Dorothy S. Woolum

Radiatively Damped Density Waves in Optically Thick Protostellar Disks

The redistribution of angular momentum and mass in circumstellar disks by spiral density waves is governed by processes that dissipate the waves. Dissipation is usually attributed to unspecified nonlinear processes or an ad hoc viscosity. In this paper, we examine dissipation by radiative losses associated with the cyclic compression and expansion of disk material by the density waves. We consider linear, discrete modes in optically thick disks and use a simplified treatment of the radiative losses in order to assess the parameter dependences and importance of radiative damping. Wave action conservation principles are generalized to describe the effect of dissipation (by any means) and the consequent coupling of wave and disk energy and momenta. A relation is derived (in the WKBJ limit) between radial mass transport of disk material and the azimuthally averaged dissipation rate. A wave amplitude equation is similarly derived, which describes the radial evolution of wave angular momentum, and therefore the radial deposition of angular momentum in the disk. Associated with the equation is a characteristic damping length, proportional to the cooling time from the base state temperature times the radial wave speed, and a factor that depends weakly on optical depth. The damping length is used to define, in terms of the disk base state temperature and surface density, regimes of strong and weak radiative damping. Waves in warm disks may be damped close to the resonances at which they are excited, while waves in cold disks can propagate to distances at which other dissipative mechanisms may dominate. It is found that the boundary between these regimes cuts across values that are thought to have characterized the primitive solar nebula. Thus both strong and weak radiative damping may be encountered in circumstellar disks in general.

A test of the smoothness of the elemental abundances of carbonaceous chondrites

The identification of CI chondrite concentrations with average solar system abundances for heavy elements is based primarily on the smoothness of the CI abundance curves for odd mass nuclei. A good test of smoothness is measurement of all elements in a given mass range in the same sample with the same technique. High precision proton-induced X-ray spectra of CI chondrites yielded analyses of 17 elements (Ni through Ru, plus Fe and Pb) with precisions better than 10% for all except As, Pb, Nb, and Ru. Excellent theoretical descriptions of the spectra were obtained. Two independent estimates of precision agree well, giving confidence in the quoted errors. Intersample differences are the largest source of variability. Within these limits good agreement with literature results are obtained, except for As and Y. Although our Y values are 10 to 30% lower than previously adopted, amonoelemental s-process peak in the abundance curve at Y is still necessary. Except for Br (higher by 59% in Ivuna), there are no significant concentration differences between Orgueil and Ivuna. In general, our results confirm previous abundance curves. The abundances are exceptionally smooth and strongly decreasing in the mass 60–75 region. From mass 75–101 a smooth curve can be drawn, within limits of intersample variability, except for the Y peak. Over the whole periodic table a large number of peaks of probable nucleosynthetic origin can be identified, some understood, some not. These smoothness deviations are 10 to 30% and set an overall limit to the smoothness argument alone in justifying using CI abundances as average solar system values.

Microprobe analyses of rare earth element fractionation in meteoritic minerals

Two meteorites were analyzed by PIXE with the Los Alamos nuclear microprobe. The enstatite achondrite Pena Blanca Spring and the ordinary chondrite St Severin were chosen as likely candidates for use in ^(244)Pu(t_(l/2)=82 my) cosmochronology and geochronology. These applications require the meteoritic minerals to have unfractionated actinides and lanthanides relative to “cosmic” elemental abundance ratios. The PIXE analyses produced evidence of actinide-lanthanide fractionation in Pena Blanca Spring oldhamite (CaS) whereas the St Severin phosphates, whitlockite and chlorapatite, do not exhibit this fractionation.

High precision thick target PIXE analyses of carbonaceous meteorites

High precision proton-induced X-ray (PIXE) thick target spectra of carbonaceous meteorites have been deconvoluted using previously described programs. Even in cases of totally overlapping peaks excellent fits to spectra were obtained. Concentrations for 15 elements in the mass range 56–100, as well as Pb, agree well in most cases with previous literature values. The total concentration range studied was over 105, with the lowest concentrations precisely analyzed being around 1 ppm.

Trace element contents of primitive meteorites; A test of solar system abundance smoothness

Elemental abundances from Cl carbonaceous chondrite meteorites are thought to represent the average solar system (“cosmic”) composition, based on the agreement between Cl and solar photospheric abundances and on the smoothness of heavy element abundances of odd mass nuclei when plotted as a function of mass number. To test Cl elemental smoothness, we have analyzed Cl meteorites, using conventional PIXE (proton probe) techniques, where we have good sensitivity for the range of elements: Ni to Mo. Preliminary analyses indicate that elemental smoothness is only approximate, with possible deviations of 30% to perhaps even 50%. This may be due to chemical fractionation. Alternatively, explanations for nonsmooth behavior may be understandable with the aid of general ideas of n-capture nucleosynthesis. As Cl abundances are refined, it could be that the lack of elemental smoothness may provide the strongest argument for the identification of Cl with primordial solar system abundances.

Bismuth and 208Pb microdistributions in enstatite chondrites

Polished sections of 5 enstatite chondrites have been irradiated with 30 MeV ^4He ions to produce the alpha-radioactive nuclei ^(211)At and ^(210)Po from ^(209)Bi and ^(208)Pb, respectively. The distribution of alpha activity can be mapped, using cellulose nitrate as an alpha track detector, to give the corresponding Bi or Pb distributions in the meteorite. No strong localization of Bi or ^(208)Pb was found; relatively uniform track distributions were observed. In particular, metal or sulfide grains are not enriched in Bi or Pb (relative to bulk), which is in agreement with the predictions of nebular condensation calculations. While the track distributions appear uniform, the results of detailed, track-by-track mappings of the Bi detectors indicate that the Bi is not totally randomly distributed; the statistical fluctuations in the observed track density are different for the cases where the Bi is totally randomly distributed and where the Bi is localized in point sources. Assuming that the Bi in a given sample is localized in identical point sources which are uniformly distributed throughout the sample, the observed relative population densities of clusters (‘stars’) of small numbers of tracks (2–5) corresponds to Bi being localized, with ~90% in grains with about 10^(−16)g-Bi (~3 × 10^5 Bi atoms), and with ~10% in 4 × 10^(−14) g-Bi sources. If these are elemental Bi, as predicted theoretically, they are ~ 10^2 A and 10^3 A in size, respectively.