Three-dimensional hydrodynamical CO⁵BOLD model atmospheres of red giant stars

Abstract

Aims. We utilize state-of-the-art three-dimensional (3D) hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB (T eff ≈ 5000 K, log g = 2.5), and characterized by four different metallicities, [M/H] = 0.0, −1.0, −2.0, −3.0. Methods. The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. The 3D hydrodynamical and classical 1D model atmospheres used in this study were calculated with the CO 5 BOLD and 1D LHD codes, respectively. Identical atmospheric parameters, chemical composition, equation of state, and opacities were used with both codes, therefore allowing a strictly differential analysis of the line formation properties in the 3D and 1D models.Results. We find that for lines of certain neutral atoms, such as Mg i, Ti i, Fe i, and Ni i, the abundance corrections strongly depend both on the metallicity of a given model atmosphere and the line excitation potential, χ. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity (≤±0.1 dex), for lines of neutral elements with low ionization potential and low-to-intermediate χ they quickly increase with decreasing metallicity, reaching in their extremes −0.6 to −0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Lines of neutral elements with higher ionization potentials (E ion 10 eV) generally behave very similarly to lines of ionized elements characterized by low ionization potentials (E ion 6 eV). In the latter case, the abundance corrections are small (generally, ≤±0.1 dex) and are caused by approximately equal contributions from the horizontal temperature fluctuations and differences between the temperature profiles in the 3D and 1D model atmospheres. Abundance corrections of molecular lines are very sensitive to the metallicity of the underlying model atmosphere and may be larger (in absolute value) than ∼−0.5 dex at [M/H] = −3.0 (∼−1.5 dex in the case of CO). At fixed metallicity and excitation potential, the abundance corrections show little variation within the wavelength range studied here, 400−1600 nm. We also find that an approximate treatment of scattering in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thin, atmosphere) leads to abundance corrections that are altered by less than ∼0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines.

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…As shown by state-of-the-art 3D hydrodynamical simulations, the use of classical model atmospheres may be a more questionable assumption, although, once again, the problem is much more acute at low metallicities according to the calculations presented by Collet et al (2007). The good level of consistency achieved between our spectroscopic parameters and independent estimates might indicate that the effect of granulation is not as severe at near-solar metallicity as anticipated by these particular models (see Dobrovolskas et al 2013).…”

“…In view of the problems plaguing the C i λ5380 abundances, they are not discussed further. Although the departures from LTE affecting the C 2 features are largely unknown (Asplund 2005), the neglect of granulation should not lead to large errors (Dobrovolskas et al 2013).…”

The early days of the Sculptor dwarf spheroidal galaxy

“…This element is only detected in the Ba  majority species, and the detected lines arise either from the ground or low-excitation levels. Dobrovolskas et al (2013) …”

The Hamburg/ESO R-process Enhanced Star survey (HERES)

“…In this study, we did not consider the influence of the 3D effects on the derived element abundances, although we took care to minimise such an influence, where possible, for example by removing low-excitation lines of Fe i and Mn i. Using the 3D hydrodynamic model atmosphere of VMP cool giant 5020/2.5/−3, Dobrovolskas et al (2013) predicted that the (3D−1D) abundance corrections are negative and large in absolute value of up to −0.6 to −0.75 dex for the lines arising from the ground state of the minority species, such as Mg i, Ti i, and Ni i, but they decrease rapidly towards higher excitation energy of the lower level and do not exceed −0.2 dex, when E exc = 2 eV. We did not determine such a large discrepancy between the E exc < 0.2 and E exc > 2 eV lines for Mg i and Ni i (Table 3).…”

“…The heavy elements beyond the Fe group are mostly observed in HE 2252−4225 for their majority species, such as Ba ii and Eu ii, and in the low-excitation lines. Dobrovolskas et al (2013) predicted that the (3D−1D) corrections for the E exc = 0 lines of Ba ii and Eu ii are small and very similar in magnitude, −0.05 dex and −0.04 dex, respectively.…”