1,077 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
The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence
(Abridged) Projected rotational velocities (vsini) have been measured for a
sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun)
located in the Orion star-forming complex. These measurements have been
supplemented with data from the literature for Orion stars with masses as low
as 0.1 Msun. The primary finding from analysis of these data is that the upper
envelope of the observed values of angular momentum per unit mass (J/M) varies
as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3
Msun. This power law extends smoothly into the domain of more massive stars (3
to 10 Msun), which in Orion are already on the ZAMS. This result stands in
sharp contrast to the properties of main sequence stars, which show a break in
the power law and a sharp decline in J/M with decreasing mass for stars with M
<2 Msun. A second result of our study is that this break is seen already among
the PMS stars in our Orion sample that are on radiative tracks, even though
these stars are only a few million years old. A comparison of rotation rates
seen for stars on either side of the convective-radiative boundary shows that
stars do not rotate as solid bodies during the transition from convective to
radiative tracks.Comment: to appear in Ap
Enhanced Perturbative Continuous Unitary Transformations
Unitary transformations are an essential tool for the theoretical
understanding of many systems by mapping them to simpler effective models. A
systematically controlled variant to perform such a mapping is a perturbative
continuous unitary transformation (pCUT) among others. So far, this approach
required an equidistant unperturbed spectrum. Here, we pursue two goals: First,
we extend its applicability to non-equidistant spectra with the particular
focus on an efficient derivation of the differential flow equations, which
define the enhanced perturbative continuous unitary transformation (epCUT).
Second, we show that the numerical integration of the flow equations yields a
robust scheme to extract data from the epCUT. The method is illustrated by the
perturbation of the harmonic oscillator with a quartic term and of the two-leg
spin ladders in the strong-rung-coupling limit for uniform and alternating rung
couplings. The latter case provides an example of perturbation around a
non-equidistant spectrum.Comment: 27 pages, 18 figures; separated methodological background from
introduction, added perturbed harmonic oscillator for additional
illustration, added explicit solution of deepCUT equation
Gas Absorption in the KH 15D System: Further Evidence for Dust Settling in the Circumbinary Disk
Na I D lines in the spectrum of the young binary KH 15D have been analyzed in
detail. We find an excess absorption component that may be attributed to
foreground interstellar absorption, and to gas possibly associated with the
solids in the circumbinary disk. The derived column density is log N_NaI = 12.5
cm^-2, centered on a radial velocity that is consistent with the systemic
velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~
12.3 cm^-2. There is no detectable change in the gas column density across the
"knife edge" formed by the opaque grain disk, indicating that the gas and
solids have very different scale heights, with the solids being highly settled.
Our data support a picture of this circumbinary disk as being composed of a
very thin particulate grain layer composed of millimeter-sized or larger
objects that are settled within whatever remaining gas may be present. This
phase of disk evolution has been hypothesized to exist as a prelude to the
formation of planetesimals through gravitational fragmentation, and is expected
to be short-lived if much gas were still present in such a disk. Our analysis
also reveals the presence of excess Na I emission relative to the comparison
spectrum at the radial velocity of the currently visible star that plausibly
arises within the magnetosphere of this still-accreting young star.Comment: Accepted for publication in ApJ, 23 pages, 6 figure
Correlated Electrons Step-by-Step: Itinerant-to-Localized Transition of Fe Impurities in Free-Electron Metal Hosts
High-resolution photoemission spectroscopy and realistic ab-initio
calculations have been employed to analyze the onset and progression of d-sp
hybridization in Fe impurities deposited on alkali metal films. The interplay
between delocalization, mediated by the free-electron environment, and Coulomb
interaction among d-electrons gives rise to complex electronic configurations.
The multiplet structure of a single Fe atom evolves and gradually dissolves
into a quasiparticle peak near the Fermi level with increasing the host
electron density. The effective multi-orbital impurity problem within the exact
diagonalization scheme describes the whole range of hybridizations.Comment: 10 pages, 4 figure
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
From observations collected with the ESPaDOnS spectropolarimeter, we report
the discovery of magnetic fields at the surface of the mildly accreting
classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in
photospheric lines and in the emission lines formed at the base of the
accretion funnels linking the disc to the protostar, and monitored over the
whole rotation cycle of V2129 Oph. We observe that rotational modulation
dominates the temporal variations of both unpolarized and circularly polarized
line profiles. We reconstruct the large-scale magnetic topology at the surface
of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to
be rather complex, with a dominant octupolar component and a weak dipole of
strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to
the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar
radial field spots located at high latitudes and coinciding with cool dark
polar spots at photospheric level. This large-scale field geometry is unusually
complex compared to those of non-accreting cool active subgiants with moderate
rotation rates. As an illustration, we provide a first attempt at modelling the
magnetospheric topology and accretion funnels of V2129 Oph using field
extrapolation. We find that the magnetosphere of V2129 Oph must extend to about
7R* to ensure that the footpoints of accretion funnels coincide with the
high-latitude accretion spots on the stellar surface. It suggests that the
stellar magnetic field succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of V2129
Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes
typical of those observed in cTTSs.Comment: MNRAS, in press (18 pages, 17 figures
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