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1.
J. L. Caswell 《Monthly notices of the Royal Astronomical Society》1999,308(3):683-690
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
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Stephen J. Blanksby rew M. McAnoy Suresh Dua John H. Bowie 《Monthly notices of the Royal Astronomical Society》2001,328(1):89-100
A recent theoretical investigation by Terzieva & Herbst of linear carbon chains, C n where n ≥ 6, in the interstellar medium has shown that these species can undergo efficient radiative association to form the corresponding anions. An experimental study by Barckholtz, Snow & Bierbaum of these anions has demonstrated that they do not react efficiently with molecular hydrogen, leading to the possibility of detectable abundances of cumulene-type anions in dense interstellar and circumstellar environments. Here we present a series of electronic structure calculations which examine possible anionic candidates for detection in these media, namely the anion analogues of the previously identified interstellar cumulenes C n H and C n −1 CH2 and heterocumulenes C n O (where n = 2–10). The extraordinary electron affinities calculated for these molecules suggest that efficient radiative electron attachment could occur, and the large dipole moments of these simple (generally) linear molecules point to the possibility of detection by radio astronomy. 相似文献
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H. C. Bhatt 《Monthly notices of the Royal Astronomical Society》1999,308(1):40-44
Optical linear polarization measurements of stars in the region of the cometary globules CG 30–31 in Vela–Puppis are presented. A polarization map representing the geometry of the magnetic field in the cometary globule complex is produced. The magnetic field is found to be nearly perpendicular to the cometary tails. This is unlike the case of the cometary globule CG 22 in which the field had earlier been found to be aligned with the tail. The observed field direction is more or less parallel to the bipolar molecular outflow from the young stellar object IRS 4 embedded in the head of CG 30. 相似文献
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Phase-referenced observations of 13 star-forming regions in the 2 Π1/2 , 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
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M. Szymczak A. J. Kus G. Hrynek 《Monthly notices of the Royal Astronomical Society》2000,312(1):211-216
We report observations of the 4765-MHz maser transition of OH (2 Π1/2 , J=1/2, F=1→0) towards 57 star-forming regions, taken with the 32-m Toruń telescope. Nine maser sources were detected, of which two had not been reported previously. The newly discovered sources in W75N and Cep A and four previously known sources were monitored over periods ranging from a few weeks to six months. Significant variations of the maser intensity occurred on time-scales of one week to two months. The relationships between the flux density and the linewidth for the new sources, established during the rise and fall phases of bursts that lasted 6–8 weeks, are consistent with a model of saturated masers. 相似文献
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Marco M. Maldoni Garry Robinson R. G. Smith W. W. Duley A. Scott 《Monthly notices of the Royal Astronomical Society》1999,309(2):325-331
We present 20–110 µm absorbance spectra of H2 O 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 H2 O ice mantles on interstellar and circumstellar grains. For H2 O 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 H2 O far-infrared spectrum. The crystallization temperature of H2 O 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. 相似文献
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D. C. Gabuzda P. YU. Kochenov R. I. Kollgaard T. V. Cawthorne 《Monthly notices of the Royal Astronomical Society》2000,315(2):229-240
Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources. 相似文献
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S. Etoka R. J. Cohen M. D. Gray 《Monthly notices of the Royal Astronomical Society》2005,360(3):1162-1170
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
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J. S.Lazendic † M.Wardle M. G.Burton F.Yusef-Zadeh J. B.Whiteoak A. J.Green M. C. B.Ashley 《Monthly notices of the Royal Astronomical Society》2002,331(2):537-544
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 12 CO, HCO+ and CS towards the maser show broad-line absorption, which is absent in the 13 CO 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. 相似文献
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L. Harvey-Smith † R. J. Cohen 《Monthly notices of the Royal Astronomical Society》2006,371(4):1550-1558
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. 相似文献
17.
K. J. Brooks † J. B. Whiteoak 《Monthly notices of the Royal Astronomical Society》2001,320(4):465-476
We have made observations of the four hyperfine transitions of the 2 Π3/2 , 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
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X. W. Zheng J. M. Moran M. J. Reid 《Monthly notices of the Royal Astronomical Society》2000,317(1):192-198
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. 相似文献
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. 相似文献
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Radio surveys of supernova remnants (SNRs) in the Galaxy have discovered 19 SNRs which are accompanied by the OH maser emission at 1720 MHz. This unusual maser is thought to be produced behind a shock front when a SNR expands into a molecular cloud. An important ingredient of this model is that the X-ray emission from the remnant enhances the production of OH molecules. In this sense, to study the characteristics of the mixed-morphology SNRs accompanied by the OH maser emission at 1720 MHz is important. By studying the X-ray characteristics of the mixed-morphology SNRs accompanied by the 1720 MHz OH maser emission, it is found that the ionization rate of X-ray is not correlated with the physical parameters , D, r, r2 and so on, but is correlated with the X-ray luminosity Lx. Meanwhile, Lx is closely correlated with the beam flux density of the weakest feature of the accompanying 1720 MHz OH maser emission. These mean that the X-ray emission from SNRs is sufficient to dissociate the water molecules behind a shock front and to produce the 1720 MHz OH masers. 相似文献
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J. L. Caswell 《Monthly notices of the Royal Astronomical Society》2003,341(2):551-568
The Parkes radio telescope has been used to study circular polarization in the spectra of masers at the 6035- and 6030-MHz transitions of excited OH. The targets were 91 previously catalogued sites of 6035-MHz maser emission. A few were not detected, primarily because of variability. However, the 6035-MHz intensity variations seldom exceed a factor of 2 over several years, with a handful of dramatic exceptions.
Towards many targets, the present observations have provided the first high-sensitivity search for the 6030-MHz transition and yielded 33 detections. All of the 6030-MHz maser features have 6035-MHz counterparts closely matching in velocity. For matching features, the 6030-MHz emission is most commonly weaker than the 6035-MHz emission by an order of magnitude but, in a few cases, is several times stronger. The detection statistics are well accounted for by very recent developments in maser modelling. However, the occasional occurrence of 6030-MHz maser emission stronger than at 6035-MHz poses a new challenge for the theory.
Spectra with good frequency resolution at 6030 and 6035 MHz yield many valuable measurements of magnetic fields. At each transition, the field can be inferred from a small frequency separation between the right-hand and left-hand circularly polarized features, attributed to the Zeeman effect in a magnetic field of a few mG. In the many instances where a 'Zeeman pair' on the 6035-MHz spectrum has features matched by the 6030-MHz spectrum, this provides convincing corroboration of the magnetic field, in both direction and magnitude.
Several prominent absorption features occur at 6035 MHz, and usually have matching absorption at 6030 MHz of similar, or slightly smaller, depth. 相似文献
Towards many targets, the present observations have provided the first high-sensitivity search for the 6030-MHz transition and yielded 33 detections. All of the 6030-MHz maser features have 6035-MHz counterparts closely matching in velocity. For matching features, the 6030-MHz emission is most commonly weaker than the 6035-MHz emission by an order of magnitude but, in a few cases, is several times stronger. The detection statistics are well accounted for by very recent developments in maser modelling. However, the occasional occurrence of 6030-MHz maser emission stronger than at 6035-MHz poses a new challenge for the theory.
Spectra with good frequency resolution at 6030 and 6035 MHz yield many valuable measurements of magnetic fields. At each transition, the field can be inferred from a small frequency separation between the right-hand and left-hand circularly polarized features, attributed to the Zeeman effect in a magnetic field of a few mG. In the many instances where a 'Zeeman pair' on the 6035-MHz spectrum has features matched by the 6030-MHz spectrum, this provides convincing corroboration of the magnetic field, in both direction and magnitude.
Several prominent absorption features occur at 6035 MHz, and usually have matching absorption at 6030 MHz of similar, or slightly smaller, depth. 相似文献