O abundances in the past are identified as (1) the lower equivalent widths adopted and (2) the choice of neglecting collisions with hydrogen atoms in the statistical equilibrium calculations for oxygen.
|
|
| 8. |
- Calvo, F., et al.
(författare)
-
Non-magnetic photospheric bright points in 3D simulations of the solar atmosphere
- 2016
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 596
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Small-scale bright features in the photosphere of the Sun, such as faculae or G-band bright points, appear in connection with small-scale magnetic flux concentrations. Aims. Here we report on a new class of photospheric bright points that are free of magnetic fields. So far, these are visible in numerical simulations only. We explore conditions required for their observational detection. Methods. Numerical radiation (magneto-) hydrodynamic simulations of the near-surface layers of the Sun were carried out. The magnetic field-free simulations show tiny bright points, reminiscent of magnetic bright points, only smaller. A simple toy model for these non-magnetic bright points (nMBPs) was established that serves as a base for the development of an algorithm for their automatic detection. Basic physical properties of 357 detected nMBPs were extracted and statistically evaluated. We produced synthetic intensity maps that mimic observations with various solar telescopes to obtain hints on their detectability. Results. The nMBPs of the simulations show a mean bolometric intensity contrast with respect to their intergranular surroundings of approximately 20%, a size of 60-80 km, and the isosurface of optical depth unity is at their location depressed by 80-100 km. They are caused by swirling downdrafts that provide, by means of the centripetal force, the necessary pressure gradient for the formation of a funnel of reduced mass density that reaches from the subsurface layers into the photosphere. Similar, frequently occurring funnels that do not reach into the photosphere, do not produce bright points. Conclusions. Non-magnetic bright points are the observable manifestation of vertically extending vortices (vortex tubes) in the photosphere. The resolving power of 4-m-class telescopes, such as the DKIST, is needed for an unambiguous detection of them.
|
|
| 9. |
- 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.
|
|
| 10. |
- Chiavassa, A., et al.
(författare)
-
Heading Gaia to measure atmospheric dynamics in AGB stars
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 617
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Asymptotic giant branch (AGB) stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The uncertainties in observed absolute magnitudes have been found to originate mainly from uncertainties in the parallaxes. The resulting motion of the stellar photocentre could have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. Methods. We use three-dimensional (3D) radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [325-1030 nm]. From those maps, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e. the photocentre) and evaluate its motion as a function of time. We extract the parallax error from Gaia data-release 2 (DR2) for a sample of semi-regular variables in the solar neighbourhood and compare it to the synthetic predictions of photocentre displacements. Results. AGB stars show a complex surface morphology characterised by the presence of few large-scale long-lived convective cells accompanied by short-lived and small-scale structures. As a consequence, the position of the photocentre displays temporal excursions between 0.077 and 0.198 AU (approximate to 5 to approximate to 11% of the corresponding stellar radius), depending on the simulation considered. We show that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error of our sample of semi-regular variables. Finally, we present evidence for a correlation between the mean photocentre displacement and the stellar fundamental parameters: surface gravity and pulsation. We suggest that parallax variations could be exploited quantitatively using appropriate radiation-hydrodynamics (RHD) simulations corresponding to the observed star.
|
|
| 11. |
- Chiavassa, A., et al.
(författare)
-
Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars
- 2020
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 640
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics, which amplifies the uncertainties on stellar parameters and distances.Aims. On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities.Methods. We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics (RHD) simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius using the average 3D intensity profile and, finally, compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70 mu m, simultaneously.Results. We measured the apparent diameter of CL Lac at two wavelengths (3.299 0.005 mas and 3.053 +/- 0.006 mas at 1.52 and 1.70 mu m, respectively) and recovered the radius (R = 307 +/- 41 and R = 284 +/- 38 R-circle dot) using a Gaia parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities.Conclusions. This work confirms the presence of convection-related surface structures on an AGB star of Gaia DR2. Our result will help us to take a step forward in exploiting Gaia measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations.
|
|
| 12. |
- Chiavassa, A., et al.
(författare)
-
Pathways For Observing Stellar Surfaces Using 3D Hydrodynamical Simulations Of Evolved Stars
- 2015
-
Ingår i: Physics Of Evolved Stars. - : EDP Sciences. - 9782759819072 ; , s. 237-242
-
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 CO5BOLD 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.
|
|
| 13. |
- Chiavassa, A., et al.
(författare)
-
Probing red supergiant dynamics through photo-center displacements measured by Gaia
- 2022
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 661
-
Tidskriftsartikel (refereegranskat)abstract
- Context: Red supergiant (RSGs) are cool massive stars in a late phase of their evolution when the stellar envelope becomes fully convective. They are the brightest stars in the universe at infrared light and can be detected in galaxies far beyond the Local Group, allowing for accurate determination of chemical composition of galaxies. The study of their physical properties is extremely important for various phenomena including the final fate of massive stars as type II supernovae and gravitational wave progenitors.Aims: We explore the well-studied nearby young stellar cluster chi Per, which contains a relatively large population of RSG stars. Using Gaia EDR3 data, we find the distance of the cluster (d & x2004;=& x2004;2.260 +/- 0.020 kpc) from blue main sequence stars and compare with RSG parallax measurements analysing the parallax uncertainties of both groups. We then investigate the variability of the convection-related surface structure as a source for parallax measurement uncertainty.Methods: We use state-of-the-art three-dimensional radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G photometric system. We calculate the variabiltiy, as a function of time, of the intensity-weighted mean (or the photo-center) from the synthetic maps. We then select the RSG stars in the cluster and compare their uncertainty on parallaxes to the predictions of photocentre displacements.Results: The synthetic maps of RSG show extremely irregular and temporal variable surfaces due to convection-related dynamics. Consequentially, the position of the photo-center varies during Gaia measurements between 0.033 and 0.130 AU (approximate to 1 to approximate to 5% of the corresponding simulation stellar radius). We argue that the variability of the convection-related surface structures accounts for a substantial part of the Gaia EDR3 parallax error of the RSG sample of chi Per.Conclusions: We suggest that the variation of the uncertainty on Gaia parallax could be exploited quantitatively using appropriate RHD simulations to extract, in a unique way, important information about the stellar dynamics and parameters of RSG stars.
|
|
| 14. |
- Chiavassa, A., et al.
(författare)
-
Radiative hydrodynamic simulations of red supergiant stars III. Spectro-photocentric variability, photometric variability, and consequences on Gaia measurements
- 2011
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 528, s. A120-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. It has been shown that convection in red supergiant stars (RSG) gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The resulting motion of the photocentre (on time scales ranging from months to years) could possibly have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the granulation on the photocentric and photometric variability. We quantify these effects in order to better characterise the error that could possibly alter the parallax. Methods. We use 3D radiative-hydrodynamics (RHD) simulations of convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps and spectra in the Gaia G band [325-1030 nm]. Results. We provide astrometric and photometric predictions from 3D simulations of RSGs that are used to evaluate the possible degradation of the astrometric parameters of evolved stars derived by Gaia. We show in particular from RHD simulations that a supergiant like Betelgeuse exhibits a photocentric noise characterised by a standard deviation of the order of 0.1 AU. The number of bright giant and supergiant stars whose Gaia parallaxes will be altered by the photocentric noise ranges from a few tens to several thousands, depending on the poorly known relation between the size of the convective cells and the atmospheric pressure scale height of supergiants, and to a lower extent, on the adopted prescription for galactic extinction. In the worst situation, the degradation of the astrometric fit caused by this photocentric noise will be noticeable up to about 5 kpc for the brightest supergiants. Moreover, parallaxes of Betelgeuse-like supergiants are affected by an error of the order of a few percents. We also show that the photocentric noise, as predicted by the 3D simulation, does account for a substantial part of the supplementary "cosmic noise" that affects Hipparcos measurements of Betelgeuse and Antares.
|
|
| 15. |
- Chiavassa, A., et al.
(författare)
-
Radiative hydrodynamics simulations of red supergiant stars IV. Gray versus non-gray opacities
- 2011
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 535, s. A22-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Red supergiants are massive evolved stars that contribute extensively to the chemical enrichment of our Galaxy. It has been shown that convection in those stars produces large granules that cause surface inhomogeneities and shock waves in the photosphere. The understanding of their dynamics is crucial for unveiling the unknown mass-loss mechanism, their chemical composition, and their stellar parameters.Aims. We present a new generation of red supergiant simulations with a more sophisticated opacity treatment performed with 3D radiative-hydrodynamics code CO5BOLD.Methods. In the code the coupled equations of compressible hydrodynamics and non-local radiation transport are solved in the presence of a spherical potential. The stellar core is replaced by a special spherical inner boundary condition, where the gravitational potential is smoothed and the energy production by fusion is mimicked by a simply producing heat corresponding to the stellar luminosity. All outer boundaries are transmitting for matter and light. The post-processing radiative transfer code OPTIM3D is used to extract spectroscopic and interferometric observables.Results. We show that if one relaxes the assumption of frequency-independent opacities, this leads to a steeper mean thermal gradient in the optical thin region that strongly affects the atomic strengths and the spectral energy distribution. Moreover, the weaker temperature fluctuations reduce the incertitude on the radius determination with interferometry. We show that 1D models of red supergiants must include a turbulent velocity that is calibrated on 3D simulations to obtain the effective surface gravity that mimics the effect of turbulent pressure on the stellar atmosphere. We provide an empirical calibration of the ad hoc micro- and macroturbulence parameters for 1D models using the 3D simulations: we find that there is no clear distinction between the different macroturbulent profiles needed in 1D models to fit 3D synthetic lines.
|
|
| 16. |
- Chiavassa, A., et al.
(författare)
-
The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE star
- 2022
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
-
Tidskriftsartikel (refereegranskat)abstract
- Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification.Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment.Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3-4 mu m), M (4.5-5 mu m), and N (8-13 mu m) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics simulations carried out with (COBOLD)-B-5 and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes OPTIM3D and RADMC3D to compute the spectral energy distribution for the L, M, and N bands, respectively.Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N-band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration.Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of VX Sgr's stellar surface and circumstellar environment across a very large spectral domain for the first time.
|
|
| 17. |
- Climent, J. B., et al.
(författare)
-
VLTI-PIONIER imaging of the red supergiant V602 Carinae
- 2020
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Red supergiant stars possess surface features and extended molecular atmospheres. Photospheric convection may be a crucial factor of the levitation of the outer atmospheric layers. However, the mechanism responsible is still poorly understood.Aims. We image the stellar surface of V602 Carinae (V602 Car) to constrain the morphology and contrast of the surface features and of the extended atmospheric layers.Methods. We observed V602 Car with the Very Large Telescope Interferometer PIONIER instrument (1.53-1.78 mu m) between May and July 2016, and April and July 2019 with different telescope configurations. We compared the image reconstructions with 81 temporal snapshots of 3D radiative-hydrodynamics (RHD) (COBOLD)-B-5 simulations in terms of contrast and morphology, using the Structural Similarity Index.Results. The interferometric data are compatible with an overall spherical disk of angular diameter 4.4 0.2 mas, and an extended molecular layer. In 2016, the reconstructed image reveals a bright arc-like feature toward the northern rim of the photospheric surface. In 2019, an arc-like feature is seen at a different orientation and a new peak of emission is detected on the opposite side. The contrasts of the reconstructed surface images are 11% +/- 2% and 9% +/- 2% for 2016 and 2019, respectively. The morphology and contrast of the two images are consistent with 3D RHD simulations, within our achieved spatial resolution and dynamic range. The extended molecular layer contributes 10-13% of the total flux with an angular diameter of 6-8 mas. It is present but not clearly visible in the reconstructed images because it is close to the limits of the achieved dynamic range. The presence of the molecular layer is not reproduced by the 3D RHD simulations.Conclusions. 3D RHD simulations predict substructures similar to the observed surface features of V602 Car at two different epochs. We interpret the structure on the stellar surface as being related to instationary convection. This structure is further convolved to larger observed patches on the stellar surface with our observational spatial resolution. Even though the simulations reproduce the observed features on the stellar surface, convection alone may not be the only relevant process that is levitating the atmosphere.
|
|
| 18. |
- Cukanovaite, Elena, et al.
(författare)
-
3D spectroscopic analysis of helium-line white dwarfs
- 2021
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 501:4, s. 5274-5293
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 <= log(H/He) <= -2.0, 7.5 <= log g <= 9.0, and less than or similar to T-eff less than or similar to 34 000 K, based on 282 3D atmospheric models calculated with the (COBOLD)-B-5 radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing Sloan Digital Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant corrections both for effective temperature and surface gravity. The 3D log g corrections are most significant for T-eff less than or similar to 18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia Data Release 2 absolute fluxes within 1 sigma-3 sigma for individual white dwarfs. By comparing our results to DA white dwarfs, we determine that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example PYTHON code.
|
|
| 19. |
- Cukanovaite, E., et al.
(författare)
-
Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs
- 2019
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 490:1, s. 1010-1025
-
Tidskriftsartikel (refereegranskat)abstract
- We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range -10.0 <= log (H/He) <= -2.0, surface gravities in the range 7.5 <= log g <= 9.0, and effective temperatures in the range 12 000K less than or similar to T-eff less than or similar to 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 <= ML2/alpha <= 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/alpha is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/alpha we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres.
|
|
| 20. |
- Cukanovaite, E., et al.
(författare)
-
Pure-helium 3D model atmospheres of white dwarfs
- 2018
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : EDP Sciences. - 0035-8711 .- 1365-2966. ; 481:2, s. 1522-1537
-
Tidskriftsartikel (refereegranskat)abstract
- We present the first grid of 3D simulations for the pure-helium atmospheres of DB white dwarfs. The simulations were computed with the co(5) BOLD radiation-hydrodynamics code and cover effective temperatures and surface gravities between 12 000 K less than or similar to T-eff less than or similar to 34 000 K and 7.5 <= log g (cgs units) <= 9.0, respectively. In this introductory work, synthetic spectra calculated from the 3D simulations are compared to appropriate 1 D model spectra under a differential approach. This results in the derivation of 3D corrections for the spectroscopically derived atmospheric parameters of DB stars with respect to the 1D ML2/alpha = 1.25 mixing-length parametrization. No significant T-eff corrections are found for the V777 Her instability strip region, and therefore no 3D revision is expected for the empirical blue and red edges of the strip. However, large log g corrections are found in the range 12 000 K < T-eff < 23 000 K for all log g values covered by the 3D grid. These corrections indicate that 1D model atmospheres overpredict log g, reminiscent of the results found from 3D simulations of pure-hydrogen white dwarfs. The next step will be to compute 3D simulations with mixed helium and hydrogen atmospheres to comprehend the full implications for the stellar parameters of DB and DBA white dwarfs.
|
|
| 21. |
- Cunningham, Tim, et al.
(författare)
-
Convective overshoot and macroscopic diffusion in pure-hydrogen-atmosphere white dwarfs
- 2019
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 488:2, s. 2503-2522
-
Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using 3D, closed-bottom, radiation hydrodynamics (COBOLD)-B-5 simulations. We rely on a new grid of deep 3D white dwarf models in the temperature range 11 400 <= T-eff <= 18 000 K where tracer particles and a tracer density are used to derive macroscopic diffusion coefficients driven by convective overshoot. These diffusion coefficients are compared to microscopic diffusion coefficients from 1D structures. We find that the mass of the fully mixed region is likely to increase by up to 2.5 orders of magnitude while inferred accretion rates increase by a more moderate order of magnitude. We present evidence that an increase in settling time of up to 2 orders of magnitude is to be expected, which is of significance for time-variability studies of polluted white dwarfs. Our grid also provides the most robust constraint on the onset of convective instabilities in DA white dwarfs to be in the effective temperature range from 18 000 to 18 250 K.
|
|
| 22. |
- Cunningham, Tim, et al.
(författare)
-
Horizontal spreading of planetary debris accreted by white dwarfs
- 2021
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 503:2, s. 1646-1667
-
Tidskriftsartikel (refereegranskat)abstract
- White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code (COBOLD)-B-5, we present the first transport coefficients in degenerate star atmospheres that describe the advection-diffusion of a passive scalar across the surface plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range of 6000-18 000K and pure-helium atmospheres in the range of 12 000-34 000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; greater than or similar to 13 000 K) and helium-rich atmospheres (DB and DBA; greater than or similar to 30 000 K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations in white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion time-scale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity that is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.
|
|
| 23. |
- Dorch, S.B.F, et al.
(författare)
-
Does Betelgeuse have a Magnetic Field?
- 2002
-
Ingår i: Modelling of Stellar Atmospheres, IAU Symposium 210. - : San Francisco: The Astronomical Society of the Pacific.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Betelgeuse (alpha Ori) is an example of a nearby cool super-giant (M1--2 Ia--Iab) that displays temporal brightness fluctuations and irregular surface structures. Recent numerical simulations by Freytag and collaborators of the entire star under realistic
|
|
| 24. |
- Dravins, Dainis, et al.
(författare)
-
Spatially resolved spectroscopy across stellar surfaces : IV. F, G, and K-stars: Synthetic 3D spectra at hyper-high resolution
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
-
Tidskriftsartikel (refereegranskat)abstract
- Context. High-precision stellar analyses require hydrodynamic 3D modeling. Such models predict changes across stellar disks of spectral line shapes, asymmetries, and wavelength shifts. For testing models in stars other than the Sun, spatially resolved observations are feasible from differential spectroscopy during exoplanet transits, retrieving spectra of those stellar surface segments that successively become hidden behind the transiting planet, as demonstrated in Papers I, II, and III. Aims. Synthetic high-resolution spectra over extended spectral regions are now available from 3D models. Similar to other ab initio simulations in astrophysics, these data contain patterns that have not been specifically modeled but may be revealed after analyses to be analogous to those of a large volume of observations. Methods. From five 3D models spanning Teff = 3964-6726 K (spectral types ~K8 V-F3 V), synthetic spectra at hyper-high resolution (λ/Δλ >1 000 000) were analyzed. Selected Fe » I and Fe » II lines at various positions across stellar disks were searched for characteristic patterns between different types of lines in the same star and for similar lines between different stars. Results. Spectral-line patterns are identified for representative photospheric lines of different strengths, excitation potentials, and ionization levels, thereby encoding the hydrodynamic 3D structure. Line profiles and bisectors are shown for various stars at different positions across stellar disks. Absolute convective wavelength shifts are obtained as differences to 1D models, where such shifts do not occur. Conclusions. Observable relationships for line properties are retrieved from realistically complex synthetic spectra. Such patterns may also test very detailed 3D modeling, including non-LTE effects. While present results are obtained at hyper-high spectral resolution, the subsequent Paper V examines their practical observability at realistically lower resolutions, and in the presence of noise.
|
|
| 25. |
- Dravins, Dainis, et al.
(författare)
-
Spatially resolved spectroscopy across stellar surfaces : V. Observational prospects: Toward Earth-like exoplanet detection
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
-
Tidskriftsartikel (refereegranskat)abstract
- Context. High-precision stellar analyses require hydrodynamic 3D modeling. Testing such models is feasible by retrieving spectral line shapes across stellar disks, using differential spectroscopy during exoplanet transits. Observations were presented in Papers I, II, and III, while Paper IV explored synthetic data at hyper-high spectral resolution for different classes of stars, identifying characteristic patterns for Fe » I and Fe » II lines. Aims. Anticipating future observations, the observability of patterns among photospheric lines of different strength, excitation potential and ionization level are examined from synthetic spectra, as observed at ordinary spectral resolutions and at different levels of noise. Time variability in 3D atmospheres induces changes in spectral-line parameters, some of which are correlated. An adequate calibration could identify proxies for the jitter in apparent radial velocity to enable adjustments to actual stellar radial motion. Methods. We used spectral-line patterns identified in synthetic spectra at hyper-high resolution in Paper IV from 3D models spanning Teff = 3964-6726 K (spectral types ~K8 V-F3 V) to simulate practically observable signals at different stellar disk positions at various lower spectral resolutions, down to λ/Δλ = 75 000. We also examined the center-to-limb temporal variability. Results. Recovery of spatially resolved line profiles with fitted widths and depths is shown for various noise levels, with gradual degradation at successively lower spectral resolutions. Signals during exoplanet transit are simulated. In addition to Rossiter-McLaughlin type signatures in apparent radial velocity, analogous effects are shown for line depths and widths. In a solar model, temporal variability in line profiles and apparent radial velocity shows correlations between jittering in apparent radial velocity and fluctuations in line depth. Conclusions. Spatially resolved spectroscopy using exoplanet transits is feasible for main-sequence stars. Overall line parameters of width, depth and wavelength position can be retrieved already with moderate efforts, but a very good signal-to-noise ratio is required to reveal the more subtle signatures between subgroups of spectral lines, where finer details of atmospheric structure are encoded. Fluctuations in line depth correlate with those in wavelength, and because both can be measured from the ground, searches for low-mass exoplanets should explore these to adjust apparent radial velocities to actual stellar motion.
|
|
| 26. |
|
|
| 27. |
|
|
| 28. |
|
|
| 29. |
|
|
| 30. |
- Freytag, Bernd
(författare)
-
Boundary conditions in CO5BOLD
- 2017
-
Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88, s. 12-21
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The declaration of boundary conditions is a crucial step in the setup of a CO5BOLD simulation (and many others) due to the physical nature of the problem, that is reflected in the mathematical description by partial differential equations, discrete versions of which are integrated by the numerical solver(s). While parameters controlling the flux of energy through the computational box are most important for all simulations of convective flows, the detailed specifications describing the behavior of energy, gas and dust densities, velocities, and magnetic fields at or just beyond the boundaries influence the flow, dynamics, and stratification within the box. Recent refinements of the treatment of boundary conditions in CO5BOLD resulted in reliably working implementations of open and closed versions for top, bottom, and ``inner'' boundaries even under conditions with strong velocity fields (waves, shocks, or downdrafts). They are implemented and available in the current version of CO5BOLD - but have to be activated properly with parameters adapted to the type of the star under consideration (by defining for instance the depth of the damping layers for the closed-bottom boundary or by specifying the damping constants for the open-bottom boundary).
|
|
| 31. |
- Freytag, Bernd, Dr., et al.
(författare)
-
Global 3D radiation-hydrodynamical models of AGB stars with dust-driven winds
- 2023
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 669
-
Tidskriftsartikel (refereegranskat)abstract
- Context: Convection and mass loss by stellar winds are two dynamical processes that shape asymptotic giant branch (AGB) stars and their evolution. Observations and earlier 3D models indicate that giant convection cells cause high-contrast surface intensity patterns, and contribute to the origin of clumpy dust clouds.Aims: We study the formation and resulting properties of dust-driven winds from AGB stars, using new global 3D simulations.Methods: The dynamical stellar interiors, atmospheres, and wind acceleration zones of two M-type AGB stars were modeled with the CO5BOLD code. These first global 3D simulations are based on frequency-dependent gas opacities, and they feature time-dependent condensation and evaporation of silicate grains.Results: Convection and pulsations emerge self-consistently, allowing us to derive wind properties (e.g., mass-loss rates and outflow velocities), without relying on parameterized descriptions of these processes. In contrast to 1D models with purely radial pulsations, the shocks induced by convection and pulsation in the 3D models cover large parts, but not the entirety, of the sphere, leading to a patchy, nonspherical structure of the atmosphere. Since dust condensation critically depends on gas density, new dust clouds form mostly in the dense wakes of atmospheric shocks, where the grains can grow efficiently. The resulting clumpy distribution of newly formed dust leads to a complex 3D morphology of the extended atmosphere and wind-acceleration zone, with simultaneous infall and outflow regions close to the star. Highly nonspherical isotherms and short-lived cool pockets of gas in the stellar vicinity are prominent features. Efficient dust formation sets in closer to the star than spherical averages of the temperature indicate, in dense regions where grain growth rates are higher than average. This can lead to weak outflows in situations where corresponding 1D models do not produce winds. For stars where the overall conditions for dust formation and wind acceleration are favorable, it is unclear whether the resulting mass-loss rates will be higher or lower than in the 1D case. The increased efficiency of dust formation in high-density clumps may be offset by a low volume coverage of the forming clouds.Conclusions: A global 3D approach is essential to make progress in understanding dynamical processes in AGB stars, and, in particular, to solve long-standing problems regarding mass loss.
|
|
| 32. |
- Freytag, Bernd, et al.
(författare)
-
Global 3D radiation-hydrodynamics models of AGB stars : Effects of convection and radial pulsations on atmospheric structures
- 2017
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 600
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales.Aims. To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations.Methods. We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters.Results. The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features.Conclusions. Our models self-consistently describe convection, convectively generated acoustic noise, fundamental-mode radial pulsations, and atmospheric shocks of various scales, which give rise to complex changing structures in the atmospheres of AGB stars.
|
|
| 33. |
|
|
| 34. |
|
|
| 35. |
- Freytag, Bernd, et al.
(författare)
-
Modelling the Entire Atmosphere of Betelgeuse with 3D Simulations
- 2002
-
Ingår i: Modelling of Stellar Atmospheres, IAU Symposium 210. - : San Francisco: The Astronomical Society of the Pacific.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The large amount of observational data on Betelgeuse points to a highly dynamical atmosphere with phenomena that have not yet been described in detail by theory. Efforts to improve theoretical understanding are being made with the help of the COBOLD compu
|
|
| 36. |
|
|
| 37. |
- Freytag, Bernd, et al.
(författare)
-
Spots on the surface of Betelgeuse -- Results from new 3D stellar convection models
- 2002
-
Ingår i: Astronomische Nachrichten. ; :323, s. 213-219
-
Tidskriftsartikel (refereegranskat)abstract
- The observed irregular brightness fluctuations of the well-known red supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by M. Schwarzschild (1975) to the changing granulation pattern formed by only a few giant convection cells covering the sur
|
|
| 38. |
|
|
| 39. |
- 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.
|
|
| 40. |
- Freytag, Bernd, et al.
(författare)
-
Three-dimensional simulations of the atmosphere of an AGB star
- 2008
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 483:2, s. 571-583
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary ("piston") simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres - convection and pulsations - are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.
|
|
| 41. |
- Freytag, Bernd, et al.
(författare)
-
Typical Scales of Structures in Numerical Models of Betelgeuse
- 2002
-
Ingår i: Modelling of Stellar Atmospheres, IAU Symposium 210. - : San Francisco: The Astronomical Society of the Pacific.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 3D radiation hydrodynamics simulations of the envelope and atmosphere of a red supergiant show giant convection cells with sometimes deep-reaching downdrafts. Smaller short-lived cells can appear in the very outer layers. The surface intensity pattern is
|
|
| 42. |
- Gallagher, A. J., et al.
(författare)
-
Enhanced methods for computing spectra from CO5BOLD models using Linfor3D. Molecular bands in metal-poor stars
- 2017
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Molecular features such as the G-band, CN-band and NH-band are important diagnostics for measuring a star's carbon and nitrogen abundances, especially in metal-poor stars where atomic lines are no longer visible in stellar spectra. Unlike atomic transitions, molecular features tend to form in bands, which cover large wavelength regions in a spectrum. While it is a trivial matter to compute carbon and nitrogen molecular bands under the assumption of 1D, it is extremely time consuming in 3D. In this contribution to the 2016 CO5BOLD workshop we review the improvements made to the 3D spectral synthesis code Linfor3D, and discuss the new challenges found when computing molecular features in 3D.
|
|
| 43. |
|
|
| 44. |
- Höfner, Susanne, et al.
(författare)
-
Exploring the origin of clumpy dust clouds around cool giants : A global 3D RHD model of a dust-forming M-type AGB star
- 2019
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 623
-
Tidskriftsartikel (refereegranskat)abstract
- Context: Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances.Aims: We explore possible causes for the formation of inhomogeneous dust layers, using 3D dynamical simulations.Methods: We modeled the outer convective envelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The simulations account for frequency-dependent gas opacities, and include a time-dependent description of grain growth and evaporation for corundum (Al2O3) and olivine-type silicates (Mg2SiO4).Results: In the inner, gravitationally bound, and corundum-dominated layers of the circumstellar envelope, a patchy distribution of the dust emerges naturally, due to atmospheric shock waves that are generated by large-scale convective flows and pulsations. The formation of silicate dust at somewhat larger distances probably indicates the outer limit of the gravitationally bound layers. The current models do not describe wind acceleration, but the cloud formation mechanism should also work for stars with outflows. Timescales of atmospheric dynamics and grain growth are similar to observed values. In spherical averages of dust densities, more easily comparable to unresolved observations and 1D models, the variable 3D morphology manifests itself as cycle-to-cycle variations.Conclusions: Grain growth in the wake of large-scale non-spherical shock waves, generated by convection and pulsations, is a likely mechanism for producing the observed clumpy dust clouds, and for explaining their physical and dynamical properties.
|
|
| 45. |
|
|
| 46. |
- Kravchenko, K., et al.
(författare)
-
Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 650
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms, such as episodic mass loss and the presence of dark spots in the photosphere, were invoked.Aims. We aim to relate the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event.Methods. We used the tomographic method which allowed us to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method was applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field were then compared with photometric and spectroscopic variations.Results. The tomographic method reveals that the succession of two shocks along our line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection and/or outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase in molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star's brightness.
|
|
| 47. |
- Kravchenko, K., et al.
(författare)
-
Tomography of cool giant and supergiant star atmospheres : I. Validation of the method
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 610
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aims. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. Methods. The tomographic method is applied to one-dimensional (1D) model atmospheres and to a realistic three-dimensional (3D) radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. Results. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of cross-correlation function profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.
|
|
| 48. |
- Kravchenko, Kateryna, et al.
(författare)
-
Tomography of cool giant and supergiant star atmospheres : III. Validation of the method on VLTI/AMBER observations of the Mira star S Ori
- 2020
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 642
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Asymptotic giant branch (AGB) stars are characterized by substantial mass loss, however the mechanism behind it not yet fully understood. The knowledge of the structure and dynamics of AGB-star atmospheres is crucial to better understanding the mass loss. The recently established tomographic method, which relies on the design of spectral masks containing lines that form in given ranges of optical depths in the stellar atmosphere, is an ideal technique for this purpose.Aims. We aim to validate the capability of the tomographic method in probing different geometrical depths in the stellar atmosphere and recovering the relation between optical and geometrical depth scales.Methods. We applied the tomographic method to high-resolution spectro-interferometric VLTI/AMBER observations of the Mira-type AGB star S Ori. The interferometric visibilities were extracted at wavelengths contributing to the tomographic masks and fitted to those computed from a uniform disk model. This allows us to measure the geometrical extent of the atmospheric layer probed by the corresponding mask. We then compared the observed atmospheric extension with others measured from available 1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD simulations.Results. While the average optical depths probed by the tomographic masks in S Ori decrease (with log tau(0) = -0.45, - 1.45, and - 2.45 from the innermost to the central and outermost layers), the angular diameters of these layers increase, from 10.59 0.09 mas through 11.84 +/- 0.17 mas, up to 14.08 +/- 0.15 mas. A similar behavior is observed when the tomographic method is applied to 1D and 3D dynamical models.Conclusions. This study derives, for the first time, a quantitative relation between optical and geometrical depth scales when applied to the Mira star S Ori, or to 1D and 3D dynamical models. In the context of Mira-type stars, knowledge of the link between the optical and geometrical depths opens the way to deriving the shock-wave propagation velocity, which cannot be directly observed in these stars.
|
|
| 49. |
- Kravchenko, K., et al.
(författare)
-
Tomography of cool giant and supergiant star atmospheres II : Signature of convection in the atmosphere of the red supergiant star mu Cep
- 2019
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 632
-
Tidskriftsartikel (refereegranskat)abstract
- Context: Red supergiants are cool massive stars and are the largest and the most luminous stars in the Universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood.Aims: The paper aims to derive the velocity field in the red supergiant star mu Cep and to relate it to the photometric variability with the help of the tomographic method.Methods: The tomographic method allows one to recover the line-of-sight velocity distribution over the stellar disk and within different optical-depth slices. This method was applied to a series of high-resolution spectra of mu Cep, and these results are compared to those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of red supergiants. Fluctuations in the velocity field are compared with photometric and spectroscopic variations, the latter were derived from the TiO band strength and serve, at least partly, as a proxy of the variations in effective temperature.Results: The tomographic method reveals a phase shift between the velocity and spectroscopic and photometric variations. This phase shift results in a hysteresis loop in the temperature - velocity plane with a timescale of a few hundred days, which is similar to the photometric one. The similarity between the hysteresis loop timescale measured in mu Cep and the timescale of acoustic waves disturbing the convective pattern suggests that such waves play an important role in triggering the hysteresis loops.
|
|
| 50. |
- Liljegren, Sofie, et al.
(författare)
-
Atmospheres and wind properties of non-spherical AGB stars
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 619
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The wind-driving mechanism of asymptotic giant branch (AGB) stars is commonly attributed to a two-step process: first, gas in the stellar atmosphere is levitated by shockwaves caused by stellar pulsation, then accelerated outwards by radiative pressure on newly formed dust, inducing a wind. Dynamical modelling of such winds usually assumes a spherically symmetric star.Aims. We explore the potential consequences of complex stellar surface structures, as predicted by three-dimensional (3D) star-in-a-box modelling of M-type AGB stars, on the resulting wind properties with the aim to improve the current wind models.Methods. Two different modelling approaches are used; the CO5BOLD 3D star-in-a-box code to simulate the convective, pulsating interior and lower atmosphere of the star, and the DARWIN one-dimensional (1D) code to describe the dynamical atmosphere where the wind is accelerated. The gas dynamics of the inner atmosphere region at distances of R ∼ 1−2 R⋆, which both modelling approaches simulate, are compared. Dynamical properties and luminosity variations derived from CO5BOLD interior models are used as input for the inner boundary in DARWIN wind models in order to emulate the effects of giant convection cells and pulsation, and explore their influence on the dynamical properties.Results. The CO5BOLD models are inherently anisotropic, with non-uniform shock fronts and varying luminosity amplitudes, in contrast to the spherically symmetrical DARWIN wind models. DARWIN wind models with CO5BOLD-derived inner boundary conditions produced wind velocities and mass-loss rates comparable to the standard DARWIN models, however the winds show large density variations on time-scales of 10–20 yr.Conclusions. The method outlined in this paper derives pulsation properties from the 3D star-in-a-box CO5BOLD models, to be used in the DARWIN models. If the current grid of CO5BOLD models is extended, it will be possible to construct extensive DARWIN grids with inner boundary conditions derived from 3D interior modelling of convection and pulsation, and avoid the free parameters of the current approach.
|
|
