Shocked molecular gas towards the supernova remnant G359.1–0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H₂) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H₂ bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H₂ emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of ¹²CO, HCO⁺ and CS towards the maser show broad-line absorption, which is absent in the ¹³CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H₂ filament is located at the boundary between the pre-shock and post-shock regions.     

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.     

CO Clouds around SNR G21.8-0.6 and G32.8-0.1

We made the first CO(I—0) mapping to SNR G21.8-0.6 and SNR G32.8-0.1, both associated with OH 1720 MHz maser.Based on the morphological correspondence and velocity and position agreement between the radio remnant and the CO clouds,we tentatively identify the clouds that are respectively interacting with the two SNRs.     

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.     

Simulations of mixed-morphology supernova remnants with anisotropic thermal conduction

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants (SNRs) through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard X-ray emission within 20 kyr, and centre-bright soft X-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard X-rays is delayed until about 60 kyr. The soft X-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology SNRs preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make large changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favourable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O  viii transitions from shell bright at early epochs to centre bright at later epochs in the evolution of an SNR expanding in a dense interstellar medium when the physics of thermal conduction is included.     

Co-propagation of maser emission at 1720 and 4765 MHz

MERLIN observations are presented of OH 4765-MHz and OH 1720-MHz masers in the massive star-forming region W3(OH). Two of the three intense spots of maser emission at 4765 MHz are spatially coincident with two similar spots at 1720 MHz in both left-hand circular (LHC) and right-hand circular (RHC) polarizations, to an accuracy of 15 mas. The spots also overlap in velocity when allowance is made for Zeeman splitting of the 1720-MHz line. We conclude that we have found two examples of masers in different rotational levels of OH which are co-propagating through the same column of gas and experiencing competitive gain effects. The third 4765-MHz maser spot was found to have no overlapping counterpart amongst the 1720-MHz masers.     

Distribution of the Molecular Gas Around SN 1572

The early-stage structure and evolution of a supernova remnant (SNR) depends largely on its ambient interstellar medium, so the interstellar medium becomes the valid probe for investigating the evolution of SNRs. We have observed the ¹²CO (J = 1 − 0) line emission around the remnant of SN 1572 with the 13.7m millimeter-wave telescope at the Qinghai Station of PMO, in order to investigate the distribution of the CO molecular gas around SN 1572 and provide some observational basis for studying the relationship of SN 1572 with its ambient molecular gas and the evolution of this SNR. The observed result indicates that the molecular gas in the velocity range of V_LSR = −69∼ −58 km/s is associated with SN 1572, and this velocity component comes from a large-scale molecular cloud. The molecular gas is distributed along the periphery of the radio shell, continually but not uniformly, and forms a semi-closed molecular shell around the SNR. The enhanced emission exists in its whole eastern half, especially the CO emission is strongest on the northeastern edge. At the emission peak position, the spectral line exhibits a broadened velocity feature (>5 km/s). Combining with available observations in the optical, infrared, X-ray and other wavebands, it is demonstrated that the fast shock wave and ejecta are expanding into the molecular gas on the northeastern edge, and interacting with the dense gas. This interaction will have an important influence on the future evolution of SN 1572.     

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.     

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.     

A model of polarized X-ray emission from twinkling synchrotron supernova shells

Synchrotron X-ray emission components were recently detected in many young supernova remnants (SNRs). There is even an emerging class – SN 1006, RX J1713.72−3946, Vela Jr and others – that is dominated by non-thermal emission in X-rays, also probably of synchrotron origin. Such emission results from electrons/positrons accelerated well above TeV energies in the spectral cut-off regime. In the case of diffusive shock acceleration, which is the most promising acceleration mechanism in SNRs, very strong magnetic fluctuations with amplitudes well above the mean magnetic field must be present. Starting from such a fluctuating field, we have simulated images of polarized X-ray emission of SNR shells and show that these are highly clumpy with high polarizations up to 50 per cent. Another distinct characteristic of this emission is the strong intermittency, resulting from the fluctuating field amplifications. The details of this 'twinkling' polarized X-ray emission of SNRs depend strongly on the magnetic field fluctuation spectra, providing a potentially sensitive diagnostic tool. We demonstrate that the predicted characteristics can be studied with instruments that are currently being considered. These can give unique information on magnetic field characteristics and high-energy particle acceleration in SNRs.     

Ground-state OH observations towards NGC 6334

We have made observations of the four hyperfine transitions of the ²Π_3/2,     ground state of OH at 1612, 1665, 1667 and 1720 MHz and the related 1.6-GHz continuum emission towards NGC 6334 using the Australia Telescope Compact Array. The observations covered all the major radio continuum concentrations aligned along the axis of NGC 6334 (V, A to F). We have detected seven OH masers plus a possible faint eighth maser; two of these masers are located towards NGC 6334-A. Absorption at 1665 and 1667 MHz was detected towards almost all the continuum distribution. All transitions show non-LTE behaviour. The 1667-/1665-MHz intensity ratios range from 1.0 to 1.2, significantly less than their LTE value of 1.8. The results of the OH 'Sum Rule' suggest that this discrepancy cannot be explained solely by high optical depths. The 1612- and 1720-MHz line profiles show conjugate behaviour whereby one line is in absorption and the other in emission. In addition, the profiles commonly showed a flip from absorption to emission and vice versa, which is interpreted as a density gradient. The OH line-to-continuum distribution, optical depth and velocity trends are consistent with a bar-like shape for the molecular gas which wraps around the continuum emission.     

Hydroxyl lines towards Puppis A

The 1667-MHz ground-state hydroxyl (OH) line has been observed towards positions on Puppis A as well as in the immediate vicinity of this supernova remnant (SNR). The spectra at two positions on Puppis A show absorption lines at velocities less than +7.6 km s¹, and emission lines above this velocity. Analysis of all four 18 cm OH lines indicates that Puppis A must lie between the OH clouds seen in absorption and emission, giving it a v _lsr of +7.6 km s¹. This implies that the kinematic distance to Puppis A is The OH clouds in front of Puppis A are slightly anomalously excited, as shown by analysis of the four 18 cm OH lines at the two observed positions on Puppis A. There is no evidence of any interaction of the SNR with the clouds in front of and behind it.     

Wind-swept clouds and possible triggered star formation associated with the supernova remnant G357.7+0.3

We present evidence for interaction between the supernova remnant (SNR) G357.7+0.3 and nearby molecular clouds, leading to the formation of wind-swept structures and bright emission rims. These features are not observed at visual wavelengths, but are clearly visible in mid-infrared mapping undertaken using the Spitzer Space Telescope . Analysis of one of these clouds, the bright cometary structure G357.46+0.60, suggests that it contains strong polycyclic aromatic hydrocarbon emission features in the 5.8 and 8.0 μm photometric bands, and that these are highly variable over relatively small spatial scales. The source is also associated with strong variations in electron density; a far-infrared continuum peak associated with dust temperatures of ∼30 K; and has previously been observed in the 1720 MHz maser transition of OH, known to be associated with SNR shock excitation of interstellar clouds. This source also appears to contain a young stellar object (YSO) within the bright rim structure, with a steeply rising spectrum between 1.25 and 24 μm. If the formation of this star has been triggered recently by the SNR, then YSO modelling suggests a stellar mass  ∼5–10 M_⊙  , and luminosity   L _YSO∼10²–2 × 10³ L_⊙  .
Finally, it is noted that a further, conical emission region appears to be associated with the Mira V1139 Sco, and it is suggested that this may represent the case of a Mira outflow interacting with a SNR. If this is the case, however, then the distance to the SNR must be ∼half of that determined from CS   J = 2–1  and 3–2 line radial velocities.     

Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for H i and OH

We present the results of a survey, the deepest to date, for H  i emission at 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720 MHz) in the direction towards the Galactic globular clusters M15, M2, NGC 6934, NGC 7006 and Pal 13. The aim is to measure the amount of hydrogen in the intracluster medium, and to find OH masers in the circumstellar envelopes of globular cluster red giants. We present a tentative detection of  0.3 M_⊙  of neutral hydrogen in M15 and possible detections of neutral hydrogen in M2 and Pal 13. We derive upper limits to the neutral hydrogen content of NGC 6934 and 7006. No OH emission is detected. We also present deep H  i data of the northern tip of the Magellanic Stream behind Pal 13.     

Maser maps and magnetic field of OH 323.459−0.079

The maser site OH 323.459−0.079 has been studied using the Long Baseline Array of the Australia Telescope National Facility. Simultaneous observations of the 1665- and 1667-MHz hydroxyl ground-state transitions yielded a series of maps at a velocity spacing of 0.18 km s⁻¹, in both senses of circular polarization, with tenth-arcsec spatial resolution. Many small-diameter maser spots were detected within a 2-arcsec region. Pairs of spots with the same position, but with right- and left-hand circular polarization offset in frequency, reveal Zeeman splitting. Six pairs were found, and in four cases, the pairs at 1667 and 1665 MHz mutually corroborate the derived values of magnetic field and (central) kinematic velocity. Over the whole site, magnetic field estimates range from +1.47 to +4.13 mG with a median value of +2.5 mG. The excited state of OH at 6035 MHz also displays Zeeman pairs revealing a similar magnetic field, and we show that the most prominent of these pairs coincides with the most prominent pair at 1665 and 1667 MHz.
We also compared the morphology and kinematics at 1665 and 1667 MHz with those of maser emission from the excited state of OH at 6035 MHz and from methanol at 6668 MHz. All three varieties of masers appear intermingled, and associated with an ultracompact H  ii region. In many respects we find that OH 323.459−0.079 is similar to W3(OH), one of the few other maser sites yet studied in comparable detail.     

Maser maps and magnetic field of OH 300.969+1.147

The southern maser site OH 300.969+1.147 has been studied using the Long Baseline Array of the Australia Telescope National Facility. The 1665- and 1667-MHz hydroxyl ground-state transitions were observed simultaneously. A series of maps with 0.1-arcsec spatial resolution, at velocity spacing  0.09 km s⁻¹  , and in both senses of circular polarization reveals 59 small diameter maser spots. The spots are scattered over 2 arcsec, coincident with a strong ultracompact H  ii region, at a distance of 4.3 kpc. 17 Zeeman pairs of oppositely polarized spots were found, all yielding magnetic field estimates towards us (negative), ranging from −1.1 to −4.7 mG, with a median value of −3.5 mG. Excited state masers of OH at 6035 and 6030 MHz at this site also display Zeeman pairs revealing a magnetic field of −5.0 mG. Weak methanol maser emission is intermingled with the OH masers, but there is no detectable closely related water maser. The consistent magnetic field direction found within this site is a striking feature of several other maser sites associated with strong H  ii regions studied in comparable detail. We interpret the site as a mature region nearing the end of the brief evolutionary stage that can support maser emission.     

OH masers, molecular outflows and magnetic fields in NGC 7538

The multi-element radio-linked interferometer network (MERLIN) measurements of 1665-, 1667- and 1720-MHz OH masers associated with NGC 7538 are presented. The masers are located at the centres of three bipolar molecular outflows associated with the infrared sources IRS 1, IRS 9 and IRS 11. The distribution of OH masers in IRS 1 is more extensive than previously reported and is displaced to the south of the methanol 6.7-GHz masers. The OH masers in IRS 9 have not previously been reported. Their distribution seems to be orthogonal to the direction of the outflow and to the distribution of H₂O masers. The maser distribution in IRS 11 is parallel to the dust ridge and orthogonal to the outflow. Full polarization measurements of the OH maser emission show systematic differences between the three sources. IRS 1 has moderate polarization, with linear polarization vectors partially aligned with the bipolar outflow; IRS 9 exhibits larger polarization, but little linear component; IRS 11 shows the strongest polarization and has linear polarization vectors aligned parallel to the outflow. There is also evidence for a toroidal component of the magnetic field around the IRS 11 outflow, orthogonal to the outflow direction. It is suggested that the differences in polarization trace a possible evolutionary sequence from oldest (IRS 1) to youngest (IRS 11).     

Spectra of OH masers at 6035 and 6030 MHz

The Parkes radio telescope has been used to study circular polarization in the spectra of masers at the 6035- and 6030-MHz transitions of excited OH. The targets were 91 previously catalogued sites of 6035-MHz maser emission. A few were not detected, primarily because of variability. However, the 6035-MHz intensity variations seldom exceed a factor of 2 over several years, with a handful of dramatic exceptions.
Towards many targets, the present observations have provided the first high-sensitivity search for the 6030-MHz transition and yielded 33 detections. All of the 6030-MHz maser features have 6035-MHz counterparts closely matching in velocity. For matching features, the 6030-MHz emission is most commonly weaker than the 6035-MHz emission by an order of magnitude but, in a few cases, is several times stronger. The detection statistics are well accounted for by very recent developments in maser modelling. However, the occasional occurrence of 6030-MHz maser emission stronger than at 6035-MHz poses a new challenge for the theory.
Spectra with good frequency resolution at 6030 and 6035 MHz yield many valuable measurements of magnetic fields. At each transition, the field can be inferred from a small frequency separation between the right-hand and left-hand circularly polarized features, attributed to the Zeeman effect in a magnetic field of a few mG. In the many instances where a 'Zeeman pair' on the 6035-MHz spectrum has features matched by the 6030-MHz spectrum, this provides convincing corroboration of the magnetic field, in both direction and magnitude.
Several prominent absorption features occur at 6035 MHz, and usually have matching absorption at 6030 MHz of similar, or slightly smaller, depth.     

Investigation of OH and H<Subscript>2</Subscript>O masers in the star-forming region G 188.946+0.886

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nançay Observatory (France) and in the H₂O line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V _LSR = 3.6 km s^?1, has a “blue” shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8–11 km s^?1. This suggests a probable connection of the observed feature in the 1720-MHz line with the “blue” wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of H₂O maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms.     

Discovery of molecular hydrogen line emission associated with methanol maser emission

We report the discovery of H₂ line emission associated with 6.67-GHz methanol maser emission in massive star-forming regions. In our UNSWIRF/AAT observations, H₂ 1–0 S(1) line emission was found associated with an ultracompact H  ii region IRAS 14567–5846 and isolated methanol maser sites in G318.95–0.20 , IRAS 15278–5620 and IRAS 16076–5134 . Owing to the lack of radio continuum in the latter three sources, we argue that their H₂ emission is shock excited, while it is UV-fluorescently excited in IRAS 14567–5846 . Within the positional uncertainties of 3 arcsec, the maser sites correspond to the location of infrared sources. We suggest that 6.67-GHz methanol maser emission is associated with hot molecular cores, and propose an evolutionary sequence of events for the process of massive star formation.