Fundamental parameters and granulation properties of Alpha Centauri A and B obtained from inversions of their spectra

Abstract

Properties of stellar granulation are obtained by inverting spectra of the late-type stars α Centauri A and B. Our inversions are based on a multi-component model of the stellar photosphere and take into account the center-to-limb variation and rotational broadening. The different atmospheric components describe the areas harboring up-, down-and horizontal flows. The inversions are constrained by fitting not only the flux profiles, but also their line bisectors, and by using a simple mass conservation scheme. The inversions return the properties of convection at the stellar surface, including the stratification of the thermodynamic parameters, as well as fundamental parameters such as the gravitational acceleration, v sin i and the element abundances. For α Cen A (G2V) the derived stratifications of the temperature and convective velocity are very similar to the Sun, while for α Cen B (K1V) we find similar up-and downflow velocities, but lower horizontal speeds and a reduced overshoot. The latter is consistent with the smaller scale height of the atmosphere, while mass conservation arguments taken with the lower horizontal speed imply that the granules on α Cen B are smaller than on the Sun. Both these properties are in good agreement with the hydrodynamic simulation of Nordlund & Dravins (1990, A&A, 228, 155). The inversions also return the fundamental parameters (T eff , log g, abundances, v sin i, etc.) of the two stars. These values are on the whole in good agreement with literature values. Also, most of them do not strongly depend on the details of the inversion. However, importantly, the element abundances are 0.1 to 0.15 dex lower when a 2-or 3-component inversion is carried out than with a 1-component inversion.

The Alpha Centauri binary system

“…Their analysis is differential with respect to the Sun, for which they adopted Table 3. Frutiger et al (2005) have also analyzed the spectra of α Cen A and B with a fundamentally different and promising technique. They have inverted high-resolution (R ∼ 10 5 ), moderately high S /N (∼250) spectra of the stars by means of a multi-component model photosphere.…”

“…They also found a very high log g for α Cen B, prompting them to attempt a larger number of inversions this for component, first fixing log g = 4.48 dex, which led to a reduced T eff = 5154 K with no appreciable change in metallicity, and then fixing the rotational velocity, which produced T eff = 5260 K, again with no significant impact on metallicity, and a new log g = 4.68 ± 0.05. The T eff s favored by the inversion method of Frutiger et al (2005) add to a complex situation. They seem to be in good agreement with our spectroscopic T eff for α Cen B when all parameters are independently derived from the inversion method, but the agreement switches to one with the Hα and photometric T eff s, when the surface gravity is fixed.…”

“…It would be very valuable to extend the novel approaches of Valenti & Fischer (2005) and Frutiger et al (2005) to additional stars for which stringent observational constraints are available, to quantify how discrepancies of the type discussed here occur for objects with atmospheric parameters increasingly different from the Sun's. At present, we can state that it is likely that our spectroscopic T eff for α Cen B is more uncertain, and systematically higher, that the other two determinations.…”

On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs

“…The reasons for this empirical fact are likely related to blends; specifically, to blend abundances (often unconstrained), lack of literature on transition parameters needed for SPINOR, uncertainties in oscillator strengths or restwavelengths, and unidentified line blends or even relative displacements of blend wavelengths due to convection. For a literature comparison of the fit quality presented here, see Frutiger et al (2005) Small systematic effects (best visible in the spectral difference between observed and computed lines profiles) complicated our analysis. The high quality of the data revealed these systematic shortcomings in reproducing real line profiles.…”

“…These profiles are described in Gray (1975) and Gr08. Thus, we obtain new annular SIPs, I (λ, θ) = I(λ, θ) * Θ(λ, θ), where " * " denotes convolution carried out as multiplication in Fourier space.…”