Microphysical Equations Research Articles

Thermal Effects in Binary Neutron Star Mergers

We study the impact of finite-temperature effects in numerical-relativity simulations of binary neutron star mergers with microphysical equations of state and neutrino transport in which we vary the effective nucleon masses in a controlled way. We find that, as the specific heat is increased, the merger remnants become colder and more compact due to the reduced thermal pressure support. Using a full Bayesian analysis, we demonstrate that this effect will be measurable in the postmerger gravitational wave signal with next-generation observatories at signal-to-noise ratios of 15.

Development of a new cloud model for Venus (MAD-VenLA) using the Modal Aerosol Dynamics approach

For decades, clouds have remained a central open question in understanding the climate system of Venus. We have developed a new microphysical model for the clouds of Venus that we describe in this paper. The model is a modal aerosol dynamical model that treats the formation and evolution of sulfuric acid solution droplets with a moderate computational cost. To this end, the microphysical equations are derived to describe the evolution of the size distribution of the particles using the moments of the distribution. We describe the derivation of the equations and their implementation in the model. We tested each microphysical process of the model separately in conditions of the Venus’ atmosphere and show that the model behaves in a physically sound manner in the tested cases. The model will be coupled in the future with a Venus Global Climate Model and used for elucidating the remaining mysteries.

Sensitivity of Sea‐Surface Enthalpy and Momentum Fluxes to Sea Spray Microphysics

AbstractAccurate estimates of air‐sea enthalpy and momentum fluxes are critically important for hurricane intensity predictions. However, calculating these fluxes is challenging due to the nature of the air‐sea transition region. At extreme wind speeds, a substantial amount of sea spray is lofted making it necessary to calculate the sea spray‐mediated enthalpy and momentum fluxes. These calculations rely on microphysical equations, which are sensitive to the details of the local environmental conditions. Here we use a microphysical model to show that there exists a threshold wind speed beyond which the net sea spray‐mediated enthalpy and momentum fluxes are well‐approximated by using the net sea spray mass flux alone. This result supports the hypothesis that at extreme wind speeds, the ratio of the air‐sea exchange coefficients becomes independent of wind speed, implying the air‐sea flux calculations can be substantially simplified.

Numerical Relativity Simulations of the Neutron Star Merger GW170817: Long-term Remnant Evolutions, Winds, Remnant Disks, and Nucleosynthesis

Abstract We present a systematic numerical relativity study of the dynamical ejecta, winds, and nucleosynthesis in neutron star (NS) merger remnants. Binaries with the chirp mass compatible with GW170817, different mass ratios, and five microphysical equations of state (EOSs) are simulated with an approximate neutrino transport and a subgrid model for magnetohydrodynamic turbulence up to 100 ms postmerger. Spiral density waves propagating from the NS remnant to the disk trigger a wind with mass flux ∼0.1–0.5 M ⊙ s−1, which persists for the entire simulation as long as the remnant does not collapse to a black hole. This wind has average electron fraction ≳0.3 and average velocity ∼0.1–0.17 c and thus is a site for the production of weak r-process elements (mass number A < 195). Disks around long-lived remnants have masses ∼0.1–0.2 M ⊙, temperatures peaking at ≲10 MeV near the inner edge, and a characteristic double-peak distribution in entropy resulting from shocks propagating through the disk. The dynamical and spiral-wave ejecta computed in our targeted simulations are not compatible with those inferred from AT2017gfo using two-components kilonova models. Rather, they indicate that multicomponent kilonova models including disk winds are necessary to interpret AT2017gfo. The nucleosynthesis in the combined dynamical ejecta and spiral-wave wind in the long-lived mergers of comparable mass robustly accounts for all the r-process peaks, from mass number ∼75 to actinides in terms of solar abundances. Total abundances are weakly dependent on the EOS, while the mass ratio affects the production of first-peak elements.

Spiral-wave Wind for the Blue Kilonova

Abstract The AT2017gfo kilonova (kN) counterpart of the binary neutron star merger event GW170817 was characterized by an early-time bright peak in optical and UV bands. Such blue kN is commonly interpreted as a signature of weak r-process nucleosynthesis in a fast expanding wind whose origin is currently debated. Numerical relativity simulations with microphysical equations of state, approximate neutrino transport, and turbulent viscosity reveal a new hydrodynamics-driven mechanism that can power the blue kN. Spiral density waves in the remnant generate a characteristic wind of mass ∼10−2 and velocity ∼0.2 c. The ejected material has an electron fraction mostly distributed above 0.25 being partially reprocessed by hydrodynamic shocks in the expanding arms. The combination of dynamical ejecta and spiral-wave wind can account for solar system abundances of r-process elements and early-time observed light curves.

The “Bag Breakup” Spume Droplet Generation Mechanism at High Winds. Part II: Contribution to Momentum and Enthalpy Transfer

AbstractIn Part I of this study, we used high-speed video to identify “bag breakup” fragmentation as the dominant mechanism by which spume droplets are generated at gale-force and hurricane wind speeds. We also constructed a spray generation function (SGF) for the bag-breakup mechanism. The distinctive feature of this new SGF is the presence of giant (~1000 μm) droplets, which may significantly intensify the exchange between the atmosphere and the ocean. In this paper, Part II, we estimate the contribution of the bag-breakup mechanism to the momentum and enthalpy fluxes, which are known to strongly affect the development and maintenance of hurricanes. We consider three contributions to the spray-mediated aerodynamic drag: 1) “bags” as obstacles before fragmentation, 2) acceleration of droplets by the wind in the course of their production, and 3) stable stratification of the marine atmospheric boundary layer due to levitating droplets. Taking into account all of these contributions indicates a peaking dependence of the aerodynamic drag coefficient on the wind speed, which confirms the results of field and laboratory measurements. The contribution of the spray-mediated flux to the ocean-to-atmosphere moist enthalpy is also estimated using the concept of “reentrant spray,” and the equation for the enthalpy flux from a single droplet to the atmosphere is derived from microphysical equations. Our estimates show that a noticeable increase in the enthalpy exchange coefficient at winds exceeding 30–35 m s−1 is due to the enhancement of the exchange processes caused by the presence of giant droplets originating from bag-breakup fragmentation.

Constraining Precipitation Susceptibility of Warm-, Ice-, and Mixed-Phase Clouds with Microphysical Equations

Abstract The strength of the effective anthropogenic climate forcing from aerosol–cloud interactions is related to the susceptibility of precipitation to aerosol effects. Precipitation susceptibility d lnP/d lnN has been proposed as a metric to quantify the effect of aerosol-induced changes in cloud droplet number N on warm precipitation rate P. Based on the microphysical rate equations of the Seifert and Beheng two-moment bulk microphysics scheme, susceptibilities of warm-, mixed-, and ice-phase precipitation and cirrus sedimentation to cloud droplet and ice crystal number are estimated. The estimation accounts for microphysical adjustments to the initial perturbation in N. For warm rain, d lnP/d lnN < −2aut/(aut + acc) is found, which depends on the rates of autoconversion (aut) and accretion (acc). Cirrus sedimentation susceptibility corresponds to the exponent of crystal sedimentation velocity with a value of −0.2. For mixed-phase clouds, several microphysical contributions that explain low precipitation susceptibilities are identified: (i) Because of the larger hydrometeor sizes involved, mixed-phase collection processes are less sensitive to changes in hydrometeor size than autoconversion. (ii) Only a subset of precipitation formation processes is sensitive to droplet or crystal number. (iii) Effects on collection processes and diffusional growth compensate. (iv) Adjustments in cloud liquid and ice amount compensate the effect of changes in ice crystal and cloud droplet number. (v) Aerosol perturbations that simultaneously affect ice crystal and droplet number have opposing effects.

Brownian coagulation of a bi-modal distribution of both spherical and fractal aerosols

The use of Global Circulation Models (GCM) or mesoscale models are now widely used to understand planetary climatic systems. The increasing complexity of the models and the more and more detailed observations of the planetary bodies necessitate a corresponding increase in the complexity of the physical processes which are included in these models. In all the planetary atmospheres in the solar system, aerosols and clouds can be found and therefore microphysical processes must be included in climate models. The most accurate models are those where aerosol and cloud droplet size distributions are described with bins. This type of sophisticated and accurate models only needs a small quantity of a priori information to be used. However, they are demanding in computational resource. They can only be used in GCM at the cost of very long, and sometime prohibitive, time of computation. Alternatively, an other class of microphysical models, based on the description of the aerosol and cloud distributions with moments of distribution can be used. But, they need first to be developed and compared with a more detailed model and they need an a priori information about the size distributions.In this article, we describe the development of the microphysical equations to treat the interaction of two populations of aerosols, with different geometrical structures, written with moment of distributions, interacting through Brownian coagulation. This problem was solved specifically for the case of Titan, the larger satellite of Saturn, where small aerosols have a spherical shape, large aerosols have a fractal aggregated structure and where aerosols bear an electric charge. The two populations interact in the mesosphere. The fractal structure of the large aerosols also has a consequence on the shape of the size distribution and on the laws of the microphysics that must be accounted a priori in the method. The case of Titan is probably one of the most complex and, in this work, we have written the set of equations in the most general way so they can be used for any other cases. Once developed, we finally compare the results yielded by our new model with the results obtained with the classical model based on a description in bins.

M=1instability and gravitational wave signal in binary neutron star mergers

We examine the development and detectability of the $m=1$ instability in the remnant of binary neutron star mergers. The detection of the gravitational mode associated with the $m=1$ degree of freedom could potentially reveal details of the equation of state. We analyze the post-merger epoch of simulations of both equal and non-equal mass neutron star mergers using three realistic, microphysical equations of state and neutrino cooling. Our studies show such an instability develops generically and within a short dynamical time to strengths that are comparable or stronger than the $m=2$ mode which is the strongest during the early post-merger stage. We estimate the signal to noise ratio that might be obtained for the $m=1$ mode and discuss the prospects for observing this signal with available Earth-based detectors. Because the $m=1$ occurs at roughly half the frequency of the more powerful $m=2$ signal and because it can potentially be long-lived, targeted searches could be devised to observe it. We estimate that with constant amplitude direct detection of the mode could occur up to a distance of roughly $14\,\mathrm{Mpc}$ whereas a search triggered by the inspiral signal could extend this distance to roughly $100\,\mathrm{Mpc}$.

How loud are neutron star mergers?

We present results from the first large parameter study of neutron star mergers using fully general relativistic simulations with finite-temperature microphysical equations of state and neutrino cooling. We consider equal and unequal-mass binaries drawn from the galactic population and simulate each binary with three different equations of state. Our focus is on the emission of energy and angular momentum in gravitational waves in the postmerger phase. We find that the emitted gravitational-wave energy in the first $\sim$$10\,\mathrm{ms}$ of the life of the resulting hypermassive neutron star (HMNS) is about twice the energy emitted over the entire inspiral history of the binary. The total radiated energy per binary mass is comparable to or larger than that of nonspinning black hole inspiral-mergers. About $0.8-2.5\%$ of the binary mass-energy is emitted at kHz frequencies in the early HMNS evolution. We find a clear dependence of the postmerger GW emission on binary configuration and equation of state and show that it can be encoded as a broad function of the binary tidal coupling constant $\kappa^T_2$. Our results also demonstrate that the dimensionless spin of black holes resulting from subsequent HMNS collapse are limited to $\lesssim0.7-0.8$. This may significantly impact the neutrino pair annihilation mechanism for powering short gamma-ray bursts (sGRB).

ANALYSIS OF TWOPHASE FLOW MODEL EQUATIONS

Abstract. In this paper, we propose closures for multi-phase owmodels, which satisfy boundary conditions and conservation con-straints. The models governing the evolution of the uid mixing arederived by applying an ensemble averaging procedure to the micro-physical equations characterized by distinct phases. We considercompressible multi species multi-phase ow with surface tensionand transport. 1. IntroductionWe discuss compressible multi-phase ow equations obtained by av-eraging microphysical equations characterized by distinct phases. Themodels we consider have distinct phase pressures and lead to hyperbolicmodels, eliminating mathematical diculties of complex characteristicsassociated with single pressure ow models having distinct velocities.In this sense they are an outgrowth of earlier studies by Stewart andWendro [23], Ransom and Hicks [20], Chen et al. [4, 12], Saltz etal. [12] and Saurel et al. [22, 1, 8]. Multiphase ow has been studiedfor many decades, with the basic physics and equations developed inthe classical treateses of Wallis [24] and Drew [9]. The derivation ofaveraged equations leads to unde ned averages of nonlinear functions ofthe primitive variables. These quantities must be modeled to close thesystem of averaged equations.The main result, achieved in part here, is to identify a closure whichsatis es all the conservation and boundary constraints for the continuityand momentum equations. In a second paper, we will close the energy

GLOBAL SOLUTIONS OF NEUTRINO DOMINATED ACCRETION FLOWS AROUND KERR BLACK HOLES

By solving a set of coupled hydrodynamical and microphysical equations with some appropriate boundary conditions, we obtain three global solutions of typical neutrino-dominated accretion flows around different spinning black holes. Our results reveal that the effect of black hole spin on the flows is restricted within inner parts of neutrino-dominated regions.

Testing approximations of thermal effects in neutron star merger simulations

We perform three-dimensional relativistic hydrodynamical calculations of neutron star mergers to assess the reliability of an approximate treatment of thermal effects in such simulations by combining an ideal-gas component with zero-temperature, microphysical equations of state. To this end we compare the results of simulations that make this approximation to the outcome of models with a consistent treatment of thermal effects in the equation of state. In particular we focus on the implications for observable consequences of merger events like the gravitational-wave signal. It is found that the characteristic gravitational-wave oscillation frequencies of the postmerger remnant differ by about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 percent) depending on the equation of state and the choice of the characteristic index of the ideal-gas component. In addition, the delay time to black hole collapse of the merger remnant as well as the amount of matter remaining outside the black hole after its formation are sensitive to the description of thermal effects.

A new open-source code for spherically symmetric stellar collapse to neutron stars and black holes

We present the new open-source spherically symmetric general-relativistic (GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR hydrodynamics (GRHD) put forth by Romero–Ibáñez–Gourgoulhon and employs radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify substantially. We discretize the GRHD equations with a finite-volume scheme, employing piecewise-parabolic reconstruction and an approximate Riemann solver. GR1D is intended for the simulation of stellar collapse to neutron stars and black holes and will also serve as a testbed for modeling technology to be incorporated in multi-D GR codes. Its GRHD part is coupled to various finite-temperature microphysical equations of state in tabulated form that we make available with GR1D. An approximate deleptonization scheme for the collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch are included and described. We also derive the equations for effective rotation in 1D and implement them in GR1D. We present an array of standard test calculations and also show how simple analytic equations of state in combination with presupernova models from stellar evolutionary calculations can be used to study qualitative aspects of black hole formation in failing rotating core-collapse supernovae. In addition, we present a simulation with microphysical equations of state and neutrino leakage/heating of a failing core-collapse supernova and black hole formation in a presupernova model of a 40 M⊙ zero-age main-sequence star. We find good agreement on the time of black hole formation (within 20%) and last stable protoneutron star mass (within 10%) with predictions from simulations with full Boltzmann neutrino radiation hydrodynamics.

Understanding macrophysical outcomes of microphysical choices in simulations of shallow cumulus convection

The sensitivity of simulations of shallow cumulus convection to their microphysical representation is explored, with a focus on the parameter space spanned by two common bulk schemes (Seifert and Beheng, SB, versus Khairoutdinov and Kogan, KK). Large-eddy simulation, simple models, and a priori analysis of the underlying microphysical equations are used as the basis for our study. The simulations are initialized using data derived from the Rain in Cumulus Over the Ocean (RICO) field study. Simulated clouds depths range between two and three kilometers. Microphysical sensitivities can largely be rationalized based on the behavior of simpler models. In particular a parcel model consisting of auto-conversion and accretion acting on a parcel condensing water at a fixed rate provides useful insight into the behavior of the microphysical schemes in the full simulation. To a first approximation the number concentration simply selects the cloud depth at which rain begins to develop, with different schemes predicting different cloud depths. Because of the interaction of auto-conversion and accretion the dependence of this cloud depth on cloud-droplet number concentration is considerably reduced from what would be deduced by an examination of auto conversion alone—suggesting a somewhat diminished role for the regulation of rain by the atmospheric aerosol. Dynamic feedbacks, such as the tendency for non-precipitating clouds to deepen more readily, can further dampen (and even reverse) the expected sensitivity of rain-rate on droplet number concentrations. Our analysis suggests that the commonly assumed exponential distribution for rain drops can strongly distort the sedimentation process in two moment microphysical schemes and that processes such as self-collection and drop break-up can not be neglected for shallow cumulus convection.

Gravitational Waves from Relativistic Neutron-Star Mergers with Microphysical Equations of State

The gravitational wave (GW) emission from a set of relativistic neutron-star (NS) merger simulations is analyzed and characteristic signal features are identified. The distinct peak in the GW energy spectrum that is associated with the formation of a hypermassive merger remnant has a frequency that depends strongly on the properties of the nuclear equation of state (EOS) and on the total mass of the binary system, whereas the mass ratio and the NS spins have a weak influence. If the total mass can be determined from the inspiral chirp signal, the peak frequency of the post-merger signal is a sensitive indicator of the EOS.

General relativistic simulations of passive-magneto-rotational core collapse with microphysics

This paper presents results from axisymmetric simulations of magneto-rotational stellar core collapse to neutron stars in general relativity using the passive field approximation for the magnetic field. These simulations are performed using a new general relativistic numerical code specifically designed to study this astrophysical scenario. The code is an extension of an existing (and thoroughly tested) hydrodynamics code, which has been applied in the recent past to study relativistic rotational core collapse. It is based on the conformally-flat approximation of Einstein’s field equations and conservative formulations of the magneto-hydrodynamics equations. The code has been recently upgraded to incorporate a tabulated, microphysical equation of state and an approximate deleptonization scheme. This allows us to perform the most realistic simulations of magneto-rotational core collapse to date, which are compared with simulations employing a simplified (hybrid) equation of state, widely used in the relativistic core collapse community. Furthermore, state-of-the-art (unmagnetized) initial models from stellar evolution are used. In general, stellar evolution models predict weak magnetic fields in the progenitors, which justifies our simplification of performing the computations under the approach that we call the passive field approximation for the magnetic field. Our results show that for the core collapse models with microphysics the saturation of the magnetic field cannot be reached within dynamical time scales by winding up the poloidal magnetic field into a toroidal one. We estimate the effect of other amplification mechanisms including the magneto-rotational instability (MRI) and several types of dynamos. We conclude that for progenitors with astrophysically expected (i.e. weak) magnetic fields, the MRI is the only mechanism that could amplify the magnetic field on dynamical time scales. The uncertainties about the strength of the magnetic field at which the MRI saturates are discussed. All our microphysical models exhibit post-bounce convective overturn in regions surrounding the inner part of the proto-neutron star. Since this has a potential impact on enhancing the MRI, it deserves further investigation with more accurate neutrino treatment or alternative microphysical equations of state.

A two-moment parameterization of aerosol nucleation and impaction scavenging for a warm cloud microphysics: description and results from a two-dimensional simulation

A new two-moment warm bulk scheme has been developed including explicitly nucleation and impaction scavenging of aerosol particles as well as all other microphysical processes. The scheme is built upon a quasispectral representation of the aerosol particle, cloud droplet and raindrop distributions. It predicts mixing ratios and number concentrations for each category. Each process is treated explicitly and independently to establish an analytic expression for each contribution for the time-dependant microphysical equations. The scheme has been tested in the dynamical framework of a two-dimensional kinematic model, developed for the Hawaiian Rainband Project (HaRP, 1990). In this frame, the scheme has performed reasonably well compared to the observations as well as to other similar parameterization schemes, and to the spectral model DESCAM. Sensitivity tests demonstrate the great sensitivity of the scheme to the initial aerosol spectrum characteristics. Moreover, they have also shown its capability to calculate nucleation and impaction scavenging and to follow the taken up particle mass in the cloud and raindrop spectra until the deposition on the ground by the rain. Therefore, the parameterization offers a possibility of treating the evolution of the liquid phase of the cloud together with the aerosol particle scavenging. However, due to the severe limitations of a two-dimensional kinematic model, the scheme needs to be further validated in a three-dimensional dynamical model.

A recipe for making hot accretion disks

view Abstract Citations (21) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Recipe for Making Hot Accretion Disks Bjornsson, Gunnlaugur ; Svensson, Roland Abstract A powerful new method to determine the structure of effectively optically thin accretion disks is described. The method reduces the set of equations needed to be numerically solved to the microphysical equations only and reduces the dimension of the parameter space needed to be explored from three to two. It is shown why proton optical depth and compactness are natural parameters in studying hot plasma clouds (HPCs), and the structure equations of geometrically thin alpha disk are studied and the accretion disk parameters are related to the HPC parameters. As an example, the method is applied to an effectively optically thin bremsstrahlung disk. It is shown how a full disk solution is constructed from the generic solution profile. Publication: The Astrophysical Journal Pub Date: April 1991 DOI: 10.1086/186004 Bibcode: 1991ApJ...371L..69B Keywords: Accretion Disks; Black Holes (Astronomy); High Temperature Plasmas; Optical Thickness; Plasma Clouds; Bremsstrahlung; X Ray Sources; Astrophysics; ACCRETION; BLACK HOLES; X-RAYS: GENERAL; X-RAYS: SOURCES full text sources ADS |

Comparison of Two Methods for the Retrieval of Thermodynamic and Microphysical Variables from Doppler Radar Measurements: Application to the Case of a Tropical Squall Line

Microphysical and thermodynamic retrieval studies using a specified wind field can provide a means for analysing the different processes occurring within an observed precipitating system. Up to now, the retrieval of microphysical variable fields or thermodynamic fields have been performed separately, though the interest of associating both types of retrieval has been already noted by several authors. The research reported here presents a new retrieval method allowing consistent and simultaneous derivation of the microphysical and thermodynamic variable fields using the whole set of governing equations (momentum, thermodynamic, and microphysical equations) with the wind field specified from Doppler radar observations. The microphysical retrieval makes use of the continuity equation for the total water substance and for the precipitating substance. Two types of precipitating particles are considered (rain and graupel), and a parameterization derived from that proposed by Kessler is chosen. In practice, the microphysical retrieval is coupled to the retrieval of thermodynamic variables, which is derived from Roux. A more classical approach taking into account the thermodynamic equation and the microphysical equations, but not the momentum equation is also used for comparison. Results obtained from both approaches in the convective region of a tropical squall line observed during COPT81 (22 June 1981) are presented and discussed. It is found that both approaches provide results in mutual agreement, and which are consistent with the observed reflectivity structure, and with surface measurements. The respective advantages and drawbacks of each approach are also discussed.