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1.
We present MERLIN observations of OH and H2O masers in the circumstellar envelope of S Per. The results are consistent with a model of a thick shell of H2O masers in a region which is still accelerating. 相似文献
2.
The large-scale distribution of interstellar H2O masers is given in this paper. Present results show that H2O masers can trace galactic spiral structure, and we also briefly discuss how to obtain a more accurate distribution of H2O masers from future observations. 相似文献
3.
《Chinese Astronomy and Astrophysics》1986,10(1):62-66
Results of infrared photometry in J, H, and K bands for 24 stellar maser sources are given. On the colour-colour diagram, nearly all the Mira variable masers are closer to the black body line than the normal M giants, which also have brighter H magnitudes. The main reason for the location of the Mira masers is the strong H2O absorption bands at 1.4 and 1.9 μm. The dust envelopes for maser sources have temperature of 600 – 700 K, so the radiation shorter than 3μm may be expected to be close to black body. The physical conditions in M supergiants are such that a peak intensity near 1 – 6 μm (the H band) may be expected.Maser-associated Mira variables have longer periods and redder colours than normal Mira stars and M giants, suggesting greater amount of mass ejection from the central stars. 相似文献
4.
A new pumping mechanism – methanol masers without population inversion is presented in this paper. It can be used to explain
the formation of a series of J
0 → J
-1
E methanol masers, while the 21 → 30
A
+ methanol masers are regarded as a driving coherent micrwave field. In the new mechanism, the intensities of J
0 - J
-1
E methanol masers are increased with the decreasing transition frequencies (or with rotational number J, approximately). These results agree with Slysh et al. (1995) and Slysh et al. (1999) J ≤ 5 observations for G3345.01+1.79 and W48, in which both J
0 → J
-1
E and 21 → 30
A
+ methanol masers are detected coincidentally. Other astronomical conditions, such as magnetic field, 21 → 30
A
+ coherent radition, incoherent pumping rate by thermal radition and so on are also discussed. The new mechanism can operate
as a complement to other ordinary maser pumping mechanisms for some class II methonal maser sources.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
在本文中较全面地述了天体脉泽源的ＶＬＢＩ观测方法，简单地提及ＶＬＢＩ观测研究的新进展，着重述评空间ＶＬＢＩ用脉泽的观测，并提及了地面ＶＬＢＩ的一些预备观测情况。 相似文献
6.
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. 相似文献
7.
D. J. van der Walt E. Churchwell M. J. Gaylard S. Goedhart 《Monthly notices of the Royal Astronomical Society》2003,341(1):270-276
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. 相似文献
8.
Interstellar H2O and OH masers associated with massive star-forming regions can be classified into three morphological types: isolated H2O masers; isolated OH masers; and spatially overlapping OH/H2O maser groups. In a large sample of star-forming regions the total number of maser groups of each type is approximately equal.
In order to account for these statistics we propose a pumping scheme based on a broadband radiative pump which produces inverted
populations of both OH and H2O masers by a process involving predissociation and dissociation of H2O. This scheme overcomes some drawbacks of earlier radiative pumping models, and may account for the association of OH and
H2O masers in massive star forming regions. 相似文献
9.
S. P. Ellingsen 《Monthly notices of the Royal Astronomical Society》2005,359(4):1498-1516
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. 相似文献
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. 相似文献
10.
J. S.Lazendic J. B.Whiteoak I.Klamer P. D.Harbison T. B. H.Kuiper 《Monthly notices of the Royal Astronomical Society》2002,331(4):969-974
Positions with subarcsecond accuracy have been measured for seven 22-GHz H2 O 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 CH3 OH). The H2 O 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 H2 O features appear to be spread over more than 1 arcsec. The velocities of the OH masers are within the spread of the H2 O velocities. The three H2 O 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. 相似文献
The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH
11.
M. D. Gray R. J. Cohen A. M. S. Richards J. A. Yates D. Field 《Monthly notices of the Royal Astronomical Society》2001,324(3):643-647
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
12.
S. L. Breen S. P. Ellingsen M. Johnston-Hollitt S. Wotherspoon I. Bains M. G. Burton M. Cunningham N. Lo C. E. Senkbeil T. Wong 《Monthly notices of the Royal Astronomical Society》2007,377(2):491-506
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 13 CO 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 13 CO and 1.2-mm dust clumps. We present statistical models that are able to predict those 13 CO and 1.2-mm dust clumps that are likely to have associated water masers, with a low misclassification rate. 相似文献
13.
14.
R. J. Ivison J. A. Yates & P. J. Hall 《Monthly notices of the Royal Astronomical Society》1998,295(4):813-818
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 H2 O maser action towards a symbiotic Mira — R Aqr. Comparison with simultaneous 22-GHz H2 O 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 H2 O and anomalously weak OH and 22-GHz H2 O 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. 相似文献
15.
G.M. Rudnitskij 《Astrophysics and Space Science》1997,251(1-2):259-262
Physical processes affecting propagation of moderate-strength shocks in the inner parts of circumstellar envelopes of Miras
are considered. In particular, the influence of dust, which is heated later than heavy gas particles, is critical for pumping
of circumstellar H2O masers. Strong shocks with Mach numbers M exceeding ∼ 10 destroy dust and molecules and partly ionize the gas. Weaker shocks
with 3 < M < 10 spend their energy mainly to dissociate molecules. Shocks with 1 < M < 3 lose their energy via heating of
dust and may stimulate formation of dust. Some implications are discussed.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
16.
J. N. McCallum S. P. Ellingsen J. E. J. Lovell C. J. Phillips J. E. Reynolds 《Monthly notices of the Royal Astronomical Society》2009,392(4):1339-1362
We have used the Long Baseline Array to observe the Circinus Galaxy H2 O 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. 相似文献
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
17.
Caswell 《Monthly notices of the Royal Astronomical Society》1998,297(1):215-235
The Australia Telescope Compact Array has been used to observe more than 200 1665-MHz hydroxyl masers south of declination −16° and derive their positions with typical rms uncertainties of 0.4 arcsec. Many of the 1665-MHz maser sites are found to have 1667-MHz OH maser counterparts which are coincident, within the errors.
The resulting position list presented here includes all well-documented, previously reported 1665-MHz masers close to the Galactic plane in the galactic longitude range 230° (through 360°) to 13°. Nearly 50 newly discovered masers are also listed, chiefly in the longitude range 312° to 356°, where the observations were conducted as an intensive survey of a continuous zone close to the Galactic plane.
Many of the maser sites are discussed briefly so as to draw attention to those possessing properties that are unusual among this large sample. Most of the masers are of the variety found in star-forming regions – at the sites of newly formed massive stars and their associated ultracompact H ii regions. The new, accurate, positions reveal coincidences of the OH masers with the continuum radio emission, with the infrared emission from dust that accompanies such regions, and with emission from other maser species such as methanol at 6668 MHz and water at 22 GHz.
By-products of the survey, also presented here, include measurements of at least 11 objects that are not associated with massive star-forming regions. They comprise several OH/IR stars (detected at the 1667- or 1665-MHz transition of OH, though commonly found to be most prominent at the 1612-MHz transition) and several unusual masers that may pinpoint other varieties of late-type stars or protoplanetary nebulae. 相似文献
The resulting position list presented here includes all well-documented, previously reported 1665-MHz masers close to the Galactic plane in the galactic longitude range 230° (through 360°) to 13°. Nearly 50 newly discovered masers are also listed, chiefly in the longitude range 312° to 356°, where the observations were conducted as an intensive survey of a continuous zone close to the Galactic plane.
Many of the maser sites are discussed briefly so as to draw attention to those possessing properties that are unusual among this large sample. Most of the masers are of the variety found in star-forming regions – at the sites of newly formed massive stars and their associated ultracompact H ii regions. The new, accurate, positions reveal coincidences of the OH masers with the continuum radio emission, with the infrared emission from dust that accompanies such regions, and with emission from other maser species such as methanol at 6668 MHz and water at 22 GHz.
By-products of the survey, also presented here, include measurements of at least 11 objects that are not associated with massive star-forming regions. They comprise several OH/IR stars (detected at the 1667- or 1665-MHz transition of OH, though commonly found to be most prominent at the 1612-MHz transition) and several unusual masers that may pinpoint other varieties of late-type stars or protoplanetary nebulae. 相似文献
18.
The data compiled for 34 very young massive compact infrared source (Becklin-Neugebauer objects) in a separately published catalogue serve as the basis for an investigation of their general properties. First, the correlations of the strength of the ice and silicate bands at 3.1 μm and 10 μm, resp., with each other and with other parameters of the sources are investigated. In accordance with theoretical expectations the strengths of both bands are not well correlated with each other. The ratio τ10/τ3 of their optical depths veries from 0.85 to 8 (and possibly up to 22). That implies for the number densities of the grains that (naked) silicate grains always are considerably more abundant than ice grains (grains with ice mantles). It follows from the discussion of the relationships between τ10, the temperature parameter of the sources, and their luminosity that relations between τ10 and temperature and possibly between τ10 and the luminosity exist. Stellar wind models for the ionized region around the embedded star are favoured after the analysis of the intensities of the hydrogen infrred emission lines and the radio continuum. The limited observational data do not yet allow a decision on a correlation between the silicate band depth and energy content and/or the velocity of the bipolar outflow detected in a number of sources. The BN objects are clearly more frequently associated with H2O masers than OH masers. The linear separations of the infrared peaks from the masers are compatible with models where the H2O masers are situated near the edges the gas-dust shells of the BN objects. 相似文献
19.
Roger J. Sylvester 《Astrophysics and Space Science》1997,251(1-2):271-274
Circumstellar hydroxyl (OH) masers are believed to be radiatively pumped (Elitzur et al. 1976). However, the proposed pumping
mechanism has not been directly observed before now, because the far-IR spectral lines which correspond to the pumping cycle
transitions are unobservable from the ground. We present ISO1 observations of the strong OH maser source IRC +10420. Our spectra clearly show the 34.6-μm doublet in absorption, and the
ensuing rotational cascade lines at 98.7, 163 and 79 μm in emission. These spectra provide the first direct confirmation of
the radiative pumping cycle for OH masers.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
20.
Daniel Okoh Jarken Esimbek Jian Jun Zhou Xin Di Tang Augustine Chukwude Johnson Urama Pius Okeke 《Astrophysics and Space Science》2014,350(2):657-681
Intriguing work on observations of 4.83 GHz formaldehyde (H2CO) absorptions and 4.87 GHz H110α radio recombination lines (RRLs) towards 6.7 GHz methanol (CH3OH) maser sources is presented. Methanol masers provide ideal sites to probe the earliest stages of massive star formation, while 4.8 GHz formaldehyde absorptions are accurate probes of physical conditions in dense (103–105 cm?3) and low temperature molecular clouds towards massive star forming regions. The work is aimed at studying feature similarities between the formaldehyde absorptions and the methanol masers so as to expand knowledge of events and physical conditions in massive star forming regions. A total of 176 methanol maser sources were observed for formaldehyde absorptions, and formaldehyde absorptions were detected 138 of them. 53 of the formaldehyde absorptions were newly detected. We noted a poor correlation between the methanol and formaldehyde intensities, an indication that the signals (though arise from about the same regions) are enhanced by different mechanisms. Our results show higher detection rates of the formaldehyde lines for sources with stronger methanol signals. The strongest formaldehyde absorptions were associated with IRAS sources and IRDCs that have developed HII regions, and that do not have EGOs. 相似文献