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We present here the first stellar models on the Hertzsprung-Russell diagram, in which convection is treated according to the new scale-free convection theory (SFC theory) by Pasetto et al. The aim is to compare the results of the new theory with those from the classical, calibrated mixing-length (ML) theory to examine differences and similarities.We integrate the equations describing the structure of the atmosphere from the stellar surface down to a few per cent of the stellar mass using both ML theory and SFC theory. The key temperature over pressure gradients, the energy fluxes, and the extension of the convective zones are compared in both theories. The analysis is first made for the Sun and then extended to other stars of different mass and evolutionary stage. The results are adequate: the SFC theory yields convective zones, temperature gradients ▿ and ▿e, and energy fluxes that are very similar to those derived from the 'calibrated' MT theory for main-sequence stars.We conclude that the old scale dependent ML theory can now be replaced with a self-consistent scale-free theory able to predict correct results, as it is more physically grounded than the ML theory. Fundamentally, the SFC theory offers a deeper insight of the underlying physics than numerical simulations. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the impact that realistic scale-dependent systematic effects may have on cosmic shear tomography. We model spatially varying residual galaxy ellipticity and galaxy size variations in weak lensing measurements and propagate these through to predicted changes in the uncertainty and bias of cosmological parameters. We show that the survey strategy -- whether it is regular or randomized -- is an important factor in determining the impact of a systematic effect: a purely randomized survey strategy produces the smallest biases, at the expense of larger parameter uncertainties, and a very regularized survey strategy produces large biases, but unaffected uncertainties. However, by removing, or modelling, the affected scales (l-modes) in the regular cases the biases are reduced to negligible levels. We find that the integral of the systematic power spectrum is not a good metric for dark energy performance, and we advocate that systematic effects should be modelled accurately in real space, where they enter the measurement process, and their effect subsequently propagated into power spectrum contributions. [ABSTRACT FROM AUTHOR]

Stellar convection is customarily described by Mixing-Length Theory, which makes use of the mixing length-scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing length-scale is taken to be proportional to the local pressure scaleheight, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to suggest that the mixing-length parameter is the same in all stars and at all evolutionary phases. The aim of this study is to present a new theory of stellar convection that does not require the mixing-length parameter. We present a self-consistent analytical formulation of stellar convection that determines the properties of stellar convection as a function of the physical behaviour of the convective elements themselves and of the surrounding medium. This new theory is formulated starting from a conventional solution of the Navier–Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame comoving with the convective elements. In our formalism, the motion of stellar convective cells inside convectively unstable layers is fully determined by a new system of equations for convection in a non-local and time-dependent formalism. We obtain an analytical, non-local, time-dependent subsonic solution for the convective energy transport that does not depend on any free parameter. The theory is suitable for the outer convective zones of solar type stars and stars of all mass on the main-sequence band. The predictions of the new theory are compared with those from the standard mixing-length paradigm for the most accurate calibrator, the Sun, with very satisfactory results. [ABSTRACT FROM AUTHOR]

X-ray observations unveiled various types of radio-silent isolated neutron stars (INSs), phenomenologically very diverse, e.g. the ∼Myr old X-ray-dim INSs (XDINSs) and the ∼kyr old magnetars. Although their phenomenology is much diverse, the similar periods (P = 2–10 s) and magnetic fields (≈1014 G) suggest that XDINSs are evolved magnetars, possibly born from similar populations of supermassive stars. One way to test this hypothesis is to identify their parental star clusters by extrapolating backwards the NS velocity vector in the Galactic potential. By using the information on the age and space velocity of the XDINS RX J1856.5−3754, we computed backwards its orbit in the Galactic potential and searched for its parental stellar cluster by means of a closest approach criterion. We found a very likely association with the Upper Scorpius OB association, for a NS age of 0.42 ± 0.08 Myr, a radial velocity VNSr = 67 ± 13 km s−1, and a present-time parallactic distance dNSπ = 123+ 11− 15 pc. Our result confirms that the ‘true’ NS age is much lower than the spin-down age (τsd = 3.8 Myr), and is in good agreement with the cooling age, as computed within standard cooling scenarios. The mismatch between the spin-down and the dynamical/cooling age would require either an anomalously large breaking index (n ∼ 20) or a decaying magnetic field with initial value B0 ≈ 1014 G. Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB association and the masses of its members, we cannot yet draw firm conclusions on the estimated mass of the RX J1856.5−3754 progenitor. [ABSTRACT FROM AUTHOR]

ABSTRACT The XMM-Newton Serendipitous Ultraviolet Source Survey (XMM-SUSS) is a catalogue of ultraviolet (UV) sources detected serendipitously by the Optical Monitor (XMM-OM) on board the XMM-Newton observatory. The catalogue contains UV-detected sources collected from 2417 XMM-OM observations in one to six broad-band UV and optical filters, made between 2000 February 24 and 2007 March 29. The primary contents of the catalogue are source positions, magnitudes and fluxes in one to six passbands, and these are accompanied by profile diagnostics and variability statistics. XMM-SUSS is populated by 753 578 UV source detections above a 3σ signal-to-noise ratio threshold limit which relate to 624 049 unique objects. Taking account of substantial overlaps between observations, the net sky area covered is 29-54 deg2, depending on UV filter. The magnitude distributions peak at mAB = 20.2, 20.9 and 21.2 in UVW2 (λeff = 2120 Å), UVM2 (λeff = 2310 Å) and UVW1 (λeff = 2910 Å), respectively. More than 10 per cent of the sources have been visited more than once using the same filter during XMM-Newton operation, and >20 per cent of sources are observed more than once per filter during an individual visit. Consequently, the scope for science based on temporal source variability on time-scales of hours to years is broad. By comparison with other astrophysical catalogues we test the accuracy of the source measurements and define the nature of the serendipitous UV XMM-OM source sample. The distributions of source colours in the UV and optical filters are shown together with the expected loci of stars and galaxies, and indicate that sources which are detected in multiple UV bands are predominantly star-forming galaxies and stars of type G or earlier. [ABSTRACT FROM AUTHOR]

ABSTRACT Cosmic shear has been identified as the method with the most potential to constrain dark energy. To capitalize on this potential, it is necessary to measure galaxy shapes with great accuracy, which in turn requires a detailed model for the image blurring by the telescope and atmosphere, the point spread function (PSF). In general, the PSF varies with wavelength and therefore the PSF integrated over an observing filter depends on the spectrum of the object. For a typical galaxy the spectrum varies across the galaxy image, thus the PSF depends on the position within the image. We estimate the bias on the shear due to such colour gradients by modelling galaxies using two co-centred, co-elliptical Sérsic profiles, each with a different spectrum. We estimate the effect of ignoring colour gradients and find the shear bias from a single galaxy can be very large depending on the properties of the galaxy. We find that halving the filter width reduces the shear bias by a factor of about 5. We show that, to the first order, tomographic cosmic shear two point statistics depend on the mean shear bias over the galaxy population at a given redshift. For a single broad filter, and averaging over a small galaxy catalogue from Simard et al., we find a mean shear bias which is subdominant to the predicted statistical errors for future cosmic shear surveys. However, the true mean shear bias may exceed the statistical errors, depending on how accurately the catalogue represents the observed distribution of galaxies in the cosmic shear survey. We then investigate the bias on the shear for two-filter imaging and find that the bias is reduced by at least an order of magnitude. Lastly, we find that it is possible to calibrate galaxies for which colour gradients were ignored using two-filter imaging of a fair sample of noisy galaxies, if the galaxy model is known. For a signal-to-noise ratio of 25 the number of galaxies required in each tomographic redshift bin is of the order of 104. [ABSTRACT FROM AUTHOR]

TheGaiamission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase-space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium-resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848- to 874-nm wavelength range, with a resolving power, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper has presented the characteristics of the RVS and its performance. The present paper details the outstanding scientific impact of this important part of theGaiasatellite on some key open questions in present-day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than, the resolving power of the RVS will give information about most of the effects that influence the position of a star in the Hertzsprung–Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterization of the many types of double, multiple and variable stars. [ABSTRACT FROM AUTHOR]

The definition and optimization studies for theGaiasatellite spectrograph, the‘radial velocity spectrometer’ (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0× 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving powerover the wavelength range[848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5-yrGaiamission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100–150 million stars up to magnitude. At the end of the mission, the RVS will provide radial velocities with precisions of∼2 km s−1 atand∼15–20 km s−1 at, for a solar-metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late-type stars ofatas well as atmospheric parameters up to. The individual abundances of elements such as silicon and magnesium, vital for the understanding of Galactic evolution, will be obtained up to. Finally, the presence of the 862.0-nm diffuse interstellar band (DIB) in the RVS wavelength range will make it possible to derive the three-dimensional structure of the interstellar reddening. [ABSTRACT FROM AUTHOR]

ABSTRACT We report simultaneous UBVRI photopolarimetric observations of the long-period (7.98 h) AM Her binary V1309 Ori. The length and shape of the eclipse ingress and egress vary from night to night. We suggest that this is a result of the variation in the brightness of the accretion stream. By comparing the phases of circular polarization zero-crossovers with previous observations, we confirm that V1309 Ori is well synchronized, and find an upper limit of 0.002 per cent for the difference between the spin and orbital periods. We model the polarimetry data using a model consisting of two cyclotron emission regions at almost diametrically opposite locations, and centred at colatitude β= 35° and 145° on the surface of the white dwarf. We also present archive X-ray observations which show that the negatively polarized accretion region is X-ray bright. [ABSTRACT FROM AUTHOR]

We report observations of the asynchronous polar BY Cam made using XMM-Newton. We find evidence for two accretion regions which have significantly different spectra. In both regions we find evidence for hard X-ray emission from the post-shock flow while we observe a distinct soft X-ray blackbody component in only one. We detect two emission lines in the RGS detectors which we attribute to nitrogen N vi and N vii: the first time that nitrogen lines have been detected in an X-ray spectrum of a cataclysmic variable. In the time interval where we observe short time-scale variability we find the hard X-ray light curve is correlated with the softest energies in the sense that the hard X-rays trail the soft X-rays and are anti-correlated. We suggest that there are a small number of dense blobs of material which impact the white dwarf without forming a shock and release an amount of optically thick material which obscures hard X-rays for a short duration. [ABSTRACT FROM AUTHOR]

In order to map the three-dimensional location and shape of the emission originating within the accretion stream in AM Her systems, we have investigated the possibilities of relaxing the hitherto-applied constraint of a predetermined stream trajectory in modelling the eclipse profiles. We use emission points which can be located anywhere in the Roche lobe of the primary, together with a regularization term which favours any curved stream structure, connected at the secondary and white dwarf primary. Our results show that, given suitable regularization constraints, such inversion is feasible. We investigate the effect of removing the regularization term, and also the sensitivity of the fit to input parameters such as inclination. [ABSTRACT FROM AUTHOR]

We present a combined analysis of XMM–Newton, Chandra and ROSAT observations of the isolated neutron star RX J0720.4-3125, spanning a total period of ∼7 yr. We develop a maximum likelihood periodogram for our analysis based on the ΔC statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked a posteriori by folding a further BeppoSAX data set with the period predicted at the time of that observation: the phase is found to be consistent. The study of the spin history and the measure of the spin-down rate are of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of P·, here measured for the first time, is ≈10[sup -14] s s[sup -1]. This value cannot be explained in terms of torque from a fossil disc. When interpreted in terms of dipolar losses, it gives a magnetic field of B≈10[sup 13] G, making it also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find it unlikely that the field decayed from a much larger value (B≈10 [sup 15] G , as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of ≈10 [sup 4] yr, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is ≈10 [sup 6] yr old, and its magnetic field has not changed substantially over the lifetime. [ABSTRACT FROM AUTHOR]

We present spectroscopic and high-speed photometric data of the eclipsing polar V895 Cen. We find that the eclipsed component is consistent with it being the accretion regions on the white dwarf. This is in contrast to Stobie et al. who concluded that the eclipsed component was not the white dwarf. Further, we find no evidence for an accretion disc in our data. From our Doppler tomography results, we find that the white dwarf has M≳0.7 M[sub ⊙]. Our indirect imaging of the accretion stream suggests that the stream is brightest close to the white dwarf. When we observed V895 Cen in its highest accretion state, emission was concentrated along field lines leading to the upper pole. There is no evidence for enhanced emission at the magnetic coupling region. [ABSTRACT FROM AUTHOR]

Using a parametrized function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf, we are also able to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match.) Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one per cent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white dwarf masses than previous formulations. [ABSTRACT FROM AUTHOR]

We present an analysis of the X-ray spectra of two strongly magnetic cataclysmic variables, DP Leo and WW Hor, made using XMM-Newton. Both systems were in intermediate levels of accretion. Hard optically thin X-ray emission from the shocked accreting gas was detected from both systems, while a soft blackbody X-ray component from the heated surface was detected only in DP Leo. We suggest that the lack of a soft X-ray component in WW Hor is owing to the fact that the accretion area is larger than in previous observations with a resulting lower temperature for the re-processed hard X-rays. Using a multi-temperature model of the post-shock flow, we estimate that the white dwarf in both systems has a mass greater than 1 M[sub ⊙]. The implications of this result are discussed. We demonstrate that the ‘soft X-ray excess’ observed in many magnetic cataclysmic variables can be partially attributed to using an inappropriate model for the hard X-ray emission. [ABSTRACT FROM AUTHOR]

We apply our technique for indirect imaging of the accretion stream to the polar HU Aqr, using eclipse profiles observed when the system was in a low-accretion state. The eclipse profile is different from that in the high state, and more variable from cycle to cycle. We find that the stream maps are brightest near the white dwarf and there is no significant brightening in the threading region. In the low state the stream threads on to the magnetic field closer to the L[sub 1] point than in the high state, with a footpoint of the accreting field line at high latitude. We then produce maps of the accretion region from polarimetry using Stokes imaging. These show that the majority of the accretion occurs near the equator. The difference between the maps may be explained if most of the stream material is not emitting significantly in the low state. If so, neither the stream eclipse mapping nor Doppler tomography techniques will trace the bulk of the accretion flow between the two stars. [ABSTRACT FROM AUTHOR]

We present high-time-resolution optical photometry of the eclipsing binary UZ For using a superconducting tunnel junction (STJ) device, a photon-counting array detector with intrinsic energy resolution. Three eclipses of the ∼18-mag 126.5-min orbital binary were observed using a 6×6 array of tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. The detector presently provides individual photon arrival-time accuracy to about 5 μs, and a wavelength resolution of about 60 nm at 500 nm, with each array element capable of counting up to ∼5000 photon s[sup -1]. The data allow us to place accurate constraints on the accretion geometry from our time- and spectrally resolved monitoring, especially of the eclipse ingress and egress. We find that there are two small accretion regions, located close to the poles of the white dwarf. The positions of these are accurately constrained, and show little movement from eclipse to eclipse, even over a number of years. The colour of the emission from the two regions appears similar, although their X-ray properties are known to be significantly different: we argue that the usual accretion shock may be absent at the non-X-ray-emitting region, and instead the flow here interacts directly with the white dwarf surface; alternatively, a special grazing occultation of this region is required. There is no evidence for any quasi-periodic oscillations on time-scales of the order of seconds, consistent with relatively stable cyclotron cooling in each accretion region. [ABSTRACT FROM AUTHOR]

We present polarimetric and spectroscopic observations of the ROSAT source RX J1141.3-6410, recently identified as a polar. The detection of circular polarization variations, with an amplitude of 10 per cent, over a 3.16-h period confirms that the system is a polar (AM Herculis star). Supporting evidence comes from the nature of the emission lines and their radial velocity variability. In addition, we observe continuum slope changes in the far-red spectral region (∼6000–8200 Å), indicative of phase dependent cyclotron emission. Polarimetric modelling at two wavelengths establishes RX J1141.3-6410 as a single-pole system, with i∼β∼70°. The accretion region is extended in magnetic longitude, and is totally self-occulted for ∼25 per cent of the orbit. The radial velocity curves derived from the emission lines show a phasing with maximum blueshift occurring with Δφ∼0.05 of maximum intensity and circular polarisation. In addition, the broader component of the lines exhibit a substantial radial velocity phase shift with respect to the narrower component, in the sense that the broad component preceeds the narrow. This can be readily understood if the narrower component is principally a result of orbital motion of the stream material and the broad component mainly a result of streaming motion near the coupling region. The phasing of the Ca ii near-infrared line radial velocities also supports this general picture. [ABSTRACT FROM AUTHOR]

We present simultaneous optical polarimetry and X-ray data of the near-synchronous polar RX J2115–5840. We model the polarization data using the Stokes imaging technique of Potter et al. We find that the data are best modelled using a relatively high binary inclination and a small angle between the magnetic and spin axes. We find that, for all spin–orbit beat phases, a significant proportion of the accretion flow is directed on to the lower hemisphere of the white dwarf, producing negative circular polarization. Only for a small fraction of the beat cycle is a proportion of the flow directed on to the upper hemisphere. However, the accretion flow never occurs near the upper magnetic pole, whatever the orientation of the magnetic poles. This indicates the presence of a non-dipole field with the field strength at the upper pole significantly higher. We find that the brightest parts of the hard X-ray emitting region and the cyclotron region are closely coincident. [ABSTRACT FROM AUTHOR]

In this paper we present an objective map of the cyclotron emission region in a magnetic cataclysmic variable from polarimetric observations of the AM Her system V347 Pav. As the system parameters of V347 Pav (for example its inclination) have not been well determined, we describe an extension to Stokes imaging of Paper I which also searches the system parameter space. We show that the cyclotron emission from V347 Pav emanates from two areas on the white dwarf, one area consisting of two relatively large regions located at opposite footprints of the same field lines, and a much smaller region located above the orbital plane on the opposite face of the white dwarf. This is different from previous less objective techniques, in which two long, thin arc-shaped cyclotron emission regions were used to model the observed polarimetric variations. [ABSTRACT FROM AUTHOR]

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I-band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I-band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters. [ABSTRACT FROM AUTHOR]

We apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1.0–1.3×10[sup 10] cm from the white dwarf; this implies a mass transfer rate in the range 8–76×10[sup 16] g s[sup -1]. The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 42° to 48° from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U, V and R. [ABSTRACT FROM AUTHOR]

ABSTRACT We calculate the temperature and density structure of the hot post-shock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1D flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We revise previous determinations of the masses of the white dwarf primary stars, and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies. [ABSTRACT FROM AUTHOR]

This article further elaborates, corrects and extends the optimal consumption of depletable natural resources. Competitive behavior in an extractive industry with increasing marginal costs of production will generate a socially efficient pattern of production over time. The authors' analysis of this result assumed that marginal extraction costs at a mine depend only on the amount extracted from the mine to date. This clarifies the author's original proof and goes on to consider more explicitly what happens when extraction costs depend not only on the amount extracted at each site but also on the firm doing the extraction. When costs of extraction at a site depend only on the cumulative amount extracted there to date, all that is required for efficiency is that there be competitive markets for the extracted commodity.