136 research outputs found
The distribution of nearby stars in phase space mapped by Hipparcos III. Clustering and streaming among A-F type stars
This paper presents the detailed results obtained in the search of density-
velocity inhomogeneities in a volume limited and absolute magnitude limited
sample of A-F type dwarfs within 125 parsecs of the Sun. A 3-D wavelet analysis
is used to extract inhomogeneities, both in the density and velocity
distributions. Having established a real picture of the phase space without
assumption we come back to previously known observational facts regarding
clusters and associations, superclusters. In the 3-D position space, well known
open clusters (Hyades, Coma Berenices and Ursa Major), associations (parts of
the Scorpio-Centaurus association) as well as the Hyades evaporation track are
retrieved. Three new probably loose clusters are identified (Bootes, Pegasus 1
and 2). The sample is relatively well mixed in the position space since less
than 7 per cent of the stars belong to structures with coherent kinematics,
most likely gravitationally bound. In the velocity space, the majority of large
scale velocity structures ( ~ 6.3 ) are Eggen's superclusters
(Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association.
A new supercluster-like structure is found with a mean velocity between the Sun
and Sirius SCl velocities. These structures are all characterized by a large
age range which reflects the overall sample age distribution. Moreover, a few
old streams of ~ 2 Gyr are also extracted at this scale with high U components.
We show that all these large velocity dispersion structures represent 46% of
the sample. Smaller scales (\sigma ~ 3.8 and 2.4 ) reveal that
superclusters are always substructured by 2 or more streams which generally
exhibit a coherent age distribution. Percentages of stars in these streams are
38% and 18% respectively.Comment: 25 pages, Latex, 29 figures, 4 tables to be published in A&A
Supplements Serie
On the vertical equilibrium of the local Galactic disk and the search for disk dark matter
Estimates of the dynamical surface mass density at the solar Galactocentric
distance are commonly derived assuming that the disk is in vertical equilibrium
with the Galactic potential. This assumption has recently been called into
question, based on the claim that the ratio between the kinetic and the
gravitational energy in such solutions is a factor of 3 larger than required if
Virial equilibrium is to hold. Here we show that this ratio between energies
was overerestimated and that the disk solutions are likely to be in Virial
equilibrium after all. We additionally demonstrate, using one-dimensional
numerical simulations, that the disks are indeed in equilibrium. Hence, given
the uncertainties, we find no reason to cast doubt on the steady-state
solutions which are traditionally used to measure the matter density of the
disk.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Does my brain want what my eyes like? - How food liking and choice influence spatio-temporal brain dynamics of food viewing.
How food valuation and decision-making influence the perception of food is of major interest to better understand food intake behavior and, by extension, body weight management. Our study investigated behavioral responses and spatio-temporal brain dynamics by means of visual evoked potentials (VEPs) in twenty-two normal-weight participants when viewing pairs of food photographs. Participants rated how much they liked each food item (valuation) and subsequently chose between the two alternative food images. Unsurprisingly, strongly liked foods were also chosen most often. Foods were rated faster as strongly liked than as mildly liked or disliked irrespective of whether they were subsequently chosen over an alternative. Moreover, strongly liked foods were subsequently also chosen faster than the less liked alternatives. Response times during valuation and choice were positively correlated, but only when foods were liked; the faster participants rated foods as strongly liked, the faster they were in choosing the food item over an alternative. VEP modulations by the level of liking attributed as well as the subsequent choice were found as early as 135-180ms after food image onset. Analyses of neural source activity patterns over this time interval revealed an interaction between liking and the subsequent choice within the insula, dorsal frontal and superior parietal regions. The neural responses to food viewing were found to be modulated by the attributed level of liking only when foods were chosen, not when they were dismissed for an alternative. Therein, the responses to disliked foods were generally greater than those to foods that were liked more. Moreover, the responses to disliked but chosen foods were greater than responses to disliked foods which were subsequently dismissed for an alternative offer. Our findings show that the spatio-temporal brain dynamics to food viewing are immediately influenced both by how much foods are liked and by choices taken on them. These valuation and choice processes are subserved by brain regions involved in salience and reward attribution as well as in decision-making processes, which are likely to influence prospective dietary choices in everyday life
The distribution of nearby stars in phase space mapped by Hipparcos: I. The potential well and local dynamical mass
Hipparcos data provide the first, volume limited and absolute magnitude
limited homogeneous tracer of stellar density and velocity distributions in the
solar neighbourhood. The density of A-type stars more luminous than
can be accurately mapped within a sphere of 125 pc radius, while proper motions
in galactic latitude provide the vertical velocity distribution near the
galactic plane. The potential well across the galactic plane is traced
practically hypothesis-free and model-free. The local dynamical density comes
out as \rho_{0}=0.076 \pm0.015~M_{\sun}~{pc}^{-3} a value well below all
previous determinations leaving no room for any disk shaped component of dark
matter.Comment: 24 pages, 13 figures, latex. To appear in A&A (main journal
Kinematic segregation of nearby disk stars from the Hipparcos database
To better understand our Galaxy, we investigate the pertinency of describing
the sys tem of nearby disk stars in terms of a two-components Schwarzschild
velocity distributio n.Using the proper motion and parallax information of
Hipparcos database, we determine t he parameters characterizing the local
stellar velocity field of a sample of 22000 disk stars. The sample we use is
essentially the same as the one described by the criteria ad opted to study the
LSR and the stream motion of the nearby stellar populationComment: 19 page
On high proper motion white dwarfs from photographic surveys
The interpretation of high proper motion white dwarfs detected by Oppenheimer
et al (2001) was the start of a lively controversy. While the discoverers
identify a large fraction of their findings as dark halo members, others
interpret the same sample as essentially made of disc and/or thick disc stars.
We use the comprehensive description of Galactic stellar populations provided
by the "Besancon" model to produce a realistic simulation of Oppenheimer et al.
data, including all observational selections and calibration biases. The
conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary
hypotheses on the local density and kinematics are sufficient to explain the
observed objects, there is no need for halo white dwarfs. This conclusion is
robust to reasonable changes in model ingredients. The main cause of the
misinterpretation seems to be that the velocity distribution of a proper motion
selected star sample is severely biased in favour of high velocities. This has
been neglected in previous analyses. Obviously this does not prove that no such
objects like halo white dwarfs can exist, but Oppenheimer et al. observations
drive their possible contribution in the dark matter halo down to an extremely
low fraction.Comment: 4 pages, 1 figure, A&A Letters, accepte
Puff-like disposable electronic cigarettes: expert consensus on their regulation using a fast-track Delphi process
Luminous and Dark Matter in the Milky Way
(Abridged) Axisymmetric models of the Milky Way exhibit strong interrelations
between the Galactic constants (R_0 and T_0), the stellar columndensity (S_*)
and the shape of the dark matter (DM) halo. Here we present analytical
relations that can be used to investigate the effects of the uncertain gaseous
velocity dispersion on the HI flaring constraints. The contribution of cosmic
rays and magnetic fields to the pressure gradients is small. A significantly
flattened dark matter halo is only possible if R_0 <~ 6.8 kpc.
If R_0 is larger than ~7 kpc, or T_0 >~ 170 km/s, we can rule out two DM
candidates that require a highly flattened DM halo: 1) decaying massive
neutrinos; and 2) a disk of cold molecular hydrogen.
It is only possible to construct self-consistent models of the Galaxy based
on the IAU-recommended values for the Galactic constants in the unlikely case
that the the stellar columndensity is smaller than ~18 M_sun/pc^2. If we assume
that the halo is oblate and S_* = 35 +/- 5 M_sun/pc^2, R_0 <~ 8 kpc and T_0 <~
200 km/s.
Combining the best kinematical and star-count estimates of S_*, we conclude
that: 25 <~ S_* <~ 45 M_sun/pc^2. Kuijken & Gilmore's (1991) determination of
the columndensity of matter with |z|<=1.1 kpc is robust and valid over a wide
range of Galactic constants.
Our mass models show that the DM density in the Galactic centre is uncertain
by a factor 1000. In the Solar neighbourhood we find: rho_DM ~0.42 GeV/c^2/cm^3
or (11 +/- 5) mM_sun/pc^3 -- roughly 15% of rho_tot.Comment: Accepted for publication in MNRA
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