Assumption Of Thermodynamic Equilibrium Research Articles

Going beyond an old shockwave conjecture for improving upon Navier-Stokes.

Nonequilibrium molecular dynamics (NEMD) computer simulations of steady shockwaves in dense fluids and rarefied gases produce detailed shockwave profiles of mechanical and thermal properties. The Boltzmann equation, under the assumption of local thermodynamic equilibrium (LTE), leads to the first-order (linear) continuum theory of hydrodynamic flow: Navier-Stokes-Fourier (NSF). (Expansion of the LTE Boltzmann equationin higher powers of gradients yields so-called Burnett second-order terms, etc.) NEMD simulations of strong shockwaves with high gradients are not well modeled by NSF theory. Many years ago, a conjecture for going "beyond Navier-Stokes" was proposed, applying the empirical observation of anisotropic thermal enhancement in the shock front to the temperature dependence of the NSF transport coefficients, whose dissipation determines the slope at the center of the shock profile: for weak shocks, the actual coefficients in NEMD simulations appear to be smaller than in NSF predictions, leading to steeper gradients being observed, while for strong shocks, the NEMD coefficients appear to be larger, leading to less steep shock rises than predicted by NSF calculations. In this paper, we show that adding significant Burnett nonlinearity into an LTE continuum theory reproduces the early shock rise and slope of NEMD profiles, for both weak and strong shocks in dense fluids, as well as strong shockwaves in the ideal gas. Moreover, we show that "Holian's conjecture" incorporates significant Burnett nonlinearity, but like all the other LTE continuum theories, it fails to describe the slow NEMD return to equilibrium beyond the shock front. We show that Maxwell relaxation has to be applied to the hydrodynamic variables themselves (rather than attempting indirect relaxation of their gradients) in order to more accurately model the entire shockwave profile. Non-LTE Maxwell relaxation is the only way to bring the entire profile into agreement with NEMD, most noticeably for strong shockwaves.

Investigation of the rotational spectrum of CH$_3$17OH and its tentative detection toward Sagittarius B2(N)

Methanol is an abundant and widespread molecule in the interstellar medium. The column density of its 18O isotopolog, CH$_3$18OH, is in some star-forming regions so high that the search for CH$_3$17OH is promising. But only very few transition frequencies of CH$_3$17OH with a microwave accuracy have been published prior to our investigation. We want to extend the very limited rotational line list of CH$_3$17OH to be able to search for this isotopolog in the interstellar medium. We recorded the rotational spectrum of CH$_3$17OH between 38 and 1095 GHz employing a methanol sample enriched in 17O to 20<!PCT!>. A torsion-rotation Hamiltonian model based on the rho-axis method was employed to fit the data, as in our previous studies. We searched for rotational transitions of CH$_3$17OH in the imaging spectral line survey ReMoCA obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming region Sgr B2(N). The observed spectra were modeled under the assumption of local thermodynamic equilibrium (LTE). The assignments cover $0 J 45$, $K_a 16$, and mainly the $ t = 0$ and 1 torsional states. The Hamiltonian model describes our data well. The model was applied to derive a line list for radio-astronomical observations. We report a tentative detection of CH$_3$17OH along with secure detections of the more abundant isotopologs of methanol toward Sgr B2(N2b). The derived column densities yield isotopic ratios 12C/13C = 25, 16O/18O = 240, and 18O/17O = 3.3, which are consistent with values found earlier for other molecules in Sgr B2. The agreement between the 18O/17O isotopic ratio that we obtained for methanol and the 18O/17O ratios reported in the past for other molecules in Sgr B2(N) strongly supports our tentative interstellar identification of CH$_3$17OH. The accuracy of the derived line list is sufficient for further radio astronomical searches for this methanol isotopolog toward other star-forming regions.

Titanium abundances in late-type stars

Context. Rapidly growing datasets from stellar spectroscopic surveys are providing unprecedented opportunities to analyse the chemical evolution history of our Galaxy. However, spectral analysis requires accurate modelling of synthetic stellar spectra for late-type stars, for which the assumption of local thermodynamic equilibrium (LTE) has been shown to be insufficient in many cases. Errors associated with LTE can be particularly large for Ti I, which is susceptible to over-ionisation, particularly in metal-poor stars. Aims. The aims of this work are to study and quantify the 1D non-LTE effects on titanium abundances across the Hertzsprung-Russell diagram for a large sample of stars. Methods. A large grid of departure coefficients, βν, were computed on standard MARCS model atmospheres. The grid extends from 3000 K to 8000 K in Teff, −0.5 dex to +5.5 dex in log g, and −5.0 to +1.0 in [Fe/H], with non-LTE effects in this grid reaching up to 0.4 dex. This was used to compute abundance corrections that were subsequently applied to the LTE abundances of over 70 000 stars selected from the GALAH survey in addition to a smaller sample of literature Keck data for metal-poor dwarfs. Results. The non-LTE effects grow towards lower [Fe/H], lower log g, and higher Teff, with a minimum and maximum ΔA(Ti)Ti I of 0.02 and 0.19 in the GALAH sample. For metal-poor giants, the non-LTE modelling reduces the average ionisation imbalance (ΔI−II) from −0.11 dex to −0.01 dex at [Fe/H] = −1.7, and the enhancement in titanium abundances from Ti I lines results in a [Ti/Fe] versus [Fe/H] trend that more closely resembles the behaviour of Ti II at low metallicities. At higher metallicities, the results are limited by the precision of the GALAH DR3 LTE abundances and the effects are within the errors. For the most metal-poor dwarfs from the Keck sample, the average ionisation imbalance increases from −0.1 dex to +0.2 dex, a shortcoming that is consistent with previous 1D non-LTE studies and which we speculate could be related to 3D effects. Conclusions. Non-LTE effects on titanium abundances are significant. Neglecting them may alter our understanding of Galactic chemical evolution. We have made our grid of departure coefficients publicly available, with the caveat that the Ti abundances of metal-poor dwarfs need further study in 3D non-LTE.

Lithium Cepheid V708 Car with an unusual chemical composition

Aims. The purpose of this work is to spectroscopically analyse the classical Cepheid V708 Car. A preliminary check of the spectrum of V708 Car showed that this is a lithium-rich supergiant. We also found that V708 Car has an unusual chemical composition in that the abundances of various elements correlate with their condensation temperatures. We tried to find an explanation of this feature, which is unusual for classical Cepheids. Methods. For the spectroscopic analysis, we used methods based on the assumption of local and non-local thermodynamic equilibrium. Results. We determined the fundamental parameters of our program star V708 Car. This long-period Cepheid has a mass of about 12 M⊙. We derived the abundances of 27 chemical elements in this star. They are clearly correlated with their condensation temperature: the higher the condensation temperature, the lower the abundance (there are exceptions for sodium and barium, however). We explain this peculiar chemical composition of the V708 Car atmosphere by the gas–dust separation in the envelope of this star. A similar mechanism leads to the observed peculiarities of the chemical composition of λ Boo, W Vir, and asymptotic giant branch stars.

Thermoelastic effects in Bragg reflectors as a potential bottleneck for XFELs with megahertz repetition rate

Bragg reflectors are essential for beam transport in X-ray free-electron laser (XFEL) facilities. On interaction with Bragg reflectors, a part of the pulse energy will be absorbed, causing the propagation of displacement waves due to rapid thermal expansion. It is suspected that these waves may cause stability problems for XFELs operating with megahertz repetition rates. Here, we experimentally investigate the displacement of a diamond Bragg reflector induced by an optical ultra-violet laser pulse, simulating XFEL pulses with mJ pulse energy, both at room temperature and cryogenic temperatures. Our experiment shows negligible damping of the displacement waves on µs timescales, which could cause disruption for subsequent XFEL pulses. We compare our measurements to a simulation framework based on the assumptions of local thermodynamic equilibrium and classical mechanics, observing reasonable agreement. Our results show that thermoelastic effects are critical for a reliable stability assessment of Bragg reflectors, but are often overlooked.

A non-equilibrium evaporation model for a droplet and spray under diesel engine-relevant conditions

Most continuum-based models are constructed based on thermodynamic equilibrium assumption. Nevertheless, the time scale in high-pressure injection is extremely short, and whether there is sufficient time to reach the equilibrium state has not been confirmed. In this study, a non-equilibrium evaporation model is proposed to investigate the non-equilibrium effect on spray evolutions. First, a non-equilibrium isobaric-isothermal flash is established based on the molecular dynamic simulations and Peng-Robinson equation of state. Then, a non-equilibrium evaporation model incorporating the non-equilibrium isobaric-isothermal flash model is implemented into the Eulerian-Lagrangian spray model. Good agreements with the measured penetration lengths of sprays have been achieved. The results reveal that the evaporation time of a single droplet and the liquid penetration length of spray are underestimated by 7.3 % and 8 %, respectively, due to an overestimation in the interfacial vapor molar fraction under the equilibrium assumption at 800 K and 2.0 K. Besides, the non-equilibrium effect is found to significantly alter the formation of lean and moderately fuel-rich mixture, while it only has a slight influence on the fuel-rich mixtures. Finally, the thermodynamic non-equilibrium effect in sprays is suggested to be considered when the ambient pressure exceeds 3.0 MPa or the ambient temperature surpasses 800 K.

Protonated acetylene in the z = 0.89 molecular absorber toward PKS 1830-211

We report the first interstellar identification of protonated acetylene, C2H3+, a fundamental hydrocarbon, in the z = 0.89 molecular absorber toward the gravitationally lensed quasar PKS 1830–211. The molecular species is identified from clear absorption features corresponding to the 212–101 (rest frequency 494.034 GHz) and 111–000 (431.316 GHz) ground-state transitions of ortho and para forms of C2H3+, respectively, in ALMA spectra toward the southwestern image of PKS 1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C2H3+ column density of 2 × 1012 cm−2 and an abundance of 10−10 compared to H2. However, formation pumping could affect the population of metastable states, yielding a C2H3+ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C2H3+, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C2H3+ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.

3D non-LTE modeling of the stellar center-to-limb variation for transmission spectroscopy studies

Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D1 and K I resonance lines on transmission spectra. Methods. We modeled the CLV of the Na I D1 and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between µ = 0.1 and µ = 1.0. Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%). Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST.

Lithium-rich Cepheid V470 Cas

In this Letter, we report the discovery of a new lithium-rich yellow supergiant star -- the Cepheid V470 Cas -- that has a high lithium abundance, $ A(Li)$ = 3.29. This is highly unusual for supergiant stars. V470 Cas is joining a very select group of lithium-rich Cepheids, with only nine members known to date in our Galaxy. For the analysis of our high-resolution echelle spectrum obtained at the Canada-France-Hawaii Telescope, methods based on both local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (NLTE) assumptions were applied. In particular, the lithium abundance was derived by analysing the equivalent width of the 6707 line. Most lithium-rich Cepheids are located near the blue edge of the instability strip near the bottom of the Hertzsprung--Russell diagram. Their main sequence progenitors are distributed over a mass range of three to five solar masses. Of the nine lithium-rich Cepheids known, six are double-mode pulsators, including our programme Cepheid. It is very likely that the stars of this small group are entering the instability strip for the first time and have not yet passed the red giant phase. Therefore, they have not experienced the large-scale mixing event that could destroy lithium in their convective zones.

Toward the ultimate efficiency of methane to syngas conversion by partial oxidation: A moving bed reactor with parallel preheating of reactants

The conversion of methane to syngas by partial oxidation combined with steam reforming in a counterflow moving bed reactor is theoretically considered. The conversion proceeds in filtration combustion mode with the non-premixed flows of steam–methane and oxidant gas. The flow of solid granular heat carrier is divided in two subflows used to preheat, respectively, steam–methane flow and oxidant gas flow prior to the reaction. The calculations are performed under assumption of thermodynamic equilibrium established in the reaction products. For the main control parameters – the flow rates of oxidant gas, steam, and solid heat carrier, related to the flow rate of methane – the parametric domain has been determined that provides a highly efficient conversion in the absence of soot in the products. Efficient heat recuperation with the solid heat carrier makes attainable conversion with the oxygen-to-methane molar ratio as low as 0.43. The flow arrangement with separate preheating of the reactants prior to reaction promises a highly efficient method of converting hydrocarbons to syngas or hydrogen.

Migration behavior of fugitive methane in porous media: Multi-phase numerical modelling of bench-scale gas injection experiments

Two-dimensional multi-phase numerical simulations based on detailed laboratory experiments are used to provide insight into the key processes of methane migration in porous media and to analyze the suitability of a continuum approach for modelling gas migration at the intermediate bench-scale. The simulations were conducted using the multi-phase numerical model DuMux, including groundwater flow, transport of free-phase methane subject to capillary and buoyancy forces, kinetic-controlled dissolution of methane into the flowing groundwater, and advective-diffusive transport of dissolved-phase methane. Mass transfer kinetics are controlled by the water-gas interfacial area, avoiding the assumption of thermodynamic equilibrium which was shown not applicable under these conditions of high aqueous velocity in sandy materials.The model is applied to a series of 2D laboratory experiments in which gas-phase methane was injected near the bottom of a 1.5 m square vertical flow cell under a background flow gradient and with homogeneous and heterogeneous configurations. Simulations are compared to the observed behavior with respect to gas-phase mass distribution over time, gas-phase saturations, and to breakthrough of the dissolved-phase methane at selected monitoring points. In this first comparative study (simulations vs real data), insights are provided into the role of spatial property distributions on methane migration, in particular gas-phase pooling below low-permeability layers. Similar to the laboratory results, the simulations confirmed that the concentrations of dissolved-phase methane are strongly dependent on the structure of the gas-phase plume which in turn is influenced by the geology. We show that at the local metre-scale, kinetic-controlled mass transfer is needed to reproduce the dissolved-phase methane concentrations. Nevertheless, an assessment of capillary parameter values typically encountered in sandy aquifer materials suggests that an equilibrium approach could still be suitable for field-scale simulations.

Irreversible Thermodynamics of Seawater Evaporation

Under typical marine conditions of about 80% relative humidity, evaporation of water from the ocean is an irreversible process accompanied by entropy production. In this article, equations are derived for the latent heat of irreversible evaporation and the related nonequilibrium entropy balance at the sea surface. To achieve this, linear irreversible thermodynamics is considered in a conceptual ocean evaporation model. The equilibrium thermodynamic standard TEOS-10, the International Thermodynamic Equation of Seawater—2010, is applied to irreversible evaporation under the assumption of local thermodynamic equilibrium. The relevance of local equilibrium conditions for irreversible thermodynamics is briefly explained. New equations are derived for the mass flux of evaporation and for the associated nonequilibrium enthalpies and entropies. The estimated entropy production rate of ocean evaporation amounts to 0.004 W m−2 K−1 as compared with the average terrestrial global entropy production of about 1 W m−2 K−1.

Shocking Sgr B2 (N1) with its own outflow

Aims. Because studies on complex organic molecules (COMs) in high-mass protostellar outflows are sparse, we want to investigate how a powerful outflow, such as that driven by the exciting source of the prominent hot core Sagittarius B2(N1), influences the gas molecular inventory of the surrounding medium with which it interacts. Identifying chemical differences to the hot core unaffected by the outflow and what causes them may help to better understand molecular segregation in other star-forming regions. Methods. We made use of the data taken as part of the 3 mm imaging spectral-line survey Re-exploring Molecular Complexity with ALMA (ReMoCA). We studied the morphology of the emission regions of simple and complex molecules in Sgr B2 (N1). For a selection of twelve COMs and four simpler species, spectra were modelled under the assumption of local thermodynamic equilibrium and population diagrams were derived at two positions, one in each lobe of the outflow. From this analysis, we obtained rotational temperatures and column densities. Abundances were subsequently compared to predictions of astrochemical models and to observations of L1157-B1, a position located in the well-studied outflow of the low-mass protostar L1157, and the source G+0.693-0.027 (G0.693), located in the Sgr B2 molecular cloud complex, which are other regions whose chemistry has been impacted by shocks. Results. Integrated intensity maps of SO and SiO emission reveal a bipolar structure with blue-shifted emission dominantly extending to the south-east from the centre of the hot core and red-shifted emission to the north-west. The morphology of both lobes is complex but can roughly be characterised by an emission component at a larger opening angle, containing most of the emission, and narrower features. The wider-angle component is also prominently observed in emission of S-bearing molecules and species that only contain N as a heavy element, including COMs, but also CH3OH, CH3CHO, HNCO, and NH2CHO. Rotational temperatures are found in the range of ~ 100–200 K. Abundances of N-bearing molecules with respect to CH3OH are enhanced in the outflow component compared to N1S, a position that is not impacted by the outflow. A comparison of molecular abundances with G+0.693–0.027 and L1157-B1 does not show any correlations, suggesting that a shock produced by the outflow impacts Sgr B2 (N1)’s material differently or that the initial conditions were different. Conclusions. The short distance of the analysed outflow positions to the centre of Sgr B2 (N1) lead us to propose a scenario in which a phase of hot-core chemistry (i.e. thermal desorption of ice species and high-temperature gas-phase chemistry) preceded a shock wave. The subsequent compression and further heating of the material resulted in the accelerated destruction of (mainly O-bearing) molecules. Gas-phase formation of cyanides seems to be able to compete with their destruction in the post-shock gas. The abundances of cyanopolyynes are enhanced in the outflow component pointing to (additional) gas-phase formation, possibly incorporating atomic N sourced from ammonia in the post-shock gas. To confirm such a scenario, chemical shock models need to be run that take into account the pre- and post-shock conditions of Sgr B2 (N1). In any case, the results provide new perspectives on shock chemistry and the importance of the environment in which it occurs.

A Low-mass line-rich Core Found in Massive Star-forming Region IRAS 16351-4722

We present Atacama Large Millimeter/submillimeter Array subarcsecond-resolution observations of both continuum and molecular lines at 345 GHz toward the massive star-forming region IRAS 16351-4722 (hereafter I16351). A total of 12 dust cores were detected based on high-spatial-resolution observations of the continuum. Among them, a high-mass core (11.6 M ⊙) and a low-mass core (1.7 M ⊙) show abundant molecular line emissions. 164 molecular transitions from 29 species and 104 molecular transitions from 25 species are identified in the high-mass and low-mass cores, respectively. Complex organic molecules (COMs) such as CH3OH, CH3OCHO, CH3OCH3, C2H5OH, and C2H5CN are detected in the two cores. Under the assumption of local thermodynamic equilibrium, the rotational temperatures and column densities of the COMs are derived with the XCLASS software. The maximum rotation temperature values in the low-mass core and the high-mass core were found to be approximately 130 K and 198 K, respectively. Additionally, the line widths in the high-mass core are larger than those in the low-mass one. The abundant COM line transitions, high gas temperatures, and smaller line widths indicate the presence of a low-mass line-rich core in the massive star formation region for the first time, while the high-mass line-rich core shows hot core properties. When comparing the molecular abundances of CH3OH, CH3OCHO, CH3OCH3, and C2H5OH of the two cores with other hot cores and hot corinos reported in the literature, we further confirm that both a hot core and a low-mass line-rich core are simultaneously detected in I16351.

Non-LTE abundance analysis of A-B stars with low rotational velocities – II. Do A-B stars with normal abundances exist?

ABSTRACT We present chemical composition and fundamental parameters (the effective temperature, surface gravity, and radius) for four sharp-lined A-type stars γ Gem (HD 41705), o Peg (HD 214994), θ Vir (HD 114330), and ν Cap (HD 193432). Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range. We refined the fundamental parameters of the stars with the sme (Spectroscopy Made Easy) package and verified their accuracy by comparing with the spectral energy distribution and hydrogen line profiles. We found Teff/log g = 9190 ± 130 K/3.56 ± 0.08, 9600 ± 50 K/3.81 ± 0.04, 9600 ± 140 K/3.61 ± 0.12, and 10200 ± 220 K/3.88 ± 0.08 for γ Gem, o Peg, θ Vir, and ν Cap, respectively. Our detailed abundance analysis employs a hybrid technique for spectrum synthesis based on classical model atmospheres calculated in local thermodynamic equilibrium (LTE) assumption together with the non-LTE (NLTE) line formation for 18 of 26 investigated species. Comparison of the abundance patterns observed in A stars of different types (normal A, Am, and Ap) with similar fundamental parameters reveals significant abundance diversity that cannot be explained by the current mechanisms of abundance peculiarity formation in stellar atmospheres. We found a rise of the heavy element (Zn, Sr, Y, Zr, and Ba) abundance excess up to +1 dex with Teff increasing from 7200 to 10 000 K, with a further decrease down to solar value at Teff = 13 000 K, indicating that stars with solar element abundances can be found among late B-type stars.

Rotational spectrum and interstellar detection of the first torsionally excited state of methylamine

Context. Methylamine (CH3NH2) was first detected in the interstellar medium (ISM) toward Sgr B2 almost 50 years ago by observation of rotational transitions in its torsional ground state. Methylamine exhibits two large-amplitude motions (LAMs), the methyl torsion and amine wagging, which complicate the spectral analysis, especially in excited vibration states. The lack of an accurate model of the two coupled LAMs has also hampered the identification in the ISM of rotational transitions in excited vibrational states. Aims. The aim of this work is to study the terahertz and microwave rotational spectra of methylamine experimentally and theoretically in order to provide a reliable basis for the detection of its rotational transitions in the first torsionally excited state, υt = 1, in the ISM. Methods. The terahertz spectrum of methylamine was measured from 150 to 1520 GHz with the Lille fast scan spectrometer. Using a new “hybrid” Hamiltonian model, we were able to analyze the nuclear quadrupole hyperfine structure and to accurately fit the rotational spectrum of the υt = 1 state of methylamine. We used the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to search for rotational transitions of methylamine in its first torsionally excited state toward the high-mass star forming region Sgr B2(N). The observed spectra are modeled under the assumption of local thermodynamic equilibrium (LTE). Results. Accurate spectral predictions were obtained for the ground and first excited states of CH3NH2. We report the first interstellar detection of methylamine in the υt = 1 state toward the offset position Sgr B2(N1S) in the hot molecular core Sgr B2(N1). The LTE parameters derived previously from the rotational emission of methylamine in its torsional ground state toward Sgr B2(N1S) yield synthetic spectra of methylamine in the υt = 1 state that are fully consistent with the ALMA spectra and allow us to identify five rotational lines of this state.

Molecular Abundance of the Circumnuclear Region Surrounding an Active Galactic Nucleus in NGC 1068 Based on an Imaging Line Survey in the 3 mm Band with ALMA

We present an imaging molecular line survey in the 3 mm band (85–114 GHz) focused on one of the nearest galaxies with an active galactic nucleus (AGN), NGC 1068, based on observations taken with the Atacama Large Millimeter/submillimeter Array. Distributions of 23 molecular transitions are obtained in the central ∼3 kpc region, including both the circumnuclear disk (CND) and starburst ring (SBR) with 60 and 350 pc resolution. The column densities and relative abundances of all the detected molecules are estimated under the assumption of local thermodynamic equilibrium in the CND and SBR. Then, we discuss the physical and chemical effects of the AGN on molecular abundance corresponding to the observation scale. We found that H13CN, SiO, HCN, and H13CO+ are abundant in the CND relative to the SBR. In contrast, 13CO is more abundant in the SBR. Based on the calculated column density ratios of N(HCN)/N(HCO+), N(HCN)/N(CN), and other molecular distributions, we conclude that the enhancement of HCN in the CND may be due to high-temperature environments resulting from strong shocks, which are traced by the SiO emission. Moreover, the abundance of CN in the CND is significantly lower than the expected value of the model calculations in the region affected by strong radiation. The expected strong X-ray irradiation from the AGN has a relatively lower impact on the molecular abundance in the CND than mechanical feedback.

The Molecular Clouds in a Section of the Third Galactic Quadrant: Observational Properties and Chemical Abundance Ratio between CO and its Isotopologues

We compare the observational properties between 12CO, 13CO, and C18O and summarize the observational parameters based on a 7069-cloud sample from the Milky Way Imaging Scroll Painting CO survey in a section of the third Galactic quadrant. We find that the 13CO angular area () generally increases with that of 12CO (), and the ratio of to is 0.38 by linear fitting. We find that the 12CO and 13CO flux are tightly correlated as with both fluxes calculated within the 13CO-bright region. This indicates that the abundance is a constant as for all samples under the assumption of local thermodynamic equilibrium (LTE). Additionally, we observed that the X-factor is approximately constant in a large sample of molecular clouds. Similarly, we find with both fluxes calculated within the C18O-bright region, which indicates that the abundance ratio stays at the same value of across the molecular clouds under the LTE assumption. The linear relationships of versus and versus hold not only for the 13CO-bright region and C18O-bright region, but also for the entire molecular cloud scale with a lower flux ratio. The abundance ratio inside the clouds shows a strong correlation with the column density and temperature. This indicates that the ratio is dominated by a combination of chemical fractionation, selective dissociation, and the self-shielding effect inside the clouds.

Fourth-order accurate numerical modeling of the multi-fluid plasma equations with adaptive mesh refinement

This work develops a multi-fluid plasma modeling capability in extension of a parallel, adaptive, fourth-order, finite-volume method, computational fluid dynamics algorithm. The governing equations of the multi-fluid plasma model are derived from moments of the Boltzmann equation, under the assumption of local thermodynamic equilibrium. Maxwell's equations describe the electromagnetic field evolution and are coupled to the fluid equations of charged species. The present work concentrates on two, charged, collisionless fluids as the first step, although the algorithm is designed to accommodate more fluids, both charged and neutral, and collisions between fluids; the extension to multiple fluids is straightforward. The numerical algorithm featuring parallel, high-order (≥4 for smooth flows), adaptive mesh refinement is expected to help cope with the inherent stiffness of the physics involved. The standard, fourth-order Runge-Kutta scheme is used to evolve the solution in time. The current study is primarily focused on verification and testing of the new, adaptive algorithm; we demonstrate 4th-order accuracy using problems with exact solutions and benchmark our solutions for common plasma simulations against references from literature. Using the 4th-order scheme, our multi-fluid model is shown to provide accurate solutions to plasma problems using half the cell count as traditional, 2nd-order methods. Furthermore, adaptive mesh refinement is successfully applied to three, plasma test problems, providing a significant reduction in mesh size. In subsequent work, an optimized version of our multi-fluid plasma algorithm will be applied to plasma test cases with more practical configurations, with a rigorous assessment of the improvements to computational efficiency afforded by our adaptive, higher-order scheme.

Local equilibrium in liquid phase shock waves.

We have assessed the assumption of local thermodynamic equilibrium in a shock wave by comparing local thermodynamic data generated with nonequilibrium molecular dynamics (NEMD) simulations with results from corresponding equilibrium simulations. The shock had a Mach number approximately equal to 2 in a Lennard-Jones spline liquid. We found that the local equilibrium assumption holds perfectly well behind the wave front, and is a very good approximation in the front itself. This was supported by calculations of the excess entropy production in the shock front with four different methods that use the local equilibrium assumption in different ways. Two of the methods assume local equilibrium between excess thermodynamic variables by treating the shock as an interface in Gibbs's sense. The other two methods are based on the local equilibrium assumption in a continuous description of the shock front. We show for the shock studied in this work that all four methods give excess entropy productions that are in excellent agreement, with an average variance of 3.5% for the nonequilibrium molecular dynamics (NEMD) simulations. In addition, we solved the Navier-Stokes (N-S) equationsnumerically for the same shock wave using an equilibrium equationof state (EoS) based on a recently developed perturbation theory. The results for the density, pressure, and temperature profiles agree well with the profiles from the NEMD simulations. For instance, the shock waves generated in the two simulations travel with almost the same speed; the average absolute Mach-number deviation of the N-S simulations relative to NEMD is 2.6% in the investigated time interval.