Probing the microarcsecond structure of the Circinus megamasers through diffractive interstellar scintillation

We have used the Long Baseline Array to observe the Circinus Galaxy H₂O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission.
The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers.
In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s⁻¹ for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success.     

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.     

Accurate positions of H2O masers in the Large Magellanic Cloud

Positions with subarcsecond accuracy have been measured for seven 22-GHz H₂O masers associated with H  ii regions in the Large Magellanic Cloud (LMC); two of the masers are new detections. Initial position measurements were obtained with the 70-m antenna of the Canberra NASA Deep Space Network during a period of more than two years in which the antenna was used to monitor the maser emission. The positions were further improved using 22-GHz observations involving three antennas of the Australia Telescope Compact Array.
The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH₃OH). The H₂O maser positions range from within 1 arcsec (270 mpc) of the centre of a compact H  ii component to beyond the boundary of significant continuum emission. Three of the four masers located near continuum peaks are close to OH masers. In two cases the positional agreement is better than 0.2 arcsec (53 mpc); in the third case the agreement is worse (0.9 arcsec) but the positions of the individual H₂O features appear to be spread over more than 1 arcsec. The velocities of the OH masers are within the spread of the H₂O velocities. The three H₂O masers offset from continuum centres are located  3–7 arcsec  from optical or infrared phenomena probably associated with very early stages of star formation; no other molecular masers are known in these directions.     

Full polarization structure of the OH main-line maser envelopes of W Hydrae

Simultaneous MERLIN observations of the OH 1665- and 1667-MHz maser lines in the circumstellar envelope of the semiregular star W Hya have been taken in all Stokes parameters. The 1665-MHz emission comes from two elongated clusters located 80 au from the star. The 1667-MHz emission arises in an incomplete shell of radius 130 au, with the blueshifted features located in the northern part of the envelope and the redshifted components clustered south of the centre. The circularly polarized maser components exhibit spatial separation along the north–south direction. The linearly polarized components were found from the near side of the envelope. Their polarization position angles indicate that the projected axis of the magnetic field at PA ≃ −20° is consistent with spatial segregation of circular polarization. The intensity of the magnetic field, estimated from a tentative measurement of Zeeman splitting, is about 0.6 mG at the location of the 1667-MHz emission, with the field pointing away from the observer. A small change of position angles of linear polarization observed in both maser lines is interpreted as a weak Faraday effect in the maser regions with an electron density of about 2 cm⁻³. The overall polarization structure of the envelope suggests an ellipsoidal or weak bipolar geometry. In such a configuration, the circumstellar magnetic field may exert a non-negligible influence on mass loss. The velocity field in the circumstellar envelope recovered from observations of SiO, H₂O, OH and CO lines at five radial distances reveals a logarithmic velocity gradient of 0.25 and 0.21 in the 1665- and 1667-MHz maser regions respectively. The acceleration within tens of stellar radii cannot be explained by the classical model of radiation pressure on dust.     

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.     

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.     

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.     

Water in the near-infrared spectrum of comet 8P/Tuttle

High-resolution spectra of comet 8P/Tuttle were obtained in the frequency range 3449.0–3462.2 cm⁻¹ on 2008 January 3 ut using CGS4 with echelle grating on United Kingdom Infrared Telescope. In addition to observing solar pumped fluorescent lines of H₂O, the long integration time (152 min on target) enabled eight weaker H₂O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called SH lines recorded in the post Deep Impact spectrum of comet Tempel 1. We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line give some clues to the physical processes involved in their production. Finally, we derive an H₂O rotational temperature of     and a water production rate of  (1.4 ± 0.3) × 10²⁸  molecules s⁻¹.     

Ice in the Southern Coalsack

To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H₂O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H₂O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H₂O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H₂O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to   A _V∼5  towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.     

Measurements of the 44-μm band of H2O ice deposited on amorphous carbon and amorphous silicate substrates

We present 20–110 µm absorbance spectra of H₂O ice, deposited on amorphous carbon and silicate substrates, obtained over the 10–140 K temperature range. The measurements have been carried out in a manner that simulates the deposition, warming and cooling of H₂O ice mantles on interstellar and circumstellar grains. For H₂O ice films deposited on these substrates we find (i) similar 44-µm-band peak wavelength temperature dependences, (ii) no bandshape differences in the respective spectra, and (iii) a structural phase transition occurring between 120 and 130 K. In comparison with published data obtained using a polyethylene substrate, the 52-µm feature (the longitudinal optical mode) observed in our spectra is less prominent. This suggests the presence of material-dependent substrate effects that can alter the appearance of the H₂O far-infrared spectrum. The crystallization temperature of H₂O ice films deposited on our amorphous silicate substrate is significantly different from that reported by Moore et al. (1994) , who found crystallization temperatures down to < 20 K for ice also deposited on an amorphous silicate substrate. This is attributed to differences in the surface structures of the respective substrates. This may indicate that, at least in the context of laboratory measurements, substrate material composition is not as significant as substrate surface structure.     

Probing the surfaces of interstellar dust grains: the adsorption of CO at bare grain surfaces

A solid-state feature was detected at around 2175 cm⁻¹ towards 30 embedded young stellar objects in spectra obtained using the Infrared Spectrometer and Array Camera at the European Southern Observatory Very Large Telescope. We present results from laboratory studies of CO adsorbed at the surface of zeolite wafers, where absorption bands were detected at 2177 and 2168 cm⁻¹ (corresponding to CO chemisorbed at the zeolite surface) and 2130 cm⁻¹ (corresponding to CO physisorbed at the zeolite surface), providing an excellent match to the observational data. We propose that the main carrier of the 2175-band is CO chemisorbed at bare surfaces of dust grains in the interstellar medium. This result provides the first direct evidence that gas–surface interactions do not have to result in the formation of ice mantles on interstellar dust. The strength of the 2175-band is estimated to be  ∼4 × 10⁻¹⁹ cm  molecule⁻¹. The abundance of CO adsorbed at bare grain surfaces ranges from 0.06 to 0.16 relative to H₂O ice, which is, at most, half of the abundance (relative to H₂O ice) of CO residing in H₂O-dominated ice environments. These findings imply that interstellar grains have a large (catalytically active) surface area, providing a refuge for interstellar species. Consequently, the potential exists for heterogeneous chemistry to occur involving CO molecules in unique surface chemistry pathways not currently considered in gas grain models of the interstellar medium.     

Thermal desorption of water ice in the interstellar medium

Water (H₂O) ice is an important solid constituent of many astrophysical environments. To comprehend the role of such ices in the chemistry and evolution of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astronomical modelling community for accurate empirical molecular data pertaining to these processes. Here we give the first results of a laboratory programme to provide such data. Measurements of the thermal desorption of H₂O ice, under interstellar conditions, are presented. For ice deposited under conditions that realistically mimic those in a dense molecular cloud, the thermal desorption of thin films (≪50 molecular layers) is found to occur with zeroth-order kinetics characterized by a surface binding energy, E _des, of 5773 ± 60 K, and a pre-exponential factor, A , of 10^30 ± 2 molecules cm⁻² s⁻¹. These results imply that, in the dense interstellar medium, thermal desorption of H₂O ice will occur at significantly higher temperatures than has previously been assumed.     

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.     

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.     

Hydrated sulphuric acid in dense molecular clouds

We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H₂SO₄ ·  n H₂O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects.     

On the detection of interstellar homonuclear diatomic molecules

The pure rotational spectrum of homonuclear diatomic molecules in the interstellar medium is strongly forbidden, and no such spectrum has been detected. In regions of high excitation, vibrational emission may occur, as is widely detected in the case of H₂ in interstellar shocks and photon-dominated regions. However, it is of considerable interest to know the abundance of homonuclear diatomics in quiescent regions. We propose that vibrational emission from homonuclear diatomic molecules in cold clouds may be detectable, where the excitation is mainly through collisions with non-thermal electrons arising from the cosmic-ray ionization of H₂. As an example, we estimate the intensity of emission from N₂ in cold, dark interstellar clouds. We show that such emission is at the limit of detectability with current technology. Other excitation mechanisms may also contribute and enhance this emission.     

Class II maser candidates in substituted methanol: CH3OD, 13CH3OH, CH318OH and CH3SH

We investigate the possibility of interstellar masers in transitions of the methanol isotopomers CH₃OD, ¹³CH₃OH and CH₃¹⁸OH, and of CH₃SH. The model used, in which masers are pumped through the first and second torsionally excited states by IR radiation, has accounted successfully for the Class II masers in main species methanol, ¹²CH₃¹⁶OH. Several potential maser candidates are identified for CH₃OD, their detectability depending on the enrichment of this species in star-forming regions. In ¹³CH₃OH and CH₃¹⁸OH the best maser candidates are direct counterparts of the well-known 6.7- and 12.2-GHz methanol masers, but the lower interstellar abundance of these substituted species means that the expected brightness is greatly reduced. The maser candidates in CH₃SH are also weak. By comparing these species we find that the large b -component of the dipole moment in methanol plays a significant role in its propensity to form masers, as does the strong torsion–rotation interaction due to the light hydroxyl frame. Thus the exceptional brightness of interstellar methanol masers is due to a favourable combination of molecular properties as well as high interstellar abundance.     

C-type shocks in the interstellar medium: profiles of CH+ and CH absorption lines

We have computed optical absorption-line profiles of CH⁺ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C⁺(H₂, H)CH⁺. Whilst CH⁺ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH⁺₃(e⁻, H₂)CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH⁺ and CH line profiles is found to be no more than approximately 2 km s⁻¹, which is smaller than has previously been assumed. We also investigate OH and HCO⁺, finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model.     

Shocked gas around Cepheus A: evidence for multiple outflows from H2S and SO2 observations

The Cepheus A star-forming region has been investigated through a multiline H₂S and SO₂ survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H₂S/SO₂ integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H₂S appears to be more abundant than SO₂ in high-velocity clumps, in agreement with chemical models. However, we also find quite small H₂S linewidths, suggestive of regions where the evaporated H₂S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks.     

Radio observations of the planetary nebula around the OH/IR star OH 354.88-0.54 (V1018 Sco)

We present radio observations of the unique, recently formed, planetary nebula (PN) associated with a very long-period OH/IR variable star V1018 Sco that is unequivocally still in its asymptotic giant branch phase. Two regions within the optical nebula are clearly detected in non-thermal radio continuum emission, with radio spectral indices comparable to those seen in colliding-wind Wolf–Rayet binaries. We suggest that these represent shocked interactions between the hot, fast stellar wind and the cold nebular shell that represents the PN's slow wind moving away from the central star. This same interface produces both synchrotron radio continuum and the optical PN emission. The fast wind is neither spherical in geometry nor aligned with any obvious optical or radio axis. We also report the detection of transient H₂O maser emission in this nebula.