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1.
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.  相似文献   

2.
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   

3.
The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH2→HNC+H, and N+CH2→HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.  相似文献   

4.
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   

5.
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.  相似文献   

6.
The photodissociation of surface species, caused by photons from the cosmic-ray-induced and background interstellar radiation fields, is incorporated into our combined gas-phase and grain-surface chemical models of quiescent dense interstellar cores. For the cores studied here, only cosmic-ray-induced photons are important. We find that photodissociation alters gas-phase and surface abundances mainly at large cloud ages (≳ 106–7 yr). The abundances of those surface species, such as H2O, that are readily reproduced on the surface following photodissociation are not strongly affected at any time. The abundances of surface species that are, on the other hand, reformed slowly via surface reactions possessing activation energy (e.g. CH3OH) are reduced, while the abundances of associated surface photoproducts (e.g. CO) increase. In the gas phase, inclusion of surface photodissociation tends to increase molecular abundances at late times, slightly improving the agreement with observation for TMC-1.  相似文献   

7.
We present the light curves of the 6.7 and 12.2 GHz methanol masers in the star-forming region G9.62+0.20E for a time-span of more than 2600 d. The earlier reported period of 244 d is confirmed. The results of monitoring the 107 GHz methanol maser for two flares are also presented. The results show that flaring occurs in all three masing transitions. It is shown that the average flare profiles of the three masing transitions are similar. The 12.2 GHz masers are the most variable of the three masers with the largest relative amplitude having a value of 2.4. The flux densities for the different masing transitions are found to return to the same level during the low phase of the masers, suggesting that the source of the periodic flaring is situated outside the masing region, and that the physical conditions in the masing region are relatively stable. On the basis of the shape of the light curve we excluded stellar pulsations as the underlying mechanism for the periodicity. It is argued that a colliding wind binary can account for the observed periodicity and provide a mechanism to qualitatively explain periodicity in the seed photon flux and/or the pumping radiation field. It is also argued that the dust cooling time is too short to explain the decay time of about 100 d of the maser flare. A further analysis has shown that for the intervals from days 48 to 66 and from days 67 to 135 the decay of the maser light curve can be interpreted as due to the recombination of a thermal hydrogen plasma with densities of approximately  1.6 × 106 cm−3  and  6.0 × 105 cm−3  , respectively.  相似文献   

8.
A southern hemisphere survey of methanol emission sources in two millimetre-wave transitions has been carried out using the ATNF Mopra millimetre telescope. 16 emission sources have been detected in the 31–40 A+ transition of methanol at 107 GHz, including six new sources exhibiting class II methanol maser emission features. Combining these results with the similar northern hemisphere survey, a total of eleven 107-GHz methanol masers have been detected. A survey of the methanol emission in the 00–1−1 E transition at 108 GHz has resulted in the detection of 16 sources; one of them showing maser characteristics. This is the first methanol maser detected at 108 GHz, presumably of class II. The results of large velocity gradient statistical equilibrium calculations confirm the classification of these new sources as class II methanol masers.  相似文献   

9.
The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + COads and H + H2C=Oads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH3OH to form efficiently at 10–20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H2C=O on the negatively charged defect are substantial, CH3OH may form efficiently during the warm-up period in star-forming regions.  相似文献   

10.
We have used the ATNF Mopra antenna and the SEST antenna to search in the directions of several class II methanol maser sources for emission from six methanol transitions in the frequency range 85–115 GHz. The transitions were selected from excitation studies as potential maser candidates. Methanol emission at one or more frequencies was detected from five of the maser sources, as well as from Orion KL. Although the lines are weak, we find evidence of maser origin for three new lines in G345.01+1.79, and possibly one new line in G9.62+0.20.
The observations, together with published maser observations at other frequencies, are compared with methanol maser modelling for G345.01+1.79 and NGC 6334F. We find that the majority of observations in both sources are consistent with a warm dust (175 K) pumping model at hydrogen density ∼106 cm−3 and methanol column density ∼ 5×1017 cm−2. The substantial differences between the maser spectra in the two sources can be attributed to the geometry of the maser region.  相似文献   

11.
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   

12.
An upper limit of the column density of the C5 linear molecule in translucent interstellar clouds is estimated from high-resolution ( R =80 000) and very high signal-to-noise ratio (∼1000) echelle spectra. It is 1012 cm−2 per E ( B − V )=1 (two orders of magnitude lower than that of C2).  相似文献   

13.
Compact regions of enhanced HCO+ and NH3 emission have been detected close to a number of Herbig–Haro objects. An interpretation of these detections is the following: a transient clump within the molecular cloud has been irradiated by the shock that generates the Herbig–Haro object. The irradiation releases icy mantles from the grains within the transient clump and initiates a photochemistry. On the basis of this picture, we have developed an extensive chemical model which predicts that a wide range of species, other than NH3 and HCO+, should also be detectable. These include CH3OH, H2S, C3H4, H2CO, SO, SO2, H2CS and NS. The chemical effects should last ∼  104 yr  .  相似文献   

14.
We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (104–105 cm−3) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC3N and SiC2 are in good agreement with observations, while those of CN, HCO+, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH2, CH2+ , HCl, OH and H2O are anticipated to be highly abundant in these objects.  相似文献   

15.
A model is constructed of the material in front of the star Cygnus OB2 no. 12 in which dense cores are embedded in diffuse clumps of gas. The model reproduces the measured abundances of C2 and CO, and predicts a column density of 91010 cm2 for HCO+.  相似文献   

16.
We have calculated synthetic spectra of perpendicular and parallel rovibronic bands of cumulene carbene molecules of the form C n H2. The perpendicular bands are consistent with a regularly spaced group of diffuse interstellar bands (DIBs) near 6850 Å. Parallel bands calculated for these molecular structures are consistent with the intrinsic profile of the associated 6614-Å DIB. Both types of bands are expected for an electronic transition that these species should have at those energies. We could not determine if the molecule was charged or if an atom other than carbon terminated the chain-end. Constraints due to molecular geometry and temperature place the chain length at 7–15 carbons to fit the 6850-Å group and 9–13 carbons to fit the 6614-Å DIB.  相似文献   

17.
A statistical estimate of the number of 6.7-GHz methanol masers in the Milky Way and their lifetime is presented. The estimate is based on the currently known number of masers, a realistic correction for sensitivity effects and generally accepted Galactic star formation rates and initial mass functions. The analysis suggests that the minimum number of masers in the Galaxy is of the order of 850 while a more realistic estimate of the total number of masers is of the order of 1200 ± 84. The lifetime is estimated to be between  2.5 × 104  and  4.5 × 104 yr  , with the variation being due to the use of different initial mass functions. The estimated lifetime agrees with that found from independent studies and agrees remarkably well with the time-scale for the chemical evolution of methanol in hot cores as well as with the dynamical time-scales of molecular outflows associated with high-mass star formation regions. It is shown that the hypothesis of the masers being associated with propagating planar shocks in cores or clumps results in lifetimes for the masers that are smaller by a factor of 2 or more compared with the lifetime of methanol masers as estimated here.  相似文献   

18.
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.  相似文献   

19.
We report observations of the 4765-MHz maser transition of OH (2Π1/2, 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   

20.
Solid CO2 is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO2 at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO2 can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.  相似文献   

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