A source of extended HCO+ emission in young stellar objects

Anomalous molecular line profile shapes are the strongest indicators of the presence of the infall of gas that is associated with star formation. Such profiles are seen for well-known tracers, such as HCO+, CS and H2CO. In certain cases, optically thick emission lines with appropriate excitation criteria may possess the asymmetric double-peaked profiles that are characteristic of infall. However, recent interpretations of the HCO+ infall profile observed towards the protostellar infall candidate B335 have revealed a significant discrepancy between the inferred overall column density of the molecule and that which is predicted by standard dark cloud chemical modelling. This paper presents a model for the source of the HCO+ emission excess. Observations have shown that, in low-mass star-forming regions, the collapse process is invariably accompanied by the presence of collimated outflows; we therefore propose the presence of an interface region around the outflow in which the chemistry is enriched by the action of jets. This hypothesis suggests that the line profiles of HCO+, as well as other molecular species, may require a more complex interpretation than can be provided by simple, chemically quiescent, spherically symmetric infall models. The enhancement of HCO+ depends primarily on the presence of a shock-generated radiation field in the interface. Plausible estimates of the radiation intensity imply molecular abundances that are consistent with those observed. Further, high-resolution observations of an infall-outflow source show HCO+ emission morphology that is consistent with that predicted by this model

Water ice deposition and growth in molecular clouds

In interstellar clouds, the deposition of water ice on to grains only occurs at visual extinctions above some threshold value (Ath). At extinctions greater than Ath, there is a (near-linear) correlation between the inferred column density of the water ice and AV. For individual cloud complexes such as Taurus, Serpens, and ρ-Ophiuchi, Ath and the gradients of the correlation are very similar along all lines of sight. We have investigated the origin of this phenomenon, with careful consideration of the various possible mechanisms that may be involved and have applied a full chemical model to analyse the behaviours and sensitivities in quiescent molecular clouds. Our key results are as follows: (i) the ubiquity of the phenomenon points to a common cause, so that the lines-of-sight probe regions with similar, advanced, chemical, and dynamical evolution; (ii) for Taurus and Serpens Ath and the slope of the correlation can be explained as resulting from the balance of freeze-out of oxygen atoms and photodesorption of H2O molecules. No other mechanism can satisfactorily explain the phenomenon; (iii) Ath depends on the local density, suggesting that there is a correlation between local volume density and column density; (iv) the different values of Ath for Taurus and Serpens are probably due to variations in the local mean radiation field strength; (v) most ice is accreted on to grains that are initially very small (<0.01\,\mum); and (vi) the very high value of Ath observed in ρ-Ophiuchi cannot be explained in the same way, unless there is complex microstructure and/or a modification to the extinction characteristics

Erratum: The chemistry of transient molecular cloud cores

We assume that some, but not all, of the structure observed in molecular clouds is associated with transient features which are not bound by self-gravity. We investigate the chemistry of a transient density fluctuation, with properties similar to those of a core within a molecular cloud. We run a multipoint chemical code through a core's condensation from a diffuse medium to its eventual dispersion, over a period of ∼1 Myr. The dynamical description adopted for our study is based on an understanding of a particular mechanism, involving slow-mode wave excitation, for transient structure formation which so far has been studied in detail only with plane-parallel models in which self-gravity has not been included. We find a significant enhancement of the chemical composition of the core material on its return to diffuse conditions, whilst the expansion of the core as it disperses moves this material out to large distances from the core centre. This process transports molecular species formed in the high-density regions out into the diffuse medium. Chemical enrichment of the cloud as a whole also occurs, as other cores of various sizes, life-spans and separations evolve throughout. Enrichment is strongly affected by freeze-out on to dust grains, which takes place in high-density, high visual extinction regions. As the core disperses after reaching its peak density and the visual extinction drops below a critical value, grain mantles are evaporated back into the gas phase, initiating more chemistry. The influence of the sizes, masses and cycle periods of cores will be large both for the level of chemical enrichment of a dark cloud and ultimately for the low-mass star formation rate. The cores in which stars form are almost certainly bound by their self-gravity and are not transient in the sense that the cores on which most of our study is focused are transient. Obviously, enrichment of the chemistry of low-density material will not take place if self-gravity prevents the re-expansion of a core. We also consider the case of a self-gravitating core, by holding its peak density conditions for a further 0.4 Myr. We find that the differences near the peak densities between transient and gravitationally bound cores are generally small, and the resultant column densities for objects near the peak densities do not provide definitive criteria for discriminating between transient and bound cores. However, increases in fractional abundances due to reinjection of mantle-borne species may provide a criterion for detection of a non-bound core

Formation of COMs in Explosions-and Their Destruction

The method(s) by which complex organic molecules are formed is a subject of much debate. Specifically, if it is assumed that they are formed through gas–grain interactions, then it is necessary to identify a mechanism that is both efficient at forming the molecules and return them to the gas phase in quiescent clouds. In this paper, we review the recent models that are based on the catastrophic recombination of radicals, stored in ice, that leads to an explosion of the ice mantles and a vigorous high density radical–radical association chemistry. We identify the strengths and weaknesses of the models in recent applications. Finally, we address the often overlooked issue of complex organic molecule (COM) destruction channels and argue that our poor knowledge of the chemical destruction mechanisms is undermining the diagnostic power of COM studies

Herschel/SPIRE observations of water production rates and ortho-to-para ratios in comets

This paper presents Herschel/SPIRE (Spectral and Photometric Imaging Receiver) spectroscopic observations of several fundamental rotational ortho- and para-water transitions seen in three Jupiter-family comets and one Oort-cloud comet. Radiative transfer models that include excitation by collisions with neutrals and electrons, and by solar infrared radiation, were used to produce synthetic emission line profiles originating in the cometary coma. Ortho-to-para ratios (OPRs) were determined and used to derived water production rates for all comets. Comparisons are made with the water production rates derived using an OPR of 3. The OPR of three of the comets in this study is much lower than the statistical equilibrium value of 3; however they agree with observations of comets 1P/Halley and C/2001 A2 (LINEAR), and the protoplanetary disc TW Hydrae. These results provide evidence suggesting that OPR variation is caused by post-sublimation gas-phase nuclear-spin conversion processes. The water production rates of all comets agree with previous work and, in general, decrease with increasing nucleocentric offset. This could be due to a temperature profile, additional water source or OPR variation in the comae, or model inaccuracies

Astrophysically important reactions involving excited hydrogen

The associative ionization reaction H(n=2) + H → H+2 + e− is found to be a greater contributor to the H2 formation rate than the direct radiative association reaction H(n=2) + H → H2 + hν in most regions of astrophysical interest. The endothermicity (≂1.1 eV) of the reaction and the high departures from LTE that are required for the H I (n=2) level to be sufficiently populated restrict its significance to regions of high excitation. The reaction H(n=2) + H+ → H+2 + hν may be significant in highly excited ionized regions, such as planetary nebulae and shocks. Chemical models of circumstellar regions have been reassessed in the light of this information. A critical examination reveals tht excitation effects are, in general, very important in many astrophysical situations. Only exceptionally, will reactions involving the higher excited states (n≳2) be as significant as those involving H(n=2)

A gas-phase primordial origin of O-2 in comet 67P/Churyumov-Gerasimenko

Recent observations made by the Rosetta/ROSINA instrument have detected molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko with abundances at the 1–10 per cent level relative to H2O. Previous studies have indicated that the likely origin of the O2 may be surface chemistry of primordial (dark cloud) origin, requiring somewhat warmer, denser, and extreme H-atom poor conditions than are usually assumed. In this study, we propose a primordial gas-phase origin for the O2 that is subsequently frozen and effectively hidden until the ice mantles are sublimated in the comet’s coma. Our study presents results from a three-phase astrochemical model that simulates the chemical evolution of ices in the primordial dark cloud phase, its gravitational collapse, and evolution in the early protosolar nebula. We find that the O2 abundance can be produced and is fairly robust to the choice of the free parameters. Good matches for the O2:H2O ratio and, to a lesser extent, the N2:CO and CO:H2O ratios are obtained, but the models significantly overproduce N2. We speculate that the low value of N2:O2 that is observed is a consequence of the specific thermal history of the comet

Excited hydrogen and the formation of molecular hydrogen via associative ionization – I. Physical processes and outflows from young stellar objects

Rates for the associative ionization reaction H(n=2)+H→H+2+e− are calculated. This reaction is found to be a greater contributor to the H2 formation rate than the direct radiative association reaction H(n=2)+H→H2+hν in most regions of astrophysical interest. Chemical models of circumstellar regions are reassessed in the light of this information. We also examine the chemical behaviour of several other excited chemical species. A critical examination reveals that excitation effects are, in general, very important in many astrophysical situations and must be incorporated into the chemistry. H2 has been detected in a variety of circumstellar regions and has a pivotal role in the overall chemistry. The method and efficiency of its formation are therefore of great importance. We test the significance of the associative ionization reaction in several models. These models include a schematic description of the radiative transfer in H I Lyα. The endothermicity (≃ 1.1 eV) of the reaction and the high departures from LTE that are required for the H I (n = 2) level to be sufficiently populated restrict its significance to regions of high excitation, such as are found in circumstellar regions. In this paper (I), we investigate the importance of the reaction in winds associated with young stellar objects. In Paper II, the investigation will be extended to include novae, supernovae, planetary nebulae and shocked regions. The results indicate that reactions involving excited atomic states may be very important in a number of circumstellar chemistries. Only exceptionally will reactions involving the higher excited states (n > 2) be as significant as those involving H(n = 2)

Gravitational instabilities in a protosolar-like disc III: molecular line detection and sensitivities

At the time of formation, protoplanetary discs likely contain a comparable mass to their host protostars. As a result, gravitational instabilities (GIs) are expected to play a pivotal role in the early phases of disc evolution. However, as these young objects are heavily embedded, confirmation of GIs has remained elusive. Therefore, we use the radiative transfer code LIME to produce line images of a 0.17 M selfgravitating protosolar-like disc. We note the limitations of using LIME and explore methods to improve upon the default gridding. We synthesise noiseless observations to determine the sensitivities required to detect the spiral flux, and find that the line flux distribution does not necessarily correlate to the abundance density distribution; hence performing radiative transfer calculations is imperative. Moreover, the spiral features are seen in absorption, due to the GI-heated midplane and high extinction, which could be indicative of GI activity. If a small beamsize and appropriate molecular line are used then spatially resolving spirals in a protosolar-like disc should be possible with ALMA for an on-source time of 30 hr. Spectrally resolving non-axisymmetric structure takes only a tenth as long for a reasonable noise level, but attributing this structure to GI-induced activity would be tentative. Finally, we find that identifying finger-like features in PV diagrams of nearly edge-on discs, which are a direct indicator of spirals, is feasible with an on-source time of 19 hr, and hence likely offers the most promising means of confirming GI-driven spiral structure in young, embedded protoplanetary discs