Chemical Nonequilibrium Research Articles

Continuum Simulations of Hypersonic Flows in Chemical and Thermal Nonequilibrium

The ability of the Navier–Stokes equations to capture the effects of strong chemical and thermal nonequilibrium on gas composition in a manner similar to the direct simulation Monte Carlo (DSMC) method is tested with the introduction of a nonequilibrium chemistry model. Although this chemistry model has the ability to reasonably reproduce measured Arrhenius rates in conditions of thermal equilibrium, it results in reaction rates that vary significantly for the most commonly used Arrhenius rates in conditions of thermal nonequilibrium. The nonequilibrium chemistry model is implemented in the three-temperature Navier–Stokes computational fluid dynamics (CFD) solver, Data-Parallel Line Relaxation (DPLR), and tested on high-altitude, hypersonic flow conditions. The results of the simulations show that the model predicts a greater amount of NO compared with previous Navier–Stokes using nonequilibrium quasi-classical trajectory derived rates and remains consistent with DSMC computations over altitudes ranging from 53.5 to 87.5 km. The most commonly used chemistry model for Navier–Stokes solvers was found unable to match this performance, indicating the importance of including nonequilibrium effects when modeling chemically reacting hypersonic flowfields.

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks. II. Metallicity Dependence of UV and X-Ray Photoevaporation

Abstract We perform a suite of radiation hydrodynamics simulations of photoevaporating disks, varying the metallicity in a wide range of . We follow the disk evolution for over ∼5000 years by solving hydrodynamics, radiative transfer, and nonequilibrium chemistry. Our chemistry model is updated from the first paper of this series by adding X-ray ionization and heating. We study the metallicity dependence of the disk photoevaporation rate and examine the importance of X-ray radiation. In the fiducial case with solar metallicity, including the X-ray effects does not significantly increase the photoevaporation rate when compared to the case with ultraviolet (UV) radiation only. At subsolar metallicities in the range of , the photoevaporation rate increases as metallicity decreases owing to the reduced opacity of the disk medium. The result is consistent with the observational trend that disk lifetimes are shorter in low metallicity environments. In contrast, the photoevaporation rate decreases at even lower metallicities of , because dust–gas collisional cooling remains efficient compared to far-UV photoelectric heating whose efficiency depends on metallicity. The net cooling in the interior of the disk suppresses the photoevaporation. However, adding X-ray radiation significantly increases the photoevaporation rate, especially at . Although the X-ray radiation itself does not drive strong photoevaporative flows, X-rays penetrate deep into the neutral region in the disk, increase the ionization degree there, and reduce positive charges of grains. Consequently, the effect of photoelectric heating by far-UV radiation is strengthened by the X-rays and enhances the disk photoevaporation.

Developing a self-consistent AGB wind model – I. Chemical, thermal, and dynamical coupling

The material lost through stellar winds of Asymptotic Giant Branch (AGB) stars is one of the main contributors to the chemical enrichment of galaxies. The general hypothesis of the mass loss mechanism of AGB winds is a combination of stellar pulsations and radiative pressure on dust grains, yet current models still suffer from limitations. Among others, they assume chemical equilibrium of the gas, which may not be justified due to rapid local dynamical changes in the wind. This is important as it is the chemical composition that regulates the thermal structure of the wind, the creation of dust grains in the wind, and ultimately the mass loss by the wind. Using a self-consistent hydrochemical model, we investigated how non-equilibrium chemistry affects the dynamics of the wind. This paper compares a hydrodynamical and a hydrochemical dust-free wind, with focus on the chemical heating and cooling processes. No sustainable wind arises in a purely hydrodynamical model with physically reasonable pulsations. Moreover, temperatures are too high for dust formation to happen, rendering radiative pressure on grains impossible. A hydrochemical wind is even harder to initiate due to efficient chemical cooling. However, temperatures are sufficiently low in dense regions for dust formation to take place. These regions occur close to the star, which is needed for radiation pressure on dust to sufficiently aid in creating a wind. Extending this model self-consistently with dust formation and evolution, and including radiation pressure, will help to understand the mass loss by AGB winds.

Steam reforming of methane in a temperature-controlled dielectric barrier discharge reactor: the role of electron-induced chemistry versus thermochemistry

While classic reforming processes rely on heat and chemical equilibrium, plasma-based reforming processes possess the ability to induce non-equilibrium and reactive chemistry at low temperatures using high energy electrons. To better understand the distinctive roles of both electron-induced chemistry and thermochemistry during plasma-assisted fuel reforming, we previously developed a temperature-controlled dielectric barrier discharge (DBD) reactor, which controlled the gas temperature and the electron temperature independently. Here, we investigate plasma-assisted steam reforming of methane using the temperature-controlled DBD reactor and electron-kinetics calculations. We investigated the individual effects of the determining factors for electron-induced chemistry (i.e. reduced electric field intensity and discharge power) and for thermochemistry (i.e. background gas temperature) by varying the discharge power, gas temperature, and pressure inside the reactor. As a result, we found that both the electron-induced chemistry and thermochemistry governed the reactant conversions. Thermochemistry positively affected the methane conversion in particular, but negatively affected the water conversion as the gas temperature increased. The electron-induced chemistry weakly affected the product distribution, while the background temperature (thermochemistry) strongly influenced the product selectivity and composition by altering the chemical pathways involving the plasma-generated reactive species at the given temperature.

New Criterion for First Grid Spacing off Wall Surface in Hypersonic Flow

In hypersonic flow calculations, the grid spacing next to the wall surface is one of the key factors in the prediction of surface heat transfer. The main tool to build the chemical nonequilibrium mathematical model is the system of the axisymmetric Navier–Stokes equations, the thermodynamic relations, and the chemical reaction model. The axisymmetric solver is developed by the implicit finite volume schemes and is validated by comparing the computational results with that in the literature. Combining the Kemp–Riddell empirical formula with the numerical results, a new criterion for the first grid spacing next to the wall surface is proposed by considering three factors such as the freestream density, the velocity, and the radius of a blunt nose. It can effectively predict the surface heat flux at the stagnation point.

Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow

Context. In the inner regions of asymptotic giant branch (AGB) outflows, several molecules have been detected with abundances much higher than those predicted from thermodynamic equilibrium chemical models. The presence of the majority of these species can be explained by shock-induced non-equilibrium chemical models, where shocks caused by the pulsating star take the chemistry out of equilibrium in the inner region. Moreover, a non-uniform density structure has been detected in several AGB outflows. Both large-scale structures, such as spirals and disks, and small-scale density inhomogeneities or clumps have been observed. These structures may also have a considerable impact on the circumstellar chemistry. A detailed parameter study on the quantitative effects of a non-homogeneous outflow has so far not been performed. Aims. We examine the effects of a non-uniform density distribution within an AGB outflow on its chemistry by considering a stochastic, clumpy density structure. Methods. We implement a porosity formalism for treating the increased leakage of light associated with radiation transport through a clumpy, porous medium. We then use this method to examine the effects from the altered UV radiation field penetration on the chemistry, accounting also for the increased reaction rates of two-body processes in the overdense clumps. The specific clumpiness is determined by three parameters: the characteristic length scale of the clumps at the stellar surface, the clump volume filling factor, and the inter-clump density contrast. In this paper, the clumps are assumed to have a spatially constant volume filling factor, which implies that they expand as they move outward in the wind. Results. We present a parameter study of the effect of clumping and porosity on the chemistry throughout the outflow. Both the higher density within the clumps and the increased UV radiation field penetration have an important impact on the chemistry, as they both alter the chemical pathways throughout the outflow. The increased amount of UV radiation in the inner region leads to photodissociation of parent species, releasing the otherwise deficient elements. We find an increased abundance in the inner region of all species not expected to be present assuming thermodynamic equilibrium chemistry, such as HCN in O-rich outflows, H2O in C-rich outflows, and NH3 in both. Conclusions. A non-uniform density distribution directly influences the chemistry throughout the AGB outflow, both through the density structure itself and through its effect on the UV radiation field. Species not expected to be present in the inner region of the outflow assuming thermodynamic equilibrium chemistry are now formed in this region, including species that are not formed in greater abundance by shock-induced non-equilibrium chemistry models. Outflows whose clumps have a large overdensity and that are very porous to the interstellar UV radiation field yield abundances comparable to those observed in O-rich and C-rich outflows for most of the unexpected species investigated. The inner wind abundances of H2O in C-rich outflows and of NH3 in O-rich and C-rich outflows are however underpredicted.

On the determination of laminar flame speed from low-pressure and super-adiabatic propagating spherical flames

Abstract The outwardly propagating spherical flame (OPF) method is popularly used to measure the laminar flame speed (LFS). Recently, great efforts have been devoted to improving the accuracy of the LFS measurement from OPF. In the OPF method, several assumptions are made. For examples, the burned gas is assumed to be static and in chemical equilibrium. However, these assumptions may not be satisfied under certain conditions. Here we consider low-pressure and super-adiabatic propagating spherical flames, for which chemical non-equilibrium exists and the burned gas may not be static. The objective is to assess the chemical non-equilibrium effects on the accuracy of LFS measurement from the OPF method. Numerical simulations considering detailed chemistry and transport are conducted. Stoichiometric methane/air flames at sub-atmospheric pressures and methane/oxygen flames at different equivalence ratios are considered. At low pressures, broad heat release zone is observed and the burned gas cannot quickly reach the adiabatic flame temperature, indicating the existence of chemical non-equilibrium of burned gas. Positive flow in the burned gas is identified and it is shown to become stronger at lower initial pressure. Consequently, the LFS measurement from OPF at low pressures is not accurate if the burned gas is assumed to be static and at chemical equilibrium. For super-adiabatic spherical flames, the burned gas speed is found to be negative due to the local temperature overshoot at the flame front. Such negative speed of burned gas can also reduce the accuracy of LFS measurement. It is recommended that the direct method measuring both flame propagation speed and flow speed of unburned gas should be used to determine the LFS at low pressures or for mixtures with super-adiabatic flame temperature.

Simulation of Laser-Induced Plasma Ignition in a Hypersonic Crossflow

A numerical study of a laser-induced plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into a hypersonic air-crossflow, compressed by a ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is attempted using a laser pulse, modeled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, wall-modeled large-eddy Simulations are used to resolve the majority of turbulent motion. Thermal nonequilibrium and detailed finite-rate chemistry models are included in the calculations. The simulations indicate that the cloud of hydroxyl radicals detected in the experiments is created on the jet’s forward surface, as the expanding plasma kernel drives a blast wave into the bowshock surrounding the jet. This event raises the temperature and pressure in the upstream mixing layer, causing spontaneous ignition of the mixture. Within tens of microseconds the laser-induced combustion detaches from the jet and combustion ceases due to unfavorable conditions for flame anchoring in this area. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate radical formation in the wake region, but no stable flame forms due to nominally low pressures in the experimental condition studied.

A simple model for molecular hydrogen chemistry coupled to radiation hydrodynamics

We introduce non-equilibrium molecular hydrogen chemistry into the radiation hydrodynamics code Ramses-RT. This is an adaptive mesh refinement grid code with radiation hydrodynamics that couples the thermal chemistry of hydrogen and helium to moment-based radiative transfer with the Eddington tensor closure model. The H2 physics that we include are formation on dust grains, gas phase formation, formation by three-body collisions, collisional destruction, photodissociation, photoionization, cosmic ray ionization, and self-shielding. In particular, we implement the first model for H2 self-shielding that is tied locally to moment-based radiative transfer by enhancing photodestruction. This self-shielding from Lyman-Werner line overlap is critical to H2 formation and gas cooling. We can now track the non-equilibrium evolution of molecular, atomic, and ionized hydrogen species with their corresponding dissociating and ionizing photon groups. Over a series of tests we show that our model works well compared to specialized photodissociation region codes. We successfully reproduce the transition depth between molecular and atomic hydrogen, molecular cooling of the gas, and a realistic Stromgren sphere embedded in a molecular medium. In this paper we focus on test cases to demonstrate the validity of our model on small scales. Our ultimate goal is to implement this in large-scale galactic simulations.

Spectra signals of gas pressure pulsations in annular and linear dual-slotted nozzles

ABSTRACTComplex results in experimental and numerical flow dynamics research and spectra signals of gas pressure pulsations in annular and linear dual-slotted nozzles are presented. Experiments are performed in a pulsed aerodynamic setup using acetylene–air mixture combustion products as working gas. Viscous flow calculations are based on the Navier–Stokes equations for multicomponent reactive gaseous medium using single-temperature chemical nonequilibrium model. As a result, the frequency Fourier spectrum of pressure pulsation at the thrust wall center is determined. Typical pulsed pressure signals obtained in the calculations and detected in the experiments are presented.

Non-equilibrium 3-D simulator applied to air injection EOR in light oil reservoirs

In this paper, a 3-D simulator based on the assumption of both chemical and thermal non-equilibrium is presented for a more realistic view of non-equilibrium process for simulating air injection EOR (enhanced oil recovery). The mathematical model is based on a non-equilibrium stage with dynamic mass transfer process between different fluid phases and energy transfer process between fluid and matrix (solid). The light crude oil oxidation and air injection process are both simulated by this simulator, through which the pore blocking, gas breakthrough and gas override process are presented through the visualization of oil, gas saturation distributions. The crude oil oxidation is refined as in-phase and interphase reactions to investigate the effect of reactant dispersion caused by the gas-liquid mass transfer. The oxygen dissolution and the evaporation of light components in crude oil are considered in the non-equilibrium process. The results without mass transfer, with dynamic mass transfer and with instantaneous mass transfer between gas and liquid phases are compared through multiplying the gas-liquid interface area 0, 1 and 100 times respectively, and the influence of these different mass transfer conditions on the migration of temperature peak front and oil recovery is revealed. In addition, the effect of local chemical and thermal non-equilibrium in different matrix permeability as well as porosity scenarios is also investigated to obtain non-equilibrium behavior in different reservoir conditions when implementing air injection EOR. In the last section, we investigated the result with different simulation timescale to see if the non-equilibrium behavior can be captured in the simulator used in this paper. This study suggests that the non-equilibrium behavior would show obvious influence on oil recovery for air injection in the lower permeability reservoir. The non-equilibrium behavior is desirable to be considered for simulating air injection EOR in light oil reservoirs. The developed 3-D simulator is robust that can better reflect the mechanism of air injection EOR in light oil reservoirs. We hope this work encouraging more investigations for the simulation side of non-equilibrium phase behavior and energy transfer in the air injection process.

Heat Transfer in Nonequilibrium Flows with Homogeneous and Heterogeneous Recombination Reactions

The aeroheating performance of hypersonic vehicles is usually significantly influenced by the chemical reaction processes. For the new-generation hypersonic cruise vehicles, the boundary-layer flow is shown to be in chemical nonequilibrium due to rarefied gas effects. As a result, there will be a competition between the homogeneous recombination of atoms in the near-wall flowfield and the heterogeneous recombination on the catalytic wall surface, which, leading to a coupling of the heat diffusion and conduction, is beyond the application scope of the classical theories developed originally for the traditional blunt reentry vehicles. In this paper, the theoretical modeling and the direct simulation Monte Carlo method are employed to study the corresponding rarefied nonequilibrium flow and heat transfer phenomena near the sharp leading edges. A generalized model is proposed to evaluate the deviation of the actual aeroheating performance from the classical description. Based on this model, an integrated nonequilibrium criterion and a simple but effective bridging function are introduced to predict the stagnation-point heat transfer under the competitive effects between the homogeneous and heterogeneous recombination of atoms. The analytical formula is validated by the numerical results under various flow conditions.

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks by Ultraviolet Radiation: Metallicity Dependence

Abstract Protoplanetary disks are thought to have lifetimes of several million yr in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low-metallicity environment. We perform a suite of radiation hydrodynamics simulations of photoevaporating protoplanetary disks to study their long-term evolution of ∼10,000 yr and the metallicity dependence of mass-loss rates. Our simulations follow hydrodynamics, extreme and far-ultraviolet (FUV) radiative transfer, and nonequilibrium chemistry in a self-consistent manner. Dust-grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk metallicity over a wide range of 10 − 4 Z ⊙ ≤ Z ≤ 10 Z ⊙ . The photoevaporation rate is lower with higher metallicity in the range of 10 − 1 Z ⊙ ≲ Z ≲ 10 Z ⊙ , because dust shielding effectively prevents FUV photons from penetrating and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10 − 2 Z ⊙ ≲ Z ≲ 10 − 1 Z ⊙ , because FUV photoelectric heating becomes less effective than dust–gas collisional cooling. The temperature in the neutral region decreases, and photoevaporative flows are excited only in an outer region of the disk. At 10 − 4 Z ⊙ ≤ Z ≲ 10 − 2 Z ⊙ , H i photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with a roughly constant rate. The typical disk lifetime is shorter at Z = 0.3 Z ⊙ than at Z = Z ⊙ , being consistent with recent observations of the extreme outer galaxy.

Observable Signatures of Wind-driven Chemistry with a Fully Consistent Three-dimensional Radiative Hydrodynamics Model of HD 209458b

Abstract We present a study of the effect of wind-driven advection on the chemical composition of hot-Jupiter atmospheres using a fully consistent 3D hydrodynamics, chemistry, and radiative transfer code, the Met Office Unified Model (UM). Chemical modeling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work, we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure, and chemical composition. In this Letter, we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude, which is directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot-Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase nonequilibrium chemistry is unlikely to explain the model–observation discrepancy in the 4.5 μm Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.

Role of Sink Density in Nonequilibrium Chemical Redistribution in Alloys.

Nonequilibrium chemical redistribution in open systems submitted to external forces, such as particle irradiation, leads to changes in the structural properties of the material, potentially driving the system to failure. Such redistribution is controlled by the complex interplay between the production of point defects, atomic transport rates, and the sink character of the microstructure. In this work, we analyze this interplay by means of a kinetic MonteCarlo (KMC) framework with an underlying atomistic model for the Fe-Cr model alloy to study the effect of ideal defect sinks on Cr concentration profiles, with a particular focus on the role of interface density. We observe that the amount of segregation decreases linearly with decreasing interface spacing. Within the framework of the thermodynamics of irreversible processes, a general analytical model is derived and assessed against the KMC simulations to elucidate the structure-property relationship of this system. Interestingly, in the kinetic regime where elimination of point defects at sinks is dominant over bulk recombination, the solute segregation does not directly depend on the dose rate but only on the density of sinks. This model provides new insight into the design of microstructures that mitigate chemical redistribution and improve radiation tolerance.

Vibrational energy transfer and dissociation in O2-N2 collisions at hyperthermal temperatures.

Simulation of vibrational energy transfer and dissociation in O2-N2 collisions is conducted using the quasi-classical trajectory method on an ab initio potential energy surface. Vibrationally resolved rate coefficients are obtained in a high-temperature region between 8000 and 20 000 K by means of the cost-efficient classical trajectory propagation method. A system of master equations is constructed using the new dataset in order to simulate thermal and chemical nonequilibrium observed in shock flows. The O2 relaxation time derived from a solution of the master equations is in good agreement with the Millikan and White correlation at lower temperatures with an increasing discrepancy toward the translational temperature of 20 000 K. At the same time, the N2 master equation relaxation time is similar to that derived under the assumption of a two-state system. The effect of vibrational-vibrational energy transfer appears to be crucial for N2 relaxation and dissociation. Thermal equilibrium and quasi-steady state dissociation rate coefficients in O2-N2 heat bath are reported.

Coupled heat transfer and thermo-mechanical behavior of hypersonic cylindrical leading edges

The analysis of heat transfer and thermo-mechanical behavior is crucial to the leading edge of hypersonic vehicles under high thermal and mechanical loads due to severe aeroheating. The present paper proposes a time-adaptive loosely coupled strategy for the modeling and analysis of hypersonic intrinsic fluid-thermal-structural characteristics. A framework of hypersonic computational coupling dynamics (HyCCD) is developed for integrating an independently developed program solving hypersonic aerothermodynamics and a finite element analysis professional software. The embedded adaptive time-step approach, hybrid interpolation strategy, and grid deformation method have taken into consideration coupling variables and their properties. A typical cylindrical leading edge is considered as the simulation model to study the effects of chemical nonequilibrium, aeroheating duration, flight trajectory, and shock interference. The time-adaptive loosely coupled analysis of aerothermostructural response provides a reliable, applicable and efficient prediction for the fluid thermal-structural coupling of hypersonic vehicles.

The Application of SSP-Runge-Kutta Method in Numerical Simulation of Hypersonic Chemical Nonequilibrium Flow

In the process of re-entry vehicle, the atmosphere around the high temperature of air can produce chemical reaction, so chemical nonequilibrium effect should be considered. Numerical simulation of this complex flow field need high efficient time discretization method. In this paper, a numerical simulation of hypersonic chemical nonequilibrium flow in 2-dsemi-cylindrical is carried out by using a ten-stage fourth order SSP-Runge-Kutta method with strong stability preserving (SSP), and which compared with a three-stage third order TVD-Runge-Kutta method and a four-stage fourth order Runge-Kutta method. The results show that the stability of the ten-stage fourth order SSP-Runge-Kutta method is good, the time-step size can be longer, and the computation efficiency is higher.

Construction of a coarse-grain quasi-classical trajectory method. II. Comparison against the direct molecular simulation method.

This work presents the analysis of non-equilibrium energy transfer and dissociation of nitrogen molecules (N2(Σg+1)) using two different approaches: the direct molecular simulation (DMS) method and the coarse-grain quasi-classical trajectory (CG-QCT) method. The two methods are used to study thermochemical relaxation in a zero-dimensional isochoric and isothermal reactor in which the nitrogen molecules are heated to several thousand degrees Kelvin, forcing the system into strong non-equilibrium. The analysis considers thermochemical relaxation for temperatures ranging from 10 000 to 25 000 K. Both methods make use of the same potential energy surface for the N2(Σg+1)-N2(Σg+1) system taken from the NASA Ames quantum chemistry database. Within the CG-QCT method, the rovibrational energy levels of the electronic ground state of the nitrogen molecule are lumped into a reduced number of bins. Two different grouping strategies are used: the more conventional vibrational-based grouping, widely used in the literature, and energy-based grouping. The analysis of both the internal state populations and concentration profiles show excellent agreement between the energy-based grouping and the DMS solutions. During the energy transfer process, discrepancies arise between the energy-based grouping and DMS solution due to the increased importance of mode separation for low energy states. By contrast, the vibrational grouping, traditionally considered state-of-the-art, captures well the behavior of the energy relaxation but fails to consistently predict the dissociation process. The deficiency of the vibrational grouping model is due to the assumption of strict mode separation and equilibrium of rotational energy states. These assumptions result in errors predicting the energy contribution to dissociation from the rotational and vibrational modes, with rotational energy actually contributing 30%-40% of the energy required to dissociate a molecule. This work confirms the findings discussed in Paper I [R. L. Macdonald et al., J. Chem. Phys. 148, 054309 (2018)], which underlines the importance of rotational energy to the dissociation process, and demonstrates that an accurate non-equilibrium chemistry model must accurately predict the deviation of rovibrational distribution from equilibrium.

GPI Spectroscopy of the Mass, Age, and Metallicity Benchmark Brown Dwarf HD 4747 B

Abstract The physical properties of brown dwarf companions found to orbit nearby, solar-type stars can be benchmarked against independent measures of their mass, age, chemical composition, and other parameters, offering insights into the evolution of substellar objects. The TRENDS high-contrast imaging survey has recently discovered a (mass/age/metallicity) benchmark brown dwarf orbiting the nearby (d = 18.69 ± 0.19 pc), G8V/K0V star HD 4747. We have acquired follow-up spectroscopic measurements of HD 4747 B using the Gemini Planet Imager to study its spectral type, effective temperature, surface gravity, and cloud properties. Observations obtained in the H-band and K 1-band recover the companion and reveal that it is near the L/T transition (T1 ± 2). Fitting atmospheric models to the companion spectrum, we find strong evidence for the presence of clouds. However, spectral models cannot satisfactorily fit the complete data set: while the shape of the spectrum can be well-matched in individual filters, a joint fit across the full passband results in discrepancies that are a consequence of the inherent color of the brown dwarf. We also find a 2σ tension in the companion mass, age, and surface gravity when comparing to evolutionary models. These results highlight the importance of using benchmark objects to study “secondary effects” such as metallicity, non-equilibrium chemistry, cloud parameters, electron conduction, non-adiabatic cooling, and other subtleties affecting emergent spectra. As a new L/T transition benchmark, HD 4747 B warrants further investigation into the modeling of cloud physics using higher resolution spectroscopy across a broader range of wavelengths, polarimetric observations, and continued Doppler radial velocity and astrometric monitoring.