7,835 research outputs found
The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs
Using the largest high-resolution spectroscopic sample to date of young, very
low mass stars (VLMS) and brown dwarfs (BDs), we investigate disk accretion in
objects ranging from just above the hydrogen-burning limit all the way to
nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or
masses from 0.15 Msun down to ~15 Jupiters. They are confirmed members of the
rho Oph, Taurus, Cha I, IC 348, R CrA, Upper Sco and TW Hydrae regions, with
ages = M6.5). We
find that: (1) classical T Tauri-like disk-accretion persists in the BD domain
down to nearly the deuterium-burning limit; (2) in addition to H-alpha,
permitted emission lines of CaII, OI and HeI are also good accretion
indicators, as in CTTs; (3) the CaII 8662A flux is an excellent quantitative
measure of the accretion rate (Mdot) in VLMS and BDs(as in CTTs); (4) Mdot
diminishes as M^2 -- our measurements support previous findings of this
correlation, and extend it to the entire range of sub-stellar masses; (5) the
accretor fraction among VLMS and BDs decreases substantially with age, as in
higher-mass stars; (6) at any given age, the VLMS and BD accretor fraction is
comparable to that in higher-mass stars; and (7) a number of sources with IR
disk excesses do not evince measurable accretion, with the incidence of such a
mismatch increasing with age: this implies that disks in the low mass regime
can persist beyond the main accretion phase, and parallels the transition from
the classical to post-T Tauri stage in more massive stars. These strong
similarities at young ages, between higher-mass stars and low-mass bodies close
to and below the hydrogen-burning limit, are consistent with a common formation
mechanism in the two mass regimes. (abridged)Comment: 64 pages, 7 figures. ApJ accepte
The kinematic relationship between disk and jet in the DG Tauri system
We present high angular resolution millimeter wavelength continuum and
13CO(2-1) observations of the circumstellar disk surrounding the TTauri star DG
Tauri. We show that the velocity pattern in the inner regions of the disk is
consistent with Keplerian rotation about a central 0.67 Msun star. The disk
rotation is also consistent with the toroidal velocity pattern in the initial
channel of the optical jet, as inferred from HST spectra of the first
de-projected 100 AU from the source. Our observations support the tight
relationship between disk and jet kinematics postulated by the popular
magneto-centrifugal models for jet formation and collimation.Comment: 4 pages, A&A Letter accepte
Synthesis, Characterization, and Magnetic Properties of gamma-NaxCoO2 (0.70 < x <0.84)
Powder NaCoO () samples were synthesized and
characterized carefully by X-ray diffraction analysis, inductive-coupled plasma
atomic emission spectroscopy, and redox titration. It was proved that
-NaCoO is formed only in the narrow range of . Nevertheless, the magnetic properties depend strongly on . We
found, for the first time, two characteristic features in the magnetic
susceptibility of NaCoO, a sharp peak at K and an
anomaly at K, as well as the transition at K and the broad
maximum at K which had already been reported. A type of weak
ferromagnetic transition seems to occur at . The transition at ,
which is believed to be caused by spin density wave formation, was observed
clearly for with constant and independent of .
On the other hand, ferromagnetic moment varies systematically depending on .
These facts suggest the occurrence of a phase separation at the microscopic
level, such as the separation into Na-rich and Na-poor domains due to the
segregation of Na ions. The magnetic phase diagram and transition mechanism
proposed previously should be reconsidered.Comment: 4 pages (2 figures included) and 2 extra figures (gif), to be
published in J. Phys. Soc. Jpn. 73 (8) with possible minor revision
A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II
We have obtained low- and medium-resolution optical spectra of 20 candidate
young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud,
using the Magellan Baade telescope. We analyze these data in conjunction with
near-infrared photometry from the 2-Micron All Sky Survey. We find that one
target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of
forbidden emission lines. These signatures are usually associated with
accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also
reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it.
Therefore, we propose that C41 is a classical T Tauri object near the
sub-stellar boundary. Thirteen other targets in our sample have continuum
spectra without intrinsic absorption or emission features, and are difficult to
characterize. They may be background giants or foreground field stars not
associated with the cloud or embedded protostars, and need further
investigation. The six remaining candidates, with moderate reddening, are
likely to be older field dwarfs, given their spectral types, lack of lithium
and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200
Resonant Production of Topological Defects
We describe a novel phenomenon in which vortices are produced due to resonant
oscillations of a scalar field which is driven by a periodically varying
temperature T, with T remaining much below the critical temperature .
Also, in a rapid heating of a localized region to a temperature {\it below}
, far separated vortex and antivortex can form. We compare our results
with recent models of defect production during reheating after inflation. We
also discuss possible experimental tests of our predictions of topological
defect production {\it without} ever going through a phase transition.Comment: Revtex, 13 pages including 5 postscript figure
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Assessing stratospheric transport in the CMAM30 simulations using ACE-FTS measurements
Stratospheric transport in global circulation models and chemistry–climate models is an important component in simulating the recovery of the ozone layer as well as changes in the climate system. The Brewer–Dobson circulation is not well constrained by observations and further investigation is required to resolve uncertainties related to the mechanisms driving the circulation. This study has assessed the specified dynamics mode of the Canadian Middle Atmosphere Model (CMAM30) by comparing to the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) profile measurements of CFC-11 (CCl3F), CFC-12 (CCl2F2), and N2O. In the CMAM30 specified dynamics simulation, the meteorological fields are nudged using the ERA-Interim reanalysis and a specified tracer was employed for each species, with hemispherically defined surface measurements used as the boundary condition. A comprehensive sampling technique along the line of sight of the ACE-FTS measurements has been utilized to allow for direct comparisons between the simulated and measured tracer concentrations. The model consistently overpredicts tracer concentrations of CFC-11, CFC-12, and N2O in the lower stratosphere, particularly in the northern hemispheric winter and spring seasons. The three mixing barriers investigated, including the polar vortex, the extratropical tropopause, and the tropical pipe, show that there are significant inconsistencies between the measurements and the simulations. In particular, the CMAM30 simulation underpredicts mixing efficiency in the tropical lower stratosphere during the June–July–August season
Nonlinear variations in axisymmetric accretion
We subject the stationary solutions of inviscid and axially symmetric
rotational accretion to a time-dependent radial perturbation, which includes
nonlinearity to any arbitrary order. Regardless of the order of nonlinearity,
the equation of the perturbation bears a form that is similar to the metric
equation of an analogue acoustic black hole. We bring out the time dependence
of the perturbation in the form of a Li\'enard system, by requiring the
perturbation to be a standing wave under the second order of nonlinearity. We
perform a dynamical systems analysis of the Li\'enard system to reveal a saddle
point in real time, whose implication is that instabilities will develop in the
accreting system when the perturbation is extended into the nonlinear regime.
We also model the perturbation as a high-frequency travelling wave, and carry
out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as
a very feeble effect. Under this approach both the amplitude and the energy
flux of the perturbation exhibit growth, with the acoustic horizon segregating
the regions of stability and instability.Comment: 15 pages, ReVTeX. Substantially revised with respect to the previous
version. One figure and a new section on travelling waves (Sec. VI) have been
added. The bibliography has been revised. arXiv admin note: substantial text
overlap with arXiv:1207.107
Implications of nonlinearity for spherically symmetric accretion
We subject the steady solutions of a spherically symmetric accretion flow to
a time-dependent radial perturbation. The equation of the perturbation includes
nonlinearity up to any arbitrary order, and bears a form that is very similar
to the metric equation of an analogue acoustic black hole. Casting the
perturbation as a standing wave on subsonic solutions, and maintaining
nonlinearity in it up to the second order, we get the time-dependence of the
perturbation in the form of a Li\'enard system. A dynamical systems analysis of
the Li\'enard system reveals a saddle point in real time, with the implication
that instabilities will develop in the accreting system when the perturbation
is extended into the nonlinear regime. The instability of initial subsonic
states also adversely affects the temporal evolution of the flow towards a
final and stable transonic state.Comment: 14 pages, ReVTeX. Substantially revised with respect to the previous
version. Three figures and a new section (Sec. VI) adde
Determination of Matter Surface Distribution of Neutron-rich Nuclei
We demonstrate that the matter density distribution in the surface region is
determined well by the use of the relatively low-intensity beams that become
available at the upcoming radioactive beam facilities. Following the method
used in the analyses of electron scattering, we examine how well the density
distribution is determined in a model-independent way by generating pseudo data
and by carefully applying statistical and systematic error analyses. We also
study how the determination becomes deteriorated in the central region of the
density, as the quality of data decreases. Determination of the density
distributions of neutron-rich nuclei is performed by fixing parameters in the
basis functions to the neighboring stable nuclei. The procedure allows that the
knowledge of the density distributions of stable nuclei assists to strengthen
the determination of their unstable isotopes.Comment: 41 pages, latex, 27 figure
Field-free orientation of CO molecules by femtosecond two-color laser fields
We report the first experimental observation of non-adiabatic field-free
orientation of a heteronuclear diatomic molecule (CO) induced by an intense
two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by
measuring fragment ion angular distributions after Coulomb explosion with an
800 nm pulse. The orientation of the molecules is controlled by the relative
phase of the two-color field. The results are compared to quantum mechanical
rigid rotor calculations. The demonstrated method can be applied to study
molecular frame dynamics under field-free conditions in conjunction with a
variety of spectroscopy methods, such as high-harmonic generation, electron
diffraction and molecular frame photoemission
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