分叉波痕在广西上泥盆统钙质浊积岩中的发现及意义

小型不对称分叉波痕(简称分叉波痕)发现于桂林杨堤剖面上泥盆统弗拉斯阶linguiformis牙形石带，桂林碳酸盐台地东南缘斜坡相钙质浊积岩鲍马序列C段。波长7．5～8．0cm，波高0．5～0．8cm，波痕指数15～10；向流面长5．0～7．0cm，背流面长2．5～4．0cm，波痕对称指数2．0～1．8；背流面向东倾；波脊较圆滑、缓曲，且具明显的分叉现象。是浊流流速减缓，密度流转化为牵引流后在低流态条件下，并叠加有推进型风暴浪作用形成的复合成因波痕。根据该波痕和寄主地层特征，推断含分叉波痕的阳朔碳酸盐盆地的最大水深约100m，极限水深小于200m。这一估计值应能代表广西乃至华南板块泥盆纪广泛发育的含牙形石动物群和钙质浊积岩碳酸盐沉积盆地的定量水深，可能也代表了弗拉斯阶一法门阶之交受集群绝灭事件重创的浅水海相生物与基本未受影响的深水海相生物的水深分界线。     

Baryonic fraction in the cold plus hot dark matter universe

The observational infrared spectra of a number of Wolf–Rayet stars of WC8–9 spectral classes are shown to be quite satisfactorily explained by making use of the detailed theoretical model of a dust shell made up of spherical amorphous carbon grains, the dynamics, growth–destruction, thermal and electrical charge balance of which are taken into account. The dust grains acquire mainly positive electrical charge, move with suprathermal drift velocities and may grow up to 100–200 Å as a result of implantation of impinging carbon ions. For most of the stars the fraction of condensed carbon does not exceed 1 per cent. While the nature of the grain nucleation remains unknown, the condensation distances and the grain seed production can be estimated by fitting the observational spectra with theoretical ones.     

The global structure of galactic discs

A statistical study of global galaxy parameters can help to improve our understanding of galaxy formation processes. In this paper we present the analysis of global galaxy parameters based on optical and near-infrared observations of a large sample of edge-on disc galaxies. We found a correlation between the ratio of the radial to vertical scale parameter and galaxy type: galaxies become systematically thinner when going from type S0 to type Sc, whereas the distribution seems to level off for later types. The observed scalelength ratios (and thus the radial colour gradients) largely represent the dust content of the galaxies. On average, the colour gradients indicated by the scalelength ratios increase from type Sa to at least type Sc. For galaxy types later than Sc, the average colour gradient seems to decrease again. The distribution of K -band (edge-on) disc central surface brightnesses is rather flat, although with a large scatter. However, the latest-type sample galaxies ( T  > 6) show an indication that their average disc central surface brightnesses may be fainter than those of the earlier types. This effect is probably not the result of dust extinction.     

Further observations of IRAS 04302+2247

We present near-infrared broad-band and H₂ images of the quadrupolar source IRAS 04302+2247. High-resolution data at 3.8 μm show that the circumstellar envelope has a high degree of axisymmetry: the asymmetry at shorter wavelengths is almost certainly the result of extinction by dust in the cavity. Photometry of the flux peaks confirms that the source is variable but little change in appearance is observed between 1995 November and 1997 September. We report the first clear detection of the molecular outflow in H₂ at 2.1 μm, revealing discrete streams which are inclined to the axis of the cavity. The motion of knots in the outflow remains the most likely cause of the variability.     

Optical emission at 5800 Å in the Red Rectangle and the 5797-Å diffuse interstellar band

An analysis of the intensity and spatial distribution of the discrete 5800-Å emission band in the spectrum of the Red Rectangle has been used to constrain the abundance and physical properties of the carrier of this emission. An origin in a large (>30 C atom) molecule is indicated. This molecule is formed in situ in the Red Rectangle, but is also a component of the diffuse interstellar medium. The UV photodissociation probability for this molecule is ≲10⁻⁵ per absorbed photon, and the luminescence efficiency is in the range 10⁻²–10⁻³. This molecule may be a product of the dissociation of carbonaceous dust.     

Internal extinction of disc galaxies – I. High-resolution extinction map of NGC 6946

Traditionally, it has been believed that extinction effects due to dust within the interstellar medium of external galaxies are rather small and can largely be ignored. Over the last 10 years, however, considerable doubt has been cast over the evidence to support this comfortable idea, and it has become clear that a more detailed analysis is required. Here, a new technique for mapping the extinction in disc galaxies with high resolution is presented.
This technique has been applied to the Sc galaxy NGC 6946. The results show that dust extinction significantly affects both the overall brightness and appearance of the galaxy. The total extinction is found to be A_B =0.45 – somewhat larger than the value of A_B =0.2 usually quoted for an Sc galaxy. When corrected for dust the morphology more closely resembles that of an Sb galaxy rather than an Sc galaxy.
The most surprising result of this work is finding interarm regions that suffer high extinction. It appears that these regions appear faint because of the high extinction and not as a result of low stellar density. There are also interarm regions that suffer little extinction; these are therefore truly regions of low stellar density.     

The poor cluster of galaxies S639

We have studied the poor southern cluster of galaxies S639. Based on new Strömgren photometry of stars in the direction of the cluster, we confirm that the Galactic extinction affecting the cluster is large. We find the extinction in Johnson B to be A_B =0.75±0.03. We have obtained new photometry in Gunn r for E and S0 galaxies in the cluster. If the Fundamental Plane is used for determination of the relative distance and the peculiar velocity of the cluster, we find a distance, in velocity units, of (5706±350) km s⁻¹, and a substantial peculiar velocity, (839±350) km s⁻¹. However, the colours and the absorption line indices of the E and S0 galaxies indicate that the stellar populations in these galaxies are different from those in similar galaxies in the two rich clusters Coma and Hydra I. This difference may severely affect the distance determination and the derived peculiar velocity. The data are consistent with a non-significant peculiar velocity for S639 and the galaxies in the cluster being on average 0.2 dex younger than similar galaxies in Coma and Hydra I. The results for S639 caution that some large peculiar velocities may be spurious and caused by unusual stellar populations.     

SCUBA imaging of the NGC 7331 dust ring

We present observations of the spiral galaxy NGC 7331 using the Submillimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at wavelengths of 450 and 850 μm. The dust ring is in good correspondence with other observations of the ring in the mid-infrared (MIR), CO and radio continuum, suggesting that the observed dust is associated with molecular gas and star formation. A B  −  K colour map shows an analogous ring structure with an asymmetry about the major axis, consistent with the extinction being produced by a dust ring. The derived temperature of the dust lies between 16 and 31 K and the gas-to-dust ratio lies between 150 and 570, depending on the assumed dust emission efficiency index (β = 1.5 or 2).     

Astronomical and terrestrial causes of physical, chemical and biological changes at geological boundaries

The boundary horizons of the Cretaceous-Tertiary (Um Sohryngkew River section, Meghalaya and Anjar section, Kutch), the Permo-Triassic (Guling, Lalung, Ganmachidam and Attargoo sections, Spiti valley) and the Eocene-Oligocene (Tapti River section, Gujarat) have been identified in the sedimentary records of the Indian subcontinent. These sections have been studied for geochemical anomalies. The results are discussed in the framework of extra-terrestrial and terrestrial causes proposed to explain the physical, chemical and mineralogical observations at these boundaries. A critical analysis suggests that although the astronomical causes, particularly the bolide impacts, can easily explain the geochemical and physical changes, the terrestrial causes (volcanism) may have played a significant role in creating the biological stress observed in fossil records (mass extinction) at or near some of these boundaries.     

Late Olenekian Radiolarians from Bedded Chert of Ashio Terrane, Northeast Japan,and Faunal Turnovers in Western Panthalassa during Early Triassic

Late Olenekian assemblages in the western Panthalassa have been recovered from bedded radiolarian chert sequences of an accretionary complex, the Ashio belt. These faunas are documented and considered in terms of radiolarian diversity and faunal turnover during the latest Permian to Middle Triassic time. The fauna includes 30 radiolarians belonging to Spumellaria or Entactinaria, with two relicts from the Late Permian. This late Olenekian fauna is markedly different from Permian and Asisian faunas, respectively, and is herein named the Minowa fauna. Study of the literaure indicates that radiolarian provinces were significantly disconnected between the western Panthalassa and eastern Tethys during late Olenekian time. Furthermore, 121 of 143 species disappeared during late Olenekian time, and in turn 118 new species appeared in the western Panthalassa around the Olenekian-Anisian boundary. It is concluded that faunal turnover occurred at least three times between the latest Permian and Middle Triassic.The first turnover is the Poalaozoie-type radiolarian extinction at the Permain-Triassic boundary,the second is the diversification of spheroidal Spumellaria and Entactinaria between early and late Olenekian time, and the third is a faunal turnover from the Minowa fauna to the true Mesozoic-type radiolarian famas that are characterized by mulit0segmented Nassellaria.