Solar and Galactic Composition
Robert F. Wimmer-Schweingruber
Physikalisches Institut, Universitdt Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Abstract. This is a personal overview of our joint SOHO/ACE workshop "Solar and Galactic Composition" held
in Bern, Switzerland, between the 6th to the 9th of March, 2001. The overview is not complete, although I have
tried to capture the spirit of the workshop. We were all confronted with results from fields we were unfamiliar
with. This broadened our horizons and showed us that our work is important to other - sometimes unexpected fields. Linking solar and galactic composition was an exciting experience.
INTRODUCTION
Let me illustrate some of the implications of solar abundance determinations. The solar system formed
from the interstellar medium as it existed at that time,
(4.57 ± 0.02) x 109 years ago. It formed in a fragment
of a giant molecular cloud, maybe similar to one of the
Hubble "pillars". We know from certain isotopic anomalies preserved in presolar grains (see Hoppe [1], this volume) that various nucleosynthetic sources such as AGB
stars, Wolf-Rayet stars (see [2]), or supernovae contributed material to the "stuff we're made of". As we
gradually begin to understand more and more about how
our solar system evolved, we see more and more clearly
that this contribution may not be accidental.
It is well possible that we would not exist if the massive, short-lived stars that formed in the same galactic
neighborhood as the solar system had not halted the accretion of the protoplanetary disk by the Sun in time for
the Earth (and the other planets) to survive the solar "cannibalism". Observations of a multitude of young stars
has shown that their surrounding protoplanetary disks
are often subject to intense UV irradiation of neighboring young, massive stars [3, 4]. Gas drag on the forming planetesimals and protoplanets makes them gradually
spiral nearer to the central star. Depending on the time of
gas removal, planets can survive in that stellar system,
or they are engulfed by the star they orbited. At least
for the outer parts of protoplanetary disks, the evaporation times by photoionization are short compared to their
viscous lifetimes [4]. This picture has consequences for
the surface composition of stars. Butler et al. [5] found
that planet-bearing stars had a higher metallicity than
the Sun, see Figure 1. So far, only stars with at least
Saturn-sized companions have been detected [5]. Smaller
planets cannot presently be detected because their influence on the central star is too small. Because giant gas
planets only form beyond the ice line, their detection
How representative of the cosmic abundances are solarsystem abundances? What are the cosmic abundances?
And how well do we know solar and solar-system abundances? How do we measure abundances, and what processes can alter them? What can we learn from composition measurements? A set of questions I am certain all of
us have pondered. The Joint SOHO/ACE workshop "Solar and Galactic Composition" addressed these questions
in a fashion that each of us could not do on his own. We
managed to unite workers in as diverse fields as solar
remote-sensing and in-situ studies, meteoritics, stellar,
interstellar, and cosmic-ray physics, as well as galactic
chemical evolution and Big Bang nucleosynthesis. This
workshop was an attempt to put the work of all of us into
a broader context and to help us understand what workers
in other fields expect to learn from us.
I will not attempt to give an objective summary of this
workshop. I freely admit that I can't. While I had many
questions at the beginning of the workshop, I believe we
have uncovered many more. Relating solar and galactic
composition was like poking into an ants nest.
We all knew that composition studies are important.
Some of us used abundance measurements to trace physical processes, others used them to study our origins. Both
subjects are interesting, fascinating, and could occupy us
for a substantial fraction of our lives. But how relevant
are they? Does a better understanding of fractionation effects in gradual solar energetic particle events tell us any
more than just that? Do measurements of the composition of the local interstellar cloud tell us more than just
that? After all, it is extremely local, viewed on a galactic
scale! Nevertheless, we found that, yes, solar composition is important, even on a "galactic scale"
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
3
thought. So the Sun lies right at the median of field
stars and below the median of planet-bearing stars solving the "old Sun" problem1. This should serve to
illustrate that solar abundances have implications beyond
just finding some reference abundances against which
to compare others. Understanding our history is quite
intimately linked to solar (and galactic) composition.
SOLAR ABUNDANCES
-0.4
-0.2
00
02
04
0.6
Metallicity [Fe/H]
FIGURE 1. Comparison of the metallicity of planet-bearing
and field stars. Data are from Butler et al. [5]. Plotted are the
number of field stars (shaded, thin line) and planet-bearing stars
(empty, thick line) in a given metallicity bin. Metallicities are
normalized to solar and given on a logarithmic scale (dex).
closer to their star means that they have migrated inward.
And any Earth-like, rocky planet must have migrated too
far... Planet-bearing stars also show a striking surface
enhancement of Li [6], an element that is largely destroyed by nucleosynthetic processes in the early phases
of the evolution of Sun-like stars. That Li is depleted in
the solar photosphere would then imply that the Sun did
not "eat up" a substantial amount of planetary matter after the formation of an outer convective zone.
Let us now consider the solar metallicity. We find that
it lies on the high side of field stars (0.2 dex, i. e. 58%,
higher than the median), and near the median of planetbearing stars. Did the Sun "eat up" some small planets in
its youth? Or is the Sun just simply metal-rich - the well
known "old Sun" problem? The latter could indeed be
the case. Low-metallicity clouds probably have less dust
in them than high-metallicity clouds, simply because the
metals condense more easily than H and He. Dust is
very likely an important ingredient in forming planetary
systems [see e. g. 7, and references therein].
This workshop brought a new twist to the "old Sun"
problem. The presently most detailed considerations of
the solar abundances of some key elements were reported
by Holweger [8]. These new determinations of solar
abundances that take into account non-equilibrium and
granulation effects seem to indicate that the Sun is quite
typical of the neighboring stars at this galactocentric
distance. For instance, the newly recommended oxygen
abundance is nearly 0.2 dex lower that that given in the
classical work of Anders and Grevesse [9]. Iron is 0.22
dex less abundant in the photosphere than previously
Let us continue to investigate solar abundances. Most of
the mass in the solar system is concentrated in the Sun.
As such, it is the reference against which solar-system
abundances must be measured. Consequently, many different methods have been used to determine solar composition. Photospheric solar abundance determinations
aren't easy. Holweger [8] (this volume) gives an account
of some of the pitfalls such as non-equilibrium and granulation effects. For chromospheric or coronal abundance
determinations, in addition to instrumental uncertainties,
the charge-state composition of the solar plasma must be
taken into account, moreover, the atomic properties governing line emission and absorption are not known with
sufficient precision (See e. g. Del Zanna et al., [13], Raymond et al. [14], von Steiger et al. [15], all in this volume). All of these uncertainties are on the order of 20%
- 30% or more. Adding quadratically we arrive at uncertainties of about 0.2 dex - the difference between field
and planet-bearing stars.
As Busemann et al. [16] show, meteoritic and solar
abundances agree remarkably well - well within the discussed photospheric abundance uncertainties. This is illustrated by Figure 2. On the right-hand side it compares photospheric and (CI) meteoritic abundances. The
logarithm of the ratio of their abundances (in the dex
notation) is plotted versus 50% condensation temperature. While some of the uncertainties are appreciable, the
overall agreement is good. The left-hand side shows a
histogram of the right-hand panel. It is well fit by a Gaussian with standard deviation of 7% (dex). How good are
the meteoritic abundances as a proxy for solar composition? Elemental abundances in meteorites are different
in the various mineral separates in different meteorites.
Possibly they have been altered by aqueous processes
or, in the differentiated meteorites, due to processes accompanying differentiation of their parent bodies. The
most primitive meteorites, the CI chondrites have retained many of their volatile elements and are the most
volatile-rich bodies in the solar system short of the Sun
1
Note however, that it still exists for isotopes. For most elements, the
Sun is isotopically lighter than the interstellar medium [Compare e. g.
10,11,12].
0.6
20 r—
0.4
15
0.2
0
Q_
10
0.0
O
o
-0.4
0
_0.6
-0.4
-0.2
0.0
0.2
0.6
photo-meteo [dex]
-0.6
500
1000
1500
2000
50% condensation temperature
FIGURE 2. Comparison of photospheric and (CI) meteoritic element abundances. The right-hand panel shows the logarithm of
the ratio between photospheric and meteoritic element abundances plotted versus 50% condensation temperature. The agreement
between the two different determinations of solar abundances is good. This is illustrated by the histogram in the left panel. It is well
fitted by a Gaussian with Gdex = 7%.
and comets. Therefore, we presume that they have preserved a faithful sample of the composition of the presolar nebula. However, as all classes of meteorites, they
have lost their primordial noble gas inventory; moreover,
because they have retained a large amount of volatiles
such as water, they are especially susceptible to aqueous
alteration. Furthermore, CI chondrites are an extremely
rare class of meteorites2. When we speak of element
abundances based on CI chondrites (in the plural), we
tend to ignore the fact that these abundances are largely
based on the analysis of one single CI chondrite, Orgueil.
Only few others have fallen onto Earth (Alais, Ivuna,
Revelstoke, and Tonk) and they often were very small or
little has been preserved3. Hence they have not been analysed as extensively as Orgueil. To put it bluntly, we base
our solar system abundances on the analysis of one single rock. The remarkable agreement between solar (photospheric and coronal and solar wind (after some corrections)) abundances and meteoritic abundances shows that
this rock was wisely chosen. Nevertheless, with the enormous progress being made in the determinations of abundances from remote-sensing and in-situ observations, we
will soon have to begin investigating possible differences.
The composition of the Sun can also be measured
more directly, by acquiring a sample of it. The Sun emits
a constant stream of particles in the form of the solar
2
Only 5 CI out of 22507 authenticated meteorites listed in the catalogue by Grady [17].
3
While more than 10 kg of Orgueil have been preserved, onlyless than
about 90 g have been preserved of the originally recovered 6 kg of
Alais. 704.5 g of Ivuna were recovered, 1 g of Revelstoke, and 7.7 g of
Tonk.[17]
wind (at energies of about 1 keV/nuc). Their abundances
can be used to measure solar isotope and element composition after correction for fractionation effects. These
affect composition mostly by first ionization potential or
time, but possibly also by other atomic properties. Lunar soils contain an archive of the ancient solar wind
(see e. g. Heber et al., [18], this volume). Solar energetic
particles are accelerated to much higher energies out of
the solar atmosphere, corona, and even the solar wind.
Their composition can be measured with high accuracy,
but again needs to be corrected for fractionation effects
(see e. g. Reames, [19] this volume).
Helioseismology has considerably improved the
knowledge about the internal structure of the Sun. From
observations of solar oscillations it is possible to derive
detailed knowledge of the solar interior [see, e.g., 20].
Standard solar models today include the effects of element diffusion [see, e.g., 21] which leads to elemental
and isotopic segregation. Models including elemental
segregation exhibit the best agreement with parameters
derived from helioseismological data. Element and
isotopic segregation are due to a competition between
two processes. Minor species in the solar interior are
influenced by various external forces, for example,
gravity, radiation pressure, etc. The particles also collide
with each other and scatter randomly. This competition
between external forces and random scattering leads to
elemental segregation [22]. Consistent solar evolution
models which include diffusion and radiative acceleration effects (apart from gravitational settling) predict a
depletion of the heavy elements at the present day solar
surface of the order of 10% or 0.04 dex [23]. This is
less than the 0.07 dex seen in Figure 2 and we are not
yet in a position to verify this theoretical prediction by
direct abundance measurements. Future mission, such
as Genesis, launched on the 8th of August, 2001, may
yield data with sufficient precision to settle this question.
However, the 0.04 dex depletion of the heavy elements is
also about the precision of meteoritic abundance determinations (typically between 0.03 and 0.05 dex). While
these are not limited by badly known atomic or possibly
instrumental properties, there is a limit due to possible
selection biases. Obviously, in spite of the proximity
of the Sun, we do not yet know its composition with
an accuracy that is sufficient to confidently relate it to
surrounding stars with a precision that is relevant to
answer some of our questions..
THE SUN AND THE INTERSTELLAR
MEDIUM
As the Sun moves through the galactic interstellar
medium, it encounters different environments. Neutral
particles enter the heliosphere where they can be measured (Salerno et al., [24] this volume). Thanks to the
large collecting area of the COLLISA experiment [24]
these workers obtain a rather precise value for the cosmologically important ratio 3He/4He. On its way through
the heliosphere, the interstellar neutral gas is ionized and
can then be measured as pick-up ions (Gloeckler and
Geiss, [25] this volume) or, after a further stage of acceleration (see Jones et al., [26]), as anomalous cosmic rays. Lunar soils may have preserved an archive of
the varying galactic environment the solar system passed
through during its history (Wimmer-Schweingruber and
Bochsler, [27] this volume).
The Sun and the interstellar medium have evolved
since the time the solar system formed. Nucleosynthesis (see Thielemann et al., [28] this volume) in stars and
supernovae contributes freshly synthesized material to
the interstellar medium. The composition of the galaxy
is slowly changed by stellar nucleosynthesis. It's evolution can be modeled (Chiappini and Matteucci, [29] this
volume) and measured (see e. g. Sofia, [30] this volume)
with new instrumentation. Galactic cosmic rays form another sample of this medium (see e. g. Wiedenbeck et al.,
[31] and the review by Klecker et al., [32] in this volume).
Remarkably, the local interstellar medium if anything
is less metallic than the Sun, as reported by Gloeckler
and Geiss [25]. This is also seen in galactic cosmic rays
[31] and in most elements in nearby interstellar clouds
[30]. Given this observational agreement, it does make
sense to speak of galactic abundances. However, we expect that Big Bang nucleosynthesis, the Sun, and the interstellar medium form a temporal sequence composition
wise [33]. We expect BBN to give us the primordial com-
position, the Sun to be representative of the composition
of the interstellar medium some 4.6 billion years ago,
and the local interstellar medium to be representative of
the present-day interstellar medium [33]. So the reported
observations make us wonder wonder what happened to
galactic chemical evolution in the past 4.6 billion years.
Was there essentially no evolution, or is the Sun currently
in a less evolved region of the galaxy?
This puzzle is augmented by the observations of Binns
et al. [2] who determined the isotope abundance ratio of
22
Ne/20Ne in the source for galactic cosmic rays (GCR).
They find a value which is five times higher the the solar
(wind) value, indicating a strong contribution by WolfRayet stars.
Other work reported here indicates that the source
of the GCR is interstellar dust grains. GCRs also show
fractionation by first ionization potential or some related
property. According to George et al. [34], this controlling
parameter is volatility, implying that interstellar dust is
the origin of the GCR, as has been proposed by Meyer et
al. and Ellison et al. [35, 36].
COMPOSITION AS A TRACER
One of the many uses of composition as a tracer is
illustrated by Figure 1 in Reinard et al. [37] in these
proceedings. Solar wind composition is a good indicator
of the coronal origin of the solar wind. In this figure,
anomalously high iron charge states indicate the passage
material ejected in a coronal mass ejection. In the solar
wind in the ecliptic plane coronal mass ejections often
have elemental composition very similar to the slow solar
wind, with the exception of He/H, which is sometimes
anomalously high.
Charge-state and elemental composition in CMEs are
determined by very different mechanisms on quite different time scales. Elements are fractionated by some process which fractionates according to their first ionization
potential and is probably intimately linked to the looplike structure of the plasma prior to onset of the CME.
Hence elements have a long time, on the order of the
life time of the loop, to be fractionated. Observations by
Feldman et al. [38] show that the FIP bias in loops increases linearly with time, ranging from no FIP bias in
emerging loops up to a factor of about 10. This allows
us to determine the average life time of the loops which
turns out to be on the order of a few days. The observation of Neukomm [39] that CMEs in the high-speed
solar wind sampled out of the ecliptic plane by Ulysses
have composition virtually indistinguishable of that of
the fast solar wind tells us that there the loop life times
must be shorter, on the order of a day or less. A similar
story seems to hold for the mass fractionation observed
in a special class of CMEs [40,41].
Ko and coworkers [42] compare solar wind abundances with measurements in the solar corona using 3dimensional MHD models. They find a likely correlation
of electron temperatures and elemental abundances in the
observed coronal region with those in the solar wind.
This may imply that the slow wind originates in regions
with mixing of closed field lines which are penetrated by
open field lines. As the open field line moves through the
close region, the resulting reconnection would then liberate the confined plasma, feeding the slow solar wind
[e.g. 43].
The work by Antonucci and Giordano [44] as well as
of Parenti et al. [45] confirm the picture that the slow
wind originates in the legs of streamers [46,47]. Streamers have long been known to be the source of the slow
solar wind [48]. They are often associated with crossings of the heliospheric current sheet [49]. Solar wind
measurements show a strong depletion of He/H at the
sector boundaries [49] accompanied by a depletion of
He/O [46, 47, 50]. Together, the optical and in-situ observations suggest that the solar wind flows out from the
"legs" of the streamer, and does not originate at the top of
streamers. On it's way to interplanetary space, this type
of slow wind is additionally fractionated by inefficient
Coulomb drag [51,52].
The work of Morris et al. [53] is another example of
the use of composition as a tracer. They observed that at
the medium energies observed with ACE/SEPICA the ratio of singly-charged to doubly-charge He increases from
the beginning to the end of corotating events. They interpret this as due to the changing magnetic connection of
the spacecraft and the corotating interaction region. The
later on in the event, the farther away lies the connection
point and the larger is the contribution of accelerated interstellar pick-up helium ions.
Let me end this section on the use of composition as a
tracer with the word of caution that we heard from Don
Reames at this workshop. His work [19] shows that variations in the elemental abundances of energetic particles
do not always imply new processes or new sources. Often, they can be explained by one single (but not necessarily simple) process, in this case wave-particle interaction between the heavy ions and the proton-generated
waves generated in the vicinity of interplanetary shocks.
That the same properties are not seen at higher energies
[see the paper by von Rosenvinge et al., 54] is simply a
consequence of the conservation of the number of particles.
CONCLUSIONS
A key result of this workshop is that solar abundances
are important to workers in a wide range of fields. A
change in solar abundances can strongly influence interpretations in vastly different questions than what they
were originally determined for. We should remember this
point when we consider audiences for future publications..
The workshop lasted for four long days, full of hard
work, as many of you will agree. Of course, this is a
short time given the background of medieval city of Bern
(founded 1191 A.D.). Yet even this time span pales
against the time scales governing the evolution of the
Sun, solar system, and the galaxy.
ACKNOWLEDGMENTS
I thank the participants of the workshop for a very stimulating time and many interesting discussions. This work
was supported by the Schweizerischer Nationalfonds and
the Kanton Bern.
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