Local Density Ratio Research Articles

Volume density maps of the 862 nm DIB carrier and interstellar dust. Hints on the role of carbon-rich ejecta from AGB stars

The carbonaceous macromolecules imprinting the numerous absorptions called diffuse interstellar bands (DIBs) in astronomical spectra are omnipresent in the Galaxy and beyond. They represent a considerable reservoir of organic matter. However, their chemical formulae, formation, and destruction sites remain unknown. Their spatial distribution and the local relation to other interstellar species is key to tracing their role in the lifecycle of organic matter. Volume density maps bring local instead of line-of-sight distributed information and allow for new diagnostics to be captured. We present the first large-scale volume (3D) density map of a DIB carrier and compare it with an equivalent map of interstellar dust. The DIB carrier map was obtained through hierarchical inversion of sim 202,000 measurements of the 8621 nm DIB obtained with the Gaia -RVS instrument. It covers about 4000 pc around the Sun in the Galactic plane. We built a dedicated interstellar dust map based on the extinction towards the same target stars. At the simeq 50 pc resolution of the maps, the shape of the 3D DIB distribution is found to be remarkably similar to the 3D distribution of dust. On the other hand, the DIB-to-dust local density ratio increases in low-dust areas. It is also increasing away from the disk, however, the minimum ratio is found to be shifted above the Galactic plane to Z=simeq +50pc. Finally, the average ratio is also surprisingly found to increase away from the Galactic Center. We suggest that the three latter trends may be indications of a dominant contribution of material from the carbon-rich category of dying giant stars to the formation of the carriers. Our suggestion is based on recent catalogs of asymptotic giant branch (AGB) stars and estimates of the mass fluxes of their C-rich and O-rich ejecta.

Stellar rotations and the disentanglement of the Galactic thin and thick discs

ABSTRACT The kinematics, mean α-elements, and ages of the stars reveal two distinct disc populations observed in the Solar neighbourhood. Although several studies have been carried out to characterize abundances of chemical elements and kinematic identifications in these populations, there are few studies that dealt with the analysis of stellar rotation and characteristics of populations focused on low-mass stars of the F and G types. In this work, we propose a new approach to classify stellar populations from the thin and thick disc for F and G dwarf stars by relating chemical abundance and rotation (v sin i) of these stars in the rotational–chemical–plane (v sin i–[α/Fe]–[Fe/H] plot). Our results show that the two rotational-chemical sequences disentangled, high- and low-[α/Fe] components, present properties (α-enhanced, ages, kinematics, space velocity gradients, local density ratio) closely linked to the stars of the chemically and kinematically defined as thin and thick disc components. This study has an impact on calibrations that relate ages to rotation (gyro-chronology) as well as age to chemical abundances (chemochronology), as well as sheds light on the understanding of the processes of Galactic disc formation.

Multiple states and transport properties of double-diffusive convection turbulence

When fluid stratification is induced by the vertical gradients of two scalars with different diffusivities, double-diffusive convection (DDC) may occur and play a crucial role in mixing. Such a process exists in many natural and engineering environments. Especially in the ocean, DDC is omnipresent since the seawater density is affected by temperature and salinity. The most intriguing phenomenon caused by DDC is the thermohaline staircase, i.e., a stack of alternating well-mixed convection layers and sharp interfaces with very large gradients in both temperature and salinity. Here we investigate DDC and thermohaline staircases in the salt finger regime, which happens when warm saltier water lies above cold fresher water and is commonly observed in the (sub)tropic regions. By conducting direct numerical simulations over a large range of parameters, we reveal that multiple equilibrium states exist in fingering DDC and staircases even for the same control parameters. Different states can be established from different initial scalar distributions or different evolution histories of the flow parameters. Hysteresis appears during the transition from a staircase to a single salt finger interface. For the same local density ratio, salt finger interfaces in the single-layer state generate very different fluxes compared to those within staircases. However, the salinity flux for all salt finger interfaces follows the same dependence on the salinity Rayleigh number of the layer and can be described by an effective power law scaling. Our findings have direct applications to oceanic thermohaline staircases.

Numerical simulation of compression corner flows at Mach number 9

A hypersonic vehicle encounters a wide range of conditions during its complete flight regime. These flight conditions may vary from low to high Mach numbers with varying angles of attack. The near-wall viscous dissipation associated with flows at combined high Mach and Reynolds numbers leads to significant wall heat transfer rates and shear stresses. The shock wave/boundary-layer interaction results in a flow separation region, which commonly augments total pressure losses in the flow and lowers the efficiency of aerodynamic control surfaces such as fins installed on a vehicle. The standard turbulence models, when used to resolve such flows, result in incorrect separation bubble size for large separated flows. Therefore, it results in an inaccurate aerodynamic load, such as the wall pressures, skin friction distribution, and heat transfer rate. In previous studies, the application of the shock-unsteadiness correction to the standard two-equation k-ω turbulence model improved the separation bubble size leading to an accurate pressure prediction and shock definition with the assumption of constant Prandtl number. In the present work, the new shock-unsteadiness modification to the k-ω turbulence model is applied to the hypersonic compression corner flows. This new model with variable Prandtl number is based on the model parameter, which depends upon the local density ratio. The computed wall pressures, heat flux and flow field are compared to the experimental data. A parametric study is carried out by varying compression deflection angles, free stream Reynolds number and wall temperatures to compute the flow field and wall data accurately, particularly in the shock boundary layer interaction region. The new shock-unsteadiness modified k-ω model with variable Prandtl number shows an accurate prediction of initial pressure rise location, pressure distribution in the plateau region and heat flux in comparison to the standard k-ω model.

Constraining the Galactic structure parameters with the XSTPS-GAC and SDSS photometric surveys

Photometric data from the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) and the Sloan Digital Sky Survey (SDSS) are used to derive the global structure parameters of the smooth components of the Milky Way. The data, which cover nearly 11,000 deg$^2$ sky area and the full range of Galactic latitude, allow us to construct a globally representative Galactic model. The number density distribution of Galactic halo stars is fitted with an oblate spheroid that decays by power law. The best-fit yields an axis ratio and a power law index $\kappa=0.65$ and $p=2.79$, respectively. The $r$-band differential star counts of three dwarf samples are then fitted with a Galactic model. The best-fit model yielded by a Markov Chain Monte Carlo analysis has thin and thick disk scale heights and lengths of $H_{1}=$ 322\,pc and $L_{1}=$2343\,pc, $H_{2}=$794\,pc and $L_{2}=$3638\,pc, a local thick-to-thin disk density ratio of $f_2=$11\,per\,cent, and a local density ratio of the oblate halo to the thin disk of $f_h=$0.16\,per\,cent. The measured star count distribution, which is in good agreement with the above model for most of the sky area, shows a number of statistically significant large scale overdensities, including some of the previously known substructures, such as the Virgo overdensity and the so-called "north near structure", and a new feature between 150\degr $< l < $ 240\degr~and $-1$5\degr $< b < $ $-$5\degr, at an estimated distance between 1.0 and 1.5\,kpc. The Galactic North-South asymmetry in the anticentre is even stronger than previously thought.

INFORMATION ON THE MILKY WAY FROM THE TWO MICRON ALL SKY SURVEY WHOLE SKY STAR COUNT: THE STRUCTURE PARAMETERS

The Ks band differential star count of the Two Micron All Sky Survey (2MASS) is used to derive the global structure parameters of the smooth components of the Milky Way. To avoid complication introduced by other fine structures and significant extinction near and at the Galactic plane, we only consider Galactic latitude |b| > 30 degree data. The star count data is fitted with a threecomponent model: double exponential thin disk and thick disk, and a power law decay oblate halo. Using maximum likelihood the best-fit local density of thin disk is n0 = 0.030 +- 0.002 stars/pc^3. The best-fit scale-height and length of the thin disk are Hz1 = 360+-10 pc and Hr1 = 3.7+-1.0 kpc, and those of the thick disk are and Hz2 = 1020+-30 pc and Hr2 = 5.0+-1.0 kpc, the local thick-to-thin disk density ratio is f2 = 7+-1%. The best-fit axis ratio, power law index and local density ratio of the oblate halo are kappa = 0.55+-0.15, p = 2.6+-0.6 and fh = 0.20+-0:10%, respectively. Moreover, we find some degeneracy among the key parameters (e.g. n0,Hz1, f2 and Hz2). Any pair of these parameters are anticorrelated to each other. The 2MASS data can be well-fitted by several possible combinations of parameters. This is probably the reason that there is a wide range of values for the structure parameters in literature similar to this study. Since only medium and high Galactic latitude data are analyzed, the fitting is very insensitive to the scale-lengths of the disks.

Voronoi analysis uncovers relationship between mosaics of normally placed and displaced amacrine cells in the thraira retina

Although neuronal dynamics is to a high extent a function of synapse strength, the spatial distribution of neurons is also known to play an important role, which is evidenced by the topographical organization of the main stations of the visual system: retina, lateral geniculate nucleus, and cortex. The coexisting systems of normally placed and displaced amacrine cells in the vertebrate retina provide interesting examples of retinotopic spatial organization. However, it is not clear whether these two systems are spatially interrelated or not. The current work applies two mathematical-computational methods-a new method involving Voronoi diagrams for local density quantification and a more traditional approach, the Ripley K function-in order to characterize the mosaics of normally placed and displaced amacrine cells in the retina of Hoplias malabaricus and search for possible spatial relationships between these two types of mosaics. The results obtained by the Voronoi local density analysis suggest that the two systems of amacrine cells are spatially interrelated through nearly constant local density ratios.

Some empirical estimates of the H2formation rate in photon-dominated regions

We combine recent ISO observations of the vibrational ground state lines of H 2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground-based telescopes in order to constrain the formation rate of H 2 on grain surfaces under the physical conditions in the layers responsible for H 2 emission. We briefly review the data available for five nearby PDRs. We use steady state PDR models in order to examine the sensitivity of different H 2 line ratios to the H 2 formation rate R f . We show that the ratio of the 0-0 S(3) to the I-0 S(1) line increases with R f but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer R f from the H 2 line data. We confirm earlier work by Habart et al. (2003) on the Oph W PDR which showed that an H 2 formation rate higher than the standard value of 3 x 10 -17 cm 3 s -1 inferred from UV observations of diffuse clouds is needed to explain the observed H 2 excitation. From comparison of the ISO and ground-based data, we find that moderately excited PDRs such as Oph W, S140 and IC 63 require an H 2 formation rate of about five times the standard value whereas the data for PDRs with a higher incident radiation field such as NGC 2023 and the Orion Bar can be explained with the standard value of R f . We compare also the H 2 1-0 S(1) line intensities with the emission in PAH features and find a rough scaling of the ratio of these quantities with the ratio of local density to radiation field. This suggests but does not prove that formation of H 2 on PAHs is important in PDRs. We also consider some empirical models of the H 2 formation process with the aim of explaining these results. Here we consider both formation on classical grains of size roughly 0.1 μm and on very small (∼ 10 A) grains by either direct recombination from the gas phase (Eley-Rideal mechanism) or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption where a physisorbed H-atom scans the grain surface before recombining with a chemisorbed H-atom is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low (below 30 K for X < 3000) temperatures may be the most promising surface for forming H 2 in PDRs.

Initiation of a doubly diffusive convection in a stable halocline

A vertically stable density stratification with temperature gradient T z < 0, salinity gradient S z < 0, and with density compensating horizontal T/S gradients is unstable to lateral intrusions because the molecular heat diffusivity is much larger than the salt diffusivity. Previous marginal instability theory is extended to the supercritical regime. The fastest growing vertical wavelength (δ z ) is obtained in a model of a T/S front with finite lateral width (L * ) and lateral salinity variation (ΔS). The value of δ z ΔS/|S z | varies only slightly with the parameters, and this result is applied to the smallest step size observed in the main thermocline of the Weddell Sea. Nonlinear considerations show that the laterally divergent heat/salt flux produced by the instability forces a mean vertical circulation which enhances the compensating lateral T/S gradients, thereby accelerating the intrusive instability, and eventually producing local density ratios sufficient to initiate strong vertical convection. This suggests that weak isopycnal T/S gradients are necessary to initiate the steps which subsequently merge into larger layers.

Sensitivity of the GFDL Modular Ocean Model to Parameterization of Double-Diffusive Processes*

Abstract The effect of double-diffusive mixing on the general circulation is explored using the GFDL MOM2 model. The motivation for this comes from the known sensitivity of the thermohaline circulation to the vertical diffusivity and the earlier work of Gargett and Holloway, who studied the effects of a simple nonunity ratio between heat and salt diffusivities in a GCM. In this work, a more realistic, yet conservative, parameterization of the double-diffusive mixing is applied, with the intensity depending on the local density ratio Rρ = αTz/βSz. A background diffusivity is used to represent non-double-diffusive turbulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and salinity at the sea surface. Three control experiments have been carried out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant but different diffusivities for heat and salt as previously considere...

Diagnostics of high-pressure arc plasmas from laser-induced fluorescence

Laser-induced fluorescence has been studied in high-pressure mercury, mercury-metal halide and sodium-mercury arcs, using a rhodamine 6G CW dye laser for the excitation. The fluorescence was observed at 90 degrees with respect to the laser beam. The fluorescence is interpreted assuming thermal equilibrium in the excited region of the arc. Quantitative evaluation of the fluorescence from the mercury arc indicates the validity of this approach. Laser-induced fluorescence can be used to determine local density ratios of arc plasma components and local emission profiles (line shapes and line shifts). As an application the electron density and the Na-Hg density ratio at the arc axis of a sodium-mercury discharge are determined from Stark shifts in the fluorescence of several transitions in the sodium atom and from fluorescence intensities.