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We present Spitzer Space Telescope observations of the "peculiar variable" DZ Cru, identified by Rushton et al. (2008, MNRAS, 386, 289) as a classical nova. A dust shell, on which are superimposed a number of features, is prominent in the 5-35micron range some 4 years after eruption. We suggest that the dust in DZ Cru is primarily hydrogenated amorphous carbon in which aliphatic bands currently predominate, and which may either become predominantly aromatic as the dust is photo-processed by ultraviolet radiation from the stellar remnant, or more likely completely destroyed., Comment: MNRAS letters, accepted

We present the first high-resolution three-dimensional simulations of the deflagration phase of Type Ia supernovae that treat the entire massive white dwarf. We report the results of simulations in which ignition of the nuclear burning occurs slightly off-center. The subsequent evolution of the nuclear burning is surprisingly asymmetric with a growing bubble of hot ash rapidly rising to the stellar surface. Upon reaching the surface, the mass of burned material is $\approx 0.075 M_\sun$ and the kinetic energy is $4.3 \times 10^{49}$ ergs. The velocity of the top of the rising bubble approaches 8000 km s$^{-1}$. The amount of the asymmetry found in the model offers a natural explanation for the observed diversity in Type Ia supernovae. Our study strongly disfavors the classic central-ignition pure deflagration scenario by showing that the result is highly sensitive to details of the initial conditions., Comment: 11 pages, 4 figures, some figures degraded to reduce size. Submitted to Astrophysical Journal Letters

In a brief review of abundances neutron-capture elements (Z > ~30) in metal-poor halo stars, attention is called to their star-to-star scatter, the dominance of r-process synthesis at lowest metallicities, the puzzle of the lighter members of this element group, and the possibility of a better r-/s-process discriminant., 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic Evolution''

Nucleocosmochronology involves the use of the abundances of radioactive nuclear species and their radiogenic decay daughters to establish the finite age of the elements and the time scale for their formation. While there exist radioactive products of several specific nucleosynthesis mechanisms that can reveal the histories of these mechanisms, it is the long lived actinide isotopes {232}Th, {235}U, and {238}U, formed in the r-process, that currently play the major role in setting the time scale of Galactic nucleosynthesis. Age determinations in the context of Galactic chemical evolution studies are constrained by intrinsic model uncertainties. Recent studies have rather taken the alternative approach of dating individual stars. Thorium/europium dating of field halo stars and globular cluster stars yields ages on the order of 15 +/- 4 Gyr. A solid lower limit on stellar ages is available as well, for stars for which one both knows the thorium abundance and has an upper limit on the uranium abundance. For the cases of the two extremely metal deficient field halo stars CS 22892-052 and HD 115444, lower limits on the nuclear age of Galactic matter lie in the range 10-11 Gyr. Th/U dating of the star CS 31082-00 gives an age of 12.5 +/- 3 Gyr. Observations of thorium and uranium abundances in globular cluster stars should make possible nuclear determinations of the ages of clusters that can be compared with ages from conventional stellar evolution considerations., Comment: 8 pages, 2 figures, 1 table; To appear in Astrophysical Ages and Time Scales, ASP Conference Series, eds. T. von Hippel, N. Manset & C. Simpson

Observations of metal-poor Galactic halo stars indicate that the abundance pattern of the (heaviest) neutron-capture elements is consistent with the scaled solar system r-process abundances. Utilizing the radioactive (r-process) element thorium, age determinations have been made for several of these same stars, placing constraints on both Galactic and cosmological age estimates., Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic Evolution''

We present first results of three-dimensional (3D-) calculations of turbulent and degenerate hydrogen-burning on top of a C+O white dwarf of one solar mass. The simulations are carried out by means of a code which solves Euler's equation for an arbitrary equation of state together with a nuclear reaction network and the energy input from nuclear reactions on a Cartesian grid covering a fraction of the white dwarf's surface and accreted atmosphere. The flow patterns we obtain are very different from those of earlier 2D simulations using the same initial conditions and the same numerical resolution. The possibility of self-enrichment of the accreted hydrogen-rich atmosphere with carbon and oxygen from the surface layers of the white dwarf during the violent phase of the burning is investigated, and it is demonstrated that self-enrichment proceeds too slowly if the accreted gas has near-solar CNO-abundances at the onset of the thermonuclear runaway. As a result, we do not find a fast nova outburst. This conclusion remains valid if the initial metallicity of the accreted gas is raised by a factor of five. Therefore we conclude that fast nova outbursts indeed require huge enrichments of C and O, as postulated from spherically symmetric models, and that the mechanism which leads to such enhancements must operate prior to the outburst., Figure 1 in higher resolution can be obtained from http://ibm-1.MPA-Garching.MPG.DE/~ask/paper_3D/ Submitted to Astronomy and Astrophysics

We present a quantitative analysis of the abundances of heavy elements in damped Ly-alpha galaxies in the sample of Lu et al. (1996). In particular, we compare the observed gas-phase abundances with those expected when the intrinsic (i.e., nucleosynthetic) pattern is the same as that in either the Sun or in Galactic halo stars and when the depletion pattern is the same as that in the warm Galactic interstellar medium, but with various values of the dust-to-metals ratio. We find that the observations are equally consistent with the solar and halo-star intrinsic patterns and that they favor some depletion, the typical dust-to-metals ratio being 40%-90% of that in the Milky Way today. However, neither intrinsic pattern matches the observations perfectly. For the solar pattern, the discrepancy is mainly with [Mn/Fe], while for the halo-star pattern, the discrepancy is with [Zn/Fe], [Ni/Fe], and possibly [Al/Fe]. Our analysis does not support the claim by Lu et al. that the damped Ly-alpha galaxies have halo-star abundance patterns and no dust depletion., Comment: latex, AAS Macros, 15 pages, 2 figures, moderately revised version, to be published in ApJ Letters, 1997 July 20

The prediction of standard chemical evolution models of higher abundances of He3 at the solar and present-day epochs than are observed indicates a possible problem with the yield of He3 for stars in the range of 1-3 solar masses. Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it is noted that galactic and stellar evolution uncertainties necessarily relax constraints based on He3. We incorporate into chemical evolution models which include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which predict that low mass stars are net destroyers of He3. Since these yields do not account for the high \he3/H ratio observed in some planetary nebulae, we also consider the possibility that some fraction of stars in the 1 - 3 solar mass range do not destroy their He3 in theirpost main-sequence phase. We also consider the possibility that the gas expelled by stars in these mass ranges does not mix with the ISM instantaneously thus delaying the He3 produced in these stars, according to standard yields, from reaching the ISM. In general, we find that the Galactic D and He3 abundances can be fit regardless of whether the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5})., 20 pages, latex, 9 ps figures

The ages of globular clusters inferred from observations depends sensitively on assumptions like the initial helium abundance and the mass loss rate. A high helium abundance (e.g., $Y\approx$0.28) or a mass loss rate of $\sim$10$^{-11}M_\odot$ yr$^{-1}$ near the main sequence turn-off region lowers the current age estimate from 14 Gyr to about 10--12 Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. Possible mechanisms for the helium enhancement in globular clusters are discussed, as are arguments for an instability strip induced mass loss near the turn-off. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is $>0.30$, which would conflict with indirect measurements of helium abundances in globular clusters., revtex 17 pages, 4 figures upon request, FERMILAB-PUB-94/060-A