861 research outputs found
Possible detection of a magnetic field in T Tauri
Medium-resolution circular spectropolarimetry of T Tauri is
presented. The star was observed twice: on November 11, 1996 and January 22,
2002. Weak circular polarization has been found in photospheric absorption
lines, indicating a mean surface longitudinal magnetic field of
G and G at the epoch of the first and second
observations respectively. While these values are near the detection limit of
our apparatus, we belive that they are real. In any case one can conclude from
our data that of T Tau does not significantly exceed 200 G, which is
much less than surface magnetic field strength of the star ( kG) found by
Guenther et al. (1999) and Johns-Krull et al. (2000). We discuss possible
reasons of this difference.Comment: 5 pages, 3 figure
Modelling the molecular Zeeman effect in M-dwarfs: methods and first results
We present first quantitative results of the surface magnetic field
measurements in selected M-dwarfs based on detailed spectra synthesis conducted
simultaneously in atomic and molecular lines of the FeH Wing-Ford
transitions. A modified version of the Molecular
Zeeman Library (MZL) was used to compute Land\'e g-factors for FeH lines in
different Hund's cases. Magnetic spectra synthesis was performed with the
Synmast code. We show that the implementation of different Hund's case for FeH
states depending on their quantum numbers allows us to achieve a good fit to
the majority of lines in a sunspot spectrum in an automatic regime. Strong
magnetic fields are confirmed via the modelling of atomic and FeH lines for
three M-dwarfs YZ~CMi, EV~Lac, and AD~Leo, but their mean intensities are found
to be systematically lower than previously reported. A much weaker field
(~kG against ~kG) is required to fit FeH lines in the spectra of
GJ~1224. Our method allows us to measure average magnetic fields in very
low-mass stars from polarized radiative transfer. The obtained results indicate
that the fields reported in earlier works were probably overestimated by about
\%. Higher quality observations are needed for more definite results.Comment: Accepted by A&A, 13 pages, 7 figures, 1 tabl
Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy
Near IR spectra obtained with ISAAC at VLT, have been used to pose
constraints on the evolutionary state and accretion properties of a sample of
five embedded YSOs located in the R CrA core. This sample includes three Class
I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6
and IRS3). Absorption lines have been detected in the medium resolution spectra
of all the observed targets, together with emission lines likely originating in
the disk-star-wind connected regions. We derived spectral types, veiling and
stellar luminosity of the five observed sources, which in turn have been used
to infer their mass and age adopting pre-main sequence evolutionary tracks. We
find that in HH100 IR and IRS2 most of the bolometric luminosity is due to
accretion, while the other three investigated sources, including the Class I
object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We
observe a general correlation between the accretion luminosity, the IR veiling
and the emission line activity of the sources. A correlation between the
accretion activity and the spectral energy distribution slope is recognizable
but with the notable exception of IRS5a. Our analysis therefore shows how the
definition of the evolutionary stage of deeply embedded YSOs by means of IR
colors needs to be more carefully refined.Comment: 17 pages, 13 figures, accepted on A&
Constrained by managerialism : caring as participation in the voluntary social services
The data in this study show that care is a connective process, underlying and motivating participation and as a force that compels involvement in the lives of others, care is at least a micro-participative process. Care or affinity not only persisted in the face of opposition, but it was also used by workers as a counter discourse and set of practices with which to resist the erosion of worker participation and open up less autonomized practices and ways of connecting with fellow staff, clients and the communities they served. The data suggest that while managerialism and taylorised practice models may remove or reduce opportunities for worker participation, care is a theme or storyline that gave workers other ways to understand their work and why they did it, as well as ways they were prepared to resist managerial priorities and directives, including the erosion of various kinds of direct and indirect participation. The degree of resistance possible, even in the highly technocratic worksite in Australia, shows that cracks and fissures exist within managerialism
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
The model of dynamo with small number of modes and magnetic activity of T Tauri stars
The model that describes operation of dynamo in fully convective stars is
presented. It is based on representation of stellar magnetic field as a
superposition of finite number of poloidal and toroidal free damping modes. In
the frame of adopted low of stellar differential rotation we estimated minimal
value of dynamo number D, starting from which generation of cyclic magnetic
field in stars without radiative core is possible. We also derived expression
for period of the cycle. It was found that dynamo cycles of fully convective
stars and stars with thin convective envelopes differ in a qualitative way: 1)
distribution of spots over latitude during the cycle is different in these
stars; 2) the model predicts that spot formation in fully convective stars
should be strongly suppressed at some phases of the cycle.
We have analyzed historical lightcurve of WTTS star V410 Tau and found that
long term activity of the star is not periodic process. Rather one can speak
about quasi cyclic activity with characteristic time of yr and chaotic
component over imposed. We concluded also that redistribution of cool spots
over longitude is the reason of long term variations of V410 Tau brightness. It
means that one can not compare directly results of photometric observations
with predictions of our axially symmetric (for simplicity) model which allows
to investigate time evolution of spot's distribution over latitude. We then
discuss what kind of observations and in which way could be used to check
predictions of the dynamo theory.Comment: 18 pages, 5 figures, accepted to Astron. Let
Magnetically channeled accretion in T Tauri stars : a dynamical process
We review observational evidence and open issues related to the process of
magnetospheric accretion in T Tauri stars. Emphasis is put on recent numerical
simulations and observational results which suggest that the interaction
between the stellar magnetosphere and the inner accretion disk is a highly time
dependent process on timescales ranging from hours to months.Comment: To appear in Open Issues in Local Star Formation and Early Stellar
Evolution, eds. J. Lepine, J. Gregorio-Hete
The analysis of magnetic field measurements of T Tau
It is shown that the existence of hot accretion spots at the surface of T Tau
practically has no effect on the accuracy of estimation of its magnetic field
strength at photospheric level.
We also found that one can interpret results of T Tau's photospheric magnetic
field strength measurements carried out via different methods in the frame of
the following alternative: 1) if T Tau's inclination angle
then magnetic field of the star may be dipolar with the angle between
rotational and magnetic axes is near 2) if it will be found (e.g.
from interferometric observations) that then magnetic field of T
Tau is essentially non-dipolar or/and non-stationary.Comment: 4 figure
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