82 research outputs found
The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high resolution near-infrared spectroscopy with the Keck telescope, we
have detected the radial velocity signatures of the cool secondary components
in four optically identified pre-main-sequence, single-lined spectroscopic
binaries. All are weak-lined T Tauri stars with well-defined center of mass
velocities. The mass ratio for one young binary, NTTS 160905-1859, is M2/M1 =
0.18+/-0.01, the smallest yet measured dynamically for a pre-main-sequence
spectroscopic binary. These new results demonstrate the power of infrared
spectroscopy for the dynamical identification of cool secondaries. Visible
light spectroscopy, to date, has not revealed any pre-main-sequence secondary
stars with masses <0.5 M_sun, while two of the young systems reported here are
in that range. We compare our targets with a compilation of the published young
double-lined spectroscopic binaries and discuss our unique contribution to this
sample.Comment: Accepted for publication in the April, 2002, ApJ; 6 figure
A Numerical Study of Brown Dwarf Formation via Encounters of Protostellar Disks
The formation of brown dwarfs (BDs) due to the fragmentation of proto-stellar
disks undergoing pairwise encounters was investigated. High resolution allowed
the use of realistic initial disk models where both the vertical structure and
the local Jeans mass were resolved. The results show that objects with masses
ranging from giant planets to low mass stars can form during such encounters
from initially stable disks. The parameter space of initial spin-orbit
orientations and the azimuthal angles for each disk was explored. An upper
limit on the initial Toomre Q value of ~2 was found for fragmentation to occur.
Depending on the initial configuration, shocks, tidal-tail structures and mass
inflows were responsible for the condensation of disk gas. Retrograde disks
were generally more likely to fragment. When the interaction timescale was
significantly shorter than the disks' dynamical timescales, the proto-stellar
disks tended to be truncated without forming objects.
The newly-formed objects had masses ranging from 0.9 to 127 Jupiter masses,
with the majority in the BD regime. They often resided in star-BD multiples and
in some cases also formed hierarchical orbiting systems. Most of them had large
angular momenta and highly flattened, disk-like shapes. The objects had radii
ranging from 0.1 to 10 AU. The disk gas was assumed to be locally isothermal,
appropriate for the short cooling times in extended proto-stellar disks, but
not for condensed objects. An additional case with explicit cooling that
reduced to zero for optically thick gas was simulated to test the extremes of
cooling effectiveness and it was still possible to form objects in this case.
Detailed radiative transfer is expected to lengthen the internal evolution
timescale for these objects, but not to alter our basic results.Comment: 18 pages, 12 figures and 2 tables. Accepted for publication in MNRA
On the properties of young multiple stars
Abridged/ We present numerical results on the properties of young binary and
multiple stellar systems. Our analysis is based on a series of SPH + Nbody
simulations of the fragmentation of small molecular clouds, that fully resolve
the opacity limit for fragmentation. We have produced a statistically
significant number of stable multiple systems, with components separations in
the range 1-1000 AU. At the end of the hydrodynamical evolution (0.5 Myr) we
find that ~60% of stars and brown dwarfs are members of multiples systems, with
about a third of these being low mass, weakly bound outliers in wide eccentric
orbits. Our results imply that in the stellar regime most stars are in
multiples (~80%) and that this fraction is an increasing function of primary
mass. After Nbody integration to 10.5 Myr, the percentage of bound objects has
dropped to ~40%, as most very low mass stars and brown dwarfs have been
released to the field. Brown dwarfs are never found to be very close companions
to stars (brown dwarf desert at very small separations), but one case exists of
a brown dwarf companion at intermediate separations (10 AU). Our simulations
can accommodate the existence of brown dwarf companions at large separations,
but only if the primaries of these systems are themselves multiples. We have
compared the outcome of our simulations with the properties of real stellar
systems as deduced from the IR CM diagram of the Praesepe cluster and from
spectroscopic and high-resolution imaging surveys of young clusters and the
field.Comment: 14 pages, 4 figures, 1 table, accepted by MNRA
The formation and evolution of binary systems. III. Low-mass binaries in the Praesepe cluster
With the aim of investigating the binary population of the 700 Myr old
Praesepe cluster, we have observed 149 G and K-type cluster members using
adaptive optics. We detected 26 binary systems with an angular separation
ranging from less than 0.08 to 3.3 arcsec (15-600 AU). After correcting for
detection biases, we derive a binary frequency (BF) in the logP (days) range
from 4.4 to 6.9 of 25.3 +/- 5.4%, which is similar to that of field G-type
dwarfs (23.8%, Duquennoy & Mayor 1991). This result, complemented by similar
ones obtained for the 2 Myr old star forming cluster IC 348 (Paper II) and the
120 Myr old Pleiades open cluster (Paper I), indicates that the fraction of
long-period binaries does not significantly evolve over the lifetime of
galactic open clusters. We compare the distribution of cluster binaries to the
binary populations of star forming regions, most notably Orion and Taurus, to
critically review current ideas regarding the binary formation process. We
conclude that it is still unclear whether the lower binary fraction observed in
young clusters compared to T associations is purely the result of the early
dynamical disruption of primordial binaries in dense clusters or whether it
reflects intrinsically different modes of star formation in clusters and
associations. We also note that if Taurus binaries result from the dynamical
decay of small-N protostellar aggregates, one would predict the existence of a
yet to be found dispersed population of mostly single substellar objects in the
Taurus cloud.Comment: 10 pages, 3 figure
A Novel Mechanism for Calmodulin Dependent Inactivation of Transient Receptor Potential Vanilloid 6
The paralogues TRPV5 and TRPV6 belong to the vanilloid subfamily of the Transient Receptor Potential (TRP) superfamily of ion channels and both play an important role in overall Cahomeostasis. The functioning of the channels centres on a tightly controlled Ca-dependent feedback mechanism where the direct binding of the universal Ca-binding protein calmodulin (CaM) to the channel's C-terminal tail is required for channel inactivation. We have investigated this interaction at the atomic level and propose that under basal cellular [CaCaM is constitutively bound to the channel's C-tail via CaM C-lobe only contacts. When cytosolic [Ca] increases charging the apo CaM N-lobe with Ca, the CaM:TRPV6 complex rearranges and the TRPV6 C-tail further engages the CaM N-lobe via a crucial interaction involving L707. In a cellular context, mutation of L707 significantly increased the rate of channel inactivation. Finally, we present a model for TRPV6 CaM-dependent inactivation, which involves a novel so-called "two-tail" mechanism whereby CaM bridges between two TRPV6 monomers resulting in closure of the channel pore
Repeated administration of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) modulates neuroinflammation and amyloid plaque load in mice bearing amyloid precursor protein and presenilin-1 mutant transgenes
BACKGROUND: Data indicates anti-oxidant, anti-inflammatory and pro-cognitive properties of noradrenaline and analyses of post-mortem brain of Alzheimer's disease (AD) patients reveal major neuronal loss in the noradrenergic locus coeruleus (LC), the main source of CNS noradrenaline (NA). The LC has projections to brain regions vulnerable to amyloid deposition and lack of LC derived NA could play a role in the progression of neuroinflammation in AD. Previous studies reveal that intraperitoneal (IP) injection of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) can modulate neuroinflammation in amyloid over-expressing mice and in one study, DSP-4 exacerbated existing neurodegeneration. METHODS: TASTPM mice over-express human APP and beta amyloid protein and show age related cognitive decline and neuroinflammation. In the present studies, 5 month old C57/BL6 and TASTPM mice were injected once monthly for 6 months with a low dose of DSP-4 (5 mg kg(-1)) or vehicle. At 8 and 11 months of age, mice were tested for cognitive ability and brains were examined for amyloid load and neuroinflammation. RESULTS: At 8 months of age there was no difference in LC tyrosine hydroxylase (TH) across all groups and cortical NA levels of TASTPM/DSP-4, WT/Vehicle and WT/DSP-4 were similar. NA levels were lowest in TASTPM/Vehicle. Messenger ribonucleic acid (mRNA) for various inflammatory markers were significantly increased in TASTPM/Vehicle compared with WT/Vehicle and by 8 months of age DSP-4 treatment modified this by reducing the levels of some of these markers in TASTPM. TASTPM/Vehicle showed increased astrocytosis and a significantly larger area of cortical amyloid plaque compared with TASTPM/DSP-4. However, by 11 months, NA levels were lowest in TASTPM/DSP-4 and there was a significant reduction in LC TH of TASTPM/DSP-4 only. Both TASTPM groups had comparable levels of amyloid, microglial activation and astrocytosis and mRNA for inflammatory markers was similar except for interleukin-1 beta which was increased by DSP-4. TASTPM mice were cognitively impaired at 8 and 11 months but DSP-4 did not modify this. CONCLUSION: These data reveal that a low dose of DSP-4 can have varied effects on the modulation of amyloid plaque deposition and neuroinflammation in TASTPM mice dependent on the duration of dosing
Rotational Velocities of Individual Components in Very Low Mass Binaries
We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s^(–1)), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349–25B and HD 130948C, are rotating at ~30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes ≾3.5 AU, have component v sin i values that differ by greater than 3σ. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948A–BC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569A–Bab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349–25AB and 2MASS 0746+20AB, are also known radio sources
Dynamic star formation in the massive DR21 filament
The formation of massive stars is a highly complex process in which it is not
clear whether the star-forming gas is in global gravitational collapse or in an
equilibrium state, supported by turbulence. By studying one of the most massive
and dense star-forming regions in the Galaxy at a distance of less than 3 kpc,
the filament containing the well-known sources DR21 and DR21(OH), we expect to
find observational signatures that allow to discriminate between the two views.
We use molecular line data from our 13CO 1-0, CS 2-1, and N2H+ 1-0 survey of
the Cygnus X region obtained with the FCRAO and high-angular resolution
observations of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM 30m
telescope. We observe a complex velocity field and velocity dispersion in the
DR21 filament in which regions of highest column-density, i.e. dense cores,
have a lower velocity dispersion than the surrounding gas and velocity
gradients that are not (only) due to rotation. Infall signatures in optically
thick line profiles of HCO+ and 12CO are observed along and across the whole
DR21 filament. From modelling the observed spectra, we obtain a typical infall
speed of 0.6 km/s and mass accretion rates of the order of a few 10^-3 Msun/yr
for the two main clumps constituting the filament. These massive (4900 and 3300
Msun) clumps are both gravitationally contracting. All observed kinematic
features in the DR21 filament can be explained if it is formed by the
convergence of flows at large scales and is now in a state of global
gravitational collapse. Whether this convergence of flows originated from
self-gravity at larger scales or from other processes can not be settled with
the present study. The observed velocity field and velocity dispersion are
consistent with results from (magneto)-hydrodynamic simulations where the cores
lie at the stagnation points of convergent turbulent flows.Comment: Astronomy and Astrophysics, in pres
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Misaligned Protoplanetary Disks in a Young Binary System
Many extrasolar planets follow orbits that differ from the nearly coplanar
and circular orbits found in our solar system; orbits may be eccentric or
inclined with respect to the host star's equator, and the population of giant
planets orbiting close to their host stars suggests significant orbital
migration. There is currently no consensus on what produces such orbits.
Theoretical explanations often invoke interactions with a binary companion star
on an orbit that is inclined relative to the planet's orbital plane. Such
mechanisms require significant mutual inclinations between planetary and binary
star orbital planes. The protoplanetary disks in a few young binaries are
misaligned, but these measurements are sensitive only to a small portion of the
inner disk, and the three-dimensional misalignment of the bulk of the
planet-forming disk mass has hitherto not been determined. Here we report that
the protoplanetary disks in the young binary system HK Tau are misaligned by
60{\deg}-68{\deg}, so one or both disks are significantly inclined to the
binary orbital plane. Our results demonstrate that the necessary conditions
exist for misalignment-driven mechanisms to modify planetary orbits, and that
these conditions are present at the time of planet formation, apparently due to
the binary formation process.Comment: Published in Nature, July 31 2014. 18 pages. This version has slight
differences from the final published version. Final version is available at
http://www.nature.com/nature/journal/v511/n7511/full/nature13521.htm
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