Comparison of theoretical radiation-driven winds from stars and discs

We compare models of line-driven winds from accretion discs and single spherical stars. We look at the problem of scaling mass-loss rates and velocities of stellar and disc winds with model parameters. We find that stellar and disc winds driven by radiation, within the CAK framework, are very similar as far as mass-loss rates and velocities are concerned. Thus we can use analytic results for stellar winds to rescale, in a first order approximation, numerical results for disc winds. We also show how the CAK stellar solutions change when we take into account effects of very low luminosities or line-driving force.Comment: LaTeX, 13 pages, including three tables, 4 Postscript files, requires mn.sty, to appear in MNRA

Non-Isobaric Thermal Instability

Multiphase media have very complex structure and evolution. Accurate numerical simulations are necessary to make advances in our understanding of this rich physics. Because simulations can capture both the linear and nonlinear evolution of perturbations with a relatively wide range of sizes, it is important to thoroughly understand the stability of condensation and acoustic modes between the two extreme wavelength limits of isobaric and isochoric instability as identified by Field. Partially motivated by a recent suggestion that large non-isobaric clouds can shatter into tiny cloudlets, we revisit the linear theory to survey all possible regimes of thermal instability. We uncover seven regimes in total, one of which allows three unstable condensation modes. Using the code Athena++, we determine the numerical requirements to properly evolve small amplitude perturbations of the entropy mode into the nonlinear regime. Our 1D numerical simulations demonstrate that for a typical AGN cooling function, the nonlinear evolution of a single eigenmode in an isobarically unstable plasma involves increasingly larger amplitude oscillations in cloud size, temperature, and density as the wavelength increases. Such oscillations are the hallmark behavior of non-isobaric multiphase gas dynamics and may be observable as correlations between changes in brightness and the associated periodic redshifts and blueshifts in systems that can be spatially resolved. Intriguingly, we discuss regimes and derive characteristic cloud sizes for which the saturation process giving rise to these oscillations can be so energetic that the cloud may indeed break apart. However, we dub this process splattering instead of shattering, as it is a different fragmentation mechanism that is triggered when the cloud suddenly lands on the stable cold branch of the equilibrium curve

Time-dependent Radiation-driven Winds

We study temporal variability of radiation-driven winds using one-dimensional, time-dependent simulations and an extension of the classic theory of line-driven winds developed by Castor Abbott & Klein. We drive the wind with a sinusoidally varying radiation field and find that after a relaxation time, determined by the propagation time for waves to move out of the acceleration zone of the wind, the solution settles into a periodic state. Winds driven at frequencies much higher than the dynamical frequency behave like stationary winds with time averaged radiation flux, whereas winds driven at much lower frequencies oscillate between the high and low flux stationary states. Most interestingly, we find a resonance frequency near the dynamical frequency that results in velocity being enhanced or suppressed by a factor comparable to the amplitude of the flux variation. Whether the velocity is enhanced or suppressed depends on the relative phase between the radiation and the dynamical variables. These results suggest that a time-varying radiation source can induce density and velocity perturbations in the acceleration zones of line-driven winds