1,376 research outputs found
On the frequency of close binary systems among very low-mass stars and brown dwarfs
We have used Monte Carlo simulation techniques and published radial velocity
surveys to constrain the frequency of very low-mass star (VLMS) and brown dwarf
(BD) binary systems and their separation (a) distribution. Gaussian models for
the separation distribution with a peak at a = 4 au and 0.6 =< sigma(log(a/au))
=< 1.0 correctly predict the number of observed binaries, yielding a close
(a<2.6 au) binary frequency of 17-30 per cent and an overall VLMS/BD binary
frequency of 32-45 per cent. We find that the available N-body models of
VLMS/BD formation from dynamically decaying protostellar multiple systems are
excluded at >99 per cent confidence because they predict too few close binary
VLMS/BDs. The large number of close binaries and high overall binary frequency
are also very inconsistent with recent smoothed particle hydrodynamical
modelling and argue against a dynamical origin for VLMS/BDs.Comment: Accepted for publication in MNRAS letters. 5 pages, 2 figure
Pre-main-sequence Lithium Depletion
In this review I briefly discuss the theory of pre-main-sequence (PMS) Li
depletion in low-mass (0.075<M<1.2 Msun) stars and highlight those uncertain
parameters which lead to substantial differences in model predictions. I then
summarise observations of PMS stars in very young open clusters, clusters that
have just reached the ZAMS and briefly highlight recent developments in the
observation of Li in very low-mass PMS stars.Comment: 8 pages, invited review at "Chemical abundances and mixing in stars
in the Milky Way and its satellites", eds. L. Pasquini, S. Randich. ESO
Astrophysics Symposium (Springer-Verlag
The NASA/MSFC global reference atmospheric model: 1990 version (GRAM-90). Part 1: Technical/users manual
A technical description of the NASA/MSFC Global Reference Atmospheric Model 1990 version (GRAM-90) is presented with emphasis on the additions and new user's manual descriptions of the program operation aspects of the revised model. Some sample results for the new middle atmosphere section and comparisons with results from a three dimensional circulation model are provided. A programmer's manual with more details for those wishing to make their own GRAM program adaptations is also presented
The NASA/MSFC global reference atmospheric model: 1990 version (GRAM-90). Part 2: Program/data listings
A new (1990) version of the NASA/MSFC Global Reference Atmospheric Model (GRAM-90) was completed and the program and key data base listing are presented. GRAM-90 incorporate extensive new data, mostly collected under the Middle Atmosphere Program, to produce a completely revised middle atmosphere model (20 to 120 km). At altitudes greater than 120 km, GRAM-90 uses the NASA Marshall Engineering Thermosphere model. Complete listings of all program and major data bases are presented. Also, a test case is included
Inelastic contribution of the resistivity in the hidden order in URu2Si2
In the hidden order of URu2Si2 the resistivity at very low temperature shows
no T^2 behavior above the transition to superconductivity. However, when
entering the antiferromagnetic phase, the Fermi liquid behavior is recovered.
We discuss the change of the inelastic term when entering the AF phase with
pressure considering the temperature dependence of the Grueneisen parameter at
ambient pressure and the influence of superconductivity by an extrapolation of
high field data.Comment: 5 pages, 2 figures, SCES conference proceedin
On the stability of gas bubbles in sea water
Laboratory experiments have been performed to determine the rate of solution and the change in composition of air bubbles in stirred sea water as a function of depth (0-200 m) and temperature (1-27° C). At each depth the shrinking of the diameter is nearly linear. This rate of solution increases with increasing pressure up to a limiting value which is almost reached at a depth of 20 m. The rate of disappearance of a bubble is only imperceptibly influenced by temperature. When a bubble of air dissolves at a given depth it becomes richer in nitrogen until a final constant composition is reached. This final nitrogen percentage increases with increasing depth. Equations have been developed, based on the theory of diffusion, which satisfactorily account for the observed facts
Multiple kinematical populations in Vela OB2 from Gaia DR1 data
Context. Recent results using radial-velocity measurements from the Gaia-ESO Survey have led to the discovery of multiple kinematic populations across the Vela OB2 association. We present here a proper-motion study of the same region.
Aims. Our aim is to test whether or not the radial-velocity populations have a counterpart in proper-motion space, and if so, how the two sets of kinematical data complement each other.
Methods. This work is based on parallaxes and proper motions from the TGAS catalogue, as part of Gaia DR1.
Results. Two distinct proper-motion populations are found dispersed across ~ 5 degrees (or ~ 30 pc at their likely distances). Their detailed correspondence to the radial-velocity populations could not be tested because of the paucity of common objects. However, compelling indications are found that one of the new proper-motion populations consists mostly of members of the young cluster NGC 2547, and the other is related to the 2 Vel cluster. Constraints on the age of the two populations, both of which appear to be only 1
Elements in Scenario-Based Simulation Associated with Nursing Studentsâ Self-Confidence and Satisfaction: A Cross-Sectional Study
Aim: To identify elements in scenarioâbased simulation associated with nursing students' satisfaction with the simulation activity and selfâconfidence in managing the simulated patient situation. The study will provide insight to improve the use of simulation as a learning strategy.
Design: A crossâsectional study.
Method: The Student Satisfaction and SelfâConfidence in Learning scale was used as the outcome measure to identify associations with elements of the Simulation Design Scale and the Educational Practices Questionnaire scale after scenarioâbased simulation using patient simulators. Firstâyear nursing students at a university college in Norway (N = 202) were invited to participate and (N = 187) responded to the questionnaires.
Results: The mean scores for selfâconfidence and satisfaction were 4.16 and 4.57, respectively. In the final multiple linear regression analysis, active learning was associated with satisfaction with the simulation activity, while clear objectives and active learning were associated with selfâconfidence in managing the simulated patient situation.publishedVersio
A New Association of Post-T Tauri Stars Near The Sun
Observing ROSAT sources in 20 x 25 deg centered at the high latitude active
star ER Eri, we found evidences for a new young nearby association (~30Myr
at~60pc), the Horologium Association (HorA), formed by at least 10 probable and
6 possible members, some being Post-T Tauri stars. We examine several
requirements that characterize a young association and they, together, create a
strong evidence for the reality of the HorA. In fact, the Li line intensities
are between those of the oldest classical T Tauri stars and the ones of the
Local Association stars. The space velocities of the HorA relative to the Sun,
U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those of
the Local Association. We suggest that some hotter and non-X-ray active stars,
with similar space velocities, could be massive members of the HorA, among
them, the nearby Be star Achernar. The maximum of the mass distribution
function of the HorA is around 0.8 solar masses. At its distance, the projected
size of the HorA, ~50 pc, would be larger than our surveyed area and many other
members could have been missed. We also observed 3 control regions, two at
northern and southern galactic latitudes and a third one in the known TW Hya
Association (TWA), and the properties and distribution of their young stars
strengthen the reality of the HorA. Contrary to the TWA, the only known
binaries in the HorA are 2 very wide systems. The HorA is much more isolated
from clouds and older than the TWA and could give some clues about the lifetime
of the disks around T Tauri stars. Actually, none of the proposed members is an
IRAS source indicating an advanced stage of the evolution of their accreting
disks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.
Are There Age Spreads in Star Forming Regions?
A luminosity spread at a given effective temperature is ubiquitously seen in
the Hertzsprung-Russell (HR) diagrams of young star forming regions and often
interpreted in terms of a prolonged period (>=10 Myr) of star formation. I
review the evidence that the observed luminosity spreads are genuine and not
caused by astrophysical sources of scatter. I then address whether the
luminosity spreads necessarily imply large age spreads, by comparing HR diagram
ages with ages from independent clocks such as stellar rotation rate, the
presence of circumstellar material and lithium depletion. I argue that whilst
there probably is a true luminosity dispersion, there is little evidence to
support age spreads larger than a few Myr. This paradox could be resolved by
brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of
Large Surveys, 8 page
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