Collisional De-excitation Research Articles

Impact of the Lyα radiation force on super-Eddington accretion on to a massive black hole

Abstract The viability of super-Eddington accretion remains a topic of intense debate, crucial for understanding the formation of supermassive black holes in the early universe. However, the impact of the Ly$\alpha$ radiation force on this issue remains poorly understood. We investigate the propagation of the Ly$\alpha$ photons and evaluate the Ly$\alpha$ radiation force within a spherically symmetric accreting H i gas on to the central black hole. We solve the radiation transfer equation, incorporating the destruction processes of Ly$\alpha$ photons through two-photon decay and collisional de-excitation. We find that the Ly$\alpha$ photons, originating in the H ii region around black holes, suffer from multiple resonance scattering before being destroyed via two-photon decay and collisional de-excitation. Hence, the Ly$\alpha$ radiation force undergoes a significant amplification, surpassing gravity at the innermost section of the H i region. This amplification, quantified as the force multiplier, reaches approximately 130 and remains nearly constant, regardless of the optical depth at the line center, provided the optical thickness of the flow is within the range of $10^{10\\!-\\!14}$. The requisite lower limit of the product of gas density and black hole mass to realize super-Eddington accretion is found to be in the range $(2\\!-\\!40) \times 10^9\, M_\odot \, {\rm cm}^{-3}$, which is a few to tens of times larger than the minimum value obtained without accounting for the Ly$\alpha$ radiation force. The pronounced amplification of the Ly$\alpha$ radiation force poses a substantial challenge to the feasibility of super-Eddington accretion.

Constraints on the densities and temperature of the Seyfert 2 narrow line region

Context. Different studies have reported the so-called temperature problem of the narrow line region (NLR) of active galactic nuclei (AGNs). Its origin is still an open issue. To properly address its cause, a trustworthy temperature indicator is required. Aims. To determine the temperature of an emission line plasma, the [O III] (λ4363Å/λ5007Å) line ratio is typically used. However, in the case of the NLR of AGNs, this ratio is not reliable when the electron density extends much above 105 cm−3 as collisional deexcitation strongly affects this ratio independently of the temperature. To verify the density regime, we need a density diagnostic that applies to high excitation plasma. Methods. We propose that the weak [Ar IV] λλ4711,40Å doublet is the appropriate tool for evaluating the density of the high excitation plasma. We subsequently made use of the recent S7 survey sample to extract reliable measurements of the weak [Ar IV] doublet in 16 high excitation Seyfert 2s. As a result we could derive the plasma density of the NLR of our Seyfert 2 sample and compared the temperature inferred from the observed [O III] (λ4363Å/λ5007Å) ratios. Results. It was found that 13 Seyfert 2s cluster near similar values as the [O III] (λ4363Å/λ5007Å) ratio, at a mean value of 0.0146 ± 0.0020. Three objects labeled outliers stand out at markedly higher [O III] values (> 0.03). Conclusions. If for each object one assumes a single density, the values inferred from the [Ar IV] doublet for the 13 clustering objects all lie below 60 000 cm−3, indicating that the [O III] (λ4363Å/λ5007Å) ratios in these objects is a valid tracer of plasma temperature. Even when assuming a continuous power-law distribution of the density, the inferred cut-off density required to reproduce the observed [Ar IV] doublet is in all cases < 105.1 cm−3. The average NLR temperature inferred for the 13 Seyfert 2s is 13 000 ± 703 K, which photoionization models have difficulty reproducing. Subsequently we considered different mechanisms to account for the observed [O III] ratios. For the three outliers, a double-bump density distribution is likely required, with the densest component having a density > 106 cm−3.

Spectroscopic measurement of increases in hydrogen molecular rotational temperature with plasma-facing surface temperature and due to collisional-radiative processes in tokamaks

Spatially resolved rotational temperature of ground state hydrogen molecules desorbed from plasma-facing surface was measured in QUEST, LTX-β, and DIII-D tokamaks, and the increases of the rotational temperature with the surface temperature and due to collisional-radiative processes in the plasmas were evaluated. The increase due to collisional-radiative processes was calculated by solving rate equations considering electron and proton collisional excitation and deexcitation and spontaneous emission. The calculation results suggest a high sensitivity for the rotational temperature to electron and proton densities, but a negligible sensitivity to the electron, proton, and surface temperatures. In the three tokamaks with different plasma parameters and plasma-facing surface materials, the spatial profile of the rotational temperature was estimated using Fulcher-α emission lines (600–608 nm). In QUEST, the spatial profile of the rotational temperature was estimated from spatially resolved spectra. In the other tokamaks, the rotational temperature was evaluated assuming a single point emission with a location determined from the Fulcher-α emission profile as measured with a filtered camera. In metal-walled devices QUEST and LTX-β, the rotational temperature increased with the surface temperature, and the calculated collisional-radiative increase is consistent with measured increase assuming that the rotational temperature at the surface is approximately 500–600 K higher than the surface temperature. In DIII-D with carbon walls, a larger collisional-radiative increase than the other tokamaks was observed because of the higher density leading to a large difference from the calculated increase compared to the other smaller tokamaks. Measurement of the Fulcher-α emission profile with higher spatial resolution in DIII-D may reduce the difference and reveal the effect of the surface temperature on the rotational temperature. These results show the increases in the rotational temperature with the surface temperature and due to the collisional-radiative processes.

An efficient Monte Carlo model for the slowing down of photoelectrons. Application to H-[formula omitted] in exoplanet atmospheres

Photoelectrons, the fast electrons produced in the photoionization of planetary atmospheres, drive transformations in the atmospheric gas that are often inhibited by energy considerations for thermal electrons. The transformations include excitation and ionization of atoms and molecules, which affect the detectability of these gases and constrain the fraction of incident stellar radiation that transforms into heat. To gain insight into these important questions, we build a Monte Carlo model that solves the slowing down of photoelectrons in a gas with arbitrary amounts of H and He atoms and thermal electrons. Our novel multi-score scheme differs from similar tools in that it efficiently handles rare collisional channels, as in the case of low-abundance excited atoms that undergo superelastic and inelastic collisions. The model is validated and its performance demonstrated. Further, we investigate whether photoelectrons might affect the population of the excited hydrogen H(2) detected at some exoplanet atmospheres by transmission spectroscopy in the H-α line. For the ultra-hot Jupiter HAT-P-32b, we find that photoelectron-driven excitation of H(2) is inefficient at the pressures probed by the line core but becomes significant (yet sub-dominant) deeper in the atmosphere where the line wings form. The contribution of photoelectrons to the destruction of H(2) either by collisional deexcitation or ionization is entirely negligible, a conclusion likely to hold for exoplanet atmospheres at large. Importantly, photoelectrons dominate the gas ionization at the altitudes probed by the H-α line, a fact that will likely affect, even if indirectly, the population of H(2) and other tracers such as metastable helium. Future modeling of these excited levels should incorporate photoelectron-driven ionization.

Observation of Collisional De-Excitation Phenomena in Plutonium

A program of research towards the high-resolution optical spectroscopy of actinide elements for the study of fundamental nuclear structure is currently ongoing at the IGISOL facility of the University of Jyväskylä. One aspect of this work is the development of a gas-cell-based actinide laser ion source using filament-based dispensers of long-lived actinide isotopes. We have observed prominent phenomena in the resonant laser ionization process specific to the gaseous environment of the gas cell. The development and investigation of a laser ionization scheme for plutonium atoms is reported, focusing on the effects arising from the collision-induced phenomena of plutonium atoms in helium gas. The gas-cell environment was observed to greatly reduce the sensitivity of an efficient plutonium ionization scheme developed in vacuum. This indicates competition between resonant laser excitation and collisional de-excitation by the gas atoms, which is likely being enhanced by the very high atomic level density within actinide elements.

TEMPERATURE DISCREPANCY WITH PHOTOIONIZATION MODELS OF THE NARROW-LINE REGION

Using published work on the narrow-line region of Active Galactic Nuclei, a comparison is carried out among the [OIII] λ4363Å/5007Å ratios (ROIII) observed in quasars, Seyfert 2's and the spatially resolved ENLR plasma. Using the weak [ArIV] λ4711Å/λ4740Å doublet observed by Koski (1978) among Seyfert 2's, we find evidence of a Narrow-Line Region (NLR) populated by low density emission clouds (≲ 104 cm-3). After considering calculations of the [Ar] and [OIII] ratios that assume a powerlaw distribution of plasma densities, no evidence of collisional deexcitation is found. The plasma temperature that is inferred is 13 500 °K, which is problematic to reproduce with standard photoionization calculations. The simplest interpretation for the near coincidence of the ROIII ratios among the ENLR and Seyfert 2 measurements (ROIII ≃ 0.017) is that the low density regime applies to both plasmas.

Inelastic, exchange, and reactive processes in rovibrationally excited collisions of HD with H

ABSTRACT The HD molecule is an important coolant in early universe chemistry models and a tracer of H2 in star-forming regions. Rate coefficients for collisional excitation and de-excitation of HD rotational and vibrational levels form important ingredients in astrophysical models. While collisions with He, H2, and H are the most important, available data for H + HD collisions are largely limited to temperatures less than 1000 K for the vibrational ground state, low-lying rotational levels of the v = 1 HD vibrational level, or computed without reactive contributions. Here, through explicit quantum scattering calculations, we report extensive data for rovibrational transitions in HD induced by H atoms for a range of rotational levels in v = 1 and some v = 0 levels for temperatures up to 1000 K. The significance of the computed results for astrophysical modeling is discussed.

The case for thermalization as a contributor to the [C ii] deficit

ABSTRACT The [C ii] deficit, which describes the observed decrease in the ratio of [C ii] 158 μm emission to continuum infrared emission in galaxies with high star formation surface densities, places a significant challenge to the interpretation of [C ii] detections from across the observable universe. In an attempt to further decode the cause of the [C ii] deficit, the [C ii] and dust continuum emission from 18 Local Volume galaxies has been split based on conditions within the interstellar medium where it originated. This is completed using the Key Insights in Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH) and Beyond the Peak (BtP) surveys and the wide-range of wavelength information, from UV to far-infrared emission lines, available for a selection of star-forming regions within these samples. By comparing these subdivided [C ii] emissions to isolated infrared emission and other properties, we find that the thermalization (collisional de-excitation) of the [C ii] line in H ii regions plays a significant role in the deficit observed in our sample.

The formation of atomic oxygen and hydrogen in atmospheric pressure plasmas containing humidity: picosecond two-photon absorption laser induced fluorescence and numerical simulations

Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio.

Isomerization and decomposition reactions of acetaldehyde relevant to atmospheric processes from dynamics simulations on neural network-based potential energy surfaces.

Acetaldehyde (AA) isomerization [to vinylalcohol (VA)] and decomposition (into either CO + CH4 or H2 + C2H2O) are studied using a fully dimensional, reactive potential energy surface represented as a neural network (NN). The NN, trained on 432 399 reference structures from MP2/aug-cc-pVTZ calculations, has a mean absolute error of 0.0453 kcal/mol and a root mean squared error of 1.186 kcal mol-1 for a test set of 27 399 structures. For the isomerization process AA → VA, the minimum dynamical path implies that the C-H vibration and the C-C-H (with H being the transferring H-atom) and the C-C-O angles are involved to surmount the 68.2 kcal/mol barrier. Using an excess energy of 93.6 kcal/mol-the typical energy available in the solar spectrum and sufficient to excite to the first electronically excited state-to initialize the molecular dynamics, no isomerization to VA is observed on the 500 ns time scale. Only with excess energies of ∼127.6 kcal/mol (including the zero point energy of the AA molecule), isomerization occurs on the nanosecond time scale. Given that collisional quenching times under tropospheric conditions are ∼1 ns, it is concluded that formation of VA following photoexcitation of AA from actinic photons is unlikely. This also limits the relevance of this reaction pathway to be a source for formic acid.

Metastable nuclear isomers as dark matter accelerators

Inelastic dark matter and strongly interacting dark matter are poorly constrained by direct detection experiments since they both require the scattering event to deliver energy from the nucleus into the dark matter in order to have observable effects. We propose to test these scenarios by searching for the collisional de-excitation of meta-stable nuclear isomers by the dark matter particles. The longevity of these isomers is related to a strong suppression of $\gamma$- and $\beta$-transitions, typically inhibited by a large difference in the angular momentum for the nuclear transition. The collisional de-excitation by dark matter is possible since heavy dark matter particles can have a momentum exchange with the nucleus comparable to the inverse nuclear size, hence lifting tremendous angular momentum suppression of the nuclear transition. This de-excitation can be observed either by searching for the direct effects of the decaying isomer, or through the re-scattering or decay of excited dark matter states in a nearby conventional dark matter detector setup. Existing nuclear isomer sources such as naturally occurring $^{180m}$Ta, $^{137m}$Ba produced in decaying Cesium in nuclear waste, $^{177m}$Lu from medical waste, and $^{178m}$Hf from the Department of Energy storage can be combined with current dark matter detector technology to search for this class of dark matter.

Horizontal and vertical structures of Jovian infrared aurora: Observation using Subaru IRCS with adaptive optics

We observed Jupiter's infrared aurora using the Subaru Infrared Camera and Spectrometer (IRCS) with adaptive optics, in order to compare horizontal and vertical emission profiles among H3+ fundamental, H3+ overtone, and H2 emissions. The fundamental and overtone lines show similar horizontal distributions around the main auroral oval for both hemispheres. There is no correlation between the fundamental lines and temperature; however, a correlation seems to exist for the overtone lines. Although both the fundamental and overtone lines are related to electron precipitation, there are differences in the precipitating electron energies that influence the emissions, because of their different emission altitudes. In addition, the morphological difference between the H2 and H3+ emission structures is significant. That is, the intensity of the H3+ emission is enhanced around the main oval, but that of the H2 emission is not. The peak altitudes of the H3+ fundamental, H3+ overtone, and H2 are 650, 870, and 830km for the north and 700, 910, and 950km for the south, with the uncertainties of approximately 100km. This finding, that the fundamental emission peak altitude is found to be lower than those of the overtone and H2 emissions, is consistent with the theoretical models and derived temperatures for the emission peak altitude. We also find similar peak altitudes for the H2 and H3+ overtone emissions. This is in accordance with previous observations, which have revealed that the altitude of H2 is higher than the model-based expectation. Collisional de-excitation of H2 can dominate at lower altitudes, which might explain the higher emission peak altitude compared to that of the model.

On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary

Abstract The blue hypergiant Cyg OB2 12 (B3Ia+) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si xiv and Mg xii. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.

Degeneracy and relativistic microreversibility relations for collisional-radiative equilibrium models.

We present the relativistic expressions of standard nonrelativistic microreversibility relations that can be used in collisional-radiative equilibrium models to calculate the transition rates including the free electron degeneracy for collisional excitation and deexcitation, collisional ionization and three-body recombination, dielectronic capture and autoionization, photoexcitation and photodeexcitation, and radiative recombination and photoionization. Semiempirical expressions or more refined calculations can be used for the cross sections of interest as long as they are calculated by taking into account either nonrelativistic, relativistic, or ultrarelativistic effects for both the bound and free electrons. The bound and the free electrons should be treated on the same footing. This is crucial for the internal consistency of the approach valid at arbitrary degeneracy and relativistic degrees.

Vertical profiles of Mars 1.27 µm O2 dayglow from MRO CRISM limb spectra: Seasonal/global behaviors, comparisons to LMDGCM simulations, and a global definition for Mars water vapor profiles

Since July of 2009, The Compact Reconnaissance Imaging Spectral Mapper (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) has periodically obtained pole-to-pole observations (i.e., full MRO orbits) of limb scanned visible/near IR spectra (λ=0.4−4.0 µ m, △λ ∼ 10 nm- Murchie et al., 2007). These CRISM limb observations support the first seasonally and spatially extensive set of Mars 1.27 µm O2(1△g) dayglow profile retrievals (∼ 1100) over ≥ 8–80 km altitudes. Their comparison to Laboratoire de Météorologie Dynamique (LMD) global climate model (GCM) simulated O2(1△g) volume emission rate (VER) profiles, as a function of altitude, latitude, and season (solar longitude, Ls), supports several key conclusions regarding Mars atmospheric water vapor (which is derived from O2(1△g) emission rates), Mars O3, and the collisional de-excitation of O2(1△g) in the Mars CO2 atmosphere. Current (Navarro et al., 2014) LMDGCM simulations of Mars atmospheric water vapor fall 2–3 times below CRISM derived water vapor abundances at 20–40 km altitudes over low-to-mid latitudes in northern spring (Ls = 30-60°), and northern mid-to-high latitudes over northern summer (Ls = 60–140°). In contrast, LMDGCM simulated water vapor is 2–5 times greater than CRISM derived values at all latitudes and seasons above 40 km, within the aphelion cloud belt (ACB), and over high-southern to mid-southern latitudes in southern summer (Ls = 190-340°) at 15–35 km altitudes. Overall, the solstitial summer-to-winter hemisphere gradients in water vapor are reversed between the LMDGCM modeled versus the CRISM derived water vapor abundances above 10–30 km altitudes. LMDGCM-CRISM differences in water vapor profiles correlate with LMDGCM-CRISM differences in cloud mixing profiles; and likely reflect limitations in simulating cloud microphysics and radiative forcing, both of which restrict meridional transport of water from summer-to-winter hemispheres on Mars (Clancy et al., 1996; Montmessin et al., 2004; Steele et al., 2014; Navarro et al., 2014) and depend on uncertain cloud microphysical properties (Navarro et al., 2014). The derived low-to-mid latitude changes in Mars water vapor vertical distributions should reduce current model-data disagreements in column O3 and H2O2 abundances over low-to-mid latitudes (e.g., within the ACB; Lefèvre et al., 2008; Encrenaz et al., 2015; Clancy et al., 2016). Lastly, the global/seasonal average comparison of CRISM and LMDGCM O2(1△g) VER below 20 km altitudes indicates a factor of ∼3 times lower value (0.25×10−20 cm3sec−1) for the CO2 collisional de-excitation rate coefficient of O2(1△g) than derived recently by Guslyakova et al. (2016).

Rotational Spectroscopy of the NH3–H2 Molecular Complex

Abstract We report the first high resolution spectroscopic study of the NH3–H2 van der Waals molecular complex. Three different experimental techniques, a molecular beam Fourier transform microwave spectrometer, a millimeter-wave intracavity jet OROTRON spectrometer, and a submillimeter-wave jet spectrometer with multipass cell, were used to detect pure rotational transitions of NH3–H2 in the wide frequency range from 39 to 230 GHz. Two nuclear spin species, (o)-NH3–(o)-H2 and (p)-NH3–(o)-H2, have been assigned as carriers of the observed lines on the basis of accompanying rovibrational calculations performed using the ab initio intermolecular potential energy surface (PES) of Maret et al. The experimental spectra were compared with the theoretical bound state results, thus providing a critical test of the quality of the NH3–H2 PES, which is a key issue for reliable computations of the collisional excitation and de-excitation of ammonia in the dense interstellar medium.

Broadening effects on opacity calculation of CH plasmas

Opacity is a function of the temperature and electron density of plasma. The plasma density can be determined by measurements of Stark-broadened K-shell spectral lines. The purpose of this work is to obtain a more detailed structure of opacity with regard to broadening effects. For this aim, the opacity frequency dependency and mean opacity of mixed plasmas are calculated under local thermodynamic equilibrium (LTE) conditions. The LTE state in inertial confinement fusion occurs when the collisional deexcitation rate from the upper level to the lower level greatly exceeds the spontaneous decay rate. Since the thermal radiation can be absorbed by the CH-ablator, by studying the behavior of the CH Polystyrene opacity, one can obtain the temperature and density of the plasma in investigations of matter found in stellar interiors, inertial fusion implosions, and Z pinches as a diagnostic technique. The main aspect of diagnostic application is spectrum broadening. The final results show that the Stark-broadened line shape is dependent on the density. Also, it is shown that the resonance peak and spectrum broadening of the opacity spectrum of a mixed plasma such as the CH-plasma is larger than a single atom plasma such as Carbon.

Experiment and computation: a combined approach to study the van der Waals complexes

A review of recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, mostly those which contain H 2 and He, is presented. In our work, we compared the experimental spectra to the theoretical bound state results, thus providing a critical test of the quality of the M–H 2 and M–He potential energy surfaces (PESs) which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH 3 , H 2 O) in the dense interstellar medium. The intermolecular interactions with He and H 2 play also an important role for high resolution spectroscopy of helium or para -hydrogen clusters doped by a probe molecule (CO, HCN). Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p -H 2 clusters.

Free electron degeneracy effects on collisional excitation, ionization, de-excitation and three-body recombination

Collisional-radiative models enable average ionization and ionization populations, plus the rates of absorption and emission of radiation to be calculated for plasmas not in thermal equilbrium. At high densities and low temperatures, electrons may have a high occupancy of the free electron quantum states and evaluations of rate coefficients need to take into account the free electron degeneracy. We demonstrate that electron degeneracy can reduce collisional rate coefficients by orders-of-magnitude from values calculated neglecting degeneracy. We show that assumptions regarding the collisional differential cross-section can alter collisional ionization and recombination rate coefficients by a further factor two under conditions relevant to inertial fusion.

PHOTOCHEMICAL HEATING OF DENSE MOLECULAR GAS

Photochemical heating is analyzed with emphasis on the heating generated by chemical reactions initiated by the products of photodissociation and photoionization. The immediate products are slowed down by collisions with the ambient gas and heat the gas. In addition to this direct process, heating is also produced by the subsequent chemical reactions initiated by these products. Some of this chemical heating comes from the kinetic energy of the reaction products and the rest from collisional de-excitation of the product atoms and molecules. In considering dense gas dominated by molecular hydrogen, we find that the chemical heating is sometimes as large if not much larger than the direct heating. In very dense gas the total photochemical heating approaches 10 eV per photodissociation (or photoionization), competitive with other ways of heating molecular gas.