The simultaneous detection of 22- and 321-GHz H2O maser emission towards the symbiotic Mira R Aquarii

We report high spatial and spectral resolution measurements of masers towards R Aqr and H1−36, both of which are examples of the sub-class of symbiotic stars that contain a long-period Mira-type variable. Our observations have resulted in the first detection of 321-GHz H₂O maser action towards a symbiotic Mira — R Aqr. Comparison with simultaneous 22-GHz H₂O maser data suggests that the masers do not have the same properties as those in the circumstellar envelopes of field Miras. R Aqr's 22-/321-GHz peak flux density and luminosity ratios are low, as is the line width ratio. Continuum and spectral-line maps indicate that the 22-GHz maser and free–free emission are aligned. Three mechanisms can reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H₂O and anomalously weak OH and 22-GHz H₂O masers. In the most convincing model, UV radiation and a fast wind from the companion remove the Mira's envelope of dusty, molecular gas, leaving a relatively small cavity of dense, neutral material within a large, ionized nebula. Excitation temperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, the 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with our high-resolution maps.     

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.     

Studies of ultracompact H ii regions — II. High-resolution radio continuum and methanol maser survey

High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS -selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H  ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H  ii region is formed around a massive star and is quickly destroyed as the UC H  ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.     

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.     

A milliarcsecond-resolution image of the OH masers in G34.3+0.2

We present measurements of the distribution of the OH masers at 1665 and 1667 MHz towards the cometary ultracompact H  ii region in the complex G34.3+0.2. The results are based on observations made in both senses of circular polarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5×20 mas². 38 maser features are identified in the region. 33 of these lie on an arc at the edge of the cometary H  ii region. Five are located in a cluster offset toward the north-east by 3 arcsec, and are probably associated with an independent ultracompact H  ii region. There is a velocity gradient of 30 km s⁻¹ pc⁻¹ across the arc. We identify five Zeeman pairs and determine that the magnetic field varies between 1 and 7 mG, but is always directed away from the Earth.
The OH masers may arise in clumps in a shell of gas in a bow shock caused by the motion of the exciting star through the molecular cloud. The stand-off distance and the thickness of the shocked shell are roughly consistent with those predicted by such a bow-shock model. Also, the position of the exciting star(s), as estimated from the focus of the parabolic bow shock, closely matches that of the peak emission from the cometary H  ii region. However, the north–south velocity gradient in the ionized material remains difficult to explain in the context of the bow-shock model.     

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.     

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.     

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.     

Methanol masers at 107.0 and 156.6 GHz

From a search of more than 80 southern class II methanol maser sites, we report measurements of 22 masers at 107.0 GHz and four at 156.6 GHz, mostly new discoveries. Class II sites, recognized by their strong emission at the 6.6-GHz methanol transition, are indirect indicators of new-born massive stars, and several hundred have been documented; only a handful of these had previously been found to exhibit maser emission at the 107.0- or 156.6-GHz transition. The present survey increases the number of known 107.0-GHz masers to 25, providing a sufficiently large sample to assess their general properties. For the stronger ones, our position measurements confirm that, to an accuracy of 5 arcsec, they coincide with the dominant maser emission at 6.6 GHz. Intensity variations exceeding 50 per cent have occurred in some 107.0-GHz maser features that we observed in both 1996 October and 1998 June.
We find that masers are rare at the 156.6-GHz transition. Two new detections increase the total now known to four. Each 156.6-GHz maser is substantially weaker than its corresponding 107.0-GHz maser. Despite the scarcity of masers, our 156.6-GHz spectra at most observed sites show emission, but apparently of a quasi-thermal variety; it is usually accompanied by somewhat weaker thermal emission at 107.0 GHz, and the intensity ratio of the transitions allows us to begin exploration of the physical characteristics of the small molecular clouds (diameter less than 60 mpc) at these sites. The thermal emission thus provides estimates of the environmental conditions that are needed to support strong masing from spots that are apparently embedded within these clouds.     

Methanol masers and their environment at high resolution

We have used the Australia Telescope Compact Array (ATCA) to make high-resolution images of the 6.7-GHz 5₁ → 6₀A⁺ maser transition of methanol towards 33 sources in the Galactic plane. Including the results from 12 methanol sources in the literature, we find that 17 out of 45 sources have curved or linear morphology. Most of the 17 have a velocity gradient along the line, which is consistent with masers lying in an edge-on circumstellar disc surrounding a massive star. We also made simultaneous continuum observations of the sources at 8.6 GHz, in order to image any associated H  ii region. 25 of the sources are associated with an ultracompact H  ii region, with a detection limit of ∼0.5 mJy beam⁻¹. We argue that the methanol sources without an associated H  ii region represent less massive embedded stars, not an earlier stage in the lifetime of the star, as previously suggested.     

MERLIN imaging of the maser flare in Markarian 348

MERLIN images of Mrk 348 at 22 GHz show H₂O maser emission at 0.02–0.11 Jy , within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is ≲ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 yr. The maser–core separation is barely resolved, but at the 3 σ significance level maser and core are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c . The correlation between the radio continuum increase and maser flare is explained as arising from high-level nuclear activity through a common excitation mechanism, although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.     

Near-infrared imaging of the environment of 6.7-GHz methanol masers

Near-infrared (NIR) images at I , J , H and K bands were made of 12 southern 6.7-GHz methanol maser sources. Astrometry accurate to 0.5 arcsec was obtained. The positions of known H  ii regions, water masers, hydroxyl masers, and mid- and far-infrared objects in the region are examined in order to try to determine the nature of the methanol maser source. Deeply embedded NIR sources were found close to seven out of 14 maser sites. In three cases, no NIR source, H  ii region, water maser or hydroxyl maser could be found in likely association with the methanol masers.     

Very Large Array observations of broad 6-cm excited-state OH lines in W49A

Using the Very Large Array (VLA), we observed all three of the 6-cm lines of the  ²Π_1/2, J = 1/2  state of OH with sub-arcsecond resolution (∼0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6-cm lines are similar to those of the ground-state (18-cm) OH lines, a large fraction of the total emission in all three 6-cm lines has large linewidths (∼5–10 km s⁻¹) and is spatially extended, very unlike typical ground-state OH masers, which typically are point-like at VLA resolutions and have linewidths ≤1 km s⁻¹. We find brightness temperatures of 5900, 4700 and ≥730 K for the 4660, 4750 and 4765-MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are ∼0.3, again very unlike the 18-cm lines, which have gains  ≥10⁴  . We compare the excited-state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low-gain maser lines. We also comment on the relationship with the 18-cm masers.     

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.     

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).     

Testing the circumstellar disc hypothesis: a search for H2 outflow signatures from massive young stellar objects with linearly distributed methanol masers

The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the  H₂ v = 1–0 S(1)  line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H₂ emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H₂ emission in these fields is not distributed randomly, but instead the majority of sources are found to have H₂ emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed.     

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 high-resolution radio study of neutral gas in the starburst galaxy NGC 520

We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ∼10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H  i ) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H  i absorption  ( N _H∼ 10²² atoms cm⁻²)  traces a velocity gradient of 0.5 km s⁻¹ pc⁻¹ across the central kpc of NGC 520. The H  i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here.     

OH maser mapping of the evolved star HD 179821: evidence for interacting outflows

The evolved star HD 179821 continues to be the subject of much debate as to whether it is a nearby     post-asymptotic giant branch (post-AGB) star or a distant     high initial mass     post-red supergiant. We have mapped the OH maser emission around HD 179821 in the 1612- and 1667-MHz lines with the MERLIN interferometer array at a resolution of 0.4 arcsec and 0.35 km s⁻¹. The OH emission lies in a thick shell with inner and outer radii of 1.3 and         and expansion velocity of 30 km s⁻¹. Although we find some evidence for acceleration and for deviations from spherical symmetry, the bulk of the maser emission is consistent with a constant-velocity spherical shell. The extent of the shell agrees with H₂O and OH dissociation models and supports a distance estimate of 6 kpc. However, the shell is incomplete and appears to have been disrupted by more recent collimated outflow activity within the last 1500 yr. We suggest that this activity is also responsible for the active envelope chemistry (in particular the presence of HCO⁺) and for the apparent offset of the star from the centre of the shell. The luminous yellow hypergiant star IRC +10420 also shows signs of recent outflows, and HD 179821 may be at a similar, perhaps slightly earlier, phase of evolution. We suggest that the SiO thermal emission arises from the same detached envelope as the OH maser emission as in IRC +10420. If so then this would strengthen the connection between these two stars and probably rule out a post-AGB status for HD 179821.     

Mid-infrared observations of methanol maser sites and ultracompact H ii regions: signposts of high-mass star formation

N -band (10.5 μm) and/or Q -band (20.0 μm) images taken with MANIAC on the ESO/MPI 2.2-m telescope are presented for 31 methanol maser sites and 19 ultracompact (UC) H  ii regions. Most of the maser sites and UC H  ii regions are coincident with mid-infrared (MIR) sources to within the positional uncertainties of ∼ 3 arcsec, consistent with the maser emission being powered by the MIR source. The IRAS source positions, however, do not always coincide with the MIR sources.
Based on an average infrared spectral energy distribution, we deduce that the MIR objects are luminous enough that they should also produce a strong ionizing radiation. Some sources are consistent with stars of later spectral type, but not all can be. A number of maser sites show no detectable radio continuum emission associated with MIR emission, despite a powering source luminous enough potentially to produce an UC H  ii region. Since no signs of an UC H  ii region are detected here, these maser sites might be produced during a very early stage of stellar evolution.
We present objects that show evidence of outflow activity stemming from a maser site, exhibiting CO and/or CS line profiles indicative of outflows coincident with the MIR source. These cases are promising examples of maser sites signposting the earliest stages of high-mass star formation.