201 research outputs found
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
Search for free-floating planetary-mass objects in the Pleiades
(Abridged) We aim at identifying the least massive population of the solar
metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the
ultimate goal of understanding the physical properties of intermediate-age,
free-floating, low-mass brown dwarfs and giant planetary-mass objects, and
deriving the cluster substellar mass function across the deuterium-burning mass
limit at ~0.012 Msol. We performed a deep photometric and astrometric J- and
H-band survey covering an area of ~0.8 deg^2. The images with completeness and
limiting magnitudes of J,H ~ 20.2 and ~ 21.5 mag were acquired ~9 yr apart
(proper motion precision of +/-6 mas/yr). J- and H-band data were complemented
with Z, K, and mid-infrared magnitudes up to 4.6 micron coming from UKIDSS,
WISE, and follow-up observations of our own. Pleiades member candidates were
selected to have proper motions compatible with that of the cluster, and colors
following the known Pleiades sequence in the interval J = 15.5-8.8 mag, and
Z_UKIDSS - J > 2.3 mag or Z nondetections for J > 18.8 mag. We found a neat
sequence of astrometric and photometric Pleiades substellar member candidates
in the intervals J = 15.5-21.2 mag and ~0.072-0.008 Msol. The faintest objects
show very red near- and mid-infrared colors exceeding those of field
high-gravity dwarfs by >0.5 mag. The Pleiades photometric sequence does not
show any color turn-over because of the presence of photospheric methane
absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by
the color-computed models. Pleiades brown dwarfs have a proper motion
dispersion of 6.4-7.5 mas/yr and are dynamically relaxed at the age of the
cluster. The Pleiades mass function extends down to the deuterium burning-mass
threshold, with a slope fairly similar to that of other young star clusters and
stellar associations.Comment: Accepted for publication in A&A. 16 page
The cosmic waltz of Coma Berenices and Latyshev 2 (Group X). Membership, phase-space structure, mass, and energy distributions
Context. Open clusters (OCs) are fundamental benchmarks where theories of
star formation and stellar evolution can be tested and validated. Coma Ber and
Latyshev 2 (Group X) are the second and third OCs closest to the Sun, making
them excellent targets to search for low-mass stars and ultra-cool dwarfs. In
addition, this pair will experience a flyby in 10-16 Myr which makes it a
benchmark to test OCs pair interactions. Aims. We aim at analysing the
membership, luminosity, mass, phase-space (i.e., positions and velocities), and
energy distributions for Coma Ber and Latyshev 2 and test the hypothesis of the
mixing of their populations at the encounter time. Methods. We develop a new
phase-space membership methodology and apply it to Gaia data. With the
recovered members we infer the phase-space, luminosity and mass distributions
using publicly available Bayesian inference codes. Then, with a publicly
available orbit integration code and members' positions and velocities, we
integrate their orbits 20 Myr into the future. Results. In Coma Ber, we
identify 302 candidate members distributed in the core and tidal tails. The
tails are dynamically cold and asymmetrically populated. The stellar system
called Group X is made of two structures: the disrupted OC Latyshev 2 (186
candidate members) and a loose stellar association called Mecayotl 1 (146
candidate members), both of them will fly by Coma Ber in Myr and
Myr, respectively, and each other in Myr. Conclusions.
We study the dynamical properties of the core and tails of Coma Ber and also
confirm the existence of the OC Latyshev 2 and its neighbour stellar
association Mecayotl 1. Although these three systems will experience encounters
we find no evidence supporting the mixing of their populations.Comment: 25 pages, 19 figures, accepted for publication in Astronomy &
Astrophysic
The CARMENES search for exoplanets around M dwarfs: Radial-velocity variations of active stars in visual-channel spectra
Previous simulations predicted the activity-induced radial-velocity (RV)
variations of M dwarfs to range from cm/s to km/s, depending on
various stellar and activity parameters. We investigate the observed relations
between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing
CARMENES high-resolution visual-channel spectra (m), which were
taken within the CARMENES RV planet survey during its first months of
operation. During this time, of the CARMENES-sample stars were observed
at least five times. From each spectrum we derived a relative RV and a measure
of chromospheric H emission. In addition, we estimated the chromatic
index (CRX) of each spectrum, which is a measure of the RV wavelength
dependence. Despite having a median number of only measurements per star,
we show that the RV variations of the stars with RV scatter of m/s and a
projected rotation velocity km/s are caused mainly by activity.
We name these stars `active RV-loud stars' and find their occurrence to
increase with spectral type: from for early-type M dwarfs
(MV) through for mid-type M dwarfs (MV) to
for late-type M dwarfs (MV). Their RV-scatter amplitude is
found to be correlated mainly with . For about half of the stars, we
also find a linear RVCRX anticorrelation, which indicates that their
activity-induced RV scatter is lower at longer wavelengths. For most of them we
can exclude a linear correlation between RV and H emission. Our results
are in agreement with simulated activity-induced RV variations in M dwarfs. The
RV variations of most active RV-loud M dwarfs are likely to be caused by dark
spots on their surfaces, which move in and out of view as the stars rotate.Comment: A&A accepte
The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004
We present results from the most recent (2002-2004) observing campaigns of
the eclipsing system KH 15D, in addition to re-reduced data obtained at Van
Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of
photometric data shows substantial evolution in the width and depth of the
eclipses. The most recent data indicate that the eclipses are now approximately
24 days in length, or half the orbital period. These results are interpreted
and discussed in the context of the recent models for this system put forward
by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set
yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord
with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et
al. Another significant peak, at 9.6 days, was found in the periodogram of the
out-of-eclipse data at two different epochs. We interpret this as the rotation
period of the visible star and argue that it may be tidally locked in
pseudosynchronism with its orbital motion. If so, application of Hut's theory
implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of
the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the
visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i)
and the measured rotation period of the star, we calculate the lower limit on
the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value
obtained by Hamilton et al. from its luminosity and effective temperature. Here
we assume that i = 90 degrees since it is likely that the spin and orbital
angular momenta vectors are nearly aligned.Comment: 55 pages, 18 figures, 1 color figure, to appear the September issue
of the Astronomical Journa
High contrast optical imaging of companions: the case of the brown dwarf binary HD-130948BC
High contrast imaging at optical wavelengths is limited by the modest
correction of conventional near-IR optimized AO systems.We take advantage of
new fast and low-readout-noise detectors to explore the potential of fast
imaging coupled to post-processing techniques to detect faint companions to
stars at small separations. We have focused on I-band direct imaging of the
previously detected brown dwarf binary HD130948BC,attempting to spatially
resolve the L2+L2 benchmark system. We used the Lucky-Imaging instrument
FastCam at the 2.5-m Nordic Telescope to obtain quasi diffraction-limited
images of HD130948 with ~0.1" resolution.In order to improve the detectability
of the faint binary in the vicinity of a bright (I=5.19 \pm 0.03) solar-type
star,we implemented a post-processing technique based on wavelet transform
filtering of the image which allows us to strongly enhance the presence of
point-like sources in regions where the primary halo dominates. We detect for
the first time the BD binary HD130948BC in the optical band I with a SNR~9 at
2.561"\pm 0.007" (46.5 AU) from HD130948A and confirm in two independent
dataset that the object is real,as opposed to time-varying residual speckles.We
do not resolve the binary, which can be explained by astrometric results
posterior to our observations that predict a separation below the NOT
resolution.We reach at this distance a contrast of dI = 11.30 \pm 0.11, and
estimate a combined magnitude for this binary to I = 16.49 \pm 0.11 and a I-J
colour 3.29 \pm 0.13. At 1", we reach a detectability 10.5 mag fainter than the
primary after image post-processing. We obtain on-sky validation of a technique
based on speckle imaging and wavelet-transform processing,which improves the
high contrast capabilities of speckle imaging.The I-J colour measured for the
BD companion is slightly bluer, but still consistent with what typically found
for L2 dwarfs(~3.4-3.6).Comment: accepted in A\&
The CARMENES search for exoplanets around M dwarfs: Wing asymmetries of H, Na I D, and He I lines
Stellar activity is ubiquitously encountered in M dwarfs and often
characterised by the H line. In the most active M dwarfs, H is
found in emission, sometimes with a complex line profile. Previous studies have
reported extended wings and asymmetries in the H line during flares. We
used a total of 473 high-resolution spectra of 28 active M dwarfs obtained by
the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths
with Near-infrared and optical Echelle Spectrographs) spectrograph to study the
occurrence of broadened and asymmetric H line profiles and their
association with flares, and examine possible physical explanations. We
detected a total of 41 flares and 67 broad, potentially asymmetric, wings in
H. The broadened H lines display a variety of profiles with
symmetric cases and both red and blue asymmetries. Although some of these line
profiles are found during flares, the majority are at least not obviously
associated with flaring. We propose a mechanism similar to coronal rain or
chromospheric downward condensations as a cause for the observed red
asymmetries; the symmetric cases may also be caused by Stark broadening. We
suggest that blue asymmetries are associated with rising material, and our
results are consistent with a prevalence of blue asymmetries during the flare
onset. Besides the H asymmetries, we find some cases of additional line
asymmetries in \ion{He}{i} D, \ion{Na}{i}~D lines, and the \ion{He}{i}
line at 10830\,\AA\, taken all simultaneously thanks to the large wavelength
coverage of CARMENES. Our study shows that asymmetric H lines are a
rather common phenomenon in M~dwarfs and need to be studied in more detail to
obtain a better understanding of the atmospheric dynamics in these objects.Comment: 28 pages, 67 figures, accepted to A&
Deep search for companions to probable young brown dwarfs
We have obtained high contrast images of four nearby, faint, and very low
mass objects 2MASSJ04351455-1414468, SDSSJ044337.61+000205.1,
2MASSJ06085283-2753583 and 2MASSJ06524851-5741376 (here after 2MASS0435-14,
SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as
probable isolated young brown dwarfs. Our goal was to search for binary
companions down to the planetary mass regime. We used the NAOS-CONICA adaptive
optics instrument (NACO) and its unique capability to sense the wavefront in
the near-infrared to acquire sharp images of the four systems in Ks, with a
field of view of 28"*28". Additional J and L' imaging and follow-up
observations at a second epoch were obtained for 2MASS0652-57. With a typical
contrast DKs= 4.0-7.0 mag, our observations are sensitive down to the planetary
mass regime considering a minimum age of 10 to 120 Myr for these systems. No
additional point sources are detected in the environment of 2MASS0435-14,
SDSS0443+00 and 2MASS0608-27 between 0.1-12" (i.e about 2 to 250 AU at 20 pc).
2MASS0652-57 is resolved as a \sim230 mas binary. Follow-up observations reject
a background contaminate, resolve the orbital motion of the pair, and confirm
with high confidence that the system is physically bound. The J, Ks and L'
photometry suggest a q\sim0.7-0.8 mass ratio binary with a probable semi-major
axis of 5-6 AU. Among the four systems, 2MASS0652-57 is probably the less
constrained in terms of age determination. Further analysis would be necessary
to confirm its youth. It would then be interesting to determine its orbital and
physical properties to derive the system's dynamical mass and to test
evolutionary model predictions.Comment: Research note, 5 pages, 2 tables and 3 figures, accepted to A&
Lucky Imaging Adaptive Optics of the brown dwarf binary GJ569Bab
The potential of combining Adaptive Optics (AO) and Lucky Imaging (LI) to
achieve high precision astrometry and differential photometry in the optical is
investigated by conducting observations of the close 0\farcs1 brown dwarf
binary GJ569Bab. We took 50000 -band images with our LI instrument FastCam
attached to NAOMI, the 4.2-m William Herschel Telescope (WHT) AO facility. In
order to extract the most of the astrometry and photometry of the GJ569Bab
system we have resorted to a PSF fitting technique using the primary star
GJ569A as a suitable PSF reference which exhibits an -band magnitude of
. The AO+LI observations at WHT were able to resolve the binary
system GJ569Bab located at 4\farcs 92 \pm 0\farcs05 from GJ569A. We measure a
separation of mas and -band magnitudes of
and and colors of 2.720.08 and 2.830.08 for
the Ba and Bb components, respectively. Our study rules out the presence of any
other companion to GJ569A down to magnitude I 17 at distances larger than
1\arcsec. The colors measured are consistent with M8.5-M9 spectral types
for the Ba and Bb components. The available dynamical, photometric and
spectroscopic data are consistent with a binary system with Ba being slightly
(10-20%) more massive than Bb. We obtain new orbital parameters which are in
good agreement with those in the literature.Comment: 13 pages, 9 figures, 7 tables, in press in MNRA
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