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The source G9.62+0.20E surprises yet again! Analysis of the associated methanol maser feature at v lsr = +8.8 km s−1 revealed a second period of P = 52.1 ± 0.3 d, along with the previously reported period P = 243 d for this source. Various explanations, harmonic flaring, Dicke's superradiance, a secondary pulsation, or two different periodically varying radio sources in the field of view with associated methanol masers are presented. The latter two hypotheses appear more plausible but it is not clear which, if any, best describe these observations. [ABSTRACT FROM AUTHOR]

We report the detection of new 12.178, 12.229, 20.347, and 23.121 GHz methanol masers in the massive star-forming region G358.93−0.03, which are flaring on similarly short time-scales (days) as the 6.668 GHz methanol masers also associated with this source. The brightest 12.178 GHz channel increased by a factor of over 700 in just 50 d. The masers found in the 12.229 and 20.347 GHz methanol transitions are the first ever reported and this is only the fourth object to exhibit associated 23.121 GHz methanol masers. The 12.178 GHz methanol maser emission appears to have a higher flux density than that of the 6.668 GHz emission, which is unusual. No associated near-infrared flare counterpart was found, suggesting that the energy source of the flare is deeply embedded. [ABSTRACT FROM AUTHOR]

In this work, we aim to characterize high-mass clumps with infall motions. We selected 327 clumps from the Millimetre Astronomy Legacy Team 90-GHz survey, and identified 100 infall candidates. Combined with the results of He et al., we obtained a sample of 732 high-mass clumps, including 231 massive infall candidates and 501 clumps where infall is not detected. Objects in our sample were classified as pre-stellar, proto-stellar, HII or photodissociation region (PDR). The detection rates of the infall candidates in the pre-stellar, proto-stellar, HII and PDR stages are 41.2 per cent, 36.6 per cent, 30.6 per cent and 12.7 per cent, respectively. The infall candidates have a higher H2 column density and volume density compared with the clumps where infall is not detected at every stage. For the infall candidates, the median values of the infall rates at the pre-stellar, proto-stellar, HII and PDR stages are 2.6 × 10-3, 7.0 × 10-3, 6.5 × 10-3 and 5.5 × 10-3 M☉ yr-1, respectively. These values indicate that infall candidates at later evolutionary stages are still accumulating material efficiently. It is interesting to find that both infall candidates and clumps where infall is not detected show a clear trend of increasing mass from the pre-stellar to proto-stellar, and to the HII stages. The power indices of the clump mass function are 2.04 ± 0.16 and 2.17 ± 0.31 for the infall candidates and clumps where infall is not detected, respectively, which agree well with the power index of the stellar initial mass function (2.35) and the cold Planck cores (2.0). [ABSTRACT FROM AUTHOR]

Massive star-forming regions with observed infall motions are good sites for studying the birth of massive stars. In this paper, 405 compact sources have been extracted from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) compact sources that also have been observed in the Millimetre Astronomy Legacy Team 90-GHz (MALT90) survey during years 1 and 2. These observations are complemented with Spitzer GLIMPSE/MIPSGAL mid-IR survey data to help classify the elected star-forming clumps into three evolutionary stages: pre-stellar, proto-stellar and UCHII regions. The results suggest that 0.05 g cm-2 is a reliable empirical lower bound for the clump surface densities required for massive-star formation to occur. The optically thick HCO+(1-0) and HNC(1-0) lines, as well as the optically thin N2H+(1-0) line were used to search for infall motions towards these sources. By analysing the asymmetries of the optically thick HCO+(1-0) and HNC(1-0) lines and the mapping observations of HCO+(1-0), a total of 131 reliable infall candidates have been identified. The HCO+(1-0) line shows the highest occurrence of obvious asymmetric features, suggesting that it may be a better infall motion tracer than other lines such as HNC(1-0). The detection rates of infall candidates towards pre-stellar, proto-stellar and UCHII clumps are 0.3452, 0.3861 and 0.2152, respectively. The relatively high detection rate of infall candidates towards UCHII clumps indicates that many UCHII regions are still accreting matter. The peak column densities and masses of the infall candidates, in general, display an increasing trend with progressing evolutionary stages. However, the rough estimates of the mass infall rate show no obvious variation with evolutionary stage. [ABSTRACT FROM AUTHOR]

Molecular emission-line observations of isolated Galactic star-forming regions are used to model the physical properties of the molecular interstellar medium in these systems. Observed line ratios are compared with the results predicted by models that incorporate gas-phase chemistry and the heating by stellar radiation and non-radiative feedback processes. The line ratios of characteristic tracer molecules may be interpreted using the contributions of two distinct components: a cold (40–50 K) and high-density (105–105.5 cm−3) photon-dominated region (PDR) with a nominal UV flux density and a warm (∼300 K) mechanical heating-dominated region (MHDR) with a slightly lower density (104.5–105 cm−3). The relative contributions of these structural components are used to model the observed line ratios. Ionized species may be better modelled by adopting an increase of the cosmic ray flux towards the Galactic Centre and the sulphur abundance should depleted by a factor of 200–400 relative to solar values. The line ratios of the Galactic sample are found to be very similar to those of the integrated signature of prominent (ultra)luminous IR Galaxies. The PDRs and MHDRs in the isolated Galactic regions may be modelled with slightly higher mean densities than in extragalactic systems and a higher MHDR temperature resulting from non-radiative mechanical heating. Multimolecular studies are effective in determining the physical and chemical properties of star formation regions by using characteristic line ratios to diagnose their environment. The addition of more molecular species will reduce the existing modelling redundancy. [ABSTRACT FROM AUTHOR]

The observed molecular properties of a sample of FIR-luminous and OH megamaser (OH-MM) galaxies have been investigated. The ratio of high and low-density tracer lines is found to be determined by the progression of the star formation in the system. The HCO+/HCN and HCO+/HNC line ratios are good proxies for the density of the gas, and PDR and XDR sources can be distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources in the sample can be explained by PDR chemistry in gas with densities higher than 105.5cm−3, confirming the classical OH-MM model of IR pumped amplification with (variable) low gains. [ABSTRACT FROM PUBLISHER]