Search

Streaming instability is hypothesized to be triggered at particular protoplanetary disk locations where the volume density of the solid particles is enriched comparable to that of the gas. A ring of planetesimals thus forms when this condition is fulfilled locally. These planetesimals collide with each other and accrete inward drifting pebbles from the outer disk to further increase masses. We investigate the growth of the planetesimals that form in a ring-belt at various disk radii. Their initial mass distributions are calculated based on the formula summarized from the streaming instability simulations. We simulate the subsequent dynamical evolution of the planetesimals with a protoplanetary disk model based either on the minimum mass solar nebula (MMSN) or on the Toomre stability criterion. For the MMSN model, both pebble accretion and planetesimal accretion are efficient at a close-in orbit of $0.3$ AU, resulting in the emergence of several super-Earth mass planets after $1$ Myr. For comparison, only the most massive planetesimals undergo substantial mass growth when they are born at $r{=}3$ AU, while the planetesimals at $r{=}30$ AU experience little or no growth. On the other hand, in the denser Toomre disk, the most massive forming planets can reach Earth mass at $t{=}1$ Myr and reach a mass between that of Neptune and that of Saturn within $3$ Myr at $30$ AU and $100$ AU. Both the pebble and planetesimal accretion rate decrease with disk radial distance. Nevertheless, planetesimal accretion is less pronounced than pebble accretion at more distant disk regions. Taken together, the planets acquire higher masses when the disk has a higher gas density, a higher pebble flux, and/or a lower Stokes number of pebbles., 11 pages, 7 figures, Accepted for publication in A&A

We have mapped the number density and mean vertical velocity of the Milky Way's stellar disk out to roughly two kiloparsecs from the Sun using Gaia Data Release 3 (DR3) and complementary photo-astrometric distance information from StarHorse. For the number counts, we carefully masked spatial regions that are compromised by open clusters, great distances, or dust extinction and used Gaussian processes to arrive at a smooth, non-parametric estimate for the underlying number density field. We find that the number density and velocity fields depart significantly from an axisymmetric and mirror-symmetric model. These departures, which include projections of the Gaia phase-space spiral, signal the presence of local disturbances in the disk. We identify two features that are present in both stellar number density and mean vertical velocity. One of these features appears to be associated with the Local Spiral Arm. It is most prominent at small heights and is largely symmetric across the mid-plane of the disk. The density and velocity field perturbations are phase-shifted by roughly a quarter wavelength, suggesting a breathing mode that is propagating in the direction of Galactic longitude $l\sim 270$ deg. The second feature is a gradient in the stellar number density and mean vertical velocity with respect to Galactocentric radius. This feature, which extends across the entire region of our analysis, may be associated with the extension of the Galactic warp into the Solar neighbourhood in combination with more localised bending waves., 13 pages, 11 figures, appendices add 12 pages, 11 figures; accepted for publication in A&A

Thermal elastic-plastic stresses and strains have been obtained for rotating annular disk by using finite difference method with Von-Mises? yield criterion and non-linear strain hardening measure. The compressibility of the disk is assumed to be varying in the radial direction. From the numerical results, we can conclude that thermal rotating disk made of functionally graded material whose thickness decreases exponentially and density increases exponentially with non-linear strain hardening measure (m = 0.2) is on the safe side of the design as compared to disk made of homogenous material. This is because of the reason that circumferential stress is less for functionally graded disk as compared to homogenous disk. Also, plastic strains are high for functionally graded disk as compared to homogenous disk. It means that disk made of functionally graded material reduces the possibility of fracture at the bore as compared to the disk made of homogeneous material which leads to the idea of stress saving.

Aims. We aim to demonstrate the scientific potential of the Gaia Early Data Release 3 (EDR3) for the study of different aspects of the Milky Way structure and evolution and we provide, at the same time, a description of several practical aspects of the data and examples of their usage., Methods. We used astrometric positions, proper motions, parallaxes, and photometry from EDR3 to select different populations and components and to calculate the distances and velocities in the direction of the anticentre. In this direction, the Gaia astrometric data alone enable the calculation of the vertical and azimuthal velocities; also, the extinction is relatively low compared to other directions in the Galactic plane. We then explore the disturbances of the current disc, the spatial and kinematical distributions of early accreted versus in situ stars, the structures in the outer parts of the disc, and the orbits of open clusters Berkeley 29 and Saurer 1., Results. With the improved astrometry and photometry of EDR3, we find that: (i) the dynamics of the Galactic disc are very complex with oscillations in the median rotation and vertical velocities as a function of radius, vertical asymmetries, and new correlations, including a bimodality with disc stars with large angular momentum moving vertically upwards from below the plane, and disc stars with slightly lower angular momentum moving preferentially downwards; (ii) we resolve the kinematic substructure (diagonal ridges) in the outer parts of the disc for the first time; (iii) the red sequence that has been associated with the proto-Galactic disc that was present at the time of the merger with Gaia-Enceladus-Sausage is currently radially concentrated up to around 14 kpc, while the blue sequence that has been associated with debris of the satellite extends beyond that; (iv) there are density structures in the outer disc, both above and below the plane, most probably related to Monoceros, the Anticentre Stream, and TriAnd, for which the Gaia data allow an exhaustive selection of candidate member stars and dynamical study; and (v) the open clusters Berkeley 29 and Saurer 1, despite being located at large distances from the Galactic centre, are on nearly circular disc-like orbits., Conclusions. Even with our simple preliminary exploration of the Gaia EDR3, we demonstrate how, once again, these data from the European Space Agency are crucial for our understanding of the different pieces of our Galaxy and their connection to its global structure and history.

Aims: Our goal is to investigate how the strength of episodic accretion bursts depends on eccentricity. Methods: We investigate the binary trigger hypothesis in longer-period (>20yr) binaries by carrying out three-dimensional magnetohydrodynamical (MHD) simulations of the formation of low-mass binary stars down to final separations of $\sim$10$\au$, including the effects of gas turbulence and magnetic fields. We ran two simulations with an initial turbulent gas core of one solar mass each and two different initial turbulent Mach numbers, M = 0.1 and M=0.2, for 6500yr after protostar formation. Results: We observe bursts of accretion at periastron during the early stages when the eccentricity of the binary system is still high. We find that this correlation between bursts of accretion and passing periastron breaks down at later stages because of the gradual circularisation of the orbits. For eccentricities greater than e=0.2, we observe episodic accretion triggered near periastron. However, we do not find any strong correlation between the strength of episodic accretion and eccentricity. The strength of accretion is defined as the ratio of the burst accretion rate to the quiescent accretion rate.We determine that accretion events are likely triggered by torques between the rotation of the circumstellar disc and the approaching binary stars. We compare our results with observational data of episodic accretion in short-period binaries and find good agreement between our simulations and the observations. Conclusions: We conclude that episodic accretion is a universal mechanism operating in eccentric young binary-star systems, independent of separation, and it should be observable in long-period binaries as well as in short-period binaries. Nevertheless, the strength depends on the torques and hence the separation at periastron., Accepted to A&A. 14 pages, 8 figures (plus 3 in appendix)

We investigate directly imaging exoplanets around eclipsing binaries, using the eclipse as a natural tool for dimming the binary and thus increasing the planet to star brightness contrast. At eclipse, the binary becomes point-like, making coronagraphy possible. We select binaries where the planet-star contrast would be boosted by $>10\times$ during eclipse, making it possible to detect a planet that is $\gtrsim10\times$ fainter or in a star system that is $\sim2$-$3\times$ more massive than otherwise. Our approach will yield insights into planet occurrence rates around binaries versus individual stars. We consider both self-luminous (SL) and reflected light (RL) planets. In the SL case, we select binaries whose age is young enough so that an orbiting SL planet would remain luminous; in U Cep and AC Sct, respectively, our method is sensitive to SL planets of $\sim$4.5$M_J$ and $\sim$9$M_J$ with current ground- or near-future space-based instruments, and $\sim$1.5$M_J$ and $\sim$6$M_J$ with future ground-based observatories. In the RL case, there are three nearby ($\lesssim50$ pc) systems -- V1412 Aql, RR Cae, RT Pic -- around which a Jupiter-like planet at a planet-star separation of $\gtrsim20$ mas might be imaged with future ground- and space-based coronagraphs. A Venus-like planet at the same distance might be detectable around RR Cae and RT Pic. A habitable Earth-like planet represents a challenge; while the planet-star contrast at eclipse and planet flux are accessible with a 6-8m space telescope, the planet-star separation is 1/3 - 1/4 of the angular separation limit of modern coronagraphy., 23 pages, 9 figures, 3 tables. To be published in AJ. Typos corrected, full decimal digits in equation 1 coefficients added

Several open questions on galaxy formation and evolution have their roots in the lack of a universal star formation law, that could univocally link the gas properties, e.g. its density, to the star formation rate (SFR) density. In a recent paper, we used a sample of nearby disc galaxies to infer the volumetric star formation (VSF) law, a tight correlation between the gas and the SFR volume densities derived under the assumption of hydrostatic equilibrium for the gas disc. However, due to the dearth of information about the vertical distribution of the SFR in these galaxies, we could not find a unique slope for the VSF law, but two alternative values. In this paper, we use the scale height of the SFR density distribution in our Galaxy adopting classical Cepheids (age$\lesssim 200$ Myr) as tracers of star formation. We show that this latter is fully compatible with the flaring scale height expected from gas in hydrostatic equilibrium. These scale heights allowed us to convert the observed surface densities of gas and SFR into the corresponding volume densities. Our results indicate that the VSF law $\rho_\mathrm{SFR} \propto \rho_\mathrm{gas}^\alpha$ with $\alpha \approx 2$ is valid in the Milky Way as well as in nearby disc galaxies., Comment: 9 pages, 4 figures, + 4 pages of Appendix, accepted by A&A

Observations of cold molecular gas reservoirs are critical for understanding the shutdown of star formation in massive galaxies. While dust continuum is an efficient and affordable tracer, this method relies upon the assumption of a "normal" molecular-gas to dust mass ratio, $\delta_{\mathrm{GDR}}$, typically of order one hundred. Recent null detections of quiescent galaxies in deep dust continuum observations support a picture where the cold gas and dust has been rapidly depleted or expelled. In this work, we present another viable explanation: a significant fraction of galaxies with low star formation per unit stellar mass are predicted to have extreme $\delta_{\mathrm{GDR}}$ ratios. We show that simulated massive quiescent galaxies at $0 < z < 3$ in the \textsc{simba} cosmological simulations have $\delta_{\mathrm{GDR}}$ values that extend $>$4 orders of magnitude. The dust in most simulated quiescent galaxies is destroyed significantly more rapidly than the molecular gas depletes, and cannot be replenished. The transition from star-forming to quiescent halts dust formation via star formation processes, with dust subsequently destroyed by supernova shocks and thermal sputtering of dust grains embedded in hot plasma. After this point, the dust growth rate in the models is not sufficient to overcome the loss of $>$3 orders of magnitude in dust mass to return to normal values of $\delta_{\mathrm{GDR}}$ despite having high metallicity. Our results indicate that it is not straight forward to use a single observational indicator to robustly pre-select exotic versus normal ratios. These simulations make strong predictions that can be tested with millimeter facilities., Comment: 9 pages, 5 figures, version accepted for publication in the Astrophysical Journal Letters

We present deep near-infrared photometry and spectroscopy of the globular cluster 2MASS-GC03 projected in the Galactic disk using MMIRS on the Clay telescope (Las Campanas Observatory) and VISTA Variables in the Via Lactea survey (VVV) data. Most probable cluster member candidates were identified from near-infrared photometry. Out of ten candidates that were followed-up spectroscopically, five have properties of cluster members, from which we calculate = -0.9 +- 0.2 and a radial velocity of v_r > = -78 +- 12km/s. A distance of 10.8kpc is estimated from 3 likely RRLyrae members. Given that the cluster is currently at a distance of 4.2kpc from the Galactic center, the cluster's long survival time of an estimated 11.3 +- 1.2Gyr strengthens the case for its globular-cluster nature. The cluster has a hint of elongation in the direction of the Galactic center., Comment: Accepted for publication, MNRAS

We present observations of the nearby (D$\sim$100\,pc) Herbig star HD~163296 taken with the vortex coronograph at Keck/NIRC2 in the L' band (3.7~$\mu$m), to search for planetary mass companions in the ringed disc surrounding this pre-main sequence star. The images reveal an arc-like region of scattered light from the disc surface layers that is likely associated with the first bright ring detected with ALMA in the $\lambda$=1.3mm dust continuum at $\sim$65~au. We also detect a point-like source at $\sim$0\farcs5 projected separation in the North-East direction, close to the inner edge of the second gap in the millimetre images. Comparing the point source photometry with the atmospheric emission models of non-accreting giant planets, we obtain a mass of 6--7~M$_J$ for a putative protoplanet, assuming a system age of 5~Myr. Based on the contrast at a 95\% level of completeness calculated on the emission-free regions of our images, we set upper limits for the masses of giant planets of 8--15~M$_J$, 4.5--6.5~M$_J$ and 2.5-4.0~M$_J$ at the locations of the first, second and third gap in the millimetre dust continuum, respectively. Further deep, high resolution thermal IR imaging of the HD~163296 system are warranted, to confirm the presence and nature of the point source and to better understand the structure of the dust disc., Comment: 9 pages, 6 figures. Accepted by MNRAS

Kiefer et al. reported the detection of infalling Ca II absorption in HD 172555, a member of the beta Pictoris Moving Group (beta PMG). We obtained HST Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph spectroscopy of this star at 2 epochs separated by a week, and we report the discovery of infalling gas in resonant transitions of Si III and IV, C II and IV, and neutral atomic oxygen. Variable absorption is seen in the C II transitions and is optically thick, with covering factors which range between 58% and 68%, similar to features seen in beta Pictoris. The O I spectral profile resembles that of C II, showing a strong low-velocity absorption to + 50 km s(-1) in the single spectral segment obtained during orbital night, as well as what may be higher-velocity absorption. Studies of the mid-IR spectrum of this system have suggested the presence of silica. The O I absorption differs from that seen in Si III, suggesting that the neutral atomic oxygen does not originate in SiO dissociation products but in a more volatile parent molecule such as CO.

The internal evolution of disk galaxies like the Milky Way are driven by non-axisymmetries (bars) and the implied angular momentum transfer of the matter; baryons are essentially driven inwards to build a more concentrated disk. This mass concentration may lead to the decoupling of a secondary bar, since the orbit precessing frequency is then much enhanced. Vertical resonances with the bar will form a box/peanut bulge in a Gyr time-scale. Gas flows due to gravity torques can lead to a young nuclear disk forming stars, revealed by a sigma-drop in velocity dispersion. These gas flows moderated by feedback produce intermittent accretion of the super-massive black hole, and cycles of AGN activity. The fountain effect due to nuclear star formation may lead to inclined, and even polar nuclear disks., Comment: 4 pages, 2 figures, SF2A 2017, C. Reyle, P. Di Matteo, F. Herpin, E. Lagadec, A. Lancon F. Royer (eds)

In Athanassoula et al. (2016), we used high resolution N-body hydrodynamical simulations to model the major merger between two disc galaxies with a hot gaseous halo each, and showed that the remnant is a spiral galaxy. The two discs are destroyed by the collision, but after the merger, accretion from the surrounding gaseous halo allows the building of a new disc in the remnant galaxy. In Peschken et al. (2017), we used these simulations to study the radial surface density profiles of the remnant galaxies with downbending profiles (type II), i.e. composed of an inner and an outer exponential disc separated by a break. We analyzed the effect of angular momentum on these profiles, and found that the inner and outer disc scalelengths, as well as the break radius, all increase linearly with the total angular momentum of the initial merging system. Following the angular momentum redistribution in our simulations, we find that the disc angular momentum is acquired via accretion from the gaseous halo. Furthermore, high angular momentum systems give more angular momentum to their discs, which affects directly their radial density profile.

Models of debris disk morphology are often focused on the effects of a planet orbiting interior to or within the disk. Nonetheless, an exterior planetary-mass perturber can also excite eccentricities in a debris disk, via Laplace-Lagrange secular perturbations in the coplanar case or Kozai-Lidov perturbations for mutually inclined companions and disks. HD 106906 is an ideal example of such a system, as it harbors a confirmed exterior 11 M_Jup companion at a projected separation of 650 au outside a resolved, asymmetric disk. We use collisional and dynamical simulations to investigate the interactions between the disk and the companion, and to use the disk's observed morphology to place constraints on the companion's orbit. We conclude that the disk's observed morphology is consistent with perturbations from the observed exterior companion. Generalizing this result, we suggest that exterior perturbers, as well as interior planets, should be considered when investigating the cause of observed asymmetries in a debris disk., 14 pages, 4 figures, accepted to ApJ Letters

The formation of planets in compact or highly eccentric binaries and the migration of hot Jupiters are two outstanding problems in planet formation. The hot Jupiter $\tau$ Boo Ab orbits the primary star in the long-period ($P \gtrsim 1000\,$yr), highly eccentric ($e \sim 0.9$) binary $\tau$ Bo\"otis. Due to the long orbital period, the orbit of the stellar binary is poorly constrained. Here we aim to constrain the orbit of the stellar binary $\tau$ Boo AB in order to investigate the formation and migration history of the system. The mutual orbital inclination of the stellar companion and the hot Jupiter has important implications for planet migration. The binary eccentricity and periastron distance are important for understanding the conditions under which $\tau$ Boo Ab formed. We combine more than 150 years of astrometric data with twenty-five years of high precision radial velocities. The combination of sky-projected and line-of-sight measurements places tight constraints on the orbital inclination, eccentricity, and periastron distance of $\tau$ Boo AB. We find an orbital inclination of $47.2^{+2.7}_{-3.7}\,$deg, periastron distance of $28.3^{+2.3}_{-3.0}\,$au and eccentricity of $0.87^{+0.04}_{-0.03}$. We find that the orbital inclinations of $\tau$ Boo Ab and $\tau$ Boo B, as well as the stellar spin-axis of $\tau$ Boo A coincide at $\sim \! 45$ degrees, a result consistent with the assumption of a well-aligned, coplanar system. The likely aligned, coplanar configuration suggests planetary migration within a well-aligned protoplanetary disc. Due to the high eccentricity and small periastron distance of $\tau$ Boo B, the protoplanetary disc was tidally truncated at $\approx \! 6\,$au. We suggest that $\tau$ Boo Ab formed near the edge of the truncated disc and migrated inwards with high eccentricity due to spiral waves generated by the stellar companion., Comment: Accepted for publication in A&A. 9 pages, 4 figures, 2 tables

We analyse the kinematics of $\sim 2000$ giant stars in the direction of the Galactic bulge, extracted from the Gaia-ESO survey in the region $-10^\circ \lesssim \ell \lesssim 10^\circ$ and $-11^\circ \lesssim b \lesssim -3^\circ$. We find distinct kinematic trends in the metal rich ($\mathrm{[M/H]}>0$) and metal poor ($\mathrm{[M/H]}, Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the Gaia-ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012.” The results presented here benefit from discussions held during the Gaia-ESO workshops and conferences supported by the European Science Foundation (ESF) through the GREAT Research Network Programme., This is the author accepted manuscript. The final version is available from IOP Publishing via http://dx.doi.org/10.3847/2041-8205/824/2/L29

According to the giant impact hypothesis, the Moon formed from a disk created by an impact between the proto-Earth and an impactor. Three major models for this hypothesis are (a) standard model: a Mars-sized impactor hit the Earth; (b) fast-spinning Earth model: a small impactor hit a rapidly spinning Earth; and (c) sub-Earths model: two half-Earth sized planets collided. These models have been supported because they can explain several observed features of the Earth-Moon system, such as the Moon's mass, angular momentum, and potentially geochemical measurements that suggest that the Earth and Moon have nearly identical isotopic ratios. However, it is uncertain if these models are consistent with other geochemical constraints. For example, isotopic measurements of Earth's rocks indicate that the early Earth's mantle was chemically heterogeneous and this signature was preserved for billions of years. However, it is not clear if the giant impact hypothesis is consistent with this geochemical constraint because the giant impact could have been so energetic that the Earth's mantle could have completely mixed and homogenized. Furthermore, the fraction of the Earth's mantle that became molten by the impact is not well known, even though it is known to have significantly affected the subsequent evolutions of the Earth's interior and atmosphere. Additionally, the water and volatile content of the Moon may have an important implication for the lunar origin. Since the Moon-forming disk formed through a giant impact process, it must have been hot and partially vaporized. From this disk, a significant amount of water and volatiles may have escaped to space. However, this idea may contradict recent geochemical studies that indicate that Moon may not be as dry as previously thought. Furthermore, moons of other planets may provide comprehensive pictures of the origin and evolution of the moons. For example, the Pluto-Charon system could have formed via a giant impact. Recent studies also suggest that Mars' satellites Phobos and Deimos could have formed in the same way. In contrast, satellites around the gas giants could have formed in situ from their proto-satellite disks and by gravitational capture. One of the Saturnian satellites, Enceladus has very unique geological and dynamical features. Cassini spacecraft observed that water plumes are emanating from cracks on the surface (so-called ``tiger stripes''). These plumes may originate from a few kilometer deep subsurface liquid ocean. Along the cracks, strong thermal emissions have been observed and these are thought to be related to the plume activities, but the connection has been still unclear. In my Ph.D. thesis, I aim to understand the origin and evolution of the Earth, Moon, and the Saturnian moon Enceladus. In order to understand the initial state of the Earth's mantle and the Moon-forming disk, I perform giant impact simulations with a method called smoothed particle hydrodynamics (SPH). I show that the Earth's mantle becomes mostly molten by the impact and that the mantle remains unmixed in (a), but it may be at least partly mixed in (b) and (c). Therefore, (a) is most consistent with the preservation of the mantle heterogeneity. As for the Moon-forming disk, my calculations show that the disk of the standard model has a relatively low temperature (up to 4500 K) and low vapor mass fraction (~20-30%) while the disk formed by other models could be much hotter (6000-7000 K) and has a higher vapor mass fraction (80-90 %). Furthermore, I investigate the structure of the Moon-forming disk and estimated the extent of water escape. I show that escape is in the diffusion-limited regime and water escape from the disk to space is minor. This result could explain recent measurements on lunar water abundance. Furthermore, I develop a dynamical model that includes flow dynamics and flow-ice wall interaction that explains the Enceladus plume mass flux, heat flux, and several observed signatures. These studies will deepen comprehensive understanding of the origin of the moons in the solar system.

The purpose of this paper is to calculate the minimal disk which contains all the eigenvalues of matrices. In order to obtain the minimal disk which contains all the eigenvalues of matrices, an algorithm is presented.

Open clusters (OCs) are routinely used as reliable tracers of the properties and evolution of the galactic disk, as they can be found at all galactocentric distances and span a wide range of ages. More than 3000 OCs are listed in catalogues, although few have been studied in details. The goal of this work is to study the properties of open clusters. This work was conducted in the framework of the Gaia-ESO Survey(GES). GES is an observational campaign targeting more than 100,000 stars in all major components of the Milky Way, including stars in a hundred open clusters. It uses the FLAMES instrument at the VLT to produce high and medium-resolution spectra, which provide accurate radial velocities and individual elemental abundances. In this framework, the goals of the Thesis are: - to study the properties of OCs and of their stars from photometry and spectroscopy to derive their age, the extinction and the chemical composition of the stars, to begin to build a homogeneous data base. Looking at literature data it is clear that different authors derive substantially different chemical compositions, and in general OC parameters. - the study of OCs and their chemical homogeneity (or inhomogeneity) can cast light on what is still an open issue: the presence of multiple populations in clusters. While multiple generations of stars are now ubiquitously found in globular clusters in the Milky Way and in the Magellanic Clouds, they have not been yet detected in open clusters. What is the main driver of the self-pollution process? - to study the cluster formation process. All, or at least a significant fraction of stars form in clusters. Young clusters (a few Myr) can retain some of the properties of the molecular cloud they originate from and give us insight about the cluster assembly process. The first GES data release contains data for the young OC Gamma Velorum, in which two (dynamically different) subpopulations have been identified. This cluster can serve as a test case for star formation mechanisms. - the study of the OCs can shed light on the disk properties, in particular on the presence of a chemical gradient. Studying the distribution of chemical elements across the Galactic disk has been a central question in astronomy for the past decade. The exact shape of this metallicity gradient, revealed by various tracers such as Cepheids, Planetary Nebulae or HII regions is not quite clear. OCs suggest a flattening of the gradient in the outer disk. Here I will investigate the issue using the GES data set. Methods: The data analysis of the GES is a complex task carried out by different groups. When dealing with a huge quantity of astronomical data, it is essential to have tools that economically process large amounts of information and produce repeatable results. As part of the GES I developed an automated tool to measure the EWs in spectra of FGK stars in a fully automatic way. This tool, called DAOSPEC Option Optimizer pipeline (DOOp), uses DAOSPEC and optimizes its key parameters in order to make the measurements as robust as possible. This tool was widely tested on synthetic and observational spectra. Stellar parameters and elemental abundances are derived with the code FAMA developed with the aim of dealing with large batches of stars. FAMA uses the widely used software MOOG and optimizes stellar parameters in order to satisfy the excitation and ionization balance, following the classical equivalent width procedure. The construction of a metallicity scale, based on high-quality spectra of benchmark stars is fundamental to interpret the spectroscopic results in the context of the Galaxy formation and evolution. We take advantage of the variety of analysis methods represented within the GES collaboration, including DOOp+ FAMA in order to produce a homogeneous metallicity scale. Those reference stars can be used to assess the precision and accuracy of a given method. Results: Using archival photometric data, I presents an in-depth study of NGC 6705 and Trumpler 20 for which I derive ages of 250-320 Myr and 1.25-1.66 Gyr (respectively). I take advantage of the wide field covered by the VPHAS data to map extinction using field red clump stars as tracers and show that the core of NGC 6705 is situated in an area of low reddening, but the outskirts are seen projected on areas of various levels of extinction, making it difficult to estimate accurately the stellar density, and making estimates of the tidal radius of NGC 6705 unreliable. Differential extinction in the region of Trumpler 20 is estimated using main sequence stars. I find a mass of 6850 +/- 1900 M for NGC 6705, from the luminosity function although strong mass segregation and and patchy extinction in the outskirts of the cluster make this number a lower limit, and derive a total mass of 6700 +/- 800 M for Tr20. These estimates are in agreement with the mass derived from the velocity dispersion. We find that the apparently broad turn-off region in the CMD of Tr20 can be explained by differential reddening, but the origin of its notorious extended red clump remains unclear. I presents a membership determination and spectroscopic study for NGC 6705, Trumpler 20, and NGC 4815. We find mean iron abundances [Fe/H]=0.10, 0.17 and 0.16 dex for these three inner disk clusters. I show that within the uncertainties, they are chemically homogeneous clusters. This result adds weight to the hypothesis that only clusters above the mass of 4x10^4 solar masses may be massive enough to form multiple populations. I study the star-forming region Gamma Velorum, which has recently been shown to be made up of two different populations (A and B) with different kinematical signatures. N-Body simulations suggest that one of the populations (population B) is unbound and rapidly expanding. In addition, the B population is less dense and less concentrated. It also appears marginally more fractal. The supervirial state of population B indicates that star formation efficiency was lower than in A, or that the parent molecular cloud was dispersed very quickly. The presence of multiple clustering of stars as in this region is not peculiar, and is often observed in large star forming regions in the Milky Way and in the Magellanic Clouds. I present abundance determinations from archived Keck (HIRES) data for four outer disk clusters (Berkeley 22, Berkeley 29, Berkeley 66, and Saurer 1). From their α-element abundances I see no clear sign that these objects are associated with the galactic anticentre structure. Complementing our sample of clusters with preliminary data from the newer GES releases, I confirm that the metallicity gradient is steep in the inner disk and much shallower (or flat) in the outer disk. The presence of a bimodal gradient has important implications on the possible scenarios of galaxy formation and evolution and is predicted by a variety of theoretical models. It comes as a natural outcome of the models involving radial migration.

We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda Galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. Initial results from this survey include: the likely non-existence of Andromeda VIII; a universal PN luminosity function, with the exception of a small amount of obscuration, and a small offset in normalization between bulge and disk components; very faint kinematically-selected photometry implying no cut-off in the disk to beyond 4 scalelengths and no halo population in excess of the bulge out to 10 effective bulge radii; disk kinematics that show significant dispersion and asymmetric drift out to large radii, consistent with a warm flaring disk; and no sign of any variation in kinematics with PN luminosity, suggesting that PNe arise from a fairly uniform population of old stars., Comment: Accepted by MNRAS. 23 pages, 37 figures. A full resolution version is available at http://www.astro.rug.nl/~pns/pns_pub.html

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution.

We describe the astrometric calibration of the Gemini NICI Planet-Finding Campaign. The Campaign requires a relative astrometric accuracy of $\approx$ 20 mas across multi-year timescales in order to distinguish true companions from background stars by verifying common proper motion and parallax with their parent stars. The calibration consists of a correction for instrumental optical image distortion, plus on-sky imaging of astrometric fields to determine the pixel scale and image orientation. We achieve an accuracy of $\lesssim 7$ mas between the center and edge of the 18$''$ NICI field, meeting the 20 mas requirement. Most of the Campaign data in the Gemini Science Archive are accurate to this level but we identify a number of anomalies and present methods to correct the errors., 4 figures. Accepted for publication in PASP

Recent observations have revealed streams of gas and stars in the halo of the Milky Way(1-3) that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies(2,4), and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory(5). Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.

The location of the gas responsible for the absorption-line system toward the nucleus of Centaurus A is a puzzle. It is generally accepted that the line features close to the systemic velocity originate in the disk. The redshifted line features in particular, however, are usually thought to be due to gas close to the nucleus or even falling toward it. We present new (CO)-C-12 (1-0), HCO+ (1-0) and HCN (1-0) absorption-line measurements, as well as an alternative interpretation of the line system. Previous papers have demonstrated that the distribution of line emission of the inner molecular and outer atomic hydrogen disk can be accounted for by a system of tilted rings with varying inclination. Using the same model, but assuming that corotating absorbing gas is located at high altitudes above the disk, one can account for all major features of the absorption-line system. In this model, the absorption takes place in high-altitude clouds that are up to about 160 pc above the molecular disk of Centaurus A at radii between 1.7 and 1.9 kpc, accounting for the two strongest central line features. In our model, the redshifted line features are due to gas associated with disk material up to about 300 pc above the disk at radii of 0.4-0.6 kpc orbiting in the non-spherically symmetric potential of the Centaurus A galaxy. In this model, the systemic velocity is at 546 km s(-1), which is the velocity of the sharp spectral feature about 6 km s(-1) to the blue of the deepest absorption line. Our new model provides a natural explanation for the general structure of the complex absorption-line system, based on a tilted-ring model that already explains the disk line emission. In this model, no significant absorbing gas component closer than 200 pc to the nonthermal radio continuum nucleus is required, and the peculiar velocity structure of the absorption-line system is due to the kinematics of the molecular gas disk.

e present resolved 200 mu m images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the 200 mu m observations with IRAS 60 mu m and 100 mu m data, we find that cold dust becomes more dominant at larger radii. We infer a grain temperature of 18-21 K for this cold component i.e. about 10 K lower than the warm dust detected by IRAS in external spirals. This value is close to theoretical predictions in the literature based on heating by the general interstellar radiation field. A comparison of the 200 mu m images with complementary B-band data also shows that the cold dust is radially more extensive than the stars. The gas-to-dust ratio of external spirals, derived using IRAS fluxes, has been claimed to be about an order of magnitude higher than the value infered for the Galaxy. By analysing the 200 mu m data for our sample, we derive a mean gas-to-dust ratio of similar to 225 which is close to the value in the solar neighborhood (150-300). It is likely that IRAS may have 'overlooked' the vast majority of grains residing in spiral disks.

We present photometry and spectroscopy of the A0IVe star 68 Ophiuchi, for which Vidal-Madjar et al. (1993) suspected the presence of a circumstellar disc of material similar to that found around beta Pic. The UV and optical continuum agrees well with a Kurucz model with parameters that are consistent with the spectral type. 68 Oph is detected at 12 and 25 mu m by IRAS, but the data show no strong evidence for thermal emission from cool circumstellar dust grains. Observations at 450 mu m and 800 mu m also indicate that there is no strong dust component. Intermediate and high resolution optical and near-IR spectra of 68 Oph confirm that the star is surrounded by a gaseous disc, seen in H alpha and Br alpha. The Ca II lambda 3933.7 line shows a broad photospheric absorption line with two narrow, blue-shifted absorption lines near line centre, one of which is also found in Na I. These absorption lines are probably of interstellar origin. We conclude that 68 Oph does not show properties observed in other proto-planetary sytems, such as red-shifted absorption features in spectral lines and a strong dust continuum.

We present the analysis of the bright X-ray binary 4U 1820-30, based mainly on XMM-Newton-RGS data, but using complementary data from XMM-Epic, Integral, and Chandra-HETG, to investigate different aspects of the source. The broad band continuum is well fitted by a classical combination of black body and Comptonized emission. The continuum shape and the high flux of the source (L/L_Edd\sim0.16) are consistent with a "high state" of the source. We do not find significant evidence of iron emission at energies >=6.4 keV. The soft X-ray spectrum contain a number of absorption features. Here we focus on the cold-mildly ionized gas. The neutral gas column density is N_H\sim1.63x10^21 cm^-2. The detailed study of the oxygen and iron edge reveals that those elements are depleted, defined here as the ratio between dust and the total ISM cold phase, by a factor 0.20\pm0.02 and 0.87\pm0.14, respectively. Using the available dust models, the best fit points to a major contribution of Mg-rich silicates, with metallic iron inclusion. Although we find that a large fraction of Fe is in dust form, the fit shows that Fe-rich silicates are disfavored. The measured Mg:Fe ratio is 2.0\pm0.3. Interestingly, this modeling may point to a well studied dust constituent (GEMS), sometimes proposed as a silicate constituent in our Galaxy. Oxygen and iron are found to be slightly over- and under-abundant, respectively (1.23 and 0.85 times the solar value) along this line of sight. We also report the detection of two absorption lines, tentatively identified as part of an outflow of mildly ionized gas (��\sim-0.5) at a velocity of \sim1200 km/s., 13 pages, 10 figures. Accepted for publication in Astronomy and Astrophysics

We study the far-infrared emission from the nearby spiral galaxy M33 in order to investigate the dust physical properties such as the temperature and the luminosity density across the galaxy. Taking advantage of the unique wavelength coverage (100, 160, 250, 350 and 500 micron) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 micron and Spitzer-MIPS 24 and 70 micron data, we construct temperature and luminosity density maps by fitting two modified blackbodies of a fixed emissivity index of 1.5. We find that the 'cool' dust grains are heated at temperatures between 11 and 28 K with the lowest temperatures found in the outskirts of the galaxy and the highest ones in the center and in the bright HII regions. The infrared/submillimeter total luminosity (5 - 1000 micron) is estimated to be 1.9x10^9 Lsun. 59% of the total luminosity of the galaxy is produced by the 'cool' dust grains (~15 K) while the rest 41% is produced by 'warm' dust grains (~55 K). The ratio of the cool-to-warm dust luminosity is close to unity (within the computed uncertainties), throughout the galaxy, with the luminosity of the cool dust being slightly enhanced in the center of the galaxy. Decomposing the emission of the dust into two components (one emitted by the diffuse disk of the galaxy and one emitted by the spiral arms) we find that the fraction of the emission in the disk in the mid-infrared (24 micron) is 21%, while it gradually rises up to 57% in the submillimeter (500 micron). We find that the bulk of the luminosity comes from the spiral arm network that produces 70% of the total luminosity of the galaxy with the rest coming from the diffuse dust disk. The 'cool' dust inside the disk is heated at a narrow range of temperatures between 18 and 15 K (going from the center to the outer parts of the galaxy)., Comment: 12 pages, 14 figures, accepted for publication in A&A

We investigate the dependence of the evolution of neutron stars with fallback disks on the strength of the magnetic dipole field of the star. Using the same model as employed by Ertan et al. (2009), we obtain model curves for different dipole fields showing that the neutron stars with magnetic dipole fields greater than ∼1013 G on the surface of the star are not likely to become anomalous X‐ray pulsars (AXPs) and soft gamma‐ray repeaters (SGRs). Other sources with conventional dipole fields evolve into the AXP phase if their disk can penetrate the light cylinder. The upper limits to the observed periods of AXP and SGRs could be understood if the disk becomes inactive below a low temperature around 100 K. We summarize our present and earlier results indicated by the evolutionary model curves of these sources with an emphasis on the importance of the minimum disk temperature and the X‐ray irradiation in the long‐term evolution of AXPs and SGRs with fallback disks.We investigate the dependence of the evolution of neutron stars with fallback disks on the strength of the magnetic dipole field of the star. Using the same model as employed by Ertan et al. (2009), we obtain model curves for different dipole fields showing that the neutron stars with magnetic dipole fields greater than ∼1013 G on the surface of the star are not likely to become anomalous X‐ray pulsars (AXPs) and soft gamma‐ray repeaters (SGRs). Other sources with conventional dipole fields evolve into the AXP phase if their disk can penetrate the light cylinder. The upper limits to the observed periods of AXP and SGRs could be understood if the disk becomes inactive below a low temperature around 100 K. We summarize our present and earlier results indicated by the evolutionary model curves of these sources with an emphasis on the importance of the minimum disk temperature and the X‐ray irradiation in the long‐term evolution of AXPs and SGRs with fallback disks.

We present a model of chemical and spectrophotometric evolution of disk galaxies based on a stochastic self-propagating star formation scenario. The model incorporates galaxy formation through the process of accretion, chemical and photometric evolution treatment, based on simple stellar populations (SSP), and parameterized gas dynamics inside the model. The model reproduces observational data of a late-type spiral galaxy M33 reasonably well. Promising test results prove the applicability of the model and the adequate accuracy for the interpretation of disk galaxy properties., 9 pages, 6 figures

Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution.Aims. We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase.Methods. We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at gimel similar to 60-190 mu m using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory.Results. We detect [OI] 63 mu m emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 mu m. Extensive models are presented in associated papers.

In this paper we discuss the large-scale structures (> 500 pc) of the neutral atomic hydrogen in the disk of nearby galaxies, with an emphasis on finding evidence for neutral gas features in the disk - halo interface. Most nearby galaxies studied in detail appear to have hole and shell type structures in their HI disks and several show evidence for gas at peculiar velocities, probably examples of gas streaming out of the disk into the halo. Not all of the observed phenomena can be explained by stellar winds and supernova explosions and other explanations will briefly be discussed.

We report on observations of circumstellar disks around young stars that have been obtained with the MIDI instrument, which is mounted on the VLT Interferometer and operates in the 10 micrometer atmospheric window. The maximum spatial resolution of 5 milli-arcsec corresponds to sub-AU scales at the distance to nearby star formation regions. Thus, we can study the disks on the spatial scales at which important processes occur, such as accretion, dust processing, and planet formation. The main results obtained so far can be summarized as follows: 1. The measured interferometric visibilities are in good qualitative agreement with those predicted by models of circumstellar disks. In particular, a predicted correlation between the strength of the far-infrared excess and the spatial structure of the disk is confirmed by direct measurements; 2. In several objects strong evidence for deviations from circular symmetry is present, indicating that an inclined disk is indeed the dominant component seen in the mid-infrared; 3. The dust properties are not uniform over the disk, but are instead a strong function of distance to the central star. The dust in the innermost disk regions is observed to be more ``processed'' than the dust further out, both in Herbig Ae star disks and in those around T-Tauri stars., Comment: 19 pages, 12 figures, invited talk at SPIE conference "Astronomical telescopes and instrumentation", Orlando, may 2006

We present a study of the HI distribution and dynamics of the nearby spiral galaxy NGC5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to 3.5 R_25, and shows a pronounced warp that starts at the end of the bright optical disk (R_25= 11.6 kpc). This very extended warp has large-scale symmetry, which along with the rotation period of its outer parts (~1.5 Gyr at 40 kpc), suggests a long-lived phenomenon. The rotation curve rises steeply in the central parts up to the maximum velocity (v_max ~ 206 km/s). Beyond the bright stellar disk (R_25), it shows a decline of about 25 km/s and then remains flat out to the last measured point. The standard analysis with luminous and dark matter components shows the dynamical importance of the disk. The best fit to the rotation curve is obtained with a ``maximum disk''. Less satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to the mass-to-light ratio in F band of the disk. Such a ``minimum disk'' contributes about 60% of the observed maximum rotational velocity. NGC5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and kinematics of the gas. The tilted ring analysis of the velocity field led us to adopt different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries disappear, both at the same time. These results point at two different dynamical regimes: an inner region dominated by the stellar disk and an outer one, dominated by a dark matter halo offset with respect to the disk., Comment: Accepted for publication in A&A. Minor corrections

We present 21-cm HI line and optical R-band observations for a sample of 26 edge-on galaxies. The HI observations were obtained with the Westerbork Synthesis Radio Telescope, and are part of the WHISP database (Westerbork HI Survey of Spiral and Irregular Galaxies). We present HI maps, optical images, and radial HI density profiles. We have also derived the rotation curves and studied the warping and lopsidedness of the HI disks. 20 out of the 26 galaxies of our sample are warped, confirming that warping of the HI disks is a very common phenomenon in disk galaxies. Indeed, we find that all galaxies that have an extended HI disk with respect to the optical are warped. The warping usually starts around the edge of the optical disk. The degree of warping varies considerably from galaxy to galaxy. Furthermore, many warps are asymmetric, as they show up in only one side of the disk or exhibit large differences in amplitude in the approaching and receding sides of the galaxy. These asymmetries are more pronounced in rich environments, which may indicate that tidal interactions are a source of warp asymmetry. A rich environment tends to produce larger warps as well. The presence of lopsidedness seems to be related to the presence of nearby companions., To appear in Astronomy & Astrophysics

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution., 10 pages, 4 figures, Invited talk in "Galaxy Evolution III: From Simple Approaches to Self-Consistent Models" (Kiel, Germany, July 16-20, 2002) eds. G. Hensler et al., in press

We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < |Z| < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars., accepted for publication in A&A

Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes., 7 pages, 2 figures, replaced with text published in Nature

We present photometry and spectroscopy of the A0IVe star 68 Ophiuchi, for which Vidal-Madjar et al. (1993) suspected the presence of a circumstellar disc of material similar to that found around beta Pic. The UV and optical continuum agrees well with a Kurucz model with parameters that are consistent with the spectral type. 68 Oph is detected at 12 and 25 mu m by IRAS, but the data show no strong evidence for thermal emission from cool circumstellar dust grains. Observations at 450 mu m and 800 mu m also indicate that there is no strong dust component. Intermediate and high resolution optical and near-IR spectra of 68 Oph confirm that the star is surrounded by a gaseous disc, seen in H alpha and Br alpha. The Ca II lambda 3933.7 line shows a broad photospheric absorption line with two narrow, blue-shifted absorption lines near line centre, one of which is also found in Na I. These absorption lines are probably of interstellar origin. We conclude that 68 Oph does not show properties observed in other proto-planetary sytems, such as red-shifted absorption features in spectral lines and a strong dust continuum.

In this paper we discuss the large-scale structures (> 500 pc) of the neutral atomic hydrogen in the disk of nearby galaxies, with an emphasis on finding evidence for neutral gas features in the disk - halo interface. Most nearby galaxies studied in detail appear to have hole and shell type structures in their HI disks and several show evidence for gas at peculiar velocities, probably examples of gas streaming out of the disk into the halo. Not all of the observed phenomena can be explained by stellar winds and supernova explosions and other explanations will briefly be discussed.

e present resolved 200 mu m images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the 200 mu m observations with IRAS 60 mu m and 100 mu m data, we find that cold dust becomes more dominant at larger radii. We infer a grain temperature of 18-21 K for this cold component i.e. about 10 K lower than the warm dust detected by IRAS in external spirals. This value is close to theoretical predictions in the literature based on heating by the general interstellar radiation field. A comparison of the 200 mu m images with complementary B-band data also shows that the cold dust is radially more extensive than the stars. The gas-to-dust ratio of external spirals, derived using IRAS fluxes, has been claimed to be about an order of magnitude higher than the value infered for the Galaxy. By analysing the 200 mu m data for our sample, we derive a mean gas-to-dust ratio of similar to 225 which is close to the value in the solar neighborhood (150-300). It is likely that IRAS may have 'overlooked' the vast majority of grains residing in spiral disks.