13,695 research outputs found
PPl 15: The First Brown Dwarf Spectroscopic Binary
PPl 15 is the first object to have been confirmed as a brown dwarf by the
lithium test (in 1995), though its inferred mass was very close to the
substellar limit. It is a member of the Pleiades open cluster. Its position in
a cluster color-magnitude diagram suggested that it might be binary, and
preliminary indications that it is a double-lined spectroscopic binary were
reported by us in 1997. Here we report on the results of a consecutive week of
Keck HIRES observations of this system, which yield its orbit. It has a period
of about 5.8 days, and an eccentricity of 0.4+/-0.05. The rotation of the stars
is slow for this class of objects. Because the system luminosity is divided
between 2 objects with a mass ratio of 0.85, this renders each of them an
incontrovertible brown dwarf, with masses between 60-70 jupiters. We show that
component B is a little redder than A by studying their wavelength-dependent
line ratios, and that this variation is compatible with the mass ratio. We
confirm that the system has lithium, but cannot support the original conclusion
that it is depleted (which would be surprising, given the new masses). This is
a system of very close objects which, if they had combined, would have produced
a low mass star. We discuss the implications of this discovery for the theories
of binary formation and formation of very low mass objects.Comment: Latex, 18 pages, 4 figures, submitted to Astron.
Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482
We present a near-infrared spectrum of the hot (
55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion
is responsible for the previously recognised infrared photometric excess. We
compare spectra of M and L dwarfs, combined with an appropriate white dwarf
model, to the data to constrain the spectral type of the secondary. We find
that uncertainties in the 2MASS photometry of the white dwarf prevent us
from distinguishing whether the secondary is stellar or substellar, and assign
a spectral type of L01 (M9-L1).Therefore, this is the hottest and youngest
( yr) DA white dwarf with a possible brown dwarf companion.Comment: 5 pages, 2 figures, accepted by MNRA
Extracting predictive models from marked-p free-text documents at the Royal Botanic Gardens, Kew, London
In this paper we explore the combination of text-mining, un-supervised and supervised learning to extract predictive models from a corpus of digitised historical floras. These documents deal with the nomenclature, geographical distribution, ecology and comparative morphology of the species of a region. Here we exploit the fact that portions of text in the floras are marked up as different types of trait and habitat. We infer models from these different texts that can predict different habitat-types based upon the traits of plant species. We also integrate plant taxonomy data in order to assist in the validation of our models. We have shown that by clustering text describing the habitat of different floras we can identify a number of important and distinct habitats that are associated with particular families of species along with statistical significance scores. We have also shown that by using these discovered habitat-types as labels for supervised learning we can predict them based upon a subset of traits, identified using wrapper feature selection
Imaging Transport Resonances in the Quantum Hall Effect
We use a scanning capacitance probe to image transport in the quantum Hall
system. Applying a DC bias voltage to the tip induces a ring-shaped
incompressible strip (IS) in the 2D electron system (2DES) that moves with the
tip. At certain tip positions, short-range disorder in the 2DES creates a
quantum dot island in the IS. These islands enable resonant tunneling across
the IS, enhancing its conductance by more than four orders of magnitude. The
images provide a quantitative measure of disorder and suggest resonant
tunneling as the primary mechanism for transport across ISs.Comment: 4 pages, 4 figures, submitted to PRL. For movies and additional
infomation, see http://electron.mit.edu/scanning/; Added scale bars to
images, revised discussion of figure 3, other minor change
Improvement of Renormalization-Scale Uncertainties Within Empirical Determinations of the b-Quark Mass
Accurate determinations of the MS-bar b-quark mass from
experimental data currently contain three
comparable sources of uncertainty; the experimental uncertainty from moments of
this cross-section, the uncertainty associated with , and the
theoretical uncertainty associated with the renormalization scale. Through
resummation of all logarithmic terms explicitly determined in the perturbative
series by the renormalization-group (RG) equation, it is shown that the
renormalization-scale dependence is virtually eliminated as a source of
theoretical uncertainty in . This resummation also reduces the
estimated effect of higher-loop perturbative contributions, further reducing
the theoretical uncertainties in . Furthermore, such resummation
techniques improve the agreement between the values of the MS-bar b-quark mass
extracted from the various moments of [], obviating the need to
choose an optimummoment for determining . Resummation techniques are
also shown to reduce renormalization-scale dependence in the relation between
b-quark MS-bar and pole mass and in the relation between the pole and
mass.Comment: 19 pages, latex2e, 6 eps figures contained in latex file. Errors
corrected in equations (20)--(22
Brown Dwarfs in the Pleiades Cluster. III. A deep IZ survey
We present the results of a deep CCD-based IZ photometric survey of a ~1 sq.
deg area in the central region of the Pleiades Galactic open cluster. The
magnitude coverage of our survey (from I~17.5 down to 22) allows us to detect
substellar candidates with masses between 0.075 and 0.03 Msol. Details of the
photometric reduction and selection criteria are given. Finder charts prepared
from the I-band images are provided.Comment: 11 pages with 8 figures, 4 of them are finder charts given in gif
format. Accepted for publication in A&AS. Also available at
http://www.iac.es/publicaciones/preprints.htm
Accumulation horizons and period-adding in optically injected semiconductor lasers
We study the hierarchical structuring of islands of stable periodic
oscillations inside chaotic regions in phase diagrams of single-mode
semiconductor lasers with optical injection. Phase diagrams display remarkable
{\it accumulation horizons}: boundaries formed by the accumulation of infinite
cascades of self-similar islands of periodic solutions of ever-increasing
period. Each cascade follows a specific period-adding route. The riddling of
chaotic laser phases by such networks of periodic solutions may compromise
applications operating with chaotic signals such as e.g. secure communications.Comment: 4 pages, 4 figures, laser phase diagrams, to appear in Phys. Rev. E,
vol. 7
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