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Kenneth G. Carpenter
Laboratory for Astronomy and Solar Physics, Code 681 NASA-GSFC,
Greenbelt, MD 20771
Richard D. Robinson
Computer Sciences Corporation @ LASP-NASA/GSFC, Code 681,
Greenbelt, MD 20771
Keywords: globular clusters,peanut clusters,bosons,bozos
UV spectra of K-M giant and supergiant stars and of carbon stars have been
acquired with the Goddard High Resolution Spectrograph (GHRS) on the Hubble
Space Telescope (HST). These spectra have been used to measure chromospheric
flow and turbulence velocities, study the acceleration of their stellar winds,
acquire constraints on the outer atmospheric structure of such stars, and
provide data needed to understand the radiative line transfer in these
atmospheres. We have observed the normal, oxygen-rich giant stars
Dra (K5 III hybrid),
Tau (K5 III),
Cru (M3.4 III),
Gem (M3 IIIab), and 30 Her (M6 III) and the supergiant
Ori
(M2 Iab), as well as the carbon stars TX Psc (N0; C6,2) and TW Hor (N0; C7,2).
The high resolution and wavelength accuracy of these data have allowed the
direct measurement of the acceleration of the stellar winds and the
macroturbulence in the chromospheres of several of these stars. The high
signal-to-noise and large dynamic range of these spectra have allowed the
detection and identification of numerous new emission features, including weak
C IV emission indicative of hot transition-region plasma in the non-coronal
giant
Tau (Carpenter, Robinson, & Judge 1994), many new
fluorescent lines of Fe II, and the first detection of molecular hydrogen and
of Ca II recombination lines in the UV spectrum of a giant star (McMurray et
al. 1996). The UV spectrum of two carbon stars have been studied with H.
Johnson et al. (see, e.g., Johnson et al. 1995) with unprecedented resolution
and reveal extraordinarily complicated Mg II lines strongly mutilated by
overlying circumstellar absorptions. Details and some examples of these
results are given below. Although, there are far too many data to be shown in
this paper, further examples (some indicated herein) can be found in Carpenter
(1996).
We have used the profiles and widths of optically-thin emission lines to
characterize the macroturbulence in the chromospheres of these stars. Our
primary diagnostic is the C II] (UV 0.01) multiplet of semi-forbidden lines
near 2325Å. These lines show no evidence for opacity broadening, but are
still much broader than one would expect on the basis of thermal
microturbulence (about 6 in these chromospheres). We have used both the
GHRS G270M and Echelle-B gratings in these observations. A sample of these
data are shown in Carpenter (1996).
Figure: Fits to C II
Lines assuming isotropic and anisotropic macroturbulence
The Echelle data, in particular, show that the lines are broadened
at the base, relative to the single Gaussian profile expected for simple
isotropic macroturbulence. Following a suggestion by David Gray, we have found
that the observed profiles are much better fit assuming an anisotropic
macroturbulence, in which the macroturbulent velocities are confined to the
radial and/or tangential directions, as one might expect at the edges and tops
of convective cells. The best fit is produced assuming pure radial
macroturbulence although tangential motions produce similar profiles and some
contribution from such motions cannot be completely ruled out on the basis of
these data. Figure 1 illustrates the best fits assuming isotropic
macroturbulence (dots) and an anisotropic, radial-tangential macroturbulence
(dashes) to the observed C II 2325Å line in Ori and
Tau.
Table 1 lists our estimates of the mean macroturbulence derived from this
fitting process (along with other results to be discussed later), assuming a
pure radial distribution of the macroturbulent velocities.
Table 1: Measured Macroturbulence, Mean Flow Velocities and Wind
Acceleration for Cool Giants and Supergiants (in km s).
A second very important diagnostic of the velocity fields in these stars are
the large variety of Fe II lines seen in the mid-UV spectral region. The
Fe II profiles are more complicated in appearance than the simple emission
lines seen in C II, in that many of the lines have one or more absorption
self-reversals superposed on the emission component. The Fe II UV 1
2586Å line in three stars is shown in Carpenter (1996), where it can be seen
that Cru shows two self-absorption components
(a strong, blue-shifted one and a weaker red-shifted one) like all K-M giant
stars,
Ori shows a single blue-shifted absorption, and
Vel
shows multiple self-absorptions, all blue-shifted. The blue-shifted components
are indicative of formation in an outflowing stellar wind and the variation of
their shift with intrinsic line strength provides a way to measure the
acceleration of the stellar wind with height. The red-shifted self-absorptions
may indicate a weaker downflow of some material or, more likely, be the result
of a subtle radiative transfer effect and a turbulence field which changes with
height (Ensman & Johnson 1995).
We have measured mean flow velocities of the C II and Fe II ions in these
chromospheres by the offset of the mean wavelength of their emission profiles,
and, for Fe II, of the self-absorptions, from the laboratory wavelength
values, adjusted for the stellar radial velocity. These flows are tabulated in
Table 1. The mean flows of the emission components of both ions correlate well
with each other and generally show either a slight inflow or slight outflow of
2--4 . The mean flows of the blue-shifted Fe II self-absorptions indicate
means outflows of several to as much as 10
, while the mean apparent
velocities of the weaker ``inflows'' are seen at 8 to 14
. We also find
that the observed blue-shift of the strong Fe II reversals increases with
increasing line strength, indicating that the outflow, i.e., the stellar wind,
is accelerating with increasing distance from the stellar photosphere (since
the self-reversals of the stronger, more opaque, lines are formed higher up in
the atmosphere). We characterize the relative line strengths by a relative
line center opacity (
), computed for a temperature of 6000 K, a hydrogen
column density of
cm
, a microturbulence of 6
, an
electron density of
cm
, and a solar Fe II/H abundance.
Figure 2 shows the observed shift of the stronger Fe II
self-absorption versus this relative log(
) in the
Cru rest
frame.
Figure: The increase of Fe II self-absorption blue-shift with increasing line
strength (height in chromosphere), reflecting the acceleration of the wind
in Cru.
We are first able to detect the wind at about 7 (where the Fe II absorption
first becomes thick enough to observe) and can follow it higher into the
atmosphere, as it accelerates up to about 15
. None of the Fe II lines are
opaque enough to allow us to sample higher and perhaps faster moving regions. In
order to sample higher regions in giant stars, we must use other, more opaque,
lines. One good diagnostic of these regions is the O I UV 2 multiplet, three
lines near 1304Å which are also self-reversed, but even stronger than the
strongest of the observed Fe II lines (the 2756Å line).
Figure 3 compares the observed profiles of these lines to the Fe II
2756Å and 2737Å lines in the K-giant
Tau. In this star, the
use of the O I lines allows us to follow the wind up to about 25
.
Figure: The acceleration
of the wind in Tau as seen in the increasing blue-shift of the
Fe II and O I self-reversals with increasing line strength.
The difference in hybrid vs. non-coronal stellar atmospheres is
well-illustrated by the differences in the spectra of Dra and
Tau around 1550Å (see Figure 4).
Figure: The region near C IV (UV 1) in a non-coronal and a hybrid giant.
We have obtained two deep exposures of this
region for the purpose of measuring the amount of flux (or better
upper limits) arising from hot (transition region) material in these stars.
The non-coronal stars (K2 and later on the giant branch) like Tau
typically show evidence for slow, massive winds, but little or no evidence of
transition region or coronal material, while the hybrid-chromosphere stars,
although later than the dividing-line spectral type of K2, look more like the
earlier-type coronal giants with evidence of hot material and faster winds. The
GHRS data we obtained were surprising in that we detected a C IV surface flux
from the non-coronal giant comparable in strength to that in the hybrid star,
suggesting a non-trivial amount of transition-region material in the
non-coronal star. However, the higher density, cooler temperatures of the
non-coronal atmosphere is spectacularly evident in the spectrum of
Tau
in the form of a myriad of narrow fluorescent emission lines from Fe II and
H
, and recombination lines of Ca II. These lines are weak or absent in the
hybrid star, but dominate the spectrum of the non-coronal star (see McMurray et al. 1996).
The absorptions superposed on the Mg II h & k emission profiles are, like the
Fe II lines, useful probes of the stellar wind, but they also frequently
contain absorptions originating in circumstellar shells and the interstellar
medium. Figure 5-A compares the profiles of the Mg II k-line in two
K5 III stars, the normal giant Tau and the hybrid-chromosphere star
Dra.
Figure: Mg II profiles in Tau,
Dra,
Ori,
Cru, and TX Psc.
The profile from the latter star differs from the former in three ways: 1) it
has a strong ISM absorption (marked ``ISM'' on the plot) to red of line center
(in Tau the ISM absorption is masked by the chromospheric
self-reversal), 2) the wind absorption extends to much higher velocities (up
to
70
vs.
30
), and 3) it lacks the sharp, weak
absorption at zero velocity (relative to the star) seen in
Tau (marked
``S'' on the plot). The visibility of the ISM absorption (item 1) is
controlled by chance, i.e., whether or not the stellar and ISM radial velocities
happen to be similar or not. The higher speed winds (item 2) are
characteristic in general of hybrid stars. The feature noted in item 3
however, has, to date, only been seen in
Tau and we believe it is
formed by a shock formed as the star passes through the interstellar medium and
not a feature intrinsic to this or other non-coronal giants.
Figure 5-B&C compare the k-line profiles in three stars, the M-giant
Cru, the M-supergiant
Ori, and the carbon star TX Psc. The
carbon star has a profile intermediate in width between the M giant and
supergiant (indicating an intermediate luminosity), although the superposed
circumstellar absorptions resemble more closely those in the supergiant. The
h-line in the carbon star is even more mutilated by overlying circumstellar
absorptions than the k-line and by features which do not appear at all in the
M-supergiant (Figure 5-D). The circumstellar absorptions
identified as Mn I UV 1 and Fe I UV 3 in
Ori show outflow velocities
consistent with previous measures of CS shell velocities (Bernat,
1977).
We are gratefully acknowledge our collaborations with H. Johnson et al. which
have produced the excellent observational data on the carbon stars and the
collaborative program with Alex Brown which obtained the data on Dra.
We also acknowledge helpful conversations with Phil Judge, Graham Harper, and
Carole Jordan.
Bernat, A. P. 1977, ApJ, 213, 756
Carpenter, K. G. 1996, in Ninth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, eds. R. Pallavicini & A. K. Dupree, ASP Conference Series, in press
Carpenter, K. G., Robinson, R. D., & Judge, P. G. 1994 BAAS, 26, 1380
Ensman, L. & Johnson, H. R. 1995, BAAS, 27, 839.
Johnson, H. R. et al. 1995, ApJ, 443, 281
McMurray, A. et al. 1996, in Ninth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, eds. R. Pallavicini & A. K. Dupree, ASP Conference Series, in press