Bibcode
Ramírez, I.; Allende Prieto, C.; Lambert, D. L.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 492, Issue 3, 2008, pp.841-855
Fecha de publicación:
12
2008
Revista
Número de citas
27
Número de citas referidas
24
Descripción
Aims: We seek to detect and quantify the effects of surface convection
(granulation) on the line spectra of K-dwarfs as a first step towards a
rigorous testing of hydrodynamic models for their atmospheres. Methods: 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 2dcoudé spectrograph on the 2.7 m telescope at
McDonald Observatory to determine wavelength shifts and asymmetries of
Fe i lines. Spectra of the same stars acquired with the High Resolution
Spectrograph (R≃120,000) on the 9.2 m Hobby Eberly Telescope were
used as radial velocity templates to calibrate the wavelength scale of
the 2dcoudé spectra. Results: 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-1 for the strongest lines (residual core flux ≃0.3),
while wavelength shifts range from about -150 m s-1 in the
weakest (EW≃10 mÅ) lines to 0 in the strongest (EW⪆100
mÅ) 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. While the first two are understood and
confirmed by our data, the relation to stellar activity, which is based
on our finding that the largest departures from the expected behavior
are seen in the most active stars, requires further investigation. For
the inactive, slow projected rotational velocity stars, we detect,
unequivocally, a plateau in the line-shifts at high EW values
(EW⪆100 mÅ), 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-1 for
four of our sample stars.