Strong Radiation Field Research Articles

Stabilization of quantum system with coulomb and short-range potentials in strong laser field

The ionization dynamics of the quantum systems with the Coulomb and short-range potentials in the presence of a strong radiation field is investigated. The features of the stabilization regime are discussed at various structures of the energy spectrum of the quantum system and the parameters of the radiation pulse. It is demonstrated that specific shapes of the atomic potential and the pulse parameters lead to either the interference stabilization or the Kramers-Henneberger stabilization. A relation between various stabilization effects and the conditions for them are analyzed.

ON THE ABSENCE OF HIGH METALLICITY-HIGH COLUMN DENSITY DAMPED Lyα SYSTEMS: MOLECULE FORMATION IN A TWO-PHASE INTERSTELLAR MEDIUM

We argue that the lack of observed damped Lyα (DLA) systems that simultaneously have high H i column densities and high metallicities results naturally from the formation of molecules in the cold phase of a two-phase atomic medium in pressure balance. Our result applies equally well in diffuse systems where the ultraviolet radiation field is dominated by the extragalactic background and in dense star-forming ones, where the local radiation field is likely to be orders of magnitude higher. We point out that such a radiation-insensitive model is required to explain the absence of high column–high metallicity systems among DLAs observed using gamma-ray burst (GRB) afterglows, since these are likely subjected to strong radiation fields created by active star formation in the GRB host galaxy. Moreover, we show that the observed relationship between the maximum atomic gas column in DLAs sets a firm upper limit on the fraction of the mass in these systems that can be in the warm, diffuse phase. Finally, we argue that our result explains the observed lack of in situ star formation in DLA systems.

TeV neutrinos from accreting x-ray pulsars

Pulsars inside binary systems can accrete matter that arrives up to the pulsar surface provided that its period is long enough. During the accretion process, matter has to be accelerated to the rotational velocity of the pulsar magnetosphere at the distance where the balance between the pressure of matter and the magnetic field is achieved. At this distance, a very turbulent and magnetized region is formed in which hadrons can be accelerated to relativistic energies. These hadrons can interact with the very strong radiation field coming from the hot polar cap on the neutron star surface created by the in-falling matter. We calculate the neutrino event rates produced in an km$^2$ detector that can be expected from accreting millisecond and classical X-ray pulsars at a typical distance within our own Galaxy.

Ultraviolet Fe ii emission inz∼ 2 quasars

We present spectra of six luminous quasars at z ∼ 2, covering rest wavelengths 1600-3200 A. The fluxes of the UV Fe II emission lines and Mg II λ2798 doublet, the line widths of Mg II and the 3000 A luminosity were obtained from the spectra. These quantities were compared with those of low-redshift quasars at z = 0.06-0.55 studied by Tsuzuki et al. In a plot of the Fe II(UV)/Mg II flux ratio as a function of the central black hole mass, Fe II(UV)/Mg II in our z ∼ 2 quasars is systematically greater than in the low-redshift quasars. We confirmed that luminosity is not responsible for this excess. It is unclear whether this excess is caused by rich Fe abundance at z ∼ 2 over low-redshift or by non-abundance effects such as high gas density, strong radiation field and high microturbulent velocity.

Fe ii emission spectra in AGN: observations and theoretical interpretation

The enrichment of Fe, relative to alpha elements such as O and Mg, represents a potential means to measure the ages of quasi-stellar object (QSO) host galaxies and probe nucleosynthesis in the early universe. QSOs exhibit prominent Fe ii features and Mg ii 2800 Å resonance doublet emission in the ultraviolet. Although chemical evolutionary models predict that the Fe/Mg abundance ratio decreases with increasing redshift, measurements of QSO Fe ii (UV)/Mg ii emission line ratios show large scatter from 1 to 20, with no redshift dependency up to z∼6.4. Before using Fe ii emission as an abundance indicator, one must ascertain how Fe ii emission varies with physical conditions. We have constructed an 830-level model atom for Fe ii and used it in a photoionization code to calculate Fe ii emission. This model is more sophisticated than previous efforts, and uses the most recent laboratory atomic data and includes the numerous transitions that are sensitive to the strong radiation field in QSOs. Predicted Fe ii(UV)/Mg ii ratios and fluxes strongly depend on non-abundance factors such as microturbulence, ionizing flux, and hydrogen density; all must be taken into account before any accurate abundance can be derived. Our calculations show that Fe ii is the dominant coolant at densities found in active galactic nucleus (AGN) broad emission line regions (BLRs), and must be included in photoionization modelling. Our close collaboration with spectroscopists at Lund University has been highly beneficial for further development of our Fe ii model, most importantly through atomic data studies that link high-energy levels in Fe ii. Additional studies of the atomic structure of Fe ii are necessary to improve our understanding of the AGN continua by accounting for the effects of the Fe ii pseudo-continuum, which blanket QSO spectra from 1000 to 10 000 Å. Predicted Fe ii emission spectra, suitable for BLRs in AGN, are available at http://iacs.cua.edu/people/verner/Feii.

Influence of phase memory effects in collisions on a resonance Raman spectrum

The influence of phase memory effects on the spectrum of resonance Raman scattering by three-level atoms with the Λ configuration of levels experiencing collisions with buffer gas atoms in a strong monochromatic radiation field is studied theoretically. Systems with a small Doppler broadening compared to the collision frequency (large buffer gas pressures) are analysed in the general case of an arbitrary change (from complete change to complete preservation) in the phase memory at any of three transitions in the Λ system. It is shown that in the absence of the collision relaxation of the low-frequency coherence at the transitions between two lower levels of the Λ system, the radiation scattering spectrum has a spectrally narrow component at the Raman frequency, which, despite the homogeneous broadening of the absorption line, exhibits a strongly pronounced anisotropy. In the direction, close to the propagation direction of exciting radiation, this line maximally narrows down. It is significant that upon optical pumping to the level unaffected by a strong field the resonance Raman spectrum noticeably differs from the spectrum in the case of the probe field. A simple expression is proposed for calculating the degree of the phase memory preservation in collisions from the relative amplitude of the Raman resonance.

ASpitzerView of Protoplanetary Disks in the γ Velorum Cluster

We present new Spitzer Space Telescope observations of stars in the young (~5 Myr)γ Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified five debris disks around A-type stars and five to six debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the infrared flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that γ Velorum stellar cluster is slightly older than reported ages and the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr.

Can Supermassive Black Holes Form in Metal‐enriched High‐Redshift Protogalaxies?

Primordial gas in protogalactic dark matter (DM) halos with virial temperatures Tvir > 10^4 K begins to cool and condense via atomic hydrogen. Provided this gas is irradiated by a strong ultraviolet (UV) flux and remains free of H2 and other molecules, it has been proposed that the halo with Tvir ~10^4 K may avoid fragmentation, and lead to the rapid formation of a supermassive black hole (SMBH) as massive as M=10^5-10^6 Msun. This ``head--start'' would help explain the presence of SMBHs with inferred masses of several x 10^9 Msun, powering the bright quasars discovered in the Sloan Digital Sky Survey at redshift z>~6. However, high-redshift DM halos with Tvir~10^4K are likely already enriched with at least trace amounts of metals and dust produced by prior star-formation in their progenitors. Here we study the thermal and chemical evolution of low-metallicity gas exposed to extremely strong UV radiation fields. Our results, obtained in one-zone models, suggest that gas fragmentation is inevitable above a critical metallicity, whose value is between Zcr~3x10^{-4} Zsun (in the absence of dust) and as low as Zcr~ 5 x 10^{-6} Zsun (with a dust-to-gas mass ratio of about 0.01 Z/Zsun). We propose that when the metallicity exceeds these critical values, dense clusters of low--mass stars may form at the halo nucleus. Relatively massive stars in such a cluster can then rapidly coalesce into a single more massive object, which may produce an intermediate-mass BH remnant with a mass up to M <~10^2-10^3 Msun.

AKARI Near-Infrared Spectroscopy: Detection of H 2 O and CO 2 Ices toward Young Stellar Objects in the Large Magellanic Cloud

We present the first results of AKARI Infrared Camera near-infrared spec- troscopic survey of the Large Magellanic Cloud (LMC). We detected absorption features of the H2O ice 3.05 um and the CO2 ice 4.27 um stretching mode toward seven massive young stellar objects (YSOs). These samples are for the first time spectroscopically confirmed to be YSOs. We used a curve-of-growth method to evaluate the column densities of the ices and derived the CO2/H2O ratio to be 0.45 pm 0.17. This is clearly higher than that seen in Galactic massive YSOs (0.17 pm 0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO2 ice abundance.

Specific features of use of wide-gap semi-insulating materials for recording of nuclear radiation

Operation of detectors in strong radiation fields or an imperfect fabrication technology gives rise to a considerable amount of structural defects in materials. Using a p +-v-n + detector structure in the unconventional forward-bias mode has been considered as a way to minimize the space-charge accumulation caused by long-term capture of carriers to deep levels. The efficiency of carrier transport and the signal amplitude spectrum were studied as functions of the applied voltage and time of signal shaping by the electronic circuit for the example of a CVD-grown 4H-SiC film containing up to 2.5 × 1017 cm−3 of primary knocked-out atoms. The detector was tested with 5.4 MeV α particles in the temperature range 20–140°C. It was demonstrated that, despite the transport of only one-fourth of the whole amount of charge, the main line of the spectrum has a high energy resolution (7%) for the given conditions.

AKARI near-infrared spectroscopy: Detection of H2O and CO2 ices toward young stellar objects in the Large Magellanic Cloud

AbstractWe present the first results of the AKARI Infrared Camera near-infrared spectroscopic survey of the Large Magellanic Cloud (LMC). The circumstellar material of young stellar objects (YSOs) are affected by galactic environments such as a metallicity or radiation field. Ices control the chemical balance of circumstellar environments of embedded YSOs. We detected absorption features of the H2O ice 3.05 μm and the CO2 ice 4.27 μm stretching mode toward seven massive YSOs in the LMC. This is the first detection of the 4.27 μm CO2 ice feature toward extragalactic YSOs. The present samples are for the first time spectroscopically confirmed to be YSOs. We used a curve-of-growth method to evaluate the column densities of the ices and derived the CO2/H2O ratio to be 0.45±0.17. This is clearly higher than that seen in Galactic massive YSOs (0.17±0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO2 ice abundance.

Ionized gas in active galactic nuclei

This review gives an introduction to the physics of ionized gas in active galactic nuclei (AGN). The main topics are: Definition of the AGN phenomenon, methods for discovering AGN, basic AGN observations, photoionization and recombination of low density plasma, energy balance and gas temperature, the spectrum of ionized gas, dust and reddening, the motion of ionized gas under the influence of strong radiation fields, the broad line region (BLR), the narrow line region (NLR), and the highly ionized outflowing gas in AGN.

Non-local thermodynamic equilibrium dust nucleation in subsaturated vapours

We use the kinetic theory of nucleation to explore the properties of dust nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical clusters have a smaller temperature compared to their vapor. This alters the dynamical balance between attachment and detachment of monomers, allowing for stable nucleation of grains in vapors that are sub-saturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n-T phase diagram. The nucleation in the non-LTE regions is likely to be at much slower rate than in the super-saturated vapors. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium.

Can photo-ionisation explain the decreasing fraction of X-ray obscured AGN with luminosity?

Chandra and XMM-Newton surveys show that the fraction of obscured AGN decreases rapidly with increasing luminosity. Although this is usually explained by assuming that the covering factor of the central engine is much smaller at luminous QSOs, the exact origin of this effect remains unknown. We perform toy simulations to test whether photo-ionisation of the obscuring screen in the presence of a strong radiation field can reproduce this effect. In particular, we create X-ray spectral simulations using a warm absorber model assuming a range of input column densities and ionisation parameters. We fit instead the simulated spectra with a simple cold absorption power-law model that is the standard practice in X-ray surveys. We find that the fraction of absorbed AGN should fall with luminosity as L −0.16±0.03 in rough agreement with the observations. Furthermore, this apparent decrease in the obscuring material is consistent with the dependence of the FeKα narrow-line equivalent width on luminosity, i.e. the X-ray Baldwin effect.

Electronic and Vibrational Spectroscopy of Diamondoids and the Interstellar Infrared Bands between 3.35 and 3.55 μm

The electronic and vibrational spectroscopic properties and ionization energies of diamondoids (nano-diamonds, microdiamonds) are computed using density functional theory (DFT). Spectra of both the neutral and cationic forms of diamondoids, ranging in size from C10H16 to C38H42, and the IR spectrum of a diamondoid-PAH hybrid molecule are presented. For the 23 neutral species, the C-H stretching bands fall near 3.47 μm and are the strongest in the spectra. Diamondoid ionization energies (IEs) are found to be quite large (about 8 eV). The electronic excitation energies of the neutral species are of the same order as the IEs and have very small oscillator strengths (f-values). For the cations, the C-H stretching peak positions differ somewhat from the neutrals and their average integrated band strengths are about half those of the neutrals. The computed electronic excitation energies for the cations are much smaller than those of the neutral species, with f-values that are very small. The neutral diamondoids will absorb most strongly near 3.47 μm, which is very close to the position of an absorption band associated with dense clouds that has been tentatively attributed to the tertiary C-H stretch of diamond-like carbon. The spectroscopic properties described here imply that 3 μm emission from highly vibrationally excited diamondoid cations and neutral species should be most intense in regions with strong radiation fields. These results, in conjunction with the observation that the 3.5 μm emission feature originates very close to the exciting star, strongly supports its assignment in HD 97048 and Elias 1 to diamondoid species. While previous work makes an excellent case for large neutral diamondoids, our work shows that some cation contribution cannot be excluded. The very small f-values explain why so few sources show this emission.

XMM-Newton observations ofζOrionis (O9.7 Ib): a collisional ionization equilibrium model

We present XMM-Newton observations of the O supergiant Zeta Orionis (O9.7 Ib). The spectra are rich in emission lines over a wide range of ionization stages. The RGS-spectra show for the first time lines of low ion stages such as C VI, N VI, N VII, and O VII. The line profiles are symmetric and rather broad (FWHM approximately 1500 km/s) and show only a slight blue shift. With the XMM-epic spectrometer several high ions are detected in this star for the first time including Ar XVII and S XV. Simultaneous multi-temperature fits and DEM-modeling were applied to the RGS and EPIC spectra to obtain emission measures, elemental abundances and plasma temperatures. The calculations show temperatures in the range of about 0.07-0.6 keV. According to the derived models the intrinsic source X-ray luminosity at a distance of 251 pc Lx=1.37(.03) times 10^{32} ergs/s, in the energy range 0.3-10 keV. In the best multi-temperature model fit, the abundances of C, N, O, and Fe are near their solar values, while the abundances of Ne, Mg, and Si appear somewhat enhanced. The sensitivity of the He-like forbidden and intercombination lines to Zeta Ori's strong radiation field is used to derive the radial distances at which lines are formed. Values of 34 R-star for N VI, 12.5 R-star for O VII, 4.8 R-star for Ne IX, and 3.9 R-star for Mg XI are obtained.

Star Formation at Very Low Metallicity. I. Chemistry and Cooling at Low Densities

We present a simplified chemical and thermal model designed to allow computationally efficient study of the thermal evolution of metal-poor gas within large numerical simulations. Our main simplification is the neglect of the molecular chemistry of the heavy elements. The only molecular chemistry retained within the model is the formation and destruction of molecular hydrogen. Despite this major simplification, the model allows for accurate treatment of the thermal evolution of the gas within a large volume of parameter space. It is valid for temperatures 50 < T < 10,000 K and metallicities 0 < Z < 0.1 Z?. In gas with a metallicity Z = 0.1 Z?, and in the absence of an incident ultraviolet radiation field, it is valid for hydrogen number densities nH 500/tchar cm-3, where tchar is the size in megayears of the characteristic physical timescale of interest in the problem. If Z 0.1 Z?, or if a strong ultraviolet radiation field is present, then the model remains accurate up to significantly higher densities. We also discuss some possible applications of this model.

The Unusual Hydrocarbon Emission from the Early Carbon Star HD 100764: The Connection between Aromatics and Aliphatics

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to obtain spectra of HD 100764, an apparently single carbon star with a circumstellar disk. The spectrum shows emission features from polycyclic aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than normally seen, as characteristic of ``class C'' systems in the classification scheme of Peeters et al. All seven of the known class C PAH sources are illuminated by radiation fields that are cooler than those which typically excite PAH emission features. The observed wavelength shifts are consistent with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We propose that the class C PAH spectra are distinctive because the carbonaceous material has not been subjected to a strong ultraviolet radiation field, allowing relatively fragile aliphatic materials to survive.

Resolving the Nature of the Rosette HH 1 Jet Facing Strong UV Dissipation

The Rosette HH1 jet is a collimated flow immersed in the strong UV radiation field of the Rosette Nebula. We investigate the physical properties of the Rosette HH1 jet using high-quality narrow-band images and high-dispersion spectroscopy. The new images show that the axis of the jet is not precisely aligned with the star near the base of the jet. The high resolution of the spectra allows us to accurately determine the contributions from the \ion{H}{2} region, jet, and star. The appoaching and receding sides of the expanding shell of the Rosette Nebula are at heliocentric velocities of 13 and 40 km s$^{-1}$, while the jet reaches a maximum velocity offset at a heliocentric velocity of $-$30 km s$^{-1}$. The [\ion{S}{2}] doublet ratios indicate an electron density of $\sim$1000 cm$^{-3}$ in the jet and $\le$100 cm$^{-3}$ in the \ion{H}{2} region. With a careful subtraction of the nebular and jet components, we find the stellar H$\alpha$ line is dominated by a broad absorption profile with little or no emission component, indicating a lack of substantial circumstellar material. The circumstellar material has most likely been photo-evaporated by the strong UV radiation field in the Rosette Nebula. The evaporation time scale is 10$^3$ -- 10$^4$ yr. The Rosette HH1 jet source provides evidence for an accelerated evolution from a CTTS to a WTTS due to the strong UV radiation field; therefore, both CTTSs and WTTSs can be spatially mixed in regions with massive star formation.

Multiwavelength Study of Young Massive Star Clusters in the Interacting Galaxy Arp 24

We made a multiwavelength study of young massive star clusters (YSCs) in the interacting galaxy Arp 24 using optical and ultraviolet images from the Hubble Space Telescope (HST), the Sloan Digital Sky Survey, and the Galaxy Evolution Explorer, mid-infrared images from the Spitzer Space Telescope, and narrowband Hα images and optical spectra from the National Astronomical Observatories of the Chinese Academy of Sciences 2.16 m telescope. Based on the HST images, we found that the brightest infrared knot in Arp 24 is associated with a complex of five YSCs within a region of ~0.95'' radius (127 pc) in size. The ages and masses of the star clusters in this complex and other regions were estimated using HST broadband photometries and the Starburst99 synthesis models. The star clusters in this complex are very young (within ages of ~3-5 Myr) and massive (masses of ~105 M⊙). The ionization parameter and metallicity of the complex were estimated using the emission-line ratios, and the star formation rates were calculated using monochromatic 24 μm, far-ultraviolet, and Hα-line luminosities. We speculate that Arp 24 may have formed in a retrograde flyby encounter indicated by its one-armed appearance and fanlike structure, and the formation of the YSCs in this galaxy was triggered by the interaction. The clusters in the YSC complex may have formed in a single giant molecular cloud simultaneously. From the ultraviolet to mid-infrared spectral energy distributions, we found that the region of the YSC complex is relatively bluer in the optical and has higher 24 μm dust emission relative to the starlight and 8 μm emission. This warm infrared color may due to a strong UV radiation field or other mechanisms (e.g., shocks) within this region that may destroy the polycyclic aromatic hydrocarbons and enhance the small-grain emission at 24 μm.