189 research outputs found
Parallax measurements of cool brown dwarfs
Accurate parallax measurements allow us to determine physical properties of
brown dwarfs, and help us to constrain evolutionary and atmospheric models,
break the age-mass degeneracy and reveal unresolved binaries.
We measured absolute trigonometric parallaxes and proper motions of 6 cool
brown dwarfs using background galaxies to establish an absolute reference
frame. We derive the absolute J-mag. The six T brown dwarfs in our sample have
spectral types between T2.5 and T7.5 and magnitudes in J between 13.9 and 18.0,
with photometric distances below 25 pc.
The observations were taken in the J-band with the Omega-2000 camera on the
3.5 m telescope at Calar Alto, during a time period of 27 months, between March
2011 and June 2013. The number of epochs varied between 11 and 12 depending on
the object. The reduction of the astrometric measurements was carried out with
respect to the field stars. The relative parallax and proper motions were
transformed into absolute measurements using the background galaxies in our
fields.
We obtained absolute parallaxes for our six brown dwarfs with a precision
between 3 and 6 mas. We compared our results in a color-magnitude diagram with
other brown dwarfs with determined parallax and with the BT-Settl 2012
atmospheric models. For four of the six targets we found a good agreement in
luminosity with objects of similar spectral types. We obtained an improved
accuracy in the parallaxes and proper motions in comparison to previous works.
The object 2MASS J11061197+2754225 is more than 1 mag overluminous in all bands
pointing to binarity or high order multiplicity.Comment: 9 pages, 15 figures. Astronomy & Astrophysics, 201
Confirming the least massive members of the Pleiades star cluster
We present optical photometry (i- and Z-band) and low-resolution spectroscopy
(640-1015 nm) of very faint candidate members (J = 20.2-21.2 mag) of the
Pleiades star cluster (120 Myr). The main goal is to address their cluster
membership via photometric, astrometric, and spectroscopic studies, and to
determine the properties of the least massive population of the cluster through
the comparison of the data with younger and older spectral counterparts and
state-of-the art model atmospheres. We confirm three bona-fide Pleiades members
that have extremely red optical and infrared colors, effective temperatures of
~1150 K and ~1350 K, and masses in the interval 11-20 Mjup, and one additional
likely member that shares the same motion as the cluster but does not appear to
be as red as the other members with similar brightness. This latter object
requires further near-infrared spectroscopy to fully address its membership in
the Pleiades. The optical spectra of two bona-fide members were classified as
L6-L7 and show features of KI, a tentative detection of CsI, hydrides and water
vapor with an intensity similar to high-gravity dwarfs of related
classification despite their young age. The properties of the Pleiades L6-L7
members clearly indicate that very red colors of L dwarfs are not a direct
evidence of ages younger than ~100 Myr. We also report on the determination of
the bolometric corrections for the coolest Pleiades members. These data can be
used to interpret the observations of the atmospheres of exoplanets orbiting
stars.Comment: Accepted for publication in MNRAS (17 pages
Testing the existence of optical linear polarization in young brown dwarfs
Linear polarization can be used as a probe of the existence of atmospheric
condensates in ultracool dwarfs. Models predict that the observed linear
polarization increases withthe degree of oblateness, which is inversely
proportional to the surface gravity. We aimed to test the existence of optical
linear polarization in a sample of bright young brown dwarfs, with spectral
types between M6 and L2, observable from the Calar Alto Observatory, and
cataloged previously as low gravity objects using spectroscopy. Linear
polarimetric images were collected in I and R-band using CAFOS at the 2.2 m
telescope in Calar Alto Observatory (Spain). The flux ratio method was employed
to determine the linear polarization degrees. With a confidence of 3,
our data indicate that all targets have a linear polarimetry degree in average
below 0.69% in the I-band, and below 1.0% in the R-band, at the time they were
observed. We detected significant (i.e. P/ 3) linear polarization
for the young M6 dwarf 2MASS J04221413+1530525 in the R-band, with a degree of
= 0.81 0.17 %.Comment: Accepted for publication in MNRA
New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs
We obtained medium-resolution near-infrared spectra of seven young M9.5-L3
dwarfs classified in the optical. We aim to confirm the low surface gravity of
the objects in the NIR. We also test whether atmospheric models correctly
represent the formation and the settling of dust clouds in the atmosphere of
young late-M and L dwarfs. We used ISAAC at VLT to obtain the spectra of the
targets. We compared them to those of mature and young BD, and young late-type
companions to nearby stars with known ages, in order to identify and study
gravity-sensitive features. We computed spectral indices weakly sensitive to
the surface gravity to derive near-infrared spectral types. Finally, we found
the best fit between each spectrum and synthetic spectra from the BT-Settl 2010
and 2013 models. Using the best fit, we derived the atmospheric parameters of
the objects and identify which spectral characteristics the models do not
reproduce. We confirmed that our objects are young BD and we found NIR spectral
types in agreement with the ones determined at optical wavelengths. The
spectrum of the L2-gamma dwarf 2MASSJ2322-6151 reproduces well the spectrum of
the planetary mass companion 1RXS J1609-2105b. BT-Settl models fit the spectra
and the 1-5 m SED of the L0-L3 dwarfs for temperatures between 1600-2000
K. But the models fail to reproduce the shape of the H band, and the NIR slope
of some of our targets. This fact, and the best fit solutions found with
super-solar metallicity are indicative of a lack of dust, in particular at high
altitude, in the cloud models. The modeling of the vertical mixing and of the
grain growth will be revised in the next version of the BT-Settl models. These
revisions may suppress the remaining non-reproducibilities.Comment: Accepted in A&A, February 6, 201
Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes
Whether BDs form as stars through gravitational collapse ("star-like") or BDs
and some very low-mass stars constitute a separate population which form
alongside stars comparable to the population of planets, e.g. through
circumstellar disk ("peripheral") fragmentation, is one of the key questions of
the star-formation problem. For young stars in Taurus-Auriga the binary
fraction is large with little dependence on primary mass above ~0.2Msun, while
for BDs it is <10%. We investigate a case in which BDs in Taurus formed
dominantly through peripheral fragmentation. The decline of the binary
frequency in the transition region between star-like and peripheral formation
is modelled. A dynamical population synthesis model is employed in which
stellar binary formation is universal. Peripheral objects form separately in
circumstellar disks with a distinctive initial mass function (IMF), own orbital
parameter distributions for binaries and a low binary fraction. A small amount
of dynamical processing of the stellar component is accounted for as
appropriate for the low-density Taurus-Auriga embedded clusters. The binary
fraction declines strongly between the mass-limits for star-like and peripheral
formation. The location of characteristic features and the steepness depend on
these mass-limits. Such a trend might be unique to low density regions hosting
dynamically unprocessed binary populations. The existence of a strong decline
in the binary fraction -- primary mass diagram will become verifiable in future
surveys on BD and VLMS binarity in the Taurus-Auriga star forming region. It is
a test of the (non-)continuity of star formation along the mass-scale, the
separateness of the stellar and BD populations and the dominant formation
channel for BDs and BD binaries in regions of low stellar density hosting
dynamically unprocessed populations.Comment: accepted for publication in A&A, 11 pages, 4 figures, 1 tabl
Adjusting scope: a computational approach to case-driven research on semantic change
Language Use in Past and Presen
Hunting for brown dwarf binaries and testing atmospheric models with X-Shooter
The determination of the brown dwarf binary fraction may contribute to the
understanding of the substellar formation mechanisms. Unresolved brown dwarf
binaries may be revealed through their peculiar spectra or the discrepancy
between optical and near-infrared spectral type classification.
We obtained medium-resolution spectra of 22 brown dwarfs with these
characteristics using the X-Shooter spectrograph at the VLT.
We aimed to identify brown dwarf binary candidates, and to test if the
BT-Settl 2014 atmospheric models reproduce their observed spectra.
To find binaries spanning the L-T boundary, we used spectral indices and
compared the spectra of the selected candidates to single spectra and synthetic
binary spectra. We used synthetic binary spectra with components of same
spectral type to determine as well the sensitivity of the method to this class
of binaries.
We identified three candidates to be combination of L plus T brown dwarfs. We
are not able to identify binaries with components of similar spectral type. In
our sample, we measured minimum binary fraction of .
From the best fit of the BT-Settl models 2014 to the observed spectra, we
derived the atmospheric parameters for the single objects. The BT-Settl models
were able to reproduce the majority of the SEDs from our objects, and the
variation of the equivalent width of the RbI (794.8 nm) and CsI (852.0 nm)
lines with the spectral type. Nonetheless, these models did not reproduce the
evolution of the equivalent widths of the NaI (818.3 nm and 819.5 nm) and KI
(1253 nm) lines with the spectral type.Comment: Accepted for publication in MNRA
Temporal Quantum Control with Graphene
We introduce a novel strategy for controlling the temporal evolution of a
quantum system at the nanoscale. Our method relies on the use of graphene
plasmons, which can be electrically tuned in frequency by external gates.
Quantum emitters (e.g., quantum dots) placed in the vicinity of a graphene
nanostructure are subject to the strong interaction with the plasmons of this
material, thus undergoing time variations in their mutual interaction and
quantum evolution that are dictated by the externally applied gating voltages.
This scheme opens a new path towards the realization of quantum-optics devices
in the robust solid-state environment of graphene.Comment: 5 pages, 2 figure
Classifying evolutionary forces in language change using neural networks
A fundamental problem in research into language and cultural change is the difficulty of distinguishing processes of stochastic drift (also known as neutral evolution) from processes that are subject to selection pressures. In this article, we describe a new technique based on deep neural networks, in which we reformulate the detection of evolutionary forces in cultural change as a binary classification task. Using residual networks for time series trained on artificially generated samples of cultural change, we demonstrate that this technique is able to efficiently, accurately and consistently learn which aspects of the time series are distinctive for drift and selection, respectively. We compare the model with a recently proposed statistical test, the Frequency Increment Test, and show that the neural time series classification system provides a possible solution to some of the key problems associated with this test.Language Use in Past and Presen
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