A. Chiavassa

The Stagger-grid: A grid of 3D stellar atmosphere models - I. Methods and general properties

Aims. We present the Stagger-grid, a comprehensive grid of time-dependent, three-dimensional (3D), hydrodynamic model atmospheres for late-type stars with realistic treatment of rad iative transfer, covering a wide range in stellar parameter s. This grid of 3D models is intended for various applications besides studies of stellar convection and atmospheres per se, including stellar parameter determination, stellar spectroscopy and abundance analysis, asteroseismology, calibration of stellar evolution mo dels, interferometry, and extrasolar planet search. In this introductory paper, w e describe the methods we applied for the computation of the grid and discuss the general properties of the 3D models as well as of their temporal and spatial averages (here denotedh 3Di models). Methods. All our models were generated with the Stagger-code, using realistic input physics for the equation of sta te (EOS) and for continuous and line opacities. Our∼ 220 grid models range in effective temperature, Teff, from 4000 to 7000 K in steps of 500 K, in surface gravity, log g, from 1.5 to 5.0 in steps of 0.5 dex, and metallicity, [Fe/H], from−4.0 to +0.5 in steps of 0.5 and 1.0 dex. Results. We find a tight scaling relation between the vertical velocit y and the surface entropy jump, which itself correlates with the constant entropy value of the adiabatic convection zone. The range in intensity contrast is enhanced at lower metallici ty. The granule size correlates closely with the pressure scale height sampled at the depth of maximum velocity. We compare theh 3Di models with currently widely applied one-dimensional (1D) atmosphere models, as well as with theoretical 1D hydrostatic models generated with the same EOS and opacity tables as the 3D models, in order to isolate the effects of using self-consistent and hydrodynamic modeling of convection, rather than the classical mixing length theory (MLT) approach. For the first time, we are able to quantify s ystematically over a broad range of stellar parameters the uncertainties of 1D models arising from the simplified treatment of physics, in particular convective energy transport. In agreement with previous fin dings, we find that the di fferences can be rather significant, especially for metal-poor stars.

The Temperatures of Red Supergiants

We present a re-appraisal of the temperatures of red supergiants (RSGs) using their optical and near-infrared spectral energy distributions (SEDs). We have obtained data of a sample of RSGs in the Magellanic Clouds using VLT+XSHOOTER, and we fit MARCS model atmospheres to different regions of the spectra, deriving effective temperatures for each star from (1) the TiO bands, (2) line-free continuum regions of the SEDs, and (3) the integrated fluxes. We show that the temperatures derived from fits to the TiO bands are systematically lower than the other two methods by several hundred kelvin. The TiO fits also dramatically overpredict the flux in the near-IR, and imply extinctions which are anomalously low compared to neighboring stars. In contrast, the SED temperatures provide good fits to the fluxes at all wavelengths other than the TiO bands, are in agreement with the temperatures from the flux integration method, and imply extinctions consistent with nearby stars. After considering a number of ways to reconcile this discrepancy, we conclude that three-dimensional effects (i.e., granulation) are the most likely cause, as they affect the temperature structure in the upper layers where the TiO lines form. The continuum, however, which forms at much deeper layers, is apparently more robust to such effects. We therefore conclude that RSG temperatures are much warmer than previously thought. We discuss the implications of this result for stellar evolution and supernova progenitors, and provide relations to determine the bolometric luminosities of RSGs from single-band photometry.

Muon ''depth-intensity'' relation measured by the LVD underground experiment and cosmic-ray muon spectrum at sea level

We present the analysis of the muon events with all muon multiplicities collected during 21804 hours of operation of the first LVD tower. The measured angular distribution of muon intensity has been converted to the `depth -- vertical intensity relation in the depth range from 3 to 12 km w.e.. The analysis of this relation allowed to derive the power index,

Radiative hydrodynamic simulations of red supergiant stars - III. Spectro-photocentric variability, photometric variability, and consequences on Gaia measurements

gamma

Three-dimensional hydrodynamical simulations of red giant stars: semi-global models for interpreting interferometric observations

, of the primary all-nucleon spectrum:

The 90-110 mu m dust feature in low to intermediate mass protostars : Calcite?

gamma=2.78 pm 0.05

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

. The `depth -- vertical intensity relation has been converted to standard rock and the comparison with the data of other experiments has been done. We present also the derived vertical muon spectrum at sea level.

The 2010 interferometric imaging beauty contest

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.

Benchmark stars for Gaia: fundamental properties of the Population II star HD 140283 from interferometric ⋆ , spectroscopic ⋆⋆ and photometric data

Context. Theoretical predictions from models of red giant branch stars are a valuable tool for various applications in astrophysics ranging from galactic chemical evolution to studies of exoplanetary systems. Aims. We use the radiative transfer code Optim3D and realistic 3D radiative-hydrodynamical (RHD) surface convection simulations of red giants to explore the impact of granulation on interferometric observables. We assess how 3D simulations of surface convection can be validated against observations. Methods. We computed intensity maps for the 3D simulation snapshots in two filters, the optical at 5000 ± 300 A and the K band 2.14±0.26 μm FLUOR filter, corresponding to the wavelength-range of instruments mounted on the CHARA interferometer. From the intensity maps, we constructed images of the stellar disks and account for center-to-limb variations. We then derived interferometric visibility amplitudes and phases. We study their behavior with position angle and wavelength, and compare them with CHARA observations of the red giant star HD 214868. Results. We provide average limb darkening coefficients for different metallicities and wavelengths ranges. We explain prospects for detecting and characterizing granulation and center-to-limb variations of red giant stars with today’s interferometers. Regarding interferometric observables, we find that the effect of convective-related surface structures depends on metallicity and surface gravity. We provide theoretical closure-phases that should be incorporated into the analysis of red giant planet companion closure phase signals. We estimate 3D−1D corrections to stellar radii determination: 3D models are ∼3.5% smaller to ∼1% larger in the optical than 1D, and roughly 0. 5t o 1.5% smaller in the infrared. Even if these corrections are small, they are needed to properly set the zero point of effective temperature scale derived by interferometry and to strengthen the confidence of existing red giant catalogs of calibrating stars for interferometry. Finally, we show that our RHD simulations provide an excellent fit to the red giant HD 214868 even though more observations are needed at higher spatial frequencies and shorter wavelength.

The Stagger-grid: A grid of 3D stellar atmosphere models - IV. Limb darkening coefficients

We present ISO spectra between 60 and 180 µm of 32 protostars of low to intermediate mass. About half of the spectra present a dust feature between ∼90 and ∼110 µm. We describe the observational characteristics of this feature, which seems to be due to one single carrier. In Class 0 sources the feature peaks around 100 µm, while in AeBe stars it peaks around 95 µm. The feature peak position seems to mostly depend on the temperature of the dust of the source, suggesting reprocessing of the dust. We present arguments for the identification of the observed feature as due to calcite, and estimate that about 10% to 30% of elemental Ca is locked up in it. Therefore, calcite seems to be formed relatively easily around protostars despite the observation that on Earth it needs aqueous solutions. This rises the question of whether conditions simulating liquid water are common around forming stars and what creates them.