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OCR for page 119
119
role of magnetic-field geometry and the mutual exclusivity
between wind flux and bright x-ray emission regions may
provide important insights into the physics of stellar
winds. Furthermore, the heating of the solar corona is
clearly magnetic in character.
Solar Nonradial Pulsations and Seismology
The unexpected discovery that the solar atmosphere
has radial wave motions with a 5-min period led to major
theoretical studies of the generation, propagation, and
damping of acoustic, gravity, and magnetic waves in stel-
lar atmospheres. Theory indicates that the oscillations
observed at the solar surface are manifestations of stand-
ing, nonracial p-mode oscillations of the outer convection
zone and predicts a well-defined observable relation
between the temporal and spatial power spectra of the
5-min oscillations. The observational confirmation of
the existence of such a relation is another triumph for
theoretical research in solar plasma dynamics and rivals
in importance the earlier confirmations of predictions of
sunspot magnetic fields and the solar wind.
m e 5-min solar oscillations are now being used to
probe solar interior structure in a manner analogous to
seismological probing of the core structure of the Earth.
These studies have already resulted in improved models of
the solar convection zone and in a measure of the rotation
of the upper convection zone. Longer time sequences of
precise solar surface velocities at the 5-min periods, as
well as at longer periods (160-min periods have been
reported), will extend this technique for "viewing" the
astrophysical processes in the solar interior to regions
closer to the solar core. Together with the solar neu-
trino observations, these seismological studies are begin-
ning to define the conditions of the interior of the Sun
and to provide truly unique insight into stellar structure
and evolution. The proposed "Star Probe n mission to the
Sun would provide sensitive new measurements of the solar
gravitational field and add profoundly to our knowledge
of the solar interior, with important additional implica-
tions for studies of stellar interiors generally.
III . SCIENCE OPPORTUNITIES FOR THE 1980' s
A. Introduction
The 1970's have been exciting years for astronomy.
Increasing instrumental sensitivity and resolution over a
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120
wide range of wavelengths has led to the discovery of
amazing objects, mechanisms, and interrelations.
During the 1980's we will build on and extend the
programs that have proved to be so fruitful during the
1970's. The Very Large Array (VLA) radio telescope, con-
structed in stages over the past decade, is now becoming
fully operational; two new 4-m optical telescopes have
gone into operation at Kitt Peak National Observatory and
Cerro Tololo Inter-American Observatory; and Space Tele-
scope (ST), together with several highly sensitive IR
space observatories, will begin to return data by the
middle of the present decade. These present and planned
facilities will require increasing levels of ground-based
optical and infrared support work if their scientific
return is to be maximized in the years ahead.
A balanced program for astronomical research in the
coming decade will thus require at least one new
optical/IA telescope larger than any yet constructed,
together with a network of smaller (yet still substan-
tial) telescopes both at the National Astronomy Centers
and at university observatories. Such a large telescope--
which was recommended in each of the two earlier decade
reviews of astronomy published by the National Academy of
Sciences--will be used to pursue frontier problems requir-
ing difficult observations of faint objects. The develop-
ment and testing of instrumentation, on the other hand,
is more advantageously carried out on smaller telescopes
in the 2.5-5-m class, which offer ample opportunities for
experimentation and modification with relatively short
lead times; many of these will be university-based facil-
ities. Equipped with modern instrumentation and detec-
tors, these more modest telescopes will be able to attack
an exciting array of research problems during the 1980's,
as well as provide important supporting observational data
for larger, forefront instruments. The new capabilities
to be presented by ST, the forthcoming series of IR space
observatories, and the 15-m-class optical/IA telescope
proposed here will be extraordinary; however, demands for
the precious observing time afforded by these powerful
facilities will be correspondingly great. Therefore,
observational programs requiring large commitments of
telescope time will, for the foreseeable future, continue
to demand the capabilities provided by substantial numbers
of ground-based telescopes in the 2.5-5-m class.
The present difficulty of access to optical/IA tele-
scopes of aperture large enough to address frontier prob-
lems in astronomy has reached crisis proportions. If
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121
healthy advances in astronomical science are to be sus-
tained in the years ahead, we must respond positively to
the challenges that confront us now. We now have the
opportunity to build, using only a very small fraction of
our national wealth, those facilities that are needed to
continue to expand the golden age of astronomical discov-
ery in which we live--an age unmatched in richness and
vitality by any other since the time of Galileo, some
350 years ago.
B. Scientific Programs
1. Galactic Astronomy
Galactic Structure
We know that we live in a spiral galaxy, although its
dimensions and detailed morphology remain a mystery. We
do not know how far our Sun is from the center, nor do we
know the rotational velocity about the center with an
accuracy sufficient to determine the Galactic distance
scale to within 20 percent. During the coming decade,
the variety of approaches available for the detailed
study of our Galaxy should produce a more coherent pic-
ture of the stellar system in which we live. Utilizing
observations from ST and the recommended 15-m facility,
dynamical studies of faint halo stars, distant globular
clusters, and outlying satellite galaxies will delineate
the extent and the mass of the Galactic system; spectro-
scopic studies will determine chemical evolution as a
function of age and of position; and radio, millimeter,
and submillimeter observations of molecular clouds will
identify regions of star formation and help us to learn
their histories and dynamics. Sophisticated theoretical
models will probe the stability of disk systems. From
these studies, astronomers should learn what effects warps
in the outer disk have on stability. The Magellanic
Stream--that extensive band of neutral hydrogen reaching
from our Galaxy to its nearest satellite neighbors--may
be understood in this context. From x-ray observations
of nearby galactic nuclei, we will study events relevant
to the energetics of our own Galaxy's nucleus. Our near-
est spiral neighbor, M31, now has 17 x-ray sources iden-
tified in its nucleus; their nature is not yet understood.
We should ultimately piece together a detailed picture of
our own Galaxy from future studies, both internal and
external.
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122
Dynamical studies of external galaxies have led to the
conclusion that disks of spiral galaxies may be stabilized
by the presence of a low-luminosity halo. Only for our
own Galaxy can we currently hope to observe both stars
and globular clusters in the halo and to determine its
composition. Broadband photographic photometry down to
magnitude 24.5 would isolate halo dwarfs and identify
them to distances of 60 kpc, as well as detect giants
throughout the total extent of the halo. Crude but still
useful abundance indicators could be obtained out to
25 kpc, together with radial velocities accurate to about
60 km/sec. For dwarfs, giants, and globular clusters, a
detailed map of the halo, complete with distances, compo-
sition, abundances, and space motions will be forthcoming
and will be the ultimate test of models of halo formation
and the possible presence of nonluminous matter in the
outer regions of our Galaxy.
m e composition of the inner spheroidal bulge; star-by-
star studies of the nuclear bulge; and details of the
physics, chemistry, and dynamics of interstellar molecules
in the Galactic nucleus will also emerge from observations
in the optical, W. IR, and radio spectral regions. A
rough rotation curve of the nucleus has recently been de-
termined from the N II line at 12 m. Continued studies
of the composition and dynamics of the Galactic center
from a 15-m ground-based telescope will produce exciting
results and perhaps even tell us if there is a black hole
at the center of our Galaxy.
b. Star Formation
Star formation is a process on which almost all
aspects of Galactic evolution hinge. Present knowledge
only scratches the surface of problems such as the
processes leading to star formation, the initial mass
functions, and how and why the rate of star formation
varies with time and location in the Galaxy. An inter-
play between observation and theory is needed in order to
understand the star-formation process; the processes by
which planets form; and the structure, chemical evolution,
and stellar populations in our own and other galaxies.
Infrared-radiation processes dominate the evolution of
interstellar gas and dust from an equilibrium temperature
of a few kelvins in molecular clouds to stellar surface
temperatures of several thousand kelvins. Infrared obser-
vations are thus vital to the study of the energy balance
and physical conditions that occur during the transition
from diffuse gas to nuclear-burning star. The structure
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123
and sizes of protostars and the dense dust clouds in
which they are immersed require high-resolution observa-
tions at all IR wavelengths, especially beyond 30 Am.
High spectral and high spatial resolution will enable us
to determine composition, density, temperature, and mass
of both the dust and gas within the molecular clouds and
in the immediate vicinity of the protostar. With a spec-
tral resolution of 105 it will be possible to measure
the radial velocity of individual components within the
system; measurements of polarization in the IR region may
give information on magnetic fields in the clouds.
Recent work has shown that there is an interplay
between the dynamics of spiral structure in a galaxy and
the processes of star formation within the disk. Star
formation occurs over large regions of space, possibly
because the suspected triggers of star formation (spiral
density waves, shock fronts from supernovas or H II
regions) are large-scale phenomena. While observations
on small spatial scales are important in learning the
details of the processes involved, large-scale radio
observations, together with the W studies of the inter-
stellar medium discussed below, are also important for an
understanding of these processes
c. Interstellar Medium
Our picture of interstellar matter has undergone sub-
stantial changes within the past decade. It now appears
that the two major components are a hot intercloud medium
and the dense molecular clouds discussed above. Observa-
tions of diffuse soft x rays made with rockets and with
HEAD-1 and observations of W absorption lines made with
Copernicus demonstrate that a substantial fraction of the
local interstellar medium is filled with hot (T = 10V K),
low-density gas, whose scale height is so great that it
merges with a Galactic corona and wind.
During the 1980's, we will study the degree of homo-
geneity of the interstellar medium with respect to tem-
perature, pressure, chemical composition, and cloud size.
Much of this information will come from high-resolution
spectroscopy with ST. The one-dimensional structure of
the clouds will be fully revealed; specific clouds will
be assigned distances, masses, and temperatures. The mass
spectrum of clouds as a function of velocity, known for
the first time, will permit detailed studies of cloud
formation and destruction. Such a program combines
ground-based work, ST observations, far- W spectroscopy,
and IR studies.
.
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124
Heating mechanisms for clouds should be quantitatively
defined by this decade. Detection of weak molecules with
ST will permit a complete grid of chemical reactions to
be built up, so chemical heat input will be defined.
X-ray heating will be determined from studies of ioniza-
tion in H II regions. Definition of the velocity struc-
ture in each cloud will allow the mechanical energy
injected by shock waves to be evaluated.
The thickness of halo gas at different temperatures
will be determined, as will the rotation properties of
the gas at high Galactic latitudes. The apparent rotation
is tied to the magnetic-field strength and the mass inflow
and outflow rates at high latitudes. The degree of
"patchiness" of the halo gas will be determined. All of
this information is relevant to the absorption lines seen
in quasar spectra, because the likelihood of seeing ab-
sorption lines through halos of foreground galaxies
depends on these same properties in galaxies along the
lines of sight to the quasars.
~ _
d. Emission Nebulas
Ultraviolet spectroscopy is highly valuable for
studies of supernova remnants (SNR's) and planetary
nebulas. It not only allows sensitive measurements of
abundances of certain elements, such as carbon, which are
difficult to study from the ground, but it also provides
sensitive determinations of electron temperature by
virtue of accessibility of strong, high-excitation
emission lines such as those of C IV, N V, and O VI.
Sensitivity below 1150 ~ is required to observe
important resonance lines such as those of O VI, S IV,
and S VI. At present, only the very brightest H II
regions have been observed with IUE, in the wavelength
range above 1900 A. Further studies of H II regions
and SNR's will require an optimized nebular spectrograph,
if possible sensitive down to 950 A. ST and the W
spectrographic telescope would be very valuable for
detailed studies of planetary nebulas, since these
objects are generally of higher surface brightness than
are H II regions and SNR's. Because planetary nebulas
are of generally small angular size, high spatial resolu-
tion in imaging spectroscopy is necessary for their study.
Of particular importance are determinations of small-scale
temperature and composition variations, with angular
resolution considerably better than achievable from the
ground. It is also important to confirm (or otherwise)
current indications of composition variations with posi-
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125
Lion of the planetary nebulas and H II regions in the
Galaxy. To observe a sufficiently large number of
objects, at sufficiently large distances to establish
abundance gradients, will require both very high sen-
sitivity and high angular resolution.
e. Outer Atmospheres of Stars
At the end of the 1970's, a revolution began in our
appreciation of the range of phenomena that occur in the
outer atmospheres of stars. The realizations that almost
all stars have hot coronas, and that at least all luminous
stars have massive winds, is changing our perspective on
stellar evolution and the interaction between stars and
the interstellar medium. Several important problems
stand out as ready for concerted attack in the coming
decade:
.
We need reliable measures of plasma character-
istics and flow velocities and of the variability of
these properties. The W spectrographs on IUE and ST
will provide valuable information on species formed in
chromospheres and winds at temperatures up to 2 X 10S K,
but to probe the hotter regions of coronas and winds
requires spectroscopic observations of ions formed at
higher temperatures. The proposed far- W spectrograph in
space with sensitivity down to at least 300 A is needed
for this task.
· We need to understand the basic mechanisms respon-
sible for the heating of stellar chromospheres and coronas
and for accelerating stellar winds. Are the outer atmo-
spheres of most stars heated by wave processes, the dissi-
pation of magnetic fields of magnetohydrodynamic waves,
or radiative instabilities? Also, why do the cool lumi-
nous stars appear not to have hot coronas? Winds may be
accelerated by thermal pressure gradients associated with
a hot corona, radiation pressure on spectral lines and
dust, or momentum deposition by waves of various types.
It is important to learn which processes dominate in dif-
ferent regions of the HR diagram and exactly how these
processes work. m e far- W spectrograph in space will
provide essential insight into these questions, but
theoretical studies and, especially, detailed studies of
the nearest star, the Sun, by SOT and the Solar Coronal
Explorer are also essential for progress.
· Recent studies of the Sun point conclusively to
the dominant ways in which magnetic fields control the
heating of the chromosphere and corona and the geometry
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126
and energy balance of the corona and wind. The magnetic
field in turn is stochastic but variable over the well-
defined cycle and presumably regenerated by dynamo
processes. We need to understand the various aspects of
the solar and stellar activity cycles. Stellar observa-
tions are critical in this regard because different
values of the important variables, such as rotational
velocity, depth of convection zone, and gravity, need to
Since very high spectral resolution, a high
be sampled. _
signal-to-noise ratio, and stability over decade-long
observing programs are needed, this program requires
a high-resolution spectrograph/telescope dedicated to
solar-stellar studies.
2. Extragalactic Astronomy
a. Galaxies and Clusters of Galaxies
-
The evolution of galaxies, the evolution of clusters
of galaxies, and the interaction between these two areas
of study offer outstanding opportunities for discovery
and understanding during the 1980's. Individual come
portents of external galaxies will be observable with
unprecedented detail. HR diagrams of the old stellar
population, colors and luminosity functions of globular
clusters, and IR spectroscopy of the interstellar medium
in external galaxies will result. These in turn will
give ages and metallicity histories of the galaxies.
Within our own Galaxy, observations of the composition
and dynamics of the nuclear bulge and of stars in the
halo, plus abundances among main-sequence stars in
globular and open clusters, will reveal the history of
nucleosynthetic enrichment in the Galaxy. Galactic
populations among the Hubble sequence are now generally
understood in terms of systematic variations in the rates
of star formation. If we are to understand Hubble types,
we must discover what factors control the gas content and
star-formation rates both now and in the past.
Strong observational evidence from the past decade
indicates that individual galaxies are frequently im-
mersed within optically undetected, massive halos. Major
photometric programs aimed at detecting faint halo stars
must be pursued, as well as new dynamical tests for unseen
matter. Important facilities here include the 21-cm
capability of VLA, as well as improved optical techniques
involving multiplexing red-shift detectors, additional
3- to 5-m-class telescopes, larger optical telescopes,
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127
and high-efficiency two-dimensional detectors. Adequate
theoretical support is also vital if we are to understand
the origin of these halos, which may even precede the
formation of visible galaxies.
m e mass we see in individual galaxies is only 10 per-
cent of the total mass contained in groups and clusters
of galaxies. Unbiased samples of accurate galaxy red
snorts out to a red split of about O.1 are necessary to
measure both the seen and unseen mass in clusters and
groups. Generous time allocations on telescopes of
moderate size are required.
It is likely that in the 1980's, the study of clusters
of galaxies will accelerate, spurred on by the wealth of
information acquired from x-ray observations. From clus-
ters in a variety of evolutionary states, we must study
the morphology of galaxies within the cluster, how galaxy
evolution (i.e., mergers, tidal stripping, gas stripping,
velocity dispersion) is influenced by the cluster evolu-
tionary state, and how clustering itself has evolved since
the epoch of formation. On still larger spatial scales,
it is important to verify the recent tentative detection
of x-ray emission from superclusters, since the implied
mass involved would dominate the gravitational potential
of the supercluster. X-ray detectors of higher sensitiv-
ity and resolution are required to map these extremely
diffuse sources of low surface brightness. Optical and
IR studies of the morphology, dynamics, and structure of
superclusters is needed to acquire the data necessary for
an understanding of our own corner of the Universe.
b. Quasars
The fundamental problems for the 1980's are the struc-
ture of the central energy source in quasars and active
galactic nuclei and the physical processes involved in
the energy release. The basic picture of the accretion
of matter onto a black hole needs to be confirmed, and
the physical process actually producing the emitted energy
remains to be understood.
Do the differences among quasi-stellar objects (QSO's),
BL Lac objects, Seyfert galaxies, and N galaxies observed
in the radio, optical, IR, and x-ray regions arise from
structural differences? A clear delineation of galaxy
types and environments in which nuclear activity occurs
will tell us much about the duration of nuclear activity
and the importance of cluster membership to galaxy activ-
ity. Ultimately, spatial resolution in the visible and
the W regions approaching that now possible with ground-
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128
based very-long-baseline interferometry (VLBI) would allow
material surround-
~ng a central object to be studied even at large red
shifts and would enable us to understand the relationship
the structure and velocity field of the
between the thermal and nonthermal components. Because
ST will clearly resolve the galactic component of nearby
quasars, it will be the single most important instrument
for studying these associated galaxies. Both ST and
large ground-based optical telescopes can contribute to
investigations of cluster membership.
Direct observational evidence on the nature of the
central energy source at high spatial resolution can
currently be obtained only with VLBI observations.
Because of its much shorter working wavelength, optical
interferometry offers significant potential gains over
radio techniques for receivers operating on identical
baselines. Ground-based optical telescopes do not cur-
rently exploit the potential for resolving power because
of blurring by atmospheric seeing.
.
. . . .
Novel techniques
Devised to overcome ants degradation are currently limited
to fairly bright objects. However, it is clear that much
of the information contained in the optical and IR spec-
tral regions is currently being wasted. The 1980's may
be an opportune time to consider whether ground-based or
space-based interferometers should be pursued first and
to undertake serious design studies and perhaps even
prototype models for optical interferometers.
Deep surveys for QSO's, Seyfert galaxies, and radio
galaxies are likely to remain our only probe of the clus-
tering properties of the Universe in the red-shift range
z = 1.5 to 4. We need to know how the clustering of QSO's
compares with the weak clumping among distant radio
sources. Optical objective-prism surveys from space and
from the ground will remain a powerful technique for these
surveys. We also need to understand the relation between
active galactic nuclei and apparently normal nuclei. Are
dormant quasars lurking within many local galaxies, inac-
tive at present because there is no current fuel supply?
ST will be able to place much better limits on the mass
and space densities in the very centers of nearby galactic
nuclei and will allow us to study the physics of the sur-
rounding, ejected, or intervening material. The direct-
imaging capabilities of ST will be adequate as currently
planned. However, to measure rotation curves and vel-
ocity dispersions close to the cores of nearby galactic
nuclei, a two-dimensional detector for spectroscopy will
be required. Such an instrument must exploit the high
angular (spatial) resolution along the slit.
How~v.?r. to m - A~:~,r" rc~tati~n Rev - i; and v`?1—
OCR for page 129
129
Many of the absorption lines in quasar spectra appear
to arise from intervening gas clouds along the line of
sight. Analysis of the distribution of these lines may
tell us about the evolution of clustering in the Universe
and how and when gas accreted into galaxies. The clouds
are so numerous that, in most quasars, the individual
components are unresolved. Further detailed study is
hampered by inadequate spectral resolution. Higher reso-
lution requires larger ground-based telescopes. For
spectral coverage in the W region, ST could provide the
necessary wavelength range, if a high-resolution echelle
spectrograph with ample spectral range were to become
available.
c. Cosmology
Because the cosmic microwave background radiation is
by far the most accessible probe of the physics of the
early Universe, it is vital to improve the accuracy of
the measurements of the spectrum and to place more strin-
gent limits on its large- and small-scale anisotropy. The
Cosmic Background Explorer (COBE) satellite will provide
excellent measurements of the spectrum and of the large
scale anisotropy (angular scales of about 5 deg or more).
Better limits on the fine-scale anisotropy, achievable
with a Large Deployable Reflector in space, are probably
the single most important measurements yet to be made on
the microwave background. Such observations are needed
to establish with certainty that the background is not
composed of point sources. They may also detect inhomo-
geneities at decoupling, which foreshadowed the eventual
formation of galaxies and clusters of galaxies. During
the 1980's we will follow up on recent, tantalizing hints
of galaxy evolution at large red shifts. Many faint gal-
axies at red shifts beyond z = 0.6 surprisingly appear to
be blue, suggesting widespread supernova events in young
galaxies. Details of the evolution of isolated galaxies
and the effects of environment on Hubble type, gas con-
tent, and star-formation rates should emerge. ST will
provide high-resolution photographs indispensable to the
morphological study of distant galaxies. Deep galaxy
counts and red-shift surveys are central to the measure-
ment of the luminosity function, rates of evolution, and
clustering properties of high-red-shift galaxies. Broad-
band energy distributions will be sensitive to the aging
of stellar populations. In addition to ST, larger ground-
based optical telescopes, more efficient ground-based
telescopes, multiplexing red-shift detectors, improved IR
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130
sensitivity out to 5 Em, and high-efficiency two-
dimensional detectors in the range 0.15-5 Em are impor-
tant for the program.
The determination of the Hubble constant to an accuracy
of better than 10 percent will require extensive observa-
tions of both old and new standard candles such as RR
Lyraes, Cepheids, and novae--all of which can be studied
in nearby galaxies with ST. Other, more novel techniques
may also be exploited, among which are the supernovae and
the diminution of the microwave background in the direc-
tion of clusters of galaxies. Improvements in millimeter
receivers are required, and perhaps ultimately a large
(10-m aperture or larger) millimeter antenna in Earth
orbit as well.
3. Astrometry
a. Stellar Census
The major program for the coming decade will be to
continue the census of the solar neighborhood, where
"neighborhood" now means within 500 parsecs rather than
50 parsecs. Parallaxes will establish the distance scale
and permit the determination of luminosities of both ordi-
nary stars and, for the first time, some fairly exotic
stars. The much larger number of proper motions also
obtained will improve the understanding of local Galactic
kinematics. m e greater number of binaries coming under
study will permit the determination of many more stellar
masses. Several clusters can be investigated for dynam-
ics and astrophysical properties. More and smaller
unseen companions will be located. The very nearest
stars will have their properties determined to high
· .
prec IS Ion.
To augment existing astrometric programs and provide
more uniform coverage, an astrometric instrument (2-m
class) will be required in the southern hemisphere, and
improved detectors (focal-plane detectors and interfero-
meters, for example) and high-grade measuring machines
(at least three) will be required at existing observa-
tories. By the end of the decade, 1-m-class telescopes
designed to achieve the very highest accuracy possible
will be required in both hemispheres. In addition, a
dedicated astrometric satellite should be under
development.
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131
b. Solar-System Model
The model of the solar system will be improved. This
will necessitate finding as many members as possible,
identifying the appropriate gravitational theories and
models of individual bodies, and determining the most
accurate values of the numerical parameters for use in
these theories. The establishment of new occultation and
laser-ranging networks will contribute significantly to
this effort.
c. Inertial Reference System
A more nearly truly inertial reference system will be
established. This will include the creation of a new fun-
damental catalog, of one or more high-precision secondary
astrographic catalogs, of tertiary catalogs of extragalac-
tic objects (in some cases tied to the radio reference
system), and of at least one survey of inertial proper
motions. Besides the intrinsic galactic-structure, navi-
gational, and geodetic applications, this reference sys-
tem is necessary as part of the other two programs.
Development of instruments with improved capability to
measure large angles and a better understanding of the
effects of the atmosphere are particularly needed.
4. Solar Physics
Solar physics is at present evolving from an intensive
study of our Sun, the closest star, into a more broadly
based inquiry into the large-scale behavior of ionized
gases in gravitational and magnetic fields. Active phe-
nomena similar to those seen on the Sun occur in many
stars and in galaxies as well. The Sun provides the only
accessible laboratory in which the processes governing
these many effects can be discerned in sufficient detail
to permit real progress in our understanding of them.
Other classes of astronomical objects complement the solar
laboratory by exhibiting more extreme activity over a
wider variety of conditions. As seen from this broadened
perspective, the major scientific problem areas in "solar
physics" that can be attacked profitably in the 1980's
are the following:
(a) The fundamental properties of the solar core, in
particular its rotation rate, chemical composition, tem-
perature distribution, and details of the processes of
nuclear energy generation.
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132
Direct tests of models of the solar interior can be
made by three techniques: the observations of the energy
spectrum and intensity of the solar neutrino flux; the
observation of the mass distribution of the Sun by the
measurement of perturbations on the orbit of a close
gravitational probe (such as the proposed Star Probe
Mission); and the observation of the global oscillations
of the Sun.
(b) The hydrodynamic structure of the solar convec-
tion zone, with particular emphasis on the character of
the solar dynamo, the nature of large-scale circulation,
and the implications of very-long-period global oscilla-
tions.
Convection is an essential factor in the evolution of
many stars because it ultimately controls the mixing of
stellar material and thereby influences the types of
thermonuclear processes occurring in the interior and the
chemical composition at the surface. Moreover, convection
drives the global circulation of cool stars and indi-
rectly, through the dynamo mechanism, generates their
magnetic activity cycles. The convection zone is thus a
basic source of nonthermal energy that may heat stellar
chromospheres and coronas and provide energy to accel-
erate stellar winds. Only the Sun can be observed in
sufficient detail to guide a theory of convection based
on first principles.
During the past decade, the rotation of the Sun has
been studied extensively from both ground and space obser-
vatories. Among the many interesting results still to be
explained are a difference in rotation rate of 5 percent
between the magnetic and nonmagnetic areas in the photo-
sphere, details of the global solar rotation, details of
magnetic and nonmagnetic areas, depth effects, latitude
effects, and long-term variations. Measurements of global
circulation patterns and of nonracial pulsations provide
important clues to the structure of the solar convection
zone.
(c) The processes involving small-scale velocity and
magnetic fields and the wave modes that determine the
thermodynamic structure and dynamics of the solar photo-
sphere, chromosphere, and corona and their implications
for stellar atmospheres.
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133
A thorough understanding of chromospheric heating is
necessary both for its own sake and to understand stellar
coronas and stellar winds. Acoustic waves generated by
photospheric turbulence provide sufficient energy to pro-
duce the initial chromospheric temperature rise, but they
probably fail to heat the upper chromosphere and corona.
Research in the 1980's should extend current work and,
most importantly, initiate fundamental new treaments of
the problem, such as detailed studies of the effects of
magnetic fields on the dissipation of all possible wave
modes.
Recent measurements by OS0-8 indicate that acoustic
waves generated in the solar convection zone carry insuf-
ficient energy to heat the corona. This discovery, as
well as the observation of a strong correlation between
atmospheric heating and the strength and configuration of
the coronal magnetic field, have stimulated theoretical
investigations into other heating mechanisms.
(d) The physical processes that drive the solar activ-
ity cycle, the variations on various time scales in the
solar radiation output, and the effect of this variabil-
ity on the Earth's upper atmosphere; the relation of this
activity to the variability of other stars.
The solar activity cycle is basically a magnetic cycle,
produced by the arrival at the surface of the Sun of mag-
netic fields generated in the convection zone below. The
complex surface effects produced by the field involve sun-
spots, plages, quiescent and active prominences, coronal
arches and loops, flares, and enhanced emission of EUV
radiation, x rays, and fast particles, as well as strong
disturbances and turbulence in the solar wind. Weir
study is of interest for the astrophysics of magnetic
field-plasma interaction on stellar scales and also is of
practical value because of the effects of long-term solar
changes on the terrestrial environment. Evidence for
solar effects on our climate is becoming increasingly
persuasive.
(e) The basic plasma-physics processes responsible
for metastable energy storage, magnetic reconnection,
particle acceleration and energy deposition in solar
flares and related nonthermal phenomena; the implications
for high-energy processes in the Universe.
OCR for page 134
134
The most difficult phenomenon of plasma and high-energy
astrophysics to understand is probably the transformation
of slow, large-scale motion and magnetic stress into im-
pulsive, energetic, small-scale events. me solar flare
represents the -best-observed model of this mechanism. A
concentrated theoretical program, in which magnetic recon-
nection and particle acceleration are major topics, coup-
led with the development of better methods for measuring
the accelerating electric field directly as a function of
position on the solar disk, is needed for the 1980's.
These objectives can be met with the proper complement of
instruments on an Advanced Solar Observatory.
Flares on dMe flare stars have temperatures, x-ray
light curves, and radio-emission properties similar to
those of solar flares but have x-ray luminosities and
coronal emission measures up to 10,000 times larger than
is typical for large solar flares. During the next dec-
ade, simultaneous observing programs involving gamma-ray,
x-ray, W. optical, and radio observations are needed to
explore what additional types of stars flare and to deter-
mine whether the basic physical properties and flare mech-
anisms of the Sun are similar to those of other stellar
types.
(f) The structure and plasma dynamics of the solar
corona, including the processes involved in heating
various coronal structures and initiating the solar wind;
the origin of coronal transients; the three-dimensional
structure of the interplanetary medium and its implica-
tions for cosmic-ray modulation; and the implications for
stellar coronas and winds and other astrophysical flows e
Two outstanding discoveries of solar physics during
the past decade are (1) the determination that coronal
holes are the source of recurrent high-speed solar-wind
streams of solar-terrestrial importance and constitute a
major fraction of the solar wind and (2) that coronal
transients are frequent mass-ejection events with large
energies (1029-103~ ergs/sec~l) that are associated with
a restructuring of the outer corona. The resonance-line
and white-light chronographs, x-ray telescope, and mag-
netograph on the proposed Solar Coronal Explorer satellite
will provide the means of determining the plasma proper-
ties of these structures as well as directly measuring
the acceleration of the solar wind deep in the corona.
Another important problem for the next decade is the
determination of the three-dimensional structure of the
Representative terms from entire chapter:
star formation
