Forming stellar grains

23 The laboratory studies directly measure the chemical and physical properties of ancient stardust. Some of these properties—for example, grain size distributions and ex­ternal morphologies—must also reflect the rigors of transit from the circumstellar birthplace to the laboratory envi­ronment

24 It turns out that the new laboratory data do not fit well with either the astronomical observations or the theory of dust in the diffuse interstellar medium (ISM). Indeed, it appears that the two dust populations are very different.

25 Presolar SiC grains, for example, often show well-de­fined crystal faces, indicating a lack of severe erosion and pulverization. Contradicting these findings, existing theo­ries of ISM dust predict that interstellar shocks quickly erode and destroy grains. Moreover, the characteristic size inferred for ISM dust from the scattering of starlight is ten times smaller than the roughly 1 micrometer size typical of presolar grains. Finally, neither very small, needle-like graphite nor silicate grains have yet been identified in the presolar grain population—even though polarization measurements indicate their presence in in­terstellar dust. A variety of possible explanations exists, but finding the right one remains a major goal.

26    Whatever the explanations, presolar grains do exist, and they can be used to infer the temperatures, pressures and times required to produce them in stellar outflows.

For example, transmission electron microscope stud­ies of ultrathin sections of presolar graphite spherules show that the spherules are often nucleated on small (5—100 nm) crystals of very refractory carbides like titanium carbide and zirconium carbide (figure lb). But they also show that less refractory carbides like SiC do not generally occur within the graphite. According to equilib­rium thermodynamics, the inferred condensation se­quence—the very refractory carbides first, then graphite, then SiC—can occur only at total pressures greater than about 0.1 Pa, which is roughly a hundred times greater than pressures generally assumed to exist in the regions of AGB atmospheres where grains condense.

27 Considering the kinetics of ideal grain growth yields the same conclusion. However, higher pressures in astellar atmosphere inevitably lead to proportionally higher rates of mass loss. To avoid contradicting astronomically measured AGB mass loss rates, the pressures inferred from presolar grain studies must be assumed to pertain not to smooth, spherically symmetric stellar outflows, but to irregularly distributed blobs of material that are sporradically ejected from stellar surfaces. Interestingly, this conclusion is in agreement with recent studies of maser emission in AGB at­mospheres that similarly require large-scale blobbiness in the outflow­ing matter.

Figure 4.

Silicon isotopic ratios measured in presolar silicon carbide grains, in terms of departures from solar ratios in parts per thousand. The grains are separable into two major compositional fields (and several additional minor fields, not shown)—namely, mainstream and X grains. The mainstream grains are thought to originate around stars on the asymptotic giant branch and have excesses of both ²⁹Si and ³⁰Si. By contrast, the X grains, which probably originate in supernova ejecta, have ²⁸Si excesses relative to either of the other two silicon isotopes. Of the total number of SiC grains analyzed, X grains represent only about 1%, whereas mainstream grains represent about 98%. The mainstream field is defined by isotopic compositions measured in about 700 individual grains. The X grain field is defined from measurements on about 100 grains located by ion imaging. (See box 3.) The inset displays the compositional trend expected when silicon of solar composition in the AGB stellar envelope is mixed with silicon produced in the helium shell. This trend cannot account for the observed distribution along a straight line of slope of 1.3 found in mainstream grains. It is thought that this spread, instead, reflects Galactic chemical evolution as sampled by multiple AGB stars of different age that contributed SiC grains to the solar mix.