M. Szymczak

The diversity of methanol maser morphologies from VLBI observations

Context. The 6.7 GHz methanol maser marks an early stage of high-mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds. Aims. We investigated which structures the methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission. Methods. We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network’s expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspected Spitzer GLIMPSE data at 3.6–8 μm from the archive. Results. High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of “ring-like” methanol masers in starforming regions appeared to be suprisingly common, 29% of the sample. Conclusions. The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by Spitzer results because the majority of the masers coincide with 4.5 μm emission to within less than 1 �� . Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed.

Masers as signposts of high-mass protostars A water maser survey of methanol maser sources

The 22 GHz H2O maser line was observed towards 79 candidate high-mass protostellar objects from a flux-limited sample of 6.7 GHz methanol sources. The emission was detected in 41 sources, towards 28 of these for the first time. The detection rate of 52% was similar to rates reported for other samples of high-mass protostars selected mainly with far-infrared (FIR) colour criteria. The median value of H2O maser luminosity of 10 −5.5 Lis equal to that of the CH3OH maser luminosity, whereas the median OH maser luminosity was found to be ∼1.5 orders of magnitude lower. Comparison of the velocity ranges showing maser emission implies that for the majority of sources the H2 Oa nd CH 3OH maser lines originate from different regions. The percentage of sources with emission in two or three of the maser species, their association with radio contin- uum and IR emission and the maser and IR luminosities are consistent with the view that evolutionary phases with H2 Oa nd CH3OH masers largely overlap and precede the OH maser phase, while at a later stage OH and CH3OH masers may coexist. Strong correlations of OH and CH3OH maser luminosities with IR luminosity and only a marginal correlation of H2 Oa nd IR luminosity confirm current pumping schemes of all three maser lines.

Polarimetric observations of OH masers in proto-planetary nebulae

The 1612 and 1667 MHz OH maser lines have been measured in all four Stokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of 42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters, respectively. The spectral extent of the 1667 MHz line overshoots that of the 1612 MHz line in about 80% of the targets. 52% and 26% of the 1612 and 1667 MHz sources, respectively, show linear polarization in at least some features. Circular polarization is more frequent, occurring in 78% and 32% of sources of the respective OH lines. The percentage polarization is usually small (<15%) reaching up to 50-80% in a few sources. Features of linearly polarized emission are usually weak (0.5-4 Jy) and narrow (0.3-0.5 km s -1 ). The strength of the magnetic field inferred from likely Zeeman pairs in two sources of a few mG is consistent with values reported elsewhere for those classes of objects. An upper limit of the electron density in the envelope of OH17.7-2.0 derived from the difference in the position angle of polarization vectors for the two OH lines is about 1 cm -3 . Distinct profiles of polarization position angle at 1612 and 1667 MHz are seen in about one third of the sources and strongly suggest that the envelopes are permeated by structured magnetic fields. The geometry of the magnetic field is implicated as an important cause of the depolarization found in some PPN candidates. For the subset of targets which show axisymmetric shells in the optical or radio images we found a dominance of magnetic field components which are orthogonal to the long axis of the nebulae. This finding supports the hypothesis that such bipolar lobes are shaped by the magnetic field.

Observations of hydroxyl ground state transitions in a complete sample of methanol sources

High sensitivity observations of all four transitions of the ground state 2 Π3/2, J = 3/ 2o f OH in both senses of circular polarization have been carried out with the Nancay radio telescope. The sample was a set of 100 star forming regions detected in a recent unbiased survey of 6668 MHz CH3OH masers. OH maser emission was found in 55 objects of which 31 were not previously catalogued. The 1665 MHz line was seen in almost all OH maser sources and was accompanied by the 1667 MHz line in about 75% of cases. Respectively 7% and 11% of OH 1665 MHz masers were accompanied by maser lines at 1612 and 1720 MHz. These two satellite line masers never occurred simultaneously in the same source nor at the same radial velocity, suggesting mutually exclusive physical conditions as predicted by models. OH maser emission usually shared the same velocity range as the 6668 MHz CH3OH maser. The intensity ratio of the 6668 MHz and 1665 MHz lines clearly divides the sample into methanol- and hydroxyl-rich sources and could be controlled by the abundance of maser molecules and the kinetic temperature. The OH maser emission was substantially polarized with a mean fractional circular polarization of 0.30 and Zeeman pair candidates were found in 15 targets. 36 sources were found in OH absorption at the main lines and 24 of them were also accompanied by OH maser emission. OH absorption features were blueshifted with regard to the related OH masers, indicating that they were formed in front of the central continuum sources. Absorption at 1720 MHz was always accompanied by emission at 1612 MHz and vice versa. The behaviour of stimulated emission and absorption in both satellite lines was thus conjugated and the 1720 MHz emission features seem to be signatures of regions of low hydrogen density and OH column density. The correlation of OH and CH3OH flux densities with the IRAS flux densities found for our sample appears to support pumping schemes of both molecules by infrared photons. Statistics of masers in the sample appear to be consistent with the scenario that the CH3OH masers appeared earlier than the OH masers.

Magnetic field in Cepheus A as deduced from OH maser polarimetric observations

We present the results of MERLIN polarization mapping of OH masers at 1665 and 1667 MHz towards the Cepheus A star-forming region. The maser emission is spread over a region of 6 arcsec by 10 arcsec, twice the extent previously detected. In contrast to the 22 GHz water masers, the OH masers associated with H II regions show neither clear velocity gradients nor regular structures. We identified ten Zeeman pairs which imply a magnetic field strength along the line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the arcsecond scale, pointing towards us in the west and away from us in the east side. The linearly polarized components, detected for the first time, show regularities in the polarization position angles depending on their position. The electric vectors of OH masers observed towards the outer parts of H II regions are consistent with the interstellar magnetic field orientation, while those seen towards the centres of H II regions are parallel to the radio-jets. A Zeeman quartet inside a southern H II region has now been monitored for 25 years; we confirm that the magnetic field decays monotonically over that period.

Periodic variability of 6.7 GHz methanol masers in G22.357 + 0.066

We report the discovery of periodic flares of 6.7GHz methanol maser in the young massive stellar object G22.357+0.066. The target was monitored in the methanol maser line over 20 months with the Torun 32m telescope. The emission was also mapped at two epochs using the EVN. The 6.7GHz methanol maser shows periodic variations with a period of 179 days. The periodic behavior is stable for the last three densely sampled cycles and has even been stable over ~12 years, as the archival data suggest. The maser structure mapped with the EVN remains unchanged at two epochs just at the putative flare maxima separated by two years. The time delays of up to ~16 days seen between maser features are combined with the map of spots to construct the 3-dimensional structure of the maser region. The emission originating in a single ~100 AU layer can be modulated by periodic changes in the infrared pumping radiation or in the free-free background emission from an HII region.

Magnetic field structure in the outer OH maser envelope of VX Sagittarii

The OH 1612 MHz maser transition from the supergiant star VX Sgr has been imaged in all Stokes parameters using MERLIN. Numerous elliptically polarised components are distributed in a roughly symmetric envelope of a size similar to that measured in total maser intensity. The polarisation vectors are generally tangential to the circumstellar envelope, indicating a globally ordered magnetic eld at the distance of about 1400 au from the supergiant. It is suggested that a predominance of projected radial magnetic lines and a displacement between the red- and blue-shifted parts of the envelope can be explained with a dipole eld tilted at 20 30 to the line of sight.

Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir ?

We present and interpret the results of a long-term OH variability study of three semiregular stars, one type a (SRa), W Hya, and two type b (SRb), R Crt and RT Vir. The 1665 and 1667 MHz OH masers of the three semiregulars were observed at intervals in the period 1982 January{1995 December using the Nan cay radio telescope, and we searched for 1612 MHz emission. The OH maser proles of the studied stars signicantly deviated from a standard double-peaked prole. The timescale of prole changes in the two SRb stars R Crt and RT Vir was as short as a month. The OH proles of the SRa star W Hya were much more stable but since November 1986 a very blue-shifted feature appeared at 1667 MHz. Our phase-lag measurements suggest that this feature comes from a detached OH shell of radius 3 10 16 cm. Faint 1612 MHz emission was found in W Hya only. Weak emission at velocities very close to the systemic velocity usually appeared during some intervals of high maser activity in R Crt and RT Vir and was almost always present in W Hya. For R Crt we estimated that this tangential emission disappeared when the kinetic temperature in the OH maser regions dropped below 150 200 K. For a few features, line narrowing and re-broadening were observed on timescales of 90 200 days. The linewidth was inversely proportional to the peak flux density, suggesting unsaturated amplication. Cyclic variations in the integrated flux density were observed in all the three stars. The OH variability curves were generally characterised by large amplitude (4 6 m ) variations over 400 800 days superimposed with 100 300-day variations of 0:2 2 m . Only the measured OH period of W Hya, of 362 7 days, was in agreement with the optical period. The two SRb stars exhibited multi-periodic OH variability including with two statistically signicant periods. The behaviour of their red- and blue-shifted emission was less correlated than in W Hya. The ratios of the flux densities at 1667 MHz to that at 1665 MHz in all the three stars were about 2 at epochs of high OH activity and usually increased during weak maser emission. Long term behaviour of the OH masers from W Hya resembled that of standard OH Miras, while that of R Crt and RT Vir suggested thin and clumpy envelopes where unsaturated emission was sustained in some clouds.

Discovery of four periodic methanol masers and updated light curve for a further one

We report the discovery of 6.7 GHz methanol maser periodic flares in four massive star forming regions and the updated light curve for the known periodic source G22.357+0.066. The observations were carried out with the Torun 32 m radio telescope between June 2009 and April 2014. Flux density variations with period of 120 to 245 d were detected for some or all spectral features. A variability pattern with a fast rise and relatively slow fall on time-scale of 30-60 d dominated. A reverse pattern was observed for some features of G22.357+0.066, while sinusoidal-like variations were detected in G25.411+0.105. A weak burst lasting ~520 d with the velocity drift of 0.24 km/s/yr occurred in G22.357+0.066. For three sources for which high resolution maps are available, we found that the features with periodic behaviour are separated by more than 500 au from those without any periodicity. This suggests that the maser flares are not triggered by large-scale homogeneous variations in either the background seed photon flux or the luminosity of the exciting source and a mechanism which is able to produce local changes in the pumping conditions is required.

Polarization properties of OH masers in AGB and post-AGB stars

Context. Ground-state OH maser emission from late-type stars is usually polarized and remains a powerful probe of the magnetic field structure in the outer regions of circumstellar envelo pes if observed with high angular and spectral resolutions. Observations in all four Stokes parameters are quite sparse and this is the most thorough, systematic study published to date. Aims. We aim to determine polarization properties of OH masers in an extensive sample of stars that show copious mass loss and search for candidate objects that are well-suited for high a ngular resolution studies. Methods. Full-polarization observations of the OH 1612 and 1667 MHz maser transitions were carried out for a sample of 117 AGB and post-AGB stars. Several targets were also observed in the 1665 MHz line. Results. A complete set of full-polarization spectra together with t he basic polarization parameters are presented. Polarized features occur in more than 75% of the sources in the complete sample and there is no intrinsic difference in the occurrence of polarized emission between the three classes of objects of different infrared characteristics. The highest fractional po larization occurs for the post-AGB+PN and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the fractional polarization between the sources at different evolutionary stages appear to be related to depolariz ation caused by blending. The alignment of the polarization angles at the extreme sides of the shell implies a regular str ucture of the magnetic field of a strength of 0.3-2.3 mG. Conclusions. Polarized OH maser features are widespread in AGB and post-AGB stars. The relationship between the circular and linear fractional polarizations for a representative samp le are consistent with the standard models of polarization f or the Zeeman splitting higher than the Doppler line width, whereas the polarized features are theσ components.