The Australia Telescope Compact Array (ATCA) has been used to image class I methanol masers at 9.9, 25 (a series from J = 2 to 9), 84, 95 and 104 GHz located in the vicinity of IRAS 16 547−4247 (G343.12−0.06), a luminous young stellar object known to harbour a radio jet. The detected maser emission consists of a cluster of six spots spread over an area of 30 arcsec. Five spots were detected in only the 84-and 95-GHz transitions (for two spots the 84-GHz detection is marginal), while the sixth spot shows activity in all 12 observed transitions. We report the first interferometric observations of the rare 9.9-and 104-GHz masers. It is shown that the spectra contain a very narrow spike (< 0.03 km s −1 ) and the brightness temperature in these two transitions exceeds 5.3 × 10 7 and 2.0 × 10 4 K, respectively. The three most southern maser spots show a clear association with the shocked gas traced by the H 2 2.12-µm emission associated with the radio jet and their velocities are close to that of the molecular core within which the jet is embedded. This fact supports the idea that the class I masers reside in the interface regions of outflows. Comparison with OH masers and infrared data reveals a potential discrepancy in the expected evolutionary state. The presence of the OH masers usually means that the source is evolved, but the infrared data suggest otherwise. The lack of any class II methanol maser emission at 6.7 GHz in the source raises an additional question, Is this source too young or too old to have a 6.7-GHz maser? We argue that both cases are possible and suggest that the evolutionary stage where the class I masers are active, may last longer and start earlier than when the class II masers are active. However, it is currently not possible to reveal the exact evolutionary status of IRAS 16 547−4247.

We report interferometric observations of nine class II methanol maser candidate lines toward W3(OH). Narrow maser emission spikes at km s~1 are present in three of the lines :

Model spectra are presented for Class II methanol masers under a variety of conditions. The model is that of Sobolev & Deguchi, which includes pumping through levels of the second and first torsionally excited states. All the currently identified Class II methanol masers appear as strong masers in one or more of the model regimes, and a number of new maser candidates are identified.

The Australia Telescope Compact Array (ATCA) has been used to make the first extensive search for the class I methanol masers at 9.9 GHz. In total, 48 regions of high-mass star formation were observed. In addition to masers in W33-Met (G12.80-0.19) and G343.12-0.06 (IRAS 16547-4247) which have already been reported in the literature, two new 9.9-GHz masers have been found towards G331.13-0.24 and G19.61-0.23. We have determined absolute positions (accurate to roughly a second of arc) for all the detected masers and suggest that some class I masers may be associated with shocks driven into molecular clouds by expanding HII regions. Our observations also imply that the evolutionary stage of a high-mass star forming region when the class I masers are present can outlast the stage when the class II masers at 6.7 GHz are detectable, and overlaps significantly with the stage when OH masers are active.Comment: accepted for publication in MNRAS, 14 pages, 3 figures, 4 table

We study how uncertainties in the rate coefficients of chemical reactions in the RATE06 database affect abundances and column densities of key molecules in protoplanetary disks. We randomly varied the gas-phase reaction rates within their uncertainty limits and calculated the time-dependent abundances and column densities using a gas-grain chemical model and a flaring steady-state disk model. We find that key species can be separated into two distinct groups according to the sensitivity of their column densities to the rate uncertainties. The first group includes CO, C$^+$, H$_3^+$, H$_2$O, NH$_3$, N$_2$H$^+$, and HCNH$^+$. For these species, the column densities are not very sensitive to the rate uncertainties but the abundances in specific regions are. The second group includes CS, CO$_2$, HCO$^+$, H$_2$CO, C$_2$H, CN, HCN, HNC and other, more complex species, for which high abundances and abundance uncertainties co-exist in the same disk region, leading to larger scatters in the column densities. However, even for complex and heavy molecules, the dispersion in their column densities is not more than a factor of ~4. We perform a sensitivity analysis of the computed abundances to rate uncertainties and identify those reactions with the most problematic rate coefficients. We conclude that the rate coefficients of about a hundred of chemical reactions need to be determined more accurately in order to greatly improve the reliability of modern astrochemical models. This improvement should be an ultimate goal for future laboratory studies and theoretical investigations.Comment: 42 pages, 6 figures, 3 tables, accepted for publication in Ap

We present the unambiguous discovery of six new class II methanol maser transitions, three of which are torsionally excited (v t =1). The newly discovered 6.18-GHz 17 2 → 18 3 E (v t =1), 7.68-GHz 12 4 → 13 3 A − (v t =0), 7.83-GHz 12 4 → 13 3 A + (v t = 0), 20.9-GHz 10 1 → 11 2 A + (v t =1), 44.9-GHz 2 0 → 3 1 E (v t =1) and 45.8-GHz 9 3 → 10 2 E (v t =0) methanol masers were detected towards G 358.931−0.030, where the known 6.68-GHz maser has recently been reported to be undergoing a period flaring. The detection of the v t =1 torsionally excited lines corroborates one of the missing puzzle pieces in class II maser pumping, but the intensity of the detected emission provides an additional challenge, especially in the case of the very highly excited 6.18-GHz line. Together with the newly detected v t = 0 lines, these observations provide significant new information which can be utilised to improve class II methanol maser modelling. We additionally present detections of 6.68-, 19.9-, 23.1-and 37.7-GHz class II masers, as well as 36.2-and 44.1-GHz class I methanol masers, and provide upper limits for the 38.3-and 38.5-GHz class II lines. Near simultaneous Australia Telescope Compact Array (ATCA) observations confirm that all 10 of the class II methanol maser detections are co-spatial to ∼0.2 arcsec, which is within the uncertainty of the observations. We find significant levels of linearly polarised emission in the 6.18-, 6.67-, 7.68-, 7.83-, 20.9-, 37.7-, 44.9-and 45.8-GHz transitions, and low levels of circular polarisation in the 6.68-, 37.7-and 45.8-GHz transitions.

With the chemical reaction rate database UMIST95 (Millar etal. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of the molecular abundances in the interstellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations were performed for the dark and translucent molecular clouds where gas phase chemistry is adequate (Terzieva & Herbst 1998). Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in them and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species. Scatter in the model abundancesTo study the influence of uncertainties in reaction rates on the model abundances we calculated 10 4 variants of each species abundance. Figure 1 shows the scatter for some representative species.We divided all species into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants.Distribution of the species within these groups depends on adopted physical conditions (see fig. 1). Scatter in logarithmic abundances of simple molecules lies within 0.5 − 1 dex. Figure 2 shows how the species are distributed by the number of atoms in different sensitivity groups. It is clear that the scatter in abundances significantly increases with the number of atoms in the molecule.

The Australia Telescope Compact Array (ATCA) has been used to search for methanol maser emission at 6.7 GHz towards the Orion Molecular Cloud 1 (OMC‐1). Two features peaking at 7.2 and −1.1 km s−1 have been detected. The former has at least two components close in both velocity and position. It is located south‐east of the Orion Kleinmann‐Low (Orion‐KL) nebula in the region of outflow traced by the 25‐GHz methanol masers and the 95‐GHz methanol emission. It is shown by modelling that, in contrast to the widespread opinion that simultaneous masing of methanol transitions of different classes is impossible, there are conditions for which simultaneous masing of the class II transition at 6.7 GHz and some class I transitions (e.g. the series at 25 GHz) is possible. A relevant example is provided, in which the pumping occurs via the first torsionally excited state and is driven by radiation of the dust intermixed with the gas in the cloud. In this regime, the dust temperature is significantly lower (T≈ 60 K) than in the case of bright 6.7‐GHz masers (T > 150 K). The narrow spectral feature at −1.1 km s−1 has a brightness temperature greater than about 1400 K, which suggests that it is probably a maser. It emanates from the Orion South region and is probably associated with the approaching part of outflow seen in CO. The 25‐GHz maser associated with OMC‐1 was observed quasi‐simultaneously with the 6.7‐GHz observations. No 25‐GHz emission associated with the −1.1 km s−1 6.7‐GHz feature towards Orion South was detected.