We present VLBI and BIMA observations of the Class II methanol maser source G 59.783+0.065. The methanol emission arises from the centre of a compact molecular core, supporting the view that methanol masers trace an early stage of massive star formation. The velocity spread of the masers is significantly larger than that of the parent molecular cloud, which is consistent with the methanol coming from a rotating structure within the molecular core. However, there is also weak evidence that the methanol is aligned with an outflow.

We report the discovery of shocked H2 emission at 2.12 μm associated with methanol masers, and discuss its implication for massive star formation.

Current models of class II methanol masers are able to describe the brightnesses of the strongest masers and provide a basis for explaining observed line ratios. Determination of the physical parameters in the source requires observational data in many maser transitions. In order to provide observational constraints for models we searched for and detected 7 new methanol masers. This allowed us to constrain the physical parameters of the 3 sources with the greatest number of detected methanol maser lines: W3(OH), NGC6334F, and G345.01 + 1.79. The models accurately account for the fluxes of the bulk of the detected maser lines. Remaining discrepancies most probably reflect the fact that the most prominent components of the different maser lines are formed under different conditions. This is supported by comparison of the line profiles. We outline directions for future studies in the field.

Results of model calculations for class II methanol masers (MMII) are presented. The model of the pumping assumes that an external dust layer provides the source of energy for maser excitation. The dependence of the emergent maser spectrum on the properties of the dust layer is studied. These properties include the chemical composition and sizes of dust grains, the temperature and optical depth of the dust layer. as well as the dilution factor of external dust emission. It is shown that, in order to reproduce the observed patterns of class II methanol maser line ratios, the pumping dust layer should be mostly composed of silicate grains with sizes smaller than 0.01 μ. It is shown that the layer of warm (> 125 K) dust effectively pumps the strongest MMII transitions over a wide range of 30 μ dust opacities (0.01–2) and dilution factors (0.02–0.5).

We study the probability density distributions of mass estimates for class II methanol masers with linear position-velocity structure, of maser cluster, using Monte Carlo tecniques. A uniform random distribution of maser sources along an edge-on rotating circumstellar-ring is assumed as a working hypothesis. It is found that in the cases with sufficiently high number (> 6) of observed maser spots it is possible to set upper limits for the mass encircled by the ring. Both turbulent motions and observational uncertainties are shown to result in the broadening of the probability density distribution of the mass estimate. As a consequence, commonly used procedures do not provide correct values for the lower mass estimate. Ten maser objects with the number of observed spots exceeding 6 were chosen for preliminary analysis. Within the framework of the working hypothesis all of these objects were found to be massive (M > 5 M⊙) with negligible probability for the mass exceeding 80 M⊙.

Results of a survey of Galactic star-forming regions in the lines of methanol 8−1 − 70E at 229.8 GHz, 3−2 − 4−1E at 230.0 GHz, 00 − 1−1E at 108.9 GHz, and a series of methanol lines J1 − J0E near 165 GHz are presented. Two masers, DR 21 (OH) and DR 21 West, and two maser candidates, L 379 IRS3 and NGC 6334I(N), as well as 16 thermal sources are found at 229.8 GHz. This is the first detection of methanol masers at a wavelength as short as 1 mm. At 108.9 GHz, masers were found towards G345.01 + 1.79 and probably, towards M 8E. Thermal emission is found towards 28 objects. Only thermal emission was found at 165 and 230.0 GHz (20 and 7 sources, respectively). The masers at 229.8 GHz belong to class I, whereas those at 108.9 GHz belong to class II, according to the classification by Menten (1991). The masers in DR 21 (OH) and DR 21 West can be roughly fitted by models with the gas kinetic temperature of the order of 50 K. The detection of the 108.9 GHz masers towards G345.01 + 1.79 and M 8E may indicate on a specific geometry of these objects. The combination of the existence of the class II J0 - J−1E masers towards W 3(OH), G345.01 + 1.79, W 48, and Cep A and our non-detection of the 3−2 − 4−1E and J1 - J0E lines is an evidence that the class II masers in these objects are pumped by the radiation of hot dust rather than by that of UC HII-regions.

Fifty-one objects in the 5−1 − 40E methanol line at 84.5 GHz was detected during a survey of Class I maser sources. Narrow maser features were found in 17 of these. Broad quasi-thermal lines were detected towards other sources. One of the objects with narrow features, the young bipolar outflow L 1157 was also observed in the 80 − 71A+ line at 95.2 GHz; a narrow line was detected at this frequency. Analysis showed that the broad lines are usually inverted. The quasi-thermal profiles imply that the line opacities are not larger than several units. These results confirm the plausibility of models in which compact Class I masers appear in extended sources as a result of an appropriate velocity field.

Measurements of linear polarization at 84.5 GHz in 13 sources were made. No polarization was found except a tentative detection of a weak polarization in M 8E.

We studied class I methanol masers in the transition 70 − 61A+ at the frequency 44 GHz with the VLA. The observations on the VLA were made with the angular resolution 0.1, which was the highest at the moment. It was shown that the masers consist of chains of unresolved spots located on curved lines or arcs. The length of such arcs is from 20 to 1000 AU and the brightness temperature of the strongest masers exceeds 3.6×108 K. The observed location of maser spots is in agreement with their position at the border line between molecular outflows and surrounding molecular clouds. The high brightness temperature implies that the maser condensations have enhanced abundance of methanol due to evaporation of methanol from the surface of dust grains. The mass of maser condensations is less than 4×10−5 M⊙ and corresponds to planetary masses.

Anomalous OH masers were detected towards supernova remnants (SNRs) more than 30 years ago. The satellite line at 1720 MHz was detected in emission, while the main lines (1665 and 1667 MHz) and the other satellite line, at 1612 MHz, occurred only in absorption. Independently, 25 years ago, a theoretical model was proposed whereby 1720 MHz OH masers could be collisionally excited by the passage of a shock through a molecular cloud. For an efficient inversion of the 1720 MHz transition, the gas should have a narrow range of physical properties, namely a kinetic temperature 50 ≤ TK ≤ 125 K, a volume density nH2 ∼ 105 cm−3, and a column density of OH gas 1016–1017 cm−2. However, it was not until 1994, with interferometric observations, that the importance of studies of these masers, formed when an SNR is in direct interaction with a molecular cloud, was realised. This discovery triggered a series of surveys to search all known Galactic SNRs. I will review the outcomes of these surveys and discuss the consequences of using 1720 MHz masers as a diagnostic tool for calculating shocked gas conditions, for magnetic field determinations (from Zeeman splitting measurements) and for measuring the properties of SNRs.

A new class of OH (1720 MHz) masers unaccompanied by main-line transitions have recently been discovered (Frail, Goss and Slysh 1994). These masers lie at the interface between supernova remnants (SNRs) interacting with molecular clouds. We discuss three new aspects of SN masers found in the direction toward the Galactic center: (i) the detection of a new −130 kms−1 OH (1720 MHz) maser in the southern lobe of the molecular ring at the Galactic center: (ii) the detection of extended OH (1720 MHz) maser emission from W28 accompanying the compact maser sources and (iii) the detection of linear polarization of the brightest OH (1720 MHz) maser in SNR G359.1-0.5.

Supernovae have a profound effect on the morphology, kinematics, and metallicity of galaxies. The impact of supernova shocks on surrounding molecular clouds is also thought to trigger new generations of star formation. A critical ingredient in such interactions and, indeed, all aspects of supernova remnant (SNR) evolution are magnetic fields. In recent years, OH (1720 MHz) masers have been used as signposts for the interaction of SNRs with molecular gas. In addition to tracing SNR/molecular cloud interactions, the OH (1720 MHz) maser line also provides a unique opportunity to measure the strength of the post-shock magnetic field via Zeeman splitting. Recent results from efforts to both detect the magnetic fields and resolve the maser spot sizes of OH (1720 MHz) masers toward W51C using the VLBA and W44 using MERLIN are presented. These observations have yielded magnetic field detections between 0.5 and 2.5 mG and large maser spot sizes of about 1015 cm.

We present CO J=2-1 observations made with the 12 Meter Telescope of NRAO using the On-The-Fly technique, towards the OH 1720 MHz maser detected in direction to the supernova remnant (SNR) Kes 69. OH 1720 MHz masers associated to SNRs are strong evidence of shocked molecular gas, and are proposed to be tracers of SNRs kinematical distances. In our images, the most conspicuous feature positionally coincident with the maser is a cloud at ∼ +41 km s−1. The difference between the velocity of the OH 1720 MHz maser and this cloud is ∼ 30 km s−1. At the systemic velocity of the OH 1720 MHz maser, we detected a weak, small clump with the maser lying at its edge, in agreement with previous findings in other SNRs. We suggest that this small clump has been shocked by the expanding SNR, and the ∼ +41 km s−1 component probably corresponds to gas accelerated by the shock front. We do not discard, however, that the ∼ +41 km s−1 component be just a quiescent, foreground cloud unrelated to Kes 69.

Maser emission from recombination lines has been detected towards the Young Stellar Object (YSO) MWC349 and the massive evolved star η Carinae. In spite of extensive searches of recombination line maser emission at millimeter wavelengths towards massive star forming regions, MWC349 remains unique. MWC349 is also a strong recombination line laser in the Far-IR with the largest amplification observed for transitions at wavelengths around 400 μm. The observational properties of the recombination line maser and laser emission from MWC349 are reviewed. Modeling of the maser and laser emission in MWC349 will be used to illustrate the potential of this kind of masers to understand the early phases of the evolution of massive stars. The impact that future instruments like the Herschel, the SMA and specially ALMA, will have in the investigation of recombination line maser emission from YSOs is briefly discussed.

Eta Car is a variable star surrounded by a cloud of gas and dust that absorbs a large part of its optical emission and that showed, in the past two hundred years, several events of mass ejection. It presents a 5.52 year period in the high excitation Heλ 10830 line intensity, indicating periodic changes in the ionizing radiation. Changes in the central velocity of optical lines, with the same periodicity, suggests the presence of a companion star, in a binary orbit. At radio frequencies, the region shows an optically thick bremsstrahlung spectrum, extending to millimeter and submillimeter wavelengths, and strong recombination lines at low quantum numbers showing maser effects. The flux densities, both from the continuum and recombination lines show a periodic behavior similar to that observed at optical wavelengths. We present observations of radio continuum and recombination lines at different millimeter wavelengths for several epochs. Based on these observations we modeled the emitting region as a thin disk and determined its temperature, electron density and velocity distributions using the radiation transfer equations in NLTE.

The high signal-to-noise ratio of the NH3(J,K) = (1,1) spectra from NGC 6334 have allowed at a first time a detailed study of departures from LTE conditions in this molecular cloud. Differences in the line shapes have shown that the surveyed region is composed of at least three overlapped sources in different stages of star formation. Comparison between physical parameters of NGC 6334 derived from LTE and non-LTE conditions are presented and discussed here.

We present NH3(J,K) = (1,1) observations toward CS(2,1) sources in the southern hemisphere, obtained with the Itapetinga Radio Telescope, that exhibit departure from LTE conditions. The mechanism of selective trapping in the hyperfine transitions of NH3(J, K) =(2,1)-(1,1) is invoked to explain the non-thermal population in the NH3(J, K) = (1,1) hyperfine states. This effect is relevant only when the width of the hyperfine lines lie between 0.3 and 0.6 km s−1. Due to this restriction, the assumption that the molecular cloud is formed by clumps, which produce spectra within this line-width interval seems to be a natural explanation for the non-thermal population. The observed spectra can be the result of the superposition of individual clump spectra with different central velocities. This model was applied to determine the physical conditions of the observed regions, providing satisfactory results for most of them. However, for some sources the model is not adequate to reproduce the observations, indicating that some other effects should be included, such as IR continuum, that was neglected in the radiative transfer calculations.

We have compared two samples of galaxies in very high and low density environments in order to study their physical properties and stablish any possible relation among them. This paper focus on the CO content as a diagnostic tool to examine galaxy properties such as the amount of molecular gas and the star formation efficiency. The similarities in star formation efficiency of the dense environments and the field galaxies suggest that the physical processes controling the formation of stars from the molecular gas are local rather than global.

I review the current state of studies of masers in the circumstellar envelopes of Asymptotic Giant Branch stars.

In recent years, it has become possible to trace rapid structural changes occurring in the extended atmospheres of red giant stars through VLBI monitoring of SiO masers. The observations reveal a complex picture. Firstly, material is predominantly outflowing, but also infalls towards the star, in a pulsating inner envelope periodically disrupted by shocks. Secondly, circular polarization measurements may indicate that the inner circumstellar envelope is permeated by a magnetic field strong enough to be of dynamical importance. Finally, towards a few stars, observations also show evidence for rotation of the SiO maser region. Current SiO maser models can reproduce many of the key features of circumstellar SiO maser emission, and have successfully predicted e.g. the infall of SiO masing gas and the observed spatial separation of v = 1 and v = 2 43 GHz masers. However, both stellar hydrodynamical and maser codes need to increase in complexity in order to model fully these regions. In this review I will discuss the current developments in circumstellar SiO maser observations and models.

Simultaneous VLBI observations of the SiO masers of the J = 1 → 0 rotational line in the v = 1 and v = 2 vibrational levels toward Mira variables are presented. Because SiO maser lines are formed deep in circumstellar envelopes they serve as a unique tool to study the innermost envelopes of evolved stars. Although the first interpretation of SiO maser emission was made in 1974, observational features are only partially explained by models which have been suggested since then. Positional coincidence of the J = 1 → 0 masers of v = 1 and v = 2 has been argued as a way to distinguish among the maser pumping models, but it requires simultaneous observations of the two lines using high resolution. We have developed the technique for such observations during the last few years and here we report successful results of our simultaneous observations of the SiO v = 1 and v = 2 J = 1 → 0 masers using the Very Long Baseline Array (VLBA). We discuss the pumping mechanism in terms of our observational results.