Dense Blob Research Articles

Settling versus mixing in stratified shear flows

We study the coupled settling, deformation and mixing dynamics of a dense blob of fluid falling in an axially (vertically) linearly stratified Taylor–Couette cell (operated in a laminar stable regime). This configuration allows the independent analysis of stretching dynamics, driven by radial (horizontal) velocity variations, and settling dynamics, driven by buoyancy forces associated with vertical density variations. As the blob settles, it is stretched in the horizontal plane and forms an elongated lamella. Through the competing effects of transverse compression of the lamella due to this shear-induced stretching and broadening due to diffusion, the lamella irreversibly mixes with ambient fluid, thus progressively adjusting its own density towards that of the ambient fluid. Eventually, the lamella settling stops at a final equilibrium position that depends on the ambient vertical density gradient and the rate at which it has been deformed by the horizontal shear. We show how this final position is determined by stretching-enhanced diffusion, i.e. mixing. We demonstrate that a theoretical mixing model compares favourably with experiments with various Froude numbers (quantifying the relative strength of the horizontal shear and the vertical stratification) and construct a new criterion for the energetic ‘efficiency’ of this mixing process that explicitly captures its inherently diffusive character.

Radiation Magnetohydrodynamic Simulations of Sub-Eddington Accretion Flows in AGNs: Origin of Soft X-Ray Excess and Rapid Time Variabilities

Abstract We investigate the origin of the soft X-ray excess component in Seyfert galaxies observed when their luminosity exceeds 0.1% of the Eddington luminosity ( ). The evolution of a dense blob in radiatively inefficient accretion flow (RIAF) is simulated by applying a radiation magnetohydrodynamic code, CANS+R. When the accretion rate onto a black hole exceeds 10% of the Eddington accretion rate ( , where c is the speed of light), the dense blob shrinks vertically because of radiative cooling and forms a Thomson thick, relatively cool (∼107–8 K) region. The cool region coexists with the optically thin, hot ( ) RIAF near the black hole. The cool disk is responsible for the soft X-ray emission, while hard X-rays are emitted from the hot inner accretion flow. Such a hybrid structure of hot and cool accretion flows is consistent with the observations of both hard and soft X-ray emissions from “changing-look” active galactic nuclei (CLAGNs). Furthermore, we find that quasi-periodic oscillations (QPOs) are excited in the soft X-ray-emitting region. These oscillations can be the origin of rapid X-ray time variabilities observed in CLAGNs.

Seeds of Life in Space (SOLIS)

Aims. The Seeds Of Life In Space IRAM/NOEMA large program aims at studying a set of crucial complex organic molecules in a sample of sources with a well-known physical structure that covers the various phases of solar-type star formation. One representative object of the transition from the prestellar core to the protostar phases has been observed toward the very low luminosity object (VeLLO) L1521F. This type of source is important to study to link prestellar cores and Class 0 sources and also to constrain the chemical evolution during the process of star formation. Methods. Two frequency windows (81.6–82.6 GHz and 96.65–97.65 GHz) were used to observe the emission from several complex organics toward the L1521F VeLLO. These setups cover transitions of ketene (H2CCO), propyne (CH3CCH), formamide (NH2CHO), methoxy (CH3O), methanol (CH3OH), dimethyl ether (CH3OCH3), and methyl formate (HCOOCH3). Results. Only two transitions of methanol (A+, E2) have been detected in the narrow window centered at 96.7 GHz (with an upper limit on E1) in a very compact emission blob (~7′′ corresponding to ~1000 au) toward the northeast of the L1521F protostar. The CS 2–1 transition is also detected within the WideX bandwidth. Consistently with what has been found in prestellar cores, the methanol emission appears ~1000 au away from the dust peak. The location of the methanol blob coincides with one of the filaments that have previously been reported in the literature. The excitation temperature of the gas inferred from methanol is (10 ± 2) K, while the H2 gas density (estimated from the detected CS 2–1 emission and previous CS 5–4 ALMA observations) is a factor >25 higher than the density in the surrounding environment (n(H2) ≥ 107 cm−3). Conclusions. Based on its compactness, low excitation temperature, and high gas density, we suggest that the methanol emission detected with NOEMA is (i) either a cold and dense shock-induced blob that formed recently (≤ a few hundred years) by infalling gas or (ii) a cold and dense fragment that may just have been formed as a result of the intense gas dynamics within the L1521F VeLLO system.

DYNAMICS OF CORONAL RAIN AND DESCENDING PLASMA BLOBS IN SOLAR PROMINENCES. I. FULLY IONIZED CASE

Observations of active regions and limb prominences often show cold, dense blobs descending with an acceleration smaller than that of free fall. The dynamics of these condensations falling in the solar corona is investigated in this paper using a simple fully ionised plasma model. We find that the presence of a heavy condensation gives rise to a dynamical rearrangement of the coronal pressure that results in the formation of a large pressure gradient that opposes gravity. Eventually this pressure gradient becomes so large that the blob acceleration vanishes or even points upwards. Then, the blob descent is characterised by an initial acceleration phase followed by an essentially constant velocity phase. These two stages can be identified in published time-distance diagrams of coronal rain events. Both the duration of the first stage and the velocity attained by the blob increase for larger values of the ratio of blob to coronal density, for larger blob mass, and for smaller coronal temperature. Dense blobs are characterised by a detectable density growth (up to 60% in our calculations) and by a steepening of the density in their lower part, that could lead to the formation of a shock. They also emit sound waves that could be detected as small intensity changes with periods of the order of 100 s and lasting between a few and about ten periods. Finally, the curvature of the falling path is only relevant when a very dense blob falls along inclined magnetic field lines.

Kelvin-Helmholtz instabilities with Godunov smoothed particle hydrodynamics

Numerical simulations for the non-linear development of Kelvin-Helmholtz instability in two different density layers have been performed with the particle-based method (Godunov SPH) developed by Inutsuka (2002). The Godunov SPH can describe the Kelvin-Helmholtz instability even with a high density contrast, while the standard SPH shows the absence of the instability across a density gradient (Agertz et al. 2007). The interaction of a dense blob with a hot ambient medium has been performed also. The Godunov SPH describes the formation and evolution of the fingers due to the combinations of Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities. The blob test result coincides well with the results of the grid-based codes. An inaccurate handling of a density gradient in the standard SPH has been pointed out as the direct reason of the absence of the instabilities. An unphysical force happens at the density gradient even in a pressure equilibrium, and repulses particles from the initial density discontinuity. Therefore, the initial perturbation damps, and a gap forms at the discontinuity. The unphysical force has been studied in terms of the consistency of a numerical scheme. Contrary to the standard SPH, the momentum equation of the Godunov SPH doesnt use the particle approximation, and has been derived from the kernel convolution or a new Lagrangian function. The new Lagrangian function used in the Godunov SPH is more analogous to the real Lagrangian function for continuum. The momentum equation of the Godunov SPH has much better linear consistency, so the unphysical force is greatly reduced compared to the standard SPH in a high density contrast.

High variability in Vela X-1: giant flares and off states

Aims. We investigate the spectral and temporal behavior of the high mass X-ray binary Vela X-1 during a phase of high activity, with special focus on the observed giant flares and off states. Methods. INTEGRAL observed Vela X-1 in a long almost uninterrupted observation for two weeks in 2003 Nov/Dec. The data were analyzed with OSA 7.0 and FTOOLS 6.2. We derive the pulse period, light curves, spectra, hardness ratios, hardness intensity diagrams, and study the eclipse. Results. In addition to an already high activity level, Vela X-1 exhibited several very intense flares, the brightest ones reaching a maximum intensity of more than 5 Crab in the 20-40 keV band and several off states where the source is no longer detected by INTEGRAL. We determine the pulse period to be 283.5320 +/- 0.0002 s which is stable throughout the whole observation. Analyzing the eclipses resulted in an improvement of the ephemeris. Spectral analysis of the flares shows that there seem to be two types of flares: relatively brief flares which can be extremely intense, and show spectral softening, contrary to high intensity states which are longer and show no softening. Conclusions. Both flares and off states are interpreted as being due to a strongly structured wind of the optical companion. When Vela X-1 encounters a cavity with strongly reduced density, the flux will drop triggering the onset of the propeller effect which inhibits further accretion, thus giving rise to off states. The required drop of the density of the material to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers on the densities in the OB star winds. The same structured wind can give rise to the giant flares when Vela X-1 encounters a dense blob in the wind.

The transition temperature of the nuclear caloric curve

Experimental studies have obtained the caloric curve of nuclear matter from heavy ion collisions as well as its dependence on the size of the fragmenting source. In particular it has been determined that smaller systems have caloric curves with higher plateau temperatures than larger systems. This work uses molecular dynamics simulations to study the thermodynamics of heavy ion collisions and to identify the main factors that determine the caloric curve. The simulations indicate that the reaction is composed of three stages: 1) an initial collision that transforms the nuclei from normal density and zero temperature and entropy, to a hot and dense blob of matter with higher values of density, temperature and entropy; 2) this is followed by a constant-entropy expansion that takes the system to the spinodal of the phase diagram; 3) where the system rapidly disassembles into fragments by the process of spinodal decomposition, and not by nucleation. These findings indicate that the plateau temperature of the caloric curve is nothing more than the temperature of the phase change and it is set by the intersection of the isentropic expansion and the spinodal. In other words, the plateau temperature is simply the temperature at which the system breaks as it enters the spinodal. This transition temperature is thus set by the entropy generated during the initial part of the collision.

Imaging and Spatially Resolved Spectroscopy of AFGL 2688 in the Thermal Infrared Region

We present ground-based high-resolution (~03) imaging of AFGL 2688 at L' (3.8 μm) and M' (4.7 μm). A wealth of structure in the central region is revealed as a consequence of less extinction in the thermal infrared. A clear border in the southern lobe at L' corresponds to the edge of the heavily obscured region in visible, indicating that there is a dense material surrounding the central region. The images also show a narrow dark lane oriented to 140° east of north with the normal at 50°. The normal position angle is inconsistent with the optical polar axis (P.A. = 15°) but is aligned to the high-velocity CO components found in the radio wavelength observations. The central star remains invisible at L' and M'. Several clumpy regions in the north lobe dominate in L' and M' luminosity. In particular, a pointlike source (peak A) at 05 northeast of the center of the nebula exhibits the highest surface brightness with a very red spectral energy distribution (SED). Based on the almost identical SED at adjacent regions, we suggest that the pointlike source is not self-luminous, as was proposed, but is a dense dusty blob reflecting thermal emission from the central star. We also present spatially resolved slit spectroscopy of the bright dusty blobs. An emission feature at 3.4 μm as well as at 3.3 μm is detected everywhere within our field of view. There is no spatial variation in the infrared emission feature throughout the observed area (02-15, or 240-1800 AU from the central source). The constant flux ratio of the emission feature relative to the continuum is consistent with the view that the blobs are mostly reflecting the light from the central star in the 3 μm region.

Microwave Detection of Shock and Associated Electron Beam Formation

We use complementary European and Japanese solar radio ground-based observations, together with Yohkoh soft X-ray and SOHO extreme-UV images, to search for the signature of flare-related waves at different heights above the Sun. The key data set for event selection is 40-800 MHz dynamic radio spectra from the Potsdam Astrophysical Institute, whose radio spectral polarimeter is sensitive to the coronal shock waves due to the associated type II radio bursts in the range between 0.2 and 1 R☉. Nobeyama Radio Heliograph images at 17 GHz show the chromosphere and the transition region to the corona with unprecedented sensitivity and time resolution (1 s image cadence). Here we focused on 17 GHz images in the time interval between flare onset and the start of the metric type II burst. The decametric-hectometric (Dm-Hm; 1-14 MHz) radio experiment on board Wind completes the radio spectral coverage. The spectra are used to check if the coronal shock wave is also continuously visible in the range 3-8 R☉ and if the corona is open or closed for electron beams exciting hectometric type III bursts. We selected two flare events that show metric type II bursts, but with different associated 17 GHz features. For both events we find flare disturbances in 17 GHz images that propagate earlier than the type II bursts: a hot, dense blob (event 1; 1997 April 2) or a cold, absorbing cloud (event 2; 1998 July 31). In event 1, the hot and dense blob preceded the formation of a wave front segment that appeared in SOHO/EIT images. In event 2, we observed the impact of the 17 GHz absorbing cloud on a preexisting quiescent prominence far out of the flaring active region after several minutes of propagation without being disturbed. We demonstrate that the spectral pattern, as well as the drift rate, of the given type II burst drastically changes shortly before the cloud's impact. The Dm-Hm spectra in event 2 reveal a typical shock-associated (SA) event in the outer corona during the interaction between the absorbing cloud and the prominence. Finally, we stress that there may be a common driver for the metric type II bursts and simultaneous decimeter reverse-drift bursts between 1 and 2 GHz recorded on the radio spectrograph of Astronomical Observatory Ondřejov.

On steady shell formation in stellar atmospheres

The energy balance of the analytical solutions of Kakouris & Moussas ([CITE]) for a steady state of an externally heated/cooled 2-D circumstellar envelope is investigated. It is found that the required heating/cooling rates are physically realistic and can be related to specific microscopic mechanisms. We find that in the subsonic region of the wind the fluid is mechanically heated. In the supersonic stellar envelope the fluid is cooled at a rate which is consistent with radiative cooling to space. The energy balance of steady shell or blob formation in the envelopes of luminous early or late type supergiants is also investigated (Kakouris & Moussas [CITE]). We find that radiative cooling occurs in the intermediate deceleration region of the three-zone envelope. Indicative of the local thermodynamic processes is the effective polytropic index α which takes values close to the star between 1 and 4, becoming 2 at larger distances. The heated subsonic region close to the stellar surface is isothermal and becomes adiabatic at the sonic transition. We find that the polytropic index α is less than unity in the vicinity of the dense blob, indicating that the region may be dominated by convection.

A Two-dimensional Magnetohydrodynamic Simulation of Chromospheric Evaporation in a Solar Flare Based on a Magnetic Reconnection Model

A two-dimensional simulation of a solar flare is performed using a newly developed magnetohydrodynamic (MHD) code that includes a nonlinear anisotropic heat conduction effect. The numerical simulation starts with a vertical current sheet that is line-tied at one end to a dense chromosphere. The flare energy is released by the magnetic reconnection mechanism that is stimulated initially by the resistivity perturbation in the corona. The released thermal energy is transported into the chromosphere by heat conduction and drives chromospheric evaporation. Owing to the heat conduction effect, the adiabatic slow-mode MHD shocks emanated from the neutral point are dissociated into conduction fronts and isothermal slow-mode shocks. We discovered two new features, i.e., (1) a pair of high-density humps on the evaporated plasma loops that are formed at the collision site between the reconnection flow and the evaporation flow, and (2) a loop-top dense blob behind the fast-mode MHD shock. We also derived a simple scaling law for the flare temperature described as where TA, B, ρ, and κ0 are the temperature at the flare loop apex, the coronal magnetic field strength, the coronal density, and the heat conduction coefficient, respectively. This formula is confirmed by the numerical simulations. Temperature and derived soft X-ray distributions are similar to the cusplike structure of long-duration-event (LDE) flares observed by the soft X-ray telescope aboard Yohkoh. Density and radio free-free intensity maps show a simple loop configuration that is consistent with the observation with the Nobeyama Radio Heliograph.

The analysis system for astrophysical plasmas (ASAP) of the Osservatorio Astronomico di Palermo

We present the software package ASAP, mainly devoted to the presentation and analysis of fluid models of astrophysical plasmas. ASAP allows to generate optically thin X-ray and UV spectra emitted by plasma of given density, temperature and velocity, by means of well established spectral synthesis codes; the emission can then be easily folded with the instrumental response of interest and compared directly with measurements, or can be used to predict the performance of planned instruments. As examples of typical applications of the ASAP package, we show high resolution emission spectra derived from 1D static models of coronal loops, detailed fitting with coronal loop models of observations made with the ROSAT/PSPC instrument, hydrodynamic simulations of a solar flaring loop as seen by the Yohkoh satellite, and the evolution of line emission from a dense blob of interstellar plasma subject to the passage of a shock. The package, designed to be highly flexible, modular, and easily expandable, is expected to evolve and grow rapidly in response to new needs and required tasks. It operates within the IDL environment, and it is under consideration for being included in the SOHO Archive System.

Self-Advection of Density Perturbations on a Sloping Continental Shelf

Bottom water movement on the continental shelf is modeled by the nonlinear interaction between long-shore bottom geostrophic flow and the density field. Bottom geostrophic velocity, subject to linear steady momentum equations with linear bottom friction, can be generated by along-isobath density variations over a sloping bottom. At the same time, the density field is slowly adveced by the velocity field. Away from boundary layers, the interplay is governed by Burgers' equation, which shows the formation and self-propulsion of strong density gradients along an isobath. The direction of propagation of a dense water blob is to have shallow water on the right- (left-) hand side facing downstream in the Northern (Southern) Hemisphere. The propagation of a light water blob is opposite to that of a dense water blob. The problem is further investigated by solving the governing equations numerically. Under forcing by localized surface cooling, the flow in the mid-shelf region shows the characteristics of the solution a Burgers' equation. A coastal buoyancy source generates a shore-hugging plume, slowly moving along the coast in the direction of Kelvin wave propagation. The flow associated with coastal dense water discharge has different characteristics: the dense water moves away from the coast initially. The accumulation of dense water on the mid-shelf then invokes the same self-advection process as found for surface cooling. The theory sheds light on bottom water movements in the Adriatic Sea over the Antarctic continental shelf, and in the Middle Atlantic Bight. It also describes the dispersion of river water and dense water outflow an the shelf. The model results agree qualitatively with the observed distribution of bottom water and give correct order-of-magnitude estimates for the propagation speed of density perturbations.