21,458 research outputs found
Search for associations containing young stars (SACY). V. Is multiplicity universal? Tight multiple systems
Context: Dynamically undisrupted, young populations of stars are crucial to
study the role of multiplicity in relation to star formation. Loose nearby
associations provide us with a great sample of close (150 pc) Pre-Main
Sequence (PMS) stars across the very important age range (5-70 Myr) to
conduct such research.
Aims: We characterize the short period multiplicity fraction of the SACY
(Search for Associations Containing Young stars) accounting for any
identifiable bias in our techniques and present the role of multiplicity
fractions of the SACY sample in the context of star formation.
Methods: Using the cross-correlation technique we identified double-lined
spectroscopic systems (SB2), in addition to this we computed Radial Velocity
(RV) values for our subsample of SACY targets using several epochs of FEROS and
UVES data. These values were used to revise the membership of each association
then combined with archival data to determine significant RV variations across
different data epochs characteristic of multiplicity; single-lined multiple
systems (SB1).
Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no
significant difference between the short period multiplicity fraction
() of the SACY sample and that of nearby star forming regions
(1-2 Myr) and the field (10%) both as a function of
age and as a function of primary mass, , in the ranges [1:200 day] and
[0.08 -].
Conclusions: Our results are consistent with the picture of universal star
formation, when compared to the field and nearby star forming regions (SFRs).
We comment on the implications of the relationship between increasing
multiplicity fraction with primary mass, within the close companion range, in
relation to star formation.Comment: 14 pages, 18 figures, published, A&A
http://dx.doi.org/10.1051/0004-6361/20142385
Symplectic quantization, inequivalent quantum theories, and Heisenberg's principle of uncertainty
We analyze the quantum dynamics of the non-relativistic two-dimensional
isotropic harmonic oscillator in Heisenberg's picture. Such a system is taken
as toy model to analyze some of the various quantum theories that can be built
from the application of Dirac's quantization rule to the various symplectic
structures recently reported for this classical system. It is pointed out that
that these quantum theories are inequivalent in the sense that the mean values
for the operators (observables) associated with the same physical classical
observable do not agree with each other. The inequivalence does not arise from
ambiguities in the ordering of operators but from the fact of having several
symplectic structures defined with respect to the same set of coordinates. It
is also shown that the uncertainty relations between the fundamental
observables depend on the particular quantum theory chosen. It is important to
emphasize that these (somehow paradoxical) results emerge from the combination
of two paradigms: Dirac's quantization rule and the usual Copenhagen
interpretation of quantum mechanics.Comment: 8 pages, LaTex file, no figures. Accepted for publication in Phys.
Rev.
Chandra observations of the millisecond X-ray pulsar IGR J00291+5934 in quiescence
In this Paper we report on our analysis of three Chandra observations of the
accretion-powered millisecond X-ray pulsar IGR J00291+5934 obtained during the
late stages of the 2004 outburst. We also report the serendipitous detection of
the source in quiescence by ROSAT during MJD 48830-48839. The detected 0.3-10
keV source count rates varied significantly between the Chandra observations
from (7.2+-1.2)x10^-3, (6.8+-0.9)x10^-3, and (1.4+-0.1)x10^-2 counts per second
for the 1st, 2nd, and 3rd Chandra observation, on MJD 53371.88, 53383.99, and
53407.57, respectively. The count rate for the 3rd observation is 2.0+-0.4
times as high as that of the average of the first two observations. The
unabsorbed 0.5-10 keV source flux for the best-fit power-law model to the
source spectrum was (7.9+-2.5)x10^-14, (7.3+-2.0)x10^-14, and
(1.17+-0.22)x10^-13 erg cm^-2 s^-1 for the 1st, 2nd, and 3rd Chandra
observation, respectively. We find that this source flux is consistent with
that found by ROSAT [~(5.4+-2.4)x10^-14 erg cm^-2 s^-1]. Under the assumption
that the interstellar extinction, N_H, does not vary between the observations,
we find that the blackbody temperature during the 2nd Chandra observation is
significantly higher than that during the 1st and 3rd observation. Furthermore,
the effective temperature of the neutron star derived from fitting an absorbed
blackbody or neutron star atmosphere model to the data is rather high in
comparison with many other neutron star soft X-ray transients in quiescence,
even during the 1st and 3rd observation. If we assume that the source quiescent
luminosity is similar to that measured for two other accretion powered
millisecond pulsars in quiescence, the distance to IGR J00291+5934 is 2.6-3.6
kpc.Comment: 7 pages, 3 Figures, accepted for publication in MNRA
Solitary Waves Under the Competition of Linear and Nonlinear Periodic Potentials
In this paper, we study the competition of linear and nonlinear lattices and
its effects on the stability and dynamics of bright solitary waves. We consider
both lattices in a perturbative framework, whereby the technique of Hamiltonian
perturbation theory can be used to obtain information about the existence of
solutions, and the same approach, as well as eigenvalue count considerations,
can be used to obtained detailed conditions about their linear stability. We
find that the analytical results are in very good agreement with our numerical
findings and can also be used to predict features of the dynamical evolution of
such solutions.Comment: 13 pages, 4 figure
Search for associations containing young stars (SACY). VI. Is multiplicity universal? Stellar multiplicity in the range 3-1000 au from adaptive-optics observations
Context. Young loose nearby associations are unique samples of close (<150
pc), young (approx 5-100 Myr) pre-main sequence (PMS) stars. A significant
number of members of these associations have been identified in the SACY
collaboration. We can use the proximity and youth of these members to
investigate key ingredients in star formation processes, such as multiplicity.
Aims. We present the statistics of identified multiple systems from 113
confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and
physical separation distributions in a consistent parameter space, and compare
our results to other PMS populations and the field.
Methods. We have obtained adaptive-optics assisted near-infrared observations
with NACO (ESO/VLT) and IRCAL (Lick Observatory) for at least one epoch of all
113 SACY members. We have identified multiple systems using co-moving
proper-motion analysis and using contamination estimates. We have explored
ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1],
respectively.
Results. We have identified 31 multiple systems (28 binaries and 3 triples).
We derive a multiplicity frequency (MF) of MF_(3-1000au)=28.4 +4.7, -3.9% and a
triple frequency (TF) of TF_(3-1000au)=2.8 +2.5, -0.8% in the separation range
of 3-1000 au. We do not find any evidence for an increase in the MF with
primary mass. The estimated mass-ratio of our statistical sample (with
power-law index gamma=-0.04 +/- 0.14) is consistent with a flat distribution
(gamma = 0).
Conclusions. We show further similarities (but also hints of discrepancies)
between SACY and the Taurus region: flat mass-ratio distributions and
statistically similar MF and TF values. We also compared the SACY sample to the
field (in the separation range of 19-100 au), finding that the two
distributions are indistinguishable, suggesting a similar formation mechanism.Comment: 16 Pages, accepted in A&A 28 May 201
Search for associations containing young stars (SACY) VII. New stellar and substellar candidate members in the young associations
The young associations offer us one of the best opportunities to study the
properties of young stellar and substellar objects and to directly image
planets thanks to their proximity (200 pc) and age (5-150 Myr).
However, many previous works have been limited to identifying the brighter,
more active members (1 M) owing to photometric survey
sensitivities limiting the detections of lower mass objects. We search the
field of view of 542 previously identified members of the young associations to
identify wide or extremely wide (1000-100,000 au in physical separation)
companions. We combined 2MASS near-infrared photometry (, , ) with
proper motion values (from UCAC4, PPMXL, NOMAD) to identify companions in the
field of view of known members. We collated further photometry and spectroscopy
from the literature and conducted our own high-resolution spectroscopic
observations for a subsample of candidate members. This complementary
information allowed us to assess the efficiency of our method. We identified 84
targets (45: 0.2-1.3 M, 17: 0.08-0.2 M, 22: 0.08 M)
in our analysis, ten of which have been identified from spectroscopic analysis
in previous young association works. For 33 of these 84, we were able to
further assess their membership using a variety of properties (X-ray emission,
UV excess, H, lithium and K I equivalent widths, radial velocities,
and CaH indices). We derive a success rate of 76-88% for this technique based
on the consistency of these properties. Once confirmed, the targets identified
in this work would significantly improve our knowledge of the lower mass end of
the young associations. Additionally, these targets would make an ideal new
sample for the identification and study of planets around nearby young stars.Comment: 28 pages, 24 figures, accepted in A&
Variability of Optical Counterparts in the Chandra Galactic Bulge Survey
We present optical lightcurves of variable stars consistent with the
positions of X-ray sources identified with the Chandra X-ray Observatory for
the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on
the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on
timescales from hr to 8 days over the of the X-ray survey
containing sources from the initial GBS catalog. Among the lightcurve
morphologies we identify are flickering in interacting binaries, eclipsing
sources, dwarf nova outbursts, ellipsoidal variations, long period variables,
spotted stars, and flare stars. of X-ray sources have at least one
potential optical counterpart. of these candidate counterparts are
detectably variable; a much greater fraction than expected for randomly
selected field stars, which suggests that most of these variables are real
counterparts. We discuss individual sources of interest, provide variability
information on candidate counterparts, and discuss the characteristics of the
variable population.Comment: Accepted for publication in the Astrophysical Journal Supplement
Microquasar models for 3EG J1828+0142 and 3EG J1735-1500
Microquasars are promising candidates to emit high-energy gamma-rays.
Moreover, statistical studies show that variable EGRET sources at low galactic
latitudes could be associated with the inner spiral arms. The variable nature
and the location in the Galaxy of the high-mass microquasars, concentrated in
the galactic plane and within 55 degrees from the galactic center, give to
these objects the status of likely counterparts of the variable low-latitude
EGRET sources. We consider in this work the two most variable EGRET sources at
low-latitudes: 3EG J1828+0142 and 3EG J1735-1500, proposing a microquasar model
to explain the EGRET data in consistency with the observations at lower
energies (from radio frequencies to soft gamma-rays) within the EGRET error
box.Comment: (1)Universitat de Barcelona, (2)Instituto Argentino de
Radioastronomia (3) Facultad de Ciencias Astronomicas y Geofisicas
(4)Lawrence Livermore National Laboratory 6 pages, 2 figures. Presented as a
poster at the V Microquasar Workshop, Beijing, June 2004. Accepted for
publication in the Chinese Journal of Astronomy & Astrophysic
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