Time-dependent Ionization Research Articles

A magnetic reconnection model for the hot explosion with both ultraviolet and Hα wing emissions

Context. Ellerman bombs (EBs) with significant Hα wing emissions and ultraviolet bursts (UV bursts) with strong Si IV emissions are two kinds of small transient brightening events that occur in the low solar atmosphere. The statistical observational results indicate that about 20% of the UV bursts connect with EBs. While some promising models exist for the formation mechanism of colder EBs in conjunction with UV bursts, the topic remains an area of ongoing research and investigation. Aim. We numerically investigated the magnetic reconnection process between the emerging arch magnetic field and the lower atmospheric background magnetic field. We aim to find out if the hot UV emissions and much colder Hα wing emissions can both appear in the same reconnection process and how they are located in the reconnection region. Methods. The open-source code NIRVANA was applied to perform the 2.5D magnetohydrodynamic (MHD) simulation. We developed the related sub-codes to include the more realistic radiative cooling process for the photosphere and chromosphere and the time-dependent ionization degree of hydrogen. The initial background magnetic field is 600 G, and the emerged magnetic field in the solar atmosphere is of the same magnitude, meaning that it results in a low- β magnetic reconnection environment. We also used the radiative transfer code RH1.5D to synthesize the Si IV and Hα spectral line profiles based on the MHD simulation results. Results. Magnetic reconnection between emerged and background magnetic fields creates a thin, curved current sheet, which then leads to the formation of plasmoid instability and the nonuniform density distributions. Initially, the temperature is below 8000 K. As the current sheet becomes more vertical, denser plasmas are drained by gravity, and hotter plasmas above 20 000 K appear in regions with lower plasma density. The mix of hot tenuous and much cooler dense plasmas in the turbulent reconnection region can appear at about the same height, or even in the same plasmoid. Through the reconnection region, the synthesized Si IV emission intensity can reach above 106 erg s−1 sr−1 cm−2 Å−1 and the spectral line profile can be wider than 100 km s−1, the synthesized Hα line profile also show the similar characteristics of a typical EB. The turbulent current sheet is always in a dense plasma environment with an optical depth larger than 6.5 × 10−5 due to the emerged magnetic field pushing high-density plasmas upward. Conclusions. Our simulation results indicate that the cold EB and hot UV burst can both appear in the same reconnection process in the low chromosphere, the EB can either appear several minutes earlier than the UV burst, or they can simultaneously appear at the similar altitude in a turbulent reconnection region below the middle chromosphere.

The persistence of high altitude non-equilibrium diffuse ionized gas in simulations of star-forming galaxies

ABSTRACT Widespread, high altitude, diffuse ionized gas with scale heights of around a kiloparsec is observed in the Milky Way and other star-forming galaxies. Numerical radiation-magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium show that gas can be driven to high altitudes above the galactic mid-plane, but the degree of ionization is often less than inferred from observations. For computational expediency, ionizing radiation from massive stars is often included as a post-processing step assuming ionization equilibrium. We extend our simulations of an Milky Way-like interstellar medium to include the combined effect of supernovae and photoionization feedback from mid-plane OB stars and a population of hot evolved low mass stars. The diffuse ionized gas has densities below 0.1 ${\rm \,cm^{-3}}$, so recombination time-scales can exceed millions of years. Our simulations now follow the time-dependent ionization and recombination of low density gas. The long recombination time-scales result in diffuse ionized gas that persists at large altitudes long after the deaths of massive stars that produce the vast majority of the ionized gas. The diffuse ionized gas does not exhibit the large variability inherent in simulations that adopt ionization equilibrium. The vertical distribution of neutral and ionized gas is close to what is observed in the Milky Way. The volume filling factor of ionized gas increases with altitude resulting in the scale height of free electrons being larger than that inferred from H $\alpha$ emission, thus reconciling the observations of ionized gas made in H $\alpha$ and from pulsar dispersion measurements.

Impact of chlorine ion chemistry on ozone loss in the middle atmosphere during very large solar proton events

Abstract. Solar coronal mass ejections can accelerate charged particles, mostly protons, to high energies, causing solar proton events (SPEs). Such energetic particles can precipitate upon the Earth's atmosphere, mostly in polar regions because of geomagnetic shielding. Here, SPE-induced chlorine activation due to ion chemistry can occur, and the activated chlorine depletes ozone in the polar middle atmosphere. We use the state-of-the-art 1D stacked-box Exoplanetary Terrestrial Ion Chemistry (ExoTIC) model of atmospheric ion and neutral composition to investigate such events in the Northern Hemisphere (NH). The Halloween SPE that occurred in late October 2003 is used as a test field for our study. This event has been extensively studied before using different 3D models and satellite observations. Our main purpose is to use such a large event that has been recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Environmental Satellite (ENVISAT) to evaluate the performance of the ion chemistry model. Sensitivity tests were carried out for different model settings with a focus on the chlorine species of HOCl and ClONO2 as well as O3 and reactive nitrogen, NOy. The model simulations were performed in the Northern Hemisphere at a high latitude of 67.5∘ N, inside the polar cap. Comparison of the simulated effects against MIPAS observations for the Halloween SPE revealed rather good temporal agreement, also in terms of altitude range for HOCl, O3 and NOy. For ClONO2, good agreement was found in terms of altitude range. The model showed ClONO2 enhancements after the peak of the event. The best model setting was the one with full ion chemistry where O(1D) was set to photo-chemical equilibrium. HOCl and ozone changes are very well reproduced by the model, especially for nighttime. HOCl was found to be the main active chlorine species under nighttime conditions, resulting in an increase of more than 0.2 ppbv. Further, ClONO2 enhancements of 0.2–0.3 ppbv have been observed during both daytime and nighttime. Model settings that compared best with MIPAS observations were applied to an extreme solar event that occurred in AD 775, presumably once in a 1000-year event. With the model applied to this scenario, an assessment can be made about what is to be expected at worst for the effects of a SPE on the middle atmosphere, concentrating on the effects of ion chemistry compared to crude parameterizations. Here, a systematic analysis comparing the impact of the Halloween SPE and the extreme event on the Earth's middle atmosphere is presented. As seen from the model simulations, both events were able to perturb the polar stratosphere and mesosphere with a high production of NOy and HOx. Longer-lasting and stronger stratospheric ozone loss was seen for the extreme event. A qualitative difference between the two events and a long-lasting impact on HOCl and HCl for the extreme event were found. Chlorine ion chemistry contributed to stratospheric ozone losses of 2.4 % for daytime and 10 % for nighttime during the Halloween SPE, as seen with time-dependent ionization rates applied to the model. Furthermore, while comparing the Halloween SPE and the extreme scenario, with ionization rate profiles applied just for the event day, the inclusion of chlorine ion chemistry added ozone losses of 10 % and 20 % respectively.

Dicarboxylic Acids Induced Tandem Transformation of Silver Nanocluster.

Structural transformation of metal nanoclusters (NCs) is of great ongoing interest regarding their synthesis, stability, and reactivity. Although sporadic examples of cluster transformations have been reported, neither the underlying transformation mechanism nor the intermediates are unambiguous. Herein, we have synthesized a flexible 54-nuclei silver cluster (Ag54) by combining soft (tBuC≡C-) and hard (nPrCOO-) ligands. The existence of weakly coordinated nPrCOO- enhances the reactivity of Ag54, thus facilitating the dicarboxylic acid to induce structural transformation. X-ray structural analyses reveal that Ag54 transforms to Ag28 cluster-based 2D networks (Ag28a and Ag28b) induced by H2suc (succinic acid) and H2glu (glutaric acid), whereas with H2pda (2,2'-(1,2-phenylene)diacetic acid), a discrete Ag28 cluster (Ag28c) is isolated. The key intermediate Ag17 that emerges during the self-dissociation of Ag54 was isolated by using cryogenic recrystallization and characterized by X-ray crystallography. The "tandem transformation" mechanism for the structure evolution from Ag54 to Ag28a is established by time-dependent electrospray ionization mass spectrometry (ESI-MS) and UV-vis spectroscopy. In addition, the catalytic activity in the 4-nitrophenol reduction follows the sequence Ag28c > Ag28b > Ag28a > Ag54 due to more bare silver sites on the surface of the Ag28 cluster unit. Our findings not only open new avenues to the synthesis of silver NCs but also shed light on a better understanding of the structural transformation mechanism from one cluster to another or cluster-based metal-organic networks induced by dicarboxylates.

Investigating the Impact of Vertically Extended Coronae on X-Ray Reverberation Mapping

Accreting black holes commonly exhibit hard X-ray emission, originating from a region of hot plasma near the central engine referred to as the corona. The origin and geometry of the corona are poorly understood, and models invoking either inflowing or outflowing material (or both) can successfully explain only parts of the observed phenomenology. In particular, recent works indicate that the time-averaged and variability property might originate in different regions of the corona. In this paper we present a model designed to move beyond the lamppost paradigm, with the goal of accounting for the vertical extent of the corona. In particular, we highlight the impact of including self-consistently a second lamppost, mimicking, for example, an extended jet base. We fully include the effect that the second source has on the time-dependent disk ionization, reflection spectrum, and reverberation lags. We also present an application of this new model to NICER observations of the X-ray binary MAXI J1820+070 near its hard-to-soft state transition. We demonstrate that in these observations, a vertically extended corona can capture both spectral and timing properties, while a single-lamppost model can not. In this scenario, the illumination responsible for the time-averaged spectrum originates close to the black hole, while the variability is likely associated with the ballistic jet.

Hydrazine-Halogen Exchange Strategy Toward N═N-Containing Compounds and Process Tracking for Mechanistic Insight.

An anhydride-promoted traceless hydrazine-I/Br exchange strategy is reported, where hydrazine hydrate and cyclic/linear iodonium, including rarely explored cyclic bromonium, are converted to benzo[c]cinnolines/azobenzenes in one pot. The reaction proceeds through diacylation (first and second C─N formation), N,N'-diarylation (third and fourth C─N formation), and deacylation/oxidation (2 C─N cleavages and 1 N═N formation). The reaction mechanism is investigated by isolating multiple intermediates and kinetic studies. Furthermore, time-dependent electrospray ionization mass spectrometry (TD ESI-MS) was applied to track the process by detecting most intermediates. The complex [CuIII(iodobiphenyl)(bipy)I]+ (Int-C) was detected for the first time, giving evidence for oxidative addition of cyclic iodonium to Cu catalyst. Another complex [CuI(PHA)(bipy)] (Int-B) via ligand-exchange between the hydrazide and Cu catalyst was also detected, indicating a two-path initial activation process.

A benchmark study of atomic models for the transition region against quiet Sun observations

ABSTRACT The use of the coronal approximation to model line emission from the solar transition region has led to discrepancies with observations over many years, particularly for Li- and Na-like ions. Studies have shown that a number of atomic processes are required to improve the modelling for this region, including the effects of high densities, solar radiation, and charge transfer on ion formation. Other non-equilibrium processes, such as time-dependent ionization and radiative transfer, are also expected to play a role. A set of models which include the three relevant atomic processes listed above in ionization equilibrium has recently been built. These new results cover the main elements observed in the transition region. To assess the effectiveness of the results, this work predicts spectral line intensities using differential emission measure modelling. Although limited in some respects, this differential emission measure modelling does give a good indication of the impact of the new atomic calculations. The results are compared to predictions of the coronal approximation and to observations of the average, quiet Sun from published literature. Significant improvements are seen for the line emission from Li- and Na-like ions, intercombination lines, and many other lines. From this study, an assessment is made of how far down into the solar atmosphere the coronal approximation can be applied, and the range over which the new atomic models are valid.

Numerical Studies of Magnetic Reconnection and Heating Mechanisms for the Ellerman Bomb

An Ellerman Bomb (EB) is a kind of small scale reconnection event, which is ubiquitously formed in the upper photosphere or the lower chromosphere. The low temperature (<10,000 K) and high density (∼1019–1022) plasma there makes the magnetic reconnection process strongly influenced by partially ionized effects and radiative cooling. This work studies the high β magnetic reconnection near the solar temperature minimum region based on high-resolution 2.5D magnetohydrodynamics simulations. The time-dependent ionization degree of hydrogen and helium are included to realize more realistic diffusivities, viscosity and radiative cooling in simulations. Numerical results show that the reconnection rate is smaller than 0.01 and decreases with time during the early quasi-steady stage, then sharply increases to a value above 0.05 in the later stage as the plasmoid instability takes place. Both the large value of η en (magnetic diffusion caused by the electron-neutral collision) and the plasmoid instability contribute to the fast magnetic reconnection in the EB-like event. The interactions and coalescence of plasmoids strongly enhance the local compression heating effect, which becomes the dominant mechanism for heating in EBs after plasmoid instability appears. However, the Joule heating contributed by η en can play a major role to heat plasmas when the magnetic reconnection in EBs is during the quasi-steady stage with smaller temperature increases. The results also show that the radiative cooling effect suppresses the temperature increase to a reasonable range, and increases the reconnection rate and generation of thermal energy.

Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion under Nonequilibrium Conditions

The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study, we tackle this old problem by using a very-high-spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and interface region imaging spectrograph (IRIS) observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent nonequilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and nonequilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg ii profiles from these high-spatial-resolution models, with and without ambipolar diffusion and NEQI, and quiet-Sun and coronal hole observations from IRIS now reveal a slightly better correspondence. The intensity of profiles is increased, and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg ii profiles are closer to those observed than in previous models, though some differences still remain.

Stepwise formation of a chemodynamic therapy agent of {Cu8} macrocyclic complex recognized by iodide ions.

An atomically precise Cu(I) macrocyclic complex Cu8I was developed for chemodynamic therapy (CDT) research. The {Cu8} macrocyclic skeleton gradually forms with the selective recognition of iodide ions, and the monitoring of intermediate fragments during Cu8I formation using time-dependent electrospray ionization mass spectrometry indicates the following possible formation process: [Cu1] → [Cu2] → [Cu3] → [Cu4] → [Cu5I] → [Cu6I] → [Cu7I] → [Cu8I] when recognized by iodide ions. Furthermore, the Cu(I)-mediated Fenton-like reaction in Cu8I catalyzes the production of toxic ˙OH from H2O2, which results in efficient tumor suppression.

TPHO: A Time-dependent Photoionization Model for AGN Outflows* *Released on ...

Outflows in active galactic nuclei (AGN) are considered a promising candidate for driving AGN feedback at large scales. However, without information on the density of these outflows we cannot determine how much kinetic power they are imparting to the surrounding medium. Monitoring the response of the ionization state of the absorbing outflows to changes in the ionizing continuum provides the recombination timescale of the outflow, which is a function of the electron density. We have developed a new self-consistent time-dependent photoionization model, tpho, enabling the measurement of the plasma density through time-resolved X-ray spectroscopy. The algorithm solves the full time-dependent energy and ionization balance equations in a self-consistent fashion for all the ionic species. The model can therefore reproduce the time-dependent absorption spectrum of ionized outflows responding to changes in the ionizing radiation of the AGN. We find that when the ionized gas is in a nonequilibrium state its transmitted spectra are not accurately reproduced by standard photoionization models. Our simulations with the current X-ray grating observations show that the spectral features identified as multicomponent warm absorbers, might in fact be features of a time-changing warm absorber and not distinctive components. The tpho model facilitates accurate photoionization modeling in the presence of a variable ionizing source, thus providing constraints on the density and in turn the location of the AGN outflows. Ascertaining these two parameters will provide important insight into the role and impact of ionized outflows in AGN feedback.

Ionization dynamics and gauge invariance

Photoionization is one of the most fundamental processes in laser-matter interaction. It plays a crucial role also from the practical point of view since the electron-density dynamics in a gaseous medium is a process that affects the field propagation. Photoionization has been addressed in different regimes: linear, multiphoton, and tunneling. The ideal tool that allows for the description of ionization, in all aforementioned regimes, is the numerical evaluation of the time-dependent Schr\"odinger equation. The determination of the electron density needs the computation of the time-dependent ionization probability which unfortunately is an ambiguous quantity due to the gauge dependence of the latter. In this paper, we show how to overcome this difficulty by properly defining the time-dependent ionization probability in the context of the resolvent operator method. We show in particular that the velocity gauge allows for a definition of adiabatic states that is suitable to define an ionization threshold at all times during the interaction to compute ionization probability. Applications to linear, multiphoton, and tunneling regimes are presented for the one-dimensional problem. The extension to the nondipole case is discussed and we show that time-dependent ionization probability cannot be defined unambiguously due to the introduction of the magnetic-field component. We also discuss the case of gauge invariance in a subspace of the eigenbasis defined by the Hamiltonian.

On the Formation of Over-ionized Plasma in Evolved Supernova Remnants

One of the outstanding mysteries surrounding the rich diversity found in supernova remnants (SNRs) is the recent discovery of over-ionized or recombining plasma from a number of dynamically evolved objects. To help decipher its formation mechanism, we have developed a new simulation framework capable of modeling the time evolution of the ionization state of the plasma in an SNR. The platform is based on a one-dimensional hydrodynamics code coupled to a fully time-dependent nonequilibrium ionization calculation, accompanied by a spectral synthesis code to generate space-resolved broadband X-ray spectra for SNRs at arbitrary ages. We perform a comprehensive parametric survey to investigate the effects of different circumstellar environments on the ionization state evolution in SNRs up to a few 104 yr. A two-dimensional parameter space, spanned by arrays of interstellar medium (ISM) densities and mass-loss rates of the progenitor, is used to create a grid of models for the surrounding environment, in which a core-collapse explosion is triggered. Our results show that a recombining plasma can be successfully reproduced in the case of a young SNR (a few 100 to 1000 yr old) expanding fast in a spatially extended low-density wind, an old SNR (>a few 1000 yr) expanding in a dense ISM, or an old SNR broken out from a confined dense wind region into a tenuous ISM. Finally, our models are confronted with observations of evolved SNRs, and an overall good agreement is found except for a couple of outliers.

Plausibility of ultraviolet burst generation in the low solar chromosphere

Context. Ultraviolet (UV) bursts and Ellerman bombs (EBs) are small-scale magnetic reconnection events taking place in the highly stratified, low solar atmosphere. The plasma density, reconnection mechanisms, radiative cooling and transfer processes clearly differ from one layer of the atmosphere to the next. In particular, EBs are believed to form in the upper photosphere or the low chromosphere. It is still not clear whether UV bursts have to be generated at a higher atmospheric layer than the EBs or whether both UV bursts and EBs can occur in the low chromosphere. Aims. We numerically studied the low β magnetic reconnection process around the solar temperature minimum region (TMR) by including more realistic physical diffusions and radiative cooling models. We aim to find out whether UV bursts may occur in the low chromosphere and to investigate the dominant mechanism that accounts for heating in the UV burst in the chromosphere. Methods. We used the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform the simulations. The time-dependent ionization degrees of hydrogen and helium are included in the code, which lead to a more realistic magnetic diffusion caused by electron-neutral collision and ambipolar diffusion. A more realistic radiative cooling model is also included in the simulations. The initial mass density and temperature are 1.66057 × 10−6 kg m−3 and 4400 K, respectively, values that are typical for the plasma environment around TMR. Results. Our results in high resolution indicate that the plasmas in the reconnection region are heated up to more than 20 000 K if the reconnecting magnetic field is as strong as 500 G, which suggests that UV bursts can be generated in the dense low chromosphere. The dominant mechanism for producing the UV burst in the low chromosphere is heating, as a result of the local compression in the reconnection process. The thermal energy occurring in the reconnection region rapidly increases after the turbulent reconnection mediated by plasmoids is invoked. The average power density of the generated thermal energy in the reconnection region can reach over 1000 erg cm−3 s−1, which is comparable to the average power density accounting for a UV burst. With the strength of the reconnecting magnetic field exceeding 900 G, the width of the synthesized Si IV 1394 Å line profile with multiple peaks can reach up to 100 km s−1, which is consistent with observations.

Vibrational wave packet dynamics of H2O+ and H2O by strong-field Fourier transform spectroscopy

The vibrational wave packet dynamics of H2O+ and H2O are investigated by pump–probe measurements using few-cycle intense laser pulses. By a Fourier transform (FT) of the time-dependent yields of the two-body Coulomb explosion pathway, H2O2+ → H+ + OH+, and the parent ion, H2O+, the vibrational excitation processes in the electronic ground states of H2O+ and H2O induced by the intense laser field are retrieved from the phases of the respective vibrational peaks in the FT spectrum. Accurate quantum-dynamics simulations on H2O+ confirm both the peak assignment and the origin of the time-dependent ionization yields of the vibrational wave packet.

The Dynamic Evolution of Solar Wind Streams Following Interchange Reconnection

Interchange reconnection is thought to play an important role in determining the dynamics and material composition of the slow solar wind that originates from near coronal-hole boundaries. To explore the implications of this process we simulate the dynamic evolution of a solar wind stream along a newly-opened magnetic flux tube. The initial condition is composed of a piecewise continuous dynamic equilibrium in which the regions above and below the reconnection site are extracted from steady-state solutions along open and closed field lines. The initial discontinuity at the reconnection site is highly unstable and evolves as a Riemann problem, decomposing into an outward-propagating shock and inward-propagating rarefaction that eventually develop into a classic N-wave configuration. This configuration ultimately propagates into the heliosphere as a coherent structure and the entire system eventually settles to a quasi-steady wind solution. In addition to simulating the fluid evolution we also calculate the time-dependent non-equilibrium ionization of oxygen in real time in order to construct in situ diagnostics of the conditions near the reconnection site. This idealized description of the plasma dynamics along a newly-opened magnetic field line provides a baseline for predicting and interpreting the implications of interchange reconnection for the slow solar wind. Notably, the density and velocity within the expanding N-wave are generally enhanced over the ambient wind, as is the O7+/O6+ ionization ratio, which exhibits a discontinuity across the reconnection site that is transported by the flow and arrives later than the propagating N-wave.

Alignment dependence of photoelectron angular distributions in the few-photon ionization of molecules by ultraviolet pulses

We probe the time-dependent ionization dynamics of impulsively excited rotational wave packets of ${\mathrm{N}}_{2}, {\mathrm{CO}}_{2}$, and ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ using broadband ultraviolet pulses centered at 262 nm. Photoelectron momentum distributions recorded by velocity-map imaging show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that substantial information about the molecular-frame photoelectron angular distribution can be obtained from the high-order laboratory-frame asymmetry parameters without any prior knowledge of the photoionization process. We also compare few-photon ionization with one-photon ionization and strong-field ionization.

Evolution of the frequency-comb structure and coherence from a Keldysh multiphoton into a tunneling regime.

We present a theoretical study of the characteristics of the frequency-comb structure and coherence via high-order harmonic generation (HHG) driven by the laser pulse trains when the ionization process is pushed from Keldysh multiphoton into tunneling regime. HHG is obtained by solving accurately the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We find that the nested comb structures are formed from each harmonic order in the Keldysh multiphoton ionization regime. But it is severely suppressed or even disappeared in the Keldysh tunneling ionization regime. It implies that the temporal coherence of the emitted frequency comb modes is very sensitive to the Keldysh ionization regime. To understand the evolution of frequency-comb structure and coherence, we perform the calculation of the time-dependent ionization probability and the spectral phase of frequency-comb HHG. We find that the frequency-comb HHG driven by the laser pulse trains in the Keldysh multiphoton regime has a good coherence because the ionization probability of the atom driven by each laser pulse is stable, leading to a phase-coherent frequency-comb structure rather than those cases in the Keldysh tunneling regime with high laser intensity. Our results shed light on current interest and significance to the experimental realization of controllable and frequency-comb vacuum-ultraviolet light sources.

Non-equilibrium Photoionization and Hydrodynamic Simulations of Starburst-driven Outflows

Starburst-driven galactic outflows in star-forming galaxies have been observed to contain complex thermal structures and emission line features that are difficult to explain by adiabatic fluid models and plasmas in photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE). We previously performed hydrodynamic simulations of starburst-driven outflows using the MAIHEM module for non-equilibrium atomic chemistry and radiative cooling functions in the hydrodynamics code FLASH, and calculated emission lines in combined CIE and PIE conditions. In the present study, we consider time-dependent non-equilibrium ionization (NEI) states produced by our MAIHEM simulations. Through extensive CLOUDY calculations made with the NEI states from our hydrodynamic simulations, we predict the UV and optical emission line profiles for starburst-driven outflows in time-evolving non-equilibrium photoionization conditions. Our hydrodynamic results demonstrate applications of non-equilibrium radiative cooling for H II regions in starburst galaxies hosting cool outflows.

Numerical Modeling of Galactic Superwinds with Time-evolving Stellar Feedback

AbstractMass-loss and radiation feedback from evolving massive stars produce galactic-scale superwinds, sometimes surrounded by pressure-driven bubbles. Using the time-dependent stellar population typically seen in star-forming regions, we conduct hydrodynamic simulations of a starburst-driven superwind model coupled with radiative efficiency rates to investigate the formation of radiative cooling superwinds and bubbles. Our numerical simulations depict the parameter space where radiative cooling superwinds with or without bubbles occur. Moreover, we employ the physical properties and time-dependent ionization states to predict emission line profiles under the assumption of collisional ionization and non-equilibrium ionization caused by wind thermal feedback in addition to photoionization created by the radiation background. We see the dependence of non-equilibrium ionization structures on the time-evolving ionizing source, leading to a deviation from collisional ionization in radiative cooling wind regions over time.