Bernd Freytag

Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere

Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere

The photospheric solar oxygen project: I. Abundance analysis of atomic lines and influence of atmospheric models

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. This paper is dedicated to the memory of Hartmut Holweger.

Spots on the surface of Betelgeuse – Results from new 3D stellar convection models

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

Radiative hydrodynamics simulations of red supergiant stars - I. interpretation of interferometric observations

Context. It has been proposed that convection in red supergiant (RSG) stars produces large-scale granules causing observable surface inhomogeneities. This convection is also extremely vigorous and is suspected to be one of the main causes of mass-loss in RSGs. It should thus be understood in detail. Evidence has accumulated of asymmetries in the photospheres of RSGs, but detailed studies of granulation are still lacking. Interferometric observations provide an innovative way of addressing this question, but they are still often interpreted using smooth symmetrical limb-darkened intensity distributions, or simple, spotted, ad hoc models. Aims. We explore the impact of the granulation on visibility curves and closure phases using the radiative transfer code OPTIM3D. We simultaneously assess how 3D simulations of convection in RSG with CO 5 BOLD can be tested by comparing with these observations. Methods. We use 3D radiative hydrodynamical (RHD) simulations of convection to compute intensity maps at various wavelengths and time, from which we derive interferometric visibility amplitudes and phases. We study their behaviour with time, position angle, and wavelength, and compare them to observations of the RSG α Ori. Results. We provide average limb-darkening coefficients for RSGs. We describe the prospects for the detection and characterization of granulation (i.e., contrast, size) on RSGs. We demonstrate that our RHD simulations provide an excellent fit to existing interferometric observations of α Ori, in contrast to limb darkened disks. This confirms the existence of large convective cells on the surface of Betelgeuse.

Radiative hydrodynamics simulations of red supergiant stars II. Simulations of convection on Betelgeuse match interferometric observations

Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection. Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band to characterize and measure the convection pattern on this star. Methods. We use a 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations. Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in both the optical and the H band based on excellent fits to the observed visibility points and closure phases. We determine that the Betelgeuse surface in the H band is covered by small to medium scale (5−15 mas) convection-related surface structures and a large (≈30 mas) convective cell. In this spectral region, H2O molecules are the main absorbers and contribute to both the small structures and the position of the first null of the visibility curve (i.e., the apparent stellar radius).

Radiative hydrodynamics simulations of red supergiant stars. IV. Gray versus non-gray opacities

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 tha ...

VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere

Aims. We aim to explore the photosphere of the very cool late-type star VX Sgr and in particular the characterization of molecular layers above the continuum forming photosphere. Methods. We obtained interferometric observations with the VLTI/AMBER interferometer using the fringe tracker FINITO in the spectral domain 1.45-2.50 mu m with a spectral resolution of approximate to 35 and baselines ranging from 15 to 88 m. We performed independent image reconstruction for different wavelength bins and fit the interferometric data with a geometrical toy model. We also compared the data to 1D dynamical models of Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG) and asymptotic giant branch (AGB) stars. Results. Reconstructed images and visibilities show a strong wavelength dependence. The H-band images display two bright spots whose positions are confirmed by the geometrical toy model. The inhomogeneities are qualitatively predicted by 3D simulations. At approximate to 2.00 mu m and in the region 2.35-2.50 mu m, the photosphere appears extended and the radius is larger than in the H band. In this spectral region, the geometrical toy model locates a third bright spot outside the photosphere that can be a feature of the molecular layers. The wavelength dependence of the visibility can be qualitatively explained by 1D dynamical models of Mira atmospheres. The best-fitting photospheric models show a good match with the observed visibilities and give a photospheric diameter of Theta = 8.82 +/- 0.50 mas. The H2O molecule seems to be the dominant absorber in the molecular layers. Conclusions. We show that the atmosphere of VX Sgr seems to resemble Mira/AGB star model atmospheres more closely than do RSG model atmospheres. In particular, we see molecular ( water) layers that are typical of Mira stars.

3D Model Atmospheres for Extremely Low-mass White Dwarfs

We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering Teff = 6000–11,500 K and log g = 5–6.5 (g in cm s−2) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., log g = 5–9). For low-mass WDs, the correction in temperature is relatively small (a few percent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + millisecond pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.

Carbon monoxide in the solar atmosphere - I. Numerical method and two-dimensional models

The radiation hydrodynamic code CO5BOLD has been supplemented with the time-dependent treatment of chemical reaction networks. Advection of particle densities due to the hydrodynamic flow field is also included. The radiative transfer is treated frequency-independently, i.e. grey, so far. The upgraded code has been applied to two-dimensional simulations of carbon monoxide (CO) in the non-magnetic solar photosphere and low chromosphere. For this purpose a reaction network has been constructed, taking into account the reactions that are most important for the formation and dissociation of CO under the physical conditions of the solar atmosphere. The network has been strongly reduced to 27 reactions, involving the chemical species H, H2, C, O, CO, CH, OH and a representative metal. The resulting CO number density is highest in the cool regions of the reversed granulation pattern at mid-photospheric heights and decreases strongly above. There, the CO abundance stays close to a value of 8.3 on the usual logarithmic abundance scale with (H) = 12 but is reduced in hot shock waves which are a ubiquitous phenomenon of the model atmosphere. For comparison, the corresponding equilibrium densities have been calculated, based on the reaction network but also under the assumption of instantaneous chemical equilibrium by applying the Rybicki & Hummer (RH) code. Owing to the short chemical timescales, the assumption holds for a large fraction of the atmosphere, in particular the photosphere. In contrast, the CO number density deviates strongly from the corresponding equilibrium value in the vicinity of chromospheric shock waves. Simulations with altered reaction networks clearly show that the formation channel via hydroxide (OH) is the most important one under the conditions of the solar atmosphere.

What causes the large extensions of red supergiant atmospheres? - Comparisons of interferometric observations with 1D hydrostatic, 3D convection, and 1D pulsating model atmospheres

B.A.T. and J.M.M. acknowledge support by the Spanish Ministry of Science and Innovation through the grants AYA2009-13036-C02-02 and AYA2012-38491-C02-01.