Thermosolutal-convective instability of a stellar atmosphere in the presence of suspended particles (original) (raw)
A Thermodynamically Induced Finite-Amplitude Convective Instability in Stellar Envelopes
The Astrophysical Journal, 2001
Stellar envelopes are subject to a finite-amplitude convective instability that originates with the reduction in the adiabatic exponent accompanying partial ionization of the principle plasma constituents, G p (d ln P/d ln r) 1 a d notably hydrogen. The instability is one-sided; low-perturbations are unstable, while high-perturbations are G G 1 1 stable. Since a partially ionized fluid has a lower adiabatic exponent than either a fully recombined or fully ionized one, convective downflows are stabilized in the upper regions of a convective envelope where the nearly fully recombined fluid is embedded in a partially ionized background. They are significantly destabilized at a depth, however, where the partially ionized downflowing fluid has a lower than does the highly ionized mean G 1 state. Convective upflows, by contrast, are stabilized at a depth where their fully ionized state contrasts with the partially ionized background and are destabilized only in the very upper layers where the mean state of the fluid is nearly fully recombined and the upflows are partially ionized. This Letter illustrates the instability mechanism, its finite-amplitude character, and its possible significance to both idealized compressible convection simulations and the solar convective envelope.
Quiescent and Catastrophic Events in Stellar Atmospheres
Mathematical Physics - Proceedings of the 12th Regional Conference, 2007
Six different phases of the 11-year starspot cycles: 0.00 (top, cycle minimum), 0.16, 0.33, 0.49 (near cycle maximum), 0.65, and 0.82 (bottom). Sample magnetograms for these phases for the simulations of the Sun-like star and of the most active star are shown in Fig. above. • The field line density within each panel is statistically proportional to field strength. Dynamic finely structured stellar atmosphere Stellar Atmospheres Pakistan, March, 2006 3 Stellar magnetic activity => wealth of phenomena: starspots, nonradiatively heated outer atmospheres, activity cycles, deceleration of rotation rates, and even, in close binaries, stellar cannibalism. Key topics : radiative transfer, convective simulations, dynamo theory, outer-atmospheric heating, stellar winds and angular momentum loss. Magnetically active stars shed angular momentum-lose mass through their asterospheric magnetic fields. This process involves the interaction of a topologically complex, evolving coronal magnetic field with embedded plasma, which is heated throughout the corona + accelerated on its way to interstellar space. Stellar observations suggest: Sun was magnetically active even before it became a hydrogen-burning star. Activity smoothly declining over billions of years-angular momentum is lost through a magnetized solar wind (e.g., Schrijver et al. 2003). Evolution of corona cartoon: gravitationally stratified layers in the 1950s (left); vertical flux tubes with chromospheric canopies (1980s, middle); fully inhomogeneous mixing of photospheric, chromospheric, TR and coronal zones by such processes as heated upflows, cooling downflows, interminent heating (ε), nonthermal electron beams (e), field line motions and reconnections, emission from hot plasma, absorption and scattering in cool plasma, acoustic waves, shock waves (right) (Shrijver 2001). Associating the traditional layers with temperature rather than height is only a little better.
The Role of Magnetic Fields in the Heating of Stellar Atmospheres — Theory
Solar and Stellar Magnetic Fields: Origins and Coronal Effects, 1983
The last ten years of observations have shown beyond doubt the fundamental role played by the magnetic field in the heating of stellar atmospheres. After the recognition of the extreme inhomogeneity of the solar corona, two basic new trends have appeared in the theoretical literature on the coronal heating problem. One is the adoption of a glob al point of view that stresses the connection of the properties of the upper layers to those of the underlying ones. In this way a general framework is provided, capable of accomodating many possible heating mechanisms that need not to be specified at this stage. The second nov elty is the explicit inclusion in the theory of the inhomogeneous nature of the stellar envelopes, as a result of the presence of magnetic fields. The present status of knowledge on the subject as determined by the above evolution of the theoretical approach will be reviewed.
Suspended Particles and the Gravitational Instability of Rotating Magnetised Medium
Beiträge aus der Plasmaphysik, 1985
The effect of uniform rotation on the self gravitational instability of an infinite homogeneous magnetised gas particle medium in the presence of suspended particles is investigated. The equations of the problem are linearized and the general dispersion relation for such system is obtained. The rotation is assumed along two different directions and separate dispersion relation for each case is obtained. The dispersion relation for propagation parallel and perpendicular to the uniform magnetic field along with rotation is derived. The effect of suspended particles on the different modes of propagation is investigated. It is found that in presence of suspended particles, magnetic field, rotation and viscosity, Jeans' criterion determines the condition of gravitational instability of gas‐particle medium.
The Magnetoviscous-Thermal Instability
The Astrophysical Journal, 2012
Accretion flows onto underluminous black holes, such as Sagittarius A* at the center of our galaxy, are dilute (mildly collisional to highly collisionless), optically thin, and radiatively inefficient. Therefore, the accretion properties of such dilute flows are expected to be modified by their large viscosities and thermal conductivities. Second, turbulence within these systems needs to transport angular momentum as well as thermal energy generated through gravitational infall outwards in order to allow accretion to occur. This is in contrast to classical accretion flows, in which the energy generated through accretion down a gravitational well is locally radiated. In this paper, using an incompressible fluid treatment of an ionized gas, we expand on previous research by considering the stability properties of a magnetized rotating plasma wherein the thermal conductivity and viscosity are not negligible and may be dynamically important. We find a class of MHD instabilities that can transport angular momentum and thermal energy outwards. They are plausible candidates to describe accretion in radiatively inefficient accretion flows (RIAFs). We finish by discussing the implications for analytic models and numerical MHD simulations of mildly dilute or collisionless astrophysical plasmas, and immediate directions for further research.
Instability of isothermal stellar wind bowshocks
New Astronomy, 1998
We present hydrodynamical simulations illustrating the instability of stellar wind bowshocks in the limit of an isothermal equation of state. In this limit, the bowshock is characterized by a thin dense shell bounded on both sides by shocks. In a time-averaged sense the shape of this bowshock shell roughly matches the steady state solution of Wilkin (1996)[ApJ, 459, L31], although the apex of the bowshock can deviate in or out by a factor of two or more. The shape of the bowshock is distorted by large amplitude kinks with a characteristic wavelength of order the standoff distance from the star. The instability is driven by a strong shear flow within the shock-bounded shell, suggesting an origin related to the nonlinear thin-shell instability. This instability occurs when both the forward bowshock and the reverse wind shock are effectively isothermal and the star is moving through the interstellar medium with a Mach number greater than a few. This work therefore suggests that ragged, clumpy bowshocks should be expected to surround stars with a slow, dense wind (which leads to rapid cooling behind the reverse wind shock), whose velocity with respect to the surrounding interstellar medium is of 21 order 60 km s (leading both to rapid cooling behind the forward bowshock and sufficiently high Mach numbers to drive the instability).
Is Thermal Instability Significant in Turbulent Galactic Gas?
The Astrophysical Journal, 2000
We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. We consider three sets of numerical simulations: a) flows in the presence of the instability only; b) flows in the presence of the instability and various types of turbulent energy injection (forcing), and c) models of the ISM including the magnetic field, the Coriolis force, self-gravity and stellar energy injection. Simulations in the first group show that the condenstion process which forms a dense phase ("clouds") is highly dynamical, and that the boundaries of the clouds are accretion shocks, rather than static density discontinuities. The density histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situations may be established, but the equilibrium is very fragile: small density fluctuations in the warm phase require large variations in that of the cold phase, probably inducing shocks into the clouds. Combined with the likely disruption of the clouds by Kelvin-Helmholtz instability (Murray et al. 1993), this result suggests that such configurations are highly unlikely. Simulations in the second group show that large-scale turbulent forcing is incapable of erasing the signature of the TI in the density PDFs, but small-scale, stellar-like forcing causes the PDFs to transit from bimodal to a single-slope power law, erasing the signature of the instability. However, these simulations do not reach stationary regimes, the TI driving an ever-increasing star formation rate. Simulations in the third group show no significant difference between the PDFs of stable and unstable cases, and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra effective pressure provided by the magnetic field and the Coriolis force overwhelm the TI as a density-structure generator in the ISM, the TI becoming a second-order effect. We emphasize that a multi-modal temperature PDF is not necessarily an indication of a multi-phase medium, which must contain clearly distinct thermal equilibrium phases, and that this "multi-phase" terminology is often inappropriately used.
THERMOSOLUTAL STABILITY OF A TWO-COMPONENT ROTATING PLASMA WITH FINITE LARMOR RADIUS
2007
The thermosolutal linear stability of a composite two-component plasma is studied in the presence of Coriolis forces, finite Larmor radius, taking into account the collisions between neutral and ionized particles. The thermosolutal instability appears due to a material convection (thermosolutal convection) in a two component fluid with different molecular diffusivities which contribute in an opposing sense to the locally vertical density gradient. The analysis shows that in the case of a stationary convection, the finite Larmor radius, stable solute gradients and rotation have stabilizing effects. Results also demonstrate that the mutual collisions between ionized and neutral particles does not affect the stationary convection.
On characterization of turbulent stellar convection
Monthly Notices of the Royal Astronomical Society, 2003
An attempt is made to characterize convective fluid motion in a stellar-type convective envelope within the framework of non-linear dynamics. For this analysis, we have simulated threedimensional turbulent compressible convection by solving numerically the full set of Navier-Stokes equations in a rectangular box with a two-layer stable-unstable stratification. The fluid motions are evolved with time until the fluid reaches a statistically steady state. A time series of the horizontally-averaged vertical velocity of the convective fluid is collected from the middle of the unstable zone. The correlation dimension and the largest Lyapunov exponent computed for this time series characterize the fluid motion as that of a low-dimensional strange attractor.
Astrophysics and Space Science
The panoptic influence of plasma q-nonextensivity and dust-charge fluctuations on the gravito-electromagnetic stability behaviour of a realistic non-thermal complex astroplasma model configuration with infinite geometrical extension is reconnoitered. It includes active viscoelasticity and dust polarization force-field effects in quasineutral hydrostatic equilibrium on the astrophysical fluid scales of space and time. The nontrivial linear model is simplified with the Jeans homogenization assumption (Jeans swindle, no zeroth-order force-field). It analytically and logically enables us to relax from the inclusion of large-scale inhomogeneities and of associated intrinsic complications. The role of boundary effects on the dynamical stability is assumed to be insignificant. We apply a standard technique of the Fourier formulaic plane-wave analysis over the basic cloud-structuring equations in a closed integrated form. It reduces the model Fourier algebraic equations decoupling into a unique form of cubic dispersion relation having mixed variable coefficients, which, indeed, explicitly, evolve on the diverse model plasma parameters. It is interestingly seen that the polarization and nonextensive effects directly play destabilizing roles. In contrast, the viscoelasticity and magnetic field create stabilizing effects on the instability. The pragmatic significance and applicability in the context of astrocosmo-galactic environments are briefly indicated aboard analytic facts and introspective faults.