7,449 research outputs found
Extracting scaling laws from numerical dynamo models
Earth's magnetic field is generated by processes in the electrically
conducting, liquid outer core, subsumed under the term `geodynamo'. In the last
decades, great effort has been put into the numerical simulation of core
dynamics following from the magnetohydrodynamic (MHD) equations. However, the
numerical simulations are far from Earth's core in terms of several control
parameters. Different scaling analyses found simple scaling laws for quantities
like heat transport, flow velocity, magnetic field strength and magnetic
dissipation time.
We use an extensive dataset of 116 numerical dynamo models compiled by
Christensen and co-workers to analyse these scalings from a rigorous model
selection point of view. Our method of choice is leave-one-out cross-validation
which rates models according to their predictive abilities. In contrast to
earlier results, we find that diffusive processes are not negligible for the
flow velocity and magnetic field strength in the numerical dynamos. Also the
scaling of the magnetic dissipation time turns out to be more complex than
previously suggested. Assuming that the processes relevant in the numerical
models are the same as in Earth's core, we use this scaling to estimate an
Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with
recent high CMB heat flux scenarios.Comment: 21 pages, 11 figure
Effects Of Kaluza-Klein Excited W On Single Top Quark Production At Tevatron
In extra dimension theories if the gauge bosons of the standard model
propagate in the bulk of the extra dimensions then they will have Kaluza-Klein
excitations that can couple to the standard model fermions. In this paper we
study the effects of the first excited Kaluza-Klein mode of the W on single top
production at the Tevatron. We find that the cross section for the single top
production can be significantly reduced if the mass of the first Kaluza-Klein
excited TeV. Hence, a measurement of the single top production cross
section smaller than the standard model prediction would not necessarily imply
or evidence of extra generation(s) of fermions mixed with the third
generation.Comment: Text added, Latex, 16 pages, 3 figures, To appear in Phys. Lett.
Accretion, disks, and magnetic activity in the TW Hya association
We present new photometric and spectroscopic data for the M-type members of
the TW Hya association with the aim of a comprehensive study of accretion,
disks and magnetic activity at the critical age of ~10 Myr where circumstellar
matter disappears.Comment: 4 pages, to appear in Proceedings IAU Symposium No. 314, Young Stars
& Planets Near the Sun, 201
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 FU Tau A
We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A,
a presumed young brown dwarf in the Taurus star forming region that has
intrigued both theorists and observers by its over-luminosity in the HR diagram
with respect to standard pre-main sequence evolutionary models. The new data,
obtained with the X-Shooter spectrograph at the Very Large Telescope, include
an unprecedented wealth of information on stellar parameters and simultaneously
observed accretion and outflow indicators for FU Tau A. We present the first
measurements of gravity (log g = 3.5 +- 0.5), radial velocity (RV = 22.5 +- 2.9
km/s), rotational velocity (v sin(i) = 20 +- 5 km/s) and lithium equivalent
width (W_Li = 430 +- 20 mAA) for FUTau A. From the rotational velocity and the
published period we infer a disk inclination of i ~ 50^deg. The lithium content
is much lower than theoretically expected for such a young very low mass
object, adding another puzzling feature to this object's properties. We
determine the mass accretion rate of FU Tau A from comparison of the
luminosities of 24 emission lines to empirical calibrations from the literature
and find a mean of log (dM/dt)_acc [M_sun/yr] = -9.9 +- 0.2. The accretion rate
determined independently from modeling of the excess emission in the Balmer and
Paschen continua is consistent with this value. The corresponding accretion
luminosity is too small to make a significant contribution to the bolometric
luminosity. The existence of an outflow in FU Tau A is demonstrated through the
first detection of forbidden emission lines from which we obtain an estimate
for the mass loss rate, log (dM/dt)_out [M_sun/yr] < -10.4. The mass outflow
and inflow rates can be combined to yield (dM/dt)_out / (dM/dt)_acc ~ 0.3, a
value that is in agreement with jet launching models.Comment: 12 pages, accepted for publication in A&
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
Extracting scaling laws from numerical dynamo models
Earth's magnetic field is generated by processes in the electrically conducting, liquid outer core, subsumed under the term ‘geodynamo'. In the last decades, great effort has been put into the numerical simulation of core dynamics following from the magnetohydrodynamic equations. However, the numerical simulations are far from Earth's core in terms of several control parameters. Different scaling analyses found simple scaling laws for quantities like heat transport, flow velocity, magnetic field strength and magnetic dissipation time. We use an extensive data set of 116 numerical dynamo models compiled by Christensen and co-workers to analyse these scalings from a rigorous model selection point of view. Our method of choice is leave-one-out cross-validation which rates models according to their predictive abilities. In contrast to earlier results, we find that diffusive processes are not negligible for the flow velocity and magnetic field strength in the numerical dynamos. Also the scaling of the magnetic dissipation time turns out to be more complex than previously suggested. Assuming that the processes relevant in the numerical models are the same as in Earth's core, we use this scaling to estimate an Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with recent high core-mantle boundary heat flux scenario
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