Post-shock Temperature Research Articles

Shock recovery experiments on dolomite and thermodynamical calculations of impact induced decarbonation

We have studied experimentally shocked dolomites and calcites by scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, and mass spectrometry analysis of the abundance ratios of the stable isotopes of carbon and oxygen. The shock‐recovery experiments have been perfomed by the multiple reverberation technique on natural dolomite rocks at 60 GPa, using steel devices and high‐explosive driver‐flyer plates as plane shock wave generators. Modified assemblies with grooves and holes were built in order to facilitate the escape of CO2, in case the conditions of breakdown of the carbonates into oxides and CO2 would be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence for outgassing, expressed, for example, by the identification of CaO or MgO, could be observed. Consequently, no isotope fractionation occured in the shocked samples. This result is consistent with the calculations of peak‐shock and postshock temperatures, as well as with the examination of outgassing conditions of carbonates, calculated in this study up to 80 GPa. We have shown that in a direct shock, outgassing in air of nonporous dolomites and calcites should occur in impacts at 55–65 GPa and 35–45 GPa, respectively. The effect of porosity, which strongly lowers these values, has been estimated. The experimental setup for shock‐recovery experiments is shown to be an important parameter: reverberated shocks lead to lower peak‐shock and postshock temperatures than direct shocks at the same pressure; differences among experimental setups might explain part of the discrepancies between previous studies. In this study, the only surviving shock‐induced phenomenon is pulverization leading to grain sizes smaller than in the starting material. The decrease in grain size has been quantified via a structure refinement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of shock effects in carbonates could probably be based on such parameters.

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Explosions of infalling comets in Jupiter's atmosphere

In view of the expected collision of comet Shoemaker-Levy 9 (1993e) with Jupiter in 1994 July, we calculate basic properties of the initial interaction for a simplified Jovian atmosphere. The comet is expected to impact Jupiter at 60 km/sec and at an angle of 45 deg to the zenith. The shock wave generated by the bolide should be optically thick once it has penetrated to an atmospheric density approximately 10(exp -6) gr/cm(exp 3), and we calculate the post-shock conditions assuming local thermodynamic equilibrium (LTE) for shock velocities v(sub sh) in the range 10 to 60 km/sec and preshock densities rho(sub a) = 10(exp -6) to 10(exp -2) gr/cm(exp 3). Our shock calculations include molecular hydrogen, atomic hydrogen, ionized hydrogen, neutral helium, and singly ionized helium. Even at the highest shock velocity, the gas is only partially ionized and the postshock temperature rises with preshock density in order to maintain the ionization. The value of the effective shock adiabatic index gamma(sub sh) varies from 1.17 (at low v(sub sh) and rho(sub a) to 1.40 (at high v(sub sh) and rho(sub a). The ablation rate is limited by the radiative flux that reaches the bolide surface. We argue that the ablated gas does not efficiently transfer its kinetic energy to the atmosphere, and it ultimately slows in a similar fashion to the comet material. As the bolide initially falls through the atmosphere, the character of the shock emission changes. At rho(sub a) approximately 10(exp -8) gr/cm (exp 3), the gas is optically thin and we expect line emission; in the optical spectrum, Balmer emission is expected from the shocked atmosphere and low-ionization metal lines from ablated cometary material. At rho(sub a) approximately 10(exp -6) gr/m(exp 3), the shocked gas is optically thick and the shock front near the bolide produces a blackbody spectrum. The temperature is favorable for ultraviolet (1000 to 3000 A) emission and the luminosity may be approximately 5 x 10(exp 23) ergs/sec for approximately 0.6 sec for a bolide 1 km in radius. At rho(sub a) approximately 10(exp -4) gr/cm(exp 3), the bolide has passed below the ultraviolet photosphere. The shock front emits considerable ionizing radiation, but it is absorbed in a narrow preshock region. The bolometric correction for the optical luminosity is large and we expect a 3000 to 8000 A luminosity of approximately 3 x 10(exp 23) ergs/sec for approximately 1 sec. The optical emission is strongly peaked in the vicinity of the bolide. The bolide does have a somewhat less luminous, optically thick trail extending greater than or equal to 10 km, but the radiation is characterized by a temperature of 4000 to 5000 K. From the fragmentation model of Chyba, Thomas, & Zahnle (1993), the bolide deposits most of its kinetic energy at rho(sub a) approximately 10(exp -3) gr/cm(exp 3) and this is the effective explosion site. The shock wave from such an explosion can move up about one density scale height. We examine the breakout of the shock front from the Jovian atmosphere and find that the shock acceleration in the decreasing density region is slow, so that the energy flux in the shock front is small. Higher velocities might be generated by shock acceleration along the channel left by the bolide if the shock motion can occur before the channel closes off as a result of radiative cooling. Hot gas created by the explosion ultimately rises due to buoyancy on a timescale of a minute. The luminosity is highest when the bubble first rises into the optically thin part of the atmosphere and may be approximately 1 x 10(exp 25) ergs/sec in the near-infrared. Roughly 1% of the initial bolide energy may be radiated in this way; the rest of the energy is lost to sound waves from the initial explosion and to work done by the bubble on the surrounding atmosphere.

H2-air and CH4-air detonations and combustions behindoblique shock waves

Two stoichiometric reactive mixtures (H 2 -air and CH 4 -air) at initial conditions of temperature and pressure, T 0 =293 K and p 0 <1 bar, respectively, very different from the point of view of their detonability, are used in order to study the conditions of the onset of combustion and/or detonation when submitted to the action of an oblique shock wave (OSW) at Mach number M of about 6 and at various angles of inclination θ. Generation of an OSW in the reactive mixture at rest is obtained via lateral expansion of the detonation products of a normal CJ detonation wave propagating in a bounded gaseous detonable mixture separated from the mixture under test by a 6- μ m-thick mylar film. Different steady combustion regimes are observed depending on θ and on the reactive mixture. Based on simple polars analysis, the possibility of observing an oblique detonation wave (ODW) are checked for the two mixtures. In the case of H 2 -air mixtures, ODWs are obtained with small degrees of overdrive ( D/D CJ ) (≊1) ofthe detonation (where D is the normal detonation velocity of the ODW) after a predetonation zone, where the OSW is followed by an oblique flame initiated at the beginning of the interaction between the two media. Minimal conditions for detonative combustion are deduced, which correspond to a postshock temperature T s ≊1000 K while the minimal condition for combustion as an oblique flame behind the OSW is T s ≊800 K. For CH 4 -air mixtures, no detonation appears even when the degree of overdrive of the theoretically possible solutions of stable ODW is increased up to 1.1 and beyond for T s up to 1480 K. Only the OSW and oblique flame configuration are observed. For T s <1000 K, no combustion occurs behind the shock wave. Thus, this mixture appears to be especially difficult to detonate.

Impact-induced phase transformations at 50–60 GPa in continental crust: an EPMA and ATEM study

Strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 Ma old Haughton impact crater were studied by optical microscopy, electron probe microanalysis (EPMA) and analytical transmission electron microscopy (ATEM). The study is focused on the substantiation of shock-induced phase transformations in rock-forming minerals, and their implications for post-shock thermal regimes and impact age dating. In the gneiss fragments, the original layering is largerly preserved, although all the minerals have experienced dramatic phase transformations. Chemically, the various shock-generated decomposition regions still reflect the pre-shock mineralogy. However, significant heterogeneities exist on the 10–100 μm scale. Four different regions of decomposition products can be distinguished (EPMA): (1) shocked biotite-rich regions with strong variations in Si, Al, K, Fe and Mg on the 10 μm scale, (2) shocked quartz with about 3% Al and K and traces of Fe and Mg, (3) a region with Al/>Si ≈ 1, which does not correspond to any pre-shock mineral, and (4) shocked K-feldspar showing a significant deficit in K, and traces of Fe and Mg. On the nanometer scale, a uniform, bimodal reaction pattern emerges: all minerals are transformed to new crystals embedded in a silica-rich glass. The shock-generated crystals are: (1) spinel in shocked biotite, (2) α-quartz, cristobalite, and coesite in shocked quartz, (3) an unknown polymorph of corundum (Al 2O 3) in feldspar and in the Al/Si ≈ 1 region, and (4) mullite in association with SiO 2 glass in shocked sillimanite grains. All crystals range in size between 0.05 and 1.0 μm. Taking into account estimated durations and pressure-temperature conditions for shock-wave passage and post-shock temperatures, the reactions observed can be ascribed to high temperatures during and after pressure release. The different mineral-glass parageneses suggest temperatures in excess of 1400°C. Moreover, given the small size of the newly formed minerals, very rapid cooling down to 1200°C is required.

Shock metamorphism of carbonaceous chondrites

We have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites. We find that sixty-nine carbonaceous chondrites can be assigned to four shock stages, SI to S4, largely on the basis of shock effects in olivine. The least shocked chondrite groups are CM2 and CO3: thirty-six out of thirty-eight members are classified as shock stage S1 (<5 GPa), the remainder are stage S2 or S3. The most strongly shocked groups are CK4-6 and CV3, which both have more than half their members in shock stages S2 and S3, the remainder are mostly stage S1. Two carbonaceous chondrites of shock stage S4 were found and none of stages S5 and S6. Opaque shock veins and melt pockets were observed in the two stage S4 chondrites, Efremovka (CV3) and EET 83311 (CK5), and a stage S3 chondrite, LEW 87009 (CK6): this is consistent with the distribution of impact melt in ordinary chondrites. Some carbonaceous chondrites appear to be breccias of material shocked to diverse shock stages prior to assembly of the material. For example, Murchison (CM2) appears to be a mixture of stage S1 and S2 material. Differences between the mean shock levels of certain groups of chondrites, e.g., CO3 and CV3, are probably due to stochastic differences in the sampling of their parent bodies. However, there is an overall tendency for the mean shock level of carbonaceous chondrites (and ordinary chondrites) to increase with increasing petrologic type that may reflect real differences in the shock level of near-surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials. Petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6. The greater porosity of types 2–3 causes higher post-shock temperatures and melting at lower pressures, and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure. We suggest that type 2–3 material shocked above 20–30 GPa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites. In the CV3 group, there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism, analogous to that discovered in ordinary chondrites by Sneyd et al. (1988), suggesting that shock rather than static overburden pressure is responsible for chondrule flattening. We infer that the correlations are due to collapse of pores under shock pressures of at least 5–10 GPa and that shock is an important process affecting many physical properties of type 2–4 chondrites.

Dynamics of shock waves in a contracting cloud

The contraction of an interstellar cloud is followed. The results indicates that there are shock waves appear during contraction. In order to study the effect of shock waves, two models have been studied. The post-shock temperature for the two models are, respectively, 3006 K and 2984 K. The density increases by more than three orders of magnitude. The gas is generally cooled by atoms, molecules, and grains. The dominant cooling process changes according to the chemical composition and the temperature of the gas. The thermal equilibrium depends on the existing physical conditions.

On hydrogen line formation in the atmospheres of pulsating giants

An analysis of the effects of radial pulsation and shock propagation on the first three hydrogen lines formed in the atmosphere of a 13.3-d W Vir model has been performed using a method allowing the solution of the transfer equations for an arbitrary velocity distribution. A three-level atom is incorporated into the comoving transfer theory, and a non-LTE self-consistent solution is obtained by the equivalent two-level-atom formalism. The calculated Hα profiles and spectroscopic velocity curve are in semi-quantitative agreement with observations of the Population II Cepheids. The emission component, formed behind the shock front, is purely thermal. Its strength and frequency position reflect, respectively, the height of the post-shock temperature peak and the post-shock gas velocity. Maximum equivalent width of Hα was about 1 Å. The absorption components may originate, mostly due to scattering, either in the pre-shock or post-shock regions, or in both, and this causes a discontinuity on the empirical velocity curve when |$\Delta\tau_l$|⁠, of the pre-shock matter becomes < 10. It is found that strong absorption line velocities are very close to 17/24 times the radial velocities and hence may be used for the radial displacement measurements.

Postshock temperatures in silica

Experimental postshock temperatures for crystalline quartz released from shock states between 86 and 127 GPa and for fused silica released from 59 to 73 GPa have been determined using previously measured spectral radiance data. The temperatures range from 3660 to 4150 K and are consistent with the interpretation based on shock temperature measurements, that the Hugoniot of SiO2 crosses the phase boundary between stishovite and liquid. The new postshock temperatures were used to determine the Grüneisen parameter of liquid silica at high pressures and temperatures and the specific volume of the release state (γ/V = 3.5 Mg/m3 and Vr = 0.266 m3/Mg, respectively), although there is some ambiguity as to whether this volume corresponds to a partial or total release state. To show the validity of these postshock temperatures, a radiative transport model was extended to times after free surface arrival of the shock wave.

Models of Shock Activity in the BN-KL Region of Orion

The observed H2, CO, OI(63 μm), SiII(34.8 μm) and H51α emission line intensities in the BN-KL region of Orion are modeled as arising from two types of shock waves. The molecular emission arises from non-dissociative shocks with low peak postshock temperatures (&lt; 3000 K), traveling at speeds of vs ∼ 40 km/s into ambient gas of density n0∼105-6 cm−3 and transverse magnetic field B0 ∼0.5 milligauss. The OI(63 μm), Sill(34.8 μm) and H51α emission arises in the cooling regions behind faster (∼80 km/s) dissociative shocks with high peak postshock temperatures (∼105 K) and with preshock densities n0 &gt;3x104 cm−3.

The soft X-ray background as a supernova blast wave viewed from inside - Solar abundance models

A model of the soft X-ray background is presented in which the Sun is assumed to be inside an active supernova blast wave. The blast wave evolves in a preexisting cavity. The broad band surface brightnesses is explained by such a blast wave with an explosion energy of E sub approx. 5 x 10 to the 50th power ergs and radius 80 to 100 pc, using solar abundances. An approach to treating the problem of large anisotropies in the ambient medium is also explored, accommodating the observed anticorrelation between the soft X-ray surface brightness and the 21 cm column density. It is found that only for post shock temperatures below 10 6 K a shock propagating into a density enhancement will be dimmer than a similar shock in a lower density region.

Analysis of Normal Shock Waves in a Carbon Particle-Laden Oxygen Gas

The propagation of a normal shock wave into a quiescent oxygen gas seeded with carbon particles is studied. Due to the elevated postshock temperature the carbon particles ignite and burn until they disappear. For evaluating the effect of the burning carbon particles on the postshock-wave flow field, i.e., the relaxation zone, the conservation equations for a steady one-dimensional reactive suspension flow are formulated and solved numerically. The solution was repeated for a similar inert suspension flow. Comparing the two solutions revealed that the carbon burning has a major effect on the suspension properties in the relaxation zone and on the eventually reached postshock equilibrium state. For example, much higher temperatures and velocities are obtained in the reactive suspension while the pressure is lower than in a similar inert case. Longer relaxation zones are obtained for the reactive suspension.

Shock metamorphism and petrography of the Shergotty achondrite

The Shergotty subsamples 1 and 12 consist of augite and pigeonite (67.5%), maskelynite (24%), ilmenite and titanomagnetite (2%), pyrrhotite (0.4%), whitlockite (1.8%), apatite (0.1%), quartz (0.5%), baddeleyite (trace), fayalite (0.4%), mesostasis (3%), and shock-induced local, polymineralic melt products (0.6%). The overall modal composition is similar to other Shergotty samples except for the rather high whitlockite content. The shock effects observed in the mineral constituents include mosaicism, deformation bands, planar fractures, and mechanical twin lamellae in clinopyroxene; isotropization of plagioclase with very rare remnants of birefringence; planar deformation structures, mosaicism, and strongly reduced birefringence in quartz; mechanical twinning of ilmenite; localized in situ melting of neighbouring minerals at the contact of low and high density phases. Based on the refractive index of maskelynite (average: 1.5467 with average An-content of 49%) and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 ± 1 GPa is derived. The inferred post-shock temperature is 200 ± 20°C. No heating event could have exceeded 400°C (DUKE, 1968). Local stress and temperature concentrations reach 60–80 GPa and 1600–2000°C. The observed shock effects can be explained by a single shock event. A second, weaker shock event as found by others appears to be highly improbable. Equilibrium shock pressures and post-shock temperatures for the other known shergottites are 31 ±2 GPa and 220 ± 50°C (Zagami), 43 ± 2 GPa and 400–800°C (ALHA 77005). The pressure estimate for EETA 79001 by Lambert (1985) is confirmed: 34 ± 1 GPa; the post-shock temperature is 250 ± 50°C. The abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence Shergotty, Zagami, EETA 79001, ALHA 77005. Undoubtedly, the melts could have been formed by the same single shock which produced the equilibrium shock effects (e.g. maskelynitization) in these meteorites. The shock-induced particle velocities inferred from Hugoniot data of basalts are in the 1.5–2.0 km/s range for the parental rocks of the shergottites. Ejection velocities are therefore in the order of 3–4 km/s. Special ejection mechanisms are required in order to exceed the escape velocity of a planet like Mars without producing higher degrees of shock (e.g. melting) than those observed in the shergottites.

The terrain of evolution of isotropic adiabatic supernova remnants

Models of the evolution of supernova remnants in isotropic media are reviewed and generalized. It is shown that properties of remnants (including cooling and volume-filling aspects) are almost history-independent functions of their instantaneous average density and postshock temperature. These properties, presented as a contour diagram in the density-temperature plane, thus define the territory through which remnant properties evolve. In different models, remnants merely take different paths across the available terrain. For the Milky Way, M101, and the LMC, the portions of this terrain which might actually be sampled by the remnant populations are discussed briefly.

Cyclotron lines in accreting magnetic white dwarfs with an application to VV Puppis

view Abstract Citations (44) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cyclotron lines in acccreting magnetic white dwarfs with an application to VV Puppis. Barrett, P. E. ; Chanmugam, G. Abstract Calculations of cyclotron line spectra for homogeneous plasmas are made for emission and absorption models. The theoretical spectra show that the visibility of cyclotron lines in the optical spectrum is limited to a small parameter space in magnetic field strength (2 - 10×107gauss), plasma temperature (⪉15 keV), and the accretion column nearly perpendicular to the line of sight for extended periods of time. These model spectra also confirm the conclusion by Wickramasinghe and Meggitt that the broad lines in VV Pup are due to cyclotron emission. The best fit to the data of VV Pup results in a polar magnetic field strength of B = 3.15×107 gauss, a postshock temperature kT = 8.7 keV, and dimensionless plasma parameter Λ ≈ 106. Publication: The Astrophysical Journal Pub Date: November 1985 DOI: 10.1086/163655 Bibcode: 1985ApJ...298..743B Keywords: Cyclotron Radiation; Plasmas (Physics); Stellar Mass Accretion; White Dwarf Stars; Absorption; Astronomical Models; Binary Stars; Emission; Magnetic Stars; Astrophysics full text sources ADS | data products SIMBAD (1)

A two-dimensional spectrum of a nonradiative shock filament in the Cygnus Loop

view Abstract Citations (38) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A two-dimensional spectrum of a nonradiative shock filament in the Cygnus Loop. Fesen, R. A. ; Itoh, H. Abstract A two-dimensional spectrum of a nonradiative filament in the Cygnus Loop covering the spectral region 4600-7500 A is presented and discussed. Emission lines of H, He II, and forbidden O III, as well as extremely faint N II and S II forbidden lines, were detected. Small spatial differences for some of the filament's emission lines and a possible velocity gradient across the filament's western edge were also observed. Measured relative line intensities are combined with previous nonradiative data and are compared to new nonradiative shock models which treat one, two, and three postshock temperature distributions. It is found that two- and three-temperature models provide a good fit to the observed relative line intensities. The observed weak N II and S II forbidden line emissions are interpreted as due to either radiative shock emissions produced by small density fluctuations in the shocked cloud (e.g., cloud cores), or faint dust grain reflection of the remnant's bright radiative filament emission. Publication: The Astrophysical Journal Pub Date: August 1985 DOI: 10.1086/163347 Bibcode: 1985ApJ...295...43F Keywords: Emission Spectra; Nebulae; Optical Emission Spectroscopy; Shock Waves; Supernova Remnants; Balmer Series; Helium; Hydrogen; Line Spectra; Nitrogen; Radial Velocity; Spectrophotometers; Sulfur; Astrophysics full text sources ADS | data products SIMBAD (2)

Shock waves in Mira variables. I - Emission-line spectra

A catalog of Balmer emission-line profiles (H-gamma, H-delta, H-zeta, and H-eta) is presented for a sample of nine Mira variables ranging in period from 150 to 394 days. These were obtained at very high dispersion with phase coverage of about half a cycle centered around maximum. Observations of emission lines of Mg, Si, and Fe are also described. Variations with phase and cycle number of line-profile shape and flux are discussed in relation to a simple shock model in which a spherically symmetric shock wave is propagating outward through the atmosphere of a pulsating Mira variable. From the simple shock model, estimates are made for the shock speed and postshock temperature at phases during which the Balmer emission lines are visible.

Chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from Lappajärvi Crater, Finland

Plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks (kärnäite) were sampled from the coherent melt sheet located in the central crater area of the Lappajärvi meteorite crater and analyzed by optical, electron optical, X ray, and microprobe methods. Both types of feldspars display strong chemical, textural, and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt. Increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt. In a lower degree of shock both primary plagioclase and microcline transform to ‘checkerboard’‐textured grains that consist of 5–10–μm‐sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz. The subgrains are strongly zoned, disordered labradorite to andesine and rimmed by sanidine. In a high degree of shock the subgrains are larger and randomly oriented. The subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place. On the basis of model calculations it is concluded that the temperature of the kärnäite melt at the time of the incorporation of the clastic debris was in the range of 1800° to 2100°C. The equilibration temperature of the clast‐melt mixture is estimated accordingly to be at least in the range of 1100° to 1140°C or higher, depending on the magnitude of the clast's postshock temperature. This temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars. The clast‐laden melt was at rest when the equilibrium temperature was reached, suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours.

The relaxation zone behind normal shock waves in a dusty reacting gas. Part 2. Diatomic gases

The propagation of a strong normal shock wave into a quiescent mixture of nitrogen gas seeded with small, spherical inert dust particles is studied. While crossing the shock front, the gaseous phase of the suspension experiences a sudden change in temperature, pressure, density and velocity. (These changes can easily be evaluated using the Rankine-Hugoniot relations.) The solid phase of the suspension (dust) is initially unaffected by the shock wave. As a result, immediately behind the shock front, one phase of the suspension (the nitrogen gas) is in a state of relatively high temperature and low velocity while the other (the dust) is in a state of relatively low temperature and high velocity. Owing to these differences in temperature and velocity, intense heat transfer and viscous interactions between the two phases take place leading eventually to a new state of equilibrium that is reached farther downstream of the shock front. The flow field where these interactions take place, the relaxation zone, is solved numerically. It is shown that the spatial extent of this zone is strongly affected by the mass concentration of the dust in the suspenson and its physical properties (size, density and specific-heat capacity). These parameters also affect the post-shock equilibrium suspension properties. It was found that increasing the dust concentration results in a shorter kinematic relaxation zone, higher post-shock suspension pressure, density and temperature, and lower velocity, as compared to a similar pure-gas case. Increasing the dust particle density or its diameter results in a longer relaxation zone and a higher post-shock equilibrium suspension pressure, density and temperature. Changes in the dust specific-heat capacity affect the extent at the thermal relaxation length and the suspension temperature and density; they do not affect the extent of the kinematic relaxation length or the post-shock suspension pressure and velocity. For the range of dust concentration, size, density, specific-heat capacity and shock-wave Mach number investigated, the kinematic relaxation zone is always longer than the thermal relaxation zone.

Velocity field and physical conditions in the active lenticular galaxy NGC 3998

A rotating and expanding flattened distribution of gas is suggested by measurements of the emission line velocity field for the line elliptical/lenticular galaxy NGC 3998, using seven long slit spectrograms in five position angles. Expanding material kinetic energy values of 10 to the 53rd to 10 to the 54th ergs, together with the flat spectrum radio source and nucleus X-ray emission, indicate pronounced nuclear activity. Spectrophotometry of the galactic nucleus shows emission line strengths typical of shocks rather than of photoionization, and line ratios indicate a postshock temperature of 60,000 K and a preshock density of 25 particles/cu cm. Both the stars and the ionized gas of the galaxy have central velocity dispersions of 260 km/s. In view of the high rotational velocity of the stars, NGC 3998 is a lenticular rather than elliptical galaxy.