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A key problem in protoplanetary disc evolution is understanding the efficiency of dust radial drift. This process makes the observed dust disc sizes shrink on relatively short timescales, implying that discs started much larger than what we see now. In this paper we use an independent constraint, the gas radius (as probed by CO rotational emission), to test disc evolution models. In particular, we consider the ratio between the dust and gas radius, $R_{\rm CO}/R_{\rm dust}$. We model the time evolution of protoplanetary discs under the influence of viscous evolution, grain growth, and radial drift. Then, using the radiative transfer code RADMC with approximate chemistry, we compute the dust and gas radii of the models and investigate how $R_{\rm CO}/R_{\rm dust}$ evolves. Our main finding is that, for a broad range of values of disc mass, initial radius, and viscosity, $R_{\rm CO}/R_{\rm dust}$ becomes large (>5) after only a short time (, 17 pages, 14 figures

Galactic bars can affect the evolution of galaxies by redistributing gas in galaxies, possibly contributing to the cessation of star formation. Recent works point to 'bar quenching' playing an important role in massive disk galaxies like the Milky Way. We construct the largest ever sample of gas rich and strongly barred disc galaxies with resolved HI observations making use of both the Giant Meter Radio Telescope (GMRT) and the Karl Jansky Very Large Array (VLA) to collect data. This sample, called HIRB (HI Rich Barred) galaxies, were identified with Galaxy Zoo - to find galaxies hosting a strong bar, and the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) blind HI survey- to identify a high HI content. We measure gas fractions, HI morphology and kinematics in each galaxy, and use archival optical data from the Sloan Digital Sky Survey (SDSS) to reveal star-formation histories and bar properties. HIRB galaxies presented here support a picture where bar quenching is playing, or will play an important role in their evolution. They also support models which show how the presence of cold gas delays and slows the development of strong bars. The galaxies with the lowest gas fractions (still high for their mass) show clear HI holes, dynamical advanced bars and low star formation rates, those with the highest gas fractions show little impact from their bar on the HI morphology, and are actively star-forming. How such unusual galaxies came to be is an open question. Several of the HIRBs have local gas rich companions. Tidal interactions with these lower mass galaxies could result in an early triggering of the bar and/or accretion of HI between them. The role of environment in the evolution of the HIRB galaxies will be explored in a future paper., 19 pages, 9 figures

Pairs of planets in a system may end up close to their host star on eccentric orbits as a consequence of planet planet scattering, Kozai or secular migration. In this scenario, general relativity and secular perturbations have comparable timescales and may interfere with each other with relevant effects on the eccentricity and pericenter evolution of the two planets. We explore, both analytically and via numerical integration, how the secular evolution is changed by general relativity for a wide range of different initial conditions. We find that when the faster secular frequency approaches the general relativity precession rate, which tipically occurs when the outer planet moves away from the inner one, it relaxes to it and a significant damping of the proper eccentricity of the inner planet occurs. The proper eccentricity of the outer planet is reduced as well due to the changes in the secular interaction of the bodies. The lowering of the peak eccentricities of the two planets during their secular evolution has important implications on their stability. A significant number of two planet systems, otherwise chaotic because of the mutual secular perturbations, are found stable when general relativity is included., Comment: Accepted for publication on MNRAS

Environment is one of the key external drivers of the galaxies, while active galactic nucleus (AGN) is one of the key internal drivers. Both of them play fundamental roles in regulating the formation and evolution of galaxies. We explore the interrelationship between environment and AGN in SDSS. At a given stellar mass, the specific star formation rate distribution of the AGN host galaxies remains unchanged with over-density, with the peak of the distribution around the Green Valley. We show that, at a given stellar mass, the AGN fraction that has been commonly used in previous studies (defined as the number of AGNs relative to all galaxies including passive and star forming ones) does decrease with increasing over-density for satellites. This is largely due to the fact that the fraction of passive galaxies strongly depends on environment. In order to investigate the intrinsic correlation between AGN and environment, especially under the assumption that AGN feedback is responsible for star formation quenching, the AGN fraction should be defined as the number of AGNs relative to the star-forming galaxies only. With the new definition, we find little dependence of AGN fraction on over-density, central/satellite, and group halo mass. There is only marginal evidence that AGN may prefer denser regions, which is possibly due to more frequent interaction of galaxies or higher merger rate in groups. Our results support the scenario that internal secular evolution is the predominant mechanism of triggering AGN activity, while external environment related processes only play a minor role., Accepted for publication in MNRAS, submitted on June 4. 11 pages, 7 figures

Rings are important and characteristic features of disc-shaped galaxies. This paper is the first in a series which re-visits galactic rings with the goals of further understanding the nature of the features and for examining their role in the secular evolution of galaxy structure. The series begins with a new sample of 3962 galaxies drawn from the Galaxy Zoo 2 citizen science database, selected because zoo volunteers recognized a ring-shaped pattern in the morphology as seen in Sloan Digital Sky Survey colour images. The galaxies are classified within the framework of the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. It is found that zoo volunteers cued on the same kinds of ring-like features that were recognized in the 1995 Catalogue of Southern Ringed Galaxies (CSRG). This paper presents the full catalogue of morphological classifications, comparisons with other sources of classifications, and some histograms designed mainly to highlight the content of the catalogue. The advantages of the sample are its large size and the generally good quality of the images; the main disadvantage is the low physical resolution which limits the detectability of linearly small rings such as nuclear rings. The catalogue includes mainly inner and outer disc rings and lenses. Cataclysmic ("encounter-driven") rings (such as ring and polar ring galaxies) are recognized in less than 1\% of the sample., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society (146 pages with 10 figures and 11 tables)

To study the secular evolution of the activity of Centaur 166P/2001 T4 (Near Earth Asteroid Tracking, NEAT) and its physical properties, we present the results of optical observations of the Centaur taken on 2009 March 29 with the Keck 10-m telescope located atop Mauna Kea, Hawaii. It was still active at r(h) = 11.9 au post-perihelion. An upper limit of the nucleus radius of a(N) < 14.32 km is derived. The colour index is B - R = 1.59 +/- 0.05. The Af rho value is measured to be 288 +/- 19 cm and the corresponding dust production rate is 252 kg s(-1). Finally, a possible mechanism of activity is discussed.

We show the effects of the perturbation caused by a passing by star on the Kuiper belt objects (KBOs) of our Solar System. The dynamics of the Kuiper belt (KB) is followed by direct $N$-body simulations. The sampling of the KB has been done with $N$ up to $131,062$, setting the KBOs on initially nearly circular orbits distributed in a ring of surface density $\Sigma \sim r^{-2}$. This modelization allowed us to investigate the secular evolution of the KB upon the encounter with the perturbing star. Actually, the encounter itself usually leads toward eccentricity and inclination distributions similar to observed ones, but tends also to excite the low-eccentricity population ($e < 0.1$ around $a\sim 40$\,$\mathrm{AU}$ from the Sun), depleting this region of low eccentricities. The following long-term evolution shows a "cooling" of the eccentricities repopulating the low-eccentricity area. In dependence on the assumed KBO mass spectrum and sampled number of bodies, this repopulation takes place in a time that goes from 0.5 Myr to 100 Myr. Due to the unavoidable limitation in the number of objects in our long-term simulations ($N \leq 16384$), we could not consider a detailed KBO mass spectrum, accounting for low mass objects, thus our present simulations are not reliable in constraining correlations among inclination distribution of the KBOs and other properties, such as their size distribution. However, our high precision long term simulations are a starting point for future larger studies on massively parallel computational platforms which will provide a deeper investigation of the secular evolution ($\sim 100\,$Myr) of the KB over its whole mass spectrum., Comment: 13 pages, 12 figures, 5 tables

Recently, the relation between the masses of the black hole ($M_{BH}$) and the host galaxy ($M_{host}$) in quasars has been probed down to the parameter space of $M_{BH}\sim10^8 M_\odot$ and $M_{host}\sim10^{11} M_\odot$ at z $, Comment: Accepted for publication by MNRAS. 9 pages, 4 figures

The recent discovery of a number of circumbinary planets lends a new tool to astrophysicists seeking to understand how and where planet formation takes place. Of the increasingly numerous circumbinary systems, Kepler-16 is arguably the most dynamically interesting: it consists of a planet on an almost perfectly circular orbit (e = 0.0069) around a moderately eccentric binary (e = 0.16). We present high-resolution 3D smoothed-particle hydrodynamics simulations of a Kepler-16 analogue embedded in a circumbinary disc, and show that the planet's eccentricity is damped by its interaction with the protoplanetary disc. We use this to place a lower limit on the gas surface density in the real disc through which Kepler-16b migrated of \Sigma_min ~ 10 g cm^-2. This suggests that Kepler-16b, and other circumbinary planets, formed and migrated in relatively massive discs. We argue that secular evolution of circumbinary discs requires that these planets likely formed early on in the lifetime of the disc and migrated inwards before the disc lost a significant amount of its original mass., Comment: 7 pages, 6 figures. Accepted for publication in MNRAS. Movies of the simulations can be found at: http://www.astro.le.ac.uk/~acd23/K16.html

Narrow-Line Seyfert 1 (NLS1) galaxies show extreme properties with respect to the other Seyfert galaxies. Indeed, they are thought to be accreting at Eddington rates and to possess low mass black holes. Therefore, they may represent a key class of objects for understanding the co-evolution of black holes and their host galaxies. We propose that NLS1s represent a class of AGN in which the black hole growth is, and has always been, dominated by secular evolution. Firstly, by looking at the NLS1 host galaxy properties in the literature, we show that the evolution of NLS1s is presently driven by secular processes, much more so than for Broad-Line Seyfert 1s (BLS1s). Secondly, we study the bulges of NLS1 and BLS1 galaxies. Our results demonstrate that NLS1 host bulges are pseudo-bulges and are statistically different from BLS1 bulges. This difference points to the particular importance of secular processes in the past evolution of their hosts. We build on this result to understand the implications on their evolution and the duration of their duty cycle. We show that NLS1s are not necessarily in a special phase of black hole growth and that several Gyr are required for their black hole masses to become similar to BLS1s. Finally, in the light of our results, we discuss the location of NLS1 galaxies on the MBH-\sigma plane and speculate about the connection between the NLS1 galaxy properties and their black hole spin.

We investigate the role of host galaxy classification and black hole mass in a heterogeneous sample of 276 mostly nearby (z 99% confidence. Using ring morphology of the host galaxy as a proxy for lack of tidal interaction, we find that AGN luminosity in host galaxies within 70Mpc is independent of host galaxy interaction for $\sim$ Gyrs, suggesting that the timescale of AGN activity due to secular evolution is much shorter than that due to tidal interactions. We find that LINER hosts have lower 12um luminosity than the median 12um luminosity of normal disk- and bulge-dominated galaxies which may represent observational evidence for past epochs of feedback that supressed star formation in LINER host galaxies. We propose that nuclear ULXs may account for the X-ray emission from LINER 2s without flat-spectrum, compact radio cores. We confirmed the robustness of our results in X-rays by comparing them with the 14-195keV 22-month BAT survey of AGN, which is all-sky and unbiased by photoelectric absorption.

We analyse V-and H-band surface photometry of a sample of 18 Sb-Sd galaxies. Combining high-resolution Hubble Space Telescope (HST) images with ground-based near-infrared observations, we extract photometric profiles, which cover the whole disc and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the HST.Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction maps in an almost model-independent way using the V-H colour map and the SAURON Mgb absorption line map of Ganda et al. The maps show that AV ranges from 0 to 2 mag, in the centre from 0 to 1.5 mag, in agreement with the models of Tuffs et al.We describe the surface brightness profiles as the superposition of an exponential disc and a Sersic bulge. The bulges are small (0.1-2.5 kpc), and show a shape parameter n ranging from approximate to 0.7 to 3, with a mean value smaller than two: well below the value for the classical de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sersic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities, such as Hubble type and stellar velocity dispersion. We compare these results with a recent paper by Graham & Worley, who present a summary of most of the NIR surface photometry of spirals in the literature. For both early-and late-type spirals, bulge luminosity strongly correlates with central velocity dispersion; at constant velocity dispersion, later type bulges are larger and less dense, and have lower Sersic n-values.

We analyse V and H-band surface photometry of a sample of 18 Sb-Sd galaxies. Combining high resolution HST images with ground-based NIR observations, we extract photometric profiles, which cover the whole disk and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the Hubble Space Telescope. Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction-maps in an almost model-independent way using the V-H colour map and the SAURON Mg b absorption line map of Ganda et al. (2007). The maps show that A_V ranges from 0 to 2 mag, in the center from 0 to 1.5 mag, in agreement with the models of Tuffs et al. (2004). We describe the surface brightness profiles as the superposition of an exponential disk and a Sersic bulge. The bulges are small (0.1-2.5 kpc), and show a shape parameter n ranging from ~ 0.7 to 3, with a mean value smaller than two: well below the value for the 'classical' de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sersic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities (abridged)., Comment: 28 pages, 12 figures and 7 tables. Accepted for publication in MNRAS. Higher resolution version available at http://www.astro.rug.nl/~peletier/ganda2009.pdf

We add the effect of turbulent viscosity via the \alpha-prescription to models of the self-consistent formation and evolution of protostellar discs. Our models are non-axisymmetric and carried out using the thin-disc approximation. Self-gravity plays an important role in the early evolution of a disc, and the later evolution is determined by the relative importance of gravitational and viscous torques. In the absence of viscous torques, a protostellar disc evolves into a self-regulated state with disk-averaged Toomre parameter Q \sim 1.5-2.0, non-axisymmetric structure diminishing with time, and maximum disc-to-star mass ratio \xi = 0.14. We estimate an effective viscosity parameter \alpha_eff associated with gravitational torques at the inner boundary of our simulation to be in the range 10^{-4}-10^{-3} during the late evolution. Addition of viscous torques with a low value \alpha = 10^{-4} has little effect on the evolution, structure, and accretion properties of the disc, and the self-regulated state is largely preserved. A sequence of increasing values of \alpha results in the discs becoming more axisymmetric in structure, being more gravitationally stable, having greater accretion rates, larger sizes, shorter lifetimes, and lower disc-to-star mass ratios. For \alpha=10^{-2}, the model is viscous-dominated and the self-regulated state largely disappears by late times. (Abridged), Comment: 13 pages, 11 figures, accepted for publication in MNRAS

We study a series of $N-$body simulations representing elliptical galaxies with central masses. Starting from two different systems with smooth centres, which have initially a triaxial configuration and are in equilibrium, we insert to them central masses of various values. Immediately after such an insertion a system presents a high fraction of particles moving in chaotic orbits, a fact causing a secular evolution towards a new equilibrium state. The chaotic orbits responsible for the secular evolution are identified. Their typical Lypaunov exponents are found to scale with the central mass as a power law $L\propto m^s$ with $s$ close to 1/2. The requirements for an effective secular evolution within a Hubble time are examined. These requirements are quantified by introducing a quantity called \emph{effective chaotic momentum} $\mathscr{L}$. This quantity is found to correlate well with the rate of the systems' secular evolution. In particular, we find that when $\mathscr{L}$ falls below a threshold value (0.004 in our $N-$body units) a system does no longer exhibit significant secular evolution.

A number of previous studies of the fragmentation of self-gravitating protostellar discs have modeled radiative cooling with a cooling timescale (t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local dynamical timescale. Such studies have delineated the `fragmentation boundary' in terms of a critical value of beta_{cool} (beta_{crit}), where the disc fragments if beta_{cool} < beta_{crit}. Such an approach however begs the question of how in reality a disc could ever be assembled with beta_{cool} < beta_{crit}. Here we adopt the more realistic approach of gradually reducing beta_{cool}, as might correspond to changes in thermal regime due to secular changes in the disc density profile. We find that when beta_{cool} is gradually reduced (on a timescale longer than t_{cool}), the disc is stabilised against fragmentation, compared with models in which beta_{cool} is reduced rapidly. We therefore conclude that a disc's ability to remain in a self-regulated, self-gravitating state (without fragmentation) is partly dependent on its thermal history, as well as its current cooling rate. Nevertheless, a slow reduction in t_{cool} appears only to lower the fragmentation boundary by about a factor two in t_{cool} and thus only permits maximum alpha values (parameterising the efficiency of angular momentum transfer in the disc) that are about a factor two higher than determined hitherto. Our results therefore do not undermine the notion of a fundamental upper limit to the heating rate that can be delivered by gravitational instabilities before the disc is subject to fragmentation. An important implication of this work, therefore, is that self-gravitating discs can enter into the regime of fragmentation via secular evolution and it is not necessary to invoke rapid (impulsive) events to trigger fragmentation., accepted for publication in MNRAS

We present deep B- and R-band surface photometry for a sample of 21 galaxies with morphological types between S0 and Sab. We present radial profiles of surface brightness, colour, ellipticity, position angle and deviations of axisymmetry for all galaxies, as well as isophotal and effective radii and total magnitudes. We have decomposed the images into contributions from a spheroidal bulge and a flat disk, using an interactive, 2D decomposition technique. We study in detail the relations between various bulge and disk parameters. In particular, we find that the bulges of our galaxies have surface brightness profiles ranging from exponential to De Vaucouleurs, with the average value of the Sersic shape parameter n being 2.5. In agreement with previous studies, we find that the shape of the bulge intensity distribution depends on luminosity, with the more luminous bulges having more centrally peaked light profiles. By comparing the ellipticity of the isophotes in the bulges to those in the outer, disk dominated regions, we are able to derive the intrinsic axis ratio q_b of the bulges. The average axis ratio is 0.55, with an rms spread of 0.12. None of the bulges in our sample is spherical, whereas in some cases, the bulges can be as flat as q_b = 0.3 - 0.4. The bulge flattening seems to be weakly coupled to luminosity, more luminous bulges being on average slightly more flattened than their lower-luminosity counterparts. Our finding that most bulges are significantly flattened and have an intensity profile shallower than R^{1/4} suggests that `pseudobulges', formed from disk material by secular processes, do not only occur in late-type spiral galaxies, but are a common feature in early-type disk galaxies as well. (abridged), Comment: 26 pages, 11 figures. Accepted for publication in MNRAS. A pdf-version with full resolution figures and the full atlas can be found at http://www.nottingham.ac.uk/~ppzen/surfphot.accepted.pdf

The secular evolution of the orbital angular momentum (OAM), the systemic mass $(M=M_{1}+M_{2})$ and the orbital period of 114 chromospherically active binaries (CABs) were investigated after determining the kinematical ages of the sub-samples which were set according to OAM bins. OAMs, systemic masses and orbital periods were shown to be decreasing by the kinematical ages. The first order decreasing rates of OAM, systemic mass and orbital period have been determined as $\dot J = 3.48 \times 10^{-10} yr^{-1}$ per systemic OAM, $\dot M = 1.30 \times 10^{-10} yr^{-1}$ per systemic mass and $\dot P = 3.96\times 10^{-10} yr^{-1}$ per orbital period respectively from the kinematical ages. The ratio of $d \log J/ d \log M = 2.68$, which were derived from the kinematics of the present sample, implies that there must be a mechanism which amplifies the angular momentum loss $\bar A = 2.68$ times in comparison to isotropic angular momentum loss of hypothetical isotropic wind from the components. It has been shown that simple isotropic mass loss from the surface of a component or both components would increase the orbital period.

I present a study of the sizes (semimajor axes) of bars in disc galaxies, combining a detailed study of 65 S0-Sb galaxies with measurements of 70 Sb-Sd galaxies from Martin (1995). As has been noted before with smaller samples, bars in early-type (S0-Sb) galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies; this is true both for relative sizes (bar length as fraction of isophotal radius R_25 or exponential disc scale length h) and absolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~3.3 kpc), ~0.2-0.8 R_25 (mean \~0.38 R_25) and ~0.5-2.5 h (mean ~1.4 h). Late-type bars extend to only \~0.5-3.5 kpc, 0.05-0.35 R_25 and 0.2-1.5 h; mean sizes are ~1.5 kpc, 0.14 R_25 and 0.6 h. Sb galaxies resemble earlier-type galaxies in terms of bar size relative to h; their smaller R_25-relative sizes may be a side effect of higher star formation, which increases R_25 but not h. For S0-Sbc galaxies, bar size correlates well with disc size (both R_25 and h); these correlations are stronger than the known correlation with M_B. All correlations appear to be weaker or absent for late-type galaxies; in particular, there seems to be no correlation between bar size and either h or M_B for Sc-Sd galaxies. I show that the bars detected in HST near-IR images at z ~ 1 by Sheth et al. (2003) have absolute sizes consistent with those in bright, nearby S0-Sb galaxies. I also compare the sizes of real bars with those produced in simulations, and discuss some possible implications for scenarios of secular evolution along the Hubble sequence. Simulations often produce bars as large as -- or larger than -- those seen in S0-Sb galaxies, but rarely any as small as those in Sc-Sd galaxies. (Abridged.), Comment: LaTeX, 22 pages, 15 EPS figures. To appear in Monthly Notices of the Royal Astronomical Society

We present the results of a photometric monitoring campaign of three well studied FU Orionis systems (FU Orionis, V1057 Cygni and V1515 Cygni) undertaken at Maidanak Observatory between 1981 and 2003. When combined with photometric data in the literature, this database provides a valuable resource for searching for short timescale variability - both periodic and aperiodic - as well as for studying the secular evolution of these systems. In the case of V1057 Cyg (which is the system exhibiting the largest changes in brightness since it went into outburst) we compare the photometric data with time dependent models. We show that prior to the end of the `plateau' stage in 1996, the evolution of V1057 Cyg in the $V-(B-V)$ colour-magnitude diagram is well represented by disc instability models in which the outburst is triggered by some agent - such as an orbiting planet - in the inner disc. Following the end of the plateau phase in 1996, the dimming and irregular variations are consistent with occultation of the source by a variable dust screen, which has previously been interpreted in terms of dust condensation events in the observed disc wind. Here we instead suggest that this effect results from the interaction between the wind and an infalling dusty envelope, the existence of this envelope having been previously invoked in order to explain the mid infrared emissio of FU Orionis systems. We discuss how this model may explain some of the photometric and spectroscopic characteristics of FU Orionis systems in general., 14 pages, 12 figures, MNRAS, accepted. Data files with detailed photometry can be found at http://www.ast.cam.ac.uk/~giuseppe/Publications/

Results from numerical simulations of normal isolated late-type spiral galaxies are presented; specifically, the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion and supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions: on the one hand, cooling and dissipation of the gas; on the other hand, gas heating by the far-ultraviolet field of young stars and supernova mechanical forcing. The disc exhibits small- and medium-scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand-design m=2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation, confirming the result of Ostriker & Peebles. The occurrence of this bar instability is further investigated. A general criterion is derived for the transition between a stable and an unstable bar, depending on the disc mass contribution and the on-disc thickness. It seems that bar stability barely depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. This demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared with the structure of some exemplary real galaxies. It is argued that the suggestion of Kormendy & Norman holds, i.e. that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.

The standard picture of CV secular evolution predicts a spike in the CV distribution near the observed short-period cutoff P_0 ~ 78 min, which is not observed. We show that an intrinsic spread in minimum (`bounce') periods P_b resulting from a genuine difference in some parameter controlling the evolution can remove the spike without smearing the sharpness of the cutoff. The most probable second parameter is different admixtures of magnetic stellar wind braking (at up to 5 times the GR rate) in a small tail of systems, perhaps implying that the donor magnetic field strength at formation is a second parameter specifying CV evolution. We suggest that magnetic braking resumes below the gap with a wide range, being well below the GR rate in most CVs, but significantly above it in a small tail.

On montre que la distribution de periode observee de variables cataclysmiques magnetiques peut etre aisement comprise sur la base des theories courantes de l'evolution seculaire de ces systemes, et en particulier la formation de saut de periode, prouvant que les naines blanches ont des moments magnetiques μ dans le domaine 10 33 -10 34 G. cm 3 ou μ