Extragalactic H<Subscript>2</Subscript>O maser sources and their properties

The properties of a sample of extragalactic H₂O maser sources over a wide spectral range are discussed. Based on a sample of 81 maser galaxies it is shown that mega- and kilomasers have completely different properties. In particular, for megamasers the strongest observed relationships are between the parameters of the H₂O line and the mass of the galactic nucleus, while the parameters of the line are uncorrelated with the x-ray, infrared, and radio emission. A weak correlation between megamaser emission and the surface (column) density of hydrogen is observed. As for kilomasers, their H₂O luminosity depends weakly on the x-ray emission, although in the case of soft x rays this dependence is significant. The H₂O luminosity of kilomasers is moderately correlated with the infrared and radio continuum luminosities, but the line parameters are independent of the mass of the nucleus and the surface density of hydrogen.     

Flares of the H<Subscript>2</Subscript>O maser in W31(2)

We analyze our monitoring data for the water-vapor maser in the source W31(2), associated with a region of vigorous star formation, a cluster of OB stars. The monitoring was performed with the 22-m radio telescope at Pushchino Radio Astronomy Observatory during 1981–2004. The variability of the H₂O maser in W31(2) was found to be cyclic, with a mean period of 1.9 yr. Two flares were most intense (superflares): in 1985–1986 and 1998–1999. In each activity cycle, we observed up to several short flares, subpeaks. The fluxes of many emission features during the flares were correlated. We also observed successive activation of individual emission features in order of increasing or decreasing radial velocity, suggesting an ordered structure and, hence, a radial-velocity gradient of the medium. There is a clear correlation of the emission peaks of the main components in the spectra at radial velocities of ?1.7, ?1.3, 0.5, and 1.3 km s^?1 with activity cycles and of the emission at V_LSR < ?8 km s^?1 with short flares. During the superflares, the emission in the low-velocity part of the H₂O spectrum and a number of other phenomena related to coherent maser-emission properties were suppressed. The maser spots are assumed to form a compact structure, to have a common pumping source, and to be associated with an accretion flow onto the cluster of OB stars.     

Triplet structure of the H<Subscript>2</Subscript>O spectra in S255

We analyzed the monitoring data for the maser S255 obtained in the H₂O line at λ=1.35 cm with the 22-m radio telescope at the Pushchino Radio Astronomy Observatory in 1981–2002. The maser was most active during 1998–2002. Since 2001, the H₂O spectra have been extended and complex; their triplet structure has been disrupted. The extent of the spectra was 24 km s^?1 (from ?6 to 18 km s^?1). We calculated orbital parameters for some of the components. We estimated the mass of the central star to be (6–7)M_⊙ and the outer Keplerian-disk radius to be ～160 AU.     

Investigation of OH and H<Subscript>2</Subscript>O masers in the star-forming region G 188.946+0.886

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nançay Observatory (France) and in the H₂O line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V _LSR = 3.6 km s^?1, has a “blue” shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8–11 km s^?1. This suggests a probable connection of the observed feature in the 1720-MHz line with the “blue” wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of H₂O maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms.     

On a new cluster of H<Subscript>2</Subscript>O maser spots in the source ON2

We present the results of our observations of the H₂O maser emission toward the complex source ON2 associated with an active star-forming region. The observations were performed in a wide range of radial velocities, from ?75 to 90 km s^?1. We have detected an emission with flux densities of 9.2, 4, and 26 Jy at radial velocities of ?33.5, ?24.4, and ?18.8 km s^?1, respectively, at which no emission has been observed previously. The detected emission is most likely associated with a hitherto unknown cluster of maser spots located between the northern (N) and southern (S) components of the source ON2 (closer to the northern one). This cluster may be associated with one of the three CO molecular outflows in the ON2 region. We have also detected an emission at ?22 and ?14.5 km s^?1 in N and at 12.6 km s^?1 in S, which has extended significantly the velocity ranges of the maser emission in these sources and allowed their models to be improved.     

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.     

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.     

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.     

Evolution of the structure of the H<Subscript>2</Subscript>O supermaser outburst region in Orion KL

During the period 1979–1999, we investigated the hyperfine structure of the H₂O supermaser region located in the core of the molecular cloud OMC-1 in Orion KL. The angular resolution is 0.1 mas, which corresponds to 0.045 AU. The detected structure, which consists of a central object, an accretion disk, a bipolar outflow, and an envelope, corresponds to the initial formation stage of a low-mass star. The accretion disk is at the stage of separation into groups of concentric rings. The bipolar outflow is a neutral, highly collimated jet of accreted material that includes H₂O molecules and dust grains in the icy envelope. The injector is a bright compact source with a size <0.05 AU and a brightness temperature T_b≈10¹⁷ K. The velocity of the bipolar outflow is v≈10 km s^?1. The rotation velocity of the jet is v_rot≈1.5 km s^?1. The jet has the shape of a conical helix due to the precession of the rotation axis. Occasionally, dense blobs (comet-shaped bullets) are ejected. The envelope amplifies the radio emission from the structures in a ～0.5 km s^?1 maser window band with velocities v≈7.65 km s^?1 by more than two orders of magnitude.     

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.     

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.     

HD/H<Subscript>2</Subscript> molecular clouds in the early Universe: The problem of primordial deuterium

We have detected new HD absorption systems at high redshifts, z _abs = 2.626 and z _abs = 1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170, respectively. Each of these systems consists of two subsystems. The HD column densities have been determined: log N _HD^A = 15.70 ± 0.07 for z _A = 2.626443(2) and log N _HD^B = 12.98 ± 0.22 for z _B = 2.626276(2) in the spectrum of J0812+3208 and log N _HD^C = 14.83 ± 0.15 for z _C = 1.77637(2) and log N _HD^D = 14.61 ± 0.20 for z _D = 1.77670(3) in the spectrum of Q1331+170. The measured HD/H₂ ratio for three of these subsystems has been found to be considerably higher than its values typical of clouds in our Galaxy.We discuss the problem of determining the primordial deuterium abundance, which is most sensitive to the baryon density of the Universe Ω_b. Using a well-known model for the chemistry of a molecular cloud, we have estimated the isotopic ratio D/H=HD/2H₂ = (2.97 ± 0.55) × 10⁻⁵ and the corresponding baryon density Ω_b h ² = 0.0205_−0.0020^+0.0025. This value is in good agreement with Ω_b h ² = 0.0226_−0.0006^0.0006 obtained by analyzing the cosmic microwave background radiation anisotropy. However, in high-redshift clouds, under conditions of low metallicity and low dust content, hydrogen may be incompletely molecularized even in the case of self-shielding. In this situation, the HD/2H₂ ratio may not correspond to the actual D/H isotopic ratio. We have estimated the cloud molecularization dynamics and the influence of cosmological evolutionary effects on it.     

The expansion field: the value of <Emphasis Type="Italic">H</Emphasis><Subscript>0</Subscript>

Any calibration of the present value of the Hubble constant (H ₀) requires recession velocities and distances of galaxies. While the conversion of observed velocities into true recession velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and cover a useful range of about 4–30 Mpc is crucial. A list of 279 such galaxy distances within v < 2,000 km s⁻¹ is given which are derived from the tip of the red-giant branch (TRGB), from Cepheids, and/or from supernovae of type Ia (SNe Ia). Their random errors are not more than 0.15 mag as shown by intercomparison. They trace a linear expansion field within narrow margins, supported also by external evidence, from v = 250 to at least 2,000 km s⁻¹. Additional 62 distant SNe Ia confirm the linearity to at least 20,000 km s⁻¹. The dispersion about the Hubble line is dominated by random peculiar velocities, amounting locally to <100 km s⁻¹ but increasing outwards. Due to the linearity of the expansion field the Hubble constant H ₀ can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of 78 galaxies above this limit give H ₀ = 63.0 ± 1.6 at an effective distance of 6 Mpc. They compensate the effect of peculiar motions by their large number. Support for this result comes from 28 independently calibrated Cepheids that give H ₀ = 63.4 ± 1.7 at 15 Mpc. This agrees also with the large-scale value of H ₀ = 61.2 ± 0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of H ₀ = 62.3 ± 1.3 is adopted. Because the value depends on two independent zero points of the distance scale its systematic error is estimated to be 6%. Other determinations of H ₀ are discussed. They either conform with the quoted value (e.g. line width data of spirals or the D _n −σ method of E galaxies) or are judged to be inconclusive. Typical errors of H ₀ come from the use of a universal, yet unjustified P–L relation of Cepheids, the neglect of selection bias in magnitude-limited samples, or they are inherent to the adopted models.     

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.     

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.     

H2O maser pumping: The effect of quasi-resonance energy transfer in collisions between H2 and H2O molecules

The effect of quasi-resonance energy transfer in collisions between H₂ and H₂O molecules in H₂O maser sources is investigated. New data on the state-to-state rate coefficients for collisional transitions for H₂O and H₂ molecules are used in the calculations. The results of ortho-H₂O level population inversion calculations for the 22.2-, 380-, 439-, and 621-GHz transitions are presented. The ortho-H₂O level population inversion is shown to depend significantly on the population distribution of the para-H₂ J = 0 and 2 rotational levels. The possibility of quasi-resonance energy transfer in collisions between H₂ molecules at highly excited rotational-vibrational levels and H₂O molecules is considered. The quasi-resonance energy transfer effect can play a significant role in pumping H₂O masers in the central regions of active galactic nuclei and in star-forming regions.     

Multi-Wavelength Studies on H2O Maser Host Galaxies

H₂O maser emissions have been found in external galaxies for more than 30 years. Main sciences associated with extragalactic H₂O masers can be summarized roughly into three parts: maser emission itself, AGN sciences and cosmology exploration. Our work in this field focusses on two projects: X-ray data analysis of individual maser source using X-ray penetrability to explore maser host obscured AGN; multi-wavelength statistical properties of the whole published H₂O maser sample. Here their nuclear radio properties were investigated in detail, based on their 6-cm and 20-cm radio observation data. Comparing the radio properties between maser-detected sources and non-detected sources at similar distance scale, we find that: (1) maser host galaxies tend to have higher nuclear radio luminosity; (2) the spectral index of both samples is comparable (～0.6), within the error ranges. In addition, for AGN-maser sources, the isotropic maser luminosity tends to increase with rising radio luminosity. Thus we propose the nuclear radio luminosity as one good indicator for searching AGN-masers in the future.     

Asymmetry in the spectrum of high-velocity H<Subscript>2</Subscript>O maser emission features in active galactic nuclei

We suggest a mechanism for the amplification of high-velocity water-vapor maser emission features from the central regions of active galactic nuclei. The model of an emitting accretion disk is considered. The high-velocity emission features originate in the right and left wings of a Keplerian disk. The hyperfine splitting of the signal levels leads to an asymmetry in the spectral profile of the water-vapor maser line at a frequency of 22.235 GHz. We show that the gain profile asymmetry must lead to an enhanced brightness of the blueshifted high-velocity emission features compared to the redshifted ones. Such a situation is observed in the source UGC 3789.     

Investigation of the OH and H2O maser emission from the semiregular variable HU puppis

We present the results of our monitoring of the semiregular variable HU Pup in the 1612, 1665, and 1667-MHz OH lines and the 22.235-GHz H₂O line. The maser emission in the 1612-MHz satellite line has been detected from this source for the first time. Strong variability of the emission has been observed in all three OH lines, including the radial-velocity drift of the two most intense features. Zeeman splitting components have been found. The longitudinal magnetic field strength has been estimated to be 1.0, 1.6 and 2.7, 3.2 mG in the 1665 and 1667-MHz lines, respectively. Our OH and H₂O observations have revealed fairly stable structures in the masing region and have allowed us to estimate the variability period of the maser emission (～1.5 yr). A possible model of the maser source in HU Pup is discussed.