The chemical composition of the interstellar medium
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10.1098/rsta.2001.0889
The chemical composition of
the interstellar medium
By A d o l f N. W i t t
Department of Physics and Astronomy, The University of Toledo,
Toledo, OH 43606, USA
Our knowledge of the abundances of heavy elements with nuclear charge Z > 2 in the
interstellar medium is surprisingly incomplete. Several factors contribute to this state
of a¬airs. A substantial but unknown fraction of heavy elements is locked up in inter-
stellar dust, but the total mass of interstellar grains, as well as their size distribution
and exact composition, are still uncertain. The use of the chemical compositions of
stellar atmospheres as a reference for the interstellar medium has become question-
able, as the range in stellar compositions is becoming more fully known. The study
of the stellar nucleosynthetic sources of heavy elements also provides only uncertain
constraints, given that many di¬erent types of processes have contributed to the
enrichment of the interstellar medium. The solution to the present dilemma may
reside in the in situ detection and chemical characterization of interstellar grains
themselves, which could be accomplished in the near future.
Keywords: interstellar abundan ces; interstellar grains; stellar reference abundances
1. Introduction
Condensed objects in galaxies, ranging from stars to planets, from comets and aster-
oids to cosmic dust grains, derive their matter and their chemical make-up from the
interstellar medium (ISM). The composition of solid objects, in particular, is depen-
dent on the abundances of chemical elements with atomic number Z > 2. Knowledge
about the chemical composition of the ISM, especially about the abundances of ele-
ments with Z > 2 in relation to the dominant light gaseous elements hydrogen and
helium, is therefore of fundamental importance. It is a disturbing fact, however, that
a direct, independent and complete determination of the chemical composition of the
ISM, even in the relatively local solar neighbourhood of the Milky Way galaxy, has
yet to be accomplished. In the remainder of this article, I will present a number of
issues related to this subject and discuss the reasons for our lack of direct information
about the chemical make-up of the ISM.
The ISM consists of gas in various stages of ionization, both atomic and molecu-
lar, as well as dust particles. These dust particles are the condensate of a substantial
fraction of the elements with Z > 2, elements often referred to as `metals’ in astro-
nomical contexts. It is customary to measure the abundance of these elements by
the relative number of nuclei of a given species on a logarithmic scale with respect
to the number of hydrogen nuclei, which is set to log N (H) = 12. The degree to
which the heavier elements are depleted from the gas phase varies from element to
element, ranging from very small fractions for some to more than 99% for others.
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The degree of depletion is also highly dependent upon the physical conditions, such
as density, temperature and local radiation elds of speci c galactic environments
(Jones 2000, 2001). While the relative abundances of elements in the gas phase can
be measured using well-developed techniques of interstellar spectroscopy, an assess-
ment of the amount and composition of the solid-phase components of the ISM is
confronted with considerable di¯ culty. Therein lies the reason for our lack of direct
information about the chemical composition of the ISM.
The traditional path out of this dilemma has been to assume that the composition
of the atmospheres of unevolved stars, hot enough to permit only the gas-phase forms
of all elements to exist, is a true re®ection of the composition of the ISM from which
these stars formed originally. High-resolution stellar or solar spectroscopy, combined
with laboratory data on transition probabilities and theoretical atmosphere models,
can produce reliable data on the relative abundance of most elements present in
the atmospheres, including those largely locked up in solids in the ISM. Given that
such data were most complete for the Sun and given that the solar data can be
complemented with abundance studies in the most primitive meteorites in the Solar
System, it has been a long-standing practice in astronomy to regard the solar and
Solar System pattern of relative abundances as `cosmic’ (see, for example, Grevesse
& Noels 1993). This approach implies the presumption that the composition of the
ISM with all solids returned to the gas phase would equal that of the solar `cosmic’
composition pattern. This practice was adopted well before it was even established
whether or not the Sun is even representative of similar stars in its vicinity, which,
as it turns out, it is not.
The Sun formed ca. 4.6 Gyr ago and has completed nearly 20 orbits about the
galactic centre since then, in an orbit known to be non-circular. It is possible and more
than likely that orbital di¬usion has carried the Sun away from the galactocentric
distance of its origin (Wielen et al. 1996), while the local ISM has undergone many
changes in composition itself, either in the form of enrichment in heavy elements from
stellar sources or in the form of dilution through the in®ow of metal-poor intergalactic
gas complexes or the admixture of new ISM material from merging galaxies. It should,
therefore, not surprise us if the solar composition does not match the composition
of the ISM surrounding the Sun today. An alternative approach, then, would be to
consider stars formed quite recently from the local ISM, e.g. local B stars and stars
in young open clusters, as a suitable reference source for the abundance pattern
expected to be present in the ISM (see, for example, So a et al. 1994). The time since
the formation of such stars, of order 10 Myr, would be insu¯ cient for a substantial
spatial separation of the stars from their birth environment, nor would the ISM
composition have been altered substantially since the stars’ formation. However, an
untested but critical assumption underlies all e¬orts to use stellar atmospheres as
the information source for the abundance pattern in the ISM: at no point in time
prior to the completion of star formation must there be a separation of the solid ISM
component, which contains much of the reservoir of elements with Z > 2, from the
gaseous component consisting mainly of hydrogen and helium.
In the following section of this paper, I will review the origins and sources of
the elements with Z > 2. The di¬erent time-scales of various processes and the
di¬erences in the spatial distribution of the sources of heavy elements provide a
natural explanation for the di¬erences in chemical abundance patterns observable
in di¬erent stars, and, to a limited extent, in the ISM. In x 3 I will examine the
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extent to which solar abundances may be considered `cosmic’ and representative
of the local ISM. I will contrast solar abundances with those of other G stars of
presumably similar age and with abundances found in stars formed more recently
from the local ISM. In x 4 I will discuss the consequences of adopting di¬erent sets
of reference abundances for models of interstellar dust, and I will look at evidence
which suggests that none of the stellar reference sources may contain as much in
the form of elements with Z > 2 as the ISM, at least in those elements strongly
represented within the solid interstellar grains.
2. The origins of heavy elements
With the exception of a small amount of lithium, which together with hydrogen and
most of the cosmically abundant helium emerged from the big bang at the origin of
the Universe, elements with Z > 2, referred to as heavy elements throughout this
paper, have been produced by nucleosynthesis in stars over time. For additional heavy
elements to appear in the ISM, we need to consider those evolutionary processes that
result in a net expulsion of heavy elements from their stellar sources. Primary among
these are the explosive nucleosynthetic processes occurring in supernovae (SNe) and
novae, supplemented by more gentle stellar mass-loss processes such as stellar winds
and the ejection of planetary nebula shells occurring in the late evolutionary stages
of lower-mass stars. Unfortunately, given the diversity of these sources, with their dif-
ferences in contributions to di¬erent parts of the elemental spectrum and their varied
time-scales, it does not appear feasible to constrain the poorly known abundances of
elements of Z > 2 in the ISM by a full assessment of their sources.
SNe are the most dramatic events leading to the enrichment of the ISM with
heavy elements, and they are the only known sources for elements heavier than
iron. Two general scenarios are thought be responsible for SNe, the core collapse of
massive (m > 8M ) stars and the thermonuclear con®agration of white-dwarf stars
reaching the Chandrasekhar mass limit for degenerate structures (m = 1:4M ) in
binary star systems. A recent account of the SNe processes and their impact on
the chemical evolution of the Universe can be found in the monograph by Arnett
(1996). The two types of SNe occur with di¬erent rates, di¬erent time-scales, di¬erent
spatial distributions and with di¬erent yields with respect to the important heavy
elements, which are the main contributors to the formation of solid objects (Arnett
1995). The progenitors to core-collapse SNe represent the massive-star end of the
initial mass function and include objects with masses covering at least one order of
magnitude (8M < m < 100M ). They reach the stage of core collapse on a nuclear
time-scale of 1{15 106 yr, which implies that they will explode close to the location
of their formation. The yield of heavy elements from a core-collapse SN depends
strongly on the initial mass and on the extent to which the mass closest to the stellar
core is incorporated into a neutron star or black-hole remnant. For the chemical
enrichment of the ISM, core-collapse SNe are important mainly for oxygen and other
intermediate-mass nuclei, while the iron produced in these events, believed to be
between 1.4 and 2:0M , is expected to be largely incorporated into the neutron star
or black-hole remnant. The sources of iron-group elements in the ISM are mainly
the white-dwarf SNe (Thielemann et al . 1993), which are not thought to produce a
remnant core. Their time-scale is determined rst by the much longer nuclear time-
scale of their lower-mass (m < 8M ) progenitors, then by the mass-transfer time-
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scale from an evolving secondary star to the white-dwarf companion. These time-
scales combine to a cumulative total of 1{2 109 yr, suggesting that these sources will
be considerably more dispersed throughout a galactic system. This also suggests both
spatial variations in the O/Fe ratio and a decline in this ratio with evolutionary time.
In relation to hydrogen, the relative abundance of all heavy elements is related to
the degree to which gas from the ISM has been cycled through stars, which decreases
with galactocentric distance in disc galaxies such as the Milky Way. This is con rmed
by observations of galactic metallicity gradients, typically 0:07 dex kpc ¡ 1 for the
range of galactocentric distances from 5 to 18 kpc (Rolleston et al . 2000).
Nucleosynthesis in novae occurs as a result of explosions of hydrogen-rich matter
arriving on the surfaces of white dwarfs through mass transfer in close binary systems,
in which the white dwarfs are the rst-evolved components (Starr eld et al . 1993;
Hernanz et al . 2001). The frequency of novae is ca. 103 times that of SNe, but due to
the small yield of an individual nova, their overall contribution to the heavy-element
abundance in the ISM is modest. They contribute mainly to the CNO elements and
their isotopes and leave a mark in the form of the interesting radioactive isotopes 22 Na
and 26 Al. Quantitatively and qualitatively di¬erent yields are expected depending
upon the mass and composition of the underlying white-dwarf star.
During late evolutionary stages of lower-mass stars, in particular during the asymp-
totic giant branch stage, convection reaching from the surface deep into the stars’
interiors dredges up processed materials. These include carbon, resulting from helium
burning, and a variety of elements produced by slow neutron capture, the so-called
s-process elements such as Zr and Ba. Continuous mass loss in the form of stellar
winds, culminating in the expulsion of the entire envelope in a planetary nebula
formation, adds these elements to the ISM over time. Given that the circumstellar
shells of these stars exhibit low temperatures (T < 2000 K), while still compara-
tively dense, many of the heavy elements condense into grains before leaving the
star. Thus these stars are major sources of new interstellar dust in the present Milky
Way galaxy. For a recent review of nucleosynthesis in asymptotic giant branch stars
and their contribution to the heavy-element enrichment of the ISM, see Busso et al .
(1999).
3. Stellar reference abundances for the interstellar medium
As long as the solid components of the ISM are beyond the reach of a direct chemical
analysis and as long as the processes leading to the injection of heavy elements into
the ISM are too varied and complex to be modelled in detail, it is tempting to use the
unmodi ed atmospheres of relatively unevolved main-sequence stars as a reference
standard for the composition of the ISM from which they formed. In this way, a
star’s composition could then be considered a snapshot of the composition of the
ISM in one point of galactic space at one point in time. This has been the basis
for using the rather well-determined abundance pattern found in the Sun and the
Solar System as the reference for galactic chemical evolution studies, for studies of
the evolution of the elements as a function of redshift, and for inferences about the
amount of solids likely to be found in the local ISM. The latter is done simply by
subtracting the observationally established amounts of gas-phase elements from the
Solar System `cosmic’ abundances.
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Solar abundances, at best, re®ect the ISM abundances in a star-forming cloud
that existed 4.6 Gyr ago. This leaves open the question of the chemical evolution of
the ISM in the intervening time, not to speak of the question of whether the Sun
is at least an average representative of the generation of stars formed at about the
same time and found today in the solar neighbourhood. The ground-breaking work
of Edvardsson et al . (1993) provides at least a partial answer to the latter question.
These authors determined the abundances of 13 elements relative to hydrogen and
the individual photometric ages of 187 nearby eld F and G dwarf stars in the
galactic disc. The application of their technique of age determination to individual
stars in the cluster M67 yielded a consistent age of 4 Gyr, instilling con dence in the
uncertainties of their derived stellar ages of ¢ log(age) = 0:07{0.15 stated by these
authors. Several of their results are important for the present issue.
(i) For stars with ages between 1.5 and 6.5 Gyr, the spread in the Fe/H ratio
increases to almost one order of magnitude.
(ii) Relative to nearby solar-type stars of similar age, the Sun has a Fe/H metallicity
ratio that is larger by 0:17 0:04 dex than the average of these stars.
These two facts have caused Wielen et al . (1996) to propose that orbital di¬usion,
over the time-scale of several Gyr, causes a real spread in the chemical composition
of stars found at any given galactocentric distance, and for the Sun in particular,
that the solar composition is indicative of a birthplace at a galactocentric distance
of 6:6 0:9 kpc, given the observed radial metallicity gradient in the Galaxy. If
correct, this would indicate that few stars of solar age currently found in the solar
neighbourhood are a reliable abundance template for the local ISM, with the Sun
being no exception to this rule.
Unevolved stars which formed much more recently from the ISM, such as main-
sequence B stars, do not yet su¬er from the orbital di¬usion problem. They also hold
out the promise that the ISM of their origin has not had su¯ cient time to be modi ed
greatly by SNe explosions or by the infall of primordial unenriched gas since their
formation. Detailed heavy-element abundance data on B stars have become available
during the past decade (Kilian 1992, 1994; Gies & Lambert 1992), which show these
stars to have heavy-element abundances which are only 50{70% solar. This fact,
by itself, appears counterintuitive, because one would have expected, on the basis
of a simple closed-system chemical evolution model, that the relative abundance
of heavy elements in the solar neighbourhood would have increased over the past
4.6 Gyr rather than decreased. A likely answer is that the system is not closed and
that the ISM has been diluted by infalling gas with a substantial underabundance of
heavy elements, as suggested by Jura et al . (1996). There is growing evidence that at
least some of the high-velocity gas currently observed to be falling into the Galaxy
has heavy-element abundances of ca. 10% solar level (Richter et al . 2000; Wakker et
al . 1999), lending support to the dilution hypothesis.
A recent abundance study by Gummersbach et al . (1998) of B main-sequence
stars ranging in galactocentric distance from 5 to 14 kpc demonstrated that the
apparent systematic di¬erence between heavy-element abundances in B stars and in
the Sun is not merely a local phenomenon. There is also a suggestion in the data
that the di¬erence between B-star abundances and solar values does not correspond
to the same factor for all elements. The di¬erence is most pronounced for carbon and
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oxygen, where it amounts to almost a factor of two, while for magnesium and silicon,
solar and average B-star abundances are almost in agreement with each other.
The use of stellar atmospheric abundances as a reference for the ISM is meaningful
only as far as dust and gas in a collapsing interstellar cloud remain perfectly mixed
throughout all stages of the star-formation process, an issue highlighted most recently
by Snow (2000). Dust grains are subject to forces that do not a¬ect the neutral gas in
equal measure, and the resulting e¬ects may cause a physical separation of dust and
gas. Radiation pressure upon grains, exerted by anisotropic radiation elds, will set
up a relative drift of grains through the gas. It has been proposed that large amounts
of dust are being swept out of quiescent galactic discs by this process (Shustov
& Vibe 1995; Alton et al. 2000) over a Hubble time. More closely related to star
formation, Ciolek & Mouschovias (1996, 1998) have shown that ambipolar di¬usion
of magnetic elds in collapsing molecular cloud cores and the interaction of magnetic
elds with charged grains can lead to a selective retention of grains from the gas ®ow,
resulting in an underabundance of the elements dominating the grain composition
(C, O, Si, Mg, Fe) in the ultimate stellar atmosphere. Alternatively, in the case of
uncharged grains, gravitational sedimentation of grains in interstellar clouds may
lead to abundance gradients within the protostar, leaving the atmosphere again with
an apparent depletion of heavy elements (Lattanzio 1984). In either event, present-
day abundances in main-sequence B stars may therefore not be a reliable re®ection
of heavy-element abundances in the ISM either; instead, the actual abundances of
elements residing in ISM solids could be higher by an unknown factor.
4. Interstellar grains and heavy-element abundances
How well can direct observations of the ISM be used to constrain the abundances
of heavy elements within it? Absorption-line observations with the Hubble Space
Telescope have yielded a substantial body of data on the abundances of gas-phase
heavy elements in the ISM (Savage & Sembach 1996). The great majority of these
elements appear with abundances that range from solar to 10¡ 4 solar. Among the
fractionally most heavily depleted elements are Si, Mg and Fe, thought to be major
components of interstellar grains by mass. However, since the dust-to-gas mass ratio
is not well determined, these observations do not provide a reliable indication of the
total abundances of the heavy elements in the ISM.
Not all heavy elements participate in the depletion into dust, and those that remain
fully in the gas make it possible, in principle at least, to assess the absolute metallicity
of the ISM. The noble gas krypton is one such element (Meyer 1997). Cardelli &
Meyer (1997) determined the abundance of krypton towards 10 stars with sightlines
through ISM with a wide range of density and found it to be constant at the 60%
solar level. If krypton is in fact undepleted, this would indicate that the B-star
abundances are indeed the more suitable reference for the ISM. Arguments to the
contrary can be supported with observations of the ISM towards the exceptionally
well-studied star ± Oph, which show a number of elements at or slightly above the
solar abundance level (S, Se, Sn, Cl, N) and the element thallium at nearly three
times solar. No strong case, one way or another, can therefore be made for a particular
set of reference abundances on the basis of gas-phase observations alone.
The solid phase of the ISM manifests its presence through a wide range of observ-
able phenomena (Mathis 2000), which can be modelled successfully. Unfortunately,
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resulting models are not unique and rely on many unproven assumptions regarding
the composition, shape, structure and size distribution of interstellar grains (Witt
2000a). Snow & Witt (1995, 1996) examined the mass requirements of a number of
models currently under discussion in the light of the availability of heavy elements.
The availability was estimated by subtracting the observed gas-phase abundances
from either the solar reference abundances or the reference abundances based on
B stars and other recently formed objects. Several serious problems emerge from
such a comparison. Virtually all models require between two and three times as
much carbon in the form of graphite, hydrogenated amorphous carbon and poly-
cyclic aromatic hydrocarbon molecules than is available with the B-star reference
abundances (Snow & Witt 1995), and only slightly smaller de ciencies arise with
respect to Si and Mg, principal ingredients for interstellar silicates (Snow & Witt
1996). Adopting solar abundances solves the `carbon crisis’, but raises a new issue,
the so-called `oxygen problem’ (Meyer 1997). In this case, the amount of oxygen
deduced as depleted into the dust phase is nearly twice as large as the amount that
can readily be accommodated by various oxygen-bearing dust compounds. It is inter-
esting that the `oxygen problem’ disappears when B-star abundances are adopted as
reference.
A potential solution to the problem posed by the uncertain amount and compo-
sition of interstellar grains is to detect and analyse grains in situ. A signi cant step
in this direction was taken by placing dust detectors on board the interplanetary
spacecraft Ulysses and Galileo, which recorded the impact of interstellar grains pen-
etrating the Solar System on hyperbolic orbits, arriving from a direction consistent
with the ®ow of gas from the local interstellar cloud (Baguhl et al . 1996; Frisch et
al . 1999). While the Ulysses and Galileo dust detectors were able to determine the
speed, direction and mass of over 600 interstellar grains (Landgraf et al. 2000), they
were not equipped to determine their chemical composition. Nevertheless, by relat-
ing the measured ®ux of interstellar solids to the gas density of the local interstellar
cloud, Frisch et al . (1999) showed that the dust-to-gas ratio of the in situ sample is
about twice as large as the value usually assumed on the basis of observed interstel-
lar reddening by dust. It is signi cant that most of the dust mass ®owing into the
Solar System is in form of grains with radii larger than 0.35 m m, extending to radii
of ca. 2 m m. These larger grains produce mainly grey extinction at visible and ultravi-
olet wavelengths and are, therefore, poorly constrained by reddening observations at
these wavelengths. As it happens, the intensity and pro les of X-ray halos observable
around point-like X-ray sources are most strongly dependent upon the largest grains
in a typical interstellar-dust size distribution. The analysis of the well-studied X-ray
halo surrounding Nova Cygni 1992 by Witt et al. (2001) supports the Frisch et al .
(1999) nding of a grain size distribution extending to radii of 2 m m, showing that
these larger grains are present on a larger galactic scale rather than being a peculiar-
ity of the local interstellar cloud. Observations with the Chandra X-ray Observatory
currently in orbit will provide an important database for additional investigations of
the size distribution of interstellar grains along many di¬erent sightlines.
Important progress in our understanding of the chemical composition and the
mineralogical character of interstellar grains is expected from the sample returns
promised by the Stardusty spacecraft, currently traversing the Solar System on a
dust (cometary as well as interstellar) collection mission. If successful, the mission’s
y Visit http://stardust.jpl.nasa.gov/science/details.html.
Phil. Trans. R. Soc. Lond. A (2001)1956 A. N. Witt
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samples will be returned to Earth in early 2006. Also addressing the present uncer-
tainties of the chemical composition of interstellar grains is the proposal for the
Galactic Duney mission. Galactic Dune, as currently proposed, would be an inter-
stellar dust observatory in high Earth orbit, capable of determining the chemical
composition of grains via a high-resolution mass spectrometer as well as observ-
ing the size distribution and mass ®ux of incoming interstellar grains. All missions,
current and proposed, face the same challenges, in that they must sort out interstel-
lar grains from a background of interplanetary dust, and the same limitations, in
that grains smaller than ca. 0.2 m m in radius are increasingly prevented from enter-
ing the Solar System by interactions with the heliopause (Frisch et al . 1999). Thus
the astrophysically important portion of the grain size distribution represented by
nanoparticles is totally excluded and would require an interstellar mission for in situ
analysis. Fortunately, while very important in numbers and highly signi cant in total
surface area, interstellar nanoparticles represent only a small fraction of the mass of
interstellar grains (Witt 2000b).
5. Conclusions
The interstellar medium consists of gas and dust. Studies of the abundance of the
gas-phase portions of the elements with nuclear charge Z > 2 relative to hydrogen
have provided evidence for elemental depletions that vary greatly from element to
element and with environmental conditions such as temperature and density of the
gas. Approximately 50% of the mass of elements with Z > 2 is in the solid phase,
but this fraction could be larger. The principal cause for this uncertainty rests with
the fact that the conventional observations of interstellar dust, such as extinction,
scattering, polarization and grain emissions, do not provide very speci c information
about the chemical composition and mass of the grains along a given line of sight in
space. Often, a given type of observation yields only fractional information on one
component of what is surely a very complex mixture of materials. Our uncertain
knowledge of interstellar grains is thus preventing us from knowing the chemical
composition of the interstellar medium to a degree that would be desirable for many
types of investigation, not the least of which is the question of what kinds of solids
are likely to form through condensation of the interstellar medium. However, there
are good prospects that in situ observations and characterizations of interstellar
grains entering the Solar System may provide the missing information in the near
future.
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Discussion
T. McDonnell (Planetary and Space Sciences Research Institute, The Open Uni-
versity, Milton Keynes, UK ). You referred to the discovery, fairly recently, of the
in®uxal interstellar grains to the Solar System. A mass spectrometer is on the star-
bus system, but also part of the Cassini Dust Analyser, with a mass spectrometer on
its journey to Saturn currently now going past Jupiter. Given this opportunity, what
clues should we look for in the elemental composition of interstellar grains detected
from the chemical analyser?
A. N. Witt. Assuming that these platforms have the capability of distinguishing
interstellar grains from zodiacal dust particles, they de nitely could assist in testing
di¬erent models for interstellar grains. Some models propose distinct populations of
silicate and carbonaceous grains, re®ecting their stellar mass out®ow origins, each
with its own mass-spectrometric response, while other models are based on composite
grains, re®ecting extensive reprocessing in interstellar clouds. The latter grains should
exhibit fairly constant ratios of all compositional components, with mass spectra
barely di¬ering from one grain to another.
H. Palme (University of Cologne, Cologne, Germany). You mentioned that larger
grains in the ISM may contain a signi cantly larger fraction of interstellar than
previously thought. What are the observational constraints on the observation of
large grains?
A. N. Witt. Indications supporting an increased fraction of larger grains in the ISM
come from the observations of higher-than-expected dust albedos in the near-infrared
Phil. Trans. R. Soc. Lond. A (2001)Downloaded fromInterstellar chemical abundances
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in dense interstellar clouds, from the pro les of X-ray scattering halos surrounding X-
ray point sources seen through long lines of sight through the di¬use ISM, and from
the analysis of dust-detection data collected onboard the Ulysses and the Galileo
spacecraft. The latter experiments have identi ed grains entering the Solar System
from the local interstellar cloud.
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