15,020 research outputs found

    A NICMOS Direct Imaging Search for Giant Planets around the Single White Dwarfs in the Hyades

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    We report preliminary results from our search for massive giant planets (6-12 Jupiter masses) around the known seven single white dwarfs in the Hyades cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had progenitor masses of about 3 solar masses, and massive gaseous giant planets should have formed in the massive circumstellar disks around these ex-Herbig A0 stars, probably at orbital separations similar or slightly larger than that of Jupiter. Such planets would have survived the post-Main-Sequence mass loss of the parent star and would have migrated outward adiabatically to a distance of about 25 AU. At the distance of the Hyades (45 pc) this corresponds to an angular separation of 0.5 arcsec. J and H magnitudes of these giants are in the range of 20.5-23.3 mag, which can be resolved with NICMOS. The achieved sensitivities and contrast ratios agree well with simulations. Preliminary evaluation of the NICMOS data set did not reveal any evidence for neither planetary mass companions with masses down to about 10 Jupiter masses nor brown dwarfs around any of the seven white dwarfs for separations larger than 0.5 arcsec.Comment: 14th European Workshop on White Dwarf

    Dynamical stability analysis of the HD202206 system and constraints to the planetary orbits

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    Long-term precise Doppler measurements with the CORALIE spectrograph revealed the presence of two massive companions to the solar-type star HD202206. Although the three-body fit of the system is unstable, it was shown that a 5:1 mean motion resonance exists close to the best fit, where the system is stable. We present here an extensive dynamical study of the HD202206 system aiming at constraining the inclinations of the two known companions, from which we derive possible ranges of value for the companion masses. We study the long term stability of the system in a small neighborhood of the best fit using Laskar's frequency map analysis. We also introduce a numerical method based on frequency analysis to determine the center of libration mode inside a mean motion resonance. We find that acceptable coplanar configurations are limited to inclinations to the line of sight between 30 and 90 degrees. This limits the masses of both companions to roughly twice the minimum. Non coplanar configurations are possible for a wide range of mutual inclinations from 0 to 90 degrees, although ΔΩ=0[π]\Delta\Omega = 0 [\pi] configurations seem to be favored. We also confirm the 5:1 mean motion resonance to be most likely. In the coplanar edge-on case, we provide a very good stable solution in the resonance, whose χ2\chi^2 does not differ significantly from the best fit. Using our method to determine the center of libration, we further refine this solution to obtain an orbit with a very low amplitude of libration, as we expect dissipative effects to have dampened the libration.Comment: 14 pages, 18 figure

    Orbits and masses in the young triple system TWA 5

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    We aim to improve the orbital elements and determine the individual masses of the components in the triple system TWA 5. Five new relative astrometric positions in the H band were recorded with the adaptive optics system at the Very Large Telescope (VLT). We combine them with data from the literature and a measurement in the Ks band. We derive an improved fit for the orbit of TWA 5Aa-b around each other. Furthermore, we use the third component, TWA 5B, as an astrometric reference to determine the motion of Aa and Ab around their center of mass and compute their mass ratio. We find an orbital period of 6.03+/-0.01 years and a semi-major axis of 63.7+/-0.2 mas (3.2+/-0.1 AU). With the trigonometric distance of 50.1+/-1.8 pc, this yields a system mass of 0.9+/-0.1 Msun, where the error is dominated by the error of the distance. The dynamical mass agrees with the system mass predicted by a number of theoretical models if we assume that TWA5 is at the young end of the age range of the TW Hydrae association. We find a mass ratio of M_Ab / M_Aa = 1.3 +0.6/-0.4, where the less luminous component Ab is more massive. This result is likely to be a consequence of the large uncertainties due to the limited orbital coverage of the observations.Comment: 9 pages, 8 figures, accepted by Astronomy and Astrophysic

    The acquisition of primary word stress in european portuguese

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    Tese de doutoramento, Linguística (Linguística Portuguesa), Universidade de Lisboa, Faculdade de Letras, 2010In this dissertation, we aim at describing the acquisition path of five Portuguese children, in respect to primary word stress (henceforth word stress). Portuguese word stress has been widely discussed, some authors arguing for a morphology‐based algorithm (Andrade & Laks, 1992; Lee, 1995; Mateus, 1983; Pereira, 1999) and others arguing for a weight‐based one (Bisol, 1992; Wetzels, 2006). Cross‐linguistic information on the acquisition of word stress suggested that children acquiring Germanic languages tend to mirror the trochaic tendency of the target system, as a result of the application of a rhythmic stress algorithm. However, in other languages, conflicting results were attested. Acoustic analyses of early words' production in some languages additionally suggested that, before 2;0, children might not master the acoustic parameters for word stress, and that they might not place stress target‐like. The acoustic analysis conducted on a speech sample of two children in our corpus showed a great initial variability in the production of the acoustic parameters and in the word shapes, both within the same child and between children. The results of the acoustic analysis further confirm that the acoustic parameters to derive word stress might not be mastered until 2;0. The results on the production of stress patterns in the speech of 5 Portuguese monolingual children show that, despite an initial difference (in which we observed a neutral emergence of trochees and iambs, and a tendency for reduplications and epentheses), EP resembles other trochaic languages (namely Dutch and English). The predominant initial production of monosyllables, along with reduplications and epentheses at the left‐edge of words, which heavily contribute to an apparent iambic tendency, lead us to propose a monosyllabic representation for early words in European Portuguese (EP), with an optional syllable at its left‐edge. Later, non‐reduplicated disyllables emerge, without favoring trochaic or iambic words. Afterwards, a preference for trochaic words was noticeable, both through the preferential truncation of iambic words, and through the preferential truncation of weakstrong‐ weak words into trochees, rather than iambs. Trisyllables were the later word shape to be acquired, especially if they had an extrametrical syllable (/SWW/). Finally, we tested the Portuguese children's sensitivity to aspects related to morphology and syllable weight. The results did not bring indisputable evidence for morphological constraints interacting with word stress acquisition, early non‐verbs are produced similarly, irrespective of their morphological constituency and the target stress pattern. Early verb forms emerge later and generally conform to a trochee and a verb theme. The results on weight‐sensitivity showed that words with final heavy stressed syllables (e.g., aMOR 'love') are acquired earlier than words with final heavy unstressed syllables (e.g., LÁpis 'pencil'), and the latter are initially truncated and later produced with a light final syllable. These results suggest that children recognize that stress in /'CV.CVC/ words is not in the final syllable, which, therefore, cannot be heavy. Overall results provide evidence for an algorithm for word stress that is sensitive to the rhythmic properties of the target language, though an interaction with morphology is not categorically rejected.The present work was sponsored by a Doctoral Grant (Ref. SFRH/BD/21696/2005) of the Science and Technology Foundation (FCT), Portugal, funded by the Portuguese Ministry of Science, Technology and Higher Educatio

    Corrections to the Boltzmann mean free path in disordered systems with finite size scatterers

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    The mean free path is an essential characteristic length in disordered systems. In microscopic calculations, it is usually approximated by the classical value of the elastic mean free path. It corresponds to the Boltzmann mean free path when only isotropic scattering is considered, but it is different for anisotropic scattering. In this paper, we work out the corrections to the so called Boltzmann mean free path due to multiple scattering effects on finite size scatterers, in the s-wave approximation, ie. when the elastic mean free path is equivalent to the Boltzmann mean free path. The main result is the expression for the mean free path expanded in powers of the perturbative parameter given by the scatterer density.Comment: 12 page

    Effect of the curing time on the numerical modelling of the behaviour of a chemically stabilised soft soil

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    The ability of the Modified Cam Clay (MCC) model combined with the Von Mises (VM) model, considering the effect of curing time on the enhancement of the mechanical properties of a chemically stabilised soft soil is examined. The evolution of the strength and stiffness over time is based on the results of undrained compressive strength (UCS) tests carried out for different curing times (from 28 days to 360 days). Initially, the MCC/VM models associated with the effect of curing time are validated by CIU triaxial tests, for curing times of 28 and 90 days. Finally, the behaviour of an embankment built on a soft soil reinforced with deep mixing columns is predicted based on the previously validated models. The results show that the increase of curing time of the DMCs slightly decreases the settlement obtained with a curing time of 28 days