Two-photon 2s ↔ 1s transitions during hydrogen recombination in the universe

Based on the standard cosmological model, we calculate the correction to the rate of two-photon 2s ? 1s transitions in the hydrogen atom under primordial hydrogen plasma recombination conditions that arises when the induced transitions under equilibrium background radiation with a blackbody spectrum and plasma recombination radiation are taken into account.     

Recombination dynamics of primordial hydrogen and helium (He I) in the universe

We calculated the ionization fraction for hydrogen and helium (He I) as a function of the redshift z by including the two-photon decays of high hydrogen and parahelium levels and the radiative transfer in the helium 2³P₁ ? 1¹S₀ intercombination line. We show that this yields corrections of no more than a few percent to the ionization fraction for hydrogen and speeds up significantly the recombination for helium compared to the recent works by Seager et al. (1999, 2000), in which these effects were disregarded.     

He II → He I recombination of primordial helium plasma including the effect of neutral hydrogen

The He II → He I recombination of primordial helium plasma (z ? 1500–3000) is considered in terms of the standard cosmological model. This process affects the formation of cosmic microwave background anisotropy and spectral distortions. We investigate the effect of neutral hydrogen on the He II → He I recombination kinetics with partial and complete redistributions of radiation in frequency in the He I resonance lines. We show that to properly compute the He II → He I recombination kinetics, it is important to take into account not only the wings in the absorption and emission profiles of the He I resonance lines, but also the mechanism of the redistribution of resonance photons in frequency. Thus, for example, the relative difference in the numbers of free electrons for the model using Doppler absorption and emission profiles and the model using a partial redistribution in frequency is 1–1.3% for the epoch z = 1770–1920. The relative difference in the numbers of free electrons for the model using a partial redistribution in frequency and the model using a complete redistribution in frequency is 1–3.8% for the epoch z = 1750–2350.     

GMRT and VLA observations at 49 cm and 20 cm of the HII region near <Emphasis Type="Italic">l</Emphasis> = 24.8°, <Emphasis Type="Italic">b</Emphasis> = 0.1°

We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M_⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm⁻³ at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V _lsr = 100 km s⁻¹. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.     

Primordial hydrogen recombination dynamics with recoil upon scattering in the Ly-<Emphasis Type="Italic">α</Emphasis> line

We show that including the recoil upon scattering in the Ly-α line can lead to a noticeable acceleration of the primordial hydrogen recombination. Thus, for example, for the ΛCDM model, the decrease in the degree of ionization exceeds 1% at redshifts z = 800–1050, reaching ≈1.3% at z = 900. The corresponding corrections to the calculated cosmic microwave background power spectra reach 1.1% and 1.7% for TT and EE spectra, respectively. The radiative transfer in these calculations was treated in the quasi-stationary approximation.We have also obtained numerical solutions (in the diffusion approximation) to the nonstationary problem of radiative transfer in the Ly-α line for a partial frequency redistribution with recoil. We trace the evolution of the local line profile and the relative number of uncompensated transitions from the 2p state to the 1s state. We show that including the nonstationarity of the Ly-α line radiative transfer can lead to an additional acceleration of the primordial hydrogen recombination.     

On the effect of collisional transitions on the cosmological hydrogen recombination spectrum

We study the effect of collisional 2s ? 2p transitions on the populations of the 2s and 2p states at the cosmological hydrogen recombination epoch and on the intensity of the recombination Hα line. Our calculations are based on a simple model hydrogen atom with four bound states (1s, 2s, 2p, and the n = 3 level with an equilibrium distribution in sublevels with different orbital quantum numbers l). We show that collisions do not lead to an equilibrium distribution in sublevels of the n = 2 level. The relative change in the cosmological Hα line intensity due to collisional transitions does not exceed 10^?3.     

A survey of recombination line emission from the galactic plane at 325 MHz

A survey of the H 272 α recombination line at 325 MHz has been made towards 53 directions in the galactic plane using the Ooty Radio Telescope (ORT). 34 of these directions correspond to well-known Hn regions, 12 to SNRs and 6 to ‘blank’ areas selected so that the 5 GHz continuum is a minimum over the telescope beam of 2ℴ x 6 arcmin. Observing procedure and spectra of 47 sources towards which lines are detected are presented. Hydrogen recombination lines have been detected towards all the observed directions havingl <40ℴ. Carbon recombination lines are identified in 12 of the directions. The hydrogen line intensities are found to correlate well with the total continuum intensity (which includes the nonthermal galactic background) indicating that most of the lines arise due to stimulated emission by the background radiation. A preliminary discussion on the nature of the line-emitting regions is also presented.     

The Radio Recombination Line Intensities Of Atoms And Ions: Helium,Carbon And Oxygen

The radio recombination line intensities of heavy elements of helium, carbon and oxygen are calculated with accounting for dielectronic recombination. Dielectronic recombination rates are determined accurate to the second order of a perturbation theory and the rates are described as function of principal quantum number for helium-like atom or ion. Balance equations are solved for the departure coefficients from LTE b_n. The collision and spontaneous transition rates are accounted for the balance equations, in which non-equilibrium distribution source is dielectronic recombination. Non-equilibrium amplification coefficients are found as functions of a medium temperature, density and ion charge z = 1–3 for radio recombination lines. Optical depths are calculated for the heavy element low-frequency lines with the numbers 300 > n > 1200. For the chosen electronic temperatures and densities T_e = 0.8× 10⁴–10× 10⁴ K, N_e = 0.05–0.1 cm⁻³ the line optical depth is determined by the values τ_L∼ 0.1× 10⁻⁴–100× 10⁻⁴. Calculated for free-free transition rates, the optical depth is given by using the value τ_ff∼ 10⁻²τ_L.     

The electron temperature and density in interstellar medium by using carbon radio recombination line observations

The intensities of carbon radio recombination lines (RRL’s) are definedallowing for the effect of dielectronic-like recombination. The rate ofdielectronic-like recombination is calculated as functions of line number,electron density and temperature accurate to 0.05. Following from the balanceequation solutions for populations, the RRL intensities are analytically foundby the method of successive approximations to an accuracy of 0.15. Theobservations of carbon RRL’s are analyzed toward Cassiopeia A. The averageelectron temperature, density, expanded CII region lengths and inaccuraciesare found with the experimental values of RRL widths and intensities.     

Time evolution of the unstable two-fluid density fluctuations in Robertson-Walker Universes

We consider density fluctuations of a two-fluid model consisting of hydrogen plasma and radiation prior to the cosmic hydrogen recombination. As investigation method that of the dispersion relations is applied, which have been derived from the general-relativistic sound-wave equations taking into account the coupling between plasma and radiation carefully. We obtain growing unstable acoustic modes within the mass range 2 · 10⁶ M _⊙ < M < 6 · 10 ¹² M _⊙. In a second step the coupled differential equations for the amplitudes of the unstable modes are integrated numerically with respect to time where the integration extends from the initial time prior to the hydrogen recombination up to the present time. We find a significant increase of the amplitudes up to 4 orders of magnitude, if the Universe is described by a cosmological model with a positive cosmological constant (Λ ? 2,2 · 10^-56 cm^-2) and a positive curvature (Lemaître-Universe) without an essential amount of cold dark matter. We conclude that the existence of galaxies confirm these statements.     

Possibilities of investigation of interstellar matter by radio recombination lines (Review)

Radio recombination lines (RRL) discovered about 25 years ago became a very effective tool for the investigation of interstellar matter (ISM). These lines are unique by the number of transitions and the spectral range observed: from 2 mm to 20 m wavelength. This allows to study the ISM under variety of physical conditions which differ by 7 to 8 orders of magnitude in density and 3 orders of magnitude in temperature. RRL allow to determine the main physical conditions in HII and HI-CII regions, to map the galactic distribution of ionized hydrogen, and to obtain the helium abundance of the ISM. Furthermore, the information contained in the RRL allows to estimate the intensity of cosmic rays and to draw conclusions on the galactic evolution. The physical properties of the RRL and the results obtained by observations of RRLs are reviewed.     

Ionization freeze-out and hydrogen excitation in the SN IIP atmosphere

We study the nonstationary recombination of hydrogen in the atmosphere of SN 1987A by taking into account ion-molecular processes. The hydrogen excitation due to nonstationary recombination is shown to be enough to explain the observed hydrogen lines in a time interval until day 30 in the absence of additional excitation mechanisms. Thus, the problem of a deficit in the hydrogen excitation that has recently been found in modeling the hydrogen spectrum of SN 1987A at an early photospheric stage by assuming statistical ionization equilibrium is resolved. The mass of the radioactive ⁵⁶Ni with a spherically symmetric distribution in the outer layers is shown to be close to 10^?6 M _⊙. Our model predicts the appearance of a blue peak in the Hα profile between days 20 and 30. This peak bears a close similarity to the observed peak known as the Bochum event. The presence of this peak in the model is attributable to nonstationary recombination and to a substantial contribution of hydrogen neutralization involving H^? and H₂.     

Physical processes in the interstellar matter determining parameters of radio recombination lines in meter and decameter ranges

We give a review of problems connected with the interpretation of meter and decameter carbon radiolines. The lines are formed inside clumps of molecular clouds in layers with a column density N ≈ 6 · 10²¹ cm⁻². These clumps are very typical structures. The distribution of physical parameters (number density, temperature, etc.) inside the clumps is poorly known. The most difficult and important question is the penetration of subcosmic rays into the clumps. Observations show that the ionization rate is ζ = (1–7) · 10⁻¹⁷ s⁻¹ inside molecular clouds and significantly greater in the diffuse gas. Long-wave radio recombination lines can probably be used for the analysis of the distribution of subcosmic rays inside molecular clouds. The interpretation is complicated by the influence of low-temperature dielectron recombination and poorl known variations of carbon depletion in the clumps.     

Nonresonant effects and hydrogen transition line shape in cosmological recombination problems

Data on the fluctuations in cosmic microwave background (CMB) radiation, whose accuracy is expected to increase in the immediate future, allow the cosmological recombination of atomic hydrogen and its interaction with the CMB radiation to be studied. Nonresonant effects play an important role in these recombination processes. We consider the quantum-mechanical foundations of the nonresonant processes and present our calculations for the differential two-photon decay rates of the 3s and 3d levels in the hydrogen atom. This article was submitted by the authors in English.     

CMB recombination lines of hydrogen: The differential spectrum

We have calculated the distortions of the cosmic microwave background (CMB) spectrum in the wavelength range 2–50 cm due to the superposition of the CMB hydrogen recombination radiation in subordinate lines. The level populations were determined by numerically solving the equation of recombination kinetics together with the statistical equilibrium equations for a 60-level model hydrogen atom. The relative distortions are ≈10^?7–10^?6, with their wavelength dependence having a low-contrast, wavy pattern. However, the contrast increases severalfold and becomes pronounced when passing to the differential distortion spectrum. We study the dependence of the distortions on cosmological parameters.     

Multiband study of NGC 7424 and its two newly discovered ultraluminous X-ray sources

The recombination of hydrogen and helium at   z ∼ 1000–7000  gives unavoidable distortions to the cosmic microwave background (CMB) spectrum. We present a detailed calculation of the line intensities arising from the Lyman α (Lyα) (2p–1s) and two-photon (2s–1s) transitions for the recombination of hydrogen, as well as the corresponding lines from helium. We give an approximate formula for the strength of the main recombination line distortion on the CMB in different cosmologies; this peak occurring at about  170 μm  . We also find a previously undescribed long-wavelength peak (which we call the pre-recombination peak) from the lines of the 2p–1s transitions, which are formed before significant recombination of the corresponding atoms occurred. Detailed calculations of the two-photon emission-line shapes are presented here for the first time. The frequencies of the photons emitted from the two-photon transition have a wide spectrum and this causes the location of the peak of the two-photon line of hydrogen to be located almost at the same wavelength as the main Lyα peak. The helium lines also give distortions at similar wavelengths, so that the combined distortion has a complex shape. The detection of this distortion would provide direct supporting evidence that the Universe was indeed once a plasma. Moreover, the distortions are a sensitive probe of physics during the time of recombination. Although the spectral distortion is overwhelmed by dust emission from the Galaxy, and is maximum at wavelengths roughly where the cosmic far-infrared background peaks, it may be able to tailor an experiment to detect its non-trivial shape.     

Cosmological hydrogen recombination: populations of the high-level substates

We present results for the spectral distortions of the cosmic microwave background (CMB) arising due to bound–bound transitions during the epoch of cosmological hydrogen recombination at frequencies down to  ν∼100 MHz  . We extend our previous treatment of the recombination problem now including the main collisional processes and following the evolution of all the hydrogen angular momentum substates for up to 100 shells. We show that, due to the low baryon density of the Universe, even within the highest considered shell full statistical equilibrium (SE) is not reached and that at low frequencies the recombination spectrum is significantly different when assuming full SE for   n > 2  . We also directly compare our results for the ionization history to the output of the recfast code, showing that especially at low redshifts rather big differences arise. In the vicinity of the Thomson visibility function the electron fraction differs by roughly −0.6 per cent which affects the temperature and polarization power spectra by ≲ 1 per cent. Furthermore, we shortly discuss the influence of free–free absorption and line broadening due to electron scattering on the bound–bound recombination spectrum and the generation of CMB angular fluctuations due to scattering of photons within the high shells.     

Determination of the primordial helium abundance from radio recombination line observations

We describe a method for determining the primordial helium abundance (Y _p) from radio recombination lines and present results of the first stage of our measurements. We analyze the observational data for hydrogen and helium radio recombination lines from six Galactic H II regions obtained at different times. We have found Y _p = 25.2−25.5% with an error of ±0.9% from four sources.     

The role of organic haze in Titan's atmospheric chemistry: I. Laboratory investigation on heterogeneous reaction of atomic hydrogen with Titan tholin

In Titan's atmosphere consisting of N₂ and CH₄, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N₂ and CH₄ gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×¹⁰⁻³×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.     

Absorption effects in sixHii regions

We have measured the profile of the 21-cm line of neutral hydrogen in the direction of sixHii regions (RCW 38, RCW 49, RCW 57 (b), RCW 74, W22, and W37). By comparison with line profiles outside the nebulae we have deduced the absorption profiles corresponding to eachHii region. We compare our results with those of other authors in the 21-cm line, in the line of OH, and in the recombination lines of hydrogen.