Radial Gas Flow Research Articles

Revisiting long-standing puzzles of the Milky Way: the Sun and its vicinity as typical outer disk chemical evolution

We present a scenario of the chemical enrichment of the solar neighborhood that solves the G-dwarf problem by taking into account constraints on a larger scale. We argue that the Milky Way disk within 10 kpc has been enriched to solar metallicity by a massive stellar population: the thick disk, which itself formed from a massive turbulent gaseous disk. While the inner disk, R ≲ 6 kpc, continued this enrichment after a quenching phase (7−10 Gyr), at larger distances radial flows of gas diluted the metals left by the thick disk formation at a time we estimate to be 7−8 Gyr ago, thus partitioning the disk into an inner and outer region characterized by different chemical evolutions. The key new consideration is that the pre-enrichment provided by the thick disk is not related to the mass fraction of this stellar population at the solar radius, as is classically assumed in inside-out scenarios, but is actually related to the formation of the entire massive thick disk, due to the vigorous gas phase mixing that occurred during its formation. Hence, the fact that this population represents only 15−25% of the local stellar surface density today, or 5−10% of the local volume density, is irrelevant for “solving” the G-dwarf problem. The only condition for this scenario to work is that the thick disk was formed from a turbulent gaseous disk that permitted a homogeneous – not radially dependent – distribution of metals, allowing the solar ring to be enriched to solar metallicity. At the solar radius, the gas flowing from the outer disk combined with the solar metallicity gas left over from thick disk formation, providing the fuel necessary to form the thin disk at the correct metallicity to solve the G-dwarf problem. Chemical evolution at R > 6 kpc, and in particular beyond the solar radius, can be reproduced with the same scheme. We suggest that the dilution, occurring at the fringe of the thick disk, was possibly triggered by the formation of the bar and the establishment of the outer Lindblad resonance (OLR), enabling the inflow of metal poorer gas from the outer disk to R ∼ 6 kpc, presumably the position of the OLR at this epoch, and at the same time isolating the inner disk from external influence. These results imply that the local metallicity distribution is not connected to the gas accretion history of the Milky Way. Finally, we argue that the Sun is the result of the evolution typical of stars in the disk beyond ∼6 kpc (i.e., also undergoing dilution), and has none of the characteristics of inner disk stars.

Effects of Nozzle Radial Position, Separation Angle, and Gas Flow Partitioning on the Mixing, Eye Area, and Wall Shear Stress in Ladles Fitted with Dual Plugs

Mixing time, slag eye area, and wall shear stress in a ladle fitted with dual plugs have been studied as a function of operating variables, namely, gas flow rate, radial nozzle position, separation angle between nozzles and flow rate partitioning. While the mixing time and slag eye area have been experimentally investigated in a scaled water model (with and without a top slag layer), the shear stress on the vessel wall has been studied computationally via a RANS-based turbulent, three-dimensional coupled Eulerian–Lagrangian (VOF-DPM), multiphase flow model. Experimental and computational results have indicated that the mixing, eye area, and wall shear stress depend on the gas flow rate, i.e., while mixing efficiency increases with the increasing gas flow rate, the slag eye area, and wall shear stress, in contrast, become more pronounced with the increasing gas flow, leading to an undesirable operating regime. The radial nozzle position, separation angle between the nozzles and flow rate partitioning at any given flow rate also influence the ladle process performance, influencing the mixing, eye area, and wall shear stress, albeit to a varied degree. Within the range of operating conditions studied, an expected inverse correspondence between mixing time and shear behavior was observed. Experimental and computational results in conjunction have indicated that the best arrangement of porous plugs or gas injection nozzles for superior ladle process performance is dependent on the gas flow rate and is specific to the desired objective (i.e., decreasing the mixing time, or slag eye area and wall shear stress). Hence, a unique gas injection practice cannot and should not be suggested as the optimum for ladle metallurgy in steelmaking. Nevertheless, if the mixing time is the parameter of primary interest, a nozzle configuration with equal flow partitioning (1:1) between the nozzles and identical nozzle radial positions (0.7R/0.7R, 45 deg) should suffice for both low and high gas flow rates. In contrast, a nonidentical nozzle radial position (0.7R/0.5R, 90 deg) and an unequal gas flow rate per nozzle (1:3) appear preferable if both ladle eye and shear stresses, rather than mixing, are the issues of concern.

WISDOM project – IV. A molecular gas dynamical measurement of the supermassive black hole mass in NGC 524

ABSTRACT We present high angular resolution (0.3 arcsec or $37\, \mathrm{pc}$) Atacama Large Millimeter/submillimeter Array observations of the CO(2–1) line emission from a central disc in the early-type galaxy NGC 524. This disc is shown to be dynamically relaxed, exhibiting ordered rotation about a compact $1.3\, \mathrm{mm}$ continuum source, which we identify as emission from an active supermassive black hole (SMBH). There is a hole at the centre of the disc slightly larger than the SMBH sphere of influence. An azimuthal distortion of the observed velocity field is found to be due to either a position angle warp or radial gas flow over the inner 2${^{\prime\prime}_{.}}$5. By forward-modelling the observations, we obtain an estimate of the SMBH mass of $4.0^{+3.5}_{-2.0}\times 10^8\,$ M⊙, where the uncertainties are at the 3σ level. The uncertainties are dominated by the poorly constrained inclination and the stellar mass-to-light ratio of this galaxy, and our measurement is consistent with the established correlation between SMBH mass and stellar velocity dispersion. Our result is roughly half that of the previous stellar dynamical measurement, but is consistent within the uncertainties of both. We also present and apply a new tool for modelling complex molecular gas distributions.

Research on operating parameters of T-groove cylindrical gas film seal based on computational fluid dynamics

Cylindrical gas film sealing technology is a new type of dry gas sealing technology. Compared with the face gas film sealing technology, the cylindrical gas film seal presents a strong floating property, which can reduce the vibration and thermal deformation of the rotor system. In this article, the effect of operating parameters such as speed, pressure difference and viscosity on the T-groove gas cylindrical film seal performance are studied in detail by the method of control variable in computational fluid dynamics software, and pressure distribution, gas film stiffness, leakage, leakage stiffness ratio and hydrodynamic force are analysed. Results show that with the increase of the rotational speed, static pressure, hydrodynamic force and film stiffness increase, but leakage decreases first and then increases. Furthermore, the results indicate that with the increase of pressure difference, the static pressure, leakage and hydrodynamic force increase. In addition, the simulations show that when the viscosity increases, the maximum pressure and film stiffness increase, but the leakage decreases. This indicates that as the rotational speed increases, the hydrodynamic effect and the amount of gas overflow in the axial direction increase, resulting in an increase of leakage. Lastly, the results also show that when the pressure difference increases, both the radial and axial gas flow rates increase, resulting in an increase in both the film stiffness and the leakage. With the increase of viscosity, the viscous shear force and fluid hydrodynamic force increase, resulting in the increase of the gas film stiffness. This study can provide a theoretical basis in industrial applications for setting the operating parameters and serving as a reference.

Packed and fluidized bed absorber modeling for carbon capture with micro-encapsulated sodium carbonate solution

Micro-Encapsulated CO2 Sorbents (MECS) are a promising technology for post-combustion carbon capture because they enable slow-reacting solvents like carbonate solution to compete with traditional amine solvents. Before scaling up MECS for pilot testing, modeling is needed to design a MECS absorber and quantify its size and energy penalty. To that end, a multi-scale model for MECS- that ranges from a single capsule to a 500 MWe power plant absorber- is developed and presented here. First, the individual capsule model is developed and fitted to experimental CO2 absorption data collected on a 0.1 g sample of capsules filled with sodium carbonate solution. This capsule model is then validated against data collected on a 25 g batch of capsules exposed to flue gas conditions in a fluidized column. This model is then scaled up to represent two absorber designs: a multi-stage, counter-flow fluidized bed and a hollow, cylindrical packed bed with radial gas flow. These two absorber bed models are first optimized for a 1 MWe pilot-scale absorber, and then optimized for a 500 MWe coal plant. This model predicts absorbers of similar dimensions and smaller energy penalties than previously modeled absorbers filled with amine solvent capsules. Furthermore, it is demonstrated here that a few reasonable improvements to capsule design would result in absorber sizes and energy penalties lower than those of a benchmark amine solvent tower. These results demonstrate that micro-encapsulated carbonate solution can compete with faster-acting amine solvents for post-combustion carbon capture.

Abundance gradients along the Galactic disc from chemical evolution models

In this paper, we study the formation and chemical evolution of the Milky Way disc with particular focus on the abundance patterns ([$\alpha$/Fe] vs. [Fe/H]) at different Galactocentric distances, the present-time abundance gradients along the disc and the time evolution of abundance gradients. We consider the chemical evolution models for the Galactic disc developed by Grisoni et al. (2017) for the solar neighborhood, both the two-infall and the one-infall ones, and we extend our analysis to the other Galactocentric distances. In particular, we examine the processes which mainly influence the formation of the abundance gradients: the inside-out scenario, a variable star formation efficiency, and radial gas flows. We compare our model results with recent abundance patterns obtained along the Galactic disc from the APOGEE survey and with abundance gradients observed from Cepheids, open clusters, HII regions and PNe. We conclude that the inside-out scenario is a key ingredient, but cannot be the only one to explain abundance patterns at different Galactocentric distances and abundance gradients. Further ingredients, such as radial gas flows and variable star formation efficiency, are needed to reproduce the observed features in the thin disc. The evolution of abundance gradients with time is also shown, although firm conclusions cannot still be drawn.

Radial elemental abundance gradients in galaxies from cosmological chemodynamical simulations

AbstractCosmological chemodynamical simulations are nowadays among the best tools to study how chemical elements are produced within galaxies, to reconstruct also the spatial distribution of the chemical elements as a function of time within different galaxy environments. Our simulation code includes the main stellar nucleosynthetic sources in the cosmos (core-collapse and Type Ia supernovae, hypernovae, asymptotic giant branch stars, and stellar winds from stars of all masses and metallicities). We present the predictions of our simulation for the evolution of the radial gradients of O/H, N/O and C/N in the gas-phase of a sample of ten star-forming disc galaxies, all characterised by very different star formation histories at the present time (see Figure 1.). On average, our simulated disc galaxies show a clear inside-out growth of the stellar mass as a function of time, and more negative slopes of the radial gas-phase O/H versus radius at earlier epochs of the galaxy evolution; we predict negative slopes of N/O and positive slopes of C/N at almost all redshifts, because of the main secondary origin of N in stars, even though the high-redshift simulation data are highly scattered because of the more turbulent conditions of the interstellar medium. Finally, we show that similar results are found with zoom-in simulations, where a spiral galaxy is re-simulated with a larger number of resolution elements. With zoom-in simulations, we study how stellar migrations (particularly old and metal-poor stellar populations migrating outwards) and radial gas flows are capable of influencing the galaxy chemical evolution at different galactic radii.

Relationships between the Charge–Discharge Methods and the Performance of a Supercapacitive Swing Adsorption Module for CO2 Separation

We report a scaled-up supercapacitive swing adsorption (SSA) module with a novel radial gas flow design to separate CO2 from a simulated flue gas mixture containing 15% CO2 and 85% N2. We define metrics that allow for a quantitative evaluation of the energetic and adsorption performance of SSA cycles, namely, the specific capacitance, the Coulombic efficiency, the energy efficiency, the energy loss, the sorption capacity, the electron efficiency, the energy consumption, the adsorption rate, and the time-energy efficiency. Using these metrics, we investigate the influence of different electrical charge–discharge methods on the energetic and adsorptive performance of the module and identify the most favorable charge–discharge method.

Relationships between the Charge-Discharge Methods and the Gas Adsorption Performance of a Supercapacitive Swing Adsorption Module for CO2 Separation

Introduction: Supercapacitive swing adsorption (SSA) is a gas separation technology that is based on selective adsorption and desorption of a gas from a gas mixture upon capacitive charging and discharging activated carbon electrodes.1,2 SSA is a promising technology due to its facile reversibility, significant sorption capacity, complete gas selectivity, and energy efficiency. In this study, we simplified the cell design and enlarged the electrode area of SSA modules. These new cell features are expected to increase the scalability of SSA technology. We defined metrics that allow for a quantitative evaluation of the energetic and adsorption performance of SSA cycles. Using these metrics, we quantitatively determined the influence of different electrical charge-discharge methods on the energetic and CO2 adsorption performance, and identified the most favorable charge-discharge method. Methods: The cross-sectional view of a SSA module is shown in Figure a. The SSA module was operated in gas flow-through mode and fed by a moistened 15% : 85% CO2 : N2 gas mixture serving as a flue gas simulant. In the SSA module, the gas was distributed by radial gas flow system, in which gas flowed from the inlet port in the center to the outlet port in the periphery of the SSA module. This design eliminated the use of gas flow channels, and simplified SSA module configuration. The electrode area was 49 cm2, which was 25 times of that in our previous study2. The CO2 concentration of SSA effluent gas was analyzed and recorded. The electrochemical behavior of the SSA module was controlled and measured using a potentiostat in a two-electrode configuration. Four different charge-discharge methods at voltages between 0 and -1 V were used to evaluate SSA performance, namely GCD (galvanostatic charging-discharging, Figure b), GCD+Pstat (GCD with potentiostatic holding steps between each GCD step, Figure c), GCD+OC (GCD with open-circuit holding steps between each GCD step, Figure d), and Combined (galvanostatic charging followed by holding step at open circuit, and galvanostatic discharging followed by potentiostatic holding step at 0 V, Figure e). Results: We introduced energetic and adsorption metrics to quantitatively evaluate SSA performance for CO2 adsorption. These metrics are specific capacitance, Coulomb efficiency, energy efficiency, energy loss, sorption capacity, electron efficiency, energy consumption, adsorption rate, and time-energy efficiency. Using these metrics, we determined how different charging methods affect the energetic and adsorption performance of SSA module. The galvanostatic charge-discharge method was most time-energy efficient due to its shortest charge time and lowest energy loss. The highest time-energy efficient for CO2 adsorption (7.5 µmol.kJ-1.s-1) was obtained with the galvanostatic charge-discharge methods at 50 mA. Increasing the constant charging current improved the energetic performance but decreased CO2 adsorption capacity. The introduction of holding steps favored CO2 adsorption at the cost of higher energy loss. A holding step of 30 min duration was sufficient for the full completion of CO2 adsorption and desorption. The highest CO2 adsorption capacity (87.5 mmol.kg-1), highest electron efficiency (0.2 molecule.electron-1), lowest energy consumption (109 kJ.mol-1) and highest CO2 adsorption rate (34 µmol.kg-1.s-1) were obtained by Combined method with 30 min holding steps. Conclusions: We demonstrated a simplified SSA module with enlarged electrode area, which is expected to benefit the scale-up of SSA technology for CO2 adsorption and separation. Our results provide systematic method to evaluate SSA performance and improve the understanding of the relationships between energetic properties and gas adsorption performance of SSA. This findings is critical to understanding the mechanism of SSA phenomena and to exploring SSA application. References 1 B. Kokoszka, N. K. Jarrah, C. Liu, D. T. Moore and K. Landskron, Angew. Chemie - Int. Ed., 2014, 53, 3698–3701. 2 C. Liu and K. Landskron, Chem. Commun., 2017, 53, 3661–3664. Figure 1

Modelling hollow fiber membrane modules for anesthesia gas separation

A membrane separation process was previously developed to remove carbon dioxide from anesthesia partial rebreathing circuits. The process investigated in this study used a cylindrical hollow fiber membrane module with radial flow of the feed gas on the shell side and axial flow of the sweep gas on the lumen side of the hollow fibers. Oxygen was used as the sweep gas in one- and two-pass configurations to facilitate carbon dioxide transfer across the membrane. The polymethylpentene membranes used in this study have previously been shown to have a high selectivity for carbon dioxide over commonly used anesthetic vapours. Therefore, the primary focus of this study was on the carbon dioxide separation performance of different module configurations and the development of a validated model that could be used for further design and optimization studies. A transient pseudo-one-dimensional design model was developed to predict the performance of the membrane system. The model was constructed by dividing the membrane into a distribution of interconnected, equally-sized, perfectly-mixed segments. Steady-state experimental data, collected for a 5% carbon dioxide (balance oxygen) feed gas and a pure oxygen sweep gas, were used to tune the number of segments in the model. The tuned model predictions were then compared to experimental data for the full range of feed and sweep flow rates (1–15 L min−1). Four different membrane modules were used to explore the impact of configuration on separation performance. The modules varied in diameter (80 mm and 100 mm), hollow fiber packing density (0.213, 0.302, and 0.454), and number of sweep passes (one and two passes). The model provided good predictions of the steady-state membrane performance for the range of operating conditions investigated in this study. Model validity was not found to be very sensitive to the number of segments over the tested range of operating conditions. The number of segments did not need to be readjusted for different flow rates, which indicates that the design model can be used for extrapolating predictions beyond the flow rates and membrane sizes investigated in the current study.

Low mass planet migration in Hall-affected disks

Recent developments in non-ideal magnetohydrodynamic simulations of protoplanetary disks suggest that instead of being traditional turbulent (viscous) accretion disks, they have a largely laminar flow with accretion driven by large-scale wind torques. These disks are possibly threaded by Hall-effect generated large-scale horizontal magnetic fields. We have examined the dynamics of the corotation region of a low mass planet embedded in such a disk and the evolution of the associated migration torque. These disks lack strong turbulence and associated turbulent diffusion, and the presence of a magnetic field and radial gas flow presents a situation outside the applicability of previous corotation torque theory. We summarize the analytical analysis of the corotation torque, give details on the numerical methods used, and in particular the relative merits of different numerical schemes for the inviscid problem.

Low-mass planet migration in magnetically torqued dead zones – II. Flow-locked and runaway migration, and a torque prescription

We examine the migration of low mass planets in laminar protoplanetary discs, threaded by large scale magnetic fields in the dead zone that drive radial gas flows. As shown in Paper I, a dynamical corotation torque arises due to the flow-induced asymmetric distortion of the corotation region and the evolving vortensity contrast between the librating horseshoe material and background disc flow. Using simulations of laminar torqued discs containing migrating planets, we demonstrate the existence of the four distinct migration regimes predicted in Paper I. In two regimes, the migration is approximately locked to the inward or outward radial gas flow, and in the other regimes the planet undergoes outward runaway migration that eventually settles to fast steady migration. In addition, we demonstrate torque and migration reversals induced by midplane magnetic stresses, with a bifurcation dependent on the disc surface density. We develop a model for fast migration, and show why the outward runaway saturates to a steady speed, and examine phenomenologically its termination due to changing local disc conditions. We also develop an analytical model for the corotation torque at late times that includes viscosity, for application to discs that sustain modest turbulence. Finally, we use the simulation results to develop torque prescriptions for inclusion in population synthesis models of planet formation.

Two dimensional radial gas flows in atmospheric pressure plasma-enhanced chemical vapor deposition

Atmospheric pressure (AP) operation of plasma-enhanced chemical vapor deposition (PECVD) is one of promising concepts for high quality and low cost processing. Atmospheric plasma discharge requires narrow gap configuration, which causes an inherent feature of AP PECVD. Two dimensional radial gas flows in AP PECVD induces radial variation of mass-transport and that of substrate temperature. The opposite trend of these variations would be the key consideration in the development of uniform deposition process. Another inherent feature of AP PECVD is confined plasma discharge, from which volume power density concept is derived as a key parameter for the control of deposition rate. We investigated deposition rate as a function of volume power density, gas flux, source gas partial pressure, hydrogen partial pressure, plasma source frequency, and substrate temperature; and derived a design guideline of deposition tool and process development in terms of deposition rate and uniformity.

Diffusive heat and mass transfer in oscillatory pipe flow

The enhancement of axial heat and mass transfer by laminar flow oscillation in pipes with axial gradients in temperature and concentration has been studied analytically for the cases of insulated and conducting walls. The axial diffusivity can exceed its molecular counterpart by many orders of magnitude, with a quadratic scaling on the pressure-gradient amplitude and the Prandtl or Schmidt number, and is a bimodal function of oscillatory frequency: quasi-steady behavior at low frequencies and a power-law decay at high frequencies. When the pipe wall is conductive and of sufficient thickness, and the flow oscillation is quasi-steady, the axial diffusivity may be enhanced by a further factor of about ten as a result of increased radial diffusion, for liquid and gas flows in pipes with walls with a wide range of thermal conductivities. Criteria for the wall thickness required to achieve this additional enhancement and for the limits placed on the validity of these solutions by viscous dissipation are also deduced. When the heat transfer per unit flow work achieved by oscillatory pipe flow is contrasted with that of a conventional parallel-flow heat exchanger, it is found to be of comparable size and the ratio of the two is shown to be a function only of the pipe geometry, heat-exchanger mean velocity, and fluid viscosity.

Quasars Probing Galaxies. I. Signatures of Gas Accretion at Redshift z ≈ 0.2∗ †

Abstract We describe the kinematics of circumgalactic gas near the galactic plane, combining new measurements of galaxy rotation curves and spectroscopy of background quasars. The sightlines pass within 19–93 kpc of the target galaxy and generally detect Mg ii absorption. The Mg ii Doppler shifts have the same sign as the galactic rotation, so the cold gas co-rotates with the galaxy. Because the absorption spans a broader velocity range than disk rotation can explain, we explore simple models for the circumgalactic kinematics. Gas spiraling inwards (near the disk plane) offers a successful description of the observations. An appendix describes the addition of tangential and radial gas flows and illustrates how the sign of the disk inclination produces testable differences in the projected line-of-sight velocity range. This inflow interpretation implies that cold flow disks remain common down to redshift z ≈ 0.2 and prolong star formation by supplying gas to the disk.

Runaway gas accretion and gap opening versus type I migration

Growing planets interact with their natal protoplanetary disc, which exerts a torque onto them allowing them to migrate in the disc. Small mass planets do not affect the gas profile and migrate in the fast type-I migration. Although type-I migration can be directed outwards for planets smaller than 20−30M⊕ in some regions of the disc, planets above this mass should be lost into the central star long before the disc disperses. Massive planets push away material from their orbit and open a gap. They subsequently migrate in the slower, type II migration, which could save them from migrating all the way to the star. Hence, growing giant planets can be saved if and only if they can reach the gap opening mass, because this extends their migration timescale, allowing them to eventually survive at large orbits until the disc itself disperses.However, most of the previous studies only measured the torques on planets with fixed masses and orbits to determine the migration rate. Additionally, the transition between type-I and type-II migration itself is not well studied, especially when taking the growth mechanism of rapid gas accretion from the surrounding disc into account. Here we use isothermal 2D disc simulations with FARGO-2D1D to study the migration behaviour of gas accreting protoplanets in discs. We find that migrating giant planets always open gaps in the disc. We further show analytically and numerically that in the runaway gas accretion regime, the growth time-scale is comparable to the type-I migration time-scale, indicating that growing planets will reach gap opening masses before migrating all the way to the central star in type-I migration if the disc is not extremely viscous and/or thick. An accretion rate limited to the radial gas flow in the disc, in contrast, is not fast enough. When gas accretion by the planet is taken into account, the gap opening process is accelerated because the planet accretes material originating from its horseshoe region. This allows an accreting planet to transition to type-II migration before being lost even if gas fails to be provided for a rapid enough growth and the classical gap opening mass is not reached.

Stellar and gaseous disc structures in cosmological galaxy equilibrium models

We present "radially-resolved-equilibrium-models" for the growth of stellar and gaseous disks in cosmologically accreting massive halos. Our focus is on objects that evolve to redshifts $z\sim 2$. We solve the time-dependent equations that govern the radially dependent star-formation rates, inflows and outflows from and to the inter- and circum-galactic medium, and inward radial gas flows within the disks. The stellar and gaseous disks reach equilibrium configurations on dynamical time scales much shorter than variations in the cosmological dark matter halo growth and baryonic accretions rates. We show analytically that mass and global angular momentum conservation naturally give rise to exponential gas and stellar disks over many radial length scales. As expected, the gaseous disks are more extended as set by the condition Toomre $Q<1$ for star-formation. The disks rapidly become baryon dominated. For massive, $5\times 10^{12}M_\odot$ halos at redshift $z=2$, we reproduced the typical observed star-formation rates of $\sim 100 \, M_\odot \, {\rm yr}^{-1}$, stellar masses $\sim 9\times 10^{10}\, M_\odot$, gas contents $\sim 10^{11}\, M_\odot$, half mass sizes of 4.5 and 5.8 kpc for the stars and gas, and characteristic surface densities of $500$ and $ 400\, M_\odot \, {\rm pc}^{-2}$ for the stars and gas.

Radial gas motions in The H i Nearby Galaxy Survey (THINGS)

The study of 21cm line observations of atomic hydrogen allows detailed insight into the kinematics of spiral galaxies. We use sensitive high-resolution VLA data from The HI Nearby Galaxy Survey (THINGS) to search for radial gas flows primarily in the outer parts (up to $3\times r_{25}$) of ten nearby spiral galaxies. Inflows are expected to replenish the gas reservoir and fuel star formation under the assumption that galaxies evolve approximately in steady state. We carry out a detailed investigation of existing tilted ring fitting schemes and discover systematics that can hamper their ability to detect signatures of radial flows. We develop a new Fourier decomposition scheme that fits for rotational and radial velocities and simultaneously determines position angle and inclination as a function of radius. Using synthetic velocity fields we show that our novel fitting scheme is less prone to such systematic errors and that it is well suited to detect radial inflows in disks. We apply our fitting scheme to ten THINGS galaxies and find clear indications of, at least partly previously unidentified, radial gas flows, in particular for NGC 2403 and NGC 3198 and to a lesser degree for NGC 7331, NGC 2903 and NGC 6946. The mass flow rates are of the same order but usually larger than the star formation rates. At least for these galaxies a scenario in which continuous mass accretion feeds star formation seems plausible. The other galaxies show a more complicated picture with either no clear inflow, outward motions or complex kinematic signatures.

EXTERNAL PHOTOEVAPORATION OF THE SOLAR NEBULA. II. EFFECTS ON DISK STRUCTURE AND EVOLUTION WITH NON-UNIFORM TURBULENT VISCOSITY DUE TO THE MAGNETOROTATIONAL INSTABILITY

The structure and evolution of protoplanetary disks, especially the radial flows of gas through them, are sensitive to a number of factors. One that has been considered only occasionally in the literature is external photoevaporation by far-ultraviolet (FUV) radiation from nearby, massive stars, despite the fact that nearly half of all disks will experience photoevaporation. Another effect apparently not considered in the literature is a spatially and temporally varying value of $\alpha$ in the disk [where the turbulent viscosity $\nu$ is $\alpha$ times the sound speed C times the disk scale height H]. Here we use the formulation of Bai \& Stone (2011) to relate $\alpha$ to the ionization fraction in the disk, assuming turbulent transport of angular momentum is due to the magnetorotational instability. We find that disk evolution is most sensitive to the surface area of dust. Typically $\alpha \lesssim 10^{-5}$ in the inner disk ($< 2$ AU), rising to $\sim 10^{-1}$ beyond 20 AU. This drastically alters the structure of the disk and the flow of mass through it: while the outer disk rapidly viscously spreads, the inner disk hardly evolves; this leads to a steep surface density profile with a slope < p > $\approx$ 2 - 5 in the 5-30 AU region) that is made steeper by external photoevaporation. We also find that the combination of variable $\alpha$ and external photoevaporation eventually causes gas as close as 3 AU, previously accreting inward, to be drawn outward to the photoevaporated outer edge of the disk. These effects have drastic consequences for planet formation and volatile transport in protoplanetary disks.

Accretion, radial flows and abundance gradients in spiral galaxies

The metal-poor gas continuously accreting onto the discs of spiral galaxies is unlikely to arrive from the intergalactic medium (IGM) with exactly the same rotation velocity as the galaxy itself and even a small angular momentum mismatch inevitably drives radial gas flows within the disc, with significant consequences to galaxy evolution. Here we provide some general analytic tools to compute accretion profiles, radial gas flows and abundance gradients in spiral galaxies as a function of the angular momentum of accreting material. We generalize existing solutions for the decomposition of the gas flows, required to reproduce the structural properties of galaxy discs, into direct accretion from the IGM and a radial mass flux within the disc. We then solve the equation of metallicity evolution in the presence of radial gas flows with a novel method, based on characteristic lines, which greatly reduces the numerical demand on the computation and sheds light on the crucial role of boundary conditions on the abundance profiles predicted by theoretical models. We also discuss how structural and chemical constraints can be combined to disentangle the contributions of inside-out growth and radial flows in the development of abundance gradients in spiral galaxies. Illustrative examples are provided throughout with parameters plausible for the Milky Way. We find that the material accreting on the Milky Way should rotate at 70-80 per cent of the rotational velocity of the disc, in agreement with previous estimates.