MACROCHONDRULES IN SOME CHONDRITES: 2. GENETIC ASPECTS

An electron microscopic and microprobe study of the macrochondrules from the chondrites Allende CV3, Krymka LL3.1, Saratov L4, "Velyka Balka" L4-5 and Château-Renard L6 allow to clear some aspects of their formation. The main of them are following: 1) The texture and mineral composition of the majority of the macrochondrules does not differ from those of ordinary chondrules, thus indicating a similarity in formation conditions, 2) It is shown for the first time that the SiO2/MgO and FeO/(FeO+MgO) ratios of the fine-grained rims of the macrochondrules and one`s of chondrules in the Krymka meteorite are well correlated. This indicates to their coexistence in the same dusty environment of the protoplanetary nebula, 3) The unique characteristics of the two firstly found macrochondrules allowed us to detail conditions of their formation and existence in the protoplanetary nebula. A polyzonal graphite-bearing macrochondrule from the Krymka meteorite formed by accretional growth in a mineralogically variable dusty environment. Its complex evolutional history involved remelting and selective evaporation of the macrochondrule surface layer, accretion of an environmental dust, late formation of a fine-grained rim and migratory intrusion of an organic protomaterial and finally a metamorphic event that produced graphite. The melted surface of a macrochondrule from the Allende chondrite was notably affected by high-energy processes and instantaneous cooling in the protoplanetary nebula, 4) The rarity of macrochondrules in chondrites compared to typical chondrules is probably related to their priority destruction. This was caused by permanent movement and collision between them and other solid objects in the pre-agglomeration period of meteorite development.

Numerical simulations of wind-driven protoplanetary nebulae. II. signatures of atomic emission

We follow up on our systematic study of axisymmetric hydrodynamic simulations of protoplanetary nebula. The aim of this work is to generate the atomic analogues of the H2 near-infrared models of Paper I with the ZEUS code modified to include molecular and atomic cooling routines. We investigate stages associated with strong $\mathrm{[Fe\, {\small II}] \, 1.64\, \mathrm{\mu m} }$ and $\mathrm{ [S\, {\small II}] \, 6716}$ Å forbidden lines, the $\mathrm{[O\, {\small I}]\, 6300}$ Å airglow line, and Hα 6563 Å emission. We simulate (80 ∼ 200 km s−1) dense (∼105 cm−3) outflows expanding into a stationary ambient medium. In the case of an atomic wind interacting with an atomic medium, a decelerating advancing turbulent shell thickens with time. This contrasts with all other cases where a shell fragments into a multitude of cometary-shaped protrusions with weak oblique shocks as the main source of gas excitation. We find that the atomic wind-ambient simulation leads to considerably higher excitation, stronger peak and integrated atomic emission as the nebula expands. The weaker emission when one component is molecular is due to the shell fragmentation into fingers so that the shock surface area is increased and oblique shocks are prevalent. Position-velocity diagrams indicate that the atomic-wind model may be most easy to distinguish with more emission at higher radial velocities. With post-AGB winds and shells often highly obscured and the multitude of configurations that are observed, this study suggests and motivates selection criteria for new surveys.

UV Monochromatic Imaging of the Protoplanetary Nebula Hen 3-1475 Using HST STIS

Collimated outflows and jets play a critical role in shaping planetary nebulae (PNe), especially in the brief transition from a spherical AGB envelope to an aspherical PN, which is called the protoplanetary nebula (pPN) phase. We present UV observations of Hen 3-1475, a bipolar pPN with fast, highly collimated jets, obtained with STIS on board the Hubble Space Telescope (HST). The deep, low-dispersion spectroscopy enabled monochromatic imaging of Hen 3-1475 in different UV nebular emission lines; this is the first of such attempt ever conducted for a pPN. The northwest inner knot (NW1) is resolved into four components in Mg ii λ 2800. Through comparison analysis with the HST optical narrowband images obtained 6 yr earlier, we found that these components of NW1 hardly move, despite of a negative gradient of high radial velocities, from −1550 km s - 1 on the innermost component to ∼−300 km s - 1 on the outermost. These NW1 knot components might thus be quasi-stationary shocks near the tip of the conical outflow of Hen 3-1475.