287 research outputs found
The double population of Chamaeleon I detected by Gaia DR2
Context. Chamaeleon I represents an ideal laboratory to study the cluster
formation in a low-mass environment. Recently, two sub clusters spatially
located in the northern and southern parts of Chamaeleon I were found with
different ages and radial velocities. Aims. In this letter we report new
insights into the structural properties, age, and distance of Chamaeleon I
based on the astrometric parameters from Gaia data-release 2 (DR2). Methods. We
identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We
determined the median distance of the cluster using Gaia parallaxes and fitted
the distribution of parallaxes and proper motions assuming the presence of two
clusters. We derived the probability of each single source of belonging to the
northern or southern sub-clusters, and compared the HR diagram of the most
probable members to pre-main sequences isochrones. Results. The median distance
of Chamaeleon I is ~190 pc. This is about 20 pc larger than the value commonly
adopted in the literature. From a Kolmogorov-Smirnov test of the parallaxes and
proper-motion distributions we conclude that the northern and southern clusters
do not belong to the same parent population. The northern population has a
distance dN = 192.7+/-0.4 pc, while the southern one dS = 186.5+/-0.7 pc. The
two sub-clusters appear coeval, at variance with literature results, and most
of the sources are younger than 3 Myr. The northern cluster is more elongated
and extends towards the southern direction partially overlapping with the more
compact cluster located in the south. A hint of a relative rotation between the
two sub-clusters is also found.Comment: Letter accepted by A&
Lithium depleted stars in the young sigma Ori cluster
Context: Knowledge of the age distribution of stars in young clusters and
associations is crucial to constrain models of star formation. HR diagrams of
different young clusters and associations suggest the presence of age spreads,
but the influence of errors on the derived ages is still largely debated.
Determination of lithium abundances in low-mass stars represents an alternative
and robust way to infer stellar ages. Aims: We measured lithium in a sample of
low mass members of the young (4-5 Myr) sigma Ori cluster with the main goal of
investigating its star formation history. Methods: Using the FLAMES
multi-object spectrograph on VLT/UT2, we obtained spectra of 98 candidate
cluster members. The spectra were used to determine radial velocities, to infer
the presence of Halpha emission, and to measure the strength of the Li 670.8 nm
absorption line. Results: Using radial velocities, Halpha and Li, together with
information on X-ray emission, we identified 59 high probability cluster
members. Three of them show severe Li depletion. The nuclear ages inferred for
these highly depleted stars exceed 10-15 Myr; for two of them these values are
in good agreement with the isochronal age, while for the third star the nuclear
age exceeds the isochronal one.Comment: 4 pages, 4 figures, Accepted for publication in A&A Letter
A 3D view of the Taurus star-forming region by <i>Gaia </i>and <i>Herschel</i>:multiple populations related to the filamentary molecular cloud
Context. Taurus represents an ideal region to study the three-dimensional distribution of the young stellar population and relate it to the associated molecular cloud. Aims. The second Gaia data release (DR2) enables us to investigate the Taurus complex in three dimensions, starting from a previously defined robust membership. The molecular cloud structured in filaments can be traced in emission using the public far-infrared maps from Herschel. Methods. From a compiled catalog of spectroscopically confirmed members, we analyze the 283 sources with reliable parallax and proper motions in the Gaia DR2 archive. We fit the distribution of parallaxes and proper motions with multiple populations described by multivariate Gaussians. We compute the cartesian Galactic coordinates (X,Y,Z) and, for the populations associated with the main cloud, also the galactic space velocity (U,V,W). We discuss the spatial distribution of the populations in relation to the structure of the filamentary molecular cloud traced by Herschel. Results. We discover the presence of six populations which are all well defined in parallax and proper motions, with the only exception being Taurus D. The derived distances range between 130 and 160 pc. We do not find a unique relation between stellar population and the associated molecular cloud: while the stellar population seems to be on the cloud surface, both lying at similar distances, this is not the case when the molecular cloud is structured in filaments. Taurus B is probably moving in the direction of Taurus A, while Taurus E appears to be moving towards them. Conclusions. The Taurus region is the result of a complex star formation history which most probably occurred in clumpy and filamentary structures that are evolving independently
Investigation on intestinal bacterial flora and Salmonella spp. presence in organic and conventional chickens
The aim of this study was to evaluate the possible differences in the intestinal microflora composition among the different rearing systems (conventional vs organic) and the Salmonella diffusion using bacteriological techniques. The results showed that the differences between the two groups at the same age, expressed by the bacterial count, are not conclusive in showing an influence of the rearing systems. Salmonella Hadar was isolated once in caeca of conventional and once in caeca of organic ones. Though the results are preliminary and referred to a well defined geographic area in Central Italy, Salmonella detection does not seem to be common in conventional and organic chicken farms
FLAMES spectroscopy of low-mass stars in the young clusters sigma Ori and lambda Ori
Aims. We performed a detailed membership selection and studied the accretion
properties of low-mass stars in the two apparently very similar young (1-10
Myr) clusters sigma Ori and lambda Ori.
Methods. We observed 98 and 49 low-mass (0.2-1.0 M_sun) stars in sigma Ori
and lambda Ori respectively, using the multi-object optical spectrograph FLAMES
at the VLT, with the high-resolution (R=17,000) HR15N grating (6470-6790 AA).
We used radial velocities, Li and Halpha to establish cluster membership and
Halpha and other optical emission lines to analyze the accretion properties of
members.
Results. We identified 65 and 45 members of the sigma Ori and lambda Ori
clusters, respectively and discovered 16 new candidate binary systems. We also
measured rotational broadening for 20 stars and estimated the mass accretion
rates in 25 stars of the sigma Ori cluster, finding values between 10^-11 and
10^-7.7 M_sun yr^-1 and in 4 stars of the lambda Ori cluster, finding values
between 10^-11 and 10^-10.1 M_sun yr-1. Comparing our results with the infrared
photometry obtained by the Spitzer satellite, we find that the fraction of
stars with disks and the fraction of active disks is larger in the sigma Ori
cluster (52+-9% and 78+-16%) than in lambda Ori (28+-8% and 40+-20%)
Conclusions. The different disk and accretion properties of the two clusters
could be due either to the effect of the high-mass stars and the supernova
explosion in the lambda Ori cluster or to different ages of the cluster
populations. Further observations are required to draw a definitive conclusion.Comment: 14 pages, 9 figures, 9 tables, accepted for publications in A&
The Gaia-ESO Survey: the selection function of the Milky Way field stars
The Gaia-ESO Survey was designed to target all major Galactic components
(i.e., bulge, thin and thick discs, halo and clusters), with the goal of
constraining the chemical and dynamical evolution of the Milky Way. This paper
presents the methodology and considerations that drive the selection of the
targeted, allocated and successfully observed Milky Way field stars. The
detailed understanding of the survey construction, specifically the influence
of target selection criteria on observed Milky Way field stars is required in
order to analyse and interpret the survey data correctly. We present the target
selection process for the Milky Way field stars observed with VLT/FLAMES and
provide the weights that characterise the survey target selection. The weights
can be used to account for the selection effects in the Gaia-ESO Survey data
for scientific studies. We provide a couple of simple examples to highlight the
necessity of including such information in studies of the stellar populations
in the Milky Way.Comment: 18 pages, 19 figures, Accepted for publication in MNRAS (April 25,
2016
The Gaia-ESO Survey: membership and Initial Mass Function of the Gamma Velorum cluster
Understanding the properties of young open clusters, such as the Initial Mass
Function (IMF), star formation history and dynamic evolution, is crucial to
obtain reliable theoretical predictions of the mechanisms involved in the star
formation process. We want to obtain a list, as complete as possible, of
confirmed members of the young open cluster Gamma Velorum, with the aim of
deriving general cluster properties such as the IMF. We used all available
spectroscopic membership indicators within the Gaia-ESO public archive together
with literature photometry and X-ray data and, for each method, we derived the
most complete list of candidate cluster members. Then, we considered
photometry, gravity and radial velocities as necessary conditions to select a
subsample of candidates whose membership was confirmed by using the lithium and
H lines and X-rays as youth indicators. We found 242 confirmed and 4
possible cluster members for which we derived masses using very recent stellar
evolutionary models. The cluster IMF in the mass range investigated in this
study shows a slope of for and
for and is consistent with a standard
IMF. The similarity of the IMF of the young population around Vel to
that in other star forming regions and the field suggests it may have formed
through very similar processes.Comment: Accepted for publication in A&A; 18 pages, 11 figures, 5 table
The Gaia-ESO Survey: Separating disk chemical substructures with cluster models
(Abridged) Recent spectroscopic surveys have begun to explore the Galactic
disk system outside the solar neighborhood on the basis of large data samples.
In this way, they provide valuable information for testing spatial and temporal
variations of disk structure kinematics and chemical evolution. We used a
Gaussian mixture model algorithm, as a rigurous mathematical approach, to
separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample from the
Gaia-ESO survey internal data release 2. We find that the sample is separated
into five groups associated with major Galactic components; the metal-rich end
of the halo, the thick disk, and three subgroups for the thin disk sequence.
This is confirmed with a sample of red clump stars from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) survey. The two
metal-intermediate and metal-rich groups of the thin disk decomposition
([Fe/H]>-0.25 dex) highlight a change in the slope at solar metallicity. This
holds true at different radial regions. The distribution of Galactocentric
radial distances of the metal-poor part of the thin disk ([Fe/H]<-0.25 dex) is
shifted to larger distances than those of the more metal-rich parts. Moreover,
the metal-poor part of the thin disk presents indications of a scale height
intermediate between those of the thick and the rest of the thin disk, and it
displays higher azimuthal velocities than the latter. These stars might have
formed and evolved in parallel and/or dissociated from the inside-out formation
taking place in the internal thin disk. Their enhancement levels might be due
to their origin from gas pre-enriched by outflows from the thick disk or the
inner halo. The smooth trends of their properties (their spatial distribution
with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by
the data indicates a quiet dynamical evolution, with no relevant merger events
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
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