Issue |
A&A
Volume 508, Number 3, December IV 2009
|
|
---|---|---|
Page(s) | 1429 - 1442 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200913149 | |
Published online | 04 November 2009 |
3D simulations of M star atmosphere velocities and their influence on molecular FeH lines
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
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 molecular line shapes. The M star
model parameters range between
of
and effective
temperatures from 2500 K to 4000 K.
Aims. Our aim is to characterize the - and
-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
, and it is our goal to give a rough estimate of the
and
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,
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 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
and
dependence of
lines, the equivalent width,
line width, and line depth are examined.
Results. The velocity fields in M-stars strongly depend on and
. They become stronger with decreasing
and
increasing
. The projected velocities from the 3D models agree
within ~100 m s-1 with the 1D micro- and macro-turbulent
velocities. The
line quantities systematically depend on
and
.
Conclusions. The influence of hydrodynamic velocity fields on line shapes of M-type
stars can well be reproduced with 1D broadening methods.
lines turn out to provide a means to measure
and
in M-type
stars. Since different
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
© ESO, 2009
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