STELLAR SURFACE STRUCTURE
Solar granulation
High spatial-resolution images of the solar photosphere show the granulation
phenomenon: a pattern of bright cells of irregular polygonal shape and
typical sizes in the range of 1000-2000 km, separated by narrow dark lanes.
Wiggly spectral lines
Granulation spectrograms reveal "wiggly"spectral lines, such that the bright
granules correlate with local blueshifts, while the darker intergranular
lanes correlate with local redshifts. These reflect the upward motion
in the brighter (hotter) granules, and downward in the darker (cooler)
intergranular lanes. After averaging over the surface, a spectral
signature remains, because the contributions from the brighter and blueshifted
areas exceed those from the darker and redshifted ones: this causes the
convective
blueshift.
Wavelength shifts
Convective blueshifts of typically 400 m/s in the Sun are expected
to reach 800 m/s in subgiants, and 1000 m/s in F-type dwarfs. For
the Sun, the blueshifts increase for weaker lines (formed deeper into the
convective layers); for ionized lines and such of high excitation potential
(formed mostly in the hotter and rising elements); and also at shorter
visual wavelengths (where the black-body contrast for a given temperature
difference increases). However, the sign reverses in the vacuum ultraviolet
to give redshifts up to 1000 m/s (lines are now formed in the region
of convective overshoot, with an inverted velocity/brightness correlation).
Observing stellar granulation
Signatures of stellar granulation are observed in high-resolution spectra
as asymmetries and convective wavelength shifts in photospheric absorption
lines. These effects originate from correlated velocity and brightness
patterns on stellar surfaces, analogous to the solar case. The dependence
of asymmetries and shifts on line strength, excitation potential, ionization
level, and wavelength region, reflects granulation properties throughout
the stellar photosphere.
Among novel types of line diagnostics, the potential of astrometric
radial velocities seems especially promising. For the Sun, its
absolute convective lineshifts can be studied because its motion is known
from planetary system dynamics: thus observed shifts can be interpreted
as originating from gravitational redshift, convective blueshift, and other
atmospheric phenomena. For other stars, absolute determinations of
their center-of-mass motion have become possible with space astrometry.
Models of stellar granulation
Since the degrees of freedom in parameterized models of three-dimensional
stellar atmospheres are potentially very many, traditional modeling techniques
involving the adjustment of successive parameters are not adequate.
Instead, numerical simulations of the three-dimensional and time-dependent
phenomena permit a modeling with considerable detail and realism.
By using the output of such simulations as sets of time- and space- dependent
model atmospheres, synthetic line profiles are obtained.
To test the validity of such model simulations, studies of "ordinary" line
profiles are not adequate. Hydrodynamic models can, however, be constrained
by second-order quantities, such as asymmetries and wavelength shifts,
and especially their differential behavior between lines of different excitation
potential, ionization stage, or height of formation.
White-dwarf surfaces
White-dwarf atmospheres have surface gravities some four orders of magnitude
greater than solar-type stars, and their granulation structure is predicted
to be correspondingly smaller and more energetic. Granular features are
expected to exist on scales down to hundreds of meters, while velocities
of tens of km/s imply characteristic timescales of perhaps 10 milliseconds,
well in the range of high-speed astrophysics.
Even if spectral lines would prove difficult to accurately observe or interpret,
granulation on white dwarfs could be studied by high-speed photometry with
large telescopes, observing the stellar microvariability in response to
the evolution of [the finite number of] granular features.
A significant subset of white dwarfs possess very strong magnetic
fields. They might well influence the gross structure of the stellar
envelope (perhaps blown up as a "magnetic balloon" due to the additional
magnetic pressure?), and most probably also the structure of photospheric
convection. Such magnetohydrodynamic issues have been studied in
collaboration with
Christian Fendt.
Publications
Researchers in stellar granulation & line formation
Carlos Allende Prieto (Austin)
Martin Asplund
(Mt.Stromlo/Uppsala)
Bernd Freytag
(Uppsala)
David
F. Gray (Western Ontario)
Dan Kiselman
(Stockholm)
Hans-Günter Ludwig
(Lund)
Åke Nordlund
(Copenhagen)
Matthias Steffen
(Potsdam)
Bob Stein
(Michigan)
Updated JD 2,452,300
