Cosmochemical Evidence for Astrophysical Processes During the Formation of Our Solar System

Cosmochemical evidence for astrophysical processes during the formation of our solar system

Glenn J. MacPhersona,1 and Alan Bossb aDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560; and bDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved October 25, 2011 (received for review June 22, 2011)

Through the laboratory study of ancient solar system materials such as meteorites and comet dust, we can recognize evidence for the same star-formation processes in our own solar system as those that we can observe now through telescopes in nearby star-forming regions. High temperature grains formed in the innermost region of the solar system ended up much farther out in the solar system, not only the asteroid belt but even in the comet accretion region, suggesting a huge and efficient process of mass transport. Bi-polar outflows, turbulent diffusion, and marginal gravitational instability are the likely mechanisms for this transport. The presence of short-lived radionuclides in the early solar system, especially 60Fe, 26Al, and 41Ca, requires a nearby supernova shortly before our solar system was formed, suggesting that the Sun was formed in a massive star-forming region similar to Orion or Carina. Solar system formation may have been “triggered” by ionizing radiation originating from massive O and B stars at the center of an expanding HII bubble, one of which may have later provided the supernova source for the short-lived radionuclides. Alternatively, a supernova shock wave may have simultaneously triggered the collapse and injected the short-lived radionuclides. Because the Sun formed in a region where many other stars were forming more or less contemporaneously, the bi-polar outflows from all such stars enriched the local region in interstellar silicate and oxide dust. This may explain several observed anomalies in the meteorite record: a near absence of detectable (no extreme isotopic properties) presolar silicate grains and a dichotomy in the isotope record between 26Al and nucleosynthetic (nonradiogenic) anomalies.

cosmochemistry | solar system origin

lanetary systems, including our meteorites, and interplanetary dust. The Collectively, the chondrites, IDPs, and own solar system, arise as a natural likely significance of this material was stardust grains give us a window to the Pbyproduct of star formation out of recognized long ago. Goldschmidt (1) and ancient past, when the solar system con- interstellar molecular clouds. In Suess and Urey (2) compiled “cosmic” sisted only of a rotating dust and gas cloud detail, many questions remain, and for abundance tables for the elements, based surrounding the proto-Sun. Moreover, which there are two complementary ap- in part on chemical analyses of chondrite these materials provide information about proaches. Astronomical observations of meteorites. Suess and Urey (2) and later different parts of the early solar system: young protostellar objects show the pro- authors (e.g., ref. 3) compared the mete- Some chondrite grains formed very near cess as is it happening essentially now, but oritic abundances with measured abun- the infant Sun and later accreted into the great distances involved limit our ability dances in the solar photosphere and found parent bodies within the asteroid belt, to resolve fine structure within pro- that, for most condensable elements (and whereas comets are thought to have toplanetary disks. Laboratory studies of other than gases), the match between CI accreted at the very fringes of the solar material from our own solar system, in chondrites in particular and the Sun was system. Surprisingly, the comet dust and particular of meteorites, comets, and in- very close. As analytical techniques have IDPs and chondrites share many compo- terplanetary dust that are preserved more improved, the match has also improved to nents in common, and this fact alone is or less untouched since the birth of our the point at which most elements agree (as detailed later) critical to understanding solar system 4.568 Ga ago, yield chemical within approximately ±10% (reviewed in major physical transport processes within and physical clues to the large-scale pro- ref. 4). (Ref. 5 presents a recent review the infant solar system. cesses and conditions extant at that time. of chondrite classification, terminology, Undoubtedly the catalyst for the modern These highly precise analytical measure- and properties.) However, although revolution in cosmochemistry was the fall, ments thus yield direct links between chondrites preserve grains from the earli- in February 1969, of the Allende mete- astronomical observations of large-scale est history of our solar system, most of orite in the northern desert of Mexico. In processes in newly forming stellar systems those grains have not remained entirely part, this is because of Allende being a rare and the same processes that occurred pristine. CI and CM chondrites, for ex- type of meteorite, a CV3 carbonaceous long ago in our solar system. ample, are composed primarily of hydrous chondrite. Even more remarkable were its The Earth and other evolved large phyllosilicates that probably formed by size (more than 2 tons recovered) and bodies in the solar system long ago lost all aqueous alteration of primary grains the timing of the fall. Grossman (ref. 6, physical traces of the primordial grains within asteroidal parent bodies. Possibly p. 559) characterized the situation aptly: from which they accreted. Very small more pristine early solar system materials “At a time when many of the world’s bodies such as asteroids and comets, how- come from interplanetary dust particles geochemical laboratories were in close ever, largely escaped planetary heating and (IDPs), which include both comet and melting and therefore do retain their asteroidal dust. Finally, we now have un- original accretionary materials in varying doubted comet dust grains collected by the Author contributions: G.J.M. and A.B. performed research degrees of preservation. Fortunately, National Aeronautics and Space Admin- and wrote the paper. comets plus collisions in the asteroid belt istration’s Stardust mission, which ren- The authors declare no conflict of interest. and elsewhere deliver some of this material dezvoused with Comet Wild 2 in 2004, This article is a PNAS Direct Submission. ’ into the inner solar system, where it falls to collected dust particles from the comet s 1To whom correspondence should be addressed. E-mail: Earth in the form of meteorites, micro- coma, and returned them to Earth in 2006. [email protected].

19152–19158 | PNAS | November 29, 2011 | vol. 108 | no. 48 www.pnas.org/cgi/doi/10.1073/pnas.1110051108 Downloaded by guest on October 2, 2021 PCA ETR:PERSPECTIVE FEATURE: SPECIAL

communication with each other and were minerals to condense are oxides and sili- The idea behind the eventual discovery perfecting their analytical techniques to cates of calcium, aluminum, magnesium, began with Urey (13), who predicted derive the maximum information from and titanium. In other words, people in that the short-lived radionuclide 26Al a minimum of sample in preparation for 1969 realized that the Allende CAIs po- might have existed in the earliest solar the return of the first lunar rocks only six tentially contain the first solid materials to system and provided the heat source nec- months thence, there were suddenly sev- have formed in our solar system. Allende essary for early planetary melting and eral tons of a single carbonaceous chon- provided us with just the right samples differentiation. At that time, neither the drite available for study.” However, the at just the right time. And because the instrumentation nor the proper samples singularity of Allende does not stop there. Apollo samples were 6 mo yet to come, existed to test the idea. Schramm et al. Like all chondrites, Allende and the other the new laboratories needed something to (14) revived the idea and measured CV3 chondrites are aggregates of pre- do. Within a few years of Allende’s fall, a number of meteoritic samples in the planetary grains of diverse character two singular discoveries based on Allende search for the daughter product of 26Al (Fig. 1). The difference is in the grain size CAIs would change the course of cosmo- decay, 26Mg, but again their analytical of some of these components. Immedi- chemistry. The first was the discovery (8) fi 16 17 techniques were not suf cient to resolve ately upon looking at a broken or cut face of excesses of O (relative to O and 18 any likely signature. Moreover, as it turned of Allende, one’s eye is drawn to the O) in Allende CAIs. The second was the out, they still were not looking at the discovery (9) of excess 26Mg that resulted prominent centimeter-sized white clasts 26 proper samples. Following the fall of Al- (Fig. 1). Composed mainly of the oxides from the in situ decay of Al at the time lende and the recognition of the CAIs as and silicates of aluminum, calcium, mag- of CAI crystallization. There quickly fol- possible solar nebular condensates, several nesium, and titanium, these clasts are lowed the discovery in a rare few CAIs laboratories began in earnest the search [referred to as FUN, for Fractionation 26 referred to as refractory inclusions or for extinct Al. Finally, Lee et al. (9) and Unidentified Nuclear isotopic effects (more commonly, and herein) calcium- demonstrated unambiguous evidence for (10)] of intrinsic nuclear anomalies that aluminum–rich inclusions (CAIs). All the in situ decay of 26Al in an Allende CAI could be traced back directly to presolar chondrite varieties contain such objects, at a level much greater than the steady- but in every other chondrite type, they are nucleosynthesis (e.g., ref. 11). Thus, the direct connection was first established state level throughout the interstellar me- both rare and very small (<1 mm). In dium. This result has been repeated and the CV3 chondrites, CAIs attain sizes as between the analysis of solar system fi objects and astrophysical processes. greatly ampli ed since then (reviewed in large as 2 cm. Had Allende been any type ref. 15; see example in Fig. 2) (16). The of chondrite other than a CV3, not only The present work is not and cannot be fi a comprehensive review. Rather, our aim is Lee et al. nding (9) of course raised the would the (rare and much smaller) CAIs to highlight how cosmochemical studies immediate question: Where did the excess have been missed, they could not have 26 have provided compelling evidence for Al come from? With a half-life of 0.71 been analyzed easily with the types of in- some large-scale astrophysical processes Myr, it must have been made no more strumentation available at the time. The during the formation of our solar system. than 1 to 2 Myr before the time of CAI last piece of the Allende miracle again At the end, we will address some remaining formation. This led Cameron and Truran relates to timing. Although the size and unanswered puzzles and speculate on one peculiar nature of the CAIs in Allende ’ possible solution. We will touch only pe- quickly caught people s attention in 1969, ripherally on the subject of the origin of interest was greatly raised because of nuclear anomalies and stellar nucleosyn- other recent events. Larry Grossman was thesis, as this topic is covered elsewhere just completing his PhD thesis at Yale in this special issue of PNAS. University, doing complete thermody- namic equilibrium calculations for the ex- Triggered Star Formation in the Birth pected condensation of solids from a of Our Solar System hot gas of solar composition (published in Star formation requires that portions of fi ref. 7). Although not the rst to attempt a interstellar molecular cloud cross a criti- such calculations, his were more compre- cal threshold of mass vs. size in order for hensive in terms of bulk chemistry and them to become gravitationally bound and fi fi were the rst to take speci cally into ac- begin collapsing. In low-mass clouds like count the changing composition of the gas Taurus–Auriga, star formation arises as as condensation proceeds. He showed that a result of turbulence-generated local the first (i.e., highest temperature) major density fluctuations, followed by loss of magnetic field support, cloud collapse and fragmentation, and continued gravitational collapse of the individual fragments. In giant molecular clouds like Orion, star formation can also be “triggered” by the effects of massive OB stars within the clouds. Numerically, most low-mass (i.e., Sun-like) stars form in giant molecular clouds (e.g., ref. 12). However, statistical considerations aside, there is direct cos- Fig. 2. (A) A cut surface of the Leoville meteorite, mochemical evidence that our own Sun showing a centimeter-sized CAI for which the magnesium isotope data (B) clearly indicate the in formed within a giant molecular cloud and situ decay of 26Al at the time that the inclusion that its formation was, indeed, triggered. formed 4.56 billion years ago. The abundance ra- The evidence primarily comes from the tio at the time of formation is given by the slope isotopic compositions of meteoritic com- of the correlation line: 26Al/27Al = (5.27 ± 0.17) × Fig. 1. A cut surface of the Allende meteorite. ponents, including CAIs and chondrules. 10−5. Data are from ref. 16.

MacPherson and Boss PNAS | November 29, 2011 | vol. 108 | no. 48 | 19153 Downloaded by guest on October 2, 2021 (17) to propose that a nearby supernova bombardment because there are no suit- Note that there is a critical difference not only triggered the collapse of the cloud able target elements, and therefore its between the newer “triggered star forma- that would become our solar system, but presence in meteorites is considered an tion model” of Hester and Desch (12) also seeded it with supernova ejecta that unequivocal fingerprint of a nucleosyn- versus the one proposed by Cameron and contained 26Al (Fig. 3 shows a modern nu- thetic origin. Shukolyukov and Lugmair Truran (17) and later investigated by Boss merical simulation of this process; ref. 18). (23, 24) first showed evidence for extinct and Keiser (18). In the Hester and Desch This proposal was countered by an al- 60Fe in the early solar system, in the form scenario (12), it is not the supernova ternative idea (19) that local proton irra- of Fe/Ni-correlated excesses of 60Ni (the shockwave itself that triggers the ongoing diation near our own infant Sun produced daughter product of 60Fe) in eucrites star formation. Rather, it is the outward- the 26Al, removing the need for any ex- (i.e., asteroidal basalts). There also is good moving shock front of the ionized hydro- ternal source. The local-irradiation model evidence (e.g., ref. 25) for extinct 60Fe in gen (HII) region, a 10,000-K plasma, has been debated at length, and the gen- chondrules, which are the small once- created by the intense stellar luminosity, eral consensus (not universal) is that it is molten silicate droplets that characterize radiation pressure, and stellar winds from unlikely for two reasons: (i) the difficulty most chondrite meteorites. The 60Fe the OB stars. In this scenario for our solar of coproducing other known short-lived plausibly originated in either a supernova system, the supernova was responsible 41 53 mainly for the late injection of short-lived radionuclides (e.g., Ca, Mn) in their or an AGB star, but the supernova source 26 41 proper abundances relative to 26Al (re- can better explain other known short-lived radionuclides such as Al and Ca into viewed in ref. 20); and (ii) 26Al is quanti- radionuclides such as 53Mn (20). A third the newly formed solar nebula, whereas in tatively not correlated with 10Be (reviewed argument in favor of a supernova source is the original scenario, the supernova shock in ref. 21), another short-lived radionu- the advantage that supernova shock waves not only injects the short-lived radionuclides in the presolar molecular cloud clide that is produced only by particle ir- have over the winds from AGB stars in 10 core, but it simultaneously triggered col- radiation. Thus, if all the Be was made simultaneously triggering collapse of the lapse of the cloud. by local irradiation, most of the 26Al was presolar cloud and injecting enough Thus, precise measurements of grains made by some other mechanism. Alumin- freshly synthesized radionuclides into within meteorites point to our sun and ium 26 does correlate strongly with 41Ca, the collapsing cloud to match the in- solar system having formed in a particular however (22)—another very short-lived ferred initial abundances in primitive — astrophysical environment: a giant molec- nuclide so those two nuclides must be meteorites (26). ular cloud, as opposed to the more quies- cogenetic. It is not possible for particle The convincing evidence for a nearby cent environment of an isolated region irradiation to produce both in their ob- supernova explosion at the time of our analogous to the Taurus–Auriga cloud. served relative proportions. Thus, plausi- solar system’s birth seemingly raises anew ble production sites for 26Al and 41Ca are one original concern with the Cameron Nebula-Wide Mass Transport of asymptotic giant branch (AGB) stars and and Truran (17) proposal, namely the Materials supernovas, but AGB stars are highly huge coincidence required. However, this Chondrules and CAIs from chondritic evolved old Sun-like stars that generally argument has now been turned upside meteorites formed at very high temper- are not located near the sites of stellar down (e.g., ref. 12) by the observation that, atures in the solar nebula, in excess of birth. Thus, the most likely source of early if our Sun formed in a giant molecular 1,200 °C, and some CAIs formed at tem- 26 41 solar system Al and Ca is supernova cloud whose evolution was largely con- peratures approaching 2,000 °C. However, injection. Because of the very short half- trolled by early-formed massive O and B the chondrites themselves formed by rel- 41 5 life of Ca, 10 years, that event must stars at its core, a supernova is actually to atively low-temperature accretion of solid have occurred within approximately 1 Myr be expected. Such massive stars have life- grains in the general region of the asteroid of the formation of CAIs in our solar times measured in millions rather than belt. The temperature in the solar nebula system. The second and even stronger in- billions of years, and they would be ex- dropped off radially away from the very dication for a nearby supernova is 60Fe. pected to explode during the episode of hot center (near the proto-Sun), and at Iron 60 cannot be produced by particle star formation in large stellar “nurseries.” distances out near the asteroid belt the temperature probably was well below 1,000 °C (27). Thus, there has always been a problem reconciling the apparent formation temperatures of CAIs and chondrules with the location of their parent bodies in the asteroid belt. Early ap- proaches to the problem involved explor- ing possible physical mechanisms that could lead to the transport of CAIs inside the disk from the very high temperatures of the inner nebula outward to much larger radial distances from the Sun (26). A somewhat different solution emerged when Shu et al. (28) proposed that CAIs and chondrules actually formed very close to the infant Sun but were expelled by magnetically driven bipolar outflows, up out of the plane of the disk (Fig. 4). Most Fig. 3. Theoretical model of a supernova shock wave (top horizontal bar) striking a dense molecular of the material from these jets escapes into interstellar space, where it is observed in cloud core, inducing dynamical collapse of the cloud, while simultaneously injecting short-lived radio- – nuclides (black contours) into the collapsing cloud (modified from ref. 18). The color scales indicate gas the form of Herbig Haro objects, but density in g/cm3. The cloud is symmetrical around the left border. Models are shown as (A) initial model, some falls back onto the nebular disk far (B) after 0.03 Myr, and (C) after 0.1 Myr. out from the Sun.

19154 | www.pnas.org/cgi/doi/10.1073/pnas.1110051108 MacPherson and Boss Downloaded by guest on October 2, 2021 solar system cannot be explained by par- sive transport of grains past Jupiter’s orbit ticle bombardment near the proto-Sun. within the nebula midplane. If Jupiter Desch et al. (34) conclude that alternative formed rapidly and cleared away the local mechanisms for producing bipolar outflow disk, and the Stardust grains did form in are more likely, that CAIs did not form the inner solar system (but see below), at the X-point but farther out, 0.1 to 0.3 it is more likely that these grains were AU from the proto-Sun, and that most of transported outwards rapidly via bipolar the short-lived radionuclides originated outflow or by a phase of gravitational from outside the solar system. Recent as- instability. tronomical evidence also argues against the X-wind model. Extremely high-reso- Presolar Silicates, the Stardust and lution spectral imaging (35) of the jet near FUN CAIs Enigmas, and the Source of the star DG Tau indicates a sharp velocity Solar System Rocky Matter Fig. 4. The X-wind model for driving bipolar gradient within the jet, whereas the Presolar grains in meteorites were first outflow in a young star. In this model, CAIs are X-wind model predicts nearly constant suspected to exist in meteorites because of envisioned to form near the X-point at Rx,from velocity. Thus, Agra-Amboage et al. (35) anomalous isotopic compositions of the which they can be launched upward and outward conclude that the X-wind model cannot noble gases in bulk chondrites. Laborious into interstellar space or the outer solar system. The explain the properties of the DG Tau jet. chemical isolation procedures finally led X-point is located less than 0.1 AU away from the That work and others (e.g., ref. 36) also to the separation and analysis of actual star, far inside the current orbit of Mercury. From fi Shu FH, Shang H, Lee T (1996) Toward an astro- speci cally note that material in jets may grains in 1987 (see ref. 44 for a historical physical theory of chondrules. Science 271:1545– come from a much wider radial distance review; also see ref. 45). This was first 1552. Reprinted with permission from AAAS. from the central star than predicted by the accomplished by dissolving >99% of the X-wind model, perhaps as much as 1 AU meteorites, tracking the isotope anomalies (an important factor in preserving dust at every step, and when the grains bearing A second piece of the puzzle fell into against evaporation). The preferred model the anomalies were finally isolated, they place with the recognition that all CAIs of Agra-Amboage et al. (35) for bipolar turned out to be chemically resistant have very similar oxygen isotopic compo- outflows is a wind driven by the magnetic phases such as diamond, graphite, and sitions no matter what kind of chondrite in structure of the disk, not the central star. SiC. Edward Anders famously described which they occur. However, the different Bipolar outflow is an attractive mass this procedure as burning down the hay- chondrite classes themselves have distinc- transport mechanism because it is observed stack to find the needle. Other presolar tively different compositions. This implies to occur in contemporary young stars. phases were later identified, including that all CAIs formed out of the same However, other transport mechanisms corundum, spinel, and hibonite. Silicates restricted nebular isotopic reservoir and have been proposed that do not involve were conspicuously missing. However, were later dispersed to their eventual bipolar outflow. Ciesla (e.g., ref. 37) the compositions of Earth and the other chondrite accretion regions. McKeegan suggested that particle diffusion within the rocky bodies require that silicates origi- et al. (29) demonstrated that CAIs disk itself led to grains being transported nally must have dominated the population contained live 10Be at the time of their significant distances outward from the of solids that went into the making of formation. Because this short-lived radio- Sun over time scales of 0.1 to 1 Myr. Boss our solar system. The assumption was that nuclide forms by particle bombardment (38) showed that a phase of marginal the silicates were less able to survive the and not by stellar nucleosynthesis, the gravitational instability is able to rapidly high-temperature processing in the early CAIs themselves must have formed very transport small particles from the inner inner solar system, and those that did near a source of intense particle bom- disk to the outer disk while simultaneously survive were later destroyed by the ex- bardment. That source likely was the in- homogenizing most initial isotopic het- treme chemical processing (described fant Sun (refs. 29–31; but see ref. 32 for erogeneity, over time scales less than 0.001 earlier) used in the laboratory to isolate a dissenting opinion). Finally, the near- Myr. It is possible that all three mecha- presolar grains. Searches for presolar sili- identity of CAI oxygen isotopic composi- nisms operated in transporting grains cates required techniques that did not tions with that of the Sun itself (33) all but outward from the Sun, but results from the destroy the host meteorite, such as auto- confirms the idea that CAIs did form close Stardust sample return mission from mated in situ isotopic scanning via sec- to the proto-Sun and were physically Comet Wild 2 comet dust may pose ondary ion MS. Presolar silicates were transported outward into the solar system a problem for the disk diffusion model. eventually found, first in IDPs (46) and by some mechanism. However, the mech- Terminal particles almost exclusively con- subsequently in chondrite meteorites (47), anism is still debated. Although bipolar sist of anhydrous silicates, chondrule at a level of 300 to 400 ppm relative to the outflow appears to be common among fragments, and even two CAIs, all mate- bulk meteorite. The existence of these very young stars, the widely cited X-wind rials commonly observed in chondritic grains is evidence that presolar silicates model of Shu et al. (28) is only one pro- (i.e., asteroidal) meteorites (39–42). Phyl- were not preferentially destroyed by high posed model for producing such outflow. losilicates and carbonates are virtually temperature processes everywhere in the Desch et al. (34) critically examined the absent (39). The implication is that high- solar system, and certainly not in the ac- details of the X-wind model and con- temperature grains from the inner solar cretion regions where the host IDPs and cluded that it is not consistent with the system were very efficiently transported primitive chondrites formed. How then properties of CAIs and chondrules. For not just to the asteroid belt but also well to explain their low abundances even in example, temperatures at the X-point beyond, to the region where Comet Wild 2 such well preserved primitive materials? were so high that no solids could have accreted. Westphal et al. (43) estimated This “abundance problem” is com- existed there at all. Further, the relatively that between 50% and 65% of the Star- pounded by the results of the Stardust oxidizing conditions at the X-point are dust grains originated in the inner solar mission, a chief motivation of which was to inconsistent with the extremely reduced system. Depending on how early the gas return samples of solar system matter that mineral assemblages in CAIs. Finally, the giants formed, especially Jupiter, these had not experienced reprocessing in the relative abundances of most short-lived planets could have posed a formidable inner solar system, such as was experi- radionuclides identified with the earliest obstacle to relatively slow outward diffu- enced by grains occurring in chondrites

MacPherson and Boss PNAS | November 29, 2011 | vol. 108 | no. 48 | 19155 Downloaded by guest on October 2, 2021 (e.g., ref. 47). Especially, there was an little escaping such processing. Second, expectation that comet dust would contain the transport mechanism from the inner a high fraction of presolar rocky material solar system to the outer solar system was preserved from the interstellar medium. remarkably efficient (43). And third, the Thus, much of the returned material was isotopic composition of condensed (i.e., expected to be amorphous and of low- rocky) matter in the solar system is defined temperature origin. However, as noted entirely by a mixture of supernova plus earlier, the returned grains instead consist nova plus red giant grains. This scenario of anhydrous crystalline materials such as may be consistent with intermittent phases olivine and pyroxene with variable but of gravitational instability in the gaseous mostly low ferrous iron contents, CAIs disk (38). (two so far), and chondrule fragments (39– Individually and together, these three 42). All these grains resemble the kinds of requirements might be met by a very inner solar system grains found in chon- different scenario. If our solar system dritic meteorites (41). Although some formed in a giant star-forming region like amorphous grains were found, it is by no Orion or Carina where triggered star birth means clear that these are primary as op- occurred in the walls of the expanding posed to being products of the high-ve- bubble, a significant source of grains would locity impacts during sample collection have been the bipolar outflow of neigh- (41). Very few presolar grains of any kind boring young stellar objects formed before have been recognized, based on the crite- (within a few million years at most) or rion of isotopic compositions. However, contemporaneously with our own. Al- this last statement may well lie at the very though grains are not directly observed in heart of the apparent abundance problem: outflows from young stellar objects, they What defines a presolar grain? are generally accepted to be present be- All presolar grains identified so far have cause of the depletions of refractory ele- extreme isotopic compositions that in- ments from the gaseous component of the Fig. 6. A portion of the Carina nebula showing dicate the grains were derived from specific jets (e.g., refs. 35, 36). This idea has been two dense gas columns, each with a young (in- freshly nucleosynthesized material, from considered before (e.g., ref. 49) but has visible) protostar at its tip; both protostars are supernovas or novas or expelled shells of not been examined in any serious way by undergoing bipolar outflow. (A) Note that the red giant stars (e.g., Fig. 5). However, the cosmochemical means. Consider Fig. 6A, outflow from the star at lower left is impinging on solar system in bulk does not have such which shows a Hubble Space Telescope the side of the neighboring column. (B) Line drawing of A shows hypothetical protostars in the compositions, and that difference enables photo of the Carina Nebula. The two large fi hidden interiors of the gas columns, and again identi cation of the presolar grains. The columns of gas represent enhanced gas with bipolar outflow. Dust from such embedded general explanation for the Stardust sam- density at the edge of the bubble created stars would become fodder for later generations ples (e.g., refs. 41, 48) is that most of the in the middle of a large molecular cloud by of protostars in the same cloud. Top photo cour- grains were formed in the hot inner re- nearby superenergetic O and B stars, and tesy of NASA, ESA, and M. Livio and the Hubble gions of the primitive solar nebula, near this is the region of triggered star forma- 20th Anniversary Team (STScI). the Sun, and then transported outward to tion. Young protostars are emerging from the comet-forming region. This imposes the tips of each of the two columns, with a number of singular requirements. First, bipolar outflows clearly visible. Likely lenge for the future, and a test of almost no presolar silicates remained in there are young protostars within the col- our model. the outermost parts of the solar system umns that are not visible (postulated in Tielens et al. (49) showed a diagram where the comets accreted. Most presolar Fig. 6B). The outflow from the protostar (Fig. 7) in which they attempted to esti- material was transported inward and pro- at the lower left of Fig. 6A is even im- mate the relative contributions to the ISM cessed in the inner solar system, with very pinging on the sides of the column to its grain population from different kinds of right, possibly propelling both gas and stellar sources. They adopted an upper grains into that column’s interior. Because star formation is enhanced in the bubble walls, there is likely to be a greatly enhanced enrichment of outflow grains relative to other kinds of interstellar grains. Moreover, because all the locally forming stars are born out of a common interstellar cloud, the bulk isotopic compositions of all of the other new solar systems would be similar to our own: The outflow-expelled grains would not be isotopically recogniz- able relative to what we think of as normal. The implication of this is that many or even most of the silicates in the most primitive meteorites or IDPs may be presolar, but they are not isotopically recog- Fig. 7. Pie diagram showing the postulated fractions of interstellar dust grains that are con- nizable by current methods. Indeed, tributed from each of plausible different stellar Fig. 5. Oxygen isotopic composition range of Bradley proposed such a model as far back sources. Young stellar objects contribute as much presolar oxide and silicate grains Figure courtesy as 1994 (50). Confirming this is an im- as 30% according to this model (Discussion). of Dr. A. N. Nguyen; data sources given in ref. 47. portant scientific and technological chal- Modified from ref. 49.

19156 | www.pnas.org/cgi/doi/10.1073/pnas.1110051108 MacPherson and Boss Downloaded by guest on October 2, 2021 limit of 30% for the fraction of interstellar Studies of chondrite meteorites indicate dust might derive from young stellar that solid grains were evaporated, con- objects; other authors have argued for densed, and melted in the hot innermost a significantly lower fraction of less than region of our solar system. Some of this 15% (e.g., ref. 51). Independent of what highly processed material ended up much the actual fraction is, in a triggered star- farther out in the solar system, not only forming region surrounding a large HII the asteroid belt but even in the comet region, the fraction could well be higher accretion region, suggesting a huge and by a factor of two or more. efficient process of mass transport. Bipolar Near-contemporaneous stellar outflows outflow is one possible mechanism for this from nearby infant stars may provide an explanation for another longstanding transport in view of the fact that it is so puzzle relating to the earliest history of our common in young stars; turbulent diffusion solar system. When the former presence of and marginal gravitational instability may short-lived 26Al was first demonstrated (9) have contributed as well, although the in an Allende CAI, those authors also effects of early-formed giant planets on analyzed a second CAI at the same time interfering with turbulent diffusion are not Fig. 8. Initial 26Al/27Al ratios in early solar system that did NOT once contain 26Al at de- yet well understood. The presence of short- — refractory objects (mainly hibonite), plotted tectable levels. This CAI and as was later against 50Ti in the same objects. Data and original lived radionuclides in the early solar sys- 26 41 shown, a small number of similar objects— diagram are from ref. 53, and the modified dia- tem, such as Al and Ca, requires contain other unusual isotopic properties, gram shown is reproduced from ref. 54. Reprinted a nearby source of nucleosynthetic mate- specifically large degrees of mass-de- from Meteorites, Comet And Planets: Treatise rial shortly before solar system formation. pendent isotopic fractionation in oxygen On Geochemistry, Vol 1, MacPherson GJ, Calcium The most likely source is a supernova, and magnesium plus nuclear (not derived aluminum-rich inclusions in chondritic meteorites, which is not only possible but probable if from radioactive decay) anomalies in Pages 201-246, Copyright 2003, with permission from Elsevier. the Sun was formed in a massive star- other elements such as calcium and tita- forming region similar to Orion or Carina. nium (e.g., refs. 11, 52). These rare (in In this case the solar system formation was CV3 chondrites) objects were named FUN been injected into the solar system after “triggered” either by a supernova shock inclusions in reference to their fraction- formation of FUN CAIs but before the wave or by ionizing radiation originating ation and unidentified nuclear effects (9). formation of normal ones. A test of With the advent of secondary ionization from massive O and B stars at the center these hypotheses rests on obtaining a high- MS (i.e., ion microprobe), it was sub- of an expanding HII bubble. Triggered precision absolute age of a FUN CAI, sequently found that much smaller objects formation of the solar system in a massive a goal that has not yet been achieved be- (mainly single hibonite crystals) with sim- star-forming cloud can explain several ilar isotopic properties occur in CM car- cause of the great rarity of large FUN observed anomalies in the meteorite re- bonaceous chondrites. Clayton et al. (53) CAIs in CV3 chondrites. However, there cord, such as the near absence of detect- reviewed (among other things) the data is a third possibility in the context of the able presolar silicate grains and a complete for FUN inclusions and presented a model presented earlier. Other contem- dichotomy in the isotope record whereby remarkable graph (Fig. 8) that shows porary infant solar systems near our own, 26Al and nucleosynthetic (i.e., nonradio- a near-perfect noncorrelation between having similar bulk chemistry, would also genic) anomalies were present in separate 26 27 50 have produced CAIs as well as other kinds initial Al/ Al and Ti, which is a nu- grain populations. The Sun would have of high-temperature grains near the re- clear anomaly that can be used to identify formed in the dense border region of the spective young stars. It is quite possible FUN inclusions. This noncorrelation re- expanding HII bubble, in close proximity to that some of those systems might have quires that the early solar system con- many other newly forming stars. Material tained (at least) two very distinct isotopic had some isotopic differences relative to our own solar system, albeit nowhere near expelled from these systems via bipolar reservoirs that did not mix, even when the outflow would become incorporated into grains were reheated and, on occasion, as great as those observed in supernova any subsequently formed stars, and it would melted. Maintaining such distinct reser- and nova and red-giant presolar grains. largelybeisotopicallyindistinguishable voirs in the gas phases is difficult, and is Such systems might, for example, largely more readily explained if the isotopic sig- have formed before the supernova that from new grains formed in those systems 26 natures were preserved entirely in grains. gave rise to the Al in our own system. because all originated from the same The existence of FUN inclusions has The nuclear anomalies seen in FUN in- molecular cloud. been a riddle ever since their discovery, and clusions may reflect heterogeneous iso- nearly every model for the evolution of the topic seeding elsewhere in the cloud by ACKNOWLEDGMENTS. We thank Dr. Ann Nguyen fi for providing Fig. 5, Drs. John Bradley and Larry early solar system has dif culty explaining red giant stars or other (much earlier?) Nittler for helpful discussions, and two anonymous 26 them. For example, their lack of Al might supernovas. reviewers for very constructive suggestions that imply that they formed long after “nor- greatly improved the paper. G.J.M. would like to ” Summary and Conclusions acknowledge the pioneering work of G. Herbig, mal CAIs. In that case, why do FUN who long ago considered the possibility that bi- CAIs preserve the nuclear anomalies that The laboratory study of ancient solar sys- polar outflow from young stars might be an im- clearly are vestiges of nucleosynthetic tem materials provides extensive un- portant source of interstellar dust. This work was processes? Alternatively, FUN CAIs might derstanding for the astrophysical processes supported by National Aeronautics and Space that formed out own solar system, pro- Administration (NASA) Grants NNX11AD43G (to have formed at a much earlier stage of G.J.M.; Cosmochemistry program), NNX09AF62G the early solar system than normal CAIs, cesses that we can also observe to be oc- (to A.P.B.; Origins of Solar Systems), and NASA 26 in which case the Al itself must have curring now in nearby star-forming regions. Astrobiology Institute NNA09DA81A (to A.P.B.).

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