The groundwork of the cosmic maser theory was laid four decades ago. The elapsed time, including the few years after the last IAU symposium dedicated to masers, did not add much to the fundamentals. In this review, I will summarize some cornerstones of the theory, with an emphasis on issues that don't seem to have received due attention in the past. I will also comment on some new developments.

Numerical modeling of molecular masers is necessary in order to understand their nature and diagnostic capabilities. Model construction requires elaboration of a basic description which allows computation, that is a definition of the parameter space and basic physical relations. Usually, this requires additional thorough studies that can consist of the following stages/parts: relevant molecular spectroscopy and collisional rate coefficients; conditions in and around the masing region (that part of space where population inversion is realized); geometry and size of the masing region (including the question of whether maser spots are discrete clumps or line-of-sight correlations in a much bigger region) and propagation of maser radiation. Output of the maser computer modeling can have the following forms: exploration of parameter space (where do inversions appear in particular maser transitions and their combinations, which parameter values describe a ‘typical’ source, and so on); modeling of individual sources (line flux ratios, spectra, images and their variability); analysis of the pumping mechanism; predictions (new maser transitions, correlations in variability of different maser transitions, and the like). Described schemes (constituents and hierarchy) of the model input and output are based mainly on the experience of the authors and make no claim to be dogmatic.

I have analysed the pumping scheme of 1720-MHz OH masers in great detail.

Maser polarization observations can reveal unique information on the magnetic field strength and structure for a large number of very different astronomical objects. As the different masers for which polarization is measured, such as silicon-monoxide, water, hydroxil and methanol, probe different physical conditions, the masers can even be used to determine for example the relation between magnetic field and density. In particular, maser polarization observations have improved our understanding of the magnetic field strength in, among others, the envelopes around evolved stars, Planetary Nebulae (PNe), massive star forming regions, supernova remnants and megamaser galaxies. This review presents an overview of maser polarization observations and magnetic field determinations of the last several years and discusses some of the theoretical considerations needed for a proper maser polarization analysis.

Magnetic fields are known to play an important role in several stages of the star formation process. Class I methanol (CH3OH) masers offer the possibility of measuring the large-scale magnetic field in star forming regions at high angular resolution, due to connections between the large-scale magnetic field in the pre-shock regions to the observed magnetic field along the outflows in the post-shock regions where these masers are formed. The detection of the Zeeman effect in the 36 GHz and 44 GHz Class I methanol maser lines by Sarma and Momjian has opened an exciting new window into the study of the star formation process, but for the results to be interpreted correctly, the Zeeman splitting factor (z) for both these lines needs to be urgently measured by experiment. Ratios between the pre-shock and post-shock magnetic fields and densities lead to the conclusion that the value of z cannot be too different from 1 Hz mG−1, unless the predicted densities at which 36 GHz and 44 GHz methanol masers are excited are drastically incorrect. Similarities between the detected fields in 36 GHz and 44 GHz Class I masers, and 6.7 GHz Class II masers, support the claim that these masers may be tracing the large-scale magnetic field or that the magnetic field remains the same during different evolutionary stages of the star formation process, provided such similarities are not just due to the assumption of a uniform nominal value for z, or result simply from selection effects due to orientation and/or the shock process. Given the exciting possibilities, a larger statistical sample of measurements in both the 36 GHz and 44 GHz lines is certainly needed.

We present observations of the circular polarization of the recombination line maser in MWC 349. Six good quality Ha30α spectra were obtained during 2010 – 2011 which show that the Zeeman features are complex, time variable, and usually different for the blue- and red-shifted maser spikes. We propose that the magnetic field, located in the corona of the circumstellar disk, has toroidal and radial components. It is plausibly generated in a disk dynamo.

We report on Very Long Baseline Array SiO maser observations of the OH/IR star OH 44.8 - 2.3. The observations show that the maser features form a ring located at a distance of 5.4 AU around the central star. The masers show high fractional linear polarization up to 100%. The polarization vectors are consistent with a dipole field morphology. Additionally, we report a tentative detection of circular polarization of 7% for the brightest maser feature. This indicates a magnetic field of 1.5 ± 0.3 G. The SiO masers and the 1612 MHz OH masers suggest a mildly preferred outflow direction in the circumstellar environment of this star. The observed polarization is consistent with magnetic field structures along the preferred outflow direction. This could indicate the possible role of the magnetic fields in shaping the circumstellar environment of this object.

A recent polarimetric survey of OH masers in a large sample of AGB and post-AGB stars revealed widespread occurrence of polarized features. We made a statistical analysis of the polarization properties of this large data set. We discuss the alignment of polarization position angles between the extreme blue- and red-shifted parts of the 1612 MHz spectrum. The average polarization angle of OH masers from the opposite sides of the envelope agrees within 20° for 80% of the sources in the sample. For two objects monitored over ~6 years the polarization position angle at 1612 MHz is constant within measurement uncertainties: this suggests a stable and a very regular structure of the circumstellar magnetic fields. Alternatively, this could indicate a galactic origin of the field which may be amplified by the stellar wind in the outermost parts of the envelopes.

Once ALMA full polarization capabilities are offered, (sub-)mm polarization studies will enter a new era. It will become possible to perform detailed studies of polarized maser emission towards for example massive star forming regions and late-type stars such as (post-) Asymptotic Giant Branch stars and young Planetary Nebulae. In these environments, SiO, H2O and HCN are molecules that can naturally generate polarized maser emission observable by ALMA. The maser polarization can then be used to derive the strength and morphology of the magnetic field in the masing regions. However, in order to derive, in particular, the magnetic field orientation from maser linear polarization, a number of conditions involving the rate of stimulated emission, molecular state decay and Zeeman splitting need to be satisfied. In this work, we discuss these conditions for the maser transitions in the ALMA frequency range and highlight the optimum transitions to further our understanding of star formation and evolved star magnetic fields.

The bright and narrow spectral line emission of masers is ideal for measuring the Zeeman-splitting as well as for determining the orientation of magnetic fields in 3-dimensions around massive protostars. Recently, polarization observations at milliarcsecond resolution of 6.7-GHz CH3OH masers have uniquely been able to resolve the morphology of magnetic fields close to massive protostars. The observations reveal that the magnetic fields are along outflows and/or on the surfaces of circumstellar tori. Here we present three different examples selected from a total number of 7 massive star-forming regions that were investigated at 6.7-GHz with the EVN in the last years.

H2O masers are important magnetic field tracers in very high density gas. We show one of the first magnetic field determinations at such high density in a low-mass protostar: IRAS 16293-2422. We used the Very Large Array (VLA) to carry out spectro-polarimetric observations of the 22 GHz Zeeman emission of H2O masers. A blend of at least three maser features can be inferred from our data. They are excited in zones of compressed gas produced by shocks between the outflows ejected by this source and the ambient gas. The post-shock particle density is in the range 1 - 3 × 109 cmt−3, and the line-of-sight component of the magnetic field is estimated as ~ 113 mG. The outflow dynamics is likely magnetically dominated.

We present results of Very Long Baseline Array (VLBA) polarimetric 22 GHz H2O maser observations of a number of low/intermediate mass evolved stars. We observed 3 Miras (Ap Lyn, IK Tau and IRC+60370), 1 semi-regular variable (RT Vir) and 1 pPN (OH231.8+4.2). Circular polarization is detected in the H2O maser region of OH231.8+4.2 and we infer a magnetic field of |B||| = ~45 mG. This implies an extrapolated magnetic field of ~2.5 G on the surface of the central star. The preliminary results on RT Vir and IRC+60370 also indicate the first detection of weak H2O maser linear polarization.

SiO masers are often observed in the near-circumstellar envelope of late-type evolved stars. The polarisation of the masers can be used as a probe of the magnetic field in this region, subject to maser polarisation radiative transfer model. Two main maser polarisation models have been developed for the weak Zeeman splitting case applicable to circumstellar SiO masers. Observational tests aimed at discriminating between these models were performed at maser component level, using VLBA observations of v=1 J=1-0, v=2 J=1-0 and v=1 J=2-1 SiO masers towards the high-luminosity source VY CMa.

Class II methanol masers are known to be tracers of an early phase of massive star formation. The 6.7- and 12.2-GHz methanol maser transitions can show a significant amount of variability, including periodic variations. Studying maser variability can lead to important insights into conditions in the maser environment but first the maser time-series need to be characterised. The results of long-term monitoring of 8 regularly-varying sources will be presented and methods of period-search discussed.

Bipolar molecular outflows have been observed and studied extensively in the past, but some recent observations of periodic variations in maser intensity pose new challenges. Even quasi-periodic maser flares have been observed and reported in the literature. Motivated by these data, we have tried to study situations in binary systems with specific attention to the two observed features, i.e., the bipolar flows and the variabilities in the maser intensity. We have studied the evolution of spherically symmetric wind from one of the bodies in the binary system, in the plane of the binary. Our approach includes the analytical study of rotating flows with numerical computation of streamlines of fluid particles using PLUTO code. We present the results of our findings assuming simple configurations, and discuss the implications.

The water maser site associated with G353.273+0.641 is classified as a dominant blueshifted H2O maser, which shows an extremely wide velocity range (± 100 km s−1) with almost all flux concentrated in the highly blueshifted emission. The previous study has proposed that this peculiar H2O maser site is excited by a pole-on jet from high mass protostellar object. We report on the monitoring of 22-GHz H2O maser emission from G353.273+0.641 with the VLBI Exploration of Radio Astrometry (VERA) and the Tomakamai 11-m radio telescope. Our VLBI imaging has shown that all maser features are distributed within a very small area of 200 × 200 au2, in spite of the wide velocity range (> 100 km s−1). The light curve obtained by weekly single-dish monitoring shows notably intermittent variation. We have detected three maser flares during three years. Frequent VLBI monitoring has revealed that these flare activities have been accompanied by a significant change of the maser alignments. We have also detected synchronized linear acceleration (−5 km s−1yr−1) of two isolated velocity components, suggesting a lower-limit momentum rate of 10−3 M⊙ km s−1yr−1 for the maser acceleration. All our results support the previously proposed pole-on jet scenario, and finally, a radio jet itself has been detected in our follow-up ATCA observation. If highly intermittent maser flares directly reflect episodic jet-launchings, G353.273+0.641 and similar dominant blueshifted water maser sources can be suitable targets for a time-resolved study of high mass protostellar jet.

In 2011 February, a burst of the 22 GHz H2O maser in Orion KL was reported. In order to identify the bursting maser features, we have been carrying out observations of the 22 GHz H2O maser in Orion KL with VERA, a Japanese VLBI network dedicated for astrometry. The bursting maser turns out to consist of two spatially different features at 7.58 and 6.95 km s−1. We determine their absolute positions and find that they are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward the southwest direction, perpendicular to the elongation of the maser features. It is most likely that the outflow from the radio source I or another young stellar object interacting with Compact Ridge is a possible origin of the H2O maser burst. We will also carry out observations with ALMA in the cycle 0 period to monitor the submillimeter H2O maser lines in the Orion Compact Ridge region. These follow-up observations will provide novel information on the physical and chemical properties of the mastering region.

The methanol masers associated with G35.20-1.74 were monitored at 12178 MHz for four years and 6668 MHz for five years using the 26m Hartebeesthoek telescope. This source showed irregular variability and a single large flare event during the monitoring window.

The work here represents the summary of observations made with the Hartebeesthoek Radio Astronomy Observatory (HartRAO) 26 m telescope of the water vapour (H2O) Masers in the Orion KL source region for a period of 8 months during the flare of 2011. The observation setup, calibration method together with the resulting maser time series are discussed.

A comparison between the observed periodic flaring of methanol maser sources in the star forming region G9.62+0.20E and the continuum emission from parts of a background HII region is made. Using a colliding wind binary (CWB) model preliminary calculations show that the CWB model results fit the maser light curves very well.