A model for the molecular accretion disk and H2O maser in the nucleus of the galaxy NGC 4258

A model is developed for the H₂O maser source observed in the circumnuclear region of the galaxy NGC 4258. The maser emission originates at distances of 0.15–0.29 pc from the center in a thin, cold accretion disk (gas-dust torus) rotating around a supermassive black hole of mass 4 × 10⁷ M _⊙. The conditions for the emergence of an inverse population of the signal 6₁₆?5₂₃ levels in ortho-H₂O working molecules are simulated numerically. The complex line profile, which includes both central and high-velocity components, is calculated. A comparison of the calculations with radiointerferometric and spectrophotometric observations allowed the physical conditions in the emitting region to be determined.     

An Accessible Model of Dynamics and Cycle for Interstellar OH and H2O Maser Associations

An accessible model for interstellar OH/H₂O maser associations is presented. It can be classified into radiative pumping model. It can close the dynamical cycle of H₂O and OH species, and can give an interpretation on interstellar OH/H₂O associations. A reasonable scheme for both regeneration and destruction of interstellar H₂O and OH molecules is argued. Our model has overcome the defects of former radiative models, and is compatible with astronomical conditions. It is shown that the rotational population of H₂O and OH in these regions is much less affected by collisions than by radiation. Some experiments have confirmed our proposal.     

The λ0 = 1.35 cm H2O maser line: The hyperfine structure and profile asymmetry

The spectra of several H₂O maser sources exhibit single λ₀ = 1.35 cm maser lines with narrow asymmetric profiles. We consider the hyperfine structure of the line that corresponds to the transition between the rotational 6₁₆ → 5₂₃ levels of ortho-H₂O molecules to account for the line asymmetry. Our numerical simulations of the maser line profile agree well with the observations if the hyperfine structure is taken into account.     

Suggestion for searh of ethylene oxide ($$-C2H4O) in a osmi objet

Ethylene oxide (\(c\)-C₂H₄O) and its isomer acetaldehyde (CH₃CHO) are important organic molecules because of their potential role in the formation of amino acids. The \(c\)-C₂H₄O molecule is a \(b\)-type asymmetric top molecule and owing to half-spin of each of the four hydrogen atoms, it has two distinct ortho (nuclear spin one) and para (nuclear spin zero and two) species. It has been detected in the Sgr B2N. Using the rotational and centrifugal distortion constants along with the electric dipole moment, we have calculated energies of 100 rotational levels of each of the ortho and para species of \(c\)-C₂H₄O molecule and the Einstein \(A\)-coefficients for radiative transitions between the levels. The values of Einstein \(A\)-coefficients along with the scaled values for the collisional rate coefficients are used for solving a set of statistical equilibrium equations coupled with the equations of radiative transfer.Brightness-temperatures of five rotational transitions of each of the ortho and para species of \(c\)-C₂H₄O molecule are investigated. Out of these ten transitions, three transitions are found to show the anomalous absorption and rest seven are found to show the emission feature. We have also investigated seven transitions observed unblended in the Sgr B2(N). We have found that the transitions \(3_{3 0} - 3_{2 1}\) (23.134 GHz), \(2_{2 0} - 2_{1 1}\) (15.603 GHz), \(3_{3 1} - 3_{2 2}\) (39.680 GHz) and \(1_{1 1} - 0_{0 0}\) (39.582 GHz) may play important role for the identification of ethylene oxide in a cosmic object.     

H2O + ions in comets: models and observations

An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H₂O ⁺ shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H₂O ⁺ ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H₂O ⁺ ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H₂O ⁺ ions. Finally, the two dimensional images of model column densities are compared with observations.     

H<Stack><Subscript>2</Subscript><Superscript>16</Superscript></Stack>O and H<Stack><Subscript>2</Subscript><Superscript>18</Superscript></Stack>O maser emission from gas-dust clouds

The collisional pumping of H₂¹⁶O and H₂¹⁸O masers in hot dense gas-dust clouds has been simulated numerically. New data on the rate coefficients for collisional transitions from Faure et al. (2007) were used in the calculations. The possibility of detecting H₂¹⁸O emission in 22.2-GHz H₂¹⁶O maser sources is investigated. The medium is shown to become optically thick in the H₂¹⁸O lines for which an inverted level population is observed at H₂O column densities of ∼1019–1020 cm⁻². A simultaneous observation of H₂¹⁸O emission and H₂¹⁶O maser emission in the same source will allow the physical conditions in the gas-dust cloud to be refined.     

H2O maser emission in circumstellar envelopes around AGB stars: Physical conditions in gas-dust clouds

The pumping of 22.2-GHz H₂O masers in the circumstellar envelopes of asymptotic giant branch stars has been simulated numerically. The physical parameters adopted in the calculations correspond to those of the circumstellar envelope around IK Tau. The one-dimensional plane-parallel structure of the gas-dust cloud is considered. The statistical equilibrium equations for the H₂O level populations and the thermal balance equations for the gas-dust cloud are solved self-consistently. The calculations take into account 410 rotational levels belonging to the five lowest vibrational levels of H₂O. The stellar radiation field is shown to play an important role in the thermal balance of the gas-dust cloud due to the absorption of emission in rotational-vibrational H₂O lines. The dependence of the gain in the 22.2-GHz maser line on the gas density and H₂O number density in the gas-dust cloud is investigated. Gas densities close to the mean density of the stellar wind, 10⁷?10⁸ cm^?3, and a high relative H₂O abundance, more than 10^?4, have been found to be the most likely physical conditions in maser sources.     

Stratospheric H2O

The present state of our knowledge and understanding of H₂O in the stratosphere is reviewed. This reveals continuing discrepancies between observations and expectations following from the Brewer-Dobson hypothesis of stratospheric circulation. In particular, available observations indicate unexplained upward and poleward directed H₂O gradients immediately downstream from the tropical tropopause and variable vertical gradients above 20 km which generally disagree with those expected from oxidation of CH₄.     

Simulating H2O Maser Emission in the Mass Outflows from Evolved Stars

The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H₂O maser emission from the circumstellar envelopes of long period variable stars. The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H₂O maser emission. As is observed in H₂O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H₂O maser regions are in conditions of n_H2 = 10⁶ − 10⁸ cm⁻³, assuming a fractional abundance of 10⁻⁴; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate based dust particles can produce the observed data on small (10-mas) angular scales. This revised version was published online in September 2006 with corrections to the Cover Date.     

Distribution and state of H2O in the high-latitude shallow subsurface of mars

A quantitative model of the state, distribution, and migration of water in the shallow Martian regolith is presented. Reported results are confined to the region of the planet greater than 40° lat. The calculations take into account (1) expected thermal variations at all depths, latitudes, and times resulting from seasonal and astronomically induced insolation variations; (2) variations in atmospheric P_H2O and P_CO2 resulting from polar insolation variations and regolith adsorptive equilibria; (3) feedback effects related to latent heat and albedo variations resulting from condensation of atmospheric constituents; (4) two possible regolith mineralogies; (5) variable total H₂O content of the regolith; (6) kinetics of H₂O transport through the Martian atmosphere and regolith; and (7) equilibrium phase partitioning of H₂O between the condensed, adsorbed, and vapor phases. Results suggest that the adsorptive capacity of the regolith is important in controlling the state and distribution of high-latitude H₂O; unweathered mafic silicates favor the development of shallow ground ice at all temperate and polar latitudes, while heavily weathered clay-like regolith materials leads to a deeper ground ice interface and far more extensive quantities of adsorbed H₂O. The capacity of the high-latitude regolith for storage of H₂O and the total mass of H₂O exchanged between the atmosphere, polar cap, and subsurface over an obliquity cycle is found to be relatively independent of mineralogy. The maximum exchanged volume is found to be 3.0 × 10⁴ km³ of ice per cycle. Implications for the history of the polar caps and the origin of the layered terrain are discussed. Results also suggest that seasonal thermal waves act to force adsorbed H₂O into the solid phase over a wide variety of latitude/obliquity conditions. Seasonal phase cycling of regolith H₂O is most common at high latitudes and obliquities. Such phase behavior is highly dependent on regolith mineralogy. In a highly weathered regolith, adsorbed H₂O is annually forced into the solid phase at all latitudes ≥40° at obliquities greater than approximately 25°. Seasonal adsorption-freezing cycles which are predicted here may produce geomorphologic signatures not unlike those produced by terrestrial freeze-thaw cycles.     

An improved model of radiative transfer for the NLTE problem in the NIR bands of CO<Subscript>2</Subscript> and CO molecules in the daytime atmosphere of Mars. 2. Population of vibrational states

The near-infrared (NIR) emission of the Martian atmosphere in the CO2 bands at 4.3, 2.7, 2.0, 1.6, 1.4, 1.3, 1.2, and 1.05 µm and in the CO bands at 4.7, 2.3, 1.6, and 1.2 µm is mainly generated under nonlocal thermodynamic equilibrium (NLTE) conditions for vibrational states, the transitions from which form the specified bands. The paper presents the results of simulations of the population of these states under NLTE for daytime conditions. In the cold high-latitude troposphere, the NLTE takes place much lower than in the troposphere under typical temperature conditions. If the NIR-radiation reflection from the surface is ignored, the population of high vibrational states substantially decreases, at least, in some layer of the lower atmosphere. However, inelastic collisions of CO₂ and CO molecules with O atoms produce no considerable influence on the values of populations. The population of vibrational states, the transitions from which form NIR bands, is also almost insensitive to possible large values of the quenching-in-collision rate constants of vibrational states higher than CO₂(00⁰1). However, very large errors in the estimates of the population of vibrational states of the CO₂ molecule (rather than the CO molecule!) can be caused by the uncertainty in the values of the rate constant of exchange between CO² molecules by the energy quantum of the asymmetric stretching vibrational mode. For this intermolecular exchange, we recommend a possible way to restrict the vibrational excitation degree of the molecule that is a collision partner and to maintain simultaneously a sufficiently high accuracy in the population estimate.     

Interstellar H2O masers. a tracer of galactic spiral structure?

The large-scale distribution of interstellar H₂O masers is given in this paper. Present results show that H₂O masers can trace galactic spiral structure, and we also briefly discuss how to obtain a more accurate distribution of H₂O masers from future observations.     

Modelling interstellar water masers

Radiative transfer calculations for interstellar H₂O have been performed using accelerated A-iteration (ALI) techniques. The results show strong maser action from known maser transitions, as well as predicting new strong maser transitions for > 1.5 THz.     

Infrared heating of Comet Halleys atmosphere

The warm circumnuclear dust in the inner cometary coma reradiates in the IR in the wavelength range of the ground state rotational band of the dominant atmospheric molecule, H₂O. However, the interaction of this radiation with H₂O has hitherto not been taken into account in cometary atmospheric models. Here we have extended our earlier two-phase, multifluid model of the dusty atmosphere by including this effect. Although this IR radiation initially pumps the rotational levels of H₂O, frequent intermolecular collisions in the inner coma transfer this energy from rotational modes to translational modes. As a result the temperature in the innermost coma no longer decreases to about 10 K, as predicted by the earlier models, but reaches a minimum of only about 120 K.     

The H2O maser toward IRAS 06308+0402

We present the monitoring results for the H₂O maser toward the infrared source IRAS 06308+0402 associated with a dense cold molecular cloud. The observations were carried out with the 22-m radio telescope at the Pushchino Radio Astronomy Observatory (Russia) during 1992–2003. The H₂O maser was discovered in May 1992 (Pashchenko 1992) during a survey of IRAS sources associated with dense cold clouds with bipolar molecular outflows. The H₂O spectrum contains many emission features, suggesting the fragmentation of the envelope around a young star. The star has a low peculiar velocity relative to the CO molecular cloud (～2.2 km s^?1). We found a cyclic variability of the total maser flux with a period from 1.8 to 3.1 yr.     

Infrared and electronic absorption spectra of formaldehyde in gas phase and astrophysical H<Subscript>2</Subscript>O ice

This work reports theoretical infrared and electronic absorption spectra of formaldehyde and its ions in gas phase and H₂O ice at different levels of theory. The vibrational frequencies from this work at B3LYP/6-311++G^** level are in agreement with the experimental determinations. The gas phase dipole moment of neutral formaldehyde 2.4 D is in excellent agreement with the experimental value of 2.33 D. An influence of ice on vibrational frequencies of neutral formaldehyde molecule was obtained using Self Consistence Isodensity Polarizable Continuum Model (SCI-PCM) with dielectric constant 78.5. Significant shift in vibrational frequencies for neutral formaldehyde molecule when studied in H₂O ice and upon ionization is observed. All the vibrational modes in cation and anion of formaldehyde in gas phase are red shifted than the corresponding modes in neutral formaldehyde. Two vibrational modes are blue shifted and all other modes are red shifted for neutral formaldehyde in H₂O ice. Time dependent density functional theory (TDDFT) is used to study electronic absorption spectrum of neutral formaldehyde and its charged states. It is found that like neutral formaldehyde, its cation and anion also display strong σ→σ ^∗ electronic transitions in vacuum and far UV regions. This study should help in detecting formaldehyde molecule and its ions in gas phase and in H₂O ice in different astronomical environment.     

A Radiative Pumping Scheme for OH and H<Subscript>2</Subscript>O Masers in Massive Star Forming Regions

Interstellar H₂O and OH masers associated with massive star-forming regions can be classified into three morphological types: isolated H₂O masers; isolated OH masers; and spatially overlapping OH/H₂O maser groups. In a large sample of star-forming regions the total number of maser groups of each type is approximately equal. In order to account for these statistics we propose a pumping scheme based on a broadband radiative pump which produces inverted populations of both OH and H₂O masers by a process involving predissociation and dissociation of H₂O. This scheme overcomes some drawbacks of earlier radiative pumping models, and may account for the association of OH and H₂O masers in massive star forming regions.     

Detection of rovibrationally excited H2 formed through the heterogeneous recombination of H atoms on a cold HOPG surface

This paper reports results from an experiment designed to measure the nascent rovibrational population of H₂ molecules that have formed through the heterogeneous recombination of H atoms on the surface of cosmic dust analogues under conditions approaching those of the interstellar medium (ISM). H₂ that has formed on a highly oriented pyrolytic graphite (HOPG) surface has been detected, using laser induced resonance-enhanced multi-photon ionization (REMPI), in the v = 1 (J= 0–3) rovibrational states at surface temperatures of 30 K and 50 K. These excited product molecules display rotational temperatures significantly higher than the target surface temperature. These first results suggest that a considerable proportion of the binding energy released on formation of the H₂ is deposited in the surface, in addition to internal excitation of the product molecules. This revised version was published online in July 2006 with corrections to the Cover Date.     

The infrared emission lines of H2

The photodissociation regions located between ionized regions and molecular clouds are studied by using a one-dimensional model where molecular H₂ are formed on the dust grains, and destructed by photodissociation. The escape probability method is used for the line transfer. The excitation of infrared emission lines of H₂ by UV fluorescence in M17, by shock heating in Orion KL and mainly by UV fluorescence in NGC 2023 are discussed.     

Ultraviolet irradiation of naphthalene in H2O ice: Implications for meteorites and biogenesis

Abstract— The polycyclic aromatic hydrocarbon (PAH) naphthalene was exposed to ultraviolet radiation in H₂O ice under astrophysical conditions, and the products were analyzed using infrared spectroscopy and high‐performance liquid chromatography. As we found in our earlier studies on the photoprocessing of coronene in H₂O ice, aromatic alcohols and ketones (quinones) were formed. The regiochemistry of the reactions is described and leads to specific predictions of the relative abundances of various oxidized naphthalenes that should exist in meteorites if interstellar ice photochemistry influenced their aromatic inventory. Since oxidized PAHs are present in carbon‐rich meteorites and interplanetary dust particles (IDPs), and ubiquitous in and fundamental to biochemistry, the delivery of such extraterrestrial molecules to the early Earth may have played a role in the origin and evolution of life.