Chemical evolution ahead of Herbig–Haro objects

Compact regions of enhanced HCO⁺ and NH₃ emission have been detected close to a number of Herbig–Haro objects. An interpretation of these detections is the following: a transient clump within the molecular cloud has been irradiated by the shock that generates the Herbig–Haro object. The irradiation releases icy mantles from the grains within the transient clump and initiates a photochemistry. On the basis of this picture, we have developed an extensive chemical model which predicts that a wide range of species, other than NH₃ and HCO⁺, should also be detectable. These include CH₃OH, H₂S, C₃H₄, H₂CO, SO, SO₂, H₂CS and NS. The chemical effects should last ∼  10⁴ yr  .     

On the relation between electron temperatures in the O+ and O++ zones in high-metallicity H ii regions

We suggest a new way to establish the relation between the electron temperature t ₃ within the [O  iii ] zone and the electron temperature t ₂ within the [O  ii ] zone in high-metallicity  (12 + log(O/H) > 8.25)  H  ii regions. The   t ₂– t ₃  diagram is constructed by applying our method to a sample of 372 H  ii regions. We find that the correlation between t ₂ and t ₃ is tight and can be approximated by a linear expression. The new   t ₂– t ₃  relation can be used to determine t ₂ and accurate abundances in high-metallicity H  ii regions with a measured t ₃. It can also be used in conjunction with the ff relation for the determination of t ₃ and t ₂ and oxygen abundances in high-metallicity H  ii regions, where the [O  iii ]λ4363 auroral line is not detected. The derived   t ₂– t ₃  relation is independent of photoionization models of H  ii regions.     

Shocked gas around Cepheus A: evidence for multiple outflows from H2S and SO2 observations

The Cepheus A star-forming region has been investigated through a multiline H₂S and SO₂ survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H₂S/SO₂ integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H₂S appears to be more abundant than SO₂ in high-velocity clumps, in agreement with chemical models. However, we also find quite small H₂S linewidths, suggestive of regions where the evaporated H₂S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks.     

On the detection of the linear C5 molecule in the interstellar medium

An upper limit of the column density of the C₅ linear molecule in translucent interstellar clouds is estimated from high-resolution ( R =80 000) and very high signal-to-noise ratio (∼1000) echelle spectra. It is 10¹² cm⁻² per E ( B − V )=1 (two orders of magnitude lower than that of C₂).     

H3+ in diffuse interstellar gas

A model is constructed of the material in front of the star Cygnus OB2 no. 12 in which dense cores are embedded in diffuse clumps of gas. The model reproduces the measured abundances of C₂ and CO, and predicts a column density of 910¹⁰ cm² for HCO⁺.     

Stability of C60 and C70 fullerenes toward corpuscular and γ radiation

The stability of C₆₀ and C₇₀ fullerenes in the interstellar medium deposited on dust surface or embedded in meteorites and comets has been simulated with γ irradiation and with He⁺ ion bombardment. It is shown by vibrational spectroscopy that a γ radiation dose of 2.6 MGy (1 Gy = 1 joule absorbed energy per kilogram) causes partial oligomerization of both C₆₀ and C₇₀ fullerenes. Oligomers are made by fullerene cages chemically connected each other which can yield back free fullerenes by a thermal treatment. The amount of irreversibly polymerized fullerenes caused by 2.6 MGy as deduced as the toluene insoluble fraction has been determined as 1.7 and 15 per cent by weight, respectively, for C₆₀ and C₇₀ fullerene. The radiation dose generated by radionuclides decay and expected to be delivered to fullerenes buried at a depth of more than 20 m in comets and meteorites is about 3 MGy per 10⁹ yr. Since fullerenes are by far resistant to such radiation dose they can survive for at least some billion years inside comets and meteorites and in fact have been detected inside certain carbonaceous chondrites. On the other hand, the direct exposure of fullerenes to cosmic rays for instance when they are adsorbed or deposited on the surface of carbon dust corresponds to the delivery of a radiation dose comprised between 30 and 65 MGy per 10⁹ yr. Experimental bombardment of both C₆₀ and C₇₀ fullerenes for instance with He⁺ ions has shown that the complete amorphization occurs at about 250 MGy. Thus in ∼4 Gyr exposure to cosmic rays it is expected a complete amorphization.     

The distribution of the warm and dense molecular gas around Cepheus A HW 2

We present VLA observations of the ( J , K )=(1, 1), (2, 2), (3, 3) and (4, 4) inversion transitions of NH₃ toward the HW 2 object in Cepheus A, with 1-arcsec angular resolution. Emission is detected in the main hyperfine line of the first three transitions. The NH₃(2, 2) emission shows a non-uniform 'ring' structure, which is more extended (3 arcsec) and intense than the emission seen in the (1, 1) and (3, 3) lines. A rotational temperature of ∼ 30–50 K and a lower limit to the mass of ∼ 1 ( X _NH3/10⁻⁸)⁻¹ M_⊙ are derived for the ring structure. The spatio-kinematical distribution of the NH₃ emission does not seem to be consistent with a simple circumstellar disc around the HW 2 thermal biconical radio jet. We suggest that it represents the remnant of the parental core from which both the inner 300-au (0.4 arcsec) disc, traced by the water maser spots previously found in the region, and the central object have formed. The complex velocity field of this core is probably produced from bound motions (similar to those of the inner disc) and from interaction with outflowing material.     

Ultra-high-resolution observations of circumstellar K i and C2 around the post-AGB star HD 56126

We have used the Ultra-High-Resolution Facility (UHRF) at the AAT, operating at a resolution of 0.35 km s⁻¹ (FWHM), to observe K  i and C₂ absorption lines arising in the circumstellar environment of the post-AGB star HD 56126. We find three narrow circumstellar absorption components in K  i , two of which are also present in C₂. We attribute this velocity structure to discrete shells resulting from multiple mass-loss events from the star. The very high spectral resolution has enabled us to resolve the intrinsic linewidths of these narrow lines for the first time, and we obtain velocity dispersions ( b -values) of 0.2–0.3 km s⁻¹ for the K  i components, and 0.54±0.03 km s⁻¹ for the strongest (and best defined) C₂ component. These correspond to rigorous kinetic temperature upper limits of 211 K for K  i and 420 K for C₂, although the b -value ratio implies that these two species do not co-exist spatially. The observed degree of rotational excitation of C₂ implies low kinetic temperatures ( T _k≈10 K) and high densities ( n ≈10⁶ to 10⁷ cm⁻³) within the shell responsible for the main C₂ component. Given this low temperature, the line profiles then imply either mildly supersonic turbulence or an unresolved velocity gradient through the shell.     

Measuring Ω0 from the entropy evolution of clusters

In this paper we have extended the entropy-driven model of cluster evolution developed by Bower in order to be able to predict the evolution of galaxy clusters for a range of cosmological scenarios. We have applied this model to recent measurements of the evolution of the L _x− T normalization and X-ray luminosity function in order to place constraints on cosmological parameters. We find that these measurements alone do not select a particular cosmological framework. An additional constraint is required on the effective slope of the power spectrum to break the degeneracy that exists between this and the background cosmology. We therefore include a theoretical calculation of the Ω₀ dependence on the power spectrum, based on the cold dark matter paradigm, which infers Ω₀<0.55 (0.1<Ω₀<0.7 for Ω₀+Λ₀=1), at the 95 per cent confidence level. Alternatively, an independent measurement of the slope of the power spectrum from galaxy clustering requires Ω₀<0.6 (Ω₀<0.65 for Ω₀+Λ₀=1), again to 95 per cent confidence. The rate of entropy evolution is insensitive to the values of Ω₀ considered, although it is sensitive to changes in the distribution of the intracluster medium.     

H2 emission from shocks in molecular outflows: the significance of departures from a stationary state

We have computed the time dependence of the H₂ rovibrational emission spectrum from molecular outflows. This emission arises in shock waves generated by the impact of jets, associated with low-mass star formation, on molecular gas. The shocks are unlikely to have attained a state of equilibrium, and so their structure will exhibit both C- and J-type characteristics. The rotational excitation diagram is found to provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is found to be approximately 1.5×10³ yr. Emission by other species, such as NH₃ and SiO, is also considered, as are the intensities of the fine-structure transitions of atoms and ions.     

Measuring Ω0 using cluster evolution

The evolution of the abundance of galaxy clusters depends sensitively on the value of the cosmological density parameter, Ω₀. Recent ASCA data are used to quantify this evolution as measured by the cluster X-ray temperature function. A χ² minimization fit to the cumulative temperature function, as well as a maximum-likelihood estimate (which requires additional assumptions about cluster luminosities), leads to the estimate Ω₀ ≈ 0.45 ± 0.25 (1σ statistical error). Various systematic uncertainties are considered, none of which significantly enhances the probability that Ω₀ = 1. These conclusions hold for models with or without a cosmological constant, i.e., with Λ₀ = 0 or Λ₀ = 1 − Ω₀. The statistical uncertainties are at least as large as any of the individual systematic errors that have been considered here, suggesting that additional temperature measurements of distant clusters will allow an improvement in this estimate. An alternative method that uses the highest redshift clusters to place an upper limit on Ω₀ is also presented and tentatively applied, with the result that Ω₀  1 can be ruled out at the 98 per cent confidence level. Whilst this method does not require a well-defined statistical sample of distant clusters, there are still modelling uncertainties that preclude a firmer conclusion at this time.     

Rotational contours of CH2X chain species compared with several diffuse interstellar bands

We have calculated synthetic spectra of perpendicular and parallel rovibronic bands of cumulene carbene molecules of the form C _n H₂. The perpendicular bands are consistent with a regularly spaced group of diffuse interstellar bands (DIBs) near 6850 Å. Parallel bands calculated for these molecular structures are consistent with the intrinsic profile of the associated 6614-Å DIB. Both types of bands are expected for an electronic transition that these species should have at those energies. We could not determine if the molecule was charged or if an atom other than carbon terminated the chain-end. Constraints due to molecular geometry and temperature place the chain length at 7–15 carbons to fit the 6850-Å group and 9–13 carbons to fit the 6614-Å DIB.     

Cumulenic and heterocumulenic anions: potential interstellar species?

A recent theoretical investigation by Terzieva & Herbst of linear carbon chains, C _n where n  ≥ 6, in the interstellar medium has shown that these species can undergo efficient radiative association to form the corresponding anions. An experimental study by Barckholtz, Snow & Bierbaum of these anions has demonstrated that they do not react efficiently with molecular hydrogen, leading to the possibility of detectable abundances of cumulene-type anions in dense interstellar and circumstellar environments. Here we present a series of electronic structure calculations which examine possible anionic candidates for detection in these media, namely the anion analogues of the previously identified interstellar cumulenes C _n H and C _{n −1}CH₂ and heterocumulenes C _n O (where n  = 2–10). The extraordinary electron affinities calculated for these molecules suggest that efficient radiative electron attachment could occur, and the large dipole moments of these simple (generally) linear molecules point to the possibility of detection by radio astronomy.     

The distance–redshift equation in quintessence cosmology and the estimation of H0 through time delays

We calculate analytically and numerically the distance–redshift equation in perfect fluid quintessence models and give an accurate fit to the numerical solutions for all the values of the density parameter and the quintessence equation of state. Then we apply our solutions to the estimation of H ₀ from multiple image time delays and find that the inclusion of quintessence modifies significantly the likelihood distribution of H ₀, generally reducing the best estimate with respect to a pure cosmological constant. Marginalizing over the other parameters (Ω _m and the quintessence equation of state) we obtain H ₀=71±6 km s⁻¹ Mpc⁻¹ for an empty beam and H ₀=64±4 km s⁻¹ Mpc⁻¹ for a filled beam. These errors, however, do not take into account the uncertainty on the modelling of the lens. We also discuss the future prospects for distinguishing quintessence from a cosmological constant with time delays.     

H 0 from an orientation-unbiased sample of Sunyaev–Zel'dovich and X-ray clusters

We have observed the Sunyaev–Zel'dovich (SZ) effect in a sample of five moderate-redshift clusters with the Ryle Telescope, and used them in conjunction with X-ray imaging and spectral data from ROSAT and ASCA to measure the Hubble constant. This sample was chosen with a strict X-ray flux limit using both the Bright Cluster Sample and the Northern ROSAT All-Sky Survey (RASS) cluster catalogues to be well above the surface brightness limit of the RASS, and hence to be unbiased with respect to the orientation of the cluster. This controls a major potential systematic effect in the SZ/X-ray method of measuring H ₀. Taking the weighted geometric mean of the results and including the main sources of error, namely the noise in the SZ measurement, the uncertainty in the X-ray temperatures and the unknown ellipticity and substructure of the clusters, we find   H ₀= 59⁺¹⁰₋₉ (random)⁺⁸₋₇(systematic) km s⁻¹ Mpc⁻¹  assuming a standard cold dark matter model with  Ω_M= 1.0, Ω_Λ= 0.0  or   H ₀= 66⁺¹¹₋₁₀ ⁺⁹₋₈ km  s⁻¹ Mpc⁻¹  if  Ω_M= 0.3, Ω_Λ= 0.7  .     

The excitation and kinematical properties of H2 and [Fe ii] in the HH 46/47 bipolar outflow

Long-slit spectra of the molecular outflow Herbig–Haro (HH) 46/47 have been taken in the J and K near-infrared bands. The observed H₂ line emission confirms the existence of a bright and extended redshifted counter-jet outflow south-west of HH 46. In contrast with the optical appearance of this object, we show that this outflow seems to be composed of two different emission regions characterized by distinct heliocentric velocities. This implies an acceleration of the counter-jet.
The observed [Fe  ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H₂ and [Fe  ii ] emission regions in the northern part of the HH 46/47 outflow. We have detected for the first time high-velocity (−250 km s⁻¹) [Fe  ii ] emission in the region bridging HH 46 to HH 47A. The two strong peaks detected can be identified with the optical positions B8 and HH 47B.
The H₂ excitation diagrams for the counter-jet shock suggest an excitation temperature for the gas of T _ex≈2600 K . The lack of emission from the higher energy H₂ lines, such as the 4–3 S(3) transition, suggests a thermal excitation scenario for the origin of the observed emission. Comparison of the H₂ line ratios with various shock models yielded useful constraints about the geometry and type of these shocks. Planar shocks can be ruled out whereas curved or bow shocks (both J- and C-type) can be parametrized to fit our data.     

Class II maser candidates in substituted methanol: CH3OD, 13CH3OH, CH318OH and CH3SH

We investigate the possibility of interstellar masers in transitions of the methanol isotopomers CH₃OD, ¹³CH₃OH and CH₃¹⁸OH, and of CH₃SH. The model used, in which masers are pumped through the first and second torsionally excited states by IR radiation, has accounted successfully for the Class II masers in main species methanol, ¹²CH₃¹⁶OH. Several potential maser candidates are identified for CH₃OD, their detectability depending on the enrichment of this species in star-forming regions. In ¹³CH₃OH and CH₃¹⁸OH the best maser candidates are direct counterparts of the well-known 6.7- and 12.2-GHz methanol masers, but the lower interstellar abundance of these substituted species means that the expected brightness is greatly reduced. The maser candidates in CH₃SH are also weak. By comparing these species we find that the large b -component of the dipole moment in methanol plays a significant role in its propensity to form masers, as does the strong torsion–rotation interaction due to the light hydroxyl frame. Thus the exceptional brightness of interstellar methanol masers is due to a favourable combination of molecular properties as well as high interstellar abundance.     

Calculations on the rates, mechanisms, and interstellar importance of the reactions between C and NH2 and between N and CH2

The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH₂→HNC+H, and N+CH₂→HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.     

IRAS 12311−3509: a carbon star with SiC2 emission

The optical spectrum of the carbon star IRAS 12311−3509 is dominated by the Merrill–Sanford emission bands of SiC₂, by absorption and emission in the Swan system of C₂, and by resonance emission lines of neutral metals. The infrared energy distribution is flat from 1 to 60 μm. These observations are interpreted as arising from a star with a cool dusty disc which is edge-on to the observer and obscures direct starlight. The infrared continuum is caused predominantly by absorption of stellar light by dust in the disc and re-emission at longer wavelengths. The optical stellar spectrum is seen by reflection off dusty material which lies out of the plane of the disc, and the molecular and atomic emission arises in the same geometry through resonance fluorescence. The object has similarities to the J-silicate stars, but may have a carbon-rich rather than oxygen-rich disc. A full spectroscopic assignment and discussion of the SiC₂ bands and their intensities are given. Modelling of the rotational contours of the     band yields a rotational temperature of 250 K, indicating very cool gas.