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.

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.

The proximity of the SiO masers to the star, makes them a powerful tool for studying the properties of the extended stellar atmosphere. This project is a long monitoring campaign of the 43 GHz SiO masers (v=1 J=1→0) around the Mira Variable TX Cam. The target source was observed with the Very Long Baseline Array (VLBA) from the 24th of May 1997 to the 25th of January 2002 in bi-weekly or monthly intervals, covering 3.06 stellar cycles. The time-span and frequency of observations helped us examine the long and short-term properties of the emission and study the dynamics of the gas, the existence of shock waves and their contribution to the morphology and kinematics. Maps from each epoch were concatenated into a 112-frame movie showing the evolution of the emission around TX Cam.

This review summarises current observations of masers around evolved stars and models for their location and behaviour, followed by some of the many highlights from the past 5 years. Some of these have been the fruition of long-term monitoring, a vital aspect of study of stars which are both periodically variable and prone to rapid outbursts or transition to a new evolutionary stage. Interferometric imaging of masers provide the highest-resolution probes of the stellar wind, but their exponential amplification and variability means that multiple observations are needed to investigate questions such as what drives the wind from the stellar surface; why does it accelerate slowly over many tens of stellar radii; what causes maser variability. VLBI parallaxes have improved our understanding of individual objects and of Galactic populations. Masers from wide range of binary and post-AGB objects are accessible to sensitive modern instruments, including energetic symbiotic systems. Masers have been detected up to THz frequencies with Herschel and ALMA's ability to resolve a wide range of maser and thermal lines will provide accurate constraints on physical conditions including during dust formation.

The recent discovery of methanol maser emission coming from ring-like distributions has led to the plausible hypothesis that they may be tracing circumstellar disks around forming high mass stars. In this article we discuss the distribution of circumstellar material around such young and massive accreting (proto)stars, and what infrared emission geometries would be expected for different disk/outflow orientations. For four targets we then compare the expected infrared geometries (as inferred from the properties of the maser rings) with actual high spatial resolution near-infrared and mid-infrared images. We find that the observed infrared emission geometries are not consistent with the masers residing in circumstellar disks.

The MeerKAT radio telescope array is the South African precursor instrument for the proposed Square Kilometer Array's mid-band frequency range. It will be the most sensitive centimetre-wavelength telescope in the southern hemisphere until the SKA is built. It will cover a broad range of astronomical science from the evolution of galaxies to tests of Einstein's theory of General Relativity, using Pulsars. The chosen frequency bands will enable sensitive southern Galactic maser surveys in the main lines of hydroxyl, 12 GHz methanol and perhaps 14.5 GHz formaldehyde lines as well as searches for redshifted water masers from red-shifts greater than about 0.5. Proposals for Large Surveys using MeerKAT were solicited in late 2009 and resulted in some 20 proposals from teams comprising 500 scientists, world-wide. The successful proposals relevant to maser research will be discussed below.

We present VLBI maps of the 6.7 GHz methanol maser emission in 32 sources obtained using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). All of the observed sources provide new VLBI maps, and the spatial morphologies have been classified into five categories similar to the results obtained from European VLBI Network observations (Bartkiewicz et al. 2009). The 32 methanol sources are being monitored to measure the relative proper motions of the methanol maser spots.

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.

We present and discuss VLA-EVLA high-sensitivity and spatial resolution observations of Water Vapor MASERs and continuum emission towards two sources that have been proposed in the literature to be high-mass star forming regions: IRAS 19217+1651 and 23151+5912. Our results indicate the presence of disks which can confirm that these regions are high-mass star forming regions.

Class II methanol masers are thought to trace the brief phase in the evolution of a massive YSO, where outflows are expected to occur. Molecular line maps of the CO isotopes of a subset of 6.7 GHz sources from the MMB catalogue were observed with the JCMT telescope. Utilising optically thick 12CO, a search was done to detect broadened line wings (initially only on the source G20.08-0.13). The physical parameters of these detected lobes were then calculated.

Four Class I maser sources were detected at 44, 84, and 95 GHz toward chemically rich outflows in the regions of low-mass star formation NGC 1333I4A, NGC 1333I2A, HH25, and L1157. One more maser was found at 36 GHz toward a similar outflow, NGC 2023. Flux densities of the newly detected masers are no more than 18 Jy, being much lower than those of strong masers in regions of high-mass star formation. The brightness temperatures of the strongest peaks in NGC 1333I4A, HH25, and L1157 at 44 GHz are higher than 2000 K, whereas that of the peak in NGC 1333I2A is only 176 K. However, a rotational diagram analysis showed that the latter source is also a maser. The main properties of the newly detected masers are similar to those of Class I methanol masers in regions of massive star formation. The former masers are likely to be an extension of the latter maser population toward low luminosities of both the masers and the corresponding YSOs.

The Class II 6.7-GHz methanol maser is a tracer of high mass young stellar objects. We present results of a 44-GHz class I methanol maser and 22-GHz water maser survey using the KVN (Korean VLBI Network) 21-m single dish radio telescopes towards 284 6.7-GHz maser sites. Class I methanol maser and water maser emission is detected towards 116 (41%) and 136 (48%) sources, respectively. About 50 sources have a peak flux density higher than 10 Jy at 44-GHz. They are candidates for VLBI studies using the KVN.

Masers in starburst galaxies are outstanding probes of a range of phenomena related to galaxy and black hole evolution, star formation, and magnetic fields. Here I briefly discuss five related topics: (1) Galactic analog water masers in nearby galaxies; (2) multiwavelength solutions to the OH megamaser puzzle in major galaxy mergers; (3) formaldehyde anti-inversion in starburst galaxies; (4) OH spoofing in HI surveys; and (5) new discovery space in radio line surveys. New insights into the physical conditions responsible for OH megamasers, including indications of a critical molecular gas density obtained from the formaldehyde “densitometer,” will be applicable to future surveys, particularly surveys for redshifted hydrogen where OH lines arising in major galaxy mergers can “contaminate” the disk population identified by the HI 21 cm line. Blind radio spectral line surveys also offer the opportunity for unexpected discoveries of new nonthermal radio lines.

I review recent developments of SiO maser observations of miras, and discuss their kinematics in the Galaxy. They are deeply related to problems in other fields, such as the noncircular motion of Galactic spiral arms, streaming motions of stars in the solar neighborhood, and the interface between the interstellar and circumstellar medium revealed by Far-IR imaging of some miras, as well as the bar structure of the Galaxy. Recent automated surveys of variable stars in optical bands have increased the number of red variables enormously. Most of these stars were not observed in the radio before because of their blue colors. We have obtained radial velocities of these long-period variables with SiO masers. The longitude-velocity diagrams have revealed interesting deviations of stellar motions from the circular motion of the Galaxy, some of which show spatial motion similar to that of the Hercules group of stars. Important developments of SiO maser observations have been made for symbiotic stars, one of which showed a nova eruption with concurrent γ ray emission in 2010 (V407 Cyg). SiO masers have been monitored for this star after the nova eruption. I summarize the SiO maser observations of individual stars also.

The massive star forming complex W33 contains several molecular clouds at different stages of star formation activity, ranging from quiescent to highly active clouds. Our trigonometric parallax observations of water masers in this complex, conducted with the VLBA at 22.2 GHz, show that all water masers have the same distance of 2.4 kpc, locating the W33 complex in the Sagittarius spiral arm.

We have carried out a search for 18-cm OH megamaser (OHM) emission with the Green Bank Telescope. The targeted galaxies comprise a sample of 121 ULIRGs at 0.09<z<1.5, making this is the first large, systematic search for OHMs at z>0.25. Nine new detections of OHMs are reported, all at redshifts z<0.25. For the remainder of the galaxies, observations constrain the upper limit on OH emission; this rules out OHMs of moderate brightness (LOH>103 L⊙) for 26% of the sample, and extremely bright OHM emission (LOH>104 L⊙) for 73% of the sample. Losses from RFI result in the OHM detection fraction being significantly lower than expected for galaxies with LIR>1012 L⊙. The new OHM detections are used to calculate an updated OH luminosity function, with Φ∝ LOH−0.66; this slope is in agreement with previous results. Non-detections of OHMs in the COSMOS field constrain the predicted sky density of OHMs; the results are consistent with a galaxy merger rate evolving as (1+z)m, where m≤6.

W Aql is a binary S-type AGB star showing SiO maser emission. The dust distribution around the star is asymmetric, possibly indicating gravitational interaction between the binary pair. There are indications that the gas distribution also exhibits asymmetries. To investigate the source of the circumstellar structures, we applied for and were rewarded VLBI time to map the distribution of the SiO masers around this source and to constrain the presence and distribution of a possible magnetic field. Using VERA observations we also aim at measuring an accurate parallax to determine the binary separation, however, from showing peak emission of 21 Jy in June 2010, the SiO(1-0) v=1 line at 43 GHz has now disappeared. We find no correlation with the stellar pulsational period.

Hydrogen radio recombination line (RRL) masers are a rare phenomenon in star forming regions. Since RRL masers were first detected in 1989 toward the emission line star MWC349A, several single-dish surveys at millimeter wavelengths have been carried out to detect other RRL maser objects. However, although RRL maser amplification is expected to appear at wavelengths <2mm, MWC349A still remains as the only RRL maser object known to date. In this contribution, I will present our recent findings of two new RRL maser objects with the Submillimeter Array (SMA) toward the massive star forming regions Cepheus A HW2 and MonR2-IRS2. Sub-millimeter observations with interferometers such as the SMA and the Atacama Large Millimeter Array (ALMA) open the possibility to detect a much larger sample of RRL maser objects, where very detailed information about the kinematics and physical structure of the innermost ionized regions can be obtained toward these objects.

To establish the existence of the galactic bar, ten methanol maser sources around the starting points of the spiral arms were observed with VERA (VLBI Exploration of radio astrometry) using the phase-referencing technique at 6.7 GHz band. For six out of ten sources, absolute proper motions were obtained with better than 3σ accuracy. Using VLBI 3-D data of eight sources, including our five sources, we compared the observed data with three galactic models and found that the model including the bar effect is better to explain the 3-D data, than a flat circular rotation model. A non-flat circular rotation model is also consistent with the VLBI data. Based on a dynamical model with a bar, we estimate an inclination angle of the bar around 35°, which is consistent with previous studies.

We present results of a VLBI experiment at a wavelength of 18 cm, which simulates the ground-space interferometer with space link to RadioAstron. An array of five antennas was used, four of them are located in the Russian Federation, plus the the 32-m radio telescope in Medicine (Italy). The 22-m radio telescope in Pushchino (Moscow Region) acted in place of the space arm. It has an effective area of 100 square meters. The three other Russian 32-m antennas are operated by the Institute of Applied Astronomy RAS; they are located at Badary, Svetloe and Zelenchukskaya (interferometer network “Quasar”). The maximum base-line, Badary-Svetloe, was about 4402 km, providing an angular resolution of about 0.009 arc seconds at a wavelength of 18 cm. The duration of the experiment was 10 hours on 02/03 February 2011. The program of observations included quasars 3C273, 3C279, 3C286 and the maser source - W3(OH). W3(OH) was observed only by the Russian telescopes and was investigated at the frequency of the 1665 MHz main line. The data were recorded on the MK5 recorder (32-m radio telescopes) and the RDR system (RadioAstron Digital Recorder) in Pushchino. The low SEFD (system equivalence of flux density) of Pushchino emulated the RadioAstron antenna. Correlation was performed with the universal software correlator of the AstroSpace Center of Lebedev Physical Institute. The correlator output format is compatible with that used by the AIPS package, which was used for data analysis. After analyzing the correlated data we obtained relative coordinates of the maser components. The main results are tabulated and presented in the figures. The data quality is sufficient for astrophysical analysis and comparison with previous observations of maser source W3(OH) on VLBI networks EVN and VLBA.