High Density Contrast Research Articles

Structural Constraints on Paraná Basalt Volcanism and their Implications on Agate Geode Mineralization (Salto do Jacuí, RS, Brazil)

The Paraná-Etendeka Continental Flood Basalt province hosts world-class agate and amethyst geode deposits in Rio Grande do Sul (Brazil; Serra Geral Fm.). Salto do Jacuí Mining District (Rio Grande do Sul, Brasil) has different types of agate geode hosted in vesicular basalt. A series of structural features has recently been investigated in the Salto do Jacuí Mining District, and indicates at least two volcanic episodes: i) normal tholeiitic basalt and dacite eruption, and ii) vesicular basalt and dacite intrusions as sills and dikes. These structural features include: basalt and aeolian sandstone xenoliths in vesicular basalts, vesicular basalt apophyses in massive basalts, sandstone and basalt breccias, sandstone dikes cutting across vesicular lavas and connected to mixed sandstone-agate geodes, sandstone assimilation by vesicular lava, and mixed sandstone and agate geodes. These features show that agate geodes were formed by melting of Botucatu sandstone xenoliths. High density contrast between vesicular basalt and Botucatu sandstone melts makes them immiscible during flow. Botucatu sandstone xenoliths melting is favored by degasing of intrusive volatile-rich basalts. The high-silica globs crystallize dynamically in a closed-system environment, giving rise to agate banding and fibrosity.

Generation of density inhomogeneities by magnetohydrodynamic waves in two dimensions

Using two dimensional simulations, we study the formation of structures with a highdensity contrast by magnetohydrodynamic waves in regions in which the ratio of thermal to magnetic pressure is small. The initial state is a uniform background perturbed by fast-mode wave. Our most significant result is that dense structures persist for far longer in a two-dimensional simulation than in the one-dimensional case. Once formed, these structures persist as long as the fast-mode amplitude remains high.

3D Photoionisation Modelling of NGC 6302

We present the first results from our 3D photoionisation modelling of NGC 6302, an extreme high-excitation bipolar planetary nebula with a dense dusty circumstellar disc. We are attempting to fully match the observed spectrum, including the high-ionization infrared coronal lines as well as the observed ISO spectrum. We use a bipolar model nebula with a dense circumstellar torus. Our initial results show that, considering the very high stellar temperature required to match the high ionization coronal lines, an extremely high density contrast between the bipolar lobes and the circumstellar disc is required to match the nebular line ratios.

Sound attenuation by small spheroidal particles due to visco-inertial coupling

The sound attenuation at ultrasonic frequencies caused by small spheroidal particles in a fluid is examined with regard to the size parameters that determine the shape of the attenuation spectrum. Our investigations are based on a coupled phase model for spheroids with arbitrary orientation, thus facilitating the calculation of average attenuation for a given orientation distribution. Since the model just considers the visco-inertial coupling, its applicability is restricted to small solid particles with high density contrast. The calculated attenuation spectra of mono-sized, randomly aligned spheroidal particles are compared with the attenuation spectra of mono-sized spheres. When the latter approximate the former to a reasonable degree the size of the spheres is called attenuation equivalent diameter. It is shown that the concept of attenuation equivalent diameter can be applied only to slightly elongated prolates. Oblates and very stretched prolates yield considerably broader attenuation spectra than mono-sized spheres. While for oblates and for slightly elongated prolates the characteristic frequency of the attenuation spectrum is determined by the volume specific surface, no such attenuation determining size parameter could be identified for very stretched prolates.

The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity

We present a first deep colour-magnitude diagram of the putative central region (0.5 deg x 0.5 deg) of the Canis Major stellar over-density (l,b)=(240,-8) found by Martin et al. (2004), which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main-sequence, extending 6 magnitudes below the turn-off to our limiting magnitude of B ~ 24.5 mag. This main sequence has very high constrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r_{1/2}) of this system based on the B-band width of the lower main sequence, obtaining 0.94 +/- 0.18 (random) +/- 0.18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar-overdensity: a flared or warped Galactic disk viewed in projection (Momamy et al. 2004). We also derived a central surface brightness of mu_{V,0}= 23.3 +/- 0.1 mag arcsec^{-2} and an absolute magnitude of M_{V}=-14.5 +/- 0.1 mag. These values place the Canis Major object in the category of dwarf galaxy in the the L_{V}--size and M_V - mu_{V} planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the[Fe/H] -M_V plane in the sense that it is too metal rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different to that of isolated dwarfs.

Molecular gas in NUclei of GAlaxies (NUGA)

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951 (Seyfert 2). These galaxies have been observed as part of the NUclei of GAlaxies–(NUGA) CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. The stellar potentials are derived using high-resolution near infrared (NIR) images and the averaged effective torques on the gas are estimated using the high-resolution (~0.5´´–2´´) CO maps of the galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. In particular, we estimate on a case-by-case basis the efficiency of viscosity versus gravity torques to drive AGN fueling. We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and ~a few 100 pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode. The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed.

Effects of photon escape on diagnostic diagrams for H II regions

In this article we first outline the mounting evidence that a significant fraction of the ionizing photons emitted by OB stars within Hii regions escape from their immediate surroundings, i.e from what is normally defined as the Hii region, and explain how an Hii region structure containing high density contrast inhomogeneities facilitates this escape. Next we describe sets of models containing inhomogeneities which are used to predict tracks in the commonly used diagnostic diagrams (based on ratios of emission lines) whose only independent variable is the photon escape fraction, . We show that the tracks produced by the models in two of the most cited of these diagrams conform well to the distribution of observed data points, with the models containing optically thick inhomogeneities (CLUMPY models) yielding somewhat better agreement than those with optically thin inhomogeneities (FF models). We show how variations in the ionization parameter U, derived from emission line ratios, could be due to photon escape, such that for a given region from which 50% of its ionizing photons leak out we would derive the same value of U as for a region with no photon escape but with an input ionizing flux almost an order of magnitude higher. This e ect will occur whether the individual inhomogeneities are optically thick or thin. Photon escape will also lead to a change in the derived value of the radiation hardness parameter, and this change di ers significantly betwen models with optically thin and optically thick clumps. Using a rather wide range of assumptions about the filling factor of dense clumps we find, for a selected set of regions observed in M 51 by D´ iaz et al. (1991) an extreme limiting range of computed photon escape fractions between near zero and 90%, but with the most plausible values ranging between 30% and 50%. We show, using oxygen as the test element, that models with di erent assumptions about the gas inhomogeneity will tend to give variations in the abundance values derived from diagnostic diagrams, but do not claim here to have a fully developed set of diagnostic tools to improve abundance determinations made in this way. We do present an important step towards an eventual improvement in abundance determinations: the combination of line ratios with the absolute H luminosity of a given Hii region, which allows us to determine the photon escape fraction, and hence resolve the degeneracy between U and . We use observational data of this type show that a large set of Hii regions in M 101 observed by Cedr´ es and Cepa (2002) all show significant photon escape with values of ranging up to 60% in the leakiest cases.

Clump Development by the Nickel Bubble Effect in Supernovae

We used one-dimensional radiative-transport radiation hydrodynamical (RHD) simulations to investigate the formation of clumping in freely expanding supernova ejecta due to the radioactive heating from the 56Ni → 56Co → 56Fe decay sequence. The heating gives rise to an inflated Ni bubble, which induces a forward shock that compresses the outer ambient gas into a shell. The radiative energy deposited by the radioactivity leaks out across the shock by radiative diffusion, and we investigate its effect on the evolution of the ejecta structure. Compared to the hydrodynamical (HD) adiabatic approximation with γ = 4/3, the preshock gas becomes accelerated by the radiation outflow. The shock is thus weakened and the shell becomes broader and less dense. The thickness of the shell takes up 4% of the radius of the bubble, and the structure of the shell can be approximately described by a self-similar solution. We compared the properties of the shell components with those of the ejecta clumps indicated by our previous HD simulations for the interaction of clumps with the outer supernova remnant. The high density contrast across the shell, χ ~ 100, is compatible with that of ejecta clumps as indicated for Tycho's knots, but there is insufficient dense gas to cause a pronounced protrusion on the outline of a core-collapse supernova remnant, like the bullets in the Vela remnant.

SU-FF-J-33: Using CT Cine Data as An Alternative Method for Respiratory Phase Analysis and a Potential Method for Tumor Motion Study

Purpose: To demonstrate the use of cine images from 4D-CT imaging as an alternative method to obtain respiratory signal for sorting of the 4D-CT data, and as a tool to understand the relationship between the externally measured respiratory signal and the internal tumor motion. Method and Materials: Six lung patients were scanned in the tumor region with coverage of approximately 8 cm. The respiratory trace for each patient was clinically acquired using the RPM system placing the RPM block near the diaphragm. The cine images were used as an alternative method to obtain respiratory signal by measuring CT density variation versus time in a region of interest chosen at the boundary of tissues with high CT density contrast. We also placed a region of interest to measure the CT number changes as a way to describe the tumor motion in either superior-inferior or anterior-posterior direction. We compared the respiratory signal from RPM and the respiratory signal from cine images and correlated the two respiratory signals with the internal tumor motion (also from cine images). Results: Images can be sorted to become 4D-CT data using the respiratory signal measured from cine images. When compared to the RPM signal, the respiratory traces measured using the cine images, showed phase differences ranging from 2% to 25%. Tumor motions observed using the cine data for three patients showed 50% phase difference in S-I direction and 0% and 25% phase differences in A-P direction from the respiratory signal of the RPM block. Conclusion: We have demonstrated that cine data can be used as an alternative way of acquiring respiratory waveform for 4DCT images registration. The tumor motion can also be studied using the cine data. The 4D-CT data can also help us understand the relationship between internal tumor motion and the externally measured respiratory signal.

Hemodynamic Compromise From a Right Coronary Artery Pseudoaneurysm After Remote Stent Placement

v a o T n c T t c C i s p f d a 62-year-old diabetic hypertensive female on chronic hemodialysis was admitted from the emergency epartment for progressive marked right-sided heart ailure, fatigue, fever, positive blood cultures for gramositive cocci, and a history of repeated vascular access hromboses with septic thrombophlebitis. She was tachyardic, tachypneic, and hypotensive. Chest x-ray was bnormal and a chest computed tomography (CT) scan as performed. The patient had suffered an acute inferior and right entricular myocardial infarction nearly 7 months earlier ith hypotension and bradyarrhythmias during an adission for Staphylococcus aureus septic thrombophlebitis. rimary angioplasty was performed. Because of a treendous thrombus burden an Angiojet was used, folowed by stenting of the proximal right coronary artery or residual stenosis and dissection. Chest CT demonstrated a 6 7 cm intrapericardial uid collection containing high-density contrast (Fig 1). ransesophageal echocardiography showed a pseudoanurysm surrounding the right coronary artery stent, ante-

The effect of dissipation on the generation of density structure by magnetohydrodynamic waves

Observations of absorption lines in diffuse interstellar clouds may indicate that significant fluctuations in physical conditions exist on scales comparable to, or even smaller than, the dissipation length associated with ambipolar diffusion. Here we describe the results of simulations of magnetohydrodynamic (MHD) wave evolution in which ambipolar diffusion plays a major role. We find that ambipolar diffusion affects the evolution of waves with wavelengths up to about a thousand times the dissipation length-scale, which means that these results are relevant to both molecular and atomic interstellar clouds. The initial conditions consist of a uniform background upon which is superposed a non-linear fast-mode ideal MHD wave with a density contrast comparable to unity or less. The ratio, β, of the thermal pressure to the magnetic pressure is assumed to be small. For initial disturbances that have wavelengths that are very large compared to the dissipation length, the generation of structures with high-density contrast is due to the excitation of slow-mode waves by non-linear steepening of the fast-mode wave. In this case, collisions between the resulting structures lead to additional density enhancement for some ranges of the initial wave amplitude. For initial disturbances that have wavelengths such that dissipation is significant, slow-mode waves are also produced by ambipolar diffusion and this can generate higher density contrasts than those found in the corresponding cases in which dissipation is negligible. Large density contrasts on scales comparable to the dissipation length are found only when the wavelength of the initial perturbation is comparatively small. Because such waves do not propagate far before they are dissipated, we suggest that the variations in absorption features with time-scales ≃10 yr can only be generated by waves if they are located at the surfaces of clouds and are excited by external disturbances such as stellar winds, supernovae, etc. In order to produce the highest density contrasts that have been detected, the waves must have even larger amplitudes than those we have examined. This places severe restrictions on the properties of waves that could produce the kind of microstructure that some believe to be responsible for the rapid variations of absorption features.

Latest techniques in head and neck CT angiography

Continuous evolution of multi row CT is increasingly making CT angiography a viable imaging modality for assessment of the supraaortic and intracranial vessels as an anatomically and functionally coherent vascular system. Extended non-invasive examinations with reduced contrast volume have become feasible with the availability of 16 and 64 row MDCT scanners. Prerequisites to obtain high resolution CT angiographies of the head and neck vessels with superior detail include the administration of low contrast volume, high contrast density (400 mg I/ml) contrast media, adequate timing and data acquisition, optimal flow rate (4 ml/s) and saline flushing. Non-invasiveness, delineation of vessel calcification, virtual independence from hemodynamic conditions, and the ability to provide quantification without needing to correct for magnification are all attributes that favour CT angiography over digital subtraction angiography and to some extent even magnetic resonance angiography as an alternative non-invasive technique. CT angiography is established as a modality of choice for the assessment of patients with acute stroke and chronic steno-occlusive disease. CT angiography may indicate the presence of extra- or intracranial acute vessel occlusion and dissection, predisposing atherosclerotic steno-occlusive disease and thus indicate thrombo-embolism or local appositional thrombosis as the principle pathogenic factor. CT angiography is used to assess anatomy, and to depict the presence, location and extent of calcified and non-calcified plaque as a cause of high grade stenosis. Despite relatively limited sensitivity CT angiography is indicated for suspected or confirmed aneurysms that demand further verification of their presence, geometry, or relationship to parent artery branches and osseous anatomic landmarks. Low volume high density contrast media have substantially increased the ability of CT angiography to depict small aneurysms, small branches, and collateral vessels, and to recognize the residual lumen in vessels with high grade stenosis or conditions such as dissection or pseudo-occlusion. Superior detail high resolution CT angiography is thus a viable alternative to DSA, relegating the latter technique to endovascular treatment applications only.

Density fluctuations in α-decay self-irradiated zircon

Small-angle x-ray scattering (SAXS) intensities measured in a series of natural zircons, from fully crystalline to fully amorphous, give evidence for nanoscale density fluctuations within the amorphous phase produced by α-decay self-irradiation. At low amorphous fractions (p<0.5), the SAXS intensity is dominated by the signal associated with the sharp crystalline/amorphous interphase. At high amorphous fractions (p>0.8) a peak at Q=3 nm−1 develops, indicating the presence of 1 nm diameter regions that show a high-density contrast with the surrounding matrix. Density fluctuations are interpreted in terms of nanometer size voids.

Small scale structure in circumstellar envelopes and the origin of globules in planetary nebulae

We analyze the small scale structure in the circumstellar envelopes of NGC 7027 and IRC+10216, using high resolution optical images in dust scattered light. We use the observations to test the proposal that globules in planetary nebulae, typified by globules in the Helix nebula, originate in high density contrast proto-globules in the atmosphere of the progenitor AGB star and are carried out in the circumstellar wind (Dyson et al. 1989). We find no evidence for the presence of proto- globules in the extended envelopes of NGC 7027 and IRC+10216 with the expected sizes and masses>10 5 M which are needed to produce globules like those in the Helix nebula. We do find azimuthal structure in the envelopes on size scales of l> 0:1d where d is the radial distance, consistent with the smoothing out of small scale structure by thermal motions in the wind acceleration region. Unless globules require special conditions not found in NGC 7027 or IRC+10216, which are among the most likely precursors of Helix-like nebulae, our results argue against their origin in the atmosphere of the central star. We suggest alternative scenarios for globule formation, including the fragmentation of the neutral circumstellar gas at the transition phase by directed outflows or jets.

On the structure of self-gravitating molecular clouds

To study the interaction of star-formation and turbulent molecular cloud structuring, we analyse numer- ical models and observations of self-gravitating clouds using the -variance as statistical measure for structural characteristics. In the models we resolve the transition from purely hydrodynamic turbulence to gravitational collapse associated with the formation and mass growth of protostellar cores. We compare models of driven and freely decaying turbulence with and without magnetic elds. Self-gravitating supersonic turbulence always pro- duces a density structure that contains most power on the smallest scales provided by collapsed cores as soon as local collapse sets in. This is in contrast to non-self-gravitating hydrodynamic turbulence where the -variance is dominated by large scale structures. To detect this eect in star-forming regions observations have to resolve the high density contrast of protostellar cores with respect to their ambient molecular cloud. Using the 3 mm contin- uum map of a star-forming cluster in Serpens we show that the dust emission traces the full density evolution. On the contrary, the density range accessible by molecular line observations is insucient for this analysis. Only dust emission and dust extinction observations are able to to determine the structural parameters of star-forming clouds following the density evolution during the gravitational collapse.

Interfacial instabilities driven by self-gravity: first numerical results

The evolution of the interstellar medium (ISM) is driven by a variety of phenomena, including turbulence, shearing flows, magnetic fields and the thermal properties of the gas. Among the most important forces at work is self-gravity, which ultimately drives protostellar collapse. As part of an ongoing study of instabilities in the ISM, Hunter, Whitaker & Lovelace have discovered another process driven by self-gravity: the instability of an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the non-linear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh–Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast, such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.

X-Ray reflectometry study of diamond-like carbon films obtained by plasma-assisted chemical vapor deposition

X-Ray reflectivity is used to determine the electron density profiles normal to the surface of diamond-like carbon (DLC) films prepared by plasma-enhanced chemical vapor deposition (PE-CVD). Average values of the scattering lengths obtained from the specular reflection data and elastic recoil detection analysis (ERDA) hydrogen measurements are used to calculate the average mass density of the films. The density is shown to be strongly dependent on the hydrogen content. This depends on the plasma parameters. Argon diluted methane plasma produces homogeneous DLC films but generally with a lower density than the films prepared from pure or He diluted plasmas. These later plasmas produce films with a high density contrast and higher densities.

Viscoelastic response of sonic band-gap materials

A brief report is presented on the effect of viscoelastic losses in a high density contrast sonic band-gap material of close-packed rubber spheres in air. The scattering properties of such a material are computed with an on-shell multiple scattering method, properties which are compared with the lossless case. The existence of an appreciable omnidirectional gap in the transmission spectrum, when losses are present, is also reported.

Dissolving star cluster candidates

We present a list of 34 neglected entries from star cluster catalogues located at relatively high galactic latitudes ( 15°) which appear to be candidate late stages of star cluster dynamical evolution. Although underpopulated with respect to usual open clusters, they still present a high number density contrast as compared to the galactic field. This was verified by means of (i) predicted model counts from different galactic subsystems in the same direction, and (ii) Guide Star Catalog equal solid angle counts for the object and surrounding fields. This suggests that the objects are physical systems, possibly star clusters in the process of disruption or their fossil remains. The sample will be useful for followup studies in view of verifying their physical nature.

On the Exchange of Kinetic and Magnetic Energy between Clouds and the Interstellar Medium

We investigate, through 2D MHD numerical simulations, the interaction of a uniform magnetic field oblique to a moving interstellar cloud. In particular we explore the transformation of cloud kinetic energy into magnetic energy as a result of field line stretching. Some previous simulations have emphasized the possible dynamical importance of a ``magnetic shield'' formed around clouds when the magnetic field is perpendicular to the cloud motion (Jones et al. 1996, Miniati et al. 1998). It was not clear, however, how dependent those findings were to the assumed field configuration and cloud properties. To expand our understanding of this effect, we examine several new cases by varing the magnetic field orientation angle with respect to the cloud motion (\theta), the cloud-background density contrast, and the cloud Mach number. We show that in 2D and with \theta large enough, the magnetic field tension can become dominant in the dynamics of the motion of high density contrast, low Mach number clouds. In such cases a significant fraction of cloud kinetic energy can be transformed into magnetic energy with the magnetic pressure at the cloud nose exceeding the ram pressure of the impinging flow. We derive a characteristic timescale for this process of energy ``conversion''. We find also that unless the cloud motion is highly aligned to the magnetic field, reconnection through tearing mode instabilities in the cloud wake limit the formation of a strong flux rope feature following the cloud. Finally we attempt to interpret some observational properties of the magnetic field in view of our results.