197 research outputs found
Orbital motion of the young brown dwarf companion TWA 5 B
With more adaptive optics images available, we aim at detecting orbital
motion for the first time in the system TWA 5 A+B. We measured separation and
position angle between TWA 5 A and B in each high-resolution image available
and followed their change in time, because B should orbit around A. The
astrometric measurement precision is about one milli arc sec. With ten year
difference in epoch, we can clearly detect orbital motion of B around A, a
decrease in separation by ~ 0.0054 arc sec per year and a decrease in position
angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter
masses (Neuh\"auser et al. 2000), but having large error bars (4 to 145 Jupiter
masses, Neuh\"auser et al. 2009). Given its large projected separation from the
primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed
star-like, and would then be a brown dwarf companion. Given the relatively
large changes in separation and position angle between TWA 5 A and B, we can
conclude that they orbit around each other on an eccentric orbit. Some evidence
is found for a curvature in the orbital motion of B around A - most consistent
with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are
detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr
period, i.e., fully consistent with the orbit of the inner close pair TWA 5
Aa+b. Measuring these residuals caused by the photocenter wobble - even in
unresolved images - can yield the total mass of the inner pair, so can test
theoretical pre-main sequence models.Comment: 6 pages, 4 figures, accepted for publication in A&A; corrected typo
in amplitude below Fig.
Confirmation of the binary status of Cha Halpha 2 - a very young low-mass binary in Chamaeleon
Neuhaeuser & Comeron (1998, 1999) presented direct imaging evidence, as well
as first spectra, of several young stellar and sub-stellar M6- to M8-type
objects in the Cha I dark cloud. One of these objects is Cha Halpha 2,
classified as brown dwarf candidate in several publications and suggested as
possible binary in Neuhaeuser et al. (2002). We have searched around Cha Halpha
2 for close and faint companions with adaptive optics imaging. Two epochs of
direct imaging data were taken with the Very Large Telescope (VLT) Adaptive
Optics instrument NACO in February 2006 and March 2007 in Ks-band. We retrieved
an earlier image from 2005 from the European Southern Observatory (ESO) Science
Archive Facility, increasing the available time coverage. After confirmation of
common proper motion, we deduce physical parameters of the objects by
spectroscopy, like temperature and mass. We find Cha Halpha 2 to be a very
close binary of ~0.16 arcsec separation, having a flux ratio of ~0.91, thus
having almost equal brightness and indistinguishable spectral types within the
errors. We show that the two tentative components of Cha Halpha 2 form a common
proper motion pair, and that neither component is a non-moving background
object. We even find evidence for orbital motion. A combined spectrum of both
stars spanning optical and near-infrared parts of the spectral energy
distribution yields a temperature of 3000+/-100 K, corresponding to a spectral
type of M6+/-1 and a surface gravity of log g= 4.0 +0.75-0.5, both from a
comparison with GAIA model atmospheres. We derive masses of ~0.110 Msun (>0.070
Msun) and ~0.124 Msun (>0.077 Msun) for the two components of Cha Halpha 2,
i.e., probably low-mass stars, but one component could possibly be a brown
dwarf.Comment: 6 pages, 8 figures, accepted for publication in A&
Astrometric confirmation of young low-mass binaries and multiple systems in the Chamaeleon star-forming regions
The star-forming regions in Chamaeleon are one of the nearest (distance ~165
pc) and youngest (age ~2 Myrs) conglomerates of recently formed stars and the
ideal target for population studies of star formation. We investigate a total
of 16 Cha targets, which have been suggested, but not confirmed as binaries or
multiple systems in previous literature. We used the adaptive optics instrument
Naos-Conica (NACO) at the Very Large Telescope Unit Telescope 4 of the Paranal
Observatory, at 2-5 different epochs, in order to obtain relative and absolute
astrometric measurements, as well as differential photometry in the J, H, and K
band. On the basis of known proper motions and these observations, we analyse
the astrometric results in our "Proper Motion Diagram" (PMD: angular separation
/ position angle versus time), to eliminate possible (non-moving) background
stars, establish co-moving binaries and multiples, and search for curvature as
indications for orbital motion. All previously suggested close components are
co-moving and no background stars are found. The angular separations range
between 0.07 and 9 arcseconds, corresponding to projected distances between the
components of 6-845 AU. Thirteen stars are at least binaries and the remaining
three (RX J0919.4-7738, RX J0952.7-7933, VW Cha) are confirmed high-order
multiple systems with up to four components. In 13 cases, we found significant
slopes in the PMDs, which are compatible with orbital motion whose periods
range from 60 to 550 years. However, in only four cases there are indications
of a curved orbit, the ultimate proof of a gravitational bond. Massive primary
components appear to avoid the simultaneous formation of equal-mass secondary
components. (abridged)Comment: 33 pages, 22 figures, accepted for publication in A&A, 2nd version:
typos and measurement unit added in Table
The sensory feedback mechanisms enabling couples to walk synchronously: An initial investigation
The inattentive eye often will not notice it, but synchronization among human walking partners is quite common. In this first investigation of this phenomenon, we studied its frequency and the mechanisms that contribute to this form of "entrainment." Specifically, by modifying the available communication links between two walking partners, we isolated the feedback mechanisms that enable couples to synchronize their stepping pattern when they walk side-by-side. Although subjects were unaware of the research aims and were not specifically asked to walk in synchrony, we observed synchronized walking in almost 50% of the walking trials, among couples who do not usually walk together. The strongest in-phase synchrony occurred in the presence of tactile feedback (i.e., handholding), perhaps because of lower and upper extremity coupling driven in part by arm swing. Interestingly, however, even in the absence of visual or auditory communication, couples also frequently walked in synchrony while 180 degrees out-of-phase, likely using different feedback mechanisms. These findings may partially explain how patients with certain gait disorders and disturbed rhythm enhance their gait when they walk with a partner and suggest alternative interventions that might improve the stepping pattern. Further, this preliminary investigation highlights the relatively ubiquitous nature of an interesting phenomenon that has not previously been studied and suggests that further work is needed to better understand the mechanisms that entrain the gait of two walking partners and allows couples to walk in synchrony with minimal or no conscious effort
A new benchmark T8-9 brown dwarf and a couple of new mid-T dwarfs from the UKIDSS DR5+ LAS
Benchmark brown dwarfs are those objects for which fiducial constraints are
available, including effective temperature, parallax, age, metallicity. We
searched for new cool brown dwarfs in 186 sq.deg. of the new area covered by
the data release DR5+ of the UKIDSS Large Area Survey. Follow-up optical and
near-infrared broad-band photometry, and methane imaging of four promising
candidates, revealed three objects with distinct methane absorption, typical of
mid- to late-T dwarfs, and one possibly T4 dwarf. The latest-type object,
classified as T8-9, shares its large proper motion with Ross 458 (BD+13o2618),
an active M0.5 binary which is 102" away, forming a hierarchical low-mass
star+brown dwarf system. Ross 458C has an absolute J-band magnitude of 16.4,
and seems overluminous, particularly in the K band, compared to similar field
brown dwarfs. We estimate the age of the system to be less than 1 Gyr, and its
mass to be as low as 14 Jupiter masses for the age of 1 Gyr. At 11.4 pc, this
new late T benchmark dwarf is a promising target to constrain the evolutionary
and atmospheric models of very low-mass brown dwarfs. We present proper motion
measurements for our targets and for 13 known brown dwarfs. Two brown dwarfs
have velocities typical of the thick disk and may be old brown dwarfs.Comment: 15 pages, 10 figures and 6 tables. Accepted by MNRAS. Uses mn2e.cls
and aas_macr
Direct evidence of a sub-stellar companion around CT Cha
In our ongoing search for close and faint companions around T Tauri stars, we
found a very faint (Ks=14.9mag, Ks_0=14.4mag) object, just ~2.67" northwest of
the Chamaeleon star-forming region member CT Cha corresponding to a projected
separation of ~440AU at 165+/-30 pc. We show that CT Cha A and this faint
object form a common proper motion pair from data of the VLT Adaptive Optics
(AO) instrument NACO taken in February 2006 and March 2007 and that the
companion is by >=4 sigma significance not a stationary background object. Our
AO integral field spectroscopy with SINFONI in J, and H+K bands yields a
temperature of 2600+/-250K for the companion and an optical extinction of
A_V=5.2+/-0.8mag, when compared to spectra calculated from Drift-Phoenix model
atmospheres. We demonstrate the validity of the model fits by comparison to
several other well-known young sub-stellar objects. Relative flux calibration
of the bands was achieved using photometry from the NACO imaging data. We
conclude that the CT Cha companion is a very low-mass member of Chamaeleon and
very likely a physical companion to CT Cha, as the probability for a by chance
alignment is <=0.01. Due to a prominent Pa-Beta emission in the J-band,
accretion is probably still ongoing onto the CT Cha companion. From temperature
and luminosity (log(Lbol/Lsun)= -2.68+/-0.21), we derive a radius of
R=2.20+0.81-0.60 R_Jup. We find a consistent mass of M=17+/-6 MJup for the CT
Cha companion from both its luminosity and temperature when placed on
evolutionary tracks. Hence, the CT Cha companion is most likely a wide brown
dwarf companion or possibly even a planetary mass object.Comment: 10 pages, 11 figures, accepted for publication in A&
Edge-on disk around the T Tauri star [MR81] Halpha 17 NE in CrA
Using the speckle camera SHARP at the 3.5m ESO NTT, K\"ohler and
collaborators found an object ~3.5 mag fainter in K only 1.3" north-east of the
T Tauri star [MR81] Ha 17 in the Corona Australis (CrA) star-forming region,
which could be either a brown dwarf or a T Tauri star with an edge-on disk. We
attempt to study this faint object in detail. We acquired deep VLT NACO
near-infrared images at three epochs to determine, whether [MR81] Ha 17 and the
nearby faint object are co-moving and to measure the infrared colors of both
objects. We obtained optical and infrared spectra of both objects with the VLT
using FORS and ISAAC, respectively, to determine spectral types and
temperatures as well as ages and masses. The T Tauri star [MR81] Ha 17 and the
faint nearby object have a projected separation of 1369.58 mas, i.e. 178 AU at
130 pc. They share the same proper motion (~5 sigma), so that they most
certainly form a bound binary pair. The apparently fainter component [MR81] Ha
17 NE has a spectral type of M2e, while the apparently brighter component
[MR81] Ha 17 SW, the previously known T Tauri star, has a spectral type of
M4-5e. We can identify a nearly edge-on disk around [MR81] Ha 17 NE by visual
inspection, which has a diameter of at least 30 to 50 AU. We are able to detect
strong emission lines in [MR81] Ha 17 NE, which are almost certainly due to
ongoing accretion. The NE object is detectable only by means of its scattered
light. If both objects are coeval (2-3 Myr) and located at the same distance
(~130 pc as CrA), then the apparently fainter [MR81] Ha 17 NE is more massive
(primary) component with a nearly edge-on disk and the apparently brighter
component [MR81] Ha 17 SW is less massive (com- panion). Both are low-mass T
Tauri stars with masses of ~0.5 and 0.23 \pm 0.05 solar masses, respectively.Comment: A&A in pres
In-depth study of moderately young but extremely red, very dusty substellar companion HD206893B
Accepted for publication in Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.The substellar companion HD206893b has recently been discovered by direct imaging of its disc-bearing host star with the SPHERE instrument. We investigate the atypical properties of the companion, which has the reddest near-infrared colours among all known substellar objects, either orbiting a star or isolated, and we provide a comprehensive characterisation of the host star-disc-companion system. We conducted a follow-up of the companion with adaptive optics imaging and spectro-imaging with SPHERE, and a multiinstrument follow-up of its host star. We obtain a R=30 spectrum from 0.95 to 1.64 micron of the companion and additional photometry at 2.11 and 2.25 micron. We carried out extensive atmosphere model fitting for the companions and the host star in order to derive their age, mass, and metallicity. We found no additional companion in the system in spite of exquisite observing conditions resulting in sensitivity to 6MJup (2MJup) at 0.5" for an age of 300 Myr (50 Myr). We detect orbital motion over more than one year and characterise the possible Keplerian orbits. We constrain the age of the system to a minimum of 50 Myr and a maximum of 700 Myr, and determine that the host-star metallicity is nearly solar. The comparison of the companion spectrum and photometry to model atmospheres indicates that the companion is an extremely dusty late L dwarf, with an intermediate gravity (log g 4.5-5.0) which is compatible with the independent age estimate of the system. Though our best fit corresponds to a brown dwarf of 15-30 MJup aged 100-300 Myr, our analysis is also compatible with a range of masses and ages going from a 50 Myr 12MJup planetary-mass object to a 50 MJup Hyades-age brown dwarf...Peer reviewedFinal Accepted Versio
Gaia GraL: Gaia DR2 Gravitational Lens Systems. V. Doubly-imaged QSOs discovered from entropy and wavelets
The discovery of multiply-imaged gravitationally lensed QSOs is fundamental to many astronomical and cosmological studies. However, these objects are rare and challenging to discover due to requirements of high-angular resolution astrometric, multiwavelength photometric and spectroscopic data. This has limited the number of known systems to a few hundred objects. We aim to reduce the constraints on angular resolution and discover multiply-imaged QSO candidates by using new candidate selection principles based on unresolved photometric time-series and ground-based images from public surveys. We selected candidates for multiply-imaged QSOs based on low levels of entropy computed from Catalina unresolved photometric time-series or Euclidean similarity to known lenses in a space defined by the wavelet power spectra of Pan-STARSS DR2 or DECaLS DR7 images, combined with multiple {\it Gaia} DR2 sources or large astrometric errors and supervised and unsupervised learning methods. We then confirmed spectroscopically some candidates with the Palomar Hale, Keck-I, and ESO/NTT telescopes. Here we report the discovery and confirmation of seven doubly-imaged QSOs and one likely double quasar. This demonstrates the potential of combining space-astrometry, even if unresolved, with low spatial-resolution photometric time-series and/or low-spatial resolution multi-band imaging to discover multiply-imaged lensed QSOs
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