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.     

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).     

Observations of methanol masers in star-forming regions

We present VLBI observations of 6.7 and 12.2 GHz methanol masers in three star-forming regions, NGC7538, W75N and S252. Our results reveal linear structures in the VLBI maps and monotonic velocity gradients in the three sources. All these results are consistent with Keplerian disks of diameter of 1000–2000 AU around young stars of mass (10–30) M.     

HCO emission from H II-molecular cloud interface regions

A survey of well-known molecular clouds in the four strongest HCO NK-,K+ = 1(01)-0(00) hyperfine transitions has been carried out to determine the prevalence of HCO and to study its chemistry. HCO emission was observed in seven molecular clouds. Three of these, NGC 2264, W49, and NGC 7538, were not previously known sources of HCO. In addition, NGC 2024 and Sgr B2 were mapped and shown to have extensive HCO emission. The survey results show the HCO abundance to be enhanced in H II-molecular cloud interface regions and support a correlation between C+ and HCO emission. The strength of the HCO emission in NGC 2024 is interpreted in terms of this enhancement and the source structure and proximity to Earth.     

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.     

Cerenkov microwave line emission of hydroxyl radical

We propose to interpret the nonthermal emission lines of molecular clouds by a Cerenkov radio line emission mechanism. Relativistic electrons passing through a cloud with population inversion can generate microwave emission lines of anomalous intensity ratios. Calculations for nonthermal emission lines of OH sources of Group I Ia associated with supernova remnants show that this nonthermal mechanism may provide a hopeful way of solving certain difficulties of the theory of celestial masers.     

A comprehensive search for extragalactic 6.7-GHz methanol masers

We have used the Australia Telescope Compact Array (ATCA) to search for 6.7-GHz methanol maser emission towards 87 galaxies. We chose the target sources using several criteria, including far-IR luminosities and the presence of known OH megamasers. In addition, we searched for methanol masers in the nearby starburst galaxy NGC 253, making a full spectral-line synthesis image. No emission was detected in any of the galaxies, with detection limits ranging from 25 to 75 mJy. This is surprising, given the close association of OH and methanol masers in Galactic star formation regions, and significantly constrains models of OH megamaser emission. This absence of maser emission may be a result of low methanol abundances in molecular clouds in starburst galaxies.     

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.     

A study of the ultra-compact H-II region NGC 7538-IRS

From an analysis of VLBI observations of H₂CO and OH maser emission in the direction of the ultra-conpact HII region NGC 7538-IRS 1, the following model is proposed: The HII region is surrounded by a thick dusty shell which breaks open at the two poles and there is a bipolar outflow. Around it is a rotating gas/dust ring and matter falls from the ring onto the surface of the HII region. The whole system, HII region and the ring, moves with a sight line velocity of −61.0 km/s inside a large cloud which moves with a sight line velocity of −57 km/s. The H₂CO and OH masers occur near the poles of the HII region and within 0.2 R_HII of the surface. The positions of the H₂O maser and other line sources are discussed in term of this model.     

Detection of a new interstellar molecular ion, H2COH+ (protonated formaldehyde)

A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+.     

An ISO survey of possible water and hydroxyl IRASER transitions towards the star-forming regions W49, W3(OH) and Sgr B2M

We report ISO LWS observations towards the star-forming regions W49N, W3(OH) and Sgr B2M in LWS04 mode (high-resolution Fabry–Perot scans). Possible far-infrared laser emission was detected in a water line at 133.55 μm towards W49N, but the spectral resolution was inadequate to establish firmly the nature of the emission. An additional water line was marginally detected in absorption at the 3 σ level, also towards W49N, at 169.74 μm. No OH lines were detected towards Sgr B2M or W3(OH) at either 134.83 or 135.95 μm, either in emission or in absorption.     

大质量分子云核的CO同位素搜寻

使用紫金山天文台 １３．７ ｍ望远镜上新安装的 ＳＩＳ系统,对 ６４个高色指数 ＩＲＡＳ源和水脉泽源进行了 ＣＯ同位素１３ＣＯ和 Ｃ１８Ｏ Ｊ＝ １－０的搜寻,并对部分源作了成图观测．结果在 ６４个源中全部测到了这一谱线对,而１３ＣＯ的辐射一般较强,说明与稠密分子区成协,其中约６０个源为首次作ＣＯ同位素谱线对巡测．用高斯拟会导出了天线温度、线心速度和谱线全半宽．对辐射强度、话线特征进行了初步分析．     

The relationship between class I and class II methanol masers

The Australia Telescope National Facility Mopra millimetre telescope has been used to search for 95.1-GHz class I methanol masers towards 62 6.6-GHz class II methanol masers. A total of 26 95.1-GHz masers were detected, 18 of these being new discoveries. Combining the results of this search with observations reported in the literature, a near complete sample of 66 6.6-GHz class II methanol masers has been searched in the 95.1-GHz transition, with detections towards 38 per cent (25 detections; not all of the sources studied in this paper qualify for the complete sample, and some of the sources in the sample were not observed in the present observations).
There is no evidence of an anticorrelation between either the velocity range, or peak flux density of the class I and II transitions, contrary to suggestions from previous studies. The majority of class I methanol maser sources have a velocity range that partly overlaps with the class II maser transitions. The presence of a class I methanol maser associated with a class II maser source is not correlated with the presence (or absence) of main-line OH or water masers. Investigations of the properties of the infrared emission associated with the maser sources shows no significant difference between those class II methanol masers with an associated class I maser and those without. This may be consistent with the hypothesis that the objects responsible for driving class I methanol masers are generally not those that produce main-line OH, water or class II methanol masers.     

Strong magnetic field in W75N OH maser flare

A flare of OH maser emission was discovered in W75N in 2000. Its location was determined with the Very Long Baseline Array (VLBA) to be within 110 au from one of the ultracompact H  ii regions, Very Large Array 2 (VLA2). The flare consisted of several maser spots. Four of the spots were found to form Zeeman pairs, all of them with a magnetic field strength of about 40 mG. This is the highest ever magnetic field strength found in OH masers, an order of magnitude higher than in typical OH masers. Three possible sources for the enhanced magnetic field are discussed: (i) the magnetic field of the exciting star dragged out by the stellar wind; (ii) the general interstellar field in the gas compressed by the magnetohydrodynamic shock; and (iii) the magnetic field of planets which orbit the exciting star and produce maser emission in gaseous envelopes.     

Modelling methanol and hydroxyl masers in star-forming regions

Class II methanol masers are found in close association with OH main-line masers in many star-forming regions, where both are believed to flag the early stages in the evolution of a massive star. We have studied the formation of masers in methanol and OH under identical model conditions for the first time. Infrared pumping by radiation from warm dust at temperatures >100 K can account for the known maser lines in both molecules, many of which develop simultaneously under a range of conditions. The masers form most readily in cooler gas (<100 K) of moderately high density  (10⁵–10⁸ cm^-3)  , although higher gas temperatures and/or lower densities are also compatible with maser action. The agreement between the current model (developed for methanol) and the established OH maser trends is very encouraging, and we anticipate that further tuning of the model will further improve such agreement.
We find the gas-phase molecular abundance to be the key determinant of observable maser activity for both molecules. Sources exhibiting both 6668-MHz methanol and 1665-MHz OH masers have a typical flux density ratio of 16; our model suggests that this may be a consequence of maser saturation. We find that the 1665-MHz maser approaches the saturated limit for OH abundances >10^−7.3, while the 6668-MHz maser requires a greater methanol abundance >10⁻⁶. OH-favoured sources are likely to be less abundant in methanol, while methanol-favoured sources may be less abundant in OH or experiencing warm (>125 K), dense (∼10⁷ cm⁻³) conditions. These abundance requirements offer the possibility of tying the appearance of masers to the age of the new-born star via models of gas-phase chemical evolution following the evaporation of icy grain mantles.     

A Possible Pumping Mechanism for Interstellar Class Ⅱ 107 GHz Methanol Masers

It is recognized that the interstellar methanol-107 GHz masers and OH-4.765 GHz masers towards Class Ⅱ sources are associated with each other and coexist towards ultracompact HⅡ regions. Therefore we suggest a new pumping mechanism-methanol masers without population inversion. It can explain the formation of 107GHz methanol masers, with the 4.765GHz OH masers acting as a driving coherent microwave field. It is argued that this mechanism is compatible with the astronomical conditions.     

The thermal and non-thermal components of sixteen nebular complexes

The nature of the fine structure of sixteen galactic nebular complexes is investigated by producing their radio spectra. The complexes are: W31, W33, W35, K39, W42, W40, W43, W48, W47, W49, K47, W51, W3, Carina, NGC 6334 and NGC 6357. Various physical parameters of the thermal components are derived by adopting a homogeneous, optically thin, spherical model. The variations of the physical parameters derived from the model as a function of the electron temperature and distance are investigated. A discussion about the thermal and non-thermal spectra is included. Some simple statistics on the physical parameters of the thermal sources (diameter, density, mass) are also presented by employing a sample of sixty-seven thermal components.     

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.     

A search for 22-GHz water masers within the giant molecular cloud associated with RCW 106

We report the results of a blind search for 22-GHz water masers in two regions, covering approximately half a square degree, within the giant molecular cloud associated with RCW 106. The complete search of the two regions was carried out with the 26-m Mount Pleasant radio telescope and resulted in the detection of nine water masers, five of which are new detections. Australia Telescope Compact Array (ATCA) observations of these detections have allowed us to obtain positions with arcsecond accuracy, allowing meaningful comparison with infrared and molecular data for the region. We find that for the regions surveyed there are more water masers than either 6.7-GHz methanol, or main-line OH masers. The water masers are concentrated towards the central axis of the star formation region, in contrast to the 6.7-GHz methanol masers which tend to be located near the periphery. The colours of the GLIMPSE point sources associated with the water masers are similar to those of 6.7-GHz methanol masers, but slightly less red. We have made a statistical investigation of the properties of the ¹³CO and 1.2-mm dust clumps with and without associated water masers. We find that the water masers are associated with the more massive, denser and brighter ¹³CO and 1.2-mm dust clumps. We present statistical models that are able to predict those ¹³CO and 1.2-mm dust clumps that are likely to have associated water masers, with a low misclassification rate.     

The Methanol Maser Emission in W51

The European VLBI Network (EVN) has been used to make phase referenced, wide-field (several arcminute) spectral line observations of the 6.7-GHz methanol maser emission towards W51. In the W51 Main region, the bulk of the methanol is offset from an UCHii region. This probably indicates the methanol emission arises at the interface of the expanding UCHii region and not from an edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region, the methanol emission is associated with a very compact, extremely embedded source supporting the hypothesis that methanol masers trace the earliest stages of massive star formation. As well as these two well-studied sites of star formation, many previously unknown regions star formation are detected, demonstrating that methanol masers are powerful means of detecting young massive stars.