Observations of OH 4765-MHz maser emission from star-forming regions

We report observations of the 4765-MHz maser transition of OH (²Π_1/2, J=1/2, F=1→0) towards 57 star-forming regions, taken with the 32-m Toruń telescope. Nine maser sources were detected, of which two had not been reported previously. The newly discovered sources in W75N and Cep A and four previously known sources were monitored over periods ranging from a few weeks to six months. Significant variations of the maser intensity occurred on time-scales of one week to two months. The relationships between the flux density and the linewidth for the new sources, established during the rise and fall phases of bursts that lasted 6–8 weeks, are consistent with a model of saturated masers.     

Discovery of large-scale methanol and hydroxyl maser filaments in W3(OH)

Images of the 6.7-GHz methanol maser emission from W3(OH) made at 50- and 100-mas angular resolution with the Multi-Element Radio-Linked Interferometer Network (MERLIN) are presented. The masers lie across the western face of the ultracompact H  ii region in extended filaments which may trace large-scale shocks. There is a complex interrelation between the 6.7-GHz methanol masers and hydroxyl (OH) masers at 1.7 and 4.7 GHz. Together the two species trace an extended filamentary structure that stretches at least 3100 au across the face of the ultracompact H  ii region. The dominant 6.7-GHz methanol emission coincides with the radio continuum peak and is populated by masers with broad spectral lines. The 6.7-GHz methanol emission is elongated at position angle 50° with a strong velocity gradient, and bears many similarities to the methanol maser disc structure reported in NGC 7538. It is surrounded by arcs of ground state OH masers at 1.7 GHz and highly excited OH masers at 13.44 GHz, some of which have the brightest methanol masers at their focus. We suggest that this region hosts the excitation centre for the ultracompact H  ii region.     

SiO maser emission in Miras

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infrared (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in very long baseline interferometry (VLBI) observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1–0.25, which is consistent with observed phase lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13 μm appear to be closely associated in three out of four phase samples.     

Evidence for Co-Propagation of 4765- and 1720-MHz OH Masers in Star-Forming Regions

Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s^–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected.     

Radio continuum emission associated with Class II methanol maser sources

Class II methanol masers are believed to be associated with high-mass star formation. Recent observations by Walsh et al. and Phillips et al. reported a very low detection rate of radio continuum emission toward a large sample of 6.7-GHz methanol masers. These results raise questions about the evolutionary phase and/or the mass range of the exciting stars of the masers. Here we report the results of a VLA search for 8.4-GHz continuum emission from the area around five Class II methanol masers, four of which were not detected by Walsh et al. at 8.6 GHz. Radio continuum emission was detected in all five fields although only two of the nine maser spot groups in the five fields were found to be superimposed on radio continuum sources that appear to be ultra-compact H  ii (UCH  ii ) regions. This suggests that continuum counterparts for some masers might be found in further surveys for which the sensitivity level is lower than  1 mJy beam⁻¹  . Considering our results as well as observations from other studies of methanol masers we conclude that masers without radio continuum counterparts are most likely associated with high-mass stars in a very early evolutionary stage, either prior to the formation of a UCH  ii region or when the H  ii region is still optically thick at centimetre wavelengths. With one exception all maser spot groups in the five fields were found to be associated with mid-infrared objects detected in the Midcourse Space Experiment survey.     

OH masers at 1612 and 1720 MHz in star-forming regions

Masers at the ground-state OH satellite transitions near 1612 and 1720 MHz are occasionally found in star-forming regions, accompanying the dominant maser of OH at 1665 MHz. The satellite lines can then be valuable diagnostics of physical conditions in star-forming regions if we can first ascertain that all maser species truly arise from the same site. For this purpose, newly measured satellite line positions with subarcsecond accuracy are reported here, and compared with masers of main-line OH at 1665 MHz, with methanol masers at 6668 MHz, and with ultracompact H  ii regions. We confirm that most of the satellite-line OH masers that we have measured are associated with star-forming regions, but a few are not: several 1612-MHz masers are associated with late-type stars, and one 1720-MHz maser is associated with a supernova remnant. The 1720-MHz masers in star-forming regions are accounted for by a pumping scheme requiring high densities, and are distinctly different from those in supernova remnants where the favoured pumping scheme operates at much lower densities.     

伴有OH 1720 MHz脉泽发射的混合形态超新星遗迹的X射线特征研究

在银河系中，超新星遗迹的射电巡视揭示了19个伴有OH1720MHz脉泽发射的超新星遗迹．在从超新星遗迹膨胀到分子云的激波波面的后面产生了这一类不寻常的脉泽源．这个模型的重要点是从超新星遗迹来的X射线促使产生OH分子．研究伴有OH1720MHz脉泽发射的混合形态超新星遗迹的X射线特征是很重要的．研究了这19个伴有OH1720MHz脉泽发射的混合形态超新星遗迹的X射线特征．得到了这些源的X射线物理参数之间的一些相关关系和反相关关系．X射线电离率ζ与θ、D、r、r^2等物理量均不相关，而与Lx之间存在着正相关关系．另外超新星遗迹的X射线光度与相伴的OH1720MHz脉泽的最弱的束流量密度之间存在着紧密的正相关关系．这些都说明从超新星遗迹来的X射线发射足够在激波波阵面后面分解水分子并产生OH1720MHz脉泽．     

The association of OH and methanol masers in W3(OH)

We present single-baseline Multi-Element Radio-Linked Interferometer Network (MERLIN) measurements of excited OH 6.0-GHz masers and methanol 6.7-GHz masers for the source W3(OH). These allow us to compare the positions of individual maser spots of these two species to ∼15 mas accuracy for the first time, and to compare these with previously published positions of ground-state OH masers near 1.7 GHz and excited-state OH masers near 4.7 GHz. There is a strong association between OH 6035-MHz and 1665-MHz masers. OH and methanol have very similar distributions, but associations of individual masers are relatively rare: most methanol 6.7-GHz masers are within 100 mas of OH 6.0-GHz masers, but only four methanol masers are within 15 mas of an OH 6.0-GHz maser. There are no correspondences of either species with excited OH 4.7-GHz masers. Zeeman splitting of the 6.0-GHz OH lines indicates an ordered magnetic field ranging from 3.2 to 14.4 mG. The magnetic fields estimated from co-propagating masers such as 6035 and 1665 MHz are generally in good agreement with each other.     

Dust radiation fields and pumping of excited state OH masers in W3(OH)

I show that it is quite possible to obtain saturating amplification in the 13.44-GHz maser which is so far unique to W3(OH), using a combined radiative and collisional pumping scheme. The dominant radiative part of the pump involves far-infrared line overlap, and the far-infrared continuum is provided by dust, modelled as either a two-component mixture or composite grains. Transport of the far-infrared radiation is carried out via the accelerated lambda iteration method. The observational link between 13.44-GHz and 6035-MHz masers is reproduced by the model. Inadequate amplification in Sobolev models probably results from the optical depth limitations imposed by this approximation. I review the dust models used in far-infrared pumping models of OH masers, and conclude that the main consequence of moving from skewed blackbody functions to more sophisticated models is a selective pumping enhancement when ice mantles are included.     

A study of the X-ray characteristics of mixed-morphology supernova remnants with 1720 MHz OH maser emission

Radio surveys of supernova remnants (SNRs) in the Galaxy have discovered 19 SNRs which are accompanied by the OH maser emission at 1720 MHz. This unusual maser is thought to be produced behind a shock front when a SNR expands into a molecular cloud. An important ingredient of this model is that the X-ray emission from the remnant enhances the production of OH molecules. In this sense, to study the characteristics of the mixed-morphology SNRs accompanied by the OH maser emission at 1720 MHz is important. By studying the X-ray characteristics of the mixed-morphology SNRs accompanied by the 1720 MHz OH maser emission, it is found that the ionization rate of X-ray is not correlated with the physical parameters , D, r, r² and so on, but is correlated with the X-ray luminosity L_x. Meanwhile, L_x is closely correlated with the beam flux density of the weakest feature of the accompanying 1720 MHz OH maser emission. These mean that the X-ray emission from SNRs is sufficient to dissociate the water molecules behind a shock front and to produce the 1720 MHz OH masers.     

Testing the 'clump' model of SiO maser emission

Building on the detection of the J =7–6 SiO maser emission in both the v =1 and v =2 vibrational states towards the symbiotic Mira R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86 GHz, represent a test of the popular thermal-instability clump models of SiO masers. The 'clump' model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance, for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J =7–6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.     

A search for 4750- and 4765-MHz OH masers in southern star-forming regions

We have used the Australia Telescope Compact Array (ATCA) to make a sensitive  (5 σ ≃100 mJy)  search for maser emission from the 4765-MHz ²Π_1/2   F =1→0  transition of OH. 55 star formation regions were searched and maser emission with a peak flux density in excess of 100 mJy was detected toward 14 sites, with 10 of these being new discoveries. In addition we observed the 4750-MHz ²Π_1/2   F =1→1  transition towards a sample of star formation regions known to contain 1720-MHz OH masers, detecting marginal maser emission from G348.550−0.979. If confirmed this would be only the second maser discovered from this transition.
The occurrence of 4765-MHz OH maser emission accompanying 1720-MHz OH masers in a small number of well-studied star formation regions has led to a general perception in the literature that the two transitions favour similar physical conditions. Our search has found that the presence of the excited-state 6035-MHz OH transition is a much better predictor of 4765-MHz OH maser emission from the same region than 1720-MHz OH maser emission is. Combining our results with those of previous high-resolution observations of other OH transitions we have examined the published theoretical models of OH masers and find that none of them predicts any conditions in which the 1665-, 6035- and 4765-MHz transitions are inverted simultaneously.     

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

Phase-referenced observations of 13 star-forming regions in the  ²Π_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  ∼10⁷ 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 × 10⁵ K  . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.     

The disappearance of the 1667-MHz OH maser in IRAS 17436+5003 (HD 161796)

We report the diminution of the 1667-MHz OH maser in the post-asymptotic giant branch star IRAS 17436+5003, by a factor of ≳17 over a period of ≲12 yr, from observations with MERLIN. This circumstellar maser was detected by Likkel in 1987, at the 13σ level of her observations with the Green Bank Telescope. We discuss a number of possible reasons for this phenomenon and conclude that it is most likely due to turbulence arising from interacting stellar winds.