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Publication Date: 2012-07-23
Journal information: Astronomische Nachrichten
E-print:
DOI: 10.1002/asna.201211702
Bibliographic code: 2012AN....333..634S
Abstract: We report the observations of 289 methanol maser sources at 6.7 GHz obtained over a two month period with the Torun 32 m telescope. The data form a catalogue of all objects north of δ = −22◦ brighter than 7.5 Jy in the peak emission. The positions of sub-arcsecond accuracy are updated for 76 % of the objects. We find that about one third of the sources show changes in the peak fluxes by a factor of two or more on time scales of 8.5–9.5 years.
Observations performed by 32-m radio telescope were used.: Yes

Publication Date: 2012-07-18
Journal information: Astrophysical Journal
E-print: arXiv:1206.0055
DOI: 10.1088/0004-637X/754/2/149
Bibliographic code: 2012ApJ...754...149D
Abstract: Milliarcsecond very long baseline interferometry maps of regions containing 6.7 GHz methanol maser emission have lead to the recent discovery of ring-like distributions of maser spots and the plausible hypothesis that they may be tracing circumstellar disks around forming high-mass stars. We aimed to test this hypothesis by imaging these regions in the near- and mid-infrared at high spatial resolution and compare the observed emission to the expected infrared morphologies as inferred from the geometries of the maser rings. In the near-infrared we used the Gemini North adaptive optics system of ALTAIR/NIRI, while in the mid-infrared we used the combination of the Gemini South instrument T-ReCS and super-resolution techniques. Resultant images had a resolution of ~150 mas in both the near-infrared and mid-infrared. We discuss the expected distribution of circumstellar material around young and massive accreting (proto)stars and what infrared emission geometries would be expected for the different maser ring orientations under the assumption that the masers are coming from within circumstellar disks. Based upon the observed infrared emission geometries for the four targets in our sample and the results of spectral energy distribution modeling of the massive young stellar objects associated with the maser rings, we do not find compelling evidence in support of the hypothesis that methanol masers rings reside in circumstellar disks.
Observations performed by 32-m radio telescope were used.: No

Publication Date: 2012-05-31
Journal information: Astronomy & Astrophysics
E-print: arXiv:1204.0806
DOI: 10.1051/0004-6361/201218999
Bibliographic code: 2012A&A...541A..72B
Abstract: Context. The 22.2 GHz water masers are often associated with the 6.7 GHz methanol masers but owing to the different excitation conditions they likely probe independent spatial and kinematic regions around the powering young massive star. Aims: We compared the emission of these two maser species on milliarcsecond scales to determine in which structures the masers arise and to test a disc-outflow scenario where the methanol emission arises in a circumstellar disc while the water emission comes from an outflow. Methods: We obtained high-angular and spectral resolution 22.2 GHz water maser observations of the two sources G31.581+00.077 and G33.641-00.228 using the EVN. Results: In both objects the water maser spots form complex and filamentary structures of sizes 18-160 AU. The emission towards the source G31.581 + 00.077 comes from two distinct regions of which one is related to the methanol maser source of ring-like shape. In both targets the main axis of methanol distribution is orthogonal to the water maser distribution. Most of water masers appear to trace shocks on a working surface between an outflow/jet and a dense envelope. Some spots are possibly related to the disc-wind interface which is as close as 100-150 AU to the regions of methanol emission.
Observations performed by 32-m radio telescope were used.: No

Publication Date: 2012-01-31
Journal information: Astronomy & Astrophysics
E-print: arXiv:1110.0773
DOI: 10.1051/0004-6361/201117263
Bibliographic code: 2012A&A...537A...5W
Abstract: 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 envelopes 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 angular 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 the 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 polarization 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 depolarization caused by blending. The alignment of the polarization angles at the extreme sides of the shell implies a regular structure 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 sample are consistent with the standard models of polarization for the Zeeman splitting higher than the Doppler line width, whereas the polarized features are the σ components.
Observations performed by 32-m radio telescope were used.: No

Publication Date: 2011-06-08
Journal information: Astronomy & Astrophysics
E-print: 2011arXiv1105.4089S
DOI: 10.1051/0004-6361/201117145
Bibliographic code: 2011A&A...531L..3S
Abstract: We report the discovery of periodic flares of 6.7 GHz 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 32 m telescope. The emission was also mapped at two epochs using the EVN. The 6.7 GHz 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.
Observations performed by 32-m radio telescope were used.: Yes

Publication Date: 2011-01-01
Journal information: Astronomy and Astrophysics, Volume 525, id.A120
E-print: http://arxiv.org/abs/1009.2334
DOI: 10.1051/0004-6361/201015235
Bibliographic code: 2011A&A...525A..120B
Abstract: 22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments they occur in. The aim was to obtain accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission recently mapped with Very Long Baseline Interferometry. A sample of 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously. Water maser clusters were detected towards 27 sites finding 15 new sources. The detection rate of water maser emission associated with methanol sources was as high as 71%. In a large number of objects (18/21) the structure of water maser is well aligned with that of the extended emission at 4.5 $mu$m confirming the origin of water emission from outflows. The sources with methanol emission with ring-like morphologies, which likely trace a circumstellar disk/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario that 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appears to trace outflows. The two types of associations might be related to different evolutionary phases.
Observations performed by 32-m radio telescope were used.: Yes

Publication Date: 2010-11-25
Journal information: Astronomy and Astrophysics, Volume 524, id.A99
E-print:
DOI: 10.1051/0004-6361/201014810
Bibliographic code: 2010A&A...524A..99S
Abstract: OH masers from the envelopes of M-type supergiants show a significant degree of polarization, implying magnetic fields of a few mG. Nothing is known about the temporal characteristics of the magnetic fields or how such changes may affect stellar mass loss. We therefore observed two supergiant stars in order to quantify the long-term polarization behaviour of the maser emission. Full-polarization spectra at 1612 and 1667 MHz were obtained with the Nançay radio telescope at intervals over periods of 4 and 6 years for S Per and VX Sgr, respectively. Time series of OH maser full polarization spectra are presented. Semiregular variations of the integrated flux densities generally follow the visual light curves as expected for radiative pumping cycles. For both sources the variability indices of individual features are higher at 1667 MHz than at 1612 MHz and their extreme values occur for the blue-shifted emission. The degrees of polarization at 1667 MHz show diverse behaviours usually uncorrelated with the total flux, whereas those at 1612 MHz are commonly stable on time scales of 4-6 yr. Several outbursts of the 1667 MHz emission on time scales of 0.5-2 yr were found in both targets. The bursting features are highly polarized and show drifts in velocity. Small changes of the degrees of polarization and smooth variations of position angle of linear polarization during the bursts were observed in S Per but they are more dramatic in VX Sgr. The OH outbursts do not seem to have any direct link to stellar events, but seem to be localized in the wind.
Observations performed by 32-m radio telescope were used.: No