819 research outputs found
VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
The physical properties of almost any kind of astronomical object can be
derived by fitting synthetic spectra or photometry extracted from theoretical
models to observational data.
We want to develop an automatic procedure to perform this kind of fittings to
a relatively large sample of members of a stellar association and apply this
methodology to the case of Collinder 69.
We combine the multiwavelength data of our sources and follow a work-flow to
derive the physical parameters of the sources. The key step of the work-flow is
performed by a new VO-tool, VOSA. All the steps in this process are done in a
VO environment.
We present this new tool, and provide physical parameters such as T, gravity, luminosity, etc. for 170 candidate members to Collinder
69, and an upper-limit for the age of this stellar association.
This kind of studies of star forming regions, clusters, etc. produces a huge
amount of data, very tedious to analyse using the traditional methodology.
Thus, they are excellent examples where to apply the VO capabilities.Comment: Accepted for publication in A&
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region
Context. Most observational studies so far point towards brown dwarfs sharing
a similar formation mechanism as the one accepted for low mass stars. However,
larger databases and more systematic studies are needed before strong
conclusions can be reached. Aims. In this second paper of a series devoted to
the study of the spectroscopic properties of the members of the Lambda Orionis
Star Forming Region, we study accretion, activity and rotation for a wide set
of spectroscopically confirmed members of the central star cluster Collinder 69
to draw analogies and/or differences between the brown dwarf and stellar
populations of this cluster. Moreover, we present comparisons with other star
forming regions of similar and different ages to address environmental effects
on our conclusions. Methods. We study prominent photospheric lines to derive
rotational velocities and emission lines to distinguish between accretion
processes and chromospheric activity. In addition, we include information about
disk presence and X-ray emission. Results. We report very large differences in
the disk fractions of low mass stars and brown dwarfs (~58%) when compared to
higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for
this dichotomy. As a byproduct, we address the implications of the spatial
distribution of disk and diskless members in the formation scenario of the
cluster itself. We have used the Halpha emission to discriminate among
accreting and non-accreting sources finding that 38+8-7% of sources harboring
disks undergo active accretion and that his percentage stays similar in the
substellar regime. For those sources we have estimated accretion rates.
Finally, regarding rotational velocities, we find a high dispersion in vsin(i)
which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and
an online tabl
Depart and approach procedures for UAS in a VFR environment
This paper assesses the depart and approach operations of Unmanned Aircraft Systems (UAS) in one of the most challenging scenarios: when flying under Visual Flight Rules (VFR). Inspired by some existing procedures for (manned) general aviation, some automatic and predefined procedures for UAS are proposed. Hence, standardized paths to specific waypoints close to the airport are defined for depart operations, just before starting the navigation phase. Conversely, and for the approach maneuvers, it is foreseen a first integration into a holding pattern near the landing runway (ideally above it) followed by a standard VFR airfield traffic pattern. This paper discusses the advantages of these operations which aim at minimizing possible conflicts with other existing aircraft while reducing the Pilot-in-Command workload. Finally, some preliminary simulations are shown where these procedures have been successfully tested with simulated surrounding traffic
Cluster membership probabilities from proper motions and multiwavelength photometric catalogues: I. Method and application to the Pleiades cluster
We present a new technique designed to take full advantage of the high
dimensionality (photometric, astrometric, temporal) of the DANCe survey to
derive self-consistent and robust membership probabilities of the Pleiades
cluster. We aim at developing a methodology to infer membership probabilities
to the Pleiades cluster from the DANCe multidimensional astro-photometric data
set in a consistent way throughout the entire derivation. The determination of
the membership probabilities has to be applicable to censored data and must
incorporate the measurement uncertainties into the inference procedure.
We use Bayes' theorem and a curvilinear forward model for the likelihood of
the measurements of cluster members in the colour-magnitude space, to infer
posterior membership probabilities. The distribution of the cluster members
proper motions and the distribution of contaminants in the full
multidimensional astro-photometric space is modelled with a
mixture-of-Gaussians likelihood. We analyse several representation spaces
composed of the proper motions plus a subset of the available magnitudes and
colour indices. We select two prominent representation spaces composed of
variables selected using feature relevance determination techniques based in
Random Forests, and analyse the resulting samples of high probability
candidates. We consistently find lists of high probability (p > 0.9975)
candidates with 1000 sources, 4 to 5 times more than obtained in the
most recent astro-photometric studies of the cluster.
The methodology presented here is ready for application in data sets that
include more dimensions, such as radial and/or rotational velocities, spectral
indices and variability.Comment: 14 pages, 4 figures, accepted by A&
Phase transformation in titania nanocrystals by the oriented attachment mechanism: the role of pH medium.
The Gaia Ultra-Cool Dwarf Sample -- II : Structure at the end of the main sequence
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We identify and investigate known late M, L, and T dwarfs in the Gaia second data release. This sample is being used as a training set in the Gaia data processing chain of the ultracool dwarfs work package. We find 695 objects in the optical spectral range M8–T6 with accurate Gaia coordinates, proper motions, and parallaxes which we combine with published spectral types and photometry from large area optical and infrared sky surveys. We find that 100 objects are in 47 multiple systems, of which 27 systems are published and 20 are new. These will be useful benchmark systems and we discuss the requirements to produce a complete catalogue of multiple systems with an ultracool dwarf component. We examine the magnitudes in the Gaia passbands and find that the G BP magnitudes are unreliable and should not be used for these objects. We examine progressively redder colour–magnitude diagrams and see a notable increase in the main-sequence scatter and a bivariate main sequence for old and young objects. We provide an absolute magnitude – spectral subtype calibration for G and G RP passbands along with linear fits over the range M8–L8 for other passbands.Peer reviewedFinal Published versio
A Possible Massive Asteroid Belt Around zeta Lep
We have used the Keck I telescope to image at 11.7 microns and 17.9 microns
the dust emission around zeta Lep, a main sequence A-type star at 21.5 pc from
the Sun with an infrared excess. The excess is at most marginally resolved at
17.9 microns. The dust distance from the star is probably less than or equal to
6 AU, although some dust may extend to 9 AU. The mass of observed dust is
\~10^22 g. Since the lifetime of dust particles is about 10,000 years because
of the Poytning-Robertson effect, we robustly estimate at least 4 10^26 g must
reside in parent bodies which may be asteroids if the system is in a steady
state and has an age of ~300 Myr. This mass is approximately 200 times that
contained within the main asteroid belt in our solar system.Comment: 12 pages, 3 figures, ApJL in pres
Activity at the Deuterium-Burning Mass Limit in Orion
We report very intense and variable Halpha emission (pseudo-equivalent widths
of ~180, 410 A) of S Ori 55, a probable free-floating, M9-type substellar
member of the young sigma Orionis open star cluster. After comparison with
state-of-the-art evolutionary models, we infer that S Ori 55 is near or below
the cluster deuterium-burning mass borderline, which separates brown dwarfs and
planetary-mass objects. We find its mass to be 0.008-0.015 Msun for ages
between 1 Myr and 8 Myr, with ~0.012 Msun the most likely value at the cluster
age of 3 Myr. The largest Halpha intensity reached the saturation level of log
L(Halpha)/L(bol) = -3. We discuss several possible scenarios for such a strong
emission. We also show that sigma Orionis M and L dwarfs have in general more
Halpha emission than their older field spectral counterparts. This could be due
to a decline in the strength of the magnetic field with age in brown dwarfs and
isolated planetary-mass objects, or to a likely mass accretion from disks in
the very young sigma Orionis substellar members.Comment: Accepted for publication in ApJ Letters. Nine pages (figures
included
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
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