Observed Ionization Rate Research Articles

The Astrochemical Impact of Cosmic Rays in Protoclusters. I. Molecular Cloud Chemistry

Abstract We present astrochemical photodissociation region models in which cosmic-ray (CR) attenuation has been fully coupled to the chemical evolution of the gas. We model the astrochemical impact of CRs, including those accelerated by protostellar accretion shocks, on molecular clouds hosting protoclusters. Our models with embedded protostars reproduce observed ionization rates. We study the imprint of CR attenuation on ions for models with different surface CR spectra and different star formation efficiencies. We find that abundances, particularly ions, are sensitive to the treatment of CRs. We show the column densities of ions are underpredicted by the “classic” treatment of CRs by an order of magnitude. We also test two common chemistry approximations used to infer ionization rates. We conclude that the approximation based on the abundance underpredicts the ionization rate, except in regions where the CRs dominate the chemistry. Our models suggest the chemistry in dense gas will be significantly impacted by the increased ionization rates, leading to a reduction in molecules such as NH3 and causing H2-rich gas to become [C ii] bright.

Importance of thermal reactivity for hexamethylenetetramine formation from simulated interstellar ices

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which, added to the standard cosmic ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear \gamma-ray line emission in the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent \gamma-ray cross section compilation for nuclear collisions, \gamma-ray line emission spectra are calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV. Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL, but a detection, especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray telescopes based on available technology. We predict also strong \gamma-ray continuum emission in the 1-8 MeV range, which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes.

DE-EXCITATION NUCLEAR GAMMA-RAY LINE EMISSION FROM LOW-ENERGY COSMIC RAYS IN THE INNER GALAXY

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated, which, added to the standard cosmic-ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV nucleon{sup -1} and several hundred MeV nucleon{sup -1} in the interstellar medium would be a detection of nuclear {gamma}-ray line emission in the range E {sub {gamma}} {approx} 0.1-10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent {gamma}-ray cross section compilation for nuclear collisions, {gamma}-ray line emission spectra are calculated alongside the high-energy {gamma}-ray emission due to {pi}{sup 0} decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E {sub {gamma}} = 0.1 GeV. Our predicted fluxes of strong nuclear {gamma}-ray lines from the inner Galaxy are well below the detection sensitivities of the International Gamma-Ray Astrophysics Laboratory, but a detection, especially of the 4.4 MeV line, seems possible with new-generationmore » {gamma}-ray telescopes based on available technology. We also predict strong {gamma}-ray continuum emission in the 1-8 MeV range, which, in a large part of our model space for low-energy cosmic rays, considerably exceeds the estimated instrument sensitivities of future telescopes.« less

The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >= 5: evidence for a photon-starved and extended epoch of reionization

Galaxies and quasars are thought to provide the bulk of the photons responsible for ionizing the hydrogen in the intergalactic medium (IGM). We use a large set of hydrodynamical simulations, combined with measurements of the Lya opacity of the IGM taken from the literature, to obtain robust estimates of the photoionization rate per hydrogen atom at z = 5 and 6. We find the photoionization rate drops by a factor of 2 and 4, respectively, compared to our recent measurements at z= 2-4. The number of ionizing photons emitted by known sources at z = 5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 A, are in reasonable agreement with the photoionization rates inferred from the Lyα forest if the escape fraction of ionizing photons from galaxies is large (≥20 per cent). The expected number of ionizing photons from observed sources at these redshifts therefore appears sufficient to maintain the IGM in its highly ionized state. Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z = 6. This strongly suggests that the epoch of reionization was photon-starved and extended. Completion of reionization at or before z = 6 requires either an emissivity which rises towards higher redshifts or one which remains constant but is dominated by sources with a rather hard spectral index. For standard assumptions, the ionizing emissivity required for completion of reionization at or before z = 6 lies at the upper end of recently reported values from searches for high-redshift galaxies at z = 8-10.

Modeling quantum dynamics of photodetachment from closed-shell anions: Static versus fluctuating well-depth models

The electronic states of halide ions are modeled by a one-dimensional Hamiltonian with a potential V(x)=−V0e. The two parameters V0 and σ are fixed by requiring V(x) to reproduce the experimentally observed ground-state ionization potentials of the halide ions concerned. The potentials so generated are shown to support only one bound state in each case. The time-dependent Fourier grid Hamiltonian method is used to follow the ionization dynamics in monochromatic light of fairly high intensities. The total Hamiltonian, in the most general case, reads H(t)=P/2m−V0e−ϵ0s(t)ex sin(ωt). For pulsed fields [s(t)=sin2(πt/tp)], the computed ionization rate constants are seen to increase with increase in the peak intensity (ϵ0) of the electric field of light. The possibility of additional transient bound states being generated at the high intensities of light and its possible consequences on the observed ionization rates are explored. The environmental effects on the dynamics are sought to be modeled by allowing the well depth (V0) to fluctuate randomly [V0(t)=V0+ΔVR(t); R(t) randomly fluctuates between +1 and −1 with time, ΔV is fixed]. The ionization rate constants (kϵ) are shown to be significantly affected by fluctuations in V0 and pass through a well-defined minimum in each case for a certain specified frequency of fluctuation. An alternative model potential V(x)=−V0e−σx is also shown to yield similar results. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 469–478, 1999

Cosmic ray ionization of the interstellar medium

We consider a cosmic ray spectrum that is a power law in momentum down to a cutoff and derive a lower cutoff corresponding to $E_{kin} \sim (30-60)$ MeV from the observed ionization rates in nearby diffuse clouds. While the real spectra of cosmic rays may not be so simple, we argue that one expects a substantial change in the spectra at such energies and that, at first approximation, a power law spectra with a lower cutoff is appropriate. Such a description of the cosmic rays in the interstellar medium is not only theoretically more attractive than the spectra used in the literature, but is also supported by recent observations.

Laser modification of ultracold atomic collisions: Theory.

Specific molecular mechanisms are proposed for associative ionization collisions of ultracold sodium atoms in a hybrid optical trap. When an intense, strongly detuned optical trap laser is on, the ionization rate is modulated by molecular bound-state resonances which are strongly affected by field dressing. When the weak, slightly detuned optical molasses lasers are on to provide cooling, an excitation mechanism which produces the two excited atoms at large internuclear separation in different hyperfine states accounts for the lower observed ionization rate.

Radio continuum observations of IRAS sources associated with dense cores

view Abstract Citations (61) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Continuum Observations of IRAS Sources Associated with Dense Cores Rodriguez, L. F. ; Myers, P. C. ; Cruz-Gonzalez, I. ; Terebey, S. Abstract Using the VLA, the continuum emission associated with IRAS sources embedded in dense cores was searched for at 6 cm. Two new sources, 04016 + 2610 in L1489 and 19243 + 2350 in L778, were detected. The embedded sources have low luminosity (L less than about 10 solar luminosities). As in other similar sources, if the emission is free-free, the stellar radiation cannot account for the observed ionization rate. Two models that have been proposed to explain the radio continuum emission from low-luminosity stars are those of (partially) ionized winds and shock-induced ionization. For the two new sources and a previously reported one (HL Tau), the associated molecular outflow is of modest power. It is shown that, under these conditions, these models, at least in their simplest versions, do not provide sufficient ionized gas. The nature of the radio continuum emission from low-luminosity young stars remains unexplained. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/168134 Bibcode: 1989ApJ...347..461R Keywords: Early Stars; Infrared Sources (Astronomy); Radio Emission; Star Formation; Infrared Astronomy Satellite; Stellar Luminosity; Stellar Radiation; Stellar Winds; Very Large Array (Vla); Astrophysics; INFRARED: SOURCES; RADIATION MECHANISMS; STARS: FORMATION; STARS: RADIO RADIATION full text sources ADS | data products SIMBAD (12)

Ionization in a dense hydrogen plasma: analytic solution of the master equation

The master equation is solved analytically for the ionization in a dense hydrogen plasma involving single-quantum transitions. The derived expression for the observed ionization rate coefficient is valid for all temperatures at which the ionization time constant is long compared with the internal relaxation time constants. At high temperature, the observed ionization rate coefficient is determined by excitation and ionization from the ground level. At low temperature, a bottleneck occurs above the first excited state, and the expression reduces to a form foreseen by Bates as the counterpart to the network-like expression for recombination. The implications with regard to the temperature dependence of the rate coefficient are discussed.

Chemi-ionization induced by cyanogen in carbon monoxide-oxygen-nitrogen flames

The primary and secondary chemi-ionization processes in the burning of cyanogen were investigated by means of a “trace technique.” To a stoichiometric carbon monoxide-oxygen-nitrogen flame, characterized by a complete absence of ions, small and varying amounts (up to 2%) of cyanogen were added. The seeding did not significantly affect the main flame parameters, such as the burning velocity and the final temperature (±2500°K). Using a saturation-current method, the chemi-ionization rate was found to be directly proportional to the initial cyanogen content; the ion yield per cyanogen molecule amounts to 10−5. A mass-spectrometric study revealed that NO+ is by far the most abundant ion; in a flame with an initial cyanogen content of 2%, [NO+]max is 2×1011 ions cm−3. The “order” of each ionic species with respect to the added cyanogen was determined from the dependence of the maximum ion concentrations on the initial cyanogen content. On the basis of these “orders” and of the ionization rate and ion concentration data, it is concluded that NO+ is the major primary ion, formed by the reaction CN+2O←CO_NO++e−. The rate constant is of the order of 10−33 cm6 molecule−2 sec−1. The NO+ ions disappear mainly by dissociative recombination with electrons; the recombination coefficient was equal to 1.6×10−7 cm3 sec−1. Another primary ion, CO+, is formed by CN*(B2Σ)+2O←NO+CO++e−. This reaction accounts only for a small fraction of the total observed ionization rate. However CO+ is responsible for the production of the secondary ions: O2+, CO2+, NO2+, and O3+. A set of reactions, fitting the observed “order” dependences is proposed for the formation and destruction of most secondary ions.

Recombination, Ionization, and Nonequilibrium Electrical Conductivity in Seeded Plasmas

New data are presented which provide direct experimental confirmation of the validity of a physical model which has been widely employed to predict the electrical conductivity of dense, two-temperature, seeded plasmas. Experimental measurements of electron temperature, and ionization and recombination rates are presented for partially ionized plasmas of potassium-seeded argon. Experimental conditions were chosen to cover those ranges of interest in connection with proposed magnetohydrodynamic energy conversion devices for which nonequilibrium electrical conductivity measurements have been previously reported, e.g., translational atom temperatures of about 2000°K, total atom densities near 1018/cm3, potassium densities of about 1016/cm3, electron densities from 1013/cm3 to 1015/cm3, and electron temperatures from 2200 to 3500°K. Measured values of electron-electron-ion recombination coefficients for potassium show good agreement with theoretical values based upon the Gryzinski classical inelastic-collision cross-section expressions. Observed ionization rates and relaxation characteristics appear to be adequately explained by a similar formulation for the ionization process.

Investigation of Early Ionization Processes in Shocked Xenon

Measurements of ionization rates in noble gases have been of considerable interest, with much of the recent experimental work performed at very high temperatures with shock-tube techniques. As a result, there have been several conflicting theories formulated to trace the history of ionization phenomena. A major difficulty has been the problem of precisely identifying the earliest stage of the ionization process. Since the probability of atom—atom collisions leading to ionization is small, another source of energy sufficient to promote the necessary early stage of the process must be provided. It appears that under conditions of relatively low temperatures, as in weak shock waves, the assumption of a photoexcitation mechanism of the gas is plausible. Biberman and Veklenko have suggested such a mechanism for the production of excited states of rare gases in shock waves. This mechanism has been used to explain the initial shock-front ionization. The nature of the processes responsible for visible emission at the shock front, which has been designated as impurity radiation, appears to be a result of the transfer of energy from the excited rare-gas atoms to the impurity molecule. Spectral observations of the shock zone in xenon have been made in a carefully outgassed shock tube (10—6 mm Hg). The absorption spectrum is typical of molecular fragments of carbon (C2) which has been shown earlier to be localized in the region of 4500 to 4960 Å. Calcium or sodium, usual alkali impurities, were not observed in any measurement. The emission appeared to be predominantly from the xenon and possible carbon impurities. Microwave absorption measurements were made normal to the shock front. By use of a plasma model which assumes a linear varying electron density, microwave attenuation in the region of cutoff was measured to determine the rate of ionization and the plasma collision frequency. The observed ionization rate was of second order with respect to xenon pressure for the temperature range of 4000° to 9000°K. The observed activation energy was 1.60 eV±0.2. This was the approximate activation energy for the following reactions in xenon: Xe*+Xe⇒Xe2++e−or Xe*+C2⇒XeC2++e−}collision of the second kind.The experimentally determined rate expression for ionization is given as ki=3.58×1014T12exp[−1.6±0.2/kT]cm3moles−1sec−1.Radiation at the shock front appears to be best described by the excitation of impurity or by the dissociation of Xe2 as suggested by Tanaka: Xe*+C2 ⇒Xe+C2*,collision of second kind,C2*(A3π) ⇒C2(X3π)+hν,radiative transition,XeC2++e−⇒XeC2*⇒XeC2+hνXe2++e− ⇒Xe2* ⇒Xe2+hν}dissociated recombination (visible emission).

Kinetics of Thermal Ionization. II. Xenon and Krypton

The rate and mechanism of the thermal ionization of xenon has been further studied by a microwave and shock-tube method. The negligible effect of impurities or background on the observed ionization rates was demonstrated. For a variety of added gases, the activation energy for the observed ionization was about 8 eV, not 12.1 eV, the ionization energy. Detailed time studies show an induction period for buildup of free electrons as a result of populating intermediate electronic states of xenon; however, the simple three-step mechanism proposed previously (Xe+M⇋Xe*+M,Xe*+M→Xe++e−+M) was shown to be insufficient to explain certain quantitative features of the data. At these high temperatures a large number of parallel paths are possible, and in general, entropy differences are more important than energy differences. On this basis it is argued that the final ionization process leads to Xe++e—+M and not MXe++e—. Limited data are presented on the ionization of krypton, and an activation energy of about 10 eV, not 14 eV (the ionization energy), is indicated. Reaction cross sections are given for xenon and krypton.

Probe Studies of Energy Distributions and Radial Potential Variations in a Low Pressure Mercury Arc

Probe measurements were made in the plasma of a low pressure mercury arc. The electron-energy distributions showed depletions from a Maxwellian distribution in the high energy range. Coupling effects between adjacent probes were investigated and were found to be quite small but in the proper direction to agree with the Langmuir-Tonks theory. Drift-current distortion of the random electron-energy distributions was measured with a bidirectional probe and compared with theory. A multisection probe extending from tube axis to tube wall allowed a determination of radial potential and density variations. Results over a pressure range from 3.4 microns to 35 microns showed good agreement with the ambipolar diffusion theory based on cumulative ionization. A direct calculation of ionization rate in the plasma was made from the ionization probability for a one-step ionizing process; comparison of this calculation with the observed ionization rate at 1.7 microns indicated that at that pressure the ionization is half direct, half cumulative. For higher arc pressures cumulative ionization evidently predominates.