OH Masers In Star-forming Regions Research Articles

Search for Collisionally Pumped 1720 MHz OH Masers in Star-forming Regions: A VLA Survey of 18 cm OH Masers toward 80 Class I Methanol Masers

Abstract Class I methanol masers (cIMMs) and 1720 MHz OH masers are believed to arise from a common collisional pumping mechanism, and both trace shocked gas regions in supernova remnants (SNRs). However, their coexistence in star formation regions (SFRs) and their association with a front of bipolar outflow (a source of shock-stimulated collisional pumping) remain unclear. To search for collisionally pumped OH(1720) masers, we conducted a Very Large Array survey of the 18 cm OH masers and continuum emission toward a sample of 80 SFRs associated with 44 GHz cIMMs. Main-line OH maser emission was detected in 50% of the sample, and OH(1720) maser emission was detected in 20%. Continuum emission was detected in 28% of sources. A catalog of the detected OH masers is presented. Individual OH masers are found in close proximity, and the regions of masers of different transitions are often overlapping. A typical linear projected offset between the OH(1720) and OH(1665) masers is ∼0.04 pc. A remarkable number of ∼81% of the OH(1720) masers are associated with the continuum emission. The median separation between the targeted 44 GHz cIMMs and OH(1720) masers is ∼0.2 pc, which is similar to reported distances between cIMMs and H ii regions. The observed properties of the detected OH(1720) masers are different from those in SNRs and indicate that a nonlocal line overlap mechanism is responsible for their excitation. Thus, while both 44 GHz cIMMs and OH(1720) masers trace shocked gas in SNRs, physical conditions favorable for excitation of cIMMs, but not OH(1720) masers, are present in SFRs.

Engineering and science highlights of the KAT-7 radio telescope

The construction of the KAT-7 array in the Karoo region of the Northern Cape in South Africa was intended primarily as an engineering prototype for technologies and techniques applicable to the MeerKAT telescope. This paper looks at the main engineering and scien- tific highlights from this effort, and discusses their applicability to both MeerKAT and other next-generation radio telescopes. In particular we found that the composite dish surface works well, but it becomes complicated to fabricate for a dish lacking circular symmetry; the Stir- ling cycle cryogenic system with ion pump to achieve vacuum works but demands much higher maintenance than an equivalent Gifford-McMahon cycle system; the ROACH (Recon- figurable Open Architecture Computing Hardware)-based correlator with SPEAD (Stream- ing Protocol for Exchanging Astronomical Data) protocol data transfer works very well and KATCP (Karoo Array Telescope Control Protocol) control protocol has proven very flexible and convenient. KAT-7 has also been used for scientific observations where it has a niche in mapping low surface-brightness continuum sources, some extended HI halos and OH masers in star-forming regions. It can also be used to monitor continuum source variability, observe pulsars, and make VLBI observations

Polarimetric survey of main-line OH masers in star-forming regions

Context. While there is evidence for considerable circular polarization of OH masers in star-forming regions, no systematic measurements of the four Stokes parameters have been done. Determining the full polarization properties of OH emission is vital to test polarization maser models and to select targets for high-angular resolution studies of magnetic fields. Aims. The polarimetric measurements of a representative sample of star-forming regions were carried out in order to better characterize the properties of OH masers. Methods. A sample of 99 star-forming regions was observed in the OH 1665 and 1667 MHz maser transitions using the Nancąy radio telescope in full-polarization mode and high spectral resolution (0.07 km s -1 ). Results. We present a complete set of polarization spectra for all 98 and 81 sources detected at 1665 and 1667 MHz, respectively. All detections have circularly polarized features, whereas linear polarization is found in 80% and 62% of the 1665 and 1667 MHz sources, respectively. The median degree of linear polarization of 16% of both transitions is a factor of ~4 lower than that of circular polarization. In consequence, the probability of linear polarization clearly drops for sources with low total flux density because of the sensitivity limit. The linear polarization is almost always accompanied by circular polarization, i.e., elliptically polarized profiles are observed. The mean line-width at half-maximum of the linear polarization profile is 0.26 km s -1 . The polarization position angle ( χ ) shows large variations across the profile with a median amplitude close to 90°. The amplitude variations of χ decrease with increasing flux density. Large variations of the total flux density found in three sources are weakly associated with changes in polarization parameters. Conclusions. The observational characteristics of linear polarization seem to be consistent with theoretical models where narrow profiles result from the overlap of σ -components traversed regions with appreciable velocity gradient and due to off resonance amplification.

A MERLIN survey of 4.7-GHz excited OH masers in star-forming regions

Phase-referenced observations of 13 star-forming regions in the 2 Π 1/2 , J = 1/2 transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of ∼10 7 K at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north-south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H II region and has a brightness temperature of ∼4 x 10 5 K. There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.

5 Centimeter OH Masers as Diagnostics of Physical Conditions in Star‐forming Regions

We demonstrate that the observed characteristics of the 5 cm OH masers in star-forming regions can be explained with the same model and the same parameters as the 18 cm and the 6 cm OH masers. In our already published study of the 18 cm and the 6 cm OH masers in star-forming regions we had examined the pumping of the 5 cm masers but did not report the results we had found because of some missing collision rate coefficients, which in principle could be important. The recently published observations on the 5 cm masers of OH encourage us to report our old calculations along with some new ones that we have performed. These calculations, in agreement with the observations, reveal the main lines at 5 cm as strong masers, the 6049 MHz satellite line as a weak maser, and the 6017 MHz satellite line as never inverted for reasonable values of the parameters.

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. II. Effects of Large Velocity Gradients and Infrared Radiation

We have studied the pumping of OH masers in star-forming regions by introducing four pumping mechanisms one at a time. The four mechanisms are collisions, local line overlap, nonlocal line overlap, and external infrared radiation field. The first two have been investigated in a previous paper (Paper I). Here we study the effects of nonlocal line overlap and of an external infrared radiation field. As in Paper I, we have used the most accurate collision rates of OH with ortho- and para-H<SUB>2</SUB>, which became available recently, and we have taken the effects of line overlap into account as accurately as possible using a recently developed formalism. We have found that, despite the significant change in the collision rates, the effects of nonlocal line overlap and of an external infrared radiation field are necessary for the explanation of the main lines in the ground state of OH. The same is true for the explanation of the general qualitative behavior of H II/OH masers. Our calculations have revealed a number of diagnostics of the physical conditions in star-forming regions. These diagnostics are enumerated in our conclusions.

A model for OH masers in star-forming regions

A new model of the OH maser environment in star-forming regions is described in detail and the predictions of the model are compared with observational data. After reviewing the status of OH maser observations, the ingredients which such a model requires are outlined. Among these we highlight rate coefficients for energy transfer, the nature of the FIR dust radiation field and the treatment of line and continuum radiation transport in the presence of FIR line overlap where the latter is based on a new theoretical description

Collisional pumping of OH masers in the 2Pi(1/2) ladder

The collisional pumping of OH masers in star-forming regions has been studied, and the first detailed calculations of such pumping since collision rates with the correct parities have become available are reported. It is found that collisions of OH with H2 pump not only the J = 1/2 Lambda-doublet but also the J = 3/2 and J = 5/2 Lambda-doublets of 2Pi(1/2). Using parameters inferred by VLBI for W3(OH), it is found that collisional pumping can easily explain the observed brightness temperatures and luminosities of the J = 1/2 Lambda-doublet masers. Furthermore, what the brightness temperature and luminosity should be for the J = 3/2 and J = 5/2 Lambda-doublets in the same region is predicted. 28 refs.