6,369 research outputs found
Search of X-ray emission from roAp stars: The case of gamma Equulei
The detection of X-ray emission from Ap stars can be an indicator for the
presence of magnetic activity and dynamo action, provided different origins for
the emission, such as wind shocks and close late-type companions, can be
excluded. Here we report on results for gamma Equu, the only roAp star for
which an X-ray detection is reported in ROSAT catalogs. We use high resolution
imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that
allow us to spatially resolve companions down to ~1" and ~0.06" separations,
respectively. The bulk of the X-ray emission is associated with a companion of
gamma Equu identified in our NACO image. Assuming coevality with the primary
roAp star (~900 Myr), the available photometry for the companion points at a
K-type star with ~0.6 M_sun. Its X-ray properties are in agreement with the
predictions for its age and mass. An excess of photons with respect to the
expected background and contribution from the nearby companion is observed near
the optical position of gamma Equu. We estimate an X-ray luminosity of log L_x
[erg/s] = 26.6 and log(L_x/L_bol) = -7.9 for this emission. A small offset
between the optical and the X-ray image leaves some doubt on its association
with the roAp star. The faint X-ray emission that we tentatively ascribe to the
roAp star is difficult to explain as a solar-like stellar corona due to its
very low L_x/L_bol level and the very long rotation period of gamma Equu. It
could be produced in magnetically confined wind shocks implying a mass loss
rate of ~10^(-14) M_sun/yr or from an additional unknown late-type companion at
separation ~0.4". If confirmed by future deeper X-ray observations this
emission could point at the origin for the presence of radioactive elements on
some roAp stars.Comment: Accepted for publication in Astronomy & Astrophysics (5 pages
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
The enigmatic young brown dwarf binary FU Tau: accretion and activity
FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have
resolved the system in a Chandra X-ray observation and detected only the
primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present
but, unexpectedly, we detect also a strong and unusually soft component from FU
Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar
to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the
dominating soft X-ray component can be explained by emission from accretion
shocks. However, the typical free-fall velocities of a brown dwarf are too low
for an interpretation of the observed X-ray temperature as post-shock region.
On the other hand, velocities in excess of the free-fall speed are derived from
archival optical spectroscopy, and independent pieces of evidence for strong
accretion in FU Tau A are found in optical photometry. The high X-ray
luminosity of FU Tau A coincides with a high bolometric luminosity confirming
an unexplained trend among young brown dwarfs. In fact, FU Tau A is
overluminous with respect to evolutionary models while FU Tau B is on the 1 Myr
isochrone suggesting non-contemporaneous formation of the two components in the
binary. The extreme youth of FU Tau A could be responsible for its peculiar
X-ray properties, in terms of atypical magnetic activity or accretion.
Alternatively, rotation and magnetic field effects may reduce the efficiency of
convection which in turn affects the effective temperature and radius of FU Tau
A shifting its position in the HR diagram. Although there is no direct prove of
this latter scenario so far we present arguments for its plausibility.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure
X-Shooter spectroscopy of young stellar objects in Lupus. Atmospheric parameters, membership and activity diagnostics
A homogeneous determination of basic stellar parameters of young stellar
object (YSO) candidates is needed to confirm their evolutionary stage,
membership to star forming regions (SFRs), and to get reliable values of the
quantities related to chromospheric activity and accretion. We used the code
ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric
parameters (Teff and logg), the veiling, the radial (RV) and projected
rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates in
the Lupus SFR. We have shown that 13 candidates can be rejected as Lupus
members based on their discrepant RV with respect to Lupus and/or the very low
logg values. At least 11 of them are background giants. The spectral
subtraction of inactive templates enabled us to measure the line fluxes for
several diagnostics of both chromospheric activity and accretion. We found that
all Class-III sources have H fluxes compatible with a pure
chromospheric activity, while objects with disks lie mostly above the boundary
between chromospheres and accretion. YSOs with transitional disks displays both
high and low H fluxes. We found that the line fluxes per unit surface
are tightly correlated with the accretion luminosity () derived
from the Balmer continuum excess. This rules out that the relationships between
and line luminosities found in previous works are simply due to
calibration effects. We also found that the CaII-IRT flux ratio,
, is always small, indicating an optically thick emission
source. The latter can be identified with the accretion shock near the stellar
photosphere. The Balmer decrement reaches instead, for several accretors, high
values typical of optically thin emission, suggesting that the Balmer emission
originates in different parts of the accretion funnels with a smaller optical
depth.Comment: 28 pages, 26 figures, accepted by A&
An S-FSCW Based Multi-Channel Reader System for Beamforming Applications using Surface Acoustic Wave Sensors
Interrogating multiple surface acoustic wave (SAW) sensors located within the same radar beam require techniques to separate the multiple superposing SAW sensor responses. The presented multi-channel reader features four parallel transceiver channels, which are based on the switched frequency-stepped continuous-wave principle and high-speed parallelized baseband electronics. Thus classical beamforming applications including angle of arrival measurement of single SAW tags and the angular separation of multiple SAW sensors are presented and compared to a multiple-input multiple-output (MIMO) approach. Due to the larger virtual array in the MIMO approach a larger aperture can be synthesized, which leads to significantly better angular separation results. The level analysis for the given system is verified by baseband-power measurements at different readout distances, considering the hardware parameters as well as the free-space propagation aspects. Finally measurements assess the maximum interrogation distance for the system
Numerical study of surface-induced reorientation and smectic layering in a nematic liquid crystal
Surface-induced profiles of both nematic and smectic order parameters in a
nematic liquid crystal, ranging from an orienting substrate to "infinity", were
evaluated numerically on base of an extended Landau theory. In order to obtain
a smooth behavior of the solutions at "infinity" a boundary energy functional
was derived by linearizing the Landau energy around its equilibrium solutions.
We find that the intrinsic wave number of the smectic structure, which plays
the role of a coupling between nematic and smectic order, strongly influences
the director reorientation. Whereas the smectic order is rapidly decaying when
moving away from the surface, the uniaxial nematic order parameter shows an
oscillatory behavior close to the substrate, accompanied by a non-zero local
biaxiality.Comment: LaTeX, 17 pages, with 4 postscript figure
Three Numerical Puzzles and the Top Quark's Chiral Weak-Moment
Versus the standard model's t --> W b decay helicity amplitudes, three
numerical puzzles occur at the 0.1 % level when one considers the amplitudes in
the case of an additional (f_M + f_E) coupling of relative strength 53 GeV. The
puzzles are theoretical ones which involve the t --> W b decay helicity
amplitudes in the two cases, the relative strength of this additional coupling,
and the observed masses of these three particles. A deeper analytic realization
is obtained for two of them. Equivalent realizations are given for the
remaining one. An empirical consequence of these analytic realizations is that
it is important to search for effects of a large chiral weak-moment of the
top-quark, the effective mass-scale is about 53 GeV. A full theoretical
resolution would include relating the origin of such a chiral weak-moment and
the mass generation of the top-quark, the W-boson, and probably the b-quark.Comment: 18 pages, 1 postscript table (revised to better explain notation,
model #1, add a little material...
The t W- Mode of Single Top Production
The t W- mode of single top production is proposed as an important means to
study the weak interactions of the top quark. While the rate of this mode is
most likely too small to be observed at Run II of the Fermilab Tevatron, it is
expected to be considerably larger at the CERN LHC. In this article the
inclusive t W- rate is computed, including O(1 / log (m_t^2 / m_b^2))
corrections, and when combined with detailed Monte Carlo simulations including
the top and W decay products, indicate that the t W- single top process may be
extracted from the considerable t tbar and W+ W- j backgrounds at low
luminosity runs of the LHC.Comment: 16 pages, 4 figure
Theory and simulation of the nematic zenithal anchoring coefficient
Combining molecular simulation, Onsager theory and the elastic description of
nematic liquid crystals, we study the dependence of the nematic liquid crystal
elastic constants and the zenithal surface anchoring coefficient on the value
of the bulk order parameter
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