Stratigraphy and composition of lava flows in Mare Nubium and Mare Cognitum     

Roberto Bugiolacchi  Paul D. Spudis  John E. Guest 《Meteoritics & planetary science》2006,41(2):285-304

Abstract— Three major periods of basaltic activity characterize the infill of the basins. Each of these periods was itself punctuated by discrete phases of widespread magma eruptions: three during both the Late Imbrian Epoch and the early Eratosthenian Period and then two in the late Eratosthenian Period. We found the youngest lavas off the eastern border of the Fra Mauro peninsula and, mantling a much larger area, over most of the central western Nubium basin. Our results place the Nubium/Cognitum basalts in the low‐Ti category (1–5 wt% TiO₂). The data indicate that the majority (?90%) of the mare terrain has iron content between 18 and 22 wt%. In particular, FeO contents tend to concentrate toward two compositional poles, each of ?20 wt%, and a much smaller one of ?15 wt%. These values are typical of nearside lunar maria. To complement our compositional data, we present a census of craters larger than 500 m using Orbiter IV images. The result was a crater count average with frequency 5.6 × 10^?2 km^?2, translating into an inferred mean age of 3300 Ma for the exposed lava flows. By combining lava chemistry with age, we find a possible correlation between the ages of the most prominent flow units and their estimated titanium content, with younger basalts becoming progressively Ti‐richer with time (from 2–3 to 4–5 wt% TiO₂).  相似文献   

The 2dF Galaxy Redshift Survey: the number and luminosity density of galaxies     

Nicholas Cross  Simon P. Driver  Warrick Couch  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Shaun Cole  Matthew Colless  Chris Collins  Gavin Dalton  Kathryn Deeley  Roberto De Propris  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Ofer Lahav  Ian Lewis  Stuart Lumsden  Steve Maddox  Darren Madgwick  Stephen Moody  Peder Norberg  John A. Peacock  Bruce A. Peterson  Ian Price  Mark Seaborne  Will Sutherland  Helen Tadros  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2001,324(4):825-841

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The 2dF Galaxy Redshift Survey: the nature of the relative bias between galaxies of different spectral type     

Edward Conway  Steve Maddox  Vivienne Wild  John A. Peacock  Ed Hawkins  Peder Norberg  Darren S. Madgwick  Ivan K. Baldry  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Shaun Cole  Matthew Colless  Chris Collins  Warrick Couch  Gavin Dalton  Roberto De Propris  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Bryn Jones  Ofer Lahav  Ian Lewis  Stuart Lumsden  Will Percival  Bruce A. Peterson  Will Sutherland  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2005,356(2):456-474

We present an analysis of the relative bias between early- and late-type galaxies in the Two-degree Field Galaxy Redshift Survey (2dFGRS) – as defined by the η parameter of Madgwick et al., which quantifies the spectral type of galaxies in the survey. We calculate counts in cells for flux-limited samples of early- and late-type galaxies, using approximately cubical cells with sides ranging from 7 to  42 h ⁻¹ Mpc  . We measure the variance of the counts in cells using the method of Efstathiou et al., which we find requires a correction for a finite volume effect equivalent to the integral constraint bias of the autocorrelation function. Using a maximum-likelihood technique we fit lognormal models to the one-point density distribution, and develop methods of dealing with biases in the recovered variances resulting from this technique. We then examine the joint density distribution function,   f (δ_E, δ_L)  , and directly fit deterministic bias models to the joint counts in cells. We measure a linear relative bias of ≈1.3, which does not vary significantly with ℓ. A deterministic linear bias model is, however, a poor approximation to the data, especially on small scales  (ℓ≤ 28  h ⁻¹ Mpc)  where deterministic linear bias is excluded at high significance. A power-law bias model with index   b ₁≈ 0.75  is a significantly better fit to the data on all scales, although linear bias becomes consistent with the data for  ℓ≳ 40  h ⁻¹ Mpc  .  相似文献   

Optical and infrared signatures of ultra-luminous X-ray sources     

Christopher Copperwheat  Mark Cropper  Roberto Soria  Kinwah Wu 《Monthly notices of the Royal Astronomical Society》2005,362(1):79-88

We have constructed a model to describe the optical emission from ultra-luminous X-ray sources (ULXs). We assume a binary model with a black hole accreting matter from a Roche lobe filling companion star. We consider the effects of radiative transport and radiative equilibrium in the irradiated surfaces of both the star and a thin accretion disc. We have developed this model as a tool with which to positively identify the optical counterparts of ULXs, and subsequently derive parameters such as the black hole mass and the luminosity class and spectral type of the counterpart. We examine the dependence of the optical emission on these and other variables. We extend our model to examine the magnitude variation at infrared wavelengths, and we find that observations at these wavelengths may have more diagnostic power than in the optical. We apply our model to existing HST observations of the candidates for the optical counterpart of ULX X-7 in NGC 4559. All candidates could be consistent with an irradiated star alone, but we find that a number of them are too faint to fit with an irradiated star and disc together. Were one of these the optical counterpart to X-7, it would display a significant temporal variation.  相似文献   

Elliptical Galaxy Nuclei Activity Powered by Infalling Globular Clusters     

Roberto Capuzzo-Dolcetta 《Astrophysics and Space Science》2004,294(1-2):95-100

Globular cluster systems evolve, in galaxies, due to internal and external dynamics and tidal phenomena. One of the causes of evolution, dynamical friction, is responsible for the orbital decay of massive clusters into the innermost galactic regions. It is found that these clusters are effective source of matter to feed a central galactic black hole such to make it grow and shine as an AGN.  相似文献   

Spectral variability in transonic discs around black holes     

Luca Zampieri  Roberto Turolla  Ewa Szuszkiewicz 《Monthly notices of the Royal Astronomical Society》2001,325(3):1266-1274

Transonic discs with accretion rates relevant to intrinsically bright Galactic X-ray sources ( L ≈10³⁸–10³⁹ erg s⁻¹) exhibit a time-dependent cyclic behaviour due to the onset of a thermal instability driven by radiation pressure. In this paper we calculate radiation spectra emitted from thermally unstable discs to provide detailed theoretical predictions for observationally relevant quantities. The emergent spectrum has been obtained by solving self-consistently the vertical structure and radiative transfer in the disc atmosphere. We focus on four particular stages of the disc evolution, the maximal evacuation stage and three intermediate stages during the replenishment phase. The disc is found to undergo rather dramatic spectral changes during the evolution, emitting mainly in the 1–10 keV band during outburst and in the 0.1–1 keV band off-outburst. Local spectra, although different in shape from a blackbody at the disc effective temperature, may be characterized in terms of a hardening factor f . We have found that f is more or less constant, both in radius and in time, with a typical value ∼ 1.65.  相似文献   

Structural parameters of nearby emission-line galaxies     

Miguel Sánchez-Portal  Ángeles I. Díaz  Elena Terlevich †  Roberto Terlevich 《Monthly notices of the Royal Astronomical Society》2004,350(3):1087-1106

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The 2dF Galaxy Redshift Survey: spherical harmonics analysis of fluctuations in the final catalogue     

Will J. Percival  Daniel Burkey  Alan Heavens  y Taylor  Shaun Cole  John A. Peacock  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Matthew Colless  Chris Collins  Warrick Couch  Gavin Dalton  Roberto De Propris  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Ofer Lahav  Ian Lewis  Stuart Lumsden  Steve Maddox  Peder Norberg  Bruce A. Peterson  Will Sutherland  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2004,353(4):1201-1218

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The circumstellar material around SN IIn 1997eg: another detection of a very narrow P Cygni profile     

IsabelSalamanca  Roberto J.Terlevich  GuillermoTenorio-Tagle 《Monthly notices of the Royal Astronomical Society》2002,330(4):844-854

We report the detection of a very narrow P Cygni profile on top of the broad emission H α and H β lines of the Type IIn Supernova 1997eg. A similar feature has been detected in SN 1997ab, SN 1998S and SN 1995G . The detection of the narrow P Cygni profile indicates the existence of a dense circumstellar material (CSM), into which the ejecta of the supernova is expanding. From the analysis of the spectra of SN 1997eg we deduce (i) that such CSM is very dense  ( n ≳5×10⁷ cm^-3)  , (ii) that it has a low expanding velocity of about 160 km s⁻¹. The origin of such dense CSM can be either a very dense progenitor wind  ( M˙ ∼10^-2 M_⊙ yr^-1)  or a circumstellar shell product of the progenitor wind expanding into a high-pressure environment.  相似文献   

Mechanical Testing in Unsaturated Soils   总被引：1，自引：0，他引：1  

Laureano R. Hoyos  Lyesse Laloui  Roberto Vassallo 《Geotechnical and Geological Engineering》2008,26(6):675-689

The state-of-the-art report presented herein is aimed at documenting, to the largest extent possible, some of the recent advances in laboratory testing of unsaturated soils for stress–strain–strength–stiffness characterization under suction-controlled isotropic, axisymmetric, and true triaxial stress states. The report is intended to be neither comprehensive nor fully inclusive, offering plenty of room for further discussion on recent refinements and techniques not yet reported in the literature. The main sections in this report are devoted to describing current methods and technologies using cylindrical triaxial systems, including advances in volume change measurements; resonant column/torsional shear systems; bender element-based systems; and suction-controlled testing under true triaxial stress states. Concluding remarks are included in the last section of the report.  相似文献