3D simulations of M star atmosphere velocities and their influence on molecular FeH lines

Abstract

Context. The measurement of line broadening in cool stars is in general a difficult task. In order to detect slow rotation or weak magnetic fields, an accuracy of 1 kms(-1) is needed. In this regime the broadening from convective motion becomes important. We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on FeH molecular line shapes. The M star model parameters range between log g of 3.0-5.0 and effective temperatures from 2500 K to 4000 K. Aims. Our aim is to characterize the T(eff)- and log g-dependence of the velocity fields and express them in terms of micro-and macro-turbulent velocities in the one dimensional sense. We present a direct comparison between 3D hydrodynamical velocity fields and 1D turbulent velocities. The velocity fields strongly affect the line shapes of FeH, and it is our goal to give a rough estimate of the log g and T(eff) parameter range in which 3D spectral synthesis is necessary and where 1D synthesis suffices. We want to distinguish between the velocity-broadening from convective motion and the rotational-or Zeeman-broadening in M-type stars we are planning to measure. For the latter, FeH lines are an important indicator. Methods. In order to calculate M-star structure models, we employ the 3D radiative-hydrodynamics (RHD) code CO(5)BOLD. The spectral synthesis in these models is performed with the line synthesis code LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian standard deviations and project them onto the line of sight to include geometrical and limb-darkening effects. The micro- and macro-turbulent velocities are determined with the "curve of growth" method and convolution with a Gaussian velocity profile, respectively. To characterize the log g and T(eff) dependence of FeH lines, the equivalent width, line width, and line depth are examined. Results. The velocity fields in M-stars strongly depend on log g and T(eff). They become stronger with decreasing log g and increasing T(eff). The projected velocities from the 3D models agree within similar to 100 m s(-1) with the 1D micro-and macro-turbulent velocities. The FeH line quantities systematically depend on log g and T(eff). Conclusions. The influence of hydrodynamic velocity fields on line shapes of M-type stars can well be reproduced with 1D broadening methods. FeH lines turn out to provide a means to measure log g and T(eff) in M-type stars. Since different FeH lines all behave in a similar manner, they provide an ideal measure for rotational and magnetic broadening.