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1.
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.  相似文献   

2.
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 2Π1/2   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 2Π1/2   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.  相似文献   

3.
Phase-referenced observations of 13 star-forming regions in the  2Π1/2, 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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
We have made observations of the four hyperfine transitions of the 2Π3/2,     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.  相似文献   

8.
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.  相似文献   

9.
We present single-baseline Multi-Element Radio-Linked Interferometer Network (MERLIN) measurements of excited OH 6.0-GHz masers and methanol 6.7-GHz masers for the source W3(OH). These allow us to compare the positions of individual maser spots of these two species to ∼15 mas accuracy for the first time, and to compare these with previously published positions of ground-state OH masers near 1.7 GHz and excited-state OH masers near 4.7 GHz. There is a strong association between OH 6035-MHz and 1665-MHz masers. OH and methanol have very similar distributions, but associations of individual masers are relatively rare: most methanol 6.7-GHz masers are within 100 mas of OH 6.0-GHz masers, but only four methanol masers are within 15 mas of an OH 6.0-GHz maser. There are no correspondences of either species with excited OH 4.7-GHz masers. Zeeman splitting of the 6.0-GHz OH lines indicates an ordered magnetic field ranging from 3.2 to 14.4 mG. The magnetic fields estimated from co-propagating masers such as 6035 and 1665 MHz are generally in good agreement with each other.  相似文献   

10.
俞志尧 《天文学报》2003,44(3):249-255
利用法国巴黎天文台的南锡射电望远镜对ON2中部以西区域OH1665和1667MHz脉泽进行观测，结合模型对中部以西区域的OH1665和1667MHz脉泽谱线频谱图进行分析和研究、利用薄盘模型，得到相应于ON2中部以西区域的OH1665和1667MHz峰的脉泽斑的位置及其Keplerian运动速度。  相似文献   

11.
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.  相似文献   

12.
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).  相似文献   

13.
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.  相似文献   

14.
Using the Nançay radio telescope of Paris Observatory, France, OH 1665 and 1667 MHz masers in the western part of ON2(C) have been observed. The observed emission spectra are analyzed with a thin-disk model, and the positions and Keplerian velocities of the maser spots corresponding to the spectral peaks are obtained.  相似文献   

15.
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.  相似文献   

16.
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.  相似文献   

17.
We present high angular resolution MERLIN observations of the 18-cm OH maser and continuum emission associated with the active core of the ultraluminous infrared galaxy Markarian 273. The continuum emission comes from three distinct regions in the central arcsecond of the galaxy. The brightest region of emission has a double-peaked structure which is spatially coincident with similar structures observed at 6 cm and 2.2 μm. The peak of the OH maser emission is spatially coincident with the peak in the continuum. For the first time the maser emission is spatially resolved, allowing us to measure the gas motion within the central 100 pc of the galaxy. Maser emission is found in both the 1665- and 1667-MHz lines, with no systematic offset found in the spatial locations of the two lines. The brighter component of the maser emission shows ordered motion and is aligned along the axis of the double-peaked structure in the brightest continuum region. The gas motion enables us to estimate the central mass density to be 850±50 M⊙ pc−3, which corresponds to a total mass of ≈1.5×108 M⊙.  相似文献   

18.
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.  相似文献   

19.
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.  相似文献   

20.
A flare of maser radio emission in the 1665-MHz OH line with a flux density of about 1000 Jy was discovered in the star-forming region W75 N in 2003. At the time of its observations, it was the strongest OH maser in the entire history of research since the discovery of cosmic OH masers in 1965. The linear polarization of the flare emission reached 100%. A weaker flare with a flux density of 145 Jy was observed in this source in 2000–2001; this was probably a precursor of the intense flare. The intensity of two other spectral features decreased when the flare emerged. This change in the intensity of the emission from maser condensations (a brightening of some of them and a weakening of others) can be explained by the passage of a magnetohydrodynamic shock through regions of enhanced gas concentration.  相似文献   

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