Ions In Boundary Layer Research Articles

Star Formation and Evolution of Blister-type H ii Region Sh2-112

Abstract We report the observational findings of the Sh2-112 H ii region by using the multiwavelength data analysis ranging from optical to radio wavelengths. This region is powered by the massive O8V-type star BD +45 3216. The surface density distribution and minimum spanning tree analyses of the young stellar object (YSO) candidates in the region reveal their groupings toward the western periphery of the H ii region. A GMRT radio continuum emission peak is found toward the northwest boundary of the H ii region and is investigated as a compact/ultracompact H ii region candidate powered by a B0–B0.5-type star. Toward the southwest direction, a prominent curved rim-like structure is found in the Hα image and GMRT radio continuum maps, where the H2 and 13CO emission is also observed. These results suggest the existence of the ionized boundary layer (IBL) on the surface of the molecular cloud. This IBL is found to be overpressured with respect to the internal pressure of the surrounding molecular cloud. This implies that the shocks are propagating/propagated into the molecular cloud, and the young stars identified within it are likely triggered due to the massive star. It is also found that this region is ionization-bounded toward the west and density-bounded toward the east. Based on the distribution of the ionized gas, molecular material, and YSO candidates, we propose that the Sh2-112 H ii region is a good candidate for the blister-type H ii region that has been evolved on the surface of a cylindrical molecular cloud.

The nature of molecular cloud boundary layers from SOFIA [O I] observations

Context. Dense highly ionized boundary layers (IBLs) outside of the neutral Photon Dominated Regions (PDRs) have recently been detected via the 122 and 205 μm transitions of ionized nitrogen. These layers have higher densities than in the Warm Ionized Medium (WIM) but less than typically found in H II regions. Observations of [C II] emission, which is produced in both the PDR and IBL, do not fully define the characteristics of these sources. Observations of additional probes which just trace the PDRs, such as the fine structure lines of atomic oxygen, are needed derive their properties and distinguish among different models for [C II] and [N II] emissison. Aims. We derive the properties of the PDRs adjacent to dense highly ionized boundary layers of molecular clouds. Methods. We combine high-spectral resolution observations of the 63 μm [O I] fine structure line taken with the upGREAT HFA-band instrument on SOFIA with [C II] observations to constrain the physical conditions in the PDRs. The observations consist of samples along four lines of sight (LOS) towards the inner Galaxy containing several dense molecular clouds. We interpret the conditions in the PDRs using radiative transfer models for [C II] and [O I]. Results. We have a 3.5-σ detection of [O I] toward one source but only upper limits towards the others. We use the [O I] to [C II] ratio, or their upper limits, and the column density of C+ to estimate the thermal pressure, Pth, in these PDRs. In two LOS the thermal pressure is likely in the range 2–5 × 105 in units of K cm−3, with kinetic temperatures of order 75–100 K and H2 densities, n(H2) ~ 2–4 × 103 cm−3. For the other two sources, where the upper limits on [O I] to [C II] are larger, Pth ≲105 (K cm−3). We have also used PDR models that predict the [O I] to [C II] ratio, along with our observations of this ratio, to limit the intensity of the Far UV radiation field. Conclusions. The [C II] and [N II] emission with either weak, or without any, evidence of [O I] indicates that the source of dense highly ionized gas traced by [N II] most likely arises from the ionized boundary layers of clouds rather than from H II regions.

Ionized gas in the Scutum spiral arm as traced in [N ii] and [C ii

Determining the properties of the warm ionized medium (WIM) at the leading edge of spiral arms is important for understanding its dynamics and cloud formation. The inner edge of the Scutum arm tangency is a unique location in which to disentangle the WIM from other components. We use high spectral resolution [C II] 158 micron and [N II] 205 micron fine structure line observations taken with the upGREAT and GREAT instruments on SOFIA, along with auxiliary HI and 13CO observations. The observations were in and out of the Galactic plane along 18 lines of sight between longitude 30deg and 32deg. We detect strong [N II] emission throughout the Scutum tangency. At VLSR = 110 to 125 km/s, where there is little, if any, 13CO, we could disentangle the [N II] and [C II] emission that arises from the WIM at the inner edge. We find an average electron density, <n(e)> about 0.9 cm{-3} in the plane, and about 0.4 cm{-3} above the plane. For VLSR < 110 km/s there is [N II] emission tracing highly ionized gas throughout the arm's molecular layer. This ionized gas has a high density, n(e) ~ 30 cm{-3}, and a few percent filling factor. Thus, [N II] and [C II] at the Scutum arm tangency reveal a highly ionized gas with <n(e)> about 10 to 20 times those of the interarm WIM, which is best explained by a model in which the interarm WIM is compressed as it falls into the potential well of the arm. The widespread distribution of [N II] in the molecular layers shows that high density ionized gas is distributed throughout the Scutum arm. The n(e) derived from [N II] for these molecular cloud regions are about 30 cm{-3}, and probably arise in the ionized boundary layers of clouds. This [N II] emission from the ionized boundary layers is probably the result of the shock compression of the WIM as it impacts the arm's neutral gas, but some could arise from extended HII regions.

C II] and [N II] from dense ionized regions in the Galaxy

The interstellar medium (ISM) consists of highly ionized and neutral atomic, as well as molecular, components. Knowledge of their distribution is important for tracing the structure and lifecycle of the ISM. Here we determine the properties of the highly ionized and neutral weakly ionized gas in the Galaxy traced by the fine-structure lines of ionized nitrogen, [N II], and ionized carbon, [C II]. To analyze the ionized ISM we utilize [C II] 158 micron and [N II] 205 micron lines taken with the high spectral resolution Heterodyne Instrument in the Far-Infrared (HIFI) on the Herschel Space Observatory along ten lines of sight towards the inner Galaxy. [N II] emission can be used to estimate the contribution of the highly ionized gas to the [C II] emission and separate the highly ionized and weakly ionized neutral gas. We find that [N II] has strong emission in distinct spectral features along all lines of sight associated with strong [C II] emission. The [N II] arises from moderate density extended HII regions or ionized boundary layers of clouds. Comparison of the [N II] and [C II] spectra in 31 separate kinematic features shows that many of the [C II] spectra are affected by absorption from low excitation gas associated with molecular clouds, sometimes strongly so. The apparent fraction of the [C II] associated with the [N II] gas is unrealistically large in many cases, most likely due to the reduction of [C II] by absorption. In a few cases the foreground absorption can be modeled to determine the true source intensity. In these sources we find that the foreground absorbing gas layer has C$^+$ column densities of order 10$^{18}$ cm$^{-2}$. [C II] emission arising from strong sources of [N II] emission is frequently absorbed by low excitation foreground gas complicating the interpretation of the properties of the ionized and neutral gas components that give rise to [C II] emission.

Testing diagnostics of triggered star formation

We produce synthetic images and SEDs from radiation hydrodynamical simulations of radiatively driven implosion. The synthetically imaged bright rimmed clouds (BRCs) are morphologically similar to those observed in star forming regions. Using nebular diagnostic line-ratios, simulated Very Large Array (VLA) radio images, H{\alpha} imaging and SED fitting we compute the neutral cloud and ionized boundary layer gas densities and temperatures and perform a virial stability analysis for each model cloud. We determine that the neutral cloud temperatures derived by SED fitting are hotter than the dominant neutral cloud temperature by 1 - 2 K due to emission from warm dust. This translates into a change in the calculated cloud mass by 8-35 %. Using a constant mass conversion factor (C{\nu}) for BRCs of different class is found to give rise to errors in the cloud mass of up to a factor of 3.6. The ionized boundary layer (IBL) electron temperature calculated using diagnostic line ratios is more accurate than assuming the canonical value adopted for radio diagnostics of 10^4 K. Both radio diagnostics and diagnostic line ratios are found to underestimate the electron density in the IBL. Each system is qualitatively correctly found to be in a state in which the pressure in the ionized boundary layer is greater than the supporting cloud pressure, implying that the objects are being compressed. We find that observationally derived mass loss estimates agree with those on the simulation grid and introduce the concept of using the mass loss flux to give an indication of the relative strength of photo-evaporative flow between clouds. The effect of beam size on these diagnostics in radio observations is found to be a mixing of the bright rim and ambient cloud and HII region fluxes, which leads to an underestimate of the cloud properties relative to a control diagnostic.

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected the MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2–He flows produces an increase of the density fluctuation intensity by up to 2dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force).

Active Control of Ionized Boundary Layers

The challenging problems, in the field of control of chaos or of transition to chaos, lie in the domain of infinite-dimensional systems. Access to all variables being impossible in this case and the controlling action being limited to a few collective variables, it will not in general be possible to drive the whole system to the desired behavior. A paradigmatic problem of this type is the control of the transition to turbulence in the boundary layer of fluid motion. By analyzing a boundary layer flow for an ionized fluid near an airfoil, one concludes that active control of the transition amounts to the resolution of an generalized integro-differential eigenvalue problem. To cope with the required response times and phase accuracy, electromagnetic control, whenever possible, seems more appropriate than mechanical control by microactuators.

An Improved Convection Scheme Applied to Recombining Divertor Plasma Flows

A high-order finite volume convection scheme, in conjunction with a monotonicity preserving flux-limiter is applied to a combined tokamak edge plasma/Navier–Stokes neutral transport model. This is a highly nonlinear system of convection–diffusion–reaction equations which describe the partially ionized boundary layer plasma of a tokamak fusion reactor. The solutions of interest contain a sharp ionization front. The improved convective discretization is applied within the context of the existing matrix-free Newton–Krylov solution algorithm. More accurate convective differencing is shown to make a significant difference on a problem of current interest. It is demonstrated that a matrix-free Newton–Krylov implementation, where the preconditioner is derived using first-order upwind convective differencing, provides savings in both memory requirements and CPU time.

Nonlinear regimes of current flow through a weakly ionized boundary layer and their stability

The problem of steady states and stability of current flow in low-temperature plasma with allowance for the Joule heat release have been considered repeatedly for the case of confined stationary plasma (see, for example, [1]). The present paper is devoted to the asymptotic solution of this problem for the case of a laminar boundary layer of weakly ionized plasma. A similar problem was previously considered in [2] in a zero-dimensional approximation.

On the interaction of MHD waves with a plasma surrounded by a cold gas-mantle

In cold gas-mantle systems a partially ionized boundary layer is formed which becomes highly resistive to the magnetoacoustic waves. Especially below the ion gyro frequency ωci the effective resistivity due to ion-neutral collisions can attain values being an order of magnitude higher than the Spitzer resistivity. Thus , a substantial part of the r.f. power available at the antenna can be damped in such a layer. The application of the r.f. power in the Alfven wave frequency range of the internal ring device FLASK IVA (FIVA) results in a relatively powerful plasma heating. The plasma-neutral gas balance is strongly modified by this power input, as well as the plasma equilibrium which becomes a sensitive function of the neutral gas content of the plasma. An r.f. power of the order of 0.5 MW is absorbed in the plasma. The parameters and the perpendicular dimension of the discharge are in the range which is typical for the layers which will form in the immediate vicinity of the wall in a fully developed mantle of a fusion device. Thus the presented data indicate that a strong interaction between the MHD waves and such cold layers should be expected.

Experimental Evidence of Temperature Gradient Effects on the Neutral Gas Penetration into a Magnetized Rotating Plasma

The experimental evidence of temperature gradient effects on the neutral gas penetration into a plasma is presented. It is found that an increased temperature gradient reduces the neutral gas content in the partially ionized boundary layer of a cold gas blanket system. A strong modification of the rotating plasma equilibrium is then observed. Such an equilibrium is a sensitive function of the neutral gas concentration both in the plasma and its boundary layers. The moderate values of the centrifugal force which act both on the ions and neutrals do not appreciably influence the plasma permeability to neutral gas. It is further observed that an acceleration of neutrals takes place in a rotating plasma.

Fracture characteristics of glass-graphite bodies in a partially ionized air flow

The accuracy of computations of the fracture characteristics of glass-graphite materials is estimated by using the analogy between the heat and mass transfer in a partially ionized boundary layer.

Plasma-Neutral Gas Interaction Effects on the Stability of Drift Modes

In quasi-steady fusion reactors partially ionized boundary layers are likely to be formed. The plasma density is then expected to be peaked close to the boundary. The possibility of large density gradients driving unstable drift modes in these layers is investigated. The expected magnetic shear, due to induced plasma currents in such systems, as well as minimum-average-B properties of the magnetic field, are probably insufficient to significantly influence the stability properties of these modes. However, effects associated with plasma-neutral gas interactions as well as with a finite Larmor radius and ion viscosity modify the behaviour of drift modes and introduce stabilizing effects.

The ionization of a low-density intercloud medium by a single O star

The ionization structure of hydrogen in a cloud-containing low-density gas surrounding an O star is calculated, emphasizing how UV photons cause the ionization of hydrogen in regions which are totally obscured from the star by neutral clouds. Particular consideration is given to the ionization of such obscured regions by diffuse Lyman continuum radiation produced when H(+) recombines directly to the ground state. The assumed physical structure of the cloud-containing neighborhood of an O star is discussed, a method is outlined for expressing the emission and absorption properties of the neutral clouds and their shadows in terms of effective volume emissivities and absorption coefficients on a line of sight, and equations of radiative transfer are derived for diffuse and stellar radiation in a cloud-containing H II region. Reradiation of Lyman continuum photons from the ionized boundary layer of a neutral cloud or shadow is analyzed. The expected emission measure across a large cloud-containing H II region is determined and found to compare well with that obtained from observations of H-alpha emission around runaway O stars. It is shown that the hydrogen in cloud shadows may be completely ionized by diffuse radiation within approximately half the Stroemgren radius, depending on the extent of that radius, the average frequency of the Lyman continuum radiation from the star, and the average cloud size.

Antenna admittance determination of electron density.

A circular-aperture flush-mounted antenna for which a theoretical description of the antenna-plasma interaction has been developed was used to obtain a comparison between theoretical and experimental antenna admittance in the presence of ionized boundary layers of low-collision frequency. The antenna was located in a flat-plate model which was in turn located in the expanding-flow environment of a reflected-shock tunnel. The electron-temperature and electron-density distributions in the plate boundary layer at the antenna location were independently measured using voltage-swept thin-wire Langmuir probes for one of the test conditions. The antenna admittance was measured using a four-probe microwave reflectometer and these measured values were found to be in good agreement with those predicted from the theory. Measurements were also performed with another type of circular-aperture antenna which did not satisfy all of the constraints of the theoretical model, although good agreement was obtained between the calculations and the experimental results.

A study of radiation properties of oxide-metal composites.

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Electric dipole radiation through a finite conical plasma sheath

The ionized boundary layer that forms around a reentry vehicle is represented by a conical sheath having a surface impedance which is determined by the integrated electron density in the boundary layer. This cone can support surface currents that are induced in it by a Hertzian dipole antenna situated on its axis. The radiation pattern of such a structure at wavelengths short compared to the slant height is calculated by a perturbation technique. The calculation holds when the surface conductivity of the sheath varies linearly with distance from the apex of the cone. Plots are presented showing the effects of sheath strength, slant height, and antenna frequency on the patterns.

Heat Transfer in Planetary Atmospheres at Super-Satellite Speeds

The equilibrium boundary layer flow at the stagnation point of a blunt body at very high enthalpies in air and in the simulated atmosphere of Venus (C02) has been calculated. The effects of dissociation and ionization are taken into account by means of the total thermodynamic and transport property concept. Correlation of the numerical results shows that a single simple equation will predict the stagnation point heat transfer rate at flight velocities between 6000 and 50,000 fps. Wall temperatures between 540° and 5400°R and stagnation pressures between 0.001 and 100 atm were considered. The same correlation equation is valid in Earth's atmosphere, as well as in the simulated atmosphere of Venus. The theoretical results are compared with experimental heat transfer data obtained in partially ionized air and in CO2. The agreement between theory and experiment is satisfactory. It is also shown that reasonable results for the heat transfer from an ionized boundary layer can be obtained from the results of Fay and Riddell by consistently neglecting the effects of ionization.