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Register a new userSpatial and velocity coincidence of 4765- and 1720-MHz OH masers in two star-forming regions Cep A and W75N
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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.
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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.
We report VLBA observations of maser emission from the rotationally excited doublet Pi 1/2, J=1/2 state of OH at 4765 MHz. We made phase-referenced observations of W3(OH) at both 4765 MHz and 1720 MHz and found emission in three fields within a about 2000 AU diameter region and verified that in two of the three fields, 4765 MHz and 1720 MHz emission arise from the same position to within about 4 mas (about 5 AU diameter emission regions along an approximately N-S arc with linear extent about 500 AU. In addition, we carried out phase-referenced observations of 4765 MHz emission from K3-50. We searched for the 4765 MHz line in W49 (without phase referencing) and W75N (phase-referenced to the strongest 4765 MHz maser feature in DR21EX); we were unable to detect these sources with the VLBA. For 2 1/2 years (including the dates of the VLBA observations), we carried out monitoring observations of 4765 MHz emission with the VLA. Constraints on models for maser emission at 1720 MHz and 4765 MHz are derived from the observations. These observations are then briefly compared with existing models.
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.
VLBI multi-epoch water maser observations are a powerful tool to study the dense, warm shocked gas very close to massive protostars. The very high-angular resolution of these observations allow us to measure the proper motions of the masers in a few weeks, and together with the radial velocity, to determine their full kinematics. In this paper we present a summary of the main observational results obtained toward the massive star-forming regions of Cepheus A and W75N, among them: (i) the identification of different centers of high-mass star formation activity at scales of 100 AU; (ii) the discovery of new phenomena associated with the early stages of high-mass protostellar evolution (e.g., isotropic gas ejections); and (iii) the identification of the simultaneous presence of a wide-angle outflow and a highly collimated jet in the massive object Cep A HW2, similar to what is observed in some low-mass protostars. Some of the implications of these results in the study of high-mass star formation are discussed.
We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic Center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km/s molecular cloud (M-0.02-0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km/s cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in the CND. We argue that the high velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently.
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