scholarly journals Effect of finite disk-thickness on swing amplification of non-axisymmetric perturbations in a sheared galactic disk

2018 ◽  
Vol 617 ◽  
pp. A47 ◽  
Author(s):  
S. Ghosh ◽  
C. J. Jog
Keyword(s):  
Galactic Disk ◽  
Finite Thickness ◽  
Dynamical Behaviour ◽  
Small Scale ◽  
Complete Suppression ◽  
Spiral Arms ◽  
Disk Thickness ◽  
Gas System ◽  
Two Fluid ◽  
Axisymmetric Perturbations

A typical galactic disk is observed to have a finite thickness. Here, we present the study of the physical effect of introduction of finite thickness on the generation of small-scale spiral arms by swing amplification in a differentially rotating galactic disk. The galactic disk is modelled first as a one-fluid system, and then as a gravitationally-coupled two-fluid (stars and gas) system where each fluid is taken as isothermal, and corotating with each other. We derived the equations governing the evolution of the non-axisymmetric perturbations in a sheared frame of reference while incorporating the effect of finite thickness of a galactic disk. We found that the finite thickness of a galactic disk has a generic trend of suppressing the growth of the non-axisymmetric perturbations via swing amplification. Moreover, even the observed range of disk-thickness values (∼300–500 pc) can lead to a complete suppression of swing amplification for Q ∼ 1.7, whereas for an infinitesimally-thin disk, the corresponding critical value is Q ∼ 2. For a two-fluid (stars and gas) system, the net amplification is shown to be set by the mutual interplay of the effect of interstellar gas in promoting the spiral features and the effect of finite thickness in preventing the spiral arms. The coexistence of these two opposite effects is shown to be capable of giving rise to diverse and complex dynamical behaviour.

scholarly journals A Search for Very Compact Structure in 17 OH Masers

1988 ◽  
Vol 129 ◽  
pp. 255-256
Author(s):  
A. J. Kemball ◽  
P. J. Diamond ◽  
F. Mantovani
Keyword(s):  
Galactic Disk ◽  
Hii Regions ◽  
Scale Structure ◽  
Small Scale ◽  
Compact Structure ◽  
Small Scale Structure ◽  
Interstellar Scattering

The apparent spot sizes of OH masers appear to be significantly broadened when seen through the inner galaxy or large extents of the galactic disk (Burke 1968). Bowers et al (1980) found evidence of small-scale structure (≲ 50 mas) in OH sources at distances of less than 5 kpc but this was characteristically absent in very distant sources (≳ 8kpc) at galactic longitudes 1 ≲ 40°. This result is typically explained in terms of interstellar scattering (ISS) by intervening diffuse HII regions.

scholarly journals The constraining effect of gas and the dark matter halo on the vertical stellar distribution of the Milky Way

2018 ◽  
Vol 617 ◽  
pp. A142 ◽  
Author(s):  
S. Sarkar ◽  
C. J. Jog
Keyword(s):  
Dark Matter ◽  
Gravitational Field ◽  
Milky Way ◽  
Galactic Disk ◽  
Outer Region ◽  
Hydrogen Gas ◽  
Dark Matter Halo ◽  
Stellar Disk ◽  
Disk Thickness ◽  
Vertical Density

We study the vertical stellar distribution of the Milky Way thin disk in detail with particular focus on the outer disk. We treat the galactic disk as a gravitationally coupled, three-component system consisting of stars, atomic hydrogen gas, and molecular hydrogen gas in the gravitational field of the dark matter halo. The self-consistent vertical distribution for stars and gas in such a realistic system is obtained for radii between 4–22 kpc. The inclusion of an additional gravitating component constrains the vertical stellar distribution toward the mid-plane, so that the mid-plane density is higher, the disk thickness is reduced, and the vertical density profile is steeper than in the one-component, isothermal, stars-alone case. We show that the stellar distribution is constrained mainly by the gravitational field of gas and dark matter halo in the inner and the outer Galaxy, respectively. We find that the thickness of the stellar disk (measured as the half-width at half-maximum of the vertical density distribution) increases with radius, flaring steeply beyond R = 17 kpc. The disk thickness is reduced by a factor of 3–4 in the outer Galaxy as a result of the gravitational field of the halo, which may help the disk resist distortion at large radii. The disk would flare even more if the effect of dark matter halo were not taken into account. Thus it is crucially important to include the effect of the dark matter halo when determining the vertical structure and dynamics of a galactic disk in the outer region.

Distribution functions for Galactic disc stellar populations in the presence of non-axisymmetric perturbations

2017 ◽  
Vol 13 (S334) ◽  
pp. 195-198
Author(s):  
B. Famaey ◽  
G. Monari ◽  
A. Siebert ◽  
J.-B. Fouvry ◽  
J. Binney
Keyword(s):  
Stellar Population ◽  
Distribution Functions ◽  
Galactic Disc ◽  
Trapping Region ◽  
Phase Mixing ◽  
Spiral Arms ◽  
First Order ◽  
The Impact ◽  
Action Variables ◽  
Axisymmetric Perturbations

AbstractThe present-day response of a Galactic disc stellar population to a non-axisymmetric perturbation of the potential, in the form of a bar or spiral arms, can be treated, away from the main resonances, through perturbation theory within the action-angle coordinates of the unperturbed axisymmetric system. The first order moments of such a perturbed distribution function (DF) in the presence of spiral arms give rise to non-zero radial and vertical mean stellar velocities, called breathing modes. Such an Eulerian linearized treatment however diverges at resonances. The Lagrangian approach to the impact of non-axisymmetries at resonances avoids this problem. It is based on the construction of new orbital tori in the resonant trapping region, which come complete with a new system of angle-action variables. These new tori can be populated by phase-averaging the unperturbed DF over the new tori. This boils down to phase-mixing the DF in terms of the new angles, such that the DF for trapped orbits only depends on the new set of actions. This opens the way to quantitatively fitting the effects of the bar and spirals to Gaia data with an action-based DF.

scholarly journals Linear analysis of the non-axisymmetric secular gravitational instability

2019 ◽  
Vol 487 (4) ◽  
pp. 5405-5415
Author(s):  
Mohsen Shadmehri ◽  
Razieh Oudi ◽  
Gohar Rastegarzadeh
Keyword(s):  
Amplification Factor ◽  
Gravitational Instability ◽  
Radial Distance ◽  
Dust Particles ◽  
Linear Perturbation ◽  
Model Parameters ◽  
Time Interval ◽  
Continuous Growth ◽  
Two Fluid ◽  
Axisymmetric Perturbations

Abstract In protoplanetary discs (PPDs) consisting of gas and dust particles, fluid instabilities induced by the drag force, including secular gravitational instability (SGI), can facilitate planet formation. Although SGI subject to the axisymmetric perturbations was originally studied in the absence of gas feedback and it then generalized using a two-fluid approach, the fate of the non-axisymmetric SGI, in either case, is an unexplored problem. We present a linear perturbation analysis of the non-axisymmetric SGI in a PPD by implementing a two-fluid model. We explore the growth of the local, non-axisymmetric perturbations using a set of linearized perturbation equations in a sheared frame. The non-axisymmetric perturbations display a significant growth during a finite time interval even when the system is stable against the axisymmetric perturbations. Furthermore, the surface density perturbations do not show the continuous growth but are temporally amplified. We also study cases where the dust component undergoes amplification whereas the gas component remains stable. The amplitude amplification, however, strongly depends on the model parameters. In the minimum mass solar nebula (MMSN), for instance, the dust fluid amplification at the radial distance 100 au occurs when the Stokes number is about unity. But the amplification factor reduces as the dust and gas coupling becomes weaker. Furthermore, perturbations with a larger azimuthal wavelength exhibit a larger amplification factor.

Two-fluid gravitational instabilities in a galactic disk

1984 ◽  
Vol 276 ◽  
pp. 114 ◽  
Author(s):  
C. J. Jog ◽  
P. M. Solomon
Keyword(s):  
Galactic Disk ◽  
Gravitational Instabilities ◽  
Two Fluid

scholarly journals Spatial Distribution of Metallicity in a Local Part of the Galactic Disk

1977 ◽  
Vol 45 ◽  
pp. 207-209
Author(s):  
Andrzej Strobel ◽  
Janina Strobel
Keyword(s):  
Spatial Distribution ◽  
Galactic Disk ◽  
Galactic Structure ◽  
Abundance Gradient ◽  
Spiral Arms ◽  
Local Part ◽  
Systematic Character ◽  
Spatial Inhomogeneities

Many recent observations have indicated variations of metallicity within our Galaxy. Apart from a general negative radial abundance gradient indirect evidences suggest the existence of spatial chemical inhomo-geneities in different regions of the galactic disk. Since this problem is of great importance in providing clues to processes of nucleosynthesis and galactic structure and evolution, it seems to be interesting to examine if these spatial inhomogeneities have any systematic character and if they are correlated with position of spiral arms.

scholarly journals The role of diffusion on the interface thickness in a ventilated filling box

2010 ◽  
Vol 652 ◽  
pp. 195-205 ◽  
Author(s):  
N. B. KAYE ◽  
M. R. FLYNN ◽  
M. J. COOK ◽  
Y. JI
Keyword(s):  
Finite Thickness ◽  
Salt Bath ◽  
Quantitative Agreement ◽  
Buoyancy Flux ◽  
Diffuse Interface ◽  
Small Scale ◽  
Interface Thickness ◽  
Cross Sectional ◽  
Reported Data

We examine the role of diffusivity, whether molecular or turbulent, on the steady-state stratification in a ventilated filling box. The buoyancy-driven displacement ventilation model of Linden et al. (J. Fluid Mech., vol. 212, 1990, p. 309) predicts the formation of a two-layer stratification when a single plume is introduced into an enclosure with vents at the top and bottom. The model assumes that diffusion plays no role in the development of the ambient buoyancy stratification: diffusion is a slow process and the entrainment of ambient fluid into the plume from the diffuse interface will act to thin the interface resulting in a near discontinuity of density between the upper and lower layers. This prediction has been corroborated by small-scale salt bath experiments; however, full-scale measurements in ventilated rooms and complementary numerical simulations suggest an interface that is not sharp but rather smeared out over a finite thickness. For a given plume buoyancy flux, as the cross-sectional area of the enclosure increases the volume of fluid that must be entrained by the plume to maintain a sharp interface also increases. Therefore the balance between the diffusive thickening of the interface and plume-driven thinning favours a thicker interface. Conversely, the interface thickness decreases with increasing source buoyancy flux, although the dependence is relatively weak. Our analysis presents two models for predicting the interface thickness as a function of the enclosure height, base area, composite vent area, plume buoyancy flux and buoyancy diffusivity. Model results are compared with interface thickness measurements based on previously reported data. Positive qualitative and quantitative agreement is observed.

Droplet-Particle Agglomeration of Maltodextrin and Salt in a Small-Scale Spray Dryer

2008 ◽  
Vol 4 (6) ◽  
Author(s):  
Tim Langrish ◽  
D. Ali ◽  
M. Asplet
Keyword(s):  
Small Scale ◽  
Inlet Temperature ◽  
Scanning Electron Micrographs ◽  
Spray Dryer ◽  
Particle Agglomeration ◽  
Collision Efficiency ◽  
Flow Rates ◽  
Air Inlet ◽  
Two Fluid ◽  
Scanning Electron

The technique of in-chamber blending in practice has been investigated, by altering the configuration of a Buchi B290 small-scale spray dryer having a two-fluid nozzle, through adding an extra pipe into the chamber to agglomerate maltodextrin (DE18) particles with 20% salt solution sprayed through the atomizer. Scanning Electron Micrographs showed the importance of splash impacts between salt droplets and maltodextrin particles in this geometry, suggesting that the collision efficiency is strongly affected by whether or not the salt droplets completely surround the dry maltodextrin particles. Changing the air inlet temperature did not affect the measured amount of collisions (from Atomic Absorption Spectroscopy) significantly, but both the main air and the nozzle air flow rates had significant effects on the collision outcomes.

Enhanced Small-Scale Faraday Rotation in the Galactic Spiral Arms

2006 ◽  
Vol 637 (1) ◽  
pp. L33-L35 ◽  
Author(s):  
M. Haverkorn ◽  
B. M. Gaensler ◽  
J. C. Brown ◽  
N. S. Bizunok ◽  
N. M. McClure-Griffiths ◽  
...  
Keyword(s):  
Faraday Rotation ◽  
Small Scale ◽  
Spiral Arms ◽  
Galactic Spiral

scholarly journals Effect of ionospheric depth on the ionospheric feedback instability: cutoff and subsequent <I>E</I><sub>||</sub> modes

2010 ◽  
Vol 28 (9) ◽  
pp. 1777-1794 ◽  
Author(s):  
R. Cosgrove ◽  
R. Doe
Keyword(s):  
Growth Rate ◽  
Electric Fields ◽  
Finite Thickness ◽  
Higher Order ◽  
Plasma Structure ◽  
Higher Order Modes ◽  
Order Of Magnitude ◽  
E Region ◽  
The One ◽  
Two Fluid

Abstract. The ionospheric feedback instability (IFI), which involves feedback between ionospheric modifications and waves reflected off the magnetosphere, has up to this point been analyzed in terms of field line integrated (FLI) ionospheric quantities, that is, with the assumption that the ionospheric thickness can be ignored. In this work we test this assumption by solving the two-fluid equations for a representative ionospheric slab of finite thickness. We find that the results are for the most part incompatible with a description in terms of FLI quantities, and that their use can easily lead to an order of magnitude overestimation of the growth rate. This occurs because the first eigenmode, which is the one compatible with an FLI description, is cutoff above a certain frequency, leaving only higher order modes with wavelengths along B that are subsumed by the slab. Taking the results at face value, the parallel electric fields associated with the higher order modes are a possible contributor to electron heating and plasma structure in the E-region ionosphere.

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