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Position: Associate Research Professor, IACS, Catholic University of America
| Address: | Code 681
Goddard Space Flight Center Greenbelt, MD 20771 |
I am primarily been involved in studies of the atmospheric physics of cool stars, particularly activity associated with magnetic fields. This includes the heating of chromospheres and coronae, the acceleration of winds, the dynamics of solar and stellar flares and the physical processes leading to starspots and prominances. I have been involved with the measurement of solar and stellar magnetic fields and am still interested in magnetic measurement techniques as well as the theory of the magnetic dynamo. Specific examples of current research topics are the following:
Winds in Cool Giant and Supergiant Stars:
Ken Carpenter and I have been analyzing GHRS spectra for a variety
of cool giant and supergiant stars obtained from GTO and GO programs. The
Fe II, Mg II and O I emission lines in these spectra show evidence for
outwardly flowing material
which increases in velocity with height, suggesting that we are observing
the onset of the stellar wind in the chromosphere. I am currently using
the SEI radiative transfer code (Sobolev with Exact Integration) to emperically
model the winds in a few well observed stars, including the K giants alpha
Tau and gamma Dra, the K supergiant lambda Vel and the M giant gamma Cru.
This code was originally developed to investigate winds in hot stars, but
has been found to provide excellent results in the investigation of cool
stars as well. In most of the cool stars studied, the C II intercombination
lines near 2330 \AA\ show
significant non-thermal broadening with FWHM of $\sim$ 25 km s
for the giants and $\sim$ 35 km s in the supergiants. This is highly
supersonic and I am currently working with Vladimir Airapetian (CSC) and
Leon Ofman (STX)
on a model for atmospheric heating and wind acceleration based upon
non-linear Alfven waves.
Flare Dynamics:
I have been collaborating with Bruce Woodgate in a search for proton
beams during the impulsive phase of stellar flares. The original observations
of the dMe flare star AU Mic consisted of rapid readout GHRS data obtained
for Woodgate's HST GO program. These detected the signature of a proton
beam. A second observation of AU Mic, taken approximately 1 year later,
observed a small flare event which occurred during the decay phase of a
much larger flare. No proton beam was detected, but a possible upper limit
to the beam energy was obtained, as well as valuable information about
the flare energy budget. A
final set of observations were obtained for the star YZ CMi as
part of a large observing campaign in Dec 1994. Approximately 29 flare
events were detected during 7 hours of on-source observing. In nearly all
cases the flares showed enhancements in both the lines and the continuum
near 1200 \AA\ . Surprisingly, nearly all of the flares primarily showed
low temperature enhancements, with little evidence for coronal emission.
The continuum enhancements complicate the search for a proton beam signature
and so far no evidence for proton beams have been seen. However, this may
be expected based numerical simulations by Jeff Brosius (STX), which show
that the lyman alpha emission from the proton beam may be rapidly suppressed
by the ionization of the atmosphere.
Transition Regions and Coronae on Active Stars:
The Fe XXI line at 1354 \AA\ is the only coronal emission line currently
available which can be resolved on a stellar source. Using observations
obtained from Steve Maran's (NASA/LASP) HST GTO program, I have investigated
the characteristics of the Fe XXI line on both the dMe flare star AU Mic
and the RS CVn binary system HR 1099. The Fe XXI line was detected in both
systems. The width of the line in AU Mic was consistent with a thermal
plasma of temperature 10$^7$ K, which is the temperature where the ionization
state is most pronounced. The observations for HR 1099 show emission from
both components of the system, as well as a much broader width, suggesting
either an extended corona or strong coronal turbulence. The O V line, seen
in the same spectrum, is unresolved in the AU Mic observation, but is well
resolved in the HR 1099 data and shows turbulent velocities with an RMS
of more than 100 km s.
Theoretical calculations by Airapetian (CSC)
and myself indicate that the observations can be accounted for by the damping
of non-linear Alfven waves in coronal loops. Apparently the heating processes
occurring in the RS CVn and dMe stars are quite different.
Magnetic Activity on Hot Stars:
X-ray emission from hot stars is typically attributed to shock waves
or the accretion of material onto a degenerate companion. The proto-typical
Be star gamma Cas may be an exception to this rule. This star has an average
X-ray emission which is much larger than typically seen in Be stars, but
still an order of magnitude smaller than that seen in X-ray binaries. Myron
Smith and
I have been investigating the properties of the star and have found
evidence that the X-rays may arise from flare-like activity near the stellar
surface. These flares are seen as short duration (10-30 seconds) X-ray
bursts superimposed on a slowly varying background. UV observations of
the star taken with the Goddard High Resolution Spectrograph aboard the
Hubble Space Telescope also show evidence for clouds of relatively cool
plasma (5,000-20,000 K) which are suspended above the surface of the star
and forced to co-rotate. These can be thought of as stellar analogs of
solar prominences and, along with the
flares, point to the presence of dynamic magnetic fields on this star.
Figure: 'GCAS_FULL_IMAGE.PS'
CAPTION: A dynamic UV spectrum of the classical Be star gamma Cas. The
image represents a time sequence of spectra which have had an average reference
spectrum subtracted so that spectral changes can be more easily observed.
In all
cases, the changes occur as absorption features. The color scale has
been selected so that red corresponds to the maximum intensity (no absorption),
while yellow, green, blue to white represent progressively larger amounts
of absorption. The maximum absorption feature in the image corresponds
to a decrease of about 20% below the local continuum intensity. The primary
features result from blue-shifted absorption by the two resonance lines
of Si IV (UV 1), which have rest wavelengths of 1393.73 Ang and 1402.73
Ang. These features have velocities of up to 1800 km/s and are often called
Descrete Absorption
Components (DACs). The dynamic spectrum is presented by Smith and Robinson
(1999, ApJ, 517, 866) and a complete analysis of the DACs will be presented
in an upcoming paper.
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