Silicon carbide in circumstellar environment

The presence of small clusters of silicon carbide (SiC) in circumstellar dust shells surrounding late-type stars is inferred from a broad emission feature peaking at around 11µm in infrared spectra [1]. These clusters are expected to condense from molecular arrangements composed of a few carbon and silicium atoms which are present in stellar winds surrounding carbon-rich late-type stars. We have searched for the possible geometric structures of SiC _n ⁺ radicals (n 5) on the basis ofab initio calculations. It is predicted that the linear structures are energetically favored compared to the other planar and 3D ones, except for SiC ₂ ⁺ where a doubt subsists and for SiC ₃ ⁺ where the rhombic structure is clearly more stable than the linear one. In the same way, a conspicuous even-odd alternation of the stability with the number of carbon atoms along the subset of linear species is shown. Vibrational frequencies of all the structures under consideration have then been determined. The possible destabilizing influence of a finite temperature effect on these structures has also been analyzed by using general considerations of thermal statistics.     

UIR Bands in Carbon Star Spectra

Unidentified infrared emission bands (UIR bands) have been attributed to polycyclic aromatic hydrocarbons (PAHs), which are believed to require ultraviolet radiation in order for the UIR bands to be excited. If, in addition to amorphous carbon and hydrogenated amorphous carbon (HAC) particles, PAHs are able to form in the outflows of cool carbon-rich stars (Cherchneff et al. 1991), then the weak UV radiation field from such stars would be unlikely to be able to excite the UIR bands and so the PAH species could remain undetected in the spectra of C-stars. However, cool carbon stars with hot companions might be exposed to strong enough UV radiation fields for UIR-band emission to be excited from PAHs. Buss et al. (1991) reported the detection of the 8 μm UIR-band (C-C stretch) in the IRAS LRS spectrum of HD 38218 (TU Tau), a carbon star with a hotter A2III companion. To investigate the phenomenon further, we have therefore obtained UKIRT CGS3 10 μm spectra of three carbon stars with hot companions, TU Tau, UV Aur and CS776. It was found that TU Tau showed the 11.25 μm and 8.6 μm UIR-bands (both attributed to C-H bend modes) at good contrast, while UV Aur clearly exhibited the 11.25 μm UIR band. No narrow UIR-band emission was detected in the spectrum of CS776. We have fitted these 10 μm region spectra using a χ²-minimization program equipped to fit stellar and dust emission continua together with the broad SiC feature and the narrow UIR-bands. The features seen in the spectra of TU Tau and UV Aur can be well fitted by a narrow 11.25 μm UIR-band sitting on top of a broad, self-absorbed 11.3 μm silicon carbide feature. Our results therefore provide strong support for the supposition that PAHs can form in carbon star outflows. This revised version was published online in September 2006 with corrections to the Cover Date.     

Silicon carbide clusters in circumstellar environment

The presence of small clusters of silicon carbide (SiC) in circumstellar dust shells surrounding late-type stars is inferred from a broad emission feature peaking at around 11 micrometre in infrared spectra (Little-Marenin ,1986 ApJ Lett. 307, L15). These clusters are expected to condense from molecular arrangements composed of a few carbon and silicium atoms which are present in stellar winds surrounding carbon-rich late-type stars. we have searched for all the possible geometric structures of SiC _n ⁺ radicals (n <= 5) with help of ab initio calculations (T = 0 K). Vibrational frequencies of the most stable species have then been determined . the destabilizing influence of a finite temperature effect on these structures has also been studied by using general considerations of thermal statistics. We show that for n >= 3 linear structures are energetically favored compared to the planar and three-dimensional ones. A comparison with other results published in this context is also made.     

On the relation between radius,luminosity, surface brightness,and surface density in elliptical galaxies

We have analyzed luminosity profiles of E galaxies studied by Strom and Strom in six clusters of galaxies. We have found a relationship between radius, luminosity, and surface brightness for galaxies in each of the clusters. A dependence of the zero point of the relation with the local projected density of galaxies is likewise found:r _{e proj} ^–0.14 L ^0.445 I _e ^–0.413 . This relationship implies (i) that there is not a universal luminosity profile for elliptical galaxies, (ii) the environmental variation of radius is larger than that produced by mergers of galaxies, (iii) distance to a galaxy can be estimated from apparent magnitude, surface brightness, angular size, and apparent local projected density of galaxies.     

An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact‐generated gases

Abstract— Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO², and H² below approximately 250–300°C during rapid cooling of trapped magmatic or impact‐generated gases. Depending on temperature, bulk composition, and oxidation‐reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO² ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01–0.001 together with high CO/CO² ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO² ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine‐grain magnetite, formation of a CO‐rich local gas phase, reduction of water vapor to H², reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high‐temperature quenching of CO‐, H²‐rich impact gases could have led to magnetite‐catalyzed hydrocarbon synthesis     

Structure of clusters with bimodal distribution of radial velocities of galaxies. IV: A1569

We report the results of study of the A1569 cluster (12 ^h 36^m.3, +16°35′) and the neighboring A1589 cluster (12 ^h 41^m.3, +18°34′), making up a pair (a supercluster) with a projected size of about 10Mpc. This study is done within the framework of our program for investigating the galaxy clusters with bimodal velocity distributions (i.e., clusters where the velocities of subsystems differ by more than Δcz ∼ 3000 km/s). In the A1569 cluster we have identified two subsystems: A1569A (cz = 20613 km/s) and A1569B (cz = 23783 km/s). These subsystems have the line-of-sight velocity dispersions of 484 km/s and 493 km/s, and dynamic masses within the R ₂₀₀ radius equal to 1.8 × 10¹⁴ and 2.0 × 10¹⁴ M _⊙, respectively. We directly estimate the distances to these subsystems using three methods applied to earlytype galaxies: the Kormendy relation, photometric plane, and fundamental plane. To this end, we use the results of our observations made with the 1-m telescope of the SAO RAS and the data adopted from the SDSS DR7 catalog. We found that A1569 consists of two independent clusters. The A1569B cluster is located at the Hubble distance corresponding to its radial velocity. The A1569A cluster has a peculiar velocity of −1290 ± 630 km/s, which can be explained by the effect of the more massive A1589 cluster (with a mass of 7.9 × 10¹⁴ M _⊙) and of the supercluster where it resides. In all the four bimodal clusters that we studied within the framework of our program, A1035, A1775, A1831, and A1569, the subsystems are independent clusters lying close to the Hubble relation between redshift and distance.     

The time-scale for core collapse in spherical star clusters

The collapse time for a cluster of equal-mass stars is usually stated to be either 330 central relaxation times (t_rc) or 12-19 half-mass relaxation times (t_rh). But the first of these times applies only to the late stage of core collapse, and the second only to low-concentration clusters. To clarify how the time depends on the density profile, the Fokker-Planck equation is solved for the evolution of a variety of isotropic cluster models, including King models, models with power-law density cusps of ρ ∼ r^−γ, and models with nuclei. The collapse times for King models vary considerably with the cluster concentration when expressed in units of t_rc or t_rh, but vary much less when expressed in units of t_rc divided by a dimensionless measure of the temperature gradient in the core. Models with cusps have larger temperature gradients and evolve faster than King models, but not all of them collapse: those with 0 < γ < 2 expand because they start with a temperature inversion. Models with nuclei collapse or expand as the nuclei would in isolation if their central relaxation times are short; otherwise their evolution is more complicated. Suggestions are made for how the results can be applied to globular clusters, galaxies, and clusters of dark objects in the centers of galaxies.Scott D. Tremaine     

Physical processes in the interstellar matter determining parameters of radio recombination lines in meter and decameter ranges

We give a review of problems connected with the interpretation of meter and decameter carbon radiolines. The lines are formed inside clumps of molecular clouds in layers with a column density N ≈ 6 · 10²¹ cm⁻². These clumps are very typical structures. The distribution of physical parameters (number density, temperature, etc.) inside the clumps is poorly known. The most difficult and important question is the penetration of subcosmic rays into the clumps. Observations show that the ionization rate is ζ = (1–7) · 10⁻¹⁷ s⁻¹ inside molecular clouds and significantly greater in the diffuse gas. Long-wave radio recombination lines can probably be used for the analysis of the distribution of subcosmic rays inside molecular clouds. The interpretation is complicated by the influence of low-temperature dielectron recombination and poorl known variations of carbon depletion in the clumps.     

Integrated luminosity distribution of Galactic open clusters

The integrated magnitudes of 221 Galactic open clusters have been used to derive the luminosity function. The completeness of the data has also been discussed. In the luminosity distribution the maximum frequency of clusters occurs nearI (M_v) = −3 _m ^. 5, and some plausible reasons for a sharp cut-off atI (M_v) = −2^m. 0 have been discussed. It is concluded that the paucity of the clusters fainter thanI (M _v) = −2 _m ^.0 is not purely due to selection effects. The surface density of the clusters for different magnitude intervals has. been obtained using the completeness radius estimated from the logN- logd plots. A relation betweenI (M^v) and surface density has been obtained which yields a steeper slope than that obtained by van den Bergh & Lafontaine (1984).     

Analysis of the properties of galaxy clusters in the Leo supercluster region

We analyze the properties of galaxy clusters in the region of the Leo supercluster using observational data from the SDSS and 2MASS catalogs. We have selected 14 galaxy clusters with a total dynamical mass of 1.77 × 10¹⁵ M _⊙ in the supercluster region 130 by 60 Mpc in the plane of the sky (z ≃ 0.037). The composite luminosity function of the supercluster is described by a Schechter function with parameters that, within the error limits, correspond to field galaxies and does not differ from the luminosity function of the richer Ursa Major (UMa) supercluster for the same luminosity range (the bright end). The luminosity functions of early-type and late-type galaxies in Leo at the faint end are characterized by a sharp decrease (α = −0.60±0.08) and a steep increase (α = −1.44± 0.10) in the number of galaxies, respectively. In the virialized cluster regions, the fraction of early-type galaxies selected by the u-r color, bulge contribution, and concentration index among the galaxies brighter than M _K ^* + 1 is, on average, 62%. This fraction is smaller than that in the UMa supercluster at a 2–3σ level. The near-infrared luminosities of galaxy clusters down to a fixed absolute magnitude correlate with their masses almost in the same way as for other samples of galaxy clusters (L _200,K ∝ M ₂₀₀^0.63±0.11)).     

Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters

Empirical evidence for both stellar mass black holes (M _•<10²  M _⊙) and supermassive black holes (SMBHs, M _•>10⁵  M _⊙) is well established. Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is correlated with the bulge mass, and even more strongly with the central stellar velocity dispersion σ _c , the M _•–σ relation. On the other hand, evidence for “intermediate-mass” black holes (IMBHs, with masses in the range 100–10⁵ M _⊙) is relatively sparse, with only a few mass measurements reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We explore the question of whether globular clusters extend the M _•–σ relationship for galaxies to lower black hole masses and find that available data for globular clusters are consistent with the extrapolation of this relationship. We use this extrapolated M _•–σ relationship to predict the putative black hole masses of those globular clusters where existence of central IMBH was proposed. We discuss how globular clusters can be used as a constraint on theories making specific predictions for the low-mass end of the M _•–σ relation.     

A calculation of structural parameters of spherical stellar systems by means of characteristic functions method. I. Theory

To calculate structural parameters of stellar systems such as an effective radius and central space (or surface) density, the method of characteristic functions is suggested. The characteristic function of the system is a Fourier image of their normalized space density profile f₃(r). In the case of spherical symmetry the probability distribution of r (Q₃(r) = (3/a³)r²f₃(r)) and its orthogonal projections have the same characteristic functions. This fact is used to calculate the effective radii of a few star cluster models (King law, Plummer model and Gausian profile). It is shown, that the characteristic function for King law clusters tends to a finite generalised function if the concentration parameter c is large. The expression for the effective radius (at c ≫ 1) is given. The formula of the effective radius in the Plummer model as well as the relation between the one-dimensional central velocity dispersion and the root mean square velocity are obtained. It is shown, that in the Gaussian model and for King law clusters the effective radius (half-mass visual radius) can differ from the effective (harmonic) radius a few times. This fact should be taken into account in estimating the mass-to-light ratio from the virial mass of such systems using the King radius.     

The role of submicrometer aerosols and macromolecules in H2 formation in the titan haze

Previous studies of the photochemistry of small molecules in Titan’s atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H₂. We describe the prompt reaction model for the formation of H₂ on aerosol surfaces and compare this with the catalytic formation of H₂ using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H₂ with a peak formation rate of ∼ 70 cm⁻³ s⁻¹ at 420 km. We also conclude that catalytic H₂ formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of ∼ 0.1 cm⁻³ s⁻¹ at 210 km) than microphysical aerosols.     

Intrinsic properties of carbon stars. I. Effective temperature scale of N-type carbon stars

It is shown that the infrared flux method for determining stellar effective temperatures (Blackwell and Shallis 1977; Blackwell, Petford and Shallis 1980) can be applied to cool carbon stars. Although the spectra of cool carbon stars are highly line blanketed, the spectral region between 3 and 4 μm (L-band in the infrared photometry system) is found to be relatively free from strong line absorption. The ratioR _L of bolometric flux toL flux can then be used as a measure of effective temperature. On the basis of the predicted line-blanketed flux based on model atmospheres, with an empirical correction for the effect of 3 μm absorption due to polyatomic species (HCN, C₂H₂), it is shown thatR _L is roughly proportional to T³ _eff. The high sensitivity ofR _L to T_eff makes it a very good measure of effective temperature, and the usual difficulty due to differential line blanketing effect in the analyses of photometric indices of cool carbon stars can be minimized. It is found that the majority of N-type carbon stars with small variability (SRb and Lb variables) are confined to the effective temperature range between 2400 and 3200 K, in contrast to M-giant stars (M0 III - M6 III, including SRb and Lb variables) that are confined to the effective temperature range between 3200 and 3900 K. The effective temperatures based on the infrared flux method show good agreement with those derived directly from angular diameter measurements of 5 carbon stars. On the basis of the new effective temperature scale for carbon stars, it is shown that the well known C-classification does not represent a temperature sequence. On the other hand, colour temperatures based on various photometric indices all show good correlations with our derived effective temperatures. An erratum to this article is available at .     

Cluster-scale magnetic fields

The search for non thermal radio emission from clusters of galaxies is a powerful tool to investigate the existence of magnetic fields on such large scale. Unfortunately, such observations are scarce thus far, mainly because of the very faint large scale radio emission expected in clusters of galaxies. In the present contribution we will first review the status of the radio observations of clusters of galaxies, carried out with the aim of detecting large scale radio emission.We will then focus on the large scale radio emission detected at 327 MHz and 610 MHz in the Coma cluster of galaxies. The features of the detected radio emission suggest that a magnetic field with an intensity of the order of ~ 10^–7 Gauss must be present on a scale of about 2 Mpc (forH _o = 100km s ^–1 Mpc ^–1). The morphology of the radio emission is similar to that of the most recent X-ray images derived with ROSAT, and follows the distribution of the galaxies in the cluster. All these pieces of information will be taken into account in the discussion on the possible origin of this large scale magnetic field.     

The effect of UV stars on the intergalactic medium

We have investigated the effect of ionizing radiation from the UV stars (hot prewhite dwarfs) on the intergalactic medium (IGM). If the UV stars are powered only by gravitational contraction they radiate most of their energy at a typical surface temperature of 1.5×10⁵ K which produces a very highly ionized IGM in which the elements carbon, nitrogen and oxygen are left with only one or two electrons. This results in these elements being very inefficient coolants. The gas is cooled principally by free-free emission and the collisional ionization of hydrogen and helium. For a typical UV star temperature ofT=1.5×10⁵ K, the temperature of the ionized gas in the IGM isT _g =1.2×10⁵ K for a Hubble constantH _o=75 km s^–1 Mpc^–1 and a hydrogen densityn _H =10^–6 cm^–3. Heating by cosmic rays and X-rays is insignificant in the IGM except perhaps inHi clouds because when a hydrogen atom recombines in the IGM it is far more likely to be re-ionized by a UV-star photon than by of the other two types of particles due to the greater space density of UV-star photons and their appreciably larger ionization cross-sections. If the UV stars radiate a substantial fraction of their energy in a helium-burning stage in which they have surface temperatures of about 5×10⁴ K, the temperature of the IGM could be lowered to about 5×10⁴ K.     

CO2 production by ion irradiation of H2O ice on top of carbonaceous materials and its relevance to the Galilean satellites

In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO₂ ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H₂O frost on carbonaceous layers with 200 keV of He⁺ and Ar⁺, and 30 keV of He⁺ at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO₂ is produced very efficiently after irradiation obtaining a maximum quantity of the order of . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO₂ molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080]. Thus we suggest that, whatever is the carbonaceous residue, CO₂ will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO₂ has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO₂ formation on the surfaces of the Galilean satellites.     

Structure of clusters with bimodal distribution of galaxy line-of-sight velocities III: A1831

We study the A1831 cluster within the framework of our program of the investigation of galaxy clusters with bimodal velocity distributions (i.e., clusters where the velocities of subsystems differ by more than Δ _cz ∼ 3000 km/s).We identify two subsystems in this cluster: A1831A (cz = 18970 km/s) and A1831B (cz = 22629 km/s) and directly estimate the distances to these subsystems using three methods applied to early-type galaxies: the Kormendy relation, the photometric plane, and the fundamental plane. To this end, we use the results of our observations made with the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and the data adopted from the SDSS DR6 catalog. We confirmed at a 99% confidence level that (1) the two subsystems are located at different distances, which are close to their Hubble distances, and (2) the two subsystems are located behind one another along the line of sight and are not gravitationally bound to each other. Both clusters have a complex internal structure, which makes it difficult to determine their dynamical parameters. Our estimates for the velocity dispersions and masses of the two clusters: 480 km/s and 1.9 × 10¹⁴ M _⊙ for A1831A, 952 km/s and 1.4 × 10¹⁵ M _⊙ for A1831B should be views as upper limits. At least three spatially and kinematically distinct groups of galaxies can be identified in the foreground cluster A1831A, and this fact is indicative of its incomplete dynamical relaxation. Neither can we rule out the possibility of a random projection. The estimate of the mass of the main cluster A1831B based on the dispersion of the line-of-sight velocities of galaxies is two-to-three times greater than the independent mass estimates based on the total K-band luminosity, temperature, and luminosity of the X-ray gas of the cluster. This fact, combined with the peculiarities of its kinematical structure, leads us to conclude that the cluster is in a dynamically active state: galaxies and groups of galaxies with large line-of-sight velocities relative to the center of the cluster accrete onto the virialized nucleus of the cluster (possibly, along the filament directed close to the line of sight).     

Catalog of isolated galaxies selected from the 2MASS survey

We search for isolated galaxies based on the automatic identification of isolated sources from the Two Micron All-Sky Survey (2MASS) followed by a visual inspection of their surroundings. We use the modified Karachentseva criterion to compile a catalog of 3227 isolated galaxies (2MIG), which contains 6% of 2MASS Extended Sources Catalog (or 2MASX) sources brighter than K _s = 12 ^m with angular diameters a _K ≥ 30″. The catalog covers the entire sky and has an effective depth of z ∼ 0.02. The 2493 very isolated objects of the catalog, which we include into the 2MVIG catalog, can be used as a reference sample to investigate the effects of the environment on the structure and evolution of galaxies located in regions with extremely low density of matter.     

The Evolution of Brightest Cluster Galaxies

In this contribution we review the properties of Brightest Cluster Galaxies (BCGs) and discuss the impact that X-ray cluster selection is having on their use as cosmological probes. BCGs form a unique galaxy population. They are located near the gravitational centre of galaxy clusters and are the most massive galaxies in the universe, being some 10 times more luminous than L* systems. Historically, BCGs have been credited with small intrinsic dispersion in their absolute magnitudes (Δ≃0.2–0.3 mag) and used as standard candles to constrain the cosmological parameters. Although indirect signs of mass accretion out to z≃1 have been observed, uncovering their full evolutionary picture has remained an elusive goal. Studies of BCGs based on serendipitiously discovered X-ray cluster samples, particularly from ROSAT, provide large numbers of unbiased clusters at z≤1. Furthermore X-ray emission guarantees the presence of a large gravitationally bound potential well and the X-ray information can be used to locate the centroids of clusters, aiding the identification of the BCG. We show that this has important consequences for studies of distance determination and large-scale streaming flows based on the optical properties of BCGs. Recent results based on X-ray selected clusters show large differences in near-IR BCG properties with their cluster environment; such that those in clusters with L _x≥1.9×10⁴⁴erg s^-1 are brighter and more uniform than those in their low-L _x counterparts. The BCGs in highL _x systems show no evidence of having undergone mass growth, whereas those in low L _x systems show a widerrange of evolution, with evidence that some have grown by a factor of 4 ormore since z≃1. These results are a direct indication of howa single homogeneous population of galaxies evolves and are a challenge to simple semi-analytical hierarchical models. If future observations at high redshift are to seriously challenge theory then better predictions of the evolutionary process are required. This revised version was published online in July 2006 with corrections to the Cover Date.