4,908 research outputs found
Metal-rich T-dwarfs in the Hyades cluster
We present the results of a search for brown dwarfs (BDs) and very low mass
(VLM) stars in the 625 Myr-old, metal-rich ([Fe/H]=0.14) Hyades cluster. We
performed a deep (I=23, z=22.5) photometric survey over 16 deg around the
cluster center. We report the discovery of the first 2 BDs in the Hyades
cluster, with a spectral type T1 and T2, respectively. Their optical and
near-IR photometry, as well as their proper motion, are consistent with them
being cluster members. According to models, their mass is about 50 Jupiter
masses at an age of 625 Myr. We also report the discovery of 3 new very low
mass stellar members and confirm the membership of 15 others
Line and continuum radiative transfer modelling of AA Tau
We present photometric and spectroscopic models of the Classical T Tauri star
AA Tau. Photometric and spectroscopic variability present in observations of AA
Tau is attributed to a magnetically induced warp in the accretion disc,
periodically occulting the photosphere on an 8.2--day timescale. Emission line
profiles show signatures of both infall, attributed to magnetospherically
accreting material, and outflow. Using the radiative transfer code TORUS, we
have investigated the geometry and kinematics of AA Tau's circumstellar disc
and outflow, which is modelled here as a disc wind. Photometric models have
been used to constrain the aspect ratio of the disc, the offset angle of the
magnetosphere dipole with respect to the stellar rotation axis, and the inner
radius of the circumstellar disc. Spectroscopic models have been used to
constrain the wind and magnetosphere temperatures, wind acceleration parameter,
and mass loss rate. We find observations are best fitted by models with a mass
accretion rate of M yr, a dipole offset of
between and , a magnetosphere that truncates the disc from
5.2 to 8.8 R, a mass-loss-rate to accretion-rate ratio of ~ 0.1, a
magnetosphere temperature of 8500 -- 9000 K, and a disc wind temperature of
8000 K.Comment: 22 pages, 32 figures, 4 tables. Accepted by MNRAS. V3: Corrected typ
A deep, wide-field search for substellar members in NGC 2264
We report the first results of our ongoing campaign to discover the first
brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region
for which our optical studies have revealed a very high density of potential
candidates - 236 in 1 deg - from the substellar limit down to at least
20 M for zero reddening. Candidate BD were first selected
using wide field () band imaging with CFH12K, by reference to current
theoretical isochrones. Subsequently, 79 (33%) of the sample were found
to have near-infrared 2MASS photometry ( 0.3 mag. or better),
yielding dereddened magnitudes and allowing further investigation by comparison
with the location of NextGen and DUSTY isochrones in colour-colour and
colour-magnitude diagrams involving various combinations of ,, and
. We discuss the status and potential substellarity of a number of
relatively unreddened (A 5) likely low-mass members in our
sample, but in spite of the depth of our observations in , we are as yet
unable to unambiguously identify substellar candidates using only 2MASS data.
Nevertheless, there are excellent arguments for considering two faint (observed
18.4 and 21.2) objects as cluster candidates with masses
respectively at or rather below the hydrogen burning limit. More current
candidates could be proven to be cluster members with masses around 0.1
M {\it via} gravity-sensitive spectroscopy, and deeper near-infrared
imaging will surely reveal a hitherto unknown population of young brown dwarfs
in this region, accessible to the next generation of deep near-infrared
surveys.Comment: 10 pages, 12 figures, accepted by A&
Dynamical Masses of Young Stars in Multiple Systems
We present recent measurements of the orbital motion in the young binaries DF
Tau and ZZ Tau, and the hierarchical triple Elias 12, that were obtained with
the Fine Guidance Sensors on the HST and at the Keck Observatory using adaptive
optics. Combining these observations with previous measurements from the
literature, we compute preliminary orbital parameters for DF Tau and ZZ Tau. We
find that the orbital elements cannot yet be determined precisely because the
orbital coverage spans only about 90 degr in position angle. Nonetheless, the
range of possible values for the period and semi-major axis already defines a
useful estimate for the total mass in DF Tau and ZZ Tau, with values of
0.90{+0.85}{-0.35} M_sun and 0.81{+0.44}{-0.25} M_sun, respectively, at a
fiducial distance of 140 pc.Comment: 26 pages, 9 figures, accepted for publication in A
Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses
Context. Brown dwarfs represent a sizable fraction of the stellar content of
our Galaxy and populate the transition between the stellar and planetary mass
regime. There is however no agreement on the processes responsible for their
formation. Aims. We have conducted a large survey of the young, nearby cluster
IC 348, to uncover its low-mass brown dwarf population and study the cluster
properties in the substellar regime. Methods. Deep optical and near-IR images
taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were
used to select photometric candidate members. A spectroscopic follow-up of a
large fraction of the candidates was conducted to assess their youth and
membership. Results. We confirmed spectroscopically 16 new members of the IC
348 cluster, including 13 brown dwarfs, contributing significantly to the
substellar census of the cluster, where only 30 brown dwarfs were previously
known. Five of the new members have a L0 spectral type, the latest-type objects
found to date in this cluster. At 3 Myr, evolutionary models estimate these
brown dwarfs to have a mass of ~13 Jupiter masses. Combining the new members
with previous census of the cluster, we constructed the IMF complete down to 13
Jupiter masses. Conclusions. The IMF of IC 348 is well fitted by a log-normal
function, and we do not see evidence for variations of the mass function down
to planetary masses when compared to other young clusters.Comment: Accepted to A&A (8 November 2012
Prediction and classification for GPCR sequences based on ligand specific features
Functional identification of G-Protein Coupled Receptors (GPCRs) is one of the current focus areas of pharmaceutical research. Although thousands of GPCR sequences are known, many of them are orphan sequences (the activating ligand is unknown). Therefore, classification methods for automated characterization of orphan GPCRs are imperative. In this study, for predicting Level 1 subfamilies of GPCRs, a novel method for obtaining class specific features, based on the existence of activating ligand specific patterns, has been developed and utilized for a majority voting classification. Exploiting the fact that there is a non-promiscuous relationship between the specific binding of GPCRs into their ligands and their functional classification, our method classifies Level 1 subfamilies of GPCRs with a high predictive accuracy between 99% and 87% in a three-fold cross validation test. The method also tells us which motifs are significant for class determination which has important design implications. The presented machine learning approach, bridges the gulf between the excess amount of GPCR sequence data and their poor functional characterization
Evolution of Angular Momentum Distribution during Star Formation
If the angular momentum of the molecular cloud core were conserved during the
star formation process, a new-born star would rotate much faster than its
fission speed. This constitutes the angular momentum problem of new-born stars.
In this paper, the angular momentum transfer in the contraction of a rotating
magnetized cloud is studied with axisymmetric MHD simulations. Owing to the
large dynamic range covered by the nested-grid method, the structure of the
cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud
experiences a run-away collapse, and a disk forms perpendicularly to the
magnetic field, in which the central density increases greatly in a finite
time-scale. In this phase, the specific angular momentum j of the disk
decreases to of the initial cloud. After the central density of
the disk exceeds , the infall on to the central
object develops. In this accretion stage, the rotation motion and thus the
toroidal magnetic field drive the outflow. The angular momentum of the central
object is transferred efficiently by the outflow as well as the effect of the
magnetic stress. In 7000 yr from the core formation, the specific angular
momentum of the central decreases a factor of 10^{-4} from the
initial value (i.e. from to ).Comment: 15 pages, 2 figures, Astrophysical Journal Letters in pres
Rotation of Low-mass Stars in Taurus with K2
We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young (~3 Myr) Taurus association, in addition to an older (~30 Myr) population of stars that is largely in the foreground of the Taurus molecular clouds. Out of 156 of the highest-confidence Taurus members, we find that 81% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all stars (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation toward both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than ~2 days. The overall amplitude of the LCs decreases with age, and higher-mass stars have generally lower amplitudes than lower-mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting LC morphologies are also different between these two classes
A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau
In searches for low-mass companions to late-type stars, correlation between
radial velocity variations and line bisector slope changes indicates
contamination by large starspots. Two young stars demonstrate that this test is
not sufficient to rule out starspots as a cause of radial velocity variations.
As part of our survey for substellar companions to T Tauri stars, we identified
the ~2 Myr old planet host candidates DN Tau and V836 Tau. In both cases,
visible light radial velocity modulation appears periodic and is uncorrelated
with line bisector span variations, suggesting close companions of several
M_Jup in these systems. However, high-resolution, infrared spectroscopy shows
that starspots cause the radial velocity variations. We also report unambiguous
results for V827 Tau, identified as a spotted star on the basis of both visible
light and infrared spectroscopy. Our results suggest that infrared follow up
observations are critical for determining the source of radial velocity
modulation in young, spotted stars.Comment: Accepted for publication in the Astrophysical Journal Letter
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