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6
Future Investigations of
Precursor Planetary Systems
The previous chapter focused on observational initiatives whose goals
are the detection and enumeration of evolved extrasolar planets. The
committee turns next to the question of enhancing current knowledge of
possible precursors of planetary systems and protoplanetaIy material—e.g.,
evolving molecular clouds, stellar nebulae, and accretion disks- through
the development of advanced laboratory and observational techniques. This
development may also be critical to the study of the physical environment
of candidate extrasolar-system planets when, and if, they are discovered.
The applicable techniques are unaging, analytic spectroscopy, and po
lamely, which will be discussed here under science objectives rather than
by technique, as in the foregoing chapter on planetary searches. The
reason is Hat clear distinctions based on technique do not exist for this
area images are taken though restrictive spectral or polarimetric filters,
whereas spectra are obtained with spatial sorting or scanning. The nature
of the instrumentation and subject area is such that planetary scientists will
cany out those programs in association with astrophysicists. Indeed, many
of the relevant projects proposed or under development spnug from the
astrophysics community.
In the following sections, three classes of study objects are discussed:
dust, protostars, and pre-main-sequence stars. The last two were identified
in Chapter 3 as the immediate precursors to evolved stellar systems, and it
is in their vicinity that we can expect to observe the early stages of planet
formation. The study of dust is singled out for special treatment because
dust is ubiquitous and plays a unique role as a tracer of many physical
57
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processes important in stellar evolution, as well as in the formation and
evolution of planetary systems.
DUST-REI^TED ASTROPHYSICAL STUDIES
Recent observations, especially in the infrared, show that dust is a
useful tracer of elements heavier than hydrogen and helium through the
processes of star formation, evolution, and death. Dust is thought to form
originally in the ejecta of evolved stars. Silicate grains have been found in
the interstellar medium in regions of star formation, in pre-main-sequence
stars around main-sequence stars, and in end stages of stellar evolution
Characteristics of astrophysical grains were described in detail in Chap-
ter 4 under Circumstellar Dust. Silicates are conspicuous to thermal in-
frared spectroscopy due to prominent 10- and 20-pm emission features.
Silicon carbide grains emit a characteristic foam spectral feature observed
around some stars. Carbon grains, responsible for Infrared continuum em~s-
sions, appear in all stages of stellar evolution as well as in the interstellar
medium.
Dust is an important solar system constituent. Dust grains, incor-
porated into comets, asteroids, and planets during the formation of the
solar system, represent material that was injected into the presolar nebula
by earlier, evolved stellar systems and processed to various extents. Dust
observed in the comae and tails of comets as it is expelled from the frozen
nucleus is the probable source of the zodiacal cloud.
Dust grains of interplanetary origin can be collected in the atmosphere
or in near-Earth space and studied in the laboratory to determine com-
position, mineralogy, and optical properties. COMPLEX has reviewed a
catalog of infrared spectra taken of individual, microscopic dust particles
that display a multiplicity of characteristic features relatable to their sep-
arateh,r determined intrinsic properties and is convinced of the efficacy of
techniques available for the establishment of ground truth. Dust, particu-
larly through its thermal and optical properties, which can be calibrated,
provides a useful diagnostic material that can probe astrophysical systems
for knowledge connected to the solar system.
Direct knowledge of the material present in our early solar system
obtained from dust-oriented studies could be compared with observations
of the dusty material thought to exist in young and evolved stellar systems.
A special emphasis should be placed on observations of young stellar
systems where planet formation is thought to be a likely process. The dusty
constituents of these systems should be careful studied to determine how
closely they resemble the material in our solar system.
Resolved observations of dust disks (or shells or clouds) can address
key questions regarding the connation and evolution of planetary systems.
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s9
Determination of locations of dust shell edges and their sharpness (e.g., at
resolutions of ~1 AU) will permit determination of how He dust is supplied,
such as through the erosion of comets, and whether clearing of the inner
edge by planets is a plausible model. For systems such as Vega, for example,
one can envision future surveys utilizing a cryogenic infrared telescope or a
large millimeter-submillimeter-wave dish in space to assess the frequency of
Vega-like dust shells, determine gas-dust compositions by spectroscopy, and
carry out direct and indirect searches for resonances and other phenomena
related to radial structure. Such work links the observation and laboratory
studies of dust with searches for planetary bodies discussed in the previous
chapter. Studies of the dust ejected into the interstellar medium by evolved
stellar systems such as supergiants, novae, and supernovae are required to
trace the injection of heavier elements into young stellar systems and to
determine the characteristics of the dust formed from the ejected material.
It Is important to establish whether or not dust grains in the vicinity of
the objects mentioned above are consistent with the hypothesis that such
grains, formed in the ejecta of evolved stars, find their way into regions
of star fonnation and young stellar systems where planets form. Obse~va-
tions, particularly in the infrared region of the spectrum, are needed to
provide detailed information on grain formation, composition, crystallo-
graphic structure, and the relative importance of various stellar sources of
interstellar dust. New observational data, combined with modeling and lab-
oratory condensation experiments, should provide a better understanding
of nucleation and grain growth. Currently there is considerable uncertainty
as to how nucleation and grain growth proceed in circumstellar environ-
ments even though these processes are actually observed in classical novae.
Signatures of carbon or silicate grains appear in nova spectra ~100 days fol-
lowing an outburst. It has been predicted that nucleation difficulties in the
pure gaseous outflow from all dust-form~ng stars could lead to high levels
of supersaturation and subsequent formation of amorphous particles, with
well-mned composition. This prediction needs to be carefully evaluated in
the context of some of the isotopic anomalies seers in meteontes.
LABORATORY STUDY OF INTERSTELLAR MATERIALS
Infrared spectra of interstellar grains are complex and difficult to in-
terpret unambiguously because there may be several constituents as well
as contn~utions to the shape of a given spectral feature from the lattice
structure of the solid matenal. Thus it is advisable to have laboratory
ground truth analysis of candidate grain samples based on the wide range
of in situ measurement techniques currently available. Such analyses will
provide a vital synergistic link between dust studied with purely astronom-
ical techniques and material that can be studied in me laboratory using
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61
low-loss EELS peak is due to plasmon effects and, in principle, is directly
related to ultraviolet spectral effects. Additional work along these lines
could also yield fresh insight into the 10-pm "silicate" feature, the SAC fea-
ture, the 3.4-pm (organic?) feature, diffuse bands, and various unidentified
infrared features.
An example of the potential of this field is the recent work on a single
interplanetary dust sample that, in addition to other features, showed an
absorption line that correlated with the 6.8-pm interstellar feature. This
interstellar feature has been postulated to be related to carbonaceous
matter, but in the dust grain, the feature was definitively shown to be
caused by carbonates. Carbonates were identified by electron microscopy
and, after dissolution of carbonates by acid treatment, the 6.~pm feature
was absent.
The search for presolar grains in the laboratory materials is limited
by the state of analytical technology for small samples. Grains detected by
astronomical techniques are generally in the 10-~3 to 10-~6-g range, while
materials analyzed in the laboratory are typically much larger. For example,
a standard high-precision isotopic analysis of meteoritic oxygen requires a
sample of over 10-3-g mass. Recently developed ion microprobe techniques
have provided isotopic analyses of carbon and hydrogen in grains in the
10-~2-g range. In addition to the ion probe, newer developing techniques
include accelerator mass spectromeny, multicollector mass spectromeDy,
synchrotron x-ray microprobe analysis, ultrasensitive organic analysis, and
high-resolution analytical electron microscopy. The sensitivity of techniques
for analysis of extraterrestrial materials has undergone orders-of-magnitude
improvement over the past mro decades, but further improvements will be
~ ~ , ~
. .
required to analyze presolar grams adequately. Such advances are needed
for work on current available extraterrestrial materials, and they will play
a critical role when the first samples of a comet nucleus are analyzed in
situ or by sample return
In parallel with these present and future investigations of extraterres-
trial materials themselves, it is extremely important to pursue laboratory
studies of the processes that are thought to act on natural extrasolar grains
during their life Cycles from condensation to destruction or accretion. Such
experiments can be conducted, using a wide variety of laboratory techniques
under appropriate physical conditions (for instance, under microgravity
conditions for studies of grain agglomeration), on simulant materials that
appear liked to represent the spectrally inferred chemical compositions,
physical states, and sizes of solid particles in stellar ejecta and in inter-
stellar and accretion-disk environments. One example, important for the
volatile inventory of planetary systems, is investigation of the condensation,
chemical-organic evolution, and erosion of icy-organic grain mantles and
their constituents in the last two of these environments.
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PROTOSTELLAR OBSERVATIONS
A primary objective of observational and theoretical studies of proto-
stellar and pre-main-sequence objects should be to piece together a unified
picture of the temporal evolution of the physical parameters of an evolving
stellar system from the onset of gravitational instability to nuclear burning
on the main sequence. Such an effort is statistical in nature; many repre-
sentative systems In various states of evolution (and, therefore, of different
ages) must be thoroughly observed. One result of such studies would be
the construction of an H-R diagram for pre-main-sequence objects.
Whereas molecular clouds and pre-main-sequence stars are common
astronomical objects, an example of the intermediate, protostellar phase
has yet to be definitely identified. An effective search for the elusive pro-
tostar is important. Candidates could be located by systematic imaging
surveys at long wavelengths. Dunng the protostellar collapse, cloud tem-
peratures as low as 10 to 50 K obtain, and such systems emit predominantly
in the far-infrared and submillimeter spectral regions. Systems operating
at these long wavelengths can and do find cold compact objects embedded
within molecular cloud cores. Initial observations, particularly by IRAS,
will be followed up using a variety of more advanced facilities that are
now being constructed or are planned. These include ground-based sub-
millimeter telescopes and interferometers, proposed airborne telescopes
like the Stratospheric Observatory for Infrared Astronomy (SOFIA), and
space-based facilities such as SIRTF and the large deployable reflector
(LDR).
Even for the nearest regions of star formation, the far-infrared and
submillimeter facilities contemplated here will only resolve details as small
as a few hundred to a few thousand astronomical units across. It will
therefore be necessary to supplement the unaging surveys by instruments
that might be expected to reveal the presence of protostellar candidates;
that is, with higher-spatial-resolution imaging studies that can be conducted
with very large ground-based telescopes operating in the near-infrared and
thermal-infrared spectral regions. For example, the once-planned National
New Technology Telescope (NNTf), with its 21-m baseline for unaging,
could have resolved condensations the size of the solar system in the 10-
to 20-pm spectral region for the closest regions of star formation. If
large space dishes and interferometers could be optimized for this spectral
region, superresolution techniques combined with interferomet~y could
enable resolutions considerably better than this.
Such infrared studies would enable us to examine in detail the spatial
morphology of the collapsing systems. It is especially important to be able
to observe evolving circumstellar disks in various stages of fragmentation
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~ order to identify sites where substellar- and planetaty-mass companions
may form.
Polarimetric imaging studies of star-forming cores should be useful for
identifying the location of central heat sources and candidate protostars,
and for mapping the spatial d~stn~ution of the dust ~ rejection nebulae.
Spectroscopic observations of atomic and molecular lines can be used
to trace the gaseous constituents and determine the spatial dependence
of the physical conditions in the collapsing cloud. Dust constituents can
be traced in the infrared by observing prominent solid-state emission-
absorption features. It is important to recognize that imaging must be
conducted at high polarimetric and spectroscopic resolutions to reveal the
dynamics of and physical conditions in collapsing systems.
Fundamental information on the processes of star formation is pro-
vided by measurements of molecular line emission (e.g., carbon-o~ygen,
oxygen-hydrogen, and hydrogen-carbon-nitrogen) at radio wavelengths.
These rotational lines are collisionally excited by hydrogen molecules and
therefore can be used to determine the density and pressures within molec-
ular clouds and the largest-scale (and thus lowest-densi~) circumstellar
disks. Doppler shift and line-width analyses provide information on ve-
locities, flows, turbulence, and chemical abundances. Such studies require
spectral resolution in the range A//~) = 103 to 106. At present, these tech-
niques alone provide direct observational support for the rotational motion
of disks. Some measurements may imply inflow within certain molecular
cloud cores; hence these techniques may eventually provide unambiguous
evidence for protostellar collapse and inflow. Radio measurements are
currently limited by both resolution and sensitivity, especially at millimeter
wavelengths where single-d~sh measurements are common
OBSERVATION OF PRE-MAIN-SEQUENCE STARS
In contrast to protostars, many pre-main-sequence stars have been
identified. They exist in regions of dense obscuration, a factor that favors
longer-wavelength studies because of lower light extinction. These objects
exhibit a range of identified or inferred phenomena- mass outflows, T Mauri
outbursts, accretion disks, Herbig-Haro objects, and expanding shocked
shells and they are accessible to the full suite of imaging, polarimetnc,
and spectroscopic studies. It is particularly important to address the key
questions related to planetary evolution in studies of such objects.
Studies of the immediate post-protostellar collapse phase, when the
central regions of the collapsing system are heating up, can be fruitfully
conducted at the highest spatial resolutions using the near-infrared and
even optical techniques on the largest telescopes. Spatial scales much
smaller than 0.1 arcsec, which represents a 15-AU resolution in the closest
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64
regions of star formation, could be probed using interferometers in space.
General imaging and photometric studies combined with polarimetry can
reveal the spatial morphologies of collapsed disk systems and the outflows
associated with pre-main-sequence stages of stellar evolution. Observations
of the smallest structures in evolving pre-main-sequence stars will require
long-baseline interferometry, perhaps from orbiting telescopes where the
deleterious effects of atmospheric "seeing" can be avoided and where
orbital precession can be used to provide complete spatial coverage.
Using the techniques of infrared spectroscopy, imaging spectroscopy,
and polarimetric studies, we can expect to obtain new structural and dy-
namical information about pre-main-sequence stars. At the highest spectral
resolution (~/~) = ~106t to 106), spectroscopic studies will provide infor-
mation about abundances and dynamical motions. The spatial resolutions
that would be available with LDR, and with ground-based facilities as
competent as the once-planned NNTI, would enable studies of turbulence
during earlier stages of stellar evolution. Observations of the differential
rotation of nebulae can be used to address the angular momentum transfer
processes.
Spectroscopic imaging studies of the atomic and molecular lines and
dust emission features can reveal important information about the ratio of
gas to dust and the chemical abundances of these species as a function
of position in a nebular system. There is ample evidence from studies of
our own solar system that spatial chemical fractionation by condensation
occurred during the nebular phase. An especial important line of inves-
tigation will be to determine the chemical evolution of nebular systems as
a function of time. Spectroscopic measurements in the near-infrared, far-
infrared, and microwave spectral regions can reveal how complex organic
molecules that are created in molecular cloud cores are incorporated into
protostellar condensation, and how these molecules evolve as the process of
stellar collapse and condensation progresses. Similar studies of the fate of
the elements themselves (such as the biogenically important carbon, nitro-
gen, and oxygen) can also be pursued. Spectral line studies will additionally
reveal the nature of the outflows in pre-main-sequence stars and can be
expected to shed light on how the outflows interact with the surrounding
molecular cloud and circumstellar environment.
The energetic phenomena associated with pre-main-sequence stars
involve large degrees of ionization and powerful magnetic fields. Radio
measurements are ideally suited to providing information on emission
mechanisms, electron temperatures and populations, and magnetic field
strengths and configurations. Energetic plasma phenomena vary rapidly in
time, and so continuous radio monitoring can in principle follow the details
of accretion~isk inflow, star spot evolution, jet acceleration, and so on.
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At present, these measurements are camed out at instruments such as the
sensitive, high-resolution (subarcsecond) Very Large Array (VLA).
SPECTROSCOPIC STUDIES OF EXIRASOLAR-SYSTEM PIANElrS
ANI) PREPLANETARY MATERIALS
Beyond its use for detection and fundamental characterization by the
techniques discussed in Chapter 5, spectroscopy is a potential tool to
elucidate physical and chemical environments of planetary and precursor
systems. For reasons discussed under the heading Direct Detection, this is
beyond the capabilities of telescopes or other systems currently In progress
or planned, although there appears to be no fundamental reason why
improved systems with the needed capabilities cannot and will not be built.
Application of analytic spectroscopy to physical and compositional studies
will require telescopes of sufficient spatial resolution to separate emitted
or reflected light from the planetary object and circumstellar material from
that of the pnma~y star.
If such systems were trained on extrasolar planets, spectroscopy could
provide the means for ascertaining the presence of an atmosphere, deter-
mining or constraining its composition, and establishing a rough tempera-
ture-pressure structure. Such measurements, combined with studies of
metal content of the parent star, could be diagnostic of the processes by
which the secondary object formed. In addition, emission spectra from
substellar objects (which should be obtainable in the infrared, for example,
from SIRES:) give information on the composition, temperature, and sur-
face gravity of these objects. Basic questions to be addressed might concern
the frequency of substeliar objects and of planets with atmospheres and
among those planets the determination of the frequency of occurrence of
major atmospheric types, such as those pnmaril~r of hydrogen or carbon
diomde, if the solar system is used as a guide, or perhaps even other
constituents. The detection of molecular oxygen as a major atmospheric
constituent may even suggest the possibility of the existence of Earth-like
biological activity.
Prerequisites for application of spectroscopic techniques include the
existence of imaging or interferometric systems with sufficient spatial res-
olution and a large enough aperture to collect the required number of
photons. Thus application of spectroscopy lo physical studies will follow
the development of light-collecting and sorting facilities in a natural way.
The development of technology for spectroscopy of extrasolar-system plan-
ets is certain to be a formidable challenge, requ~nng extensive laboratory
development of narrow-band spectrometers for the visible and infrared that
are analogous to the radio-frequency spectrometers used with radio tele-
scope arrays. For example, development of heterodyne spectrometers for
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the far infrared has progressed marked in recent years. Future extensions
to shorter wavelengths should find wide applicability to the spectroscopy
of protoplanetary nebulae and other systems whose content is primarily un
molecular form. It is likely that new molecular and atomic laboratory data
will be necessary to support analysis of these observations. Thus, if NASA
is to be ready for further studies when other planetary systems are located,
it must begin to refine spectroscopic capabilities for that purpose.
SUMMARY RECOMMENDATIONS
· The committee strong supports coniinua~on and expansion of ob-
serving programs directed toward investigation of preplaneta~y environments
and precursor materials, and for development of enhanced imaging, spectro-
scopic, and polw7me~c z~st~ument capabilities relevant to such programs.
In addinon, COMPLEXmakes three more specific recommendations with
respect to the dust-related research discussed above, and later in Chapter 7.
· Observanona] studies, collection programs and techniques, and labo-
ratory u~vesagahons focused on e~raterrestriz~1 dust should be continued and
refined
· rhythm this general area of study, He technology of laboratory analysis
of small e~raterrestnal samples should be developed to the point that submicron
Gains carrying "exoac'' isotopic sig~amres suggestive of presolar origin can be
individually identified arid analyze
· Final, active encouragement should be given to theoreacal, observa-
tional' and laboratory simulation studies of the condensing and chemical
nature of dust grams us czrcumstellar environments, of grain interactions with
and survival in the interstellar medium arid during infall into accretion disks,
and of the chemical and physical properties of icy~rganic grain mantles
Representative terms from entire chapter:
evolved stellar
