On the correlation between metallicity and the presence of giant planets

The correlation between stellar metallicity and the presence of giant planets is well established. It has been tentatively explained by the possible increase of planet formation probability in stellar disks with enhanced amount of metals. However, there are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed towards metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars. None of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk, a property that has been largely neglected until now. We illustrate that a planet-metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of molecular hydrogen in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this region.Comment: Accepted in ApJ

Kinematics of Nearby Subdwarf Stars

We present an analysis of the space motions of 742 subdwarf stars based on the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper motions and Tycho2 proper motions were combined with radial velocities and metallicities from CLLA. The kinematical behavior is discussed in particular in relation to their metallicities. The majority of these sample stars have metal abundances of [Fe/H] >-1 and represent the thick disk population. The halo component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity and a radially elongated velocity ellipsoid. In the intermediate metallicity range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with disklike kinematics. We interpret this population of stars as a metal-weak thick disk population.Comment: 6 pages, 7 figures, accepted by Astronomy & Astrophysic

Starbursts in multiple galaxy mergers

We numerically investigate stellar and gaseous dynamical evolution of mergers between five identical late-type disk galaxies with the special emphasis on star formation history and chemical evolution of multiple galaxy mergers. We found that multiple encounter and merging can trigger repetitive massive starbursts (typically $\sim$ 100 $M_{\odot}$ ${\rm yr}^{-1}$) owing to the strong tidal disturbance and the resultant gaseous dissipation during merging. The magnitude of the starburst is found to depend on initial virial ratio (i.e., the ratio of total kinematical energy to total potential energy) such that the maximum star formation rate is larger for the merger with smaller virial ratio. Furthermore we found that the time interval between the epochs of the triggered starbursts is longer for the merger with the larger virial ratio. The remnant of a multiple galaxy merger with massive starbursts is found to have metal-poor gaseous halo that is formed by tidal stripping during the merging. We accordingly suggest that metal-poor gaseous halo in a field elliptical galaxy is a fossil record of the past multiple merging events for the galaxy.Comment: 23 pages 16 figures,2000,ApJ,545 in press. For all ps figures, see http://newt.phys.unsw.edu.au/~bekki/res.dir/paper.dir/mul.dir/fig.tar.g

Radial mixing and the transition between the thick and thin Galactic discs

The analysis of the kinematics of solar neighbourhood stars shows that the low and high metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high metallicity dispersion in the age-metallicity distribution. Most importantly, it is shown that the population of low metallicity wanderers of the thin disc (-0.7<[Fe/H]<-0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about -0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that 'transition' objects, which have alpha-element abundance intermediate between that of the thick and thin discs, are found in the range [-0.4,-0.2] dex. Once the metal-poor thin disc stars are recognised for what they are - wanderers from the outer thin disc - the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [alpha/Fe] and [Fe/H] . Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [-0.2,+0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.Comment: 11 pages, 12 figures, accepted in MNRA

Lifetimes of tidally limited star clusters with different radii

We study the escape rate, dN/dt, from clusters with different radii in a tidal field using analytical predictions and direct N-body simulations. We find that dN/dt depends on the ratio R=r_h/r_j, where r_h is the half-mass radius and r_j the radius of the zero-velocity surface. For R>0.05, the "tidal regime", there is almost no dependence of dN/dt on R. To first order this is because the fraction of escapers per relaxation time, t_rh, scales approximately as R^1.5, which cancels out the r_h^1.5 term in t_rh. For R<0.05, the "isolated regime", dN/dt scales as R^-1.5. Clusters that start with their initial R, Ri, in the tidal regime dissolve completely in this regime and their t_dis is insensitive to the initial r_h. We predicts that clusters that start with Ri<0.05 always expand to the tidal regime before final dissolution. Their t_dis has a shallower dependence on Ri than what would be expected when t_dis is a constant times t_rh. For realistic values of Ri, the lifetime varies by less than a factor of 1.5 due to changes in Ri. This implies that the "survival" diagram for globular clusters should allow for more small clusters to survive. We note that with our result it is impossible to explain the universal peaked mass function of globular cluster systems by dynamical evolution from a power-law initial mass function, since the peak will be at lower masses in the outer parts of galaxies. Our results finally show that in the tidal regime t_dis scales as N^0.65/w, with w the angular frequency of the cluster in the host galaxy. [ABRIDGED

Tracing the Galactic thick disk to Solar metallicities

We show that the Galactic thick disk reaches at least solar metallicities, and that it experienced strong chemical enrichment during a period of ~3 Gyr, ending around 8-9 Gyr ago. This finding puts further constraints on the relation and interface between the thin and thick disks, and their formation processes. Our results are based on a detailed elemental abundance analysis of 261 kinematically selected F and G dwarf stars in the solar neighborhood: 194 likely members of the thick disk and 67 likely members of the thin disk, in the range -1.3<[Fe/H]<+0.4.Comment: Accepted for publication in ApJ Letter

Formation and evolution of dusty starburst galaxies I. A new method for deriving spectral energy distribution

We present a new numerical code which is designed to derive a spectral energy distribution (SED) for an arbitrary spatial distribution of stellar and gaseous components in a dusty starburst galaxy. We apply a ray tracing method to numerical simulations and thereby estimate extinction and reemission of stellar light by dusty gas in an explicitly self-consistent manner. By using this code, we can investigate simultaneously dynamical and photometric evolution of a dusty galaxy based on stellar and gaseous dynamical simulations. As an example, we demonstrate when and how a major galaxy merger with dusty starburst becomes an ultra-luminous infrared galaxy owing to strong internal dust extinction. We furthermore discuss advantages and disadvantages of the present new code in clarifying the nature and the origin of low and high redshift dusty starburst galaxies.Comment: 44 pages 19 figures (11 color), accepted by Ap

Conjugate Meningococcal Vaccines Development: GSK Biologicals Experience

Meningococcal diseases are serious threats to global health, and new vaccines specifically tailored to meet the age-related needs of various geographical areas are required. This paper focuses on the meningococcal conjugate vaccines developed by GSK Biologicals. Two combined conjugate vaccines were developed to help protect infants and young children in countries where the incidence of meningococcal serogroup C or serogroup C and Y disease is important: Hib-MenC-TT vaccine, which offers protection against Haemophilus influenzae type b and Neisseria meningitidis serogroup C diseases, is approved in several countries; and Hib-MenCY-TT vaccine, which adds N. meningitidis serogroup Y antigen, is currently in the final stages of development. Additionally, a tetravalent conjugate vaccine (MenACWY-TT) designed to help protect against four meningococcal serogroups is presently being evaluated for global use in all age groups. All of these vaccines were shown to be highly immunogenic and to have clinically acceptable safety profiles

Calibration of radii and masses of open clusters with a simulation

Context: A recent new approach to apply a simple dynamical mass estimate of tidally limited star clusters is based on the identification of the tidal radius in a King profile with the dynamical Jacobi radius. The application to an unbiased open cluster catalogue yields significantly higher cluster masses compared to the classical methods. Aims: We quantify the bias in the mass determination as function of projection direction and cluster age by analysing a simulated star cluster. Methods: We use direct $N$-body simulations of a star cluster including stellar evolution in an analytic Milky Way potential and apply a best fit to the projected number density of cluster stars. Results: We obtain significantly overestimated star cluster masses which depend strongly on the viewing direction. The overestimation is typically in the range of 10-50 percent and reaches a factor of 3.5 for young clusters. Mass segregation reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic

High-velocity white dwarfs: thick disk, not dark matter

We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al (2001) in a high-latitude astrometric survey. We argue that the velocity distribution of the majority of the sample is more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars showing retrograde motion gives a local density close to that expected for white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark-matter, heavy halo. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic Disk.Comment: 11 pages, 5 figures; accepted for ApJ, 29 May 200