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.     

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.     

Merlin and Puschino Observations of H2o Masers in Outer Galactic SFR S128N

We have imaged H₂O maser emission from the star-forming region S128 at milli-arcsec resolution using MERLIN, to complement 20 years of monitoring data from the Puschino radio telescope. The drift velocities of the masers and the velocity and location of a new maser region add depth to the model of two colliding CO clouds triggering collapse. Some H₂O masers appear to originate directly from this shock front. The brightest maser appears typical of a YSO jet and remains unsaturated close to peak intensity. The distribution of maser clumps has a fractal dimension 0.4; combined with analysis of drift velocity variations this suggests that the masers trace the dissipation of supersonic turbulence. The spatial distribution of velocities shows that this is in parts more structured than the Kolmogorov cascade.     

The circumstellar envelope of VX Sgr

By use of the data for VLBI observation of SiO masrs, and MERLIN observation of H₂O and OH masers of VX Sgr, we found that the SiO, H₂O, OH 1665/7, OH 1612 masers occur on a few separated shells, and the order of magnitude of the radii of their corresponding shells is 10¹⁴, 10¹⁵, 10¹⁶ cm, respectively. The field of the expanding velocity of the shells is obtained. It is important to note ice-layer covers on the surface of dust and grain for outlayer envelopes.     

Low-Luminosity Extra-Galactic H2O Masers

Recent high-resolution observations of low-luminosity extra-galactic H₂O masers towards M 82, M 51, and NGC 4051 are presented in this contribution. These observations were conducted using the very large array (VLA). The general observational properties of these masers are discussed.     

Cs studies of dense cores in regions of high mass star formation: a survey of southern molecular masers

We searched for the CSJ = 2 – 1 emission towards 29 southern H₂O and H₂O/OH masers and 1 OH maser with the SEST radio telescope. We detected and mapped 24 CS emitting regions probably associated with 27 H₂O masers. The C³⁴SJ = 2 – 1 and COJ = 1 – 0 lines were also observed at the grid positions closest to the CS peaks. Four cores were mapped in the CSJ = 5 – 4 and C³⁴SJ = 2 – 1 lines.     

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.     

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.     

On the properties of very young massive infrared sources (Mitteilungen der Universitäts-Sternwarte zu Jena Nr. 169)

The data compiled for 34 very young massive compact infrared source (Becklin-Neugebauer objects) in a separately published catalogue serve as the basis for an investigation of their general properties. First, the correlations of the strength of the ice and silicate bands at 3.1 μm and 10 μm, resp., with each other and with other parameters of the sources are investigated. In accordance with theoretical expectations the strengths of both bands are not well correlated with each other. The ratio τ₁₀/τ₃ of their optical depths veries from 0.85 to 8 (and possibly up to 22). That implies for the number densities of the grains that (naked) silicate grains always are considerably more abundant than ice grains (grains with ice mantles). It follows from the discussion of the relationships between τ₁₀, the temperature parameter of the sources, and their luminosity that relations between τ₁₀ and temperature and possibly between τ₁₀ and the luminosity exist. Stellar wind models for the ionized region around the embedded star are favoured after the analysis of the intensities of the hydrogen infrred emission lines and the radio continuum. The limited observational data do not yet allow a decision on a correlation between the silicate band depth and energy content and/or the velocity of the bipolar outflow detected in a number of sources. The BN objects are clearly more frequently associated with H₂O masers than OH masers. The linear separations of the infrared peaks from the masers are compatible with models where the H₂O masers are situated near the edges the gas-dust shells of the BN objects.     

VLBI observations of high-opacity HI gas in NGC 5793

We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×10²² cm⁻² assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm⁻³). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H₂O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO.     

Evidence for the effect of the interstellar far UV-radiation on circumstellar shells

The interstellar UV field at 1565 Å is calculated around nearby OH/IR sources. The front-back asymmetry observed in the 1612 MHz maser line profile is well correlated with anisotropy of the interstellar UV flux. For some sources the spatial positions of the 1612 MHz masers are confined to the position angles for which stronger UV radiation occurs. These facts strongly support the theory of the photoproduction of OH from H₂O induced by ambient interstellar UV photons penetrating the circumstellar shell. A simple model of the 1612 MHz maser with OH photoproduction suggests that the influence of the UV field on the observed maser profiles is governed by the mass loss rate and the relative abundances of OH and H₂O molecules.     

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.     

Molecular gas around symbiotic miras: masers in R aquarii and H1-36

The concept of studying symbiotic Miras via their molecular emission holds the promise of an improved understanding of the circumbinary environment in symbiotics, but following the largely ineffective maser surveys conducted during the 1970s and 80s there has been a widespread misconception that this avenue of study is a dead end. Here, we present spectral line and continuum maps of R Aqr and H1-36, two objects in which OH and H₂O masers were detected during a recent survey of symbiotic Miras.     

The research of the time variation of H2O maser

To explain the intensity fluctuations observed from bursts of W3(OH) and Cep A H₂O masers in the period of 50–60 days, we propose a new idea that energy of the maser pump is injected in the maser region with pulse form not only once but several times, and that it is supplied by the radiation diffusing through the gas cloud, in spite of the pump mechanism being radiation pump or collision pump. With our model the fits are obviously better than what many authors obtained in previous literature.     

High-Resolution Observations in B1-IRS: Ammonia, Ccs and Water Masers

We present a study of the structure and dynamics of the star-forming region B1-IRS (IRAS 03301+3057) using the properties of different molecules at high angular resolution (4). We have used VLA observations of NH₃, CCS, and H₂O masers at 1 cm. CCS emission shows three clumps around the central source, with a velocity gradient from red to blue-shifted velocities towards the protostar, probably due to the interaction with outflowing material. Water maser emission is elongated in the same direction as a reflection nebula detected at 2 m by 2MASS, with the maser spots located in a structure of some hundreds of AU from the central source, possibly tracing a jet. We propose a new outflow model to explain all our observations, consisting of a molecular outflow near the plane of the sky. Ammonia emission is extended and anticorrelated with CCS. We have detected for the first time this anticorrelation at small scales (1400 AU) in a star-forming region.     

A Survey and Statistics of Interstellar OH and H_2O Masers

We present a statistical analysis of a sky survey of interstellar H2O and OH masers. These masers can be classified into three categories: isolated H2O masers, isolated OH masers, and simple OH/H2O maser associations. The total number of sources in each category is of the same order of magnitude, and as an evolutionary phase they can maintain -105 yr. An improved radiative pumping mechanism is proposed. This model avoids some of the deficiencies of previous radiative models, such as shortage of exciting photons. The statistical results obtained from the survey can be interpreted by the new mechanism together with the evolutionary model in which the gravitational force of the central stellar objects is responsible for the HII region.     

Characteristics of Massive Star-forming Molecular Cores: The Spectral Observations of CO,CO and CO and the Statistical Comparison

With the 13.7 m millimeter wave telescope of Purple Mountain Observatory at Qinghai Station, the simultaneous mapping observations at the ¹²CO(J=1-0), ¹³CO(J=1-0) and C¹⁸O(J=1-0) lines were performed towards the 24 Galactic high-mass star-forming cores, which are associated with water masers and have available Spitzer's infrared data. The average mapping range was 8′ × 8′. The C¹⁸O line emission was detected in all the cores, in which 11 cores were observed to the half maximum of their C¹⁸O integrated intensities and the rather extended (5′ − 8′) C¹⁸O maps were obtained, while the others were failed to make such a large scale mapping because of the low SNR or the intrinsically extended morphology of the cores. On the 11 completely mapped dense cores, we analyzed their characteristics and made the statistics and comparisons on the integrated intensity ratios between ¹²CO and ¹³CO (R_12/13), ¹³CO and C¹⁸O(R_13/18), as well as ¹²CO and C¹⁸O(R_12/18). We concluded that as a tracer of dense gas, C¹⁸O is absolutely optically thin and can be used to detect the detailed structures of the cores, and that in general the 3 ratios increase gradually from the core center to the periphery. We found that the integrated intensity ratio R_12/13 ranges from 2 to 6; R_13/18 fluctuates between 4 and 20, but in central regions it is concentrated in the range 6–12 with a small fluctuation; and R_12/18 occupies a wider range 13–90, but it is concentrated between 13 and 50 in the denser regions of the cores.     

The bends in the slopes of radial abundance gradients in the disks of spiral galaxies – do they exist?

Spiral galaxies with a reported bend in the slope of the oxygen abundance O/H_{R 23}, derived with the traditionally used R₂₃-method, are examined. It is shown that the artificial origin of the reported bends can be naturally explained. Two causes of the false bend in the slope of O/H_{R 23} are indicated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Injection of molecules from the surfaces of grains into the gas phase chemistry of an expanding cloud

We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction A _v during the expansion. We consider separately injection at two different values of A _v, and include a representative large hydrocarbon, C₆H, and sulphur-bearing molecule, H₂SO₄, as well as H₂O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred.