Presolar Silicon Carbide Grains in Meteorites
Peter Hoppe
Max-Planck-Institut für Chemie, Abteilung Kosmochemie,
P.O. Box 3060, D-55020 Mainz, Germany
[email protected]
1. Introduction
Primitive meteorites contain small concentrations (ppb to ppm) of presolar dust grains
that have survived largely unaltered the
processes that led to the formation of the solar
system. The most important presolar minerals
identified to date are diamonds, silicon carbide
(SiC), graphite, silicon nitride (Si3N4), and
corundum (Al2O3). Diamonds are most abundant (with concentrations of up to 1000 ppm),
followed by SiC and graphite (several ppm),
corundum (< ppm), and Si3N4 (ppb). Diamonds
are only 2 nm in size. All other types of grains
are larger and range from ≈ 0.2 to 20 µm (Fig.
1). The isotopic compositions of presolar grains
are highly anomalous and vary over many
orders of magnitude, indicative of a stellar
origin of the grains. The laboratory study of
presolar grains can thus provide information on
stellar nucleosynthesis and evolution, the
galactic chemical evolution, grain growth in
stellar atmospheres or in the ejecta of stellar
explosions, and on the inventory of stars that
contributed dust to the solar system (see Fig. 2).
porated in the host minerals at the time of grain
formation. Most important are 26Al and 44Ti that
are diagnostic with respect to stellar nucleosynthesis and stellar source. It is thus possible
to get information on the presence and
abundance of radionuclides in certain types of
stars.
Stellar nucleosynthesis
and formation of
dust grains
Star
Star
Protosolar
nebula
4
He
Star
4
4
He
He
12
C
Formation of
solar system
Meteorites
Chemical & physical
separation
of dust grains
Mass
Laboratory
analyses
Figure 2. Path of presolar dust grains from
their stellar source to the laboratory. Their
isotopic compositions are determined by the
starting composition of and internal stellar
nucleosynthesis in the parent stars. After
passage through the ISM such grains became
part of the protosolar nebula. They survived the
episode of solar system formation inside small
planetary bodies and they were carried to the
Earth by meteorites from which they can be
separated by chemical and physical procedures.
Finally, they can be analyzed for their isotopic
and structural properties in the laboratory.
Figure 1. Presolar SiC grain separated from the
Murchison meteorite. The grain diameter is
about 0.5 µm.
All types of presolar dust contain the decay
products of radionuclides that were incor-
Silicon carbide is the best studied presolar
mineral phase. The reason for this is its
relatively high abundance in primitive
meteorites and its comparatively large grain
size that allows isotopic analyses of the major
and of many trace elements in individual
AGB stars, or more specifically carbon stars,
are believed to be source of the SiC mainstream
grains. Arguments in favour of C stars are: (i)
the SiC mainstream grains have 12C/13C ratios
similar to those found in the atmosphere of C
stars (see Fig. 3), (ii) many trace elements
present in the SiC mainstream grains carry the
signature of the s-process, (iii) C stars have
been successfully invoked to explain the
abundances and isotopic patterns of the grains’s
nobel gases, (iv) they are considered to be the
most prolific injectors of carbonaceous dust
into the ISM, and (v) they are observed to show
the 11.3 µm emission feature typical of SiC. On
the other hand, based on their C-, N-, and Siisotopic compositions, presence of now extinct
44
Ti at the time of grain formation, and the
signature of r-process nucleosynthesis in the
isotopic patterns of heavy trace elements, the X
grains are believed to have formed in the ejecta
of type II supernova explosions.
For more detailed informations the reader is
referred to some recent reviews on the subject
of presolar grains found in meteorites [1-6].
2. Carbon-, Nitrogen-, and Silicon-isotopic
Compositions of Presolar SiC
Mainstream Grains: The SiC mainstream
grains have 12C/13C ratios between 10 and 100
and 14N/15N ratios between 50 and 20,000 (Fig.
3). Most of these grains show the imprint of the
CNO cycle, i.e., enrichments in 13C and 14N
relative to solar abundances. The Si-isotopic
compositions of most mainstream grains are
characterized by enrichments in the heavy Si
isotopes of up to 200 ‰ relative to their solar
abundances, falling along a line with slope ≈
1.3 in a three-isotope-representation (Fig. 4). It
is the preferred interpretation today that the
slope 1.3 Si correlation line primarily reflects
the GCE of the Si isotopes, both in time and
space, and represents the starting compositions
of a large number of C stars.
105
SiC Mainstr.
SiC A&B
SiC X
SiC Y
SiC Z
CNO cycle
14
N/15N
104
Extra-mixing
(e.g., CBP)
103
C stars
102
10
He burning
in massive stars
Novae
100
10-1
100
102
10
12
103
104
105
C/13C
Figure 3. C- and N-isotopic compositions of
presolar SiC grains.
400
slope 1.34
line
200
δ29Si/28Si (‰)
grains. In addition to the major elements C and
Si isotope data exist for N, Mg, Ca, Ti, the
noble gases, and heavy refractory elements
(e.g., Sr, Zr, Mo, Ba, Nd, Sm). On the basis of
the isotopic compositions of C, N, Si, and the
abundance of radiogenic 26Mg six different
populations of SiC grains can be discerned: The
mainstream grains, which make up the majority
of the grains (≈ 90% of the total), and the minor
types A, B, X, Y, and Z. The mainstream and
type X grains are of particular interest and this
paper will focus mainly on those grains.
0
-200
-400
-600
-800
-1000
To pure
28
Si
SiC Mainstream
SiC X grains
-1000 -800 -600 -400 -200
0
200
400
δ Si/ Si (‰)
30
28
Figure 4. Si-isotopic compositions of presolar
SiC grains. The iSi/28Si ratios are given as
permil deviation from the solar system ratios.
Type X grains: The X grains typically show the
opposite isotopic signatures of those observed
in the mainstream grains (Figs. 3 and 4). Their
12 13
C/ C ratios range from 18 to 7000 but most
of them have higher than solar 12C/13C ratios.
With one exception all X grains show lower
than solar 14N/15N ratios (down to 0.05x solar).
From the nucleosynthetic point of view high
12 13
C/ C and low 14N/15N ratios are the signature
of He burning. Silicon generally shows
3. Extinct Radioactivities
After nitrogen, aluminum and titanium are the
most abundant trace elements contained in
presolar SiC (with concentrations on the order
of permil to percent). Since the proposed stellar
sources of the mainstream and type X grains
produce 26Al (C stars, SNe) and 44Ti (SNe),
radiogenic 26Mg and 44Ca can be expected to be
present in presolar SiC.
44Ti-rich
SiC X grains
SN mixing
100
Ni zone
C- and
Si-rich
zones
44
10-1
10-2
Al/27Al
Type II SN
mixing
10-2
26
10
10-1
100
10-3
10-6
AGB stars
10-3
10-5
10-4
10-3
10-2
10-1
44
Ti/Si
10-4
10-5
100
performed in the laboratory reveal strong
excesses in 26Mg and in some cases Mg even
consists of monoisotopic 26Mg. Because 26Mg
excesses are very large and because 25Mg/24Mg
ratios are close to solar it is widely accepted
that the 26Mg excesses found in presolar SiC
are in fact due to the decay of radioactive 26Al.
The inferred initial 26Al/27Al ratios of the
mainstream grains are typically between 10-4
and 10-2, roughly compatible with the
expectations for AGB stars (Fig. 5). The type X
grains have very high inferred initial 26Al
abundances with 26Al/27Al ratios of up to 0.6
and they are clearly distinguished from the
mainstream grains (Fig. 5). When compared
with expectations from type II SN mixing
models the Al- and C-isotopic ratios of many X
grains are adequately reproduced by the models
although an explanation for the highest
26
Al/27Al is still lacking.
Ti/48Ti
depletion in the n-rich isotopes 29Si and 30Si (or
alternatively enrichment in 28Si) of up to a
factor of 5 relative to solar, the signature of
advanced nuclear burning stages. The C- and
Si-isotopic compositions of the X grains are
well explained by mixing of matter from the Cand Si-rich zones (which experienced H and He
burning and, respectively, Ne and O burning) in
a type II SN. On the other hand, the
enrichments in 15N and very high N contents of
up to several wt% in the X grains are hard to
understand in view of current SN models. This
might point to deficiencies in the current
understanding of the production of 15N in SNe
or of the condensation behaviour of N in SN
ejecta.
Mainstream
Type X
Type A&B
102
10
103
104
12
C/13C
Figure 5. Inferred initial
presolar SiC grains.
26
Al/27Al ratios of
Aluminum-26: During condensation of SiC
trace elements like Mg and Al are incorporated
into the growing grains. Due to different
condensation behaviours Al is strongly
favoured over Mg leading to high Al/Mg ratios
in the SiC grains. Magnesium isotope analyses
Figure 6. Inferred initial 44Ti/48Ti and
ratios of type X SiC grains.
44
Ti/Si
Titanium-44: Similar to Mg and Al, Ca and Ti
condense along with the growing SiC grains.
While all mainstream grains have close to solar
44
Ca/40Ca ratios, some of the X grains (≈ 20%)
show large excesses in 44Ca of up to a factor of
20 relative to its solar abundance. Because the
44
Ca excesses are very large and since other Ca
isotope ratios are close to solar the 44Ca
excesses are best explained as being due to the
decay of radioactive 44Ti. The inferred initial
44
Ti/48Ti ratios range up to 0.6 and 44Ti
concentrations can be as high as 0.1wt% (Fig.
6). The data on 44Ti are in reasonable
agreement with the predictions from type II SN
mixing models. Since any mixture of the Cand Si-rich zones (that supply the C and Si for
the growing SiC grains) cannot account for the
44
Ti/Si ratios of the X grains contributions from
the innermost Ni-rich zone in the SN, which is
richest in 44Ti, appear to be necessary, implying
deep mixing of the SN ejecta.
4. Outlook
Future isotope studies on presolar SiC grains
will focus on smaller grains (≈ 0.1-0.5 µm)
since grains in this size range make up the
majority of presolar SiC. In addition, we will
investigate the homogeneity of the distribution
of extinct 26Al and 44Ti in micrometer-sized
grains. This aspect is closely related to the
question on the presence of 26Al- and 44Ti-rich
subgrains and it might be possible to get new
insights into the mixing processes in SN ejecta.
Such studies will be possible with a new
secondary ion mass spectrometer, the Nanosims
50, which will allow us to measure isotopic
compositions with a lateral resolution of down
to 50 nm. This intrument will be delivered to
the Washington University at St. Louis and to
the Max-Planck-Institute for Chemistry at
Mainz in the year 2000.
References
[1] E. Anders and E. Zinner, Interstellar grains
in primitive meteorites: Diamond, silicon
carbide, and graphite, Meteoritics 28, 490-514,
1993.
[2] U. Ott, Interstellar grains in meteorites,
Nature 364, 25-33, 1993.
[3] P. Hoppe and U. Ott, Mainstream silicon
carbide
grains
from
meteorites,
in:
Astrophysical Implications of the Laboratory
Study of Presolar Materials, T.J. Bernatowicz
and E. Zinner, eds., pp. 27-58, AIP, New York,
1997.
[4] S. Amari and E. Zinner, Supernova grains
from meteorites, in: Astrophysical Implications
of the Laboratory Study of Presolar Materials,
T.J. Bernatowicz and E. Zinner, eds., pp. 287305, AIP, New York, 1997.
[5] E. Zinner, Stellar nucleosynthesis and the
isotopic composition of presolar grains from
primitive meteorites, Ann. Rev. Earth and
Planet. Sci. 26, 147-188, 1998.
[6] P. Hoppe and E. Zinner, Presolar dust
grains from meteorites and their stellar sources,
Journal of Geophysical Research-Space
Physics, in press, 1999.