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Register a new userMAGMO: Polarimetry of 1720-MHz OH Masers towards Southern Star Forming Regions
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C. S. Ogbodo
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From targeted observations of ground-state OH masers towards 702 Multibeam (MMB) survey 6.7-GHz methanol masers, between Galactic longitudes 186$^{circ}$ through the Galactic centre to 20$^{circ}$, made as part of the `MAGMO project, we present the physical and polarisation properties of the 1720-MHz OH maser transition, including the identification of Zeeman pairs. We present 10 new and 23 previously catalogued 1720-MHz OH maser sources detected towards star formation regions. In addition, we also detected 16 1720-MHz OH masers associated with supernova remnants and two sites of diffuse OH emission. Towards the 33 star formation masers, we identify 44 Zeeman pairs, implying magnetic field strengths ranging from $-$11.4 to $+$13.2 mG, and a median magnetic field strength of $|B_{LOS}|$ $sim$ 6 mG. With limited statistics, we present the in-situ magnetic field orientation of the masers and the Galactic magnetic field distribution revealed by the 1720-MHz transition. We also examine the association statistics of 1720-MHz OH SFR masers with other ground-state OH masers, excited-state OH masers, class I and class II methanol masers and water masers, and compare maser positions with mid-infrared images of the parent star forming regions. Of the 33 1720-MHz star formation masers, ten are offset from their central exciting sources, and appear to be associated with outflow activity.
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We present the first maps of 4765-MHz OH masers in two star-forming regions Cepheus A and W75N, made with Multi-Element Radio Linked Interferometer Network. In Cep A the emission has an arc-like structure of size 40 mas with a clear velocity gradient, and lies at the edge of H II region 3b Over a period of 8 weeks the maser diminished in intensity by a factor of 7 but its structure remained stable. This structure coincides with a newly mapped 1720-MHz maser in Cep A within the positional error, and matches it in velocity. The 1720-MHz line exhibits Zeeman splitting that corresponds to a magnetic field strength of -17.3 mG. In W75N the excited 4765-MHz OH maser has a linear structure of size 45 mas with a well defined velocity gradient, and lies at the edge of H II region VLA 1. This structure coincides in position and velocity with the 1720-MHz masers. We conclude that in both sources the 4765-MHz emission coexists with 1720-MHz emission in the same volume of gas. In such a case the physical conditions in these regions are tightly constrained by the maser-pumping models.
Maser emission plays an important role as a tool in star formation studies. It is widely used for deriving kinematics, as well as the physical conditions of different structures, hidden in the dense environment very close to the young stars, for example associated with the onset of jets and outflows. We will summarize the recent observational and theoretical progress on this topic since the last maser symposium: the IAU Symposium 242 in Alice Springs.
We present results of spectral line observations of the ground state transitions of hydroxyl(OH) toward supernova remnant IC 443 carried out with the Green Bank Telescope. At a spatial resolution of 7.2 arcminutes we detect weak, extended OH(1720 MHz) maser emission with OH(1667/5,1612 MHz) absorption along the southern extent of the remnant, where no bright compact maser sources have been observed previously. These newly detected SNR-type masers are coincident with known molecular clumps and a ridge of shocked molecular hydrogen indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission including excitation temperature, OH column density and filling factor. Observed line profiles suggest the shock is largely propagating toward the line-of-sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.
Using the Very Long Baseline Array (VLBA) we performed a high resolution OH maser survey in Galactic star-forming regions (SFRs). We observed all the ground state spectral lines: the main lines at 1665 and 1667 MHz and the satellite lines at 1612 and 1720 MHz. Due to the exceptionality of finding satellite lines in SFRs, we will focus our discussion on those lines. In our sample of 41 OH maser sources, five (12%) showed the 1612 MHz line and ten (24%) showed the 1720 MHz line, with only one source showing both lines. We find that 1720 MHz emission is correlated with the presence of HII regions, suggesting that this emission could be used to diagnose or trace high-mass star formation. We include an analysis of the possible mechanisms that could be causing this correlation as well as assessing the possible relationships between lines in our sample. In particular, the presence of magnetic fields seems to play an important role, as we found Zeeman splitting in four of our sources (W75 N, W3(OH), W51 and NGC 7538). Our results have implications for current understanding of the formation of high-mass stars as well as on the masing processes present in SFRs.
We present the pilot results of the `MAGMO project, targeted observations of ground-state hydroxyl masers towards sites of 6.7-GHz methanol maser emission in the Carina-Sagittarius spiral arm tangent, Galactic longitudes 280 degrees to 295 degrees. The `MAGMO project aims to determine if Galactic magnetic fields can be traced with Zeeman splitting of masers associated with star formation. Pilot observations of 23 sites of methanol maser emission were made, with the detection of ground-state hydroxyl masers towards 11 of these and six additional offset sites. Of these 17 sites, nine are new detections of sites of 1665-MHz maser emission, three of them accompanied by 1667-MHz emission. More than 70% of the maser features have significant circular polarization, whilst only ~10% have significant linear polarization (although some features with up to 100% linear polarization are found). We find 11 Zeeman pairs across six sites of high-mass star formation with implied magnetic field strengths between -1.5 mG and +3.8 mG and a median field strength of +1.6 mG. Our measurements of Zeeman splitting imply that a coherent field orientation is experienced by the maser sites across a distance of 5.3+/-2.0 kpc within the Carina-Sagittarius spiral arm tangent.
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