Granulation in K-type Dwarf Stars. I. Spectroscopic observations

Very high resolution (R~160,000-210,000), high signal-to-noise ratio (S/N>300) spectra of nine bright K-dwarfs were obtained with the 2dcoude spectrograph on the 2.7m Telescope at McDonald Observatory to determine wavelength shifts and asymmetries of Fe I lines. The observed shapes and positions of Fe I lines reveal asymmetries and wavelength shifts that indicate the presence of granulation. In particular, line bisectors show characteristic C-shapes while line core wavelengths are blueshifted by an amount that increases with decreasing equivalent width (EW). On average, Fe I line bisectors have a span that ranges from nearly 0 for the weakest lines (residual core flux > 0.7) to about 75 m/s for the strongest lines (residual core flux ~ 0.3) while wavelength shifts range from about -150 m/s in the weakest (EW ~ 10 mA) lines to 0 in the strongest (EW > 100 mA) features. A more detailed inspection of the bisectors and wavelength shifts reveals star-to-star differences that are likely associated with differences in stellar parameters, projected rotational velocity, and stellar activity. For the inactive, slow projected rotational velocity stars, we detect, unequivocally, a plateau in the line-shifts at large EW values (EW > 100 mA), a behavior that had been identified before only in the solar spectrum. The detection of this plateau allows us to determine the zero point of the convective blueshifts, which is useful to determine absolute radial velocities. Thus, we are able to measure such velocities with a mean uncertainty of about 60 m/s.

Center-to-Limb Variation of Solar 3-D Hydrodynamical Simulations

We examine closely the solar Center-to-Limb variation of continua and lines and compare observations with predictions from both a 3-D hydrodynamic simulation of the solar surface (provided by M. Asplund and collaborators) and 1-D model atmospheres. Intensities from the 3-D time series are derived by means of the new synthesis code ASSET, which overcomes limitations of previously available codes by including a consistent treatment of scattering and allowing for arbitrarily complex line and continuum opacities. In the continuum, we find very similar discrepancies between synthesis and observation for both types of model atmospheres. This is in contrast to previous studies that used a ``horizontally and time averaged representation of the 3-D model and found a significantly larger disagreement with observations. The presence of temperature and velocity fields in the 3-D simulation provides a significant advantage when it comes to reproduce solar spectral line shapes. Nonetheless, a comparison of observed and synthetic equivalent widths reveals that the 3-D model also predicts more uniform abundances as a function of position angle on the disk. We conclude that the 3-D simulation provides not only a more realistic description of the gas dynamics, but, despite its simplified treatment of the radiation transport, it also predicts reasonably well the observed Center-to-Limb variation, which is indicative of a thermal structure free from significant systematic errors.

Signatures of Granulation in the Spectra of K-Dwarfs

Very high resolution (R>150,000) spectra of a small sample of nearby K-dwarfs have been acquired to measure the line asymmetries and central wavelength shifts caused by convective motions present in stellar photospheres. This phenomenon of granulation is modeled by 3D hydrodynamical simulations but they need to be confronted with accurate observations to test their realism before they are used in stellar abundance studies. We find that the line profiles computed with a 3D model agree reasonably well with the observations. The line bisectors and central wavelength shifts on K-dwarf spectra have a maximum amplitude of only about 200 m/s and we have been able to resolve these granulation effects with a very careful observing strategy. By computing a number of iron lines with 1D and 3D models (assuming local thermodynamic equilibrium), we find that the impact of 3D-LTE effects on classical iron abundance determinations is negligible.