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PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

We investigate a repulsion mechanism between two low-mass planets migrating in a protoplanetary disk, for which the relative migration switches from convergent to divergent. This mechanism invokes density waves emitted by one planet transferring angular momentum to the coorbital region of the other and then directly to it through the horseshoe drag. We formulate simple analytical estimates, which indicate when the repulsion mechanism is effective. One condition for a planet to be repelled is that it forms a partial gap in the disk and another is that this should contain enough material to support angular momentum exchange with it. Using two-dimensional hydrodynamical simulations we obtain divergent migration of two super-Earths embedded in a protoplanetary disk because of repulsion between them and verify these conditions. To investigate the importance of resonant interaction we study the migration of planet pairs near first-order commensurabilities. It appears that proximity to resonance is significant but not essential. In this context we find repulsion still occurs when the gravitational interaction between the planets is removed sugesting the importance of angular momentum transfer through waves excited by another planet. This may occur through the scattering of coorbital material (the horseshoe drag), or material orbiting close by. Our results indicate that if conditions favor the repulsion between two planets described above, we expect to observe planet pairs with their period ratios greater, often only slightly greater, than resonant values or possibly rarity of commensurability., Comment: 35 pages, 31 figures, to be published in ApJ

When two planets are born in a protoplanetary disk, they may enter into a mean-motion resonance as a consequence of the convergent planetary migration. The formation of mean-motion resonances is important for understanding how the planetary systems are shaped in the disk environments. Motivated by recent progress in the comprehension of the migration of partial gap-opening planets, we have investigated the orbital evolution of the planet pairs in a wide range of masses and disk properties with the aim to find out when the resonance capture is likely to happen. Using the formula for the migration timescale of a gap-opening planet developed in our previous work, we have derived a simple criterion that allows us to predict when the migration will be convergent (divergent). Further, we have verified the criterion using two-dimensional hydrodynamic simulations. We have found that the resonant pair of planets formed at the early phase of evolution can depart from the resonance at later times because the migration speed of the outer planet slows down due to the gap formation. Moreover, adopting our formula of the migration timescale, we have also carried out three-body simulations, which confirm the results of hydrodynamic simulations. Finally, we have compared our predictions with the observations, selecting a sample of known two-planet systems., Comment: 23 pages, 21 figures, 2 tables, accepted for publication in The Astrophysical Journal

We study the formation of the 9:7 mean motion resonance in a system of two low-mass planets ($m_{1}=m_{2}=3M_{\oplus}$) embedded in a gaseous protoplanetary disk employing a full 2D hydrodynamic treatment of the disk-planet interactions. Our aim is to determine the disk properties that favor a capture of two equal-mass super-Earths into this second-order resonance. For this purpose, we have performed a series of numerical hydrodynamic simulations of the system of two super-Earths migrating in disks with a variety of different initial parameters and found conditions for the permanent or temporary locking in the 9:7 resonance. We observe that capture occurs during the convergent migration of planets if their resonance angle at the moment of arrival at the resonance assumes values in a certain range (inside a window of capture). The width of such a window depends on the relative migration and circularization rates that are determined by the disk parameters. The window is wide if the relative migration rate is slow, and it becomes narrower as the relative migration rate increases. The window will be closed if the migration rate is sufficiently high, and the capture will not take place. We illustrate also how the 9:7 resonance window of capture is affected by the initial eccentricities and the initial orbits of the planets., Comment: 16 pages, 18 figures. Accepted to ApJ

A large planet orbiting a star in a protoplanetary disk opens a density gap along its orbit due to the strong disk-planet interaction and migrates with the gap in the disk. It is expected that in the ideal case, a gap-opening planet migrates at the viscous drift speed, which is referred to as type II migration. However, recent hydrodynamic simulations have shown that in general, the gap-opening planet is not locked to the viscous disk evolution. A new physical model is required to explain the migration speed of such a planet. For this reason, we re-examined the migration of a planet in the disk, by carrying out the two-dimensional hydrodynamic simulations in a wide parameter range. We have found that the torque exerted on the gap-opening planet depends on the surface density at the bottom of the gap. The planet migration slows down as the surface density of the bottom of the gap decreases. Using the gap model developed in our previous studies, we have constructed an empirical formula of the migration speed of the gap-opening planets, which is consistent with the results given by the hydrodynamic simulations performed by us and other researchers. Our model easily explains why the migration speed of the gap-opening planets can be faster than the viscous gas drift speed. It can also predict the planet mass at which the type I migration is no longer adequate due to the gap development in the disk, providing a gap formation criterion based on planetary migration., Comment: Accepted for publication in ApJ, 15 pages, 12 figures

We study the dynamical evolution of the TRAPPIST-1 system under the influence of orbital circularization through tidal interaction with the central star. We find that systems with parameters close to the observed one evolve into a state where consecutive planets are linked by first order resonances and consecutive triples, apart from planets c, d and e, by connected three body Laplace resonances. The system expands with period ratios increasing and mean eccentricities decreasing with time. This evolution is largely driven by tides acting on the innermost planets which then influence the outer ones. In order that deviations from commensurability become significant only on $Gy$ time scales or longer, we require that the tidal parameter associated with the planets has to be such that $Q' > \sim 10^{2-3}.$ At the same time, if we start with two subsystems, with the inner three planets comprising the inner one, $Q'$ associated with the planets has to be on the order (and not significantly exceeding) $10^{2-3}$ for the two subsystems to interact and end up in the observed configuration. This scenario is also supported by modelling of the evolution through disk migration which indicates that the whole system cannot have migrated inwards together. Also in order to avoid large departures from commensurabilities, the system cannot have stalled at a disk inner edge for significant time periods. We discuss the habitability consequences of the tidal dissipation implied by our modelling, concluding that planets d, e and f are potentially in habitable zones., Comment: 27 pages, 15 figures, accepted for publication in MNRAS

We study the orbital architecture, physical characteristics of planets, formation and long-term evolution of the Kepler-30 planetary system, detected and announced in 2012 by the KEPLER team. We show that the Kepler-30 system belongs to a particular class of very compact and quasi-resonant, yet long-term stable planetary systems. We re-analyse the light curves of the host star spanning Q1-Q17 quarters of the KEPLER mission. A huge variability of the Transit Timing Variations (TTV) exceeding 2 days is induced by a massive Jovian planet located between two Neptune-like companions. The innermost pair is near to the 2:1 mean motion resonance (MMR), and the outermost pair is close to higher order MMRs, such as 17:7 and 7:3. Our re-analysis of photometric data allows us to constrain, better than before, the orbital elements, planets' radii and masses, which are $9.2 \pm 0.1$, $536 \pm 5$, and $23.7 \pm 1.3$ Earth masses for Kepler-30b, Kepler-30c and Kepler-30d, respectively. The masses of the inner planets are determined within $\sim 1\%$ uncertainty. We infer the internal structures of the Kepler-30 planets and their bulk densities in a wide range from $ (0.19 \pm 0.01)$ g$\cdot$cm$^{-3}$ for Kepler-30d, $(0.96\pm0.15)$ g$\cdot$cm$^{-3}$ for Kepler-30b, to $(1.71 \pm 0.13)$ g$\cdot$cm$^{-3}$ for the Jovian planet Kepler-30c. We attempt to explain the origin of this unique planetary system and a deviation of the orbits from exact MMRs through the planetary migration scenario. We anticipate that the Jupiter-like planet plays an important role in determining the present dynamical state of this system., Comment: 38 pages, 34 figures, corrected version re-submitted to Monthly Notices of the RAS (MNRAS)

We investigate the dynamical stability of the Kepler-60 planetary system with three super-Earths. We first determine their orbital elements and masses by Transit Timing Variation (TTV) data spanning quarters Q1-Q16 of the KEPLER mission. The system is dynamically active but the TTV data constrain masses to ~4 Earth masses and orbits in safely wide stable zones. The observations prefer two types of solutions. The true three-body Laplace MMR exhibits the critical angle librating around 45 degrees and aligned apsides of the inner and outer pair of planets. In the Laplace MMR formed through a chain of two-planet 5:4 and 4:3 MMRs, all critical angles librate with small amplitudes of ~30 degrees and apsidal lines in planet's pairs are anti-aligned. The system is simultaneously locked in a three-body MMR with librations amplitude of ~10 degrees. The true Laplace MMR can evolve towards a chain of two-body MMRs in the presence of planetary migration. Therefore the three-body MMR formed in this way seems to be more likely state of the system. However, the true three-body MMR cannot be disregarded a priori and it remains a puzzling configuration that may challenge the planet formation theory., Comment: 5+1 pages, 5 figures, accepted in Monthly Notices of the Royal Astronomical Society Letters (MNRAS)

In this paper we investigate the possibility of a migration-induced resonance locking in systems containing three planets, namely an Earth analog, a super-Earth and a gas giant. The planets have been listed in order of increasing orbital periods. All three bodies are embedded in a locally isothermal gaseous disc and orbit around a solar mass star. We are interested in answering the following question: Will the low-mass planets form the same resonant structures with each other in the vicinity of the gas giant as in the case when the gas giant is absent? When there is no gas giant in the system, it has been already shown that if the two low-mass planets undergo a convergent differential migration, they will capture each other in a mean-motion resonance. For the choices of disc parameters and planet masses made in this paper, the formation of the 5:4 resonance in the absence of the Jupiter has been observed. In this work we add a gas giant on the most external orbit of the system in such a way that its differential migration is convergent with the low-mass planets. We show that the result of this set-up is the speeding up of the migration of the super-Earth and, after that, all three planets become locked in a triple mean-motion resonance. However, this resonance is not maintained due to the low-mass planet eccentricity excitation, a fact that leads to close encounters between planets and eventually to the ejection from the internal orbits of one or both low-mass planets., Comment: 10 pages, 11 figures, to be published in MNRAS

The dynamical interactions that occur in newly formed planetary systems may reflect the conditions occurring in the protoplanetary disk out of which they formed. With this in mind, we explore the attainment and maintenance of orbital resonances by migrating planets in the terrestrial mass range. Migration time scales varying between millions of years and thousands of years are considered. In the former case, for which the migration time is comparable to the lifetime of the protoplanetary gas disk, a 2:1 resonance may be formed. In the latter, relatively rapid migration regime commensurabilities of high degree such as 8:7 or 11:10 may be formed. However, in any one large-scale migration several different commensurabilities may be formed sequentially, each being associated with significant orbital evolution. We also use a simple analytic theory to develop conditions for first order commensurabilities to be formed. These depend on the degree of the commensurability, the imposed migration and circularization rates, and the planet mass ratios. These conditions are found to be consistent with the results of our simulations., Comment: 11 pages with 4 figures, pdflatex, to appear in the proceedings of the conference "Extra-solar Planets in Multi-body Systems: Theory and Observations"; eds. K. Gozdziewski, A. Niedzielski and J. Schneider, EAS Publication Series

The aim of this talk is to present the most recent advances in establishing plausible planetary system architectures determined by the gravitational tidal interactions between the planets and the disc in which they are embedded during the early epoch of planetary system formation. We concentrate on a very well defined and intensively studied process of the disc-planet interaction leading to the planet migration. We focus on the dynamics of the systems in which low-mass planets are present. Particular attention is devoted to investigation of the role of resonant configurations. Our studies, apart from being complementary to the fast progress occurring just now in observing the whole variety of planetary systems and uncovering their structure and origin, can also constitute a valuable contribution in support of the missions planned to enhance the number of detected multiple systems., Comment: 10 pages with 5 figures, pdflatex, to appear in the proceedings of the conference "Extra-solar Planets in Multi-body Systems: Theory and Observations"; eds. K. Gozdziewski, A. Niedzielski and J. Schneider, EAS Publication Series

This paper is an extension of the work done by Pierens & Nelson (2008) in which they have investigated the behaviour of a two-planet system embedded in a protoplanetary disc. They have put a Jupiter mass gas giant on the internal orbit and a lower mass planet on the external one. We consider here a similar problem taking into account a gas giant with masses in the range of 0.5 to 1 Jupiter mass and a Super-Earth as the outermost planet. By changing disc parameters and planet masses we have succeeded in getting the convergent migration which allows for the possibility of their resonant locking. However, in the case in which the gas giant has the mass of Jupiter, before any mean motion first order commensurability could be achieved, the Super-Earth is caught in a trap when it is very close to the edge of the gap opened by the giant planet. This confirms the result obtained by Pierens & Nelson (2008) in their simulations. Additionally, we have found that, in a very thin disc, an apsidal resonance is observed in the system if the Super-Earth is captured in the trap. Moreover, the eccentricity of the small planet remains low, while that of the gas giant increases slightly due to the imbalance between Lindblad and corotational resonances. We have also studied analogous systems in which the gas giant is allowed to take Sub-Jupiter masses. In this case, after performing an extensive survey over all possible parameters, we have succeeded in getting the 1:2 mean motion resonant configuration only in a disc with low aspect ratio and low surface density. However, the resonance is maintained just for few thousand orbits. Thus, we conclude that for typical protoplanetary discs the mean motion commensurabilities are rare if the Super-Earth is located on the external orbit relative to the gas giant. (abridged), Comment: 10 pages, 11 figures, accepted for publication in MNRAS

Cosmic rays represent one of the most fascinating research themes in modern astronomy and physics. After almost a century since their discovery, a huge amount of scientific literature has been written on this topic and it is not always easy to extract from it the necessary information for somebody who approaches the subject for the first time. This has been the main motivation for preparing this article, which is a concise and self-contained review for whoever is interested in studying cosmic rays. The priority has been given here to well established facts, which are not at risk to get obsolete in a few years due to the fast progress of the research in this field. Also many data are presented, which are useful to characterize the doses of ionizing radiation delivered to organisms living on the Earth due to cosmic rays. The technical terms which are often encountered in the scientific literature are explained in a separate appendix., Comment: 32 pages, 8 figures in eps format, plain LaTeX + package graphicx

We are carrying out a programme of non-linear time-dependent numerical calculations to study the evolution of the thermal instability driven by radiation pressure in transonic accretion discs around black holes. In our previous studies we first investigated the original version of the slim-disc model with low viscosity (parameter alpha = 0.001) for a stellar-mass (10 solar masses) black hole, comparing the behaviour seen with results from local stability analysis (which were broadly confirmed). In some of the unstable models, we saw a violently evolving shock-like feature appearing near to the sonic point. Next, we retained the original model simplifications but considered a higher value of alpha = 0.1 and demonstrated the existence of limit-cycle behaviour under suitable circumstances. The present paper describes more elaborate calculations with a more physical viscosity prescription and including a vertically integrated treatment of acceleration in the vertical direction. Limit-cycle behaviour is still found for a model with alpha = 0.1, giving a strong motivation to look for its presence in observational data., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

Thermal instability driven by radiation pressure might be relevant for intrinsically bright accreting sources. The most promising candidate where this instability seems to be at work is one of the two known galactic superluminal sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relevance, this scenario has not yet been confirmed by proper time-dependent modelling. Non-linear time-dependent calculations performed by Szuszkiewicz and Miller (1998) show that thermally unstable discs undergo limit-cycle behaviour with successive evacuation and refilling of the central parts of the disc. This evolution is very similar to the one proposed by Belloni et al. (1997) in their phenomenological model. Further investigations are needed to confirm the thermal instability being operational in this source. First of all the spectra emitted from the disc during its evolution should be calculated and compared with observations. Here such spectra are computed assuming local blackbody emission from the best studied transonic disc model., Comment: 4 pages, 2 figures, To be published in `Proceedings of the Third Microquasar Workshop: Granada Workshop on galactic relativistic jet sources', Eds: A. J. Castro-Tirado, J. Greiner and J. M. Paredes, Astrophysics and Space Science, in press

We derive the mass function of the relic black holes and compared with that of the Massive Dark Objects in galaxies. Under the assumption that accretion onto massive BH's powers the Active Galactic Nuclei, the mass function of the BH responsibile for the past activity of QSO/AGN is computed. Our results support the scenario in which the QSO phase has exclusively occurred in every proto-elliptical., Comment: 10 pages, 8 Figures. Version improved with referee comments. J. Accepted on MNRAS

In our recent paper (Salucci et al. 1998) we have investigated the mass distribution function of massive dark objects in galaxies, exploiting the available optical and radio observations. Under the assumption that massive black holes power active galactic nuclei, we have compared the mass functions of massive dark objects and black holes responsible for the observed activity. We have found that a scenario with a single short burst per active galactic nucleus is in a good agreement with the available data. Here we summarize and discuss the main points of our study., Comment: 4 pages, Latex, 1 Postscrpt figure, uses paspconf.sty, to appear in `Observational Cosmology: The development of galaxy systems', ed. G. Giuricin et al., ASP Conference Series

Nonlinear time-dependent calculations are being carried out in order to study the evolution of vertically-integrated models of non-selfgravitating, transonic accretion discs around black holes. In this paper we present results from a new calculation for a high-alpha model similar to one studied previously by Honma, Matsumoto and Kato who found evidence for limit-cycle behaviour connected with thermal instability. Our results are in substantial agreement with theirs but, in our calculation, the disc material does not always remain completely optically thick and we include a suitable treatment for this. We followed the evolution for several cycles and determined the period of the cycle as being about 780 seconds. Advective cooling is dominant in the region just behind the outward-moving peak of surface density. The behaviour of this model is significantly different from what we saw earlier for low-alpha models (which we discussed in a previous paper) and we contrast and compare the two situations., Comment: 15 pages, 11 figures, to be published in MNRAS

We consider transient behavior in low-mass X-ray binaries. In short-period neutron-star systems (orbital period less than ~ 1d) irradiation of the accretion disk by the central source suppresses this except at very low mass transfer rates. Formation constraints however imply that a significant fraction of these neutron star systems have nuclear-evolved main-sequence secondaries and thus mass transfer rates low enough to be transient. But most short-period low-mass black-hole systems will form with unevolved main-sequence companions and have much higher mass transfer rates. The fact that essentially all of them are nevertheless transient shows that irradiation is weaker, as a direct consequence of the fundamental black-hole property - the lack of a hard stellar surface., Comment: 13 pages (including 3 figures); accepted for publication in ApJ

Nonlinear time-dependent calculations have been carried out in order to study the evolution of the thermal instability for optically thick, transonic, slim accretion discs around black holes. In the present calculations we have investigated only the original version of the slim disc model with low viscosity. This version does not yet contain several important non-local effects but our aim is to use it as a standard reference against which to compare the results from forthcoming studies in which additional effects will be added one by one thus giving a systematic way of understanding the contribution from each of them. A range of results for different cases is presented showing a number of interesting features. One preliminary conclusion is that the stabilizing effect of advection seems not to be strong enough in these low viscosity models to allow for limit cycle behaviour to occur., Comment: 23 pages, Latex, 11 Postscript figures, accepted by MNRAS

We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate, the spectra of the high-accretion-rate disks extend into the soft X-rays. A comparison with observations shows that such disks could produce the soft X-ray excesses claimed in some AGNs. We show also that the sequence of our models with fixed mass and different accretion rates can explain the time evolution of the observed spectra in Fairall 9., Comment: LaTeX file, 21 pages, 21 figures (on request from ewa@sissa.it) accepted by Astrophysical Journal

We consider the current flow in an accreting rotating neutron star (X-ray pulsator) and propose that the active magnetosphere is limited by the Alfv\`en surface. Formulae for the luminosity and torque due to currents circulating in the neutron star are given, and astrophysical consequences for systems like Her X-1 and LMXRB are shortly discussed., Comment: LATEX file, 8 pages