Radiative feedback from ionized gas

H₂ formation in metal-free gas occurs via the intermediate  H⁻  or  H⁺₂  ions. Destruction of these ions by photodissociation therefore serves to suppress  H₂  formation. In this paper, I highlight the fact that several processes that occur in ionized primordial gas produce photons energetic enough to photodissociate  H⁻  or  H⁺₂  and outline how to compute the photodissociation rates produced by a particular distribution of ionized gas. I also show that there are circumstances of interest, such as during the growth of H  ii regions around the first stars, in which this previously overlooked form of radiative feedback is of considerable importance.     

The X-ray spectrum of NGC 7213 and the Seyfert–LINER connection

We present an XMM–Newton observation of the Seyfert–LINER (low-ionization nuclear emission-line region) galaxy NGC 7213. The RGS soft X-ray spectrum is well fitted with a power law plus soft X-ray collisionally ionized thermal plasma  ( kT = 0.18^+0.03_−0.01 keV)  . We confirm the presence of Fe  i , Fe  xxv and Fe  xxvi Kα emission in the EPIC spectrum and set tighter constraints on their equivalent widths of  82⁺¹⁰₋₁₃, 24⁺⁹₋₁₁  and 24⁺¹⁰₋₁₃ eV, respectively. We compare the observed properties together with the inferred mass accretion rate of NGC 7213 with those of other Seyfert and LINER galaxies. We find that NGC 7213 has intermediate X-ray spectral properties lying between those of the weak active galactic nucleus found in the LINER M81 and higher-luminosity Seyfert galaxies. There appears to be a continuous sequence of X-ray properties from the Galactic Centre through LINER galaxies to Seyferts, probably determined by the amount of material available for accretion in the central regions.     

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.     

ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies

We present an analysis of four off-axis ROSAT Position Sensitive Proportional Counter (PSPC) observations of the Perseus cluster of galaxies (Abell 426). We detect the surface brightness profile to a radius of 80 arcmin (∼2.4 h⁻¹₅₀ Mpc) from the X-ray peak. The profile is measured in various sectors and in three different energy bands. First, a colour analysis highlights a slight variation of N _H over the region, and cool components in the core and in the eastern sector. We apply the β-model to the profiles from different sectors and present a solution to the, so-called, β-problem. The residuals from an azimuthally-averaged profile highlight extended emission both in the east and in the west, with estimated luminosities of about 8 and 1 ×10⁴³ erg s⁻¹, respectively. We fit several models to the surface brightness profile, including the one obtained from the Navarro, Frenk &38; White potential. We obtain the best fit with the gas distribution described by a power law in the inner, cooling region and a β-model for the extended emission. Through the best-fitting results and the constraints from the deprojection of the surface brightness profiles, we define the radius where the overdensity inside the cluster is 200 times the critical value, r ₂₀₀, at 2.7 h⁻¹₅₀ Mpc. Within 2.3  h⁻¹₅₀ Mpc (0.85 r ₂₀₀), the total mass in the Perseus cluster is 1.2 × 10¹⁵ M_⊙ and its gas fraction is about 30 per cent.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – III. Multimolecular line mapping

We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H  ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO⁺ and N₂H⁺ are anticorrelated.     

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.     

X-ray observations of the ultraluminous infrared galaxy IRAS 19254−7245 (the Superantennae)

We present ROSAT High Resolution Imager (HRI) and ASCA observations of the well-known ultraluminous infrared galaxy (ULIRG) IRAS 19254−7245 (the 'Superantennae' ). The object is not detected by ROSAT , implying a 3 σ upper limit of X-ray luminosity L _X∼8×10⁴¹ erg s⁻¹ in the 0.1–2 keV band. However, we obtain a clear detection by ASCA , yielding a luminosity in the 2–10 keV band of 2×10⁴² erg s⁻¹. The X-ray spectrum of IRAS 19254−7245 is very hard, equivalent to a photon index of Γ=1.0±0.35. We therefore attempt to model the X-ray data using a 'scatterer' model, in which the intrinsic X-ray emission along our line of sight is obscured by an absorbing screen while some fraction, f , is scattered into our line of sight by an ionized medium; this is the standard model for the X-ray emission in obscured (but non Compton-thick) Seyfert galaxies. We obtain an absorbing column density of N _H=2×10²³ cm⁻² for a power-law photon index of Γ=1.9, an order of magnitude above the column estimated on the basis of optical observations; the percentage of the scattered emission is high (∼20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus ( N _H>10²⁴ cm⁻²) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90 per cent) to the equivalent width of the 6.4 keV line is high (∼3 keV). Overall , the results suggest that most of the X-ray emission originates in a highly obscured Seyfert 2 nucleus.     

Anomalous mixing of the w2Po3/2 and x6Po3/2 levels in Fe ii

The neighbouring lines a ⁶D_5/2– x ⁶P^o_3/2 (1272.617 Å) and a ⁶D_5/2– w ²P^o_3/2 (1272.657 Å) have been observed in the UV spectrum of χ Lupi to be of comparable intensity. The latter, Δ S  = 2, transition would be expected to be very weak. The two upper states should display negligible mixing. We give a detailed, quantitative discussion of how the two upper states are in fact strongly mixed through their mixing with 3d⁶(³D)4p ⁴P^o_3/2, and hence we explain the relative strengths of the two UV lines.     

HE 1127–1304: the largest radio quasar

By considering the propagation of low-amplitude magnetohydrodynamic waves in partially ionized plasmas, it is shown that the ion-neutral drift (ambipolar diffusion) induced by the waves can have specific effects on the molecular chemistry of cold material. The chemistry occurring in gas swept by Alfvén waves is described and it is shown that this leads to spatial variations in the deuterium fractionation ratios of, for example, HCO⁺ and N₂H⁺, on spatial scales of a few hundredths of a parsec, depending upon the fractional ionization of the ambient medium. The possibility of detecting interstellar Alfvén waves by molecular spectroscopy and their effect of producing small-scale chemical abundance gradients in molecular clouds are briefly discussed.     

Single star scidar: atmospheric parameters profiling using the simulated annealing algorithm

In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed   v ( h _i )  , the standard deviation of the horizontal wind speed fluctuations  σ _v ( h_i )  and the integrated value of   C ² _n   over the thickness  Δ h_i   of the turbulent layer   C ² _n ( h_i )Δ h_i   , where   h_i   is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using   C ² _n ( h_i )  and   v ( h_i )  values. The results of other techniques support this new method.     

Inhomogeneous reionization of the intergalactic medium regulated by radiative and stellar feedbacks

We study the inhomogeneous reionization in a critical density CDM universe resulting from stellar sources, including Population III objects. The spatial distribution of the sources is obtained from high-resolution numerical N -body simulations. We calculate the source properties, taking into account a self-consistent treatment of both radiative (i.e. ionizing and H₂-photodissociating photons) and stellar (i.e. SN explosions) feedbacks regulated by massive stars. This allows us to describe the topology of the ionized and dissociated regions at various cosmic epochs, and to derive the evolution of H, He and H₂ filling factors, soft UV background, cosmic star formation rate and the final fate of ionizing objects. The main results are: (i) galaxies reionize the intergalactic medium by z ≈10 (with some uncertainty related to the gas clumping factor), whereas H₂ is completely dissociated already by z ≈25; (ii) reionization is mostly caused by the relatively massive objects which collapse via H line cooling, while objects the formation of which relies on H₂ cooling alone are insufficient for this purpose; (iii) the diffuse soft UV background is the major source of radiative feedback effects for z ≤15; at higher z direct flux from neighbouring objects dominates; (iv) the match of the calculated cosmic star formation history with that observed at lower redshifts suggests that the conversion efficiency of baryons into stars is ≈1 per cent; (v) we find that a very large population of dark objects which failed to form stars is present by z ≈8. We discuss and compare our results with similar previous studies.     

Hydrogenated fulleranes and the anomalous microwave emission of the dark cloud LDN 1622

We study the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes under the physical conditions of the dark cloud (DC) LDN 1622. The abundance of fullerenes is estimated by fitting theoretical photoabsorption spectra to the characteristics of the ultraviolet (UV) bump extinction in DCs. The UV bump appears to be well reproduced by a mixture of fullerenes following a size-distribution power law, which gives progressively lower abundances as the radius of the fullerene increases. We infer abundances of the order of  0.2 × 10⁻⁶  n (H ₂)  for C₆₀. A significant fraction of these molecules are expected to be hydrogenated. We compute the electric dipole rotational emission from these fullerene hydrides, taking into account rotational excitation and damping processes. The recent detection of anomalous microwave emission (5–60 GHz) in LDN 1622 by Casassus et al. can be explained as the result of electric dipole radiation from hydrogenated fullerenes. The bulk of the emission (10–30 GHz) appears to be associated with 60–80 carbon atom fulleranes with a degree of hydrogenation of C:H ≈ 3:1. A small contribution (∼10 per cent) of these molecules residing in the surrounding cold neutral medium and/or photodissociation region of the cloud is required to fit the high-frequency tail (40–60 GHz) of the emission.     

The minimum mass ratio of W UMa-type binary systems

When the total angular momentum of a binary system   J _tot= J _orb+ J _spin  is at a certain critical (minimum) value, a tidal instability occurs which eventually forces the stars to merge into a single, rapidly rotating object. The instability occurs when   J _orb= 3 J _spin  , which in the case of contact binaries corresponds to a minimum mass ratio   q _min≈  0.071–0.078. The minimum mass ratio is obtained under the assumption that stellar radii are fixed and independent. This is not the case with contact binaries where, according to the Roche model, we have   R ₂= R ₂( R ₁, a , q )  . By finding a new criterion for contact binaries, which arises from  d J _tot= 0  , and assuming   k ²₁≠ k ²₂  for the component's dimensionless gyration radii, a theoretical lower limit   q _min= 0.094–0.109  for overcontact degree   f = 0–1  is obtained.     

Upper limit on dimming of cosmological sources by intergalactic grey dust from the soft X-ray background

Active galactic nuclei (AGN) produce a dominant fraction  ( F _AGN∼ 80 per cent)  of the soft X-ray background (SXB) at photon energies  0.5 < E < 2 keV  . If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray haloes around AGN. Taking account of known galaxies and galaxy clusters, only a fraction   F _halo≲ 10 per cent  of the SXB can be in the form of diffuse X-ray haloes around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains, with radii in the range   a = 0.2–2.0 μm  , to a level  τ_GD≲ 0.15( F _halo/10 per cent)(F_AGN/80 per cent)⁻¹  to a redshift   z ∼ 1  . Our results are only weakly dependent on the grain size distribution in this size range or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.     

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.     

PDS 456: an extreme accretion rate quasar?

We present quasi-simultaneous ASCA and RXTE observations of the most luminous known active galactic nucleus in the local ( z <0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted that show that PDS 456 has a bolometric luminosity of ∼10⁴⁷ erg s⁻¹ peaking in the ultraviolet part of the spectrum. In the X-ray band the 2–10 keV (rest-frame) luminosity is 10⁴⁵ erg s⁻¹. The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionized iron K edge, observed at 8.7 keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionized matter, probably the inner accretion disc. PDS 456 appeared to show a strong (factor of ∼2.1) outburst in just ∼17 ks, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with a light-crossing-time corresponding to ∼2 R _S . The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a 'super-Eddington' or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionized material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.     

Soft X-ray spectroscopy of Compton-thick Seyfert 2 galaxies with BeppoSAX

We present an X-ray spectroscopic study of the bright Compton-thick Seyfert 2 galaxies NGC 1068 and the Circinus Galaxy, performed with BeppoSAX . Matt et al. interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC 1068 spectrum is rich in emission lines, mainly owing to K _α transitions of He-like elements from oxygen to iron, plus a K _α fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, N _warm, of the warm scatterer less than a few×10²¹ cm⁻². In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with U _X≲5 and N _warm∼ a few×10²² cm⁻². In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC 1068 an optically thin plasma emission with kT ≃500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed.     

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⁺.     

The distribution of C2S and NH3 in two Bok globules at different evolutionary stages: CB246 and CB34

Two Bok globules, L1253 (CB246) and CB34, have been mapped in the C₂S (2₁–1₀) transition and in the NH₃ (1, 1) and NH₃ (2, 2) inversion transitions, respectively. By comparing the C₂S map of L1253 (CB246) with the NH₃ map of the same globule from Lemme et al., a clumped onion structure results as a consequence of the chemical and dynamical evolution of the object. From the derived parameters it appears that both L1253 (CB246) and CB34 are close to virial equilibrium.     

Smooth matter and source size in microlensing simulations of gravitationally lensed quasars

Several gravitationally lensed quasars are observed with anomalous magnifications in pairs of images that straddle a critical curve. Simple theoretical arguments suggest that the magnification of these images should be approximately equivalent, whereas one image is observed to be significantly demagnified. Microlensing provides a possible explanation for this discrepancy. There are two key parameters when modelling this effect. The first, the fraction of smooth matter in the lens at the image positions, has been explored by Schechter & Wambsganss. They have shown that the anomalous flux ratio observed in the lensed quasar MG 0414+0534 is a priori a factor of 5 more likely if the assumed smooth matter content in the lens model is increased from 0 to 93 per cent. The second parameter, the size of the emission region, is explored in this paper, and shown to be more significant. We find that the broadening of the magnification probability distributions due to smooth matter content is washed out for source sizes that are predicted by standard models for quasars. We apply our model to the anomalous lensed quasar MG 0414+0534, and find a 95 per cent upper limit of  2.62 × 10¹⁶  h ^−1/2₇₀ ( M /M_⊙)^1/2 cm  on the radius of the I -band emission region. The smooth matter percentage in the lens is unconstrained.