The relationship between class I and class II methanol masers

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.     

Space–VLBI observations of the OH maser OH 34.26+0.15: low interstellar scattering

We report on the first space–VLBI observations of the OH 34.26+0.15 maser in two main-line OH transitions at 1665 and 1667 MHz. The observations involved the space radio telescope on board the Japanese satellite HALCA and an array of ground radio telescopes. The map of the maser region and images of individual maser spots were produced with an angular resolution of 1 mas, which is several times higher than the angular resolution available on the ground. The maser spots were only partly resolved and a lower limit to the brightness temperature     was obtained. The maser seems to be located in the direction of low interstellar scattering, an order of magnitude lower than the scattering of a nearby extragalactic source and pulsar.     

New models of interstellar gas–grain chemistry – III. Solid CO2

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.     

Modelling line profiles in infalling cores

Results are presented from a model of molecular line formation in collapsing star-forming cores. The study includes, for the first time, a self-consistent chemical and dynamical model which is then directly coupled to an appropriate radiative transfer model. The assumptions of chemical uniformity or simple monotonic variations within such cores are shown to be unacceptable. The results show that the abundance variations and the line profiles are highly sensitive to the assumed values of the free parameters in the chemical model. Extreme caution is therefore advised in the quantitative analysis of emission-line profiles from infall sources. The implied degeneracy can be overcome by multiple line-of-sight observations of many species and transitions.     

Far-ultraviolet extinction and diffuse interstellar bands

We relate the equivalent widths of the major diffuse interstellar bands (DIBs) near 5797 and 5780 Å with different colour excesses, normalized by E ( B − V ) , which characterize the growth of interstellar extinction in different wavelength ranges. It is demonstrated that the two DIBs correlate best with different parts of the extinction curve, and the ratio of these diffuse bands is best correlated with the far-ultraviolet (UV) rise. A number of peculiar lines of sight are also found, indicating that the carriers of some DIBs and the far-UV extinction can be separated in certain environments, e.g. towards the Per OB2 association.     

Electron-impact rotational excitation of linear molecular ions

Molecular R -matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO⁺, HCO⁺, NO⁺ and H₂⁺ up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Δ j =1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Δ j >1 are found to have appreciable rates and are found to be entirely dominated by short-range effects.     

Organic carbon concentrations and transport in small mountain rivers,Panama

Tropical small mountainous rivers (SMRs) are increasingly recognized for their role in the global export of dissolved organic carbon (DOC) to the oceans. Here we utilize the Isthmus of Panama as an ideal place to provide first-order estimates of DOC yields across a wide assemblage of bedrock lithologies and land cover practices. Samples for dissolved organic carbon (DOC) analysis were collected across Panama along an E–W transect from the central Panama area to the Costa Rican border for 24 mainstem rivers, 3 large tributary rivers, and one headwater stream. Sampling occurred during both the wet and the dry seasons. DOC concentrations during the wet season are higher than during the dry season in all but three of the rivers. Concentrations vary greatly from river to river and from season to season, with values as low as 0.64 mg l⁻¹ to greater than >25 mg l⁻¹ with the highest concentrations observed for the rivers draining Tertiary marine sedimentary rocks in the Burica and Azuero peninsulas. DOC yields from Panamanian rivers (2.29–7.97 tons/km²/y) are similar to or slightly lower than those determined for other tropical SMR systems. Areas underlain by Tertiary aged sediments exhibited significantly higher mean DOC yields compared to their igneous counterparts, despite maintaining substantially lower aboveground carbon densities, suggesting the important influence of lithology. Finally, regression analyses between DOC yields and select watershed parameters revealed a negative and statistically significant relationship with maximum and mean gradient suggesting lower soil retention times may be linked to lower DOC yields.     

Predominance of archaea-derived hydrocarbons in an Early Triassic microbialite

We investigated the distribution of lipids in Lower Triassic sedimentary rocks (252–247 myr) from South China, including a shallow water microbialite in the uppermost section of the outcrop. Archaeal derived hydrocarbons were the major constituents of the microbialite from the latest Early Triassic. Among these, we detected (i) abundant C₄₀ acyclic and monocyclic biphytanes (possibly derived from glycerol dialkyl glycerol tetraether lipids) and their degradation products, C_30–39 pseudohomologues and (ii) a C₂₅ head-to-tail linked (regular) isoprenoid hydrocarbon [possibly derived from dialkyl glycerol diether lipids (DGDs)] and its degradation products, C_21–24 pseudohomologues and abundant pristane and phytane. Through combination of compound-specific stable carbon isotope analysis of isoprenoid hydrocarbons, which had average δ¹³C values of −35‰ to −30‰, and their molecular distribution, it was not possible to unambiguously define the archaeal source for the biphytanes in the microbialite. The δ¹³C values for pristane and phytane were similar to those for head-to-tail linked C_21–25 isoprenoids; potential source organisms for these compounds were halophilic archaea. Except for methane seep microbialites, no other ancient or recent phototrophic microbialites have been reported to contain predominantly archaeal isoprenoid hydrocarbons. Our findings suggest the presence of a new type of microbialite.