Download CP2: KUPKA et al.: Observational signatures of atmospheric velocity

Observational Signatures of Atmospheric Velocity Fields
in Main Sequence Stars
F.
1,3
Kupka ,
2,3
Landstreet ,
2,3
Sigut ,
2
Bildfell ,
J. D.
A.
C.
2
2
2
T. Officer , J. Silaj , A. Townshend
A.
2
Ford ,
1Max-Planck-Institut
fur Astrophysik, Garching, Germany
2Department of Physics and Astronomy, University of Western Ontario
3 Guest Investigator, Canada-France-Hawaii Telescope, Hawaii
Abstract
This star appears to be an
example of a hot extension of
the HgMn and He-wk PGa
sequence. Its rotational velocity
is essentially undetectably
small, and using the three
strong lines of Si III at 4552,
4567 and 4574 we find that the
microturbulence is not
significantly different from 0
km/s. The computed line
profiles match the observed
ones very closely; although the
star has very small v sin i, it
does not furnish any useful
information about a possible
atmospheric velocity field.
In stars with sufficiently small projected rotational velocities (less than a few km/s), it is often possible to
detect signatures of the atmospheric velocity field in line profiles. These signatures may be as
subtle as small asymmetries in the profile ("line bisector curvature") or as obvious as profile shapes that
strongly depart from those predicted even by simple microturbulence models. We have recently carried
out a high resolution survey of sharp-line stars to search for these symptoms of local velocity fields. This
poster will report the first results of a comparison of models with the observed profiles.
Introduction
One of us (FK) has been developing new models of convection based on equations describing local mean values of
moments of the distributions of velocity, temperature, density, pressure, etc. These model are reaching the point
where it is useful to compare them with velocity fields observed through their effects on stellar line profiles in a
variety of stars. We have acquired at the CFHT a sample of spectra of sharp-line B, A and F stars for the purpose.
As a first step in comparing them to the new convection models, we model the spectra with conventional line
profile synthesis with microturbulence, and search for line profiles which deviate from this model in ways that
provide further information about the atmospheric velocity fields. This poster gives a progress report on this
work.
HD 175640 is a single-line
spectroscopic binary and a
cool HgMn star. It has one
of the smallest values of v
sin i known among A and B
stars, but also appears to
have no significant
atmospheric
convection. The computed
and observed profiles match
very closely; again we have
no information about
velocity fields.
This star is a typical sharpline F star. The red wings
of the observed profiles of
the strong lines generally
are a little lower than the
computed profiles; this
corresponds to the "C"shaped bisector curvature
found in solar-type stars.
A fit to the spectrum of the sharp-line F4V star theta Cyg = HD 185395. The
model atmosphere has T_e = 6500 and log g = 4.2. The best
microturbulence is 2 km/s, and v sin i = 7 km/s. Abundances were fitted for
Ca, Ti, Cr and Fe.
Fit to spectrum of the B9 III HgMn star HD 175640. The model
used has T_e = 10500 K and log g = 4.0. The v sin i value is 2.9
km/s. The microturbulence is not significantly different from
0. Abundances are found for Ti, Cr, Fe and Ba; Cr is significantly
more abundant than the solar value, while Fe is underabundant.
Fit to spectrum of the B3p star HD 174179. The model
atmosphere has T_e = 17400 K and log g = 3.7. The best fit v sin i = 0.5
km/s, and the best fit microturbulence parameter is xi = 0 km/s. From
this window we get abundances of Al, Si, S, Cr and Fe. Al and S are
about 1 dex below solar; the other elements are close to or slightly
below solar.
iota Her is a single-line
spectroscopic binary, and a
beta Cep pulsating variable.
However, no systematic
discrepancies between
computed and observed line
profiles are found; it appears
that even the very small v sin i
(iota Her is one of the
sharpest-line normal B stars
known) is enough to mask the
signature of the atmospheric
convective velocity field, and
the same small v sin i
prevents the beta Cep
oscillations from appearing in
the profiles.
Fit to spectrum of HD 160762 = iota Her, a normal B3 IV
star. Our model atmosphere has T_e = 17000 K and log g = 4.0. The best
fit v sin i = 8.5 km/s, and the microturbulence parameter is found to
be 1.2 km/s, clearly different from 0. Abundances are
generally fairly close to solar values except for Si which appears
overabundant, but this may be a non-LTE effect.
Conclusions
From the data we have obtained so far, four conclusions stand out.
Fit to spectrum of the A4 II star o Sco = HD 147084.
The model has T_e = 7400 K and log g = 1.5. The
microturbulence parameter is found to be 0 km/s.
Abundances are found for Ti, Cr and Fe; all are
slightly below solar abundances.
This star has been suspected as
a member of the Sco-Cen OB
association, but the Hipparcos
parallax shows that it is too far
away. It is intermediate
between a giant and a
supergiant. The line profiles
clearly show the effects of what
is usually modelled as
macroturbulence. The observed
lines have depressed wings
compared to the models, and
clear bisector curvature. The v
sin i that fits the strong lines
seems too large for the weak
lines.
This star shows clearly
the departure of the line
profiles from the simple
model used. All the
strong lines have a
depressed blue line wing
compared to the model
lines. Further, the v sin i
value that reproduces the
strong lines seems too
large for the weak lines.
(Numerous weak lines
not modeled are from the
secondary star.)
A fit to the spectrum of the A3 V star HD 103578. The star is an SB2
system. The spectrum has been corrected by subtracting 9% of the
light to remove the secondary contribution. The model atmosphere
has T_e = 8500 K and log g = 4.0, giving about the right ionization
balance. The best microturbulence parameter is about 1.5 km/s, and
v sin i = 7 km/s.
* The stars whose profiles reveal the atmospheric velocity field are very
rare among early-type stars on or near the main sequence.
* The classical model with height-independent microturbulence fits the
line profiles of some early-type stars (even ones with very small v sin i)
extremely well.
* In a few stars, this model does not give a consistent fit to the detailed
line profiles. These are stars whose spectra contain further information
about the atmospheric velocity fields.
* The effects of the atmospheric velocity field on line profiles are
usually quite subtle, and require high spectral resolution, high signalto-noise data, and careful examination to identify.
The next step will be to use the new convection models to try to model
the profiles of stars for which the classical model gives unsatisfactory
results.