3D simulations of M star atmosphere velocities and their influence on molecular FeH lines (original) (raw)

A&A 508, 1429-1442 (2009)

1 Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany e-mail: sewende@astro.physik.uni-goettingen.de; Ansgar.Reiners@phys.uni-goettingen.de
2 GEPI, CIFIST, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France e-mail: Hans.Ludwig@obspm.fr

Received: 19 August 2009
Accepted: 13 October 2009

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 km s-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 $\element[][]{FeH}$ 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 $T_{\mathrm{eff}}$ - 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 $\element[][]{FeH}$ , and it is our goal to give a rough estimate of the $\log{g}$ and $T_{\mathrm{eff}}$ 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, $\element[][]{FeH}$ lines are an important indicator.

Methods. In order to calculate M-star structure models, we employ the 3D radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis in these models is performed with the line synthesis codeLINFOR3D. 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 $T_{\mathrm{eff}}$ dependence of $\element[][]{FeH}$ lines, the equivalent width, line width, and line depth are examined.

Results. The velocity fields in M-stars strongly depend on $\log{g}$ and $T_{\mathrm{eff}}$ . They become stronger with decreasing $\log{g}$ and increasing $T_{\mathrm{eff}}$ . The projected velocities from the 3D models agree within ~100 m s-1 with the 1D micro- and macro-turbulent velocities. The $\element[][]{FeH}$ line quantities systematically depend on $\log{g}$ and $T_{\mathrm{eff}}$ .

Conclusions. The influence of hydrodynamic velocity fields on line shapes of M-type stars can well be reproduced with 1D broadening methods. $\element[][]{FeH}$ lines turn out to provide a means to measure $\log{g}$ and $T_{\mathrm{eff}}$ in M-type stars. Since different $\element[][]{FeH}$ lines all behave in a similar manner, they provide an ideal measure for rotational and magnetic broadening.

Key words: hydrodynamics: stars: low-mass, brown dwarfs / line: profiles / turbulence / stars: late-type