OH masers at 1612 and 1720 MHz in star-forming regions

Masers at the ground-state OH satellite transitions near 1612 and 1720 MHz are occasionally found in star-forming regions, accompanying the dominant maser of OH at 1665 MHz. The satellite lines can then be valuable diagnostics of physical conditions in star-forming regions if we can first ascertain that all maser species truly arise from the same site. For this purpose, newly measured satellite line positions with subarcsecond accuracy are reported here, and compared with masers of main-line OH at 1665 MHz, with methanol masers at 6668 MHz, and with ultracompact H  ii regions. We confirm that most of the satellite-line OH masers that we have measured are associated with star-forming regions, but a few are not: several 1612-MHz masers are associated with late-type stars, and one 1720-MHz maser is associated with a supernova remnant. The 1720-MHz masers in star-forming regions are accounted for by a pumping scheme requiring high densities, and are distinctly different from those in supernova remnants where the favoured pumping scheme operates at much lower densities.     

Searching for high-redshift centimeter-wave continuum, line and maser emission using the Square Kilometer Array

We discuss the detection of redshifted line and continuum emission at radio wavelengths using a Square Kilometer Array (SKA), specifically from low-excitation rotational molecular line transitions of CO and HCN (molecular lines), the recombination radiation from atomic transitions in almost-ionized hydrogen (radio recombination lines; RRLs), OH and H₂O maser lines, as well as from synchrotron and free–free continuum radiation and HI 21-cm line radiation. The detection of radio lines with the SKA offers the prospect to determine the redshifts and thus exact luminosities for some of the most distant and optically faint star-forming galaxies and active galactic nuclei, even those galaxies that are either deeply enshrouded in interstellar dust or shining prior to the end of reionization. Moreover, it provides an opportunity to study the astrophysical conditions and resolved morphologies of the most active regions in galaxies during the most active phase of star formation at redshift z 2. A sufficiently powerful and adaptable SKA correlator will enable wide-field three-dimensional redshift surveys at chosen specific high redshifts, and will allow new probes of the evolution of large-scale structure (LSS) in the distribution of galaxies. The detection of molecular line radiation favours pushing the operating frequencies of SKA up to at least 26 GHz, and ideally to 40 GHz, while very high redshift maser emissions requires access to about 100 MHz. To search for LSS the widest possible instantaneous field of view would be advantageous.     

Positions of hydroxyl masers at 1665 and 1667 MHz

The Australia Telescope Compact Array has been used to observe more than 200 1665-MHz hydroxyl masers south of declination −16° and derive their positions with typical rms uncertainties of 0.4 arcsec. Many of the 1665-MHz maser sites are found to have 1667-MHz OH maser counterparts which are coincident, within the errors.
The resulting position list presented here includes all well-documented, previously reported 1665-MHz masers close to the Galactic plane in the galactic longitude range 230° (through 360°) to 13°. Nearly 50 newly discovered masers are also listed, chiefly in the longitude range 312° to 356°, where the observations were conducted as an intensive survey of a continuous zone close to the Galactic plane.
Many of the maser sites are discussed briefly so as to draw attention to those possessing properties that are unusual among this large sample. Most of the masers are of the variety found in star-forming regions – at the sites of newly formed massive stars and their associated ultracompact H ii regions. The new, accurate, positions reveal coincidences of the OH masers with the continuum radio emission, with the infrared emission from dust that accompanies such regions, and with emission from other maser species such as methanol at 6668 MHz and water at 22 GHz.
By-products of the survey, also presented here, include measurements of at least 11 objects that are not associated with massive star-forming regions. They comprise several OH/IR stars (detected at the 1667- or 1665-MHz transition of OH, though commonly found to be most prominent at the 1612-MHz transition) and several unusual masers that may pinpoint other varieties of late-type stars or protoplanetary nebulae.     

Evidence for Co-Propagation of 4765- and 1720-MHz OH Masers in Star-Forming Regions

Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s^–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected.     

Australia Telescope Compact Array 1.2-cm observations of the massive star-forming region G305.2+0.2

We report on Australia Telescope Compact Array observations of the massive star-forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non-local thermodynamic equilibrium excitation calculation suggests a kinematic temperature of the order of 200 K. A time-dependent chemical model is also used to model the gas-phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between  2 × 10⁴  and  1.5 × 10⁵ yr  . We also report on a feature to the south-east of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3 arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two H  ii regions. The implications of the non-detection of radio continuum emission towards G305A and G305B are also discussed.     

A possible formation mechanism of the asymmetry in the H2O maser emission line

We present a possible formation mechanism of the asymmetry in the maser emission line of H₂O sources associated with star-forming regions. Observations with the RT-22 radio telescope at the Pushchino Radio Astronomy Observatory are used. We analyze the line profiles of emission features in the sources G43.8-0.1, NGC 2071, and ON1. If the line is asymmetric, the left (low-velocity) wing is higher than the right wing. The proposed mechanism accounts for the observed asymmetry and makes it possible to estimate some physical parameters of the medium in the vicinity of a maser spot.     

Periodic class II methanol masers in G9.62+0.20E

We present the light curves of the 6.7 and 12.2 GHz methanol masers in the star-forming region G9.62+0.20E for a time-span of more than 2600 d. The earlier reported period of 244 d is confirmed. The results of monitoring the 107 GHz methanol maser for two flares are also presented. The results show that flaring occurs in all three masing transitions. It is shown that the average flare profiles of the three masing transitions are similar. The 12.2 GHz masers are the most variable of the three masers with the largest relative amplitude having a value of 2.4. The flux densities for the different masing transitions are found to return to the same level during the low phase of the masers, suggesting that the source of the periodic flaring is situated outside the masing region, and that the physical conditions in the masing region are relatively stable. On the basis of the shape of the light curve we excluded stellar pulsations as the underlying mechanism for the periodicity. It is argued that a colliding wind binary can account for the observed periodicity and provide a mechanism to qualitatively explain periodicity in the seed photon flux and/or the pumping radiation field. It is also argued that the dust cooling time is too short to explain the decay time of about 100 d of the maser flare. A further analysis has shown that for the intervals from days 48 to 66 and from days 67 to 135 the decay of the maser light curve can be interpreted as due to the recombination of a thermal hydrogen plasma with densities of approximately  1.6 × 10⁶ cm⁻³  and  6.0 × 10⁵ cm⁻³  , respectively.     

The brightest OH maser in the sky: A flare of emission in W75 N

A flare of maser radio emission in the 1665-MHz OH line with a flux density of about 1000 Jy was discovered in the star-forming region W75 N in 2003. At the time of its observations, it was the strongest OH maser in the entire history of research since the discovery of cosmic OH masers in 1965. The linear polarization of the flare emission reached 100%. A weaker flare with a flux density of 145 Jy was observed in this source in 2000–2001; this was probably a precursor of the intense flare. The intensity of two other spectral features decreased when the flare emerged. This change in the intensity of the emission from maser condensations (a brightening of some of them and a weakening of others) can be explained by the passage of a magnetohydrodynamic shock through regions of enhanced gas concentration.     

Radio sources in the 2dF Galaxy Redshift Survey – II. Local radio luminosity functions for AGN and star-forming galaxies at 1.4 GHz

We have cross-matched the 1.4-GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 deg² (about 20 per cent of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio identifications – the largest and most homogeneous set of radio source spectra ever obtained. The 2dFGRS radio sources span the redshift range     to 0.438, and are a mixture of active galaxies (60 per cent) and star-forming galaxies (40 per cent). About 25 per cent of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We make a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star formation density of         .     

Discovery of molecular hydrogen line emission associated with methanol maser emission

We report the discovery of H₂ line emission associated with 6.67-GHz methanol maser emission in massive star-forming regions. In our UNSWIRF/AAT observations, H₂ 1–0 S(1) line emission was found associated with an ultracompact H  ii region IRAS 14567–5846 and isolated methanol maser sites in G318.95–0.20 , IRAS 15278–5620 and IRAS 16076–5134 . Owing to the lack of radio continuum in the latter three sources, we argue that their H₂ emission is shock excited, while it is UV-fluorescently excited in IRAS 14567–5846 . Within the positional uncertainties of 3 arcsec, the maser sites correspond to the location of infrared sources. We suggest that 6.67-GHz methanol maser emission is associated with hot molecular cores, and propose an evolutionary sequence of events for the process of massive star formation.     

An analysis of the line shape for H2O maser emission peaks in star-forming regions

We analyze the line shape for emission peaks of H₂O maser sources associated with star-forming regions by using the spectra obtained with the RT-22 radio telescope at the Pushchino Radio Astronomy Observatory. For five sources, we found the line profile of emission peaks to be asymmetric. In all cases, the left (high-frequency) line wing is higher than the right wing. Our analysis of the line shape yielded additional information on the structure and evolution of the maser sources under study. In G43.8-0.1, the emission feature was found to split up into two components. To explain the evolution of the 16.8 km s^?1 line in NGC 2071, we propose a model in which the line-of-sight velocity gradient changes under the effect of a (non-shock) wave. The observed short-duration flares of individual emission features in W75N can emerge due to a chance projection of the numerous clumps of matter involved in Keplerian motion onto each other.     

A Radiation Mechanism for Interstellar Methanol 6.7GHz Masers

All interstellar methanol maser sources can be divided into two classes. Their spectra are distinctive. In particular, the prominent Class II maser transitions, 6.7GHz and 12.2GHz lines show enhanced absorption toward Class I sources. We notice that the 6.7 and 12.2GHz methanol masers toward Class II sources are associated with each other and coexist toward ultracompact HII regions. Therefore we suggest a new pumping mechanism – methanol masers without population inversion. It can be used to explain the formation of 6.7GHz methanol masers while the 12.2GHz methanol masers are regarded as driving coherent microwave field. In this paper, we demonstrate that the new mechanism is associated with astronomical conditions and does not contradict with other mechanisms. This revised version was published online in July 2006 with corrections to the Cover Date.     

RadioAstron Science Program Five Years after Launch: Main Science Results

The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on baselines of 100–200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure testifying to recollimation shocks on scales of 100–250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer baselines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which considerably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20–60 μas correspond to linear sizes of about 5–10 million km (several solar diameters).     

A MERLIN survey of 4.7-GHz excited OH masers in star-forming regions

Phase-referenced observations of 13 star-forming regions in the  ²Π_1/2, J = 1/2  transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of  ∼10⁷ K  at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H  ii region and has a brightness temperature of  ∼4 × 10⁵ K  . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.     

MERLIN imaging of the maser flare in Markarian 348

MERLIN images of Mrk 348 at 22 GHz show H₂O maser emission at 0.02–0.11 Jy , within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is ≲ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 yr. The maser–core separation is barely resolved, but at the 3 σ significance level maser and core are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c . The correlation between the radio continuum increase and maser flare is explained as arising from high-level nuclear activity through a common excitation mechanism, although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.     

A sample of radio-loud active galactic nuclei in the Sloan Digital Sky Survey

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the National Radio Astronomy Observatories (NRAO) Very Large Array (VLA) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey. The comparison is carried out in a multistage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multicomponent radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95 per cent completeness and 98.9 per cent reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud active galactic nuclei (AGN) and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000-Å break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star-forming galaxies brighter than 5 mJy at 1.4 GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-infrared (FIR) correlation, the radio luminosity function of star-forming galaxies is also compared to the luminosity function derived in the FIR. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to FIR correlation breaks down below about 10²² W Hz⁻¹ at 1.4 GHz.     

Methanol masers at 107.0 and 156.6 GHz

From a search of more than 80 southern class II methanol maser sites, we report measurements of 22 masers at 107.0 GHz and four at 156.6 GHz, mostly new discoveries. Class II sites, recognized by their strong emission at the 6.6-GHz methanol transition, are indirect indicators of new-born massive stars, and several hundred have been documented; only a handful of these had previously been found to exhibit maser emission at the 107.0- or 156.6-GHz transition. The present survey increases the number of known 107.0-GHz masers to 25, providing a sufficiently large sample to assess their general properties. For the stronger ones, our position measurements confirm that, to an accuracy of 5 arcsec, they coincide with the dominant maser emission at 6.6 GHz. Intensity variations exceeding 50 per cent have occurred in some 107.0-GHz maser features that we observed in both 1996 October and 1998 June.
We find that masers are rare at the 156.6-GHz transition. Two new detections increase the total now known to four. Each 156.6-GHz maser is substantially weaker than its corresponding 107.0-GHz maser. Despite the scarcity of masers, our 156.6-GHz spectra at most observed sites show emission, but apparently of a quasi-thermal variety; it is usually accompanied by somewhat weaker thermal emission at 107.0 GHz, and the intensity ratio of the transitions allows us to begin exploration of the physical characteristics of the small molecular clouds (diameter less than 60 mpc) at these sites. The thermal emission thus provides estimates of the environmental conditions that are needed to support strong masing from spots that are apparently embedded within these clouds.     

Amplification by Stimulated Emission in Rydberg Matter Clusters as the Source of Intense Maser Lines in Interstellar Space

The detailed processes giving maser line radiation from various molecules in space are not well understood, as can be seen from many recent detailed studies of maser line emission with high spatial and velocity resolution, and with polarization measurements. We now propose an improved maser mechanism based on amplification of the original molecular line emission by stimulated emission in Rydberg Matter (RM) clouds in HII regions, containing clusters H _N and (H₂) _N . This mechanism will amplify the molecular lines, depending on the position, velocity, cluster size and state of excitation of the clusters in the RM cloud. RM will only support certain frequencies, corresponding to rotational transitions of the clusters. The bond lengths in the RM clusters are known within 1% from radio frequency emission measurements in the laboratory, and it is now shown that all the commonly studied maser lines agree well with stimulated emission transitions in several types of RM clusters simultaneously. This may explain the strongly varying intensities of neighboring or related maser lines, an important effect that is not well understood previously. It is also pointed out that the magnetic field due to RM is of the same order of magnitude as observed from the Zeeman splitting in maser lines; thus, the molecules that are the original sources of the lines may be embedded in the RM clouds, for example in dense HII regions that are likely to be RM regions.     

Determining the period of the long-period activity of the water-vapor maser in W75N

We present the results of our study of the H₂O maser emission from the source W75N, which is associated with a star-forming region, between November 1994 and March 1999. The observations were carried out with the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Lebedev Physical Institute). The maser emission in 1994–1999 can be represented as a superposition of flares of separate components with a duration from two to six months, which occurred mainly in the radial-velocity range 8–17.5 km s^?1. We detected a regular drift of the velocity centroid from 13 to 9 km s^?1 and an abrupt change in its velocity from 9 to 5 km s^?1, which took place at the initial stage of maser activity. Based on the variability of the total H₂O flux in all years of our observations of W75N (from December 1979 through March 1999), we conclude that the long-period variability of the water-vapor maser emission has a period of ～11.5 years. We give arguments that this variability is mainly associated with the most compact group of maser spots, whose positions coincide with the position of the continuum source VLA 2.     

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.