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Register a new userDynamics of stellar wind in a Roche potential: implications for (i) outflows & periodicities relevant to astronomical masers, and (ii) generation of baroclinicity
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We study the dynamics of stellar wind from one of the bodies in the binary system, where the other body interacts only gravitationally. We focus on following three issues: (i) we explore the origin of observed periodic variations in maser intensity; (ii) we address the nature of bipolar molecular outflows; and (iii) we show generation of baroclinicity in the same model setup. From direct numerical simulations and further numerical modelling, we find that the maser intensity along a given line of sight varies periodically due to periodic modulation of material density. This modulation period is of the order of the binary period. Another feature of this model is that the velocity structure of the flow remains unchanged with time in late stages of wind evolution. Therefore the location of the masing spot along the chosen sightline stays at the same spatial location, thus naturally explaining the observational fact. This also gives an appearance of bipolar nature in the standard position-velocity diagram, as has been observed in a number of molecular outflows. Remarkably, we also find the generation of baroclinicity in the flow around binary system, offering another site where the seed magnetic fields could possibly be generated due to the Biermann battery mechanisms, within galaxies.
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We consider the radiation properties and processes of a gas with a population inversion using the formalism based on the Maxwell-Bloch equations. We focus on the maser action and Dickes superradiance to establish their relationship in the overall radiation process during the temporal evolution of the system as a function of position. We show that the maser action and superradiance are not competing phenomena but are rather complementary, and define two distinct limits for the intensity of radiation. Masers characterise the quasi-steady state limit, when the population inversion density and the polarisation amplitude vary on time-scales longer than those of non-coherent processes affecting their evolution (e.g., collisions), while superradiance defines the fast transient regime taking place when these conditions are reversed. We show how a transition from a maser regime to superradiance will take place whenever a critical threshold for the column density of the population inversion is reached, at which point a strong level of coherence is established in the system and a powerful burst of radiation can ensue during the transient regime. This critical level also determines the spatial region where a transition from the unsaturated to the saturated maser regimes will take place; superradiance can thus be seen as the intermediary between the two. We also quantify the gain in radiation intensity attained during the superradiance phase relative to the two maser regimes, and show how the strong coherence level during superradiance is well suited to explain observations that reveal intense and fast radiation flares in maser-hosting regions.
Orbital evolution of binary systems in dense stellar clusters is important in a variety of contexts: origin of blue stragglers, progenitors of compact object mergers, millisecond pulsars, and so on. Here we consider the general problem of secular evolution of the orbital elements of a binary system driven by the smooth tidal field of an axisymmetric stellar cluster (globular, nuclear, etc.) in which the binary orbits. We derive a secular Hamiltonian (averaged over both the inner Keplerian orbit of the binary and its outer orbit within the cluster) valid to quadrupole order for an arbitrary cluster potential and explore its characteristics. This doubly-averaged tidal Hamiltonian depends on just two parameters, which fully absorb the information about the background cluster potential and the binarys orbit within it: a dimensional parameter $A$ setting the secular timescale, and a dimensionless parameter $Gamma$ which determines the phase portrait of the binarys inner orbital evolution. We examine the dependence of $A$ and $Gamma$ on cluster potential (both spherical and axisymmetric) and on the binary orbit within the cluster. Our theory reproduces known secular results - such as Lidov-Kozai evolution and the effect of the Galactic tide on Oort Cloud comets - in appropriate limits, but is more general. It provides a universal framework for understanding dynamical evolution of various types of binaries driven by the smooth tidal field of any axisymmetric potential. In a companion paper (Hamilton & Rafikov 2019b) we provide a detailed exploration of the resulting orbital dynamics.
Radiation-dust driven outflows, where radiation pressure on dust grains accelerates gas, occur in many astrophysical environments. Almost all previous numerical studies of these systems have assumed that the dust was perfectly-coupled to the gas. However, it has recently been shown that the dust in these systems is unstable to a large class of resonant drag instabilities (RDIs) which de-couple the dust and gas dynamics and could qualitatively change the nonlinear outcome of these outflows. We present the first simulations of radiation-dust driven outflows in stratified, inhomogeneous media, including explicit grain dynamics and a realistic spectrum of grain sizes and charge, magnetic fields and Lorentz forces on grains (which dramatically enhance the RDIs), Coulomb and Epstein drag forces, and explicit radiation transport allowing for different grain absorption and scattering properties. In this paper we consider conditions resembling giant molecular clouds (GMCs), HII regions, and distributed starbursts, where optical depths are modest ($lesssim 1$), single-scattering effects dominate radiation-dust coupling, Lorentz forces dominate over drag on grains, and the fastest-growing RDIs are similar, such as magnetosonic and fast-gyro RDIs. These RDIs generically produce strong size-dependent dust clustering, growing nonlinear on timescales that are much shorter than the characteristic times of the outflow. The instabilities produce filamentary and plume-like or horsehead nebular morphologies that are remarkably similar to observed dust structures in GMCs and HII regions. Additionally, in some cases they strongly alter the magnetic field structure and topology relative to filaments. Despite driving strong micro-scale dust clumping which leaves some gas behind, an order-unity fraction of the gas is always efficiently entrained by dust.
We identify multiple periodic dusty structures in Saturns Roche Division, a faint region spanning the $sim3000$ km between the A and F rings. The locations and extent of these features vary over Cassinis tour of the Saturn system, being visible in 2006 and 2016-2017, but not in 2012-2014. These changes can be correlated with variations in Saturns magnetospheric periods. In 2006 and 2016-2017, one of the drifting magnetospheric periods would produce a 3:4 resonance within the Roche Division, but in 2012-2014 these resonances would move into the A ring as the magnetospheric periods converged. A simple model of magnetic perturbations indicates that the magnetic field oscillations responsible for these structures have amplitudes of a few nanotesla, comparable to the magnetic field oscillation amplitudes of planetary period oscillations measured by the magnetometer onboard Cassini. However, some previously unnoticed features at higher radii have expected pattern speeds that are much slower than the magnetospheric periodicities. These structures may reflect an unexpectedly long-range propagation of resonant perturbations within dusty rings.
Following up on our discovery of terahertz water masers, reported in 2017, we report two further detections of water maser emission at frequencies above 1 THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz $8_{27}-7_{34}$ transition of water toward two additional oxygen-rich evolved stars, omicron Ceti (Mira) and R Crateris, and obtained an upper limit on the 1.296 THz line emission from U Orionis. Toward these three sources, and toward the red supergiant star VY Canis Majorae from which 1.296 THz line emission was reported previously, we have also observed several lower-frequency (sub)millimeter water maser transitions using the APEX 12-m telescope along with the 22 GHz transition using the Effelsberg 100-m telescope. We have used a simple model to analyse the multi-transition data thereby obtained. Adopting, as a prior, independent literature estimates of the mass-loss-rates in these four sources and in W Hydrae, we infer water abundances in a remarkably narrow range: $n({rm H_2O})/n({rm H_2}) = 1.4 - 2.5 times 10^{-4}$. For o Cet, VY CMa, and W Hya, the model is successful in predicting the maser line fluxes to within a typical factor $sim 1.6 - 3$. For R Crt and U Ori, the model is less successful, with typical line flux predictions lying an order of magnitude above or below the observations; such discrepancies are perhaps unsurprising given the exponential nature of maser amplification.
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