The winds of change in Sri Lanka? Rajapaksa's charisma and foreign factors in Sri Lankan politics

Whilst the external support received by the government in Sri Lanka from India and the West may play a pivotal role in the coming months in terms of securing the grip of power in the parliament, the outcome of the local elections in Sri Lanka has created a political storm in the island, write Eshan Jayawardena and Punsara Amarasinghe

Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds

Aims: We study the evolution of stellar rotation and wind properties for low-mass main-sequence stars. Our aim is to use rotational evolution models to constrain the mass loss rates in stellar winds and to predict how their properties evolve with time on the main-sequence. Methods: We construct a rotational evolution model that is driven by observed rotational distributions of young stellar clusters. Fitting the free parameters in our model allows us to predict how wind mass loss rate depends on stellar mass, radius, and rotation. We couple the results to the wind model developed in Paper I of this series to predict how wind properties evolve on the main-sequence. Results: We estimate that wind mass loss rate scales with stellar parameters as $\dot{M}_\star \propto R_\star^2 \Omega_\star^{1.33} M_\star^{-3.36}$. We estimate that at young ages, the solar wind likely had a mass loss rate that is an order of magnitude higher than that of the current solar wind. This leads to the wind having a higher density at younger ages; however, the magnitude of this change depends strongly on how we scale wind temperature. Due to the spread in rotation rates, young stars show a large range of wind properties at a given age. This spread in wind properties disappears as the stars age. Conclusions: There is a large uncertainty in our knowledge of the evolution of stellar winds on the main-sequence, due both to our lack of knowledge of stellar winds and the large spread in rotation rates at young ages. Given the sensitivity of planetary atmospheres to stellar wind and radiation conditions, these uncertainties can be significant for our understanding of the evolution of planetary environments.Comment: 26 pages, 14 figures, 2 tables, to be published in A&

The Role of Galactic Winds on Molecular Gas Emission from Galaxy Mergers

We assess the impact of starburst and AGN feedback-driven winds on the CO emission from galaxy mergers, and, in particular, search for signatures of these winds in the simulated CO morphologies and emission line profiles. We do so by combining a 3D non-LTE molecular line radiative transfer code with smoothed particle hydrodynamics (SPH) simulations of galaxy mergers that include prescriptions for star formation, black hole growth, a multiphase interstellar medium (ISM), and the winds associated with star formation and black hole growth. Our main results are: (1) Galactic winds can drive outflows of masses ~10^8-10^9 Msun which may be imaged via CO emission line mapping. (2) AGN feedback-driven winds are able to drive imageable CO outflows for longer periods of time than starburst-driven winds owing to the greater amount of energy imparted to the ISM by AGN feedback compared to star formation. (3) Galactic winds can control the spatial extent of the CO emission in post-merger galaxies, and may serve as a physical motivation for the sub-kiloparsec scale CO emission radii observed in local advanced mergers. (4) Secondary emission peaks at velocities greater than the circular velocity are seen in the CO emission lines in all models. In models with winds, these high velocity peaks are seen to preferentially correspond to outflowing gas entrained in winds, which is not the case in the model without winds. The high velocity peaks seen in models without winds are typically confined to velocity offsets (from the systemic) < 1.7 times the circular velocity, whereas the models with AGN feedback-driven winds can drive high velocity peaks to ~2.5 times the circular velocity.Comment: Accepted by ApJ; Minor revisions; Resolution tests include

Evidence for simultaneous jets and disk winds in luminous low-mass X-ray binaries

Recent work on jets and disk winds in low-mass X-ray binaries (LMXBs) suggests that they are to a large extent mutually exclusive, with jets observed in spectrally hard states and disk winds observed in spectrally soft states. In this paper we use existing literature on jets and disk winds in the luminous neutron star (NS) LMXB GX 13+1, in combination with archival Rossi X-ray Timing Explorer data, to show that this source is likely able to produce jets and disk winds simultaneously. We find that jets and disk winds occur in the same location on the source's track in its X-ray color-color diagram. A further study of literature on other luminous LMXBs reveals that this behavior is more common, with indications for simultaneous jets and disk winds in the black hole LMXBs V404 Cyg and GRS 1915+105 and the NS LMXBs Sco X-1 and Cir X-1. For the three sources for which we have the necessary spectral information, we find that the simultaneous jets/winds all occur in their spectrally hardest states. Our findings indicate that in LMXBs with luminosities above a few tens of percent of the Eddington luminosity, jets and disk winds are not mutually exclusive, and that the presence of disk winds does not necessarily result in jet suppression.Comment: Updated to match published version (2016, ApJ, 830, L5

Laboratory simulations of local winds in the atmospheric boundary layer via image analysis

In the atmospheric boundary layer, under high pressure conditions and negligible geostrophic winds, problems associated with pollution are the most critical. In this situation local winds play a major role in the evaluation of the atmospheric dynamics at small scales and in dispersion processes. These winds originate as a result of nonuniform heating of the soil, either when it is homogeneous or in discontinuous terrain in the presence of sea and/or slopes. Depending on the source of the thermal gradient, local winds are classified into convective boundary layer, sea and land breezes, urban heat islands, and slope currents. Local winds have been analyzed by (i) simple analytical models; (ii) numerical models; (iii) field measurements; (iv) laboratory measurements through which it is impossible to completely create the necessary similarities, but the parameters that determine the phenomenon can be controlled and each single wind can be separately analyzed. The present paper presents a summary of laboratory simulations of local winds neglecting synoptic winds and the effects of Coriolis force. Image analysis techniques appear suitable to fully describe both the individual phenomenon and the superposition of more than one local wind. Results do agree with other laboratory studies and numerical experiments

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

Winds from accretion disks driven by the radiation and magnetocentrifugal force

We study the 2-D, time-dependent hydrodynamics of radiation-driven winds from luminous accretion disks threaded by a strong, large-scale, ordered magnetic field. The radiation force is due to spectral lines and is calculated using a generalized multidimensional formulation of the Sobolev approximation. The effects of the magnetic field are approximated by adding a force that emulates a magnetocentrifugal force. Our approach allows us to calculate disk winds when the magnetic field controls the flow geometry, forces the flow to corotate with the disk, or both. In particular, we calculate models where the lines of the poloidal component of the field are straight and inclined to the disk at a fixed angle. Our numerical calculations show that flows which corotate with the disk have a larger mass loss rate than their counterparts which conserve specific angular momentum. The difference in the mass loss rate between these two types of winds can be several orders of magnitude for low disk luminosities but vanishes for high disk luminosities. Winds which corotate with the disk have much higher velocities than angular momentum conserving winds. Fixing the wind geometry stabilizes winds which are unsteady when the geometry is derived self-consistently. The inclination angle between the poloidal velocity and the normal to the disk midplane is important. Non-zero inclination angles allow the magnetocentrifugal force to increase the mass loss rate for low luminosities, and increase the wind velocity for all luminosities. Our calculations also show that the radiation force can launch winds from magnetized disks. The line force can be essential in producing MHD winds from disks where the thermal energy is too low to launch winds or where the field lines make an angle of < 30^o with respect to the normal to the disk.Comment: LaTeX, 11 pages, 6 color postscript or PJEG files, to appear in Ap

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