arXiv subjects

Eliot Quataert

Publications and source records attributed to Eliot Quataert.

At least 325 records · Page 18Linked to original sources

arXiv2013

Setting the Stage for Circumstellar Interaction in Core-Collapse Supernovae II: Wave-Driven Mass Loss in Supernova Progenitors

Supernovae (SNe) powered by interaction with circumstellar material provide evidence for intense stellar mass loss during the final years leading up to core collapse. We have argued that during and after core neon burning, internal gravity waves excited by core convection can tap into the core fusion power and transport a super-Eddington energy flux out to the stellar envelope, potentially unbinding up to ~ 1 solar mass of material. In this work, we explore the internal conditions of SN progenitors using the MESA 1-D stellar evolution code, in search of those most susceptible to wave-driven mass loss. We focus on simple, order of magnitude considerations applicable to a wide range of progenitors. Wave-driven mass loss during core neon and oxygen fusion happens preferentially in either lower mass (<~ 20 solar mass ZAMS) stars or massive, sub-solar metallicity stars. Roughly 20 per cent of the SN progenitors we survey can excite ~ 10^46 - 10^48 erg of energy in waves that can potentially drive mass loss within a few months to a decade of core collapse. This energy can generate a circumstellar environment with 10^-3 - 1 solar mass reaching ~ 100 AU before explosion. We predict a correlation between the energy associated with pre-SN mass ejection and the time to core collapse, with the most intense mass loss preferentially happening closer to core collapse. During silicon burning, a ~ 5 day long phase for our progenitor models, wave energy may inflate ~ 10^-3 - 1 solar mass of the stellar envelope to ~ 10 - 100s of solar radii. This suggests that some nominally compact SN progenitors (Type Ibc progenitors) will have a significantly different SN shock breakout signature than traditionally assumed. We discuss the implications of our results for the core-collapse SN mechanism, Type IIn SNe, Type IIb SNe from extended progenitors (e.g., SNe 1993j and 2011dh), and observed pre-SN outbursts.

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arXiv2013

The Efficiency of Second-Order Fermi Acceleration by Weakly Compressible MHD Turbulence

We investigate the effects of pitch-angle scattering on the efficiency of particle heating and acceleration by MHD turbulence using phenomenological estimates and simulations of non-relativistic test particles interacting with strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering rate, which is meant to approximate the effects of high frequency plasma waves and/or velocity space instabilities. We focus on plasma parameters similar to those found in the near-Earth solar wind, though most of our results are more broadly applicable. An important control parameter is the size of the particle mean free path lambda_{mfp} relative to the scale of the turbulent fluctuations L. For small scattering rates, particles interact quasi-resonantly with turbulent fluctuations in magnetic field strength. Scattering increases the long-term efficiency of this resonant heating by factors of a few-10, but the distribution function does not develop a significant non-thermal power-law tail. For higher scattering rates, the interaction between particles and turbulent fluctuations becomes non-resonant, governed by particles heating and cooling adiabatically as they encounter turbulent density fluctuations. Rapid pitch-angle scattering can produce a power-law tail in the proton distribution function but this requires fine-tuning of parameters. Moreover, in the near-Earth solar wind, a significant power-law tail cannot develop by this mechanism because the particle acceleration timescales are longer than the adiabatic cooling timescale set by the expansion of the solar wind. Our results thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin of the v^{-5} tail in the proton distribution function observed in the solar wind.

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arXiv2013

Dense Molecular Gas: A Sensitive Probe of Stellar Feedback Models

We show that the mass fraction of GMC gas (n>100 cm^-3) in dense (n>>10^4 cm^-3) star-forming clumps, observable in dense molecular tracers (L_HCN/L_CO(1-0)), is a sensitive probe of the strength and mechanism(s) of stellar feedback. Using high-resolution galaxy-scale simulations with pc-scale resolution and explicit models for feedback from radiation pressure, photoionization heating, stellar winds, and supernovae (SNe), we make predictions for the dense molecular gas tracers as a function of GMC and galaxy properties and the efficiency of stellar feedback. In models with weak/no feedback, much of the mass in GMCs collapses into dense sub-units, predicting L_HCN/L_CO(1-0) ratios order-of-magnitude larger than observed. By contrast, models with feedback properties taken directly from stellar evolution calculations predict dense gas tracers in good agreement with observations. Changing the strength or timing of SNe tends to move systems along, rather than off, the L_HCN-L_CO relation (because SNe heat lower-density material, not the high-density gas). Changing the strength of radiation pressure (which acts efficiently in the highest density gas), however, has a much stronger effect on L_HCN than on L_CO. We predict that the fraction of dense gas (L_HCN/L_CO(1-0)) increases with increasing GMC surface density; this drives a trend in L_HCN/L_CO(1-0) with SFR and luminosity which has tentatively been observed. Our results make specific predictions for enhancements in the dense gas tracers in unusually dense environments such as ULIRGs and galactic nuclei (including the galactic center).

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arXiv2013

What Sets Temperature Gradients in Galaxy Clusters? Implications for non-thermal pressure support and mass-observable scaling relations

We present a spherically symmetric model for the origin and evolution of the temperature profiles in the hot plasma filling galaxy groups and clusters. We find that the gas in clusters is generically not isothermal, and that the temperature declines with radius at large distances from the cluster center (outside the core- and scale radii). This temperature profile is determined by the accretion history of the halo, and is not quantitatively well-described by a polytropic model. We explain quantitatively how the large-scale temperature gradient persists in spite of thermal conduction and convection. These results are a consequence of the cosmological assembly of clusters and cannot be reproduced with non-cosmological simulations of isolated halos. We show that the variation in halo assembly histories produces a ~10% scatter in temperature at fixed mass. On top of this scatter, conduction decreases the temperature of the gas near the scale radius in massive clusters, which may bias hydrostatic mass estimates inferred from x-ray and SZ observations. As an example application of our model profiles, we use mixing-length theory to estimate the turbulent pressure support created by the magnetothermal instability (MTI): in agreement with our earlier MHD simulations, we find that the convection produced by the MTI can provide ~5% non-thermal pressure support near r_500. The magnitude of this turbulent pressure support is likely to be non-monotonic in halo mass, peaking in ~10^14.5 M_sun halos.

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arXiv2013

Internal Gravity Wave Excitation by Turbulent Convection

We calculate the flux of internal gravity waves (IGWs) generated by turbulent convection in stars. We solve for the IGW eigenfunctions analytically near the radiative-convective interface in a local, Boussinesq, and cartesian domain. We consider both discontinuous and smooth transitions between the radiative and convective regions and derive Green's functions to solve for the IGWs in the radiative region. We find that if the radiative-convective transition is smooth, the IGW flux depends on the exact form of the buoyancy frequency near the interface. IGW excitation is most efficient for very smooth interfaces, which gives an upper bound on the IGW flux of ~ F_conv (d/H), where F_conv is the flux carried by the convective motions, d is the width of the transition region, and H is the pressure scale height. This can be much larger than the standard result in the literature for a discontinuous radiative-convective transition, which gives a wave flux ~ F_conv M$, where M is the convective Mach number. However, in the smooth transition case, the most efficiently excited perturbations will break in the radiative zone. The flux of IGWs which do not break and are able to propagate in the radiative region is at most F_conv M^(5/8) (d/H)^(3/8), larger than the discontinuous transition result by (MH/d)^(-3/8). The transition region in the Sun is smooth for the energy-bearing waves; as a result, we predict that the IGW flux is a few to five times larger than previous estimates. We discuss the implications of our results for several astrophysical applications, including IGW driven mass loss and the detectability of convectively excited IGWs in main sequence stars.

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arXiv2012

The Observational Signatures of Convectively Excited Internal Gravity Modes in Main Sequence Stars

We predict the flux and surface velocity perturbations produced by convectively excited gravity modes (g-modes) in main sequence stars. Core convection in massive stars can excite g-modes to sufficient amplitudes to be detectable with high precision photometry by Kepler and CoRoT, if the thickness of the convective overshoot region is < 30 per cent of a pressure scale height. The g-modes manifest as excess photometric variability, with amplitudes of ~ 10 micromagnitudes at frequencies ~ 10 microHz (0.8 1/d) near the solar metallicity zero-age main sequence. The flux variations are largest for stars with M > 5 solar masses, but are potentially detectable down to M ~ 2 - 3 solar masses. During the main sequence evolution, radiative damping decreases such that ever lower frequency modes reach the stellar surface and flux perturbations reach up to ~ 100 micromagnitudes at the terminal-age main sequence. Using the same convective excitation model, we confirm previous predictions that solar g-modes produce surface velocity perturbations of < 0.3 mm/s. This implies that stochastically excited g-modes are more easily detectable in the photometry of massive main sequence stars than in the Sun.

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arXiv2012

Thermal Instability in Gravitationally-Stratified Plasmas: Implications for Multi-Phase Structure in Clusters and Galaxy Halos

We study the interplay among cooling, heating, conduction, and magnetic fields in gravitationally stratified plasmas using simplified, plane-parallel numerical simulations. Since the physical heating mechanism remains uncertain in massive halos such as groups or clusters, we adopt a simple, observationally-motivated prescription which enforces global thermal equilibrium when averaged over large scales. The plasma remains susceptible to local thermal instability, however, and cooling drives an inward flow of material. In contrast to previous results, we argue that the thermal stability of the plasma is independent of its convective stability. We find that the ratio of the cooling timescale to the dynamical timescale t_cool/t_ff controls the saturation of the thermal instability: when t_cool/t_ff < 1, the plasma develops extended multi-phase structure, whereas when t_cool / t_ff > 1 it does not. (In a companion paper, we show that the criterion for thermal instability in a spherical potential is somewhat less stringent, t_cool / t_ff < 10.) When thermal conduction is anisotropic with respect to the magnetic field, the criterion for multi-phase structure is essentially independent of the thermal conductivity of the plasma. Our criterion for local thermal instability to produce multi-phase structure is an extension of the cold vs. hot accretion modes in galaxy formation that applies at all radii in hot halos, not just to the virial shock. We show that this criterion is consistent with data on multi-phase gas in the ACCEPT sample of clusters; in addition, when t_cool / t_ff > 1, the net cooling rate to low temperatures and the mass flux to small radii are suppressed enough relative to models without heating to be qualitatively consistent with star formation rates and x-ray line emission in groups and clusters.

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arXiv2012

The Viscous Evolution of White Dwarf Merger Remnants

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $α$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of the He envelope in some WD mergers, potentially producing a Type Ia supernova via a double detonation scenario. Our calculations provide the proper initial conditions for studying the long-term thermal evolution of WD merger remnants. This is important for understanding WD mergers as progenitors of Type Ia supernovae, neutron stars, R Coronae Borealis stars and other phenomena.

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arXiv2012

Stellar Feedback & Bulge Formation in Clumpy Disks

We use numerical simulations of isolated galaxies to study the effects of stellar feedback on the formation and evolution of giant star-forming gas 'clumps' in high-redshift, gas-rich galaxies. Such galactic disks are unstable to the formation of bound gas-rich clumps whose properties initially depend only on global disk properties, not the microphysics of feedback. In simulations without stellar feedback, clumps turn an order-unity fraction of their mass into stars and sink to the center, forming a large bulge and kicking most of the stars out into a much more extended stellar envelope. By contrast, strong radiative stellar feedback disrupts even the most massive clumps after they turn ~10-20% of their mass into stars, in a timescale of ~10-100 Myr, ejecting some material into a super-wind and recycling the rest of the gas into the diffuse ISM. This suppresses the bulge formation rate by direct 'clump coalescence' by a factor of several. However, the galactic disks do undergo significant internal evolution in the absence of mergers: clumps form and disrupt continuously and torque gas to the galactic center. The resulting evolution is qualitatively similar to bar/spiral evolution in simulations with a more homogeneous ISM.

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arXiv2012

On the Structure of Hot Gas in Halos: Implications for the Lx-Tx Relation & Missing Baryons

We present one-dimensional models of the hot gas in dark-matter halos, which both predict the existence of cool cores and explain their structure. Our models are directly applicable to semi-analytic models (SAMs) of galaxy formation. We have previously argued that filaments of cold (~10^4 K) gas condense out of the intracluster medium (ICM) in hydrostatic and thermal equilibrium when the ratio of the thermal instability timescale to the free-fall time $t_{TI}/t_{ff}$ falls below 5-10. This criterion corresponds to an upper limit on the density of the ICM and motivates a model in which a density core forms wherever $t_{TI}/t_{ff} \lesssim 10$. Consistent with observations and numerical simulations, this model predicts larger and more tenuous cores for lower-mass halos---while the core density in a cluster may be as large as ~ 0.1 cm^{-3}, the core density in the Galactic halo should not exceed ~ 10^{-4} cm^{-3}. We can also explain the large densities in smaller mass halos (galactic `coronae') if we include the contribution of the central galaxy to the gravitational potential. Our models produce a favorable match to the observational X-ray luminosity-temperature (Lx-Tx) relation. For halo masses $\lesssim 10^{13}$ Msun the core size approaches the virial radius. Thus, most of the baryons in such halos cannot be in the hot ICM, but either in the form of stars or in the form of hot gas beyond the virial radius. Because of the smaller mass in the ICM and much larger mass available for star formation, the majority of the baryons in low mass halos ($\lesssim 10^{13}$ Msun) can be expelled beyond the virial radius due to supernova feedback. This can account for the baryons `missing' from low mass halos, such as the Galactic halo.

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arXiv2012

3-D Radiative Transfer Calculations of Radiation Feedback from Massive Black Holes: Outflow of Mass from the Dusty "Torus"

Observational and theoretical arguments suggest that the momentum carried in mass outflows from AGN can reach several times L / c, corresponding to outflow rates of hundreds of solar masses per year. Radiation pressure on lines alone may not be sufficient to provide this momentum deposition, and the transfer of reprocessed IR radiation in dusty nuclear gas has been postulated to provide the extra enhancement. The efficacy of this mechanism, however, will be sensitive to multi-dimensional effects such as the tendency for the reprocessed radiation to preferentially escape along sight-lines of lower column density. We use Monte Carlo radiative transfer calculations to determine the radiation force on dusty gas residing within approximately 10 parsecs from an accreting super-massive black hole. We calculate the net rate of momentum deposition in the surrounding gas and estimate the mass-loss rate in the resulting outflow as a function of solid angle for different black hole luminosities, sightline-averaged column densities, clumping parameters, and opening angles of the dusty gas. We find that these dust-driven winds carry momentum fluxes of 1-5 times L / c and correspond to mass-loss rates of 10-100 solar masses per year for a 10^8 solar mass black hole radiating at or near its Eddington limit. These results help to explain the origin of high velocity molecular and atomic outflows in local ULIRGs, and can inform numerical simulations of galaxy evolution including AGN feedback.

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arXiv2012

Resonance Broadening and Heating of Charged Particles in Magnetohydrodynamic Turbulence

The heating, acceleration, and pitch-angle scattering of charged particles by MHD turbulence are important in a wide range of astrophysical environments, including the solar wind, accreting black holes, and galaxy clusters. We simulate the interaction of high-gyrofrequency test particles with fully dynamical simulations of subsonic MHD turbulence, focusing on the parameter regime with beta ~ 1, where beta is the ratio of gas to magnetic pressure. We use the simulation results to calibrate analytical expressions for test particle velocity-space diffusion coefficients and provide simple fits that can be used in other work. The test particle velocity diffusion in our simulations is due to a combination of two processes: interactions between particles and magnetic compressions in the turbulence (as in linear transit-time damping; TTD) and what we refer to as Fermi Type-B (FTB) interactions, in which charged particles moving on field lines may be thought of as beads spiralling around moving wires. We show that test particle heating rates are consistent with a TTD resonance which is broadened according to a decorrelation prescription that is Gaussian in time. TTD dominates the heating for v_s >> v_A (e.g. electrons), where v_s is the thermal speed of species s and v_A is the Alfven speed, while FTB dominates for v_s << v_A (e.g. minor ions). Proton heating rates for beta ~ 1 are comparable to the turbulent cascade rate. Finally, we show that velocity diffusion of collisionless, large gyrofrequency particles due to large-scale MHD turbulence does not produce a power-law distribution function.

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arXiv2012

The Structure of the Interstellar Medium of Star Forming Galaxies

We present numerical methods for including stellar feedback in galaxy-scale simulations. We include heating by SNe (I & II), gas recycling and shock-heating from O-star & AGB winds, HII photoionization, and radiation pressure from stellar photons. The energetics and time-dependence are taken directly from stellar evolution models. We implement these in simulations with pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to massive z~2 starburst disks. Absent feedback, gas cools and collapses without limit. With feedback, the ISM reaches a multi-phase steady state in which GMCs continuously form, disperse, and re-form. Our primary results include: (1) Star forming galaxies generically self-regulate at Toomre Q~1. Most of the volume is in diffuse hot gas with most of the mass in dense GMC complexes. The phase structure and gas mass at high densities are much more sensitive probes of stellar feedback physics than integrated quantities (Toomre Q or gas velocity dispersion). (2) Different feedback mechanisms act on different scales: radiation & HII pressure are critical to prevent runaway collapse of dense gas in GMCs. SNe and stellar winds dominate the dynamics of volume-filling hot gas; however this primarily vents out of the disk. (3) The galaxy-averaged SFR is determined by feedback. For given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect; but changing feedback mechanisms directly translates to shifts off the Kennicutt-Schmidt relation. (4) Self-gravity leads to marginally-bound GMCs with an ~M^-2 mass function with a cutoff at the Jeans mass; they live a few dynamical times before being disrupted by stellar feedback and turn ~1-10% of their mass into stars (increasing from dwarfs through starburst galaxies). Low-mass GMCs are preferentially unbound.

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arXiv2012

Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds

Feedback from massive stars is believed to play a critical role in driving galactic super-winds that enrich the IGM and shape the galaxy mass function and mass-metallicity relation. In previous papers, we introduced new numerical methods for implementing stellar feedback on sub-GMC through galactic scales in galaxy simulations. This includes radiation pressure (UV through IR), SNe (Type-I & II), stellar winds ('fast' O-star through 'slow' AGB winds), and HII photoionization. Here, we show that these feedback mechanisms drive galactic winds with outflow rates as high as ~10-20 times the galaxy SFR. The mass-loading efficiency (wind mass loss rate divided by SFR) scales inversely with circular velocity, consistent with momentum-conservation expectations. We study the contributions of each feedback mechanism to galactic winds in a range of galaxy models, from SMC-like dwarfs & MW-analogues to z~2 clumpy disks. In massive, gas-rich systems (local starbursts and high-z galaxies), radiation pressure dominates the wind generation. For MW-like spirals and dwarf galaxies the gas densities are much lower, and shock-heated gas from SNe and stellar winds dominates production of large-scale outflows. In all models, however, winds have a multi-phase structure that depends on interactions between multiple feedback mechanisms operating on different spatial & time scales: any single mechanism fails to reproduce the winds observed. We provide fitting functions for wind mass-loading and velocities as a function of galaxy properties, for use in cosmological simulations and semi-analytic models. These differ from typically-adopted formulae with explicit dependence on gas surface density that can be very important in both low-density dwarf galaxies and high-density gas-rich galaxies.

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arXiv2012

An Analytic Model of Angular Momentum Transport by Gravitational Torques: From Galaxies to Massive Black Holes

We present analytic calculations of angular momentum transport and gas inflow in galaxies, from scales of ~kpc to deep in the potential of a central black hole (BH). We compare these analytic calculations to numerical simulations and use them to develop a sub-grid model of BH growth that can be incorporated into semi-analytic models or cosmological simulations. Both analytic calculations and simulations argue that the strongest torque on gas arises when non-axisymmetric perturbations to the stellar gravitational potential produces orbit crossings and shocks in the gas. This is true both at large radii ~0.01-1 kpc, where bar-like modes dominate the non-axisymmetric potential, and at smaller radii <10 pc, where a lopsided/eccentric disk dominates. The traditional orbit crossing criterion is not always adequate to predict the locations of, and inflow due to, shocks in gas+stellar disks with finite sound speeds. We derive a modified criterion that predicts the presence of shocks in stellar dominated systems even absent formal orbit crossing. We then derive analytic expressions for the loss of angular momentum and the resulting gas inflow rates in the presence of such shocks. We test our analytic predictions using hydrodynamic simulations at a range of galactic scales, and show that they successfully predict the mass inflow rates and quasi-steady gas surface densities with small scatter (0.3 dex). We use our analytic results to construct a new estimate of the BH accretion rate given galaxy properties at larger radii. This captures the key scalings in the numerical simulations. Alternate estimates such as the local viscous accretion rate or the spherical Bondi rate fail systematically to reproduce the simulations.

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arXiv2012

An Explanation for the Slopes of Stellar Cusps in Galaxy Spheroids

The stellar surface mass density profiles at the centers of typical ~L* and lower-mass spheroids exhibit power law 'cusps' with $Σ\propto R^(-n)$, where 0.5<n<1 for radii ~1-100 pc. Observations and theory support models in which these cusps are formed by dissipative gas inflows and nuclear starbursts in gas-rich mergers. At these comparatively large radii, stellar relaxation is unlikely to account for or strongly modify the cuspy stellar profiles. We argue that the power-law surface density profiles observed are a natural consequence of the gravitational instabilities that dominate angular momentum transport in the gravitational potential of a central massive black hole. The dominant mode at these radii is an m=1 lopsided/eccentric disk instability, in which stars torquing the gas can drive rapid inflow and accretion. Such a mode first generically appears at large radii and propagates inwards by exciting eccentricities at smaller and smaller radii, where M*(<R)<<M_BH. When the stellar surface density profile is comparatively shallow with n<1/2, the modes cannot efficiently propagate to R=0 and so gas piles up and star formation steepens the profile. But if the profile is steeper than n=1, the inwards propagation of eccentricity is strongly damped, suppressing inflow and bringing n down again. Together these results produce an equilibrium slope of 1/2 < n < 1 in the potential of the central black hole. These physical arguments are supported by nonlinear numerical simulations of gas inflow in galactic nuclei. Together, these results naturally explain the observed stellar density profiles of 'cusp' elliptical galaxies.

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arXiv2012

Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal state (TD) to the higher variability, non-thermal steep power law state (SPL). The disc component in all states is typically modeled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability, and gravitational microlensing observations of active galactic nuclei (AGNs), the supermassive analogs of BHBs. An inhomogeneous disc (ID) model with large (~0.4 dex) temperature fluctuations in each radial annulus can qualitatively explain all of these AGN observations. The inhomogeneity may be a consequence of instabilities in radiation dominated discs, and therefore may be present in BHBs as well. We show that ID models can explain many features of the TD and SPL states of BHBs. The observed relationships between spectral hardness, disc fraction, and rms variability amplitude in BHBs are reproduced with temperature fluctuations similar to those inferred in AGNs, suggesting a unified picture of luminous accretion discs across orders of magnitude in black hole mass. This picture can be tested with spectral fitting of ID models, X-ray polarization observations, and radiation MHD simulations. If BHB accretion discs are indeed inhomogeneous, only the most disc dominated states (disc fraction > 0.95) can be used to robustly infer black hole spin using current continuum fitting methods.

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arXiv2012

Nonlinear Tides in Close Binary Systems

We study the excitation and damping of tides in close binary systems, accounting for the leading order nonlinear corrections to linear tidal theory. These nonlinear corrections include two distinct effects: three-mode nonlinear interactions and nonlinear excitation of modes by the time-varying gravitational potential of the companion. This paper presents the formalism for studying nonlinear tides and studies the nonlinear stability of the linear tidal flow. Although the formalism is applicable to binaries containing stars, planets, or compact objects, we focus on solar type stars with stellar or planetary companions. Our primary results include: (1) The linear tidal solution often used in studies of binary evolution is unstable over much of the parameter space in which it is employed. More specifically, resonantly excited gravity waves are unstable to parametric resonance for companion masses M' > 10-100 M_Earth at orbital periods P = 1-10 days. The nearly static equilibrium tide is, however, parametrically stable except for solar binaries with P < 2-5 days. (2) For companion masses larger than a few Jupiter masses, the dynamical tide causes waves to grow so rapidly that they must be treated as traveling waves rather than standing waves. (3) We find a novel form of parametric instability in which a single parent wave excites a very large number of daughter waves (N = 10^3[P / 10 days]) and drives them as a single coherent unit with growth rates that are ~N times faster than the standard three wave parametric instability. (4) Independent of the parametric instability, tides excite a wide range of stellar p-modes and g-modes by nonlinear inhomogeneous forcing; this coupling appears particularly efficient at draining energy out of the dynamical tide and may be more important than either wave breaking or parametric resonance at determining the nonlinear dissipation of the dynamical tide.

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