Electron-impact excitation of CrII: A theoretical calculation of effective collision strengths for optically allowed transitions.

In this paper, we present electron-impact excitation collision strengths and Maxwellian averaged effective collision strengths for the complicated iron-peak ion Cr II. We consider specifically the allowed lines for transitions from the 3d(5) and 3d(4)4s even parity configuration states to the 3d(4)4p odd parity configuration levels. The parallel suite of R-Matrix packages, RMATRX II, which have recently been extended to allow for the inclusion of relativistic effects, were used to compute the collision cross sections. A total of 108 LS pi/280 J pi levels from the basis configurations 3d(5), 3d(4)4s, and 3d(4)4p were included in the wavefunction representation of the target including all doublet, quartet, and sextet terms. Configuration interaction and correlation effects were carefully considered by the inclusion of seven more configurations and a pseudo-corrector (4d) over bar type orbital. The 10 configurations incorporated into the Cr II model thus listed are 3d(5), 3d(4)4s, 3d(4)4p, 3d(3)4s(2), 3d(3)4p(2), 3d(3)4s4p, 3d(4)(4d) over bar, 3d(3)4s (4d) over bar, 3d(3)4p (4d) over bar, and 3d(3)(4d) over bar (2), constituting the largest Cr II target model considered to date in a scattering calculation. The Maxwellian averaged effective collision strengths are computed for a wide range of electron temperatures 2000-100,000 K which are astrophysically significant. Care has been taken to ensure that the partial wave contributions to the collision strengths for these allowed lines have converged with "top-up" from the Burgess-Tully sum rule incorporated. Comparisons are made with the results of Bautista et al. and significant differences are found for some of the optically allowed lines considered.

Limits on HDS/H2S abundance ratios in hot molecular cores

We have searched for HDS emission in a small number of hot cores. Using observations of (H2S)-S-34, we have derived upper limits to the [HDS]/[H2S] abundance ratio. The upper limits, which are close to 10(-3) can be interpreted in two ways, depending on whether grain surface reactions contribute to the formation of H2S. If grains do not dominate, then the H2S observed is formed in hot, post-shocked gas and a ratio close to the cosmic [D]/[H] ratio is expected for [HDS]/[H2S]. This scenario is consistent with our upper limits and with the relatively low abundance ratio found for [HDO]/[H2O] in hot cores but does not seem to account for all of the molecular [D]/[H] ratios observed in hot cores. If grains do dominate the formation of HBS, then the observed upper limit to the ratio is consistent with the formation of 'hot core ices' at a temperature of 60-80 K, close to the temperature at which cometary ices are thought to form.

A 330-360 GHz spectral survey of G34.3+0.15 - III. The outer halo

We have undertaken a 330-360 GHz molecular line survey of the halo gas surrounding the hot core associated with G34.26+0.15. In contrast to our molecular line survey of the hot core itself, where 338 lines from at least 38 species were detected, only 18 lines from 9 species were detected in the halo. The lines are mainly single transitions of simple di atomic and triatomic molecules. Lower limits to their column densities have been evaluated by an LTE method. In the case of methanol, where four transitions were detected, the rotation temperature and column density have been evaluated by the rotation diagram technique. We have modified the previous depth-dependent chemical model developed in Paper II to calculate the column densities observed along a general line of sight drawn through the model cloud. The model is also extended to produce beam-averaged column densities for better comparison with those observed. We compare the model column densities with those observed and make recommendations for future depth-dependent chemical modelling of hot cores.

A survey of molecular line emission towards ultracompact HII regions

We have used the JCMT to survey molecular line emission towards 14 ultracompact HII regions (G5.89, G9.62, G10.30, G10.47, G12.21, G13.87, G29.96, G31.41, G34.26, G43.89, G45.12, G45.45, G45.47, and G75.78). For each source, we observed up to ten 1 GHz bands between 200 and 350 GHz, covering lines of more than 30 species including multiple transitions of CO isotopes, CH3OH, CH3CCH, CH3CN and HCOOCH3, and sulphuretted molecules. The number of transitions detected varied by a factor of 20 between sources; which were chosen following observations of high-excitation ammonia (Cesaroni et al. 1994a) and methyl cyanide (Olmi et al. 1993). In half our sample (the line-poor sources), only (CO)-O-17: (CO)-O-18, SO, (CS)-S-34 and CH3OH were detected. In the line-rich sources, we detected over 150 lines, including high excitation lines of CH3CN, HCOOCH3; C2H5CN, CH3OH, and CH3CCH. We have calculated the physical conditions of the molecular gas. To reproduce the emission from the line-rich sources requires both a hot, dense compact core and an ambient cloud consisting of less dense, cooler gas. The hot cores, which are less than 0.1 pc in size; reach densities of at least 10(8) cm(-3) and temperatures of more than 80 K. The line-poor sources can be modelled without a hot core by a 20-30 K, 10(5) cm(-3) cloud. We find no correlation between the size of the HII region and the current physical conditions in the molecular environment. A comparison with chemical models (Millar et al. 1997) confirms that grain surface chemistry is important in hot cores.

Gas and grain chemistry in a protoplanetary disk

The chemistry in a protoplanetary accretion disk is modelled between a radius of 100 and 0.1 AU of the central object. We find that interaction of the gas with the dust grains is very important, both by removing a large fraction of the material from the gas in the outer regions and through the chemical reactions which can occur on the dust grain surfaces. In addition, collision with grains neutralises gaseous ions effectively and keeps the ionization fraction low. This results in a chemistry which is dominated by neutral-neutral reactions, even if ionization is provided by cosmic rays or by the decay of radioactive isotopes. We model the effects of two desorption processes with very different efficiencies and find that while these produce similar results over much of the disk for many species, some molecules are extremely sensitive to the nature of the desorption and may one day be used as an observational test for the desorption process.

Sulphur chemistry and evolution in hot cores

We compare the results of our JCMT spectral line survey of molecular gas towards ultracompact HII regions with the predictions of models of sulphur chemistry in hot cores. We investigate the range of evolutionary models that are consistent with the observed physical conditions and chemical abundances, and see to what extent it is possible to constrain core ages by comparing abundances with the predictions of chemical models. The observed abundance ratios vary little from source to source, suggesting that all the sources are at a similar evolutionary stage. The models are capable of predicting the observed abundances of H2S, SO, SO2, and CS. The models fail to predict the amount of OCS observed, suggesting that an alternative formation route is required. An initial H2S abundance from grain mantle evaporation of similar to 10(-7) is preferred.

The DCN/HCN abundance ratio in hot molecular cores

We have observed the 3-2 transitions of DCN and (HCN)-N-15 in a number of hot molecular cores previously surveyed by us with the interesting result that the DCN/HCN ratio is low, a few times 10(-3), in the hot cores. The abundance ratio of DCN/HCN is derived both 'on-core' and 'off-core' and, in general is larger at the 'off-core' positions. Comparision with chemical models of these sources indicates that DCN liberated from evaporated ices can be destroyed rapidly in the hot gas by reaction with atomic hydrogen, which works to reset the the initial DCN/HCN ratio in the ice to the gas-phase atomic D/H ratio. The low DCN/HCN abundance ratio we measure can be reached in less than 10(4) years, consistent with previous estimates of the core ages, if the activation energy of the reaction is less than 500 K.

Molecular abundances in the magellanic clouds III. LIRS 36, a star-forming region in the Small Magellanic Cloud

Detections of CO, CS, SO, C2H, HCO+, HCN, HNC, H2CO, and C3H2 are reported from LIRS 36, a star-forming region in the Small Magellanic Cloud. (CO)-O-18, NO, CH3OH, and most notably CN have not been detected, while the rare isotopes (CO)-C-13 and, tentatively, (CS)-S-34 ar,seen. This is so far the most extensive molecular multiline study of an interstellar medium with a heavy element depletion exceeding a factor of four.

A 330-360 GHz spectral survey of G34.3+0.15 .2. Chemical modelling

We describe a detailed depth-and time-dependent model of the molecular cloud associated with the ultracompact H II region G 34.3+0.15. Previous work on observations of NH3 and CS indicates that the molecular cloud has three distinct physical components:- an ultracompact hot core, a compact hot core and an extended halo. We have used the physical parameters derived from these observations as input to our detailed chemical kinetic modelling. The results of the model calculations are discussed with reference to the different chemistries occuring in each component and are compared with abundances derived from our recent spectral line survey of G 34.3+0.15 (Paper I).

Chemistry in oxygen-rich circumstellar envelopes

We have considered the chemistry occuring in the circumstellar envelope surrounding an oxygen-rich AGE star and have specifically modelled 4 sources; R Dor, TX Cam, OH231.8+4.2 and IK Tau. Methane has been assumed to be a parent molecule and the resulting carbon chemistry is investigated. We find that carbon chain molecules up to C2H4 can be abundant as can CH3CN and CH3OH. Our model extends previous work by including the chemistry of silicon, chlorine and phosphorus. The presence of CH4 as a parent and hence its daughter species CH3 and CH3+ leads to other carbon-bearing species such as H2CS, SiCH2, H2CN and CCl.

Molecular abundances in the magellanic clouds .1. A multiline study of five cloud cores

Nine H II regions of the LMC were mapped in (CO)-C-13(1-0) and three in (CO)-C-12(1-0) to study the physical properties of the interstellar medium in the Magellanic Clouds. For N113 the molecular core is found to have a peak position which differs from that of the associated H II region by 20 ''. Toward this molecular core the (CO)-C-12 and (CO)-C-13 peak T-MB line temperatures of 7.3 K and 1.2 K are the highest so far found in the Magellanic Clouds. The molecular concentrations associated with N113, N44BC, N159HW, and N214DE in the LMC and LIRS 36 in the SMC were investigated in a variety of molecular species to study the chemical properties of the interstellar medium. I(HCO+)/I(HCN) and I(HCN)/I(HNC) intensity ratios as well as lower limits to the I((CO)-C-13)/I((CO)-O-18) ratio were derived for the rotational 1-0 transitions. Generally, HCO+ is stronger than HCN, and HCN is stronger than HNC. The high relative HCO+ intensities are consistent with a high ionization flux from supernovae remnants and young stars, possibly coupled with a large extent of the HCO+ emission region. The bulk of the HCN arises from relatively compact dense cloud cores. Warm or shocked gas enhances HCN relative to HNC. From chemical model calculations it is predicted that I(HCN)/I(HNC) close to one should be obtained with higher angular resolution (less than or similar to 30 '') toward the cloud cores. Comparing virial masses with those obtained from the integrated CO intensity provides an H-2 mass-to-CO luminosity conversion factor of 1.8 x 10(20) mol cm(-2) (K km s(-1))(-1) for N113 and 2.4 x 10(20) mol cm(-2) (K km s(-1))(-1) for N44BC. This is consistent with values derived for the Galactic disk.

The UMIST database for astrochemistry 1995

We report the release of a new version of the UMIST database for astrochemistry. The database contains the rate coefficients of 3864 gas-phase reactions important in interstellar and circumstellar chemistry and involves 395 species and 12 elements. The previous (1990) version of this database has been widely used by modellers. In addition to the rate coefficients, we also tabulate permanent electric dipole moments of the neutral species and heats of formation. A numerical model of the chemical evolution of a dark cloud is calculated and important differences to that calculated with the previous database noted.

A search for interstellar gas-phase CO2 gas: Solid state abundance ratios

We present searches for gas-phase CO2 features in the ISO-SWS infrared spectra of four deeply embedded massive young stars, which all show strong solid CO2 absorption. The abundance of gas-phase CO2 is at most 2. 10(-7) with respect to H-2, and is less than 5% of that in the solid phase. This is in strong contrast to CO, which is a factor of 10-100 more abundant in the gas than in solid form in these objects. The gas/solid state ratios of CO2, CO and H2O are discussed in terms of the physical and chemical state of the clouds.