The origin and abundances of the chemical elements

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by  Virginia Trimble

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Title
The origin and abundances of the chemical elements
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https://escholarship.org/uc/item/86s95592
Journal
Reviews of Modern Physics, 47(4)
ISSN
0034-6861
Author
Trimble, V
Publication Date
1975
DOI
10.1103/RevModPhys.47.877
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The origin and abundances of the cheriiical eleriients*&
Virginia Trimble~
Astronomy Program, University ofMaryland, College Park, Maryland 20742
and Department ofPhysics, University ofCalifornia, IrvineC, alifornia 92664
A NATO Advanced Studies Institute on the Origins and Abundances ofthe
Chemical Elements was held at the Institute ofAstronomy, Cambridge~ England
from 23 July to 9 August 1974.The problems discussed there are the basis for
this review. Three major areas can be identified. First, the normal distribution of
chemical and isotopic abundances and deviations from it must be determined
observationally. Second, the relevant nuclear processes and their sites and
products must be identified. Third, a model ofthe evolution ofgalaxies which
ixsgludes the relevant processes and produces the observed abundances must be
formulated. There has been rapid progress recently in all three ofthese areas.
CONTENTS F. Other Galaxies and QSOs 914
1.Helium 914
I. Introduction and History 877 2. Mean metal abundance 915
II. Observed Abundances 879 3. Abundance gradients in galaxies 915
A. The problem ofthe retrieval of meaningful data 879 III. Nuclear Processes, Their Sites and Products 917
1. D/H in molecules 879 A. Cosmological nucleosynthesis 917
2. The sola'r corona/photosphere iron discrepancy 879 B, Cosmic ray spallation and other versions ofthe xprocess 920
3. Weak He in QSOs and the galactic center 879 1. Supermassive stars 920
4. Peculiar A stars 880 2. Shell burning and shell flashes 920
5. Low energy solar flare particles 881 3. Novae 920
6. Galactic cosmic rays (as observed) 882 4. Supernova shock waves 920
Solar system abundances 882 5. Spallation by cosmic rays 921
1.The Earth—Moon system 882 C. Hydrostatic processes in stars 922
2. Meteorites 883 1.Hydrogen burning and the hot CNO cycle 922
a. The standard elemental abundance distribution 883 2. Helium burning 925
b. Abundances of the nuclides 887 3. Hydrostatic carbon, oxygen, and silicon burning 927
c. Helium, neon, carbon, and oxygen 887 D. Violent processes in stars 929
d. Xenonology and nucleocosmochronology 889 1. Explosive carbon burning 929
3. Solar photosphere and solar corona 891 a. Carbon detonation in degenerate cores 929
4. Solar wind and solar flares 895 b. Explosive carbon burning in massive stars 930
Other stars 896 2. Explosive oxygen and silicon burning and the e
1. Unevolved stellar compositions 896 process 932
a. Atmospheric compositions ofindividual stars 896 E. Processes which build heavy elements and rare isotopes 935
b. Stellar helium abundances 898 1.The sprocess 936
c. Stellar metal abundance as afunction ofage 899 2. The r process 942
d. The number ofstars as afunction ofmetal 3. The p process 948
abundance 901 F. Summing over the processes to get the yield per star and
e. Supermetallicity 902 the yield per generation ofheavy elements 950
2. Evolved stars 903 IV. The Evolution of Galaxies 953
a. Carbon stars and other giants 903 A. Observations ofgalaxies 953
b. Helium stars, R CrB variables, and other 1.Classification and morphology 954
hydrogen-de6cient stars 905 2. Luminosity distribution 954
c. Algol-type and other binaries 906 3. Colors and populations 955
d. White dwarfs 906 4. Masses 956
e. Supernovae 906 5. Gas mass and infall 956
D. The interstellar medium 907 6. Star formation rates 958
1.Hn regions 907 7. Supernova rates 960
2. Planetary nebulae 908 B.Homogeneous, one-zone models with constant IMF 961
3. Supernova remnants 90S C. Models with variable IMF 963
4. The general interstellar medium 909 D. Models with infall orinhomogeneity 964
E. Cosmic ray source composition{s) 910 E. Dynamical models ofgalactic evolution 964
1.The arriving and "source" compositions at 1. Spherical galaxies 964
intermediate energy 910 2. Disk galaxies 965
432..~, TTIshhoeetophloiiwgcheceronmeerpgnoyersgitcyioosnmcoiscmricayracyomcpoomspitoiosintion 999111344 V. ACocknnc3lo.uwsMiloeodndgsemlseanntwdsitEhxhmoarstsaitvioenshalos 999666786
References 968
~The literature search for this review was terminated on 1 April
1975.
fDedicated to my father, Lyne Starling Trimble, vvho taught me I. INTRODUCTION AND HISTORY
that, when you have along story totell, you must start atthe beginning,
keep going until you get to the end, and then stop, and to my late The problems of determining the abundances of the
mother, Virginia Farmer Trimble, who listened with great love and
patience to my early e8orts at story-telling. chemical elements and their isotopes and accounting for
fAlfred P.Sloan Ioundation Research Fellow 1972—1974. them in terms of a theory of nucleosynthesis and the
Reviews of Modern Physics, Vol. 47, No. 4, October 1975 Copyright 1975American Physical Society
878 Virginia Trimble: The origin and abundances of the chemical elements
evolution of our Galaxy involves virtually every branch which show the least evidence of remelting after formationj
of astronomy, and many related sciences, from objective are very similar to the solar ratios. This is even true to
prism spectroscopy and neutron activation techniques to a lesser extent for terrestrial rocks, given that the sid-
shell models of the nucleus and computerized histories of erophilic and chalcophilic elements are preferentially hidden
the Galaxy. It is not perhaps surprising then that no in the core, and that volatile materials have-again been
completely unique, clear picture of nucleosynthesis and lost. Moon rocks are similarly made up of only the more
evolution has yet emerged. refractory elements, but again in standard proportions.
Meteoritic, terrestrial, and lunar samples are particularly
In order to be able to account for, the abundances of valuable because isotopic ratios (which are generally not
the chemical elements, we must first know what they are. much affected by chemical fractionation) can be accurately
The first attempt at compiling a table of "cosmic abun- determined in them, which is not generally possible from
dances" (we will discuss shortly what this is supposed solar or stellar spectroscopy. Thus, it is possible to arrive
to mean) was made by Goldschmidt (1937), who relied at a representative set of Solar System abundances, which
primarily on data taken from meteorites and stellar spectra. (with a few exceptions) represents the surface of the Sun
He distinguished elements as siderophilic, chalcophilic, or now, and which should represent the original solar nebula
lithophilic, depending on whether they were found primarily out of which the Sun and planets were formed, in the
in the metal, sulphide, or silicate phases of meteorites, and conventional view of things.
adopted a very high abundance of Li, Be, and B on the
basis of igneous rocks. In addition, abundances very similar to those in the
Solar System are found in the interstellar gas in Hu regions
Other important compilations were those of Brown around newly formed stars (called Hn regions because the
(1949)and Suess and Urey (1956),who were able to make hydrogen there has been ionized by the ultraviolet radia-
considerable improvements in combining the data from tion from the hot young stars) and in planetary nebulae
various sources because of better understanding of the (the ejected outer.layers of older low mass stars). Among
ratio of metal to silicate phases in meteorites and the the stars as well, abundances are quite similar, provided
Earth, the ratio of volatile to refractory elements in the that we confine our attention to stars which (a) are not
Sun and stars (which allows you to link up stellar and evolved off the main sequence so that their surfaces have
meteorite data), and the ratios of isotopes which were by not been polluted by the products of their own nuclear
then also regarded as containing important information. reactions, (b) have convective atmospheres or are rapidly
rotating, so that their surface layers are kept well mixed,
The current standard abundance distributions to which and (c) belong to Population I or the disk population of
theoretical predictions are usually compared are those by stars which, on kinematic grounds, must have formed after
Cameron (1968, 1973).The latter of these does not differ the first billion years or so of the life of our galaxy. Even
very much from the former and is not regarded by every- the very old Population II stars, although their total
one as being an improvement. In fact the standard abun- metal' abundance is very low, have those heavy elements
dances have been remarkably stable. Only 8 elements out that can be detected present in nearly solar relative pro-
of the 81 tabulated had abundances which changed by portions. The integrated spectra of other galaxies and
factors of 3or more between 1956and 1968:C, Ga, and Pb quasars do not exhibit gross differences from "normal"
were revised upward by factors of 3, 4, and 8.5, respec- abundances either.
tively, and Be, La, Sm, Ta, and W downward by factors
of 25, 4, 3, 3, and 3, respectively. Thus, it is possible to define a standard abundance
distribution which applies to the Solar System and many
The most surprising thing, in a way, is that such a other objects, and, then, todiscuss how other objects deviate
compilation is possible at all. To what extent can we from this norrp. This will be done in Sec. II, special'at-
expect to represent with a single set of numbers objects tention being given to the problem of differentiating
as diverse as the Earth (more than half of whose a,toms, abundance anomalies that reflect nuclear processes from
at least in the crust, are oxygen), the Sun (mostly hy- those that result from fractionation, diffusion, and so forth.
drogen), and the iron meteorites (whose name is self- Such a distribution will be referred to hereinaf ter as
explanatory)'? If we aim at "cosmic" rather than Solar "cosmic" or "normal" abundances.
System abundances, the situation might appear even worse.
There are stars like R CrB (whose atmosphere is evidently The nIext step is to account for the observed abundances
about half helium and half carbon) and a2 CVn (whose in terms of nuclear processes and their products. Modern
spectrum shows 1000 times as much Eu as our best guess discussions of this problem build on two fundamental
at the solar value), and large numbers of Population II approaches. The first is synthesis in an early, dense phase
stars whose entire heavy element abundance, (i.e. every- of the universe, as formulated in the work of Gamow and
thing but hydrogen and helium) is a percent or less of his colleagues (Alpher et al., 1948, often called Alpha,
the solar. Beta, and Gamma; Alpher and Herman, 1950, 1953).This
is now believed to be responsible for most of the synthesis
of the very light nuclides ('H, 'He, 4He, and very pos-
But, in fact, once allowance is made for some secondary sibly I.i). The second is nucleosynthesis in stars, as for-
processes, abundances are remarkably similar nearly every-
where we look. The meteorites, for instance, have obviously mulated by Burbidge, Burbidge, Fowler, and Hoyle (1957,
lost much of their volatile material (H, He, C, N, O, and
'Astronomers hav"e customarily called all elements but hydrogen
the rare gases), but the relative proportions of the heavier and helium "metals. There isprobably nothing that can bedone about
elements in the Type I carbonaceous chondrites (those this.
Rev. Mod. Phys., Vol. 47, No. 4, October 1975
Virginia Trimble: The origin and abundances of the chemical elements 87S
usually known as B'FH) and Cameron (1957).They con- et a/., 1973). We should not, therefore, be surprised by
sidered seven processes which are (with some modifications) the high D/H ratios (up to 6 && 10 ') found for HCN in
now believed to occur in stars, the earlier ones (hydrogen the Orion Nebula by Jefferts et al (.1973)and interpreted,
and helium burning) being associated with well-defined again in terms of exchange reactions, by Solomon and
evolutionary stages and definite positions of the stars on Woolf (1973) as being consistent with the value of D/H
a color-magnitude (Hertzsprung —Russell) diagram. The = 1.4 Q 10 ' in the atomic phase of the present inter-
others appear to occur in later, more rapid stages of evo- stellar medium (Rogerson and York, 1973).
lution, generally of massive stars. Finally, they postu-
lated an x process, which was necessary to account for 2. The solar corona/photosphere iron discrepancy
the observed amount of Li, Be, and B (which tend to
For many years, the coronal iron abundance (as well
be destroyed rather than produced in stars because they
as the meteoritic) seemed to be about 7.8 (Pottasch, 1964),
burn at low te'mperatures). This is now attributed to =
0 on the logarithmic scale where H j.2.00, while the
spallation of C, N, and in the interstellar medium photospheric abundance was more like 6.5, prompting
by passing cosmic ray protons and alpha particles (and
various theories of separation in the solar nebula as well
conversely). These processes and their probable sites and
products are discussed in Sec. III. The section concludes as in the Sun itself. Improved values of the oscillator
with a discussion of how to sum over the ~arious zones strengths (f values) for the permitted lines of Fe (e.g.,
of a star to determine its total yield of heavy elements, Bcrreiadsgeeds athned pWhoietoses,phe1r9ic70;aWbuonldnaicnkce et taol.,7.139—771.6) haanvde tihne-
and how to sum over stars of various masses to get the
discrepancy to a tolerable level. Additional recent de-
yield per generation.
terminations of transition probabilities for Fe are given by
Smith et al. (1970), Whaling et al. (1970), and Martinez-
Thus, very broadly, we must think in terms of material,
Garcia et al. (1971), and the solar abundance derived
with an initial composition determined by the Big Bang,
from them by Smith and Whaling (1973). Ni has had
condensing into galaxies. Then the first generations of f
a similar, but less spectacular history. The values are
stars more massive than the Sun undergo nuclear reactions
and return the products to the interstellar medium. Suc- given by Lennard et al. (1973) and the solar abundance
derived by Lennard et al (1975)..
cessive generations of stars, including the Sun and its
planetary system, will then be formed from the material
It might seem that most of the determinations of rela-
thus enriched in heavy elements. The pioneering work of
tive metal richness or poorness in stars, which are usually
Eggen, Lynden-Bell, and Sandage (1962) made clear that
expressed and frequently measured as log(Fe/H) relative
this process was accompanied, in the case of our Galaxy
to the same ratio in the Sun, would now be seen to be
(and presumably other spirals), by the collapse of an nonsense. But luckily the same (bad) f values were used
initially spherical gas distribution to a Aat disk of gas
for both solar and stellar studies, so that relative abun-
and stars. Models of the chemical and dynamical evolution
of galaxies of various types will be discussed in Sec. IV. dances are nearly unaffected.
Section V summarizes the present state of these various 3. Weak He lines in QSOs and at the
problems and concludes with exhortations to observers, galactic center
experimenters, and theorists. The helium emission lines are sometimes much weaker
relative to the hydrogen lines in quasistellar objects than
II. OBSERVED ABUNDANCES
in typical emission nebulae (e.g., Wampler, 1968).Bahcall
A. The problem of the retrieval of meaningful data and Oke (1971), among others, interpreted the observed
line strengths as implying a ratio by number He/H as
The chief difhculty in detenmning the observed stan- low as 0.003 (for 3C273) and 0.01 for some other objects.
dard abundance distributions is deciding just which bits of If this represented the original material of the QSO's,
the wealth of available raw data really belong to the it would be a serious objection to synthesis of helium in
problem and which are telling us about some process other ' the.early universe. Williams (1971)points out, however,
than nucleosynthesis. Unfortunately, not everyone agrees that a blob of gas which is (a) ionized by a synchrotron
on which bits of data belong in which pile. YVe discuss (power-law) spectrum of ultraviolet photons and (b) op-
here briefly data which the majority of speakers at the tically thick in the Iyman continuum of both Ho and He+
Institute (NATO, 1974) believe do not belong to the will show weak He lines even for a "normal" ratio He/H
problem. = 0.1. The assumptions are likely ones in light of the
1. D/H in molecules radio and (probably) optical synchrotron radiation of
QSOs and the large amount of gas required to produce
For many years, the only available abundance for deu- their emission lines. Shields (1974) has pointed out that
terium came from terrestrial (Craig, 1961) and meteoritic the apparent variation in He/H in Seyfert galaxies (He/H
(Boato, 1954) water, which give D/H = (1.5—1.6) )& 10 4, = 0.09—0.23 from conventional analysis) is probably also
an uncomfortably high value for some theorists. This in- an ionization effect.
spired a discussion by Geiss and Reeves (1972) of the
probable preferential formation of HDO over H~O by Early reports (Mezger et a/., 1970) of a low He/H ratio
molecular exchange interactions. They estimated a pre- at the galactic center, based on radio recombination lines,
'
fractionation ratio of 2.5 )& 10 for the solar nebula. This also caused some theoretical distress to believers in "cosmic"
=
is more or less confirmed by a measurement of D/H 2 helium, But again, the distress seems to have been un-
'
)& 10 in the hydrogen molecules in Jupiter (Trauger necessary. The catch in these low resolution observations
Rev. Mod. Phys., Vol. 47, No. 4, October 1975
Virginia Trimble: The origin and abundances of the chemical elements
is that the He+ and H+ lines really come from two Strom- 4. Peculiar A stars (including helium-weak and
gren spheres of ionized helium and hydrogen, respectively. helium-strong B stars, Mn—Hg stars, Ap stars of
(Lines from He++ are never seen in norma. l HII regions. ) the Si x 4200 and Eu—Cr—Sr types, Am stars, and
There is direct observational evidence that this is a prob- their Population il analogs)
lem. The measured He/H ratio in the Orion Nebula is
normal near the center and gradually declines to essentially This heterogeneous and improbable group comprises
zero about one parsec away from the center (Churchwell, main sequence (or almost so) Population I stars from
in NATO, 1974),implying that the He line emitting region about 80 to F5 and some Population II stars of similar
is smaller than the H line emitting region. With poorer temperature, which display a wide variety of surface
resolution, this would look like low He/H. There are at abundance anomalies not attributable to unusual ioniza-
least two circumstances under which the helium Stromgren tion or excitation conditions. They include a few percent
sphere is expected to be smaller than the hydrogen one. of the early and mid B stars, about 10% of the late B
These have been discussed by Mezger el al. (1974) and and early A, and up to 30% of the main sequence near
the observations reexamined in light of the discussion by A5—6, declining to a low percentage of all stars again
Churchwell et al (19.74). First, a star with an equivalent by F2. They have in common low rotation velocities
blackbody temperature less than about 37000'K (later (less than about 100km/sec compared to about 200 km/sec
than 09) will emit enough fewer helium-ionizing photons for normal hot main sequence stars) often indistinguishable
(228—504 A) than hydrogen-ionizing photons (504—912A) from zero rotation, and ratios of radiation pressure to
surface gravity exceeding that of a 1.5 Ko unevolved
to give a smaller He sphere. This may be relevant for some
star. The incidence of spectroscopic binaries appears to
spiral arm Hn regions and perhaps 30 Doradus in the
be 100% among the Am's (Abt, 1965), normal among
Large Magellanic Cloud (C.Cesarsky, in NATO, 1974)but the Mn—Hg stars, and unusually low (20% or less)
probably not for the galactic center. The problem is further among the Si and Eu—Cr—Sr types (Abt and Snowden,
discussed by Rodriguez et ul. (1975), who include the 1974). The visual binary frequency is normal for the
e6ects of helium abundance in the ionizing star. latter two groups. Most or all Ap's show evidence of
magnetic fields in excess of 1000 G when the lines are
The second thing that can mislead us in this way is sharp enough to measure, and they are frequently variable
dust which preferentially absorbs the shorter-wavelength in. luminosity, field strength, and spectrum with the same
helium-ionizing photons. Mezger et al. (1974) point out period for all variations. The other types have magnetic
that, where this is important, there should also be con- fields less than 100Gand no strong evidence for variability.
siderable infrared emission. Again, there is evidence for Observations of Ap stars have been reviewed by Preston
(1974) and by Jaschek and Jaschek (1974). Among tQ,
such a correlation. We can, for iristance, compare M17,
which shows essentially normal helium (He/H = 0.092) types of anomalies found are:
and little or no infrared emission by dust, with W3, which (a) Enhanced helium, accompanied by abnormally high
is a strong ir source and shows variable He/H across the 0 and N, but normal C, Si, Mg, and Al in a few very
nebula with the He disappearing completely in places hot B stars, for instance fT Ori E, studied by Osmer and
(Churchwell, in NATO, 1974). This mechanism can ac- Peterson (1974).
count for the apparent absence of He at the galactic center
if the ratio of dust to gas there is about 1.0 times what (b) Helium apparently deficient by a factor of 5 or more
it is in spiral arm regions. Alternatively, or in addition, among cooler B stars (Norris, 1971), accompanied by
the dust could have a different dependence of absorption other anomalies in P, Ti, and CNO. An interesting example
coe%cient on wavelength there, so that it was about 10 is 3 Cen A, in which 'He/'He is greater than unity (Sargent
times more effective at absorbing He-ionizing photons. and Jugaku, 1961) and P is greatly enriched. A more
typical example is ~ Sco in which weak He ('He!) is ac-
There is observational evidence for both effects. The ir
companied by enhancements of Sr, Cr, and Ti (Jugaku
emission from the galactic center suggests that the dust and Sargent, 1961).No stars are found to have weak He
to gas ratio there is 2—3 times higher than in spiral arms and be otherwise normal.
(Cesarsky, in NATO, 1974), while the anomalously high
strength of the 9.7p, silicate absorption feature in the (c) An overabundance of Mn and Hg (which defines
direction of the galactic center compared to the visual the class as Mn or Mn—Hg stars) among late B stars with
extinction there suggests unusual wavelength dependence a general enhancement of the iron peak and heavier ele-
of the absorption cross section. LThe visible and infrared ments (except Cr), especially gallium and yttrium. Typical
extinction by small silicate particles has recently been examples are 53Tau (Aller and Bidelman, 1964)and K Cnc
remeasured by Day et al. (1974), who find the infrared (Sargent, 1964).Helium is somewhat underabundant (Auer
e%ciency at least 3 times higher than had previously el al., 1966). Mn stars are discussed by Wolff and Wolff
j (1974), and Cowley and Aikman (1975) have pointed out
been supposed. Thus the low He/H ratio observed for
that some of the anomalies cannot be accounted for by any
- some Hu regions near the galactic center probably reflects reasonable nuclear process, because odd Z's are more
dust distribution rather than low abundance. A genuinely abundant than even Z's.
low He abundance cannot be conclusively excluded by
these considerations, and more work is clearly required. (d) An overabundance of Si in late B to middle A stars
He/H appears norinal in one galactic center Hn region, (defining. the class as Si stars or X 4200 stars), accom-
Sgr A West (Pauls eE a/., 1974). panied by weak He, C, O, and. Ne and an enhanced iron
Rev. Mod. Phys., VoI. 47, No. 4, October 1975
Virginia Trimble: The origin and abundances of the chemical elements
peak except for Mn and Ni. These are probably a higher It now seems probable that all:of these theories should
temperature variant of the next group. be rejected. The arguments discussed by Michaud (1970),
by Pikel'ner and Khokhlova (1971),and by Vauclair and
(e) Large enhancements of the elements beyond Fe, Reeves (1972) with special reference to 'He production
especially the rare earths (hence the name europium stars), by spallation, include the large amounts of energy required
accompanied by normal iron peak abundances, among A for surface nuclear reactions, the absen,ce of stars with
stars. Adelman (1973) has studied a number of these normal spectra and strong magnetic fields, and the details
classic Ap stars, ofwhich the best known example iso,2CVn, of the anomalies and their correlations with the tempera-
and finds that many of the anomalies correlate with ef- ture and surface gravity of the stars rather than with
fective temperature. Even more surprising is the great evolutionary status.
relative excess of the heavy isotopes of Hg (Preston, 1971)
and Pt (Dworetsky and Vaughan, 1973), amounting in A promising alternative mechanism involving diffusion
'"Pt.
some cases to almost 100% cf '04Hg or The excess in the stars' atmospheres was originally proposed by
of neutron rich isotopes is greatest in the cooler stars. Greenstein et al. (1967) to account for weak He in old
halo B stars. Michaud (1970) considered diffusion in the
(f) Deficiencies of Ca and Sc, sometimes accompanied atmospheres of Ap stars and was able to account for the
0
by a general enhancement of Fe and heavier elements. deficiencies of He and and the overabundance of Si, Mn,
Conti (1970) has discussed both the subtle Am stars like and many other heavy elements in the temperature regions
Sirius and the more extreme ones like r U Ma and picked where they are observed, provided that the atmospheric
out weak Scas the defining characteristic of the class. turbulence is sufhciently low. The excess of 3He in 3 Cen A
and other stars can also be accounted for in this way
(g) Apparent weakness of everything but hydrogen (Vauclair et a/., 1974), since being lighter, it is less prone
among A and early F stars that are, dynamically, Popula- to sink -than 4He. A modification of the mechanism by
tion I and rapid rotators. The prototype star, P Boo, was Smith (1971)and Watson (1971)also considers the lifting
discovered by Morgan et al. (1943), and the class is dis- of certain elements by radiation pressure and the sinking
cussed by Sargent (1968). of others under the inAuence of gravity, but in a stable
radiative zone above the Hen ionization region and below
(h) Population II analogies of most of the above types. stheeenhyindroAgemn stcaornsveacstioan fuzonncteionin Aofsttaerms.peTrahtuereabunadreancweesll.
wFoearkinssttaanr;ce3,8FDeriagceon8i6s (aSMargnensttara;ndanSdeaBrlDe, +1956'84)26is8aaHSei reproduced, even at the low temperature end (Smith, 1973).
4200 star.
P Further work (Michaud, 1973, 1974, and in NATO,
1974) seems to indicate that the anomalies can be built
There are several reasons to conclude that only the up on a reasonable time scale (e.g., 10' years) and that
surface layers of these stars are involved in the abundance large excesses of either the heaviest or the lightest isotopes
anomalies. First, they all have temperatures such that of some elements will result, depending on whether the
there will be no large surface convection zone, and nearly dominant effect is lifting an element into the line forma-
all are slow rotators or have strong magnetic fields (or tion zone or blowing it off the star completely. Atoms
both), which also tend to reduce mixing. Second, even a
can be trapped in the line formation region either by
few supernova explosions of stars whose whole volume magnetic fields or by recombining to an ionization state
was as much enriched in rare earths as the surface of
that has few lines in the part of the spectrum where the
~' CVn would raise the average abundance of these things star is brightest. Additional work, including non-LTE
in the galaxy well above what is observed in the solar effects and detailed models for particular stars, is in
system. The concentration of certain of the enhanced
progress.
abundances in spots on the stellar surface in the spectrum
variable Ap stars, the close correlation of some anomalies
with surface temperature, and the similarity of the e6ects 5. Low energy solar flare particles
in Population I and Population II stars also seem incon-
It has recently become possible to determine the com-
sistent with the whole mass of the star being involved,
position of particles from solar Hares do'wn to energies
as in the model of Fowler er al. (1965), in which surface
less than 1 MeV/amu. Typical trends include a ratio
spallation and interior nuclear reactions are followed by
Fe/He gradually increasing from close to the photospheric
mixing. Explanations involving nuclear reactions have, value at 20—30 MeV/amu to 10 or 20 times it at a few
therefore, concentrated either on surface nuclear reactions,
as discussed by Brancazio and Cameron (1967, who con- McV/RIllll RIld thc appeal allcc of fluctuations 111Hc/CNO
sidered reactions of seed nuclei with alpha particles ac- below 1 MeV/amu which do not occur at higher energies
celerated in asurface magnetic field of 10' ' G) and Xovikov (Price, 1973). Since we know that these particles come
and Sunyaev (1967, for 'He) and more recently by Ku- from the Sun, there is no ambiguity in attributing abun-
chowicz (1971)and Tjin a Djie et al,. '(1973), with special dance variations to the acceleration mechanism. This must,
of course, also be true for more energetic solar Rare par-
reference to promethium, or on contamination due to the
ticles, but since they provide our primary standard for
supernova of a binary companio, the accretion being
influenced by the magnetic field of the Ap star, as dis- the Solar System abundance of He and Ne and may also
cussed in two series of papers by van den Heuvel (1967) have something to tell us about how acceleration affects
and by Guthrie (1970,1971a,b, 1972).Drobyshevskii (1974) cosmic ray abundances, they are discussed in Sec. II.8.4.
has presented yet another model of this type. below.
Rev. Mod. Phys., VoI. 47, No. 4, October 1975
Virginia Trirnble: The origin and abundances of the chemical elements
6. Galactic cosmic rays (as observed) finement must be made on the basis of an independent
measurement of the age of the cosmic rays. A suitable
The gross distribution of the chemical elements in the clock exists in the form of ' Be, which is produced at a
cosmic rays coming from outside the Solar System in
well-known ratio to the other spallation products, and
those energ—y ranges where we have detailed information then decays with a half-life of 1.6 Q 106 yr. Other sec-
(100 MeV a few GeV/amu) is very much like that of ondary nuclei with half-lives between 10' and 2 10'
everything else: a great predominance of H and He (with &&
Haned/H0,aabnoduta0g.r1adbuyalnudmecbreera)s,e aobfouatbun1d%ancaes mwuitchh aCto,mNic, "yCealr,s awndhich"Mmni.ghtUnafolsrotunbaeteluys,efulindinivitdhuiasl regisaordtopeareab"uAn-l,
dances cannot be measured for any of these at the present
number, interrupted by the Fe peak, towar=d Z ' 45, time, but the total abundance ratio, Be/(Li+ B) is con-
wfoiltlhoiwneda bfayctoar loefvel1in0g) beoyffondat tXha,(Zt.)/BXu(tFet)here 1a0re al(stoo sistent with part, but not all, of the ' Be having decayed,
sthoemne HsignainfidcanHtediafrfeerednecpelse.tedIfwbeynoarbmoaultizea afat,cteo.rg.,ocfar3b—o1n0,, a1n9d71a).confinement time of order 107 yr (Meneguzzi et al'.,
while the iron peak and heavier elements are enhanced by
similar factors. The cause of these effects is not clear. Above a few GeV/amu, the average ratio of secondary
It is discussed below in Sec. II.E on cosmic ray source nenuecrligdye.s Ftoorpirnismtaanricees, g(Zrad=ual1ly7—2d5e)c/r(eFasee+s Nwii)thdrionpcsreasfrinogm
composition.
about 1.2 at 1 GeV/amu to 0.2 or less above 10GeV/amu.
In addition, the large "holes" in the cosmic abundances The ratios (Li+ Be+ B)/(C + N + 0) and N/(C + 0)
at Li, Be, and B, F, and Sc to Mn are partially filled in, also seem to decline with increasing energy. In addition,
and the odd—even effects partly smoothed over. This filling (C+ 0)/(Fe+ Ni) decreases with increasing energy, al-
in of gaps is reasonably well understood on the basis of though all of these are primaries. Recent data are given
spallation, heavy cosmic ray nuclei being fragmented into by Ormes and Balasubrahmanyan (1973), Webber et at.
lighter ones by collisions with protons and helium nuclei (1973),Juliusson et al. (1972),and O'Sullivan et al. (1973),
in the interstellar medium. Thus, 4He gives rise to 3He and discussed by Audouze and Cesarsky (1973),Meneguzzi
and 'H; C, N, and 0 to Li, Be, and B;Ne, Si, and Mg (1973), and Price (1973).A consistent explanation of all
to F; Fe to Sc, Ti, and V; nuclei in the Pb and Pt peaks the data is obtained if the mean path length traversed
to nuclei in the range Z = 60—75, and so forth. %eshould, by the cosmic rays decreases from 6—7 g/cm' at 1—2 GeV/
therefore, be able, at least in principle, to take the ob- amu to less than 1g/cm' at 100 GeV/amu, with xo varying
served abundance of each isotope and extract the abun- as E ' to E '. The effect of this on secondary to primary
dances that must have left the source. In practice, this is ratios is obvious. (C+ 0)/(Fe + Ni) is also affected
not a trivial operation, since there are, for instance, about because the iron peak nuclei are more readily broken up
1000paths that lead from Fe to Li, most of them involving by spallation than carbon and oxygen, thus at low ener-
spallation cross sections that have never been measured. gies, where much of the Fe and Ni have been eroded
A general scheme for extracting the source abundances away, the (C+ 0)/(Fe+ Ni) ratio looks artificially high.
ias sdeimsciuesmsepdiricably Sahpapprioraochandto Sgilebtteirnbgergma(n1y970o,f 1th9e71n),ecaensd- Tfahcatt ththaits tihsethreaticoorr(ecCt+ex0p)la/n(aFtieon+ Nisi+indZica=ted17—b2y5)thies
sary cross sections by Silberberg and Tsao (1973a,,b). about the same at all energies observed in the range 1—50
Shirk et al. (1973) have adopted the alternative approach GeV/amu. This means that Fe and Ni are gra,dually
postuferlelaasrsrumsppirnoagcceessaa)nndainncdiotimaplproarpciaongmgaptiontsghietionrietsufol(tersw.ga.wr,ditShotlhatrrhoeuSgyhosbtseeminrvteerdo-r witnrhadniilcsefaotertmhdeedbdyesttr(ouLcitlii+ognhtBeroef+nCu8c+l)e/i(0Cwii+sthle0sis)n,cirmweahpsioicnrhtganitsspleaLslaslastitiohtna,nis
abundances. The derived source abundances are discussed 0.3 a,t all energiesg.
in Sec. II.E.
The ratios at the highest measured energies tend to
The amount of interstellar matter that must be tra- resemble those in the calculated source abundances. It
versed to produce the observed amount of spallation varies therefore seems hopeful that detailed composition measure-
with both the atomic number and the energy range con- ments at sufficiently high energies may eventually be able
sidered. For .instance, to produce the Li, Be, and 8 seen to tell us the source abundances directly. The highest
from C, N, and 0 requires 5—7 g/cm', assuming He/H energies presently available preserve the main features
= 0.1 in the interstellar medium (Shapiro and Silberberg, of H and He depleted by about a factor of 10, and Fe
1971),while Comstock et al. (1972) find that the observed and heavier elements enhanced by a factor of 3—10 if we
'H and 'He can be 'made from 'He by 4 g/cm'. The per- normalize at carbon.
sistence of nuclei as heavy as U and Th requires that
some'material have come through much less than 1 g/cm', B. Solar system abundaoces
since they are very easily destroyed. All of the data for 1. The Earth —Moon system
energies below a few GeV/amu can be fitted by an ex-
=
ponential distribution of path lengths, N(x) (1/x) By far the most convenient sample of Solar System
)& exp(—x/xo), with xo in the range 4—6g/cm' (Shirk et at., material available for analysis is the Earth. Unfortunately,
1973; Price, 1973).This corresponds to the cosmic rays 4.6 billion years of geologic processing have produced so
being confined in the galactic disk (with an average den- much chemical fractionation that the only statements we
sity of 1 atom/cm') for 3 million years or in the galactic can make naively are of the form that silicon seems to
halo (with an average density of 0.01 atom/cm') for 300 be a good deal commoner than europium. But on the
million years. A decision between the two kinds of con- same basis we might have said that oxygen seems to be
Rev. Mod. Phys., VoI. 47, No. 4, October 1975
Virginia Trimbie: The origin and abundances of the chemical elements
a good deal commoner than heliuTn, which will turn out adopted elemental abundances, and the same data is shown
to be entirely wrong for the Solar System as a whole, so graphically in Fig. 1. Slightly modified values for K, Ca,
that although a great deal is known about the composi- Fe, Zr, Hf, and Th, recommended by Anders (in NATO,
tion of various components of the Earth's crust (see, for 1974) are included. Some elements (especially those which
instance, the papers in Sec. V of Ahrens, 1968), the Earth are gases under normal circumstances) are greatly depleted
is not a pnmary standard for any chemical abundances. in all types of meteorites, and their abundances must be
Rather, effort has gone into trying to account for the obtained in other ways. We discuss these first, before
composition of the Earth in terms of condensation from returning to the general question of the reliability and
a solar nebula with a composition determined in other meaning of the meteoritic data.
ways, and subsequent geological processing.
0
H, C, N, and are observed spectroscopically in the
Terrestrial materials are, however, our primary standard Sun. Thus, if we believe that the solar surface is a good
for the ratios of isotopic abundances within individual sample of its total composition Lneglecting, for instance,
chemical elements. All isotopic abundances in the com- Joss's (1974) suggestion that the surface is much enhanced
pilation by Cameron (1973) discussed in Sec. II.B.2 are in heavy elements due to the accretion of comet-like
terrestria, l, except those of H, He, Ne, Xe, and "Ar (the material, and Hoyle's classic remarks about the surface
last being an interpolation). In addition, those isotopic composition of chimney sweeps), the only problem re-
abundances vhich involve the parent or daughter -nuclide maining is one of normalization. This arises because solar
. of radioactive decay processes have been corrected back abundances are determined fairly directly with respect to
to the birth of the Solar System. It is belie~ed that isotopic hydrogen (which supplies most of the continuous opacity)
ratios (except for the very light elements; see the dis- and are usually expressed on a scale in v,hich logX(H)
cussion of D/H in Sec. II.A.1) are not greatly affected = 12.00, while meteoritic abundances are customarily
by chemical processes. A typical result is that of Reynolds cletermined and given vith respect to silicon, with X(Si)
(1953), who found that the ratio 'Si/SSi did. not vary = 10', Cameron's (1973) solution to the problem was to
by more than 0.2% from his standard value for olivine take the average of the ratios of the meteoritic to solar
(obtained from dunite from North Carolina) in any of (as given by Withbroe, 1971) abundances of the eight
.a wide variety of minerals. The standard compilation of elements Mg, Al, Si, P, S, Ca, Fe, and Ni as his normal
isotopic abundances, which was used by Cameron (1968, ization constant. This results in X(meteoritic scale)
1973), is that of Fuller and Nier (1965).The same data = 0.03175 X 1V(solar scale) for any element. Other authors
is also available in, e.g., any recent edition of The Ham/- have normalized at Sialone, which. appears less satisfactory.
book of Chemistry and Physics or The American Institute
of Physics Hamtbook. He and Ne do not appear in either meteorites or the
solar photosphere (because of their very high first excita-
The Moon appears to be even more severely fractiona ted tion potentials), but do occur in energetic particles from
than the Earth, being relatively more depleted in volatile solar Aares. The observations are discussed in Sec. II.B.4.
elements and enriched in those with low condensation Cameron has adopted the results of Bertsch et al. (1972),
temperatures, by factors. of about 10 and 3, respectively who give Ne/0 = 0.16~ 0.03 and He/0 = 103~ 10. In
(Ganapathy and Anders, 1974). Again the primary effort viev of the variable composition of solar Rare particles,
has gone into accounting for the observed surface corn- these are clearly:. rather uncertain, and cannot be thought
position in terms of condensation processes and a theory of as inconsistent with any other values (e.g., solar corona).
of subsequent fractionation. A recent review of the prob- He/H is well determined in other stars and the inter-
lems is given by Anderson (1973);it is not even absolutely stellar medium (see Secs. II.C and II.D) and appears to
clear whether the interior of the Moon is richer or poorer be constant at 0.1 where not aRected by processes in the
in iron and associated elements than the surface. Of course, objects themselves. Ne is much more difficult, and "well
I.
the same thing could be said of the Earth, if yttleton's determined" modern values range at least from 2.5 )& 10'
(1973 and references therein) phase change theory is (for nearby Hn regions, Peimbert and Costero, 1969) to
allov ed as a viable alternative to the conventional iron— 1.3 X 10' (for v. Sco, analyzed by Hardorp and Scholz,
nickel core. There are no significant (for nucleosynthesis) 1970), v ith the solar coronal value (10') on the low end
Earth—Moon differences in isotope ratios, apart from those of the range. It is hard to take the variations very seriously.
involved in radioactive decays, cosmic ray production
and the like. Four elements, Ar, Kr, Nb, and Xe, are not observed
v ell anough anywhere in the solar system for their abun-
2. Meteorites
dances to be measured. Cameron's adopted values for
Meteoritic data is the primary source for the standard these are interpolations between nearby nuclei on the
solar system elemental abundances. It also contains in- basis of the systematics of nuclear reactions. Their abun-
formation on possible interstellar components in the Solar dances cannot, therefore, be used as tests of theories of
System and on extinct radioactivities. nucleosynthesis, although their isotope ratios can be.
Kr, Nb, and Xe are not well enough observed anywhere
a. The standard elemental abundance distribution. The else in the universe for convicts to arise, but there is some
standard Solar System abundance distribution (Cameron, evidence (discussed at the end of Sec. II.B.3) that the
1973) is obtained primarily from 'meteoritic data, espe- adopted value for argon may be too high by a factor of
cially the Type I carbonaceous chondrites (those which about 4. The adopted values for S, Cl, and K have also
show the least evidence of remelting after c6ndensation). been influenced by considerations of nuclear systematics,
The first column of Table I shows essentially Cameron's so that the adopted values for Cl and K are about a f'actor
Rev. Mod. Phys., Vol. 47, No. 4, October 1975
884 Virginia Trimble: The origin and abundances of the chemical elements
TABLEX. Relative abundances by number of the chemical elements in the meteorites, solar photosphere, solar corona, and galactic cosmic ray
sourc'es. The abundances of Th and U have been corrected to the time of formation of the Solar System. The solar abundances are normalized
at hydrogen and the cosmic ray source at carbon. The meteoritic data are those of Cameron (1973)and include solar values for boron and the
volatile elements; the photospheric data are those of Engvold and Hauge (1974);the coronal data are those of Withbroe (1971)modi6ed by
subsequent measurements discussed by Engvold and Hauge (1974)and byWalker eIal. (1974a,b).The cosmic ray source data are those ofShapiro
and Silberberg {1974}and Price (1973).Qis the quality ofthe photospheric determinations as estimated by Engvold and Hauge (1974).arepre-
sents errors ofless than 30/q, b less than afactor of2.5,and cless than afactor of 10.Unstable elements are omitted.
So1ar Solar
Z, Element Meteorites photosphere corona Cosmic ray sources
H 3.18 X 10" 2.5 X 10" 2.5 X 10" 4.1 X 10'
2 He 2.21 X 109 2 X 109 b 2.0X 109 3.1 X 108
3 Li 49.5 0.2 b 0~~
4 Be 0.81 0.2 b ~~~
5 B 3.2 &4.0 b ~~ ~~~
6 C 1.18 X 10' 107 1.4 X 10~ 1.18X 10'
7 N 3.74 X 10' 3 X 10' 2.8X 106 1.3 X
8 0 2.15 X 107 1.6X 10~ 2.0X 10' 1.3 X 107
190 NFe 3-44245X0 106 1010006 cb 1.7~X~~106 1 8~X~1~ 0s
11 Na 6.0 X 10' X 10 5.3 X 104 9.4 X 104
12 Mg 1.061X 10' 8 X 10' 9.4X 10' 2.7 X 10'
13 Al 8.5 X 104 X 10' 7.9X 104 2.4 X 10'
14 Si X 10' 10' 1.1 X 10' 2.4 X 10'
15 P 9600 10' b 7100 2.4 X 10'
16 S 5.0 X 10' 4 X 105 3.5 X 105 3.5 X 10'
17 Cl 5700 8 X 10' b ~~~
18 Ar 1.172X 10' 2.4 X 104 c 8 X 104 8.3 X 104
19 K 3790 8 X 10' c 1.4 X 104
20 Ca X 104 X 10' 6-3X 104 2.6 X 105
21 Sc 35 30 b 315
22 Tl 2775 1600 b 5000
23 V 262 250 b 1.6X 104
24 Cr 1.27 X 104 1.6X 10' b 2.8 X 10~ 3.5 X 104
25 Mn 9300 6000 b 8900 2.4 X 104
26 Fe 8.9 X 10' 6 X 10~ b 8.2 X 10' 2.6 X 10'
27 Co 2210 800 b 5600
28 Ni 4.80 X 10' 8 X 104 a 8.5 X 10' 9.4 X 104
3209 ZCnU 1524404 460300 bb 1~0~0~0 977.540 X(Z1=0'30—31)
31 Ga 48 16 b
32 Ge 115 80 b ~X~ ~ &0 (Z = 32 —34)
33 As 6.6 No lines
34 Se 67.2 No lines
35 Br 13.5 No lines 113(Z = 35 —39)
36 Kr 46.8 No lines
37 Rb 5.88 10 b
38 Sr 26.9 20
39 Y 4.8 1.6 b
40 Zl 15.1 16 b 61 (Z =40 —44)
41 Nb 1.4 5 b
42 Mo 4.0 8 b
4445- RRuh 01.9 02..85 ,bb ~~~ 0.9 (Z = 45 —49)
46 Pd 1.3 0'6 b
47 Ag 0.45 0.2 b
48 Cd 1.48 2 b
4590 XSnn 03..6189 01..38 bb 14.4 (Z = 50—54}
51 Sb 0.316 0.25 b
TIe 61..4029 NNoo lliinneess ~~~~~~
54 Xe 5.38 No lines
55 Cs 0.387 &2.0 10.6 {Z= 55 —59)
56 Ba 2.5
57 La 0.445 1.6
58 Ce 1.18 2.0
59 Pr 0;149 1.0
60 Nd 0.78 1.6 0.5 (Z = 60—64)
62 Sm 0.2~6 1.3
63 Eu 0.085 0.13
64 Gd 0.297 0.3
65 Tb 0.055 Nofvalues 0.2 (Z = 65 —69)
66 Dy 0.36 0.3
Ho 0.079 Nofvalues
Er 0.225 0.16
Rev. Mod. Phys., Vol. 47, No. 4, October 1975
Virginia Trimble: The origin and abundances of the chemical elements 885
TABLE I. (Continued)
Solar Solar
Z, Element Meteorites photosphere Q corona Cosmic ray sources
69 Tm 0.034 0.06 b
70 Yh 0.126 0.16 c ~ ~~ 1.2 (z = 7o —44)
71 Lu 0.036 0.16 b
72 Hf 0.15 0.2 c
73 Ta 0.021 No lines
7754 WRVe 00..01563 &100.01 ch 13.4 (Z = 75 —79)
76 Os 0.75 0.16 c
77 Ir 0.717 4.0 c
78 Pt 1.4 3.2 c
8709 AHug 00.4202 &03.13 bb 5.6 (Z = 80 —84)
81 Tl 0.192 0.2 b
82 Pb 4 2.0 a
83 Bi 0.143 &2.0 b
90 Th 0.045 0.20 c 6.1 (Z = 9O —94)
92 U 0.0262 &0.27 c
of 2 higher than the average of all measured values, while the majority of the elements, only Type I carbonaceous
Sis about the same as the measurements in Cl meteorites. chondrites are considered. Sc, Os, and-Pt come from the
mean of all types of chondrites. The relative abundances
The boron value in Table I is that of Boesgaard et al. of the rare earths are taken from ordinary chondrites, but
(1974) for Vega, while that in Fig. 1 is a modified mete- their sum normalized to the value for C1's. Rh is taken
=
oritic abundance (B 350) given by Cameron (1973). from ensta,tites and high-iron ordinary chondrites (in the
Cameron now believes this meteoritic abundance to be absence of C1 data); and Hg from enstatites, in view of
too high (private communication to H. Reeves, 1974).We the great variability of the C1 data.
will return to this point at the end of Sec.II.B.4.
The use of meteorites, especially Type I carbonaceous
All other adopted values are based entirely on mete- chondrites, as the primary Solar System abundance stan-
oritic data, whose sources Cameron (1973) references. For dard has been vigorously and convincingly defended by
I I I I I I I
IO
z 5—
0
0—
-2 5l IOI l5I 2l0 2I5 3I0 3I5 4l0 4l5 5l0 5I5 6l0 6l5 7I0 7I5 8I0 8l5 9I0
ATOMIC NUMBER
FIG.1. The abundances by number ofthe chemical elements in Solar System material. The abundances are largely derived from Type Icarbon-
aceous chondritic meteorites and are those given by Cameron (1973),except forslight revisions in the values for K,Ca, Fe,Zr, Hf, and Thwhich
were suggested by Aaders (in NATO, 1974).Boron is probably too high by a factor of about 100. Breaks in the line represent elements all of
whose isotopes have half-lives short compared to the age ofthe Solar System. Sources ofthe abundances are further discussed in Sec.II.B.
Rev. Mod. Phys., Vol. 47, No. 4, October 1975