3,596 research outputs found
An infrared imaging search for low-mass companions to members of the young nearby beta Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby stars in
the Tucana/Horologium and beta Pic associations, all ~ 10 to 35 Myrs young and
at ~10 to 60 pc distance. Such young nearby stars are well-suited for direct
imaging searches for brown dwarf and even planetary companions, because young
sub-stellar objects are still self-luminous due to contraction and accretion.
We performed our observations at the ESO 3.5m NTT with the normal infrared
imaging detector SofI and the MPE speckle camera Sharp-I. Three arc sec north
of GSC 8047-0232 in Horologium a promising brown dwarf companion candidate is
detected, which needs to be confirmed by proper motion and/or spectroscopy.
Several other faint companion candidates are already rejected by second epoch
imaging. Among 21 stars observed in Tucana/Horologium, there are not more than
one to five brown dwarf companions outside of 75 AU (1.5" at 50 pc); most
certainly only < 5 % of the Tuc/HorA stars have brown dwarf companions (13 to
78 Jupiter masses) outside of 75 AU. For the first time, we can report an upper
limit for the frequency of massive planets (~ 10 M_jup) at wide separations (~
100 AU) using a meaningfull and homogeneous sample: Of 11 stars observed
sufficiently deep in beta Pic (12 Myrs), not more than one has a massive planet
outside of ~ 100 AU, i.e. massive planets at large separations are rare (< 9
%).Comment: Astronomische Nachrichten, in pres
The QCD equation of state at finite density from analytical continuation
We determine the equation of state of QCD at finite chemical potential, to
order , for a system of 2+1 quark flavors. The simulations are
performed at the physical mass for the light and strange quarks on several
lattice spacings; the results are continuum extrapolated using lattices of up
to temporal resolution. The QCD pressure and interaction measure are
calculated along the isentropic trajectories in the plane
corresponding to the RHIC Beam Energy Scan collision energies. Their behavior
is determined through analytic continuation from imaginary chemical potentials
of the baryonic density. We also determine the Taylor expansion coefficients
around from the simulations at imaginary chemical potentials.
Strangeness neutrality and charge conservation are imposed, to match the
experimental conditions.Comment: 5 pages, 4 figure
Inclusion of turbulence in solar modeling
The general consensus is that in order to reproduce the observed solar p-mode
oscillation frequencies, turbulence should be included in solar models.
However, until now there has not been any well-tested efficient method to
incorporate turbulence into solar modeling. We present here two methods to
include turbulence in solar modeling within the framework of the mixing length
theory, using the turbulent velocity obtained from numerical simulations of the
highly superadiabatic layer of the sun at three stages of its evolution. The
first approach is to include the turbulent pressure alone, and the second is to
include both the turbulent pressure and the turbulent kinetic energy. The
latter is achieved by introducing two variables: the turbulent kinetic energy
per unit mass, and the effective ratio of specific heats due to the turbulent
perturbation. These are treated as additions to the standard thermodynamic
coordinates (e.g. pressure and temperature). We investigate the effects of both
treatments of turbulence on the structure variables, the adiabatic sound speed,
the structure of the highly superadiabatic layer, and the p-mode frequencies.
We find that the second method reproduces the SAL structure obtained in 3D
simulations, and produces a p-mode frequency correction an order of magnitude
better than the first method.Comment: 10 pages, 12 figure
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
Space and Ground Based Pulsation Data of Eta Bootis Explained with Stellar Models Including Turbulence
The space telescope MOST is now providing us with extremely accurate low
frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this
paper that these data, when combined with ground based measurements of the high
frequency p-mode spectrum, can be reproduced with stellar models that include
the effects of turbulence in their outer layers. Without turbulence, the l=0
modes of our models deviate from either the ground based or the space data by
about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including
turbulence in the models and we can exactly match 12 out of 13 MOST frequencies
that we identified as l=0 modes in addition to 13 out of 21 ground based
frequencies within their observational 2 sigma tolerances. The better agreement
between model frequencies and observed ones depends for the most part on the
turbulent kinetic energy which was taken from a 3D convection simulation for
the Sun.Comment: 13 pages, 7 figures, ApJ in pres
A systematic review of pedagogies that support, engage and improve the educational outcomes of Aboriginal students
This review analyses studies that identify pedagogies to support, engage and improve Aboriginal and Torres Strait Islander student outcomes. Some studies focus on pedagogies to support and engage, while others describe pedagogies that are designed to improve engagement, attendance and academic skills. The role of context emerges as a key theme, particularly in remote areas. In larger studies, Aboriginal students are often a subset of a larger student group, included because of socio-economic status and achievement levels. Key findings indicate a disconnect between practice and outcomes where links to improved outcomes are by implication rather than evidence. Further, definitions and detail about pedagogies are mostly absent, relying on âcommon understandingsâ of what pedagogy means. This review highlights that most of the research identifies effective pedagogies to engage and support Aboriginal students rather than to improve their educational outcomes
Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry
We have simultaneously monitored the photometric and polarimetric variations
of the Classical T Tauri star AA Tau during the fall of 2002. We combine these
data with previously published polarimetric data covering two earlier epochs.
The phase coverage is complete, although not contiguous. AA Tau clearly shows
cyclic variations coupled with the rotation of the system. The star-disk system
produces a repeatable polarisation curve where the polarisation increases with
decreasing brightness. The data fit well with the model put forward by Bouvier
et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively
dumping material onto the central star via magnetospheric accretion. The inner
edge of the disk is deformed by its interaction with the tilted magnetosphere,
producing eclipses as it rotates and occults the photosphere periodically. From
the shape of the polarisation curve in the QU-Plane we confirm that the
accretion disk is seen at a large inclination, almost edge-on, and predict that
its position angle is PA~90 deg., i.e., that the disk's major axis is oriented
in the East-West direction.Comment: Astron. Astrophys., in pres
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