2,742 research outputs found
Cross-reaction of naturally-produced β-lactamases from Citrobacter farmeri and Citrobacter amalonaticus with immunological detection of CTX-M enzymes.
The NG-Test CTX-M MULTI immunochromatographic assay has been developed to identify CTX-M-type β-lactamases in Enterobacterales, being the most widespread extended-spectrum β-lactamases. We showed here that the chromosomally-encoded ß-lactamases from Citrobacter farmeri and Citrobacter amalonaticus generated false-positive NG-Test CTX-M MULTI results, compromising the specificity of the test
The formation and evolution of binary systems. III. Low-mass binaries in the Praesepe cluster
With the aim of investigating the binary population of the 700 Myr old
Praesepe cluster, we have observed 149 G and K-type cluster members using
adaptive optics. We detected 26 binary systems with an angular separation
ranging from less than 0.08 to 3.3 arcsec (15-600 AU). After correcting for
detection biases, we derive a binary frequency (BF) in the logP (days) range
from 4.4 to 6.9 of 25.3 +/- 5.4%, which is similar to that of field G-type
dwarfs (23.8%, Duquennoy & Mayor 1991). This result, complemented by similar
ones obtained for the 2 Myr old star forming cluster IC 348 (Paper II) and the
120 Myr old Pleiades open cluster (Paper I), indicates that the fraction of
long-period binaries does not significantly evolve over the lifetime of
galactic open clusters. We compare the distribution of cluster binaries to the
binary populations of star forming regions, most notably Orion and Taurus, to
critically review current ideas regarding the binary formation process. We
conclude that it is still unclear whether the lower binary fraction observed in
young clusters compared to T associations is purely the result of the early
dynamical disruption of primordial binaries in dense clusters or whether it
reflects intrinsically different modes of star formation in clusters and
associations. We also note that if Taurus binaries result from the dynamical
decay of small-N protostellar aggregates, one would predict the existence of a
yet to be found dispersed population of mostly single substellar objects in the
Taurus cloud.Comment: 10 pages, 3 figure
The Distribution of Stellar Mass in the Pleiades
As part of an effort to understand the origin of open clusters, we present a
statistical analysis of the currently observed Pleiades. Starting with a
photometric catalog of the cluster, we employ a maximum likelihood technique to
determine the mass distribution of its members, including single stars and both
components of binary systems. We find that the overall binary fraction for
unresolved pairs is 68%. Extrapolating to include resolved systems, this
fraction climbs to about 76%, significantly higher than the accepted field-star
result. Both figures are sensitive to the cluster age, for which we have used
the currently favored value of 125 Myr. The primary and secondary masses within
binaries are correlated, in the sense that their ratios are closer to unity
than under the hypothesis of random pairing. We map out the spatial variation
of the cluster's projected and three-dimensional mass and number densities.
Finally, we revisit the issue of mass segregation in the Pleiades. We find
unambiguous evidence of segregation, and introduce a new method for quantifying
it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa
Photometry and membership for low mass stars in the young open cluster NGC 2516
We present the results of a 0.86 square degree CCD photometric survey of the
open cluster NGC 2516, which has an age of about 150 Myr and may have a much
lower metallicity than the similarly-aged Pleiades. We select a preliminary
catalogue of 1254 low mass (between 0.2 and 2.0M_{sun}) cluster candidates, of
which about 70--80 percent are expected to be genuine. The mass function is
metallicity dependent, but consistent with a Salpeter-like law (dN/dlog M ~
M^{-alpha}, alpha=+1.47+/-0.11 or alpha=+1.67+/-0.11 for solar and half-solar
metallicities) between 0.7 and 3.0M_{sun}. At lower masses (between 0.3 and
0.7M_{sun}) there is a sharp fall in the mass function, with alpha=-0.75+/-0.20
(solar metallicity) or alpha=-0.49+/-0.13 (half-solar metallicity), which seems
inconsistent with the much flatter mass functions seen in the Pleiades and
field populations. We explain this by demonstrating that mass segregation has
been at work in NGC 2516 -- more than half the cluster low mass stars are
expected to lie outside out survey. The mass of NGC 2516 stars with mass
greater than 0.3M_{sun} inside our survey is 950-1200M_{sun}, depending on
metallicity and what corrections are applied for unresolved binarity.
Correcting for mass segregation increases this to ~1240-1560M_{sun}, about
twice the total mass of the Pleiades.Comment: 27 pages, accepted for Astronomy & Astrophysic
On the universal outcome of star-formation: Is there a link between stars and brown-dwarfs?
(abridged) The recent evidence obtained by Briceno et al. that star-formation
in Taurus-Auriga (TA) may be producing significantly fewer brown dwarfs (BDs)
per star than the ONC is investigated by setting up a realistic model stellar
plus BD population and explicitly taking into account a high binary proportion
and dynamical evolution in the TA groups and the ONC. The Briceno result is
reproduced almost exactly despite an identical IMF in both systems because many
BD-BD and star-BD binaries are disrupted in the ONC thus freeing BDs, while the
TA groups remain unevolved dynamically. However, the resulting populations do
not have the correct star-star, star-BD and expecially BD-BD binary properties,
even if a variable BD IMF is allowed for. The conclusion is therefore that BDs
need to be added as a separate population which has its own binary properties.
Such an extra population can have various origins which are briefly discussed
in this contribution but more fully in an associated paper.Comment: MNRAS, accepted, 23 pages, 14 figures, LaTeX, two references adde
On the Rotation of Post-T Tauri Stars in Associations
Nearby associations are excellent objects for the study of the initial spin
up phase during the PMS evolution. An empirical approach is adopted here to
infer their rotations properties and relations to X-ray emission. Three nearby
associations are considered. The TW Hya association with an age of 8 Myr, the
Beta Pictoris moving group with 12 Myr and a combination of Tucana and
Horologium associations (30 Myr). Two low and high rotation modes are
considered for each association with stellar masses less than 1.5M and greater
than 1.5M respectively. We infer representative equatorial rotation velocities
Vo from the observed distribution of projected rotational velocities vsini. A
spin up is found for the high rotation mode, whereas in the low rotation mode
the Vo do not increase significantly. This insufficient increase of Vo is
probably the cause of a decrease of the total mean specific angular momentum
for the low mass stars between 8 and 30 Myr. However, for the high mass stars,
where a sufficient spin up is present, the specific angular momentum is
practically conserved in this same time interval. By supposing that the
distribution of masses of these three associations follows a universal mass
function, we estimate the number of members of these associations that remain
to be detected. The analysis of rotation and stellar masses using luminosity
X-rays indicators present similar properties, as the dependence on stellar mass
and rotation, at least for the younger associations, to those obtained for T
Tauri stars in the ONC. A strong desaturation effect appears at 30 Myr. This
effect seems to be provoked by the minimum configuration of the stellar
convection layers, attained for the first time for the higher mass stars at 30
Myr. The desaturation appears to be independent of rotation at this stage.Comment: 37 pages, 10 figures, 4 tables. Accepted by the Astronomical Journa
The Mass and Structure of the Pleiades Star Cluster from 2MASS
We present the results of a large scale search for new members of the
Pleiades star cluster using 2MASS near-infrared photometry and proper motions
derived from POSS plates digitized by the USNO PMM program. The search extends
to a 10 degree radius around the cluster, well beyond the presumed tidal
radius, to a limiting magnitude of R ~ 20, corresponding to ~ 0.07 M_sun at the
distance and age of the Pleiades. Multi-object spectroscopy for 528 candidates
verifies that the search was extremely effective at detecting cluster stars in
the 1 - 0.1 M_sun mass range using the distribution of H_alpha emission
strengths as an estimate of sample contamination by field stars.
When combined with previously identified, higher mass stars, this search
provides a sensitive measurement of the stellar mass function and dynamical
structure of the Pleiades. The degree of tidal elongation of the halo agrees
well with current N body simulation results. Tidal truncation affects masses
below ~ 1 M_sun. The cluster contains a total mass ~ 800 M_sun. Evidence for a
flatter mass function in the core than in the halo indicates the depletion of
stars in the core with mass less than ~ 0.5 M_sun, relative to stars with mass
\~1 - 0.5 M_sun, and implies a preference for very low mass objects to populate
the halo or escape. The overall mass function is best fitted with a lognormal
form that becomes flat at ~ 0.1 M_sun. Whether sufficient dynamical evaporation
has occurred to detectably flatten the initial mass function, via preferential
escape of very low mass stars and brown dwarfs, is undetermined, pending better
membership information for stars at large radial distances.Comment: 19 pages, 14 figures, 2 tables, accepted by AJ, to appear April 200
The velocity distribution of nearby stars from Hipparcos data II. The nature of the low-velocity moving groups
The velocity distribution of nearby stars contains many "moving groups" that
are inconsistent with the standard assumption of an axisymmetric,
time-independent, and steady-state Galaxy. We study the age and metallicity
properties of the low-velocity moving groups based on the reconstruction of the
local velocity distribution in Paper I of this series. We perform stringent,
conservative hypothesis testing to establish for each of these moving groups
whether it could conceivably consist of a coeval population of stars. We
conclude that they do not: the moving groups are not trivially associated with
their eponymous open clusters nor with any other inhomogeneous star formation
event. Concerning a possible dynamical origin of the moving groups, we test
whether any of the moving groups has a higher or lower metallicity than the
background population of thin disk stars, as would generically be the case if
the moving groups are associated with resonances of the bar or spiral
structure. We find clear evidence that the Hyades moving group has higher than
average metallicity and weak evidence that the Sirius moving group has lower
than average metallicity, which could indicate that these two groups are
related to the inner Lindblad resonance of the spiral structure. Further we
find weak evidence that the Hercules moving group has higher than average
metallicity, as would be the case if it is associated with the bar's outer
Lindblad resonance. The Pleiades moving group shows no clear metallicity
anomaly, arguing against a common dynamical origin for the Hyades and Pleiades
groups. Overall, however, the moving groups are barely distinguishable from the
background population of stars, raising the likelihood that the moving groups
are associated with transient perturbations. [abridged
Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?
[Abridged] In this paper, we present the results of a photometric survey to
identify low mass and brown dwarf members of the old open cluster Praesepe (age
of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer
its mass function which we compare with that of other clusters. We have
performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric
survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection
limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses
from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol
down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in
agreement with the mass function derived by previous studies, including a
survey based on proper motion and photometry. Comparing our mass function with
that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we
see a significant difference. Possible reasons are that dynamical evaporation
has not influenced the Hyades and Praesepe in the same way, or that the
clusters did not have the same initial mass function, or that dynamical
interactions have modified the evolution of one or both clusters. Although a
difference in the binary fractions of the clusters could cause the observed
(i.e. system) mass functions to differ, measurements in the literature give no
evidence for a significant difference in the binary fractions of the two
clusters. Of our cluster candidates, six have masses predicted to be equal to
or below the stellar/substellar boundary at 0.072Msol.Comment: 11 pages, 11 figures, accepted for publication in A&A. Higher
resolution of Figures 2-3-4-5 in A&A published version. Revised version
corrected for Englis
RACE-OC Project: Rotation and variability in young stellar associations within 100 pc
Our goal is to determine the rotational and magnetic-related activity
properties of stars at different stages of evolution. We have focussed our
attention on 6 young loose stellar associations within 100 pc and ages in the
range 8-70 Myr: TW Hydrae (~8 Myr), beta Pictoris (~10 Myr), Tucana/Horologium,
Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional data on alpha
Persei and the Pleiades from the literature is also considered. Rotational
periods of stars showing rotational modulation due to photospheric magnetic
activity (i.e. starspots) have been determined applying the Lomb-Scargle
periodogram technique to photometric time-series obtained by the All Sky
Automated Survey (ASAS). The magnetic activity level has been derived from the
amplitude of the V lightcurves. We detected the rotational modulation and
measured the rotation periods of 93 stars for the first time, and confirmed the
periods of 41 stars already known from the literature. For further 10 stars we
revised the period determinations by other authors. The sample was augmented
with periods of 21 additional stars retrieved from the literature. In this way,
for the first time we were able to determine largest set of rotation periods at
ages of ~8, ~10 and ~30 Myr, as well as increase by 150\% the number of known
periodic members of AB Dor.The analysis of the rotation periods in young
stellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264
data from the literature, has allowed us to find that in the 0.6 - 1.2 solar
masses range the most significant variations of the rotation period
distribution are the spin-up between 9 and 30 Myr and the spin-down between 70
and 110 Myr. Variations between 30 and 70 Myr are rather doubtful, despite the
median period indicates a significant spin-up.Comment: Accepted by Astronomy and Astrophysic
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