Laboratory studies of stardust: Experimental astrophysics

CHINESE ASTRONOMY AND ASTROPHYSICS ELSEVIER

Chinese Astronomy and Astrophysics 30 (2006) 235 242

L a b o r a t o r y S t u d i e s of Stardust: E x p e r i m e n t a l Astrophysics t * HSU Wei-biao Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

Abstract Primitive meteorites contain microscopic pre-solar s t a r d u s t grains t h a t originated from stellar outflows and supernova ejecta. Identified phases include nano-diamond, graphite, silicon carbide, corundum, spinel, hibonite, nitride, and silicates. Their stellar origin was manifested by their enormous isotopic ratio variations c o m p a r e d to solar system materials. T h e y are solid samples from various stellar sources, including red giant stars, A G B stars, novae, and supernovae. L a b o r a t o r y isotopic analyses of these grains provide unique insights into stellar evolution, nucleosynthesis and mixing processes, dust formation in stellar envelopes, and galactic chemical evolution. Pre-solar grains open a new observational window for astrophysical researches. K e y w o r d s : solar system: f o r m a t i o n - - s t a r s : evolution nucleosynthesis--meteorites

Galaxy: e v o l u t i o n - -

1. I N T R O D U C T I O N In 1957, four research scientists at Caltech published a classic p a p e r t h a t established the foundation for the origin of elements and their chemical evolution [1]. It was later referred as the BSFH theory, citing the initial of the authors' last names. T h e y argued t h a t except for H and p a r t of He and Li, all other elements were synthesized by nuclear reactions within stars. The Big Bang p r o d u c e d large amounts of basic particles and H atoms. W h e n the first generation stars formed, the m a j o r components of stars were H and a small a m o u n t of He t Supported by National Natural Science Foundation ~z "One-Hundred-Talent Program" of Chinese Academy of Sciences Received 2005 03-09; revised version 2005-06-28 * A translation of Acta Astron. Sin. Vol. 47, No. 1, pp. 1-8, 2006

0275-1062~06~S-seefront matter (~) 2006 Elsevier B. V. All rights reserved. DOI: 10.1016/j.chinastron.2006.07.001

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and Li. The abundances of other elements were zero. Nucleosyntheses within stars produced He, C, N, O, etc. through H-H and CNO nuclear reactions. As a result, the metallicities (the abundance of elements heavier than He) of stars increased. When the first generation stars died, their materials were ejected into interstellar medium by stellar winds or explorations and became the source for the formation of next generation stars. This process continues and the chemical abundances of elements change with time and space within the galaxy. This has been known as the galactic evolution trend (GET). Prior to the discovery of meteoritic stardust, the elemental abundances and GET were studied through the observation of emission spectrum and ages of stars. This kind of observation is not easy in practice. First, old and distant stars are very rare; second, the precision of spectrum analysis is very low. To analyze isotopic compositions of stars is even more difficult. The astrophysical studies were hampered by these limitations. In the late 1970s and early 1980s, cosmochemists isolated and identified microscopic stellar particles from primitive meteorites. Using modern micro- and nano-analytical techniques, they analyzed the elemental abundances and isotopic compositions of these particles and obtained for the first time high precision data on chemical abundances and isotopic compositions of stars. This opens a new observational window for astrophysical research: experimental astrophysics [2]. Pre-solar grains of stardust are solid samples of stars that can be studied in terrestrial laboratories. These grains provide us with a deeper understanding of the chemical evolution of the galaxy, stellar nucleosynthesis and convective mixing processes, as well as dust formation in stellar envelopes, dust processing in the interstellar medium, the formation of the solar system, and aqueous alteration and thermal processes within meteorite parent bodies [2-5].

2. T Y P E S O F P R E - S O L A R G R A I N S A N D T H E I R I S O T O P I C CHARACTERISTICS

When a star has evolved into the late stage of giant star, solid particles condense from the cooling circumstellar atmosphere and form a dust shell around the star. The thermal emission from cool dust around post-AGB objects exhibits characteristic features at 11.2, 9.7 and 18.5#m, corresponding to SiC and silicate grains [6'7]. These grains are expelled into the interstellar medium by stellar outflows and become the raw material from which new generation of stars formed. For the solar system, a large variety of stars contribute material to the solar nebula. Most of the mass was molecular gas. It was well mixed and yielded average element abundances and isotopic compositions of the solar system. The composition of the solar system does not represent any single star, but reflects the result of mixing various stellar materials. Being solid, the pre-solar grains can retain the original chemical and isotopic signatures of the parent stars. These grains were incorporated into asteroids and comets and are well preserved in the most primitive meteorites. It has long been suggested that stellar material may exist in the solar system. Early work showed that isotopic compositions of major elements in meteorites are very similar to those of terrestrial samples. Therefore, stardust in the solar nebula would have been completely vaporized and well mixed with others. Information about their parent stars has been lost [s]. In the early 1960s, Xe isotopic anomalies were observed in primitive meteorites [9].

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This largely encouraged people searching for stellar material in meteorites. In 1973, Clayton et al. [1°] observed 4% 160 enrichment in refractory inclusions of carbonaceous chondrites, demonstrating that stellar material exists in the solar system. After 20 years' hard work, pre-solar grains were finally isolated and identified from primitive chondrites[ 11]. Early identified pre-solar grains are C-rich objects, including diamond, graphite, and SiC [11-14]. Pre-solar oxide [15] and nitride [16] were later observed. Recently, pre-solar silicate grains were identifiedI17,1s]. Pre-solar grains are very small in size, ranging from several nm to several #m (Fig. 1). The abundances of pre-solar grains are also very low in meteorites at ppm to ppb levels (Table 1). Laboratory study of pre-solar grains is not an easy task. It is very time-consuming and requires modern micro-scale analysis techniques. Meteorites are first mechanically ground into fine powders, which are then treated with various chemicals to dissolve the silicates, sulfides and organics. Solid grains from the acid residues are then examined with an electron microscope and candidates will be analyzed for their isotopic compositions with an ion microprobe.

Fig. 1 Scanning electron micrograph of pre-solar grains, a) SiC; b) graphite; c) corundum; d) TEM image of nanodiamonds. Photos are from [5]. Pre-solar grains have distinctively different isotopic compositions from solar materials. For example, the variation of O isotopic compositions in the solar system is relatively small ( - 4 0 to 20 0/00 for 6170 and ~180), while for pre-solar grains, the O isotopic variation is enormous, from - 5 0 0 to 8000 0/00 for 6170 and from -990 to 4000 0/00 for 51SO. Isotopic fractionation by any physical and chemical process could not produce such large variations, which represent results of stellar nucleosynthesis. The isotopic composition is the major criterion to identify pre-solar grains. Fig. 2 shows the O isotopic variations of pre-solar corundum (A1304) and hibonite (CaA112019) grains from various stellar sources. The O

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isotopic compositions of pre-solar grains reflect characteristics of their p a r e n t stars, such as red giant stars a n d A G B ( a s y m p t o t i c giant b r a n c h ) stars. Table 1

Types of pre-solar grains in meteorites**

Types of pre-solar grains Content in meteorites (ppm) Size 2 nm nanodiamond 1400 0.1 20 zm SiC ~ 10 Graphite ~ 10 1 - - 20 zm TiC, ZrC, MoC, RuC, FeC, Fe-Ni metal ? 5 - - 220 nm N 1 ~tm Si3N4 ~ 0.01 0.5 3 zm Corundum, A1304 ~ 0.2 0.1 - - 3 zm Spinel, MgA1204 ~ 1 N2J, m Hibonite, CaAl12019 ? (5 grains) ~ 1 l,m TiO2 ? (1 grain) 0.1 - - 0.9 ~m Silicates ? (15 grains) Data are from [4,5].

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Fig. 2 Oxygen isotopic compositions of corundum (circles) and hibonite (triangles) pre-solar grains. Most grains are enriched in 170 and depleted in 1So, relative to the solar composition (170/160 = 3.82×10 -4 , lSo/a60 = 2.01×10-3), marked by a star.

3. T H E S O U R C E S

OF PRE-SOLAR

GRAINS

Stars s p e n d most of their life t i m e at the m a i n sequence stage. D u r i n g this period, H is synthesized into He in the core. W h e n H in the core is b u r n e d out, i n t e r n a l nuclear reaction stops a n d the star ends its m a i n sequence stage. T h e stars t h e n cools d o w n a n d e x p a n d s into a red giant star. D u r i n g this period, convective mixing of the p r o d u c t s of H - b u r n i n g into the stellar envelope changes the surface isotopic compositions (the first dredge up). At this stage, there is still some nuclear reaction occurring in the t h i n H shell j u s t outside the He core. T h e H shell b u r n i n g heats u p the He core a n d triggers He b u r n i n g to p r o d u c e 12C. T h e He b u r n i n g in the core lasts a very short time relative to the H b u r n i n g . Eventually, the core He is exhausted. For low a n d i n t e r m e d i a t e mass stars ( M < 8M®, M o the solar mass), t h e t e m p e r a t u r e a n d pressure in t h e core are n o t high e n o u g h to trigger further nuclear fusion reactions. T h e outer layers e x p a n d a n d the star becomes a red giant again,

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now known as an asymptotic giant branch (AGB) star. H-burning and He-burning continue in the thin H and He shells outside the core to produce large amounts of 12C and 160. Heavy elements were synthesized by slow capture of neutrons by lighter nuclides (s-process). Periodic convective mixing processes (the second and third dredges up) within the star bring the nuclear products up to the envelope. The abundances of 12C and s-process isotopes in the envelope gradually increase. Stardust condenses from the cooling gas in stellar winds and preserves the star's chemical and isotopic compositions. For massive stars (M > 8Mo) , nuclear fusion reactions in the core will continue to make heavy elements and form an onion-shell internal structure. When the core becomes a Fe core, no more nuclear fusion could occur. There will not be enough thermal pressure to compensate the gravity and the core collapses and rebounds. The star explodes and becomes a supernova. Intensive shock waves trigger a series of explosive nucleosyntheses to make heavy elements especially those heavier than Fe by way of photodisintegration and proton capture (p-process) and rapid neutron capture (r-process). The chemical and isotopic compositions of stardust grains are dependent on the metallicity and mass of the parent star. They provide important clues to stellar synthesis and convective mixing process within stars. These grains have been proved to originate in red giant stars, AGB stars, supernovae and novae. Nano-diamond is the most abundant pre-solar grain (Fig. ld). But its stellar origin is still under debate. Nano-diamond is very small, only about 2.5 nm. Analyses of individual grain are very difficult. Bulk isotopic analysis shows that nano-diamonds display Xe and Te isotopic anomalies. They are probably from supernovae. However, their C isotopic composition is similar to that of the solar system. Further analysis shows that only about one in a million diamond grains carries a single Xe atom. Most nanodiamonds probably form within the solar system and only a few are from supernovae. SiC is the best studied species of pre-solar grain, ranging in size from 0.1 to 20 #m with most around 1 #m (Fig. la). Several thousands of SiC grains have been analyzed and they display large isotopic anomalies of major and minor elements, such as C, S, N, Mg, Ca, Ti, Ne, Xe, Kr, Ba, Nd, Sr, Sm, Dy, Mo, Zr, and Ru. Fig. 3 shows the C, N, and Si isotopic compositions of pre-solar SiC grains. They are divided into several subgroups on the basis of the isotopic signatures. 90 % of the grains belong to the mainstream group, and the rest are X, Y, Z and anomalous groups. The distribution patterns of pre-solar SiC grains reflect the physical and chemical environments of their parent stars. Results of isotopic analysis and theoretical calculation indicate that the mainstream SiC grains are from C-rich AGB stars and the SiC X-grains from supernovae. Pre-solar graphite has abundance and size distributions similar to SiC, but is less well understood. Graphite is of two morphological types: "cauliflowers" (aggregates of submicron grains) and "onions" (Fig. lb, concentric layers of relatively well-graphitized C). Most pre-solar graphite grains contain tiny sub-grains (20 to 500nm) of TiC, ZrC, MoC, and Fe-Ni metal [19'2°]. Ion microprobe isotopic analyses show that the variation of 1 2 c / l a c in pre-solar graphite grains is similar to that of SiC, but the distribution pattern is different. Most pre-solar graphite grains are enriched in 12C, similar to SiC X grains. But the "mainstream" SiC grains are enriched in 13C. Another difference is the N isotopic composition. Most pre-solar graphite grains do not display N isotopic anomaly, probably reflecting contamination. The density of pre-solar graphite grains varies greatly, from 1.6 g/era a to

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2 . 2 g / c m 3. Higher-density graphite grains are smaller t h a n lower-density ones, and their C and noble gas isotopic compositions are also different [21]. The isotopic compositions of lower-density graphite grains are in many ways similar to those of the rare SiC X grains. T h e y p r o b a b l y originated in supernovae. Only a small fraction of graphite grains is from A G B stars. This is an unsolved question. Because stars t h a t produce SiC grains also make graphite grains. However, most SiC grains are from A G B stars whereas the m a j o r i t y of graphite grains originate in supernovae.

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500 1000 1500 83°Si(%o) Fig. 3 C, N, Si isotopic compositions of pre-solar SiC grains. They are divided into several subgroups on the basis of the isotopic signatures. The dashed lines represent the solar ratios. Most grains belong to the mainstream group, which are believed to have formed in C-rich AGB stars. Figures are from 12C/13 C

[5].

In contrast to thousands of well-studied SiC and graphite grains, only about 200 presolar oxide grains have been identified. These include 198 c o r u n d u m grains (Fig. lc)[ 22], 41 spinel [231, 5 hibonite [24]. The difficulty in locating these grains is due to the fact t h a t most oxide grains in meteorites formed within the solar system. On average, only about one in ten thousand oxide grains is a pre-solar grain. Most pre-solar oxide grains are enriched in 170 b u t depleted inlSO (Fig. 2). T h e y p r o b a b l y originated in red giant stars or A G B stars [22-24]. Only one pre-solar oxide grain is from supernova. W i t h the newly developed Nano-SIMS technique, more t h a n one hundred additional tiny pre-solar oxide grains (< 1 #m) were identified, including 150 spinel grains and 30 corundum grains [25]. Infrared astronomical observation of late stage stars shows evidence of abundant silicate grains in their envelopes (9.7 and 18.5#m characteristic features). To identify pre-solar silicate grains in meteorites is not an easy task because silicates are the m a j o r mineral phases t h a t formed in the solar system. W i t h a Nano-SIMS, Messenger et al. [17] found 6 sub-micron pre-solar silicate grains in interplanetary dusts, and Nguyen and Zinner [ls] identified 9 presolar silicate grains in a carbonaceous chondrite. The O isotopic compositions of pre-solar silicate grains are very similar to t h a t of pre-solar oxides. T h e y p r o b a b l y have the same origin.

4.

SCIENTIFIC

PERSPECTIVE

OF

PRE-SOLAR

GRAINS

As mentioned above, the m a j o r i t y of pre-solar SiC grains originated in C-rich AGB stars, whereas most pre-solar oxide grains were from O-rich red giant and A G B stars. C and O

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isotopic compositions of pre-solar grains are very similar to the results from astronomical observations of red giant and AGB stars [26]. And they are also compatible with the theoretical calculations [27]. The infrared observation of red giant and AGB stars confirms the existence of SiC and corundum grains in their envelopes [6]. Laboratory study of pre-solar grains extends the traditional macro-scale telescope observation of distant stars to the microscale chemical and isotopic analyses of pre-solar stardust. With the advent of astrophysical theory and modern geochemical analysis techniques, the ability to study stellar materials in the laboratory has opened a new scientific frontier in astrophysics. The isotopic analysis results of pre-solar grains greatly enhance our understanding of stellar nucleosynthesis and convective mixing processes within stars. Traditional stellar models can account for some of the results, but not all. For example, C and N isotopic compositions of pre-solar SiC grains are significantly different from theoretical calculation results based on the traditional AGB stellar model. The discrepancy has prompted a modification of internal mixing process within AGB stars: this is so-called cool-bottom processing (CBP), an extra mixing process beneath the convective layer [2sl. CBP can account for the variation of C, N, O, and A1 isotopic compositions in low-mass red giant and AGB stars. The study of pre-solar grains also plays an important role in the field of galactic chemical evolution. The mainstream SiC grains and Y and Z grains originated in AGB stars. But their C and Si isotopic compositions differ significantly. The parent stars of SiC Y and Z grains have very low metallicity, between one-half and one-third of the solar value. These pre-solar grains can help us to better understand the initial stellar nucleosynthesis processes within stars with a large range of metallicity. Laboratory study of C, O, Si, and Ti isotopic compositions of pre-solar grains provides firsthand data to verify the theory of galactic chemical evolution [29]. Pre-solar SiC X grains, lower-density graphite and some oxide grains probably originated in supernovae. They are enriched in 15N, 12C, 28Si, and 26A1, especially in 44Ca[3°]. 44Ca is the decay product of 44Ti (half life 60 years). 44Ti can only be synthesized in supernovae. Recently, new evidence of 49V (half life 330 days) was observed in SiC X grains [31]. This nuclide is also produced in supernovae. The major stellar synthesis processes in supernovae are p-process and r-process. Numerous products of p-process and r-process have been found in pre-solar grains. They provide unique insights into convective mixing and explosive nucleosynthesis in supernovae. Pre-solar grains condensed from cooling circumstellar atmospheres. They reflect the physical and chemical conditions of stellar envelopes. Different grains have different condensation temperatures, and their internal crystal structures are results of physical processes during condensation. Many pre-solar grains contain sub-grain "core". They provide us critical information about chemical composition (e.g., C/O ratio), pressure, and mass-loss rates. Supernova explosion is a transient event, during which pre-solar grains are formed through numerous complex physical processes under highly varied conditions. Trace element analyses of pre-solar grains reveal that chemical fractionation occurred during the condensation. Laboratory study of pre-solar grains has rapidly emerged as a new area of astronomy. In just 30 years, its existence has evolved from a bewildering new discovery into the best of all techniques available for measuring isotopic abundance ratios with high precision in stars. Every single pre-solar grain contains a wealth of information about the physical and chemical conditions of its parent star. It is the only solid sample we can have from a remote ancient

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star. F i n d i n g a n e w t y p e of pre-solar g r a i n will c e r t a i n l y a d d new i n f o r m a t i o n t o t h e d a t a set. Recently, a few silicate pre-solar grains were f o u n d in p r i m i t i v e m e t e o r i t e s [17'18]. A c c o r d i n g to t h e i n f r a r e d a s t r o n o m i c a l o b s e r v a t i o n of stellar envelopes, t h e r e should be m o r e pre-solar silicate grains in m e t e o r i t e s . W i t h t h e a d v e n t of m o d e r n in situ isotopic analysis techniques, s u b - m i c r o (50 2 0 0 n m ) grains can also be a n a l y z e d . In fact, pre-solar silicate grains were identified w i t h t h e N a n o - S I M S technique. R e s o n a n c e i o n i z a t i o n mass s p e c t r o m e t r y ( R I M S ) can p e r f o r m t r a c e e l e m e n t a n d isotopic analyses on a single grain. T h i s largely e n h a n c e s our u n d e r s t a n d i n g of t h e physics and c h e m i s t r y of stars. L a b o r a t o r y s t u d y of pre-solar grains involves b o t h g e o c h e m i s t r y and astrophysics. C o l l a b o r a t i o n b e t w e e n t h e s e two fields will c o n t r i b u t e to a b e t t e r u n d e r s t a n d i n g of pre-solar grains, stellar nucleosynthesis, c o n v e c t i v e mixing, a n d galactic c h e m i c a l evolution.

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