6,909 research outputs found
Using the Sun to estimate Earth-like planets detection capabilities.I. Impact of cold spots
Stellar spots may in some cases produce radial velocity (RV) signatures
similar to those of exoplanets. To further investigate the impact of spots, we
aim at studying the detectability of Earth mass planets in the habitable zone
(HZ) of solar type stars, if covered by spots similar to the sunspots. We have
used the Sunspots properties recorded over one solar cycle between 1993 and
2003 to build the RV curve that a solar type star seen edge-on would show, if
covered by such spots with Tsun -Tspot = 550K. We also simulate the RV of such
a spotted star surrounded by an Earth mass planet located in the HZ. Under
present assumptions, the detection of a 1 M Earth planet located between 0.8
and 1.2 AU requires an intensive monitoring (weekly or better), during several
years of low activity phasis. The temporal sampling is more crucial than the
precision of the data (assuming precisions in the range [1-10] cm/s). Cooler
spots may become a problem for such detections. Also, we anticipate that
plages, not considered in this paper, could further complicate or even
compromise the detections
A new function of hydrocarbons in insect communication : maternal care and offspring signalling in the European earwig
International audienc
A new method of correcting radial velocity time series for inhomogeneous convection
Magnetic activity strongly impacts stellar RVs and the search for small
planets. We showed previously that in the solar case it induces RV variations
with an amplitude over the cycle on the order of 8 m/s, with signals on short
and long timescales. The major component is the inhibition of the convective
blueshift due to plages. We explore a new approach to correct for this major
component of stellar radial velocities in the case of solar-type stars. The
convective blueshift depends on line depths; we use this property to develop a
method that will characterize the amplitude of this effect and to correct for
this RV component. We build realistic RV time series corresponding to RVs
computed using different sets of lines, including lines in different depth
ranges. We characterize the performance of the method used to reconstruct the
signal without the convective component and the detection limits derived from
the residuals. We identified a set of lines which, combined with a global set
of lines, allows us to reconstruct the convective component with a good
precision and to correct for it. For the full temporal sampling, the power in
the range 100-500~d significantly decreased, by a factor of 100 for a RV noise
below 30 cm/s. We also studied the impact of noise contributions other than the
photon noise, which lead to uncertainties on the RV computation, as well as the
impact of the temporal sampling. We found that these other sources of noise do
not greatly alter the quality of the correction, although they need a better
noise level to reach a similar performance level. A very good correction of the
convective component can be achieved providing very good RV noise levels
combined with a very good instrumental stability and realistic granulation
noise. Under the conditions considered in this paper, detection limits at 480~d
lower than 1 MEarth could be achieved for RV noise below 15 cm/s.Comment: Accepted in A&A 18 July 201
Spontaneous polarization and piezoelectricity in boron nitride nanotubes
Ab initio calculations of the spontaneous polarization and piezoelectric
properties of boron nitride nanotubes show that they are excellent
piezoelectric systems with response values larger than those of piezoelectric
polymers. The intrinsic chiral symmetry of the nanotubes induces an exact
cancellation of the total spontaneous polarization in ideal, isolated nanotubes
of arbitrary indices. Breaking of this symmetry by inter-tube interaction or
elastic deformations induces spontaneous polarization comparable to those of
wurtzite semiconductors.Comment: 5 pages in PRB double column format, 3 figure
Reconstructing the solar integrated radial velocity using MDI/SOHO
Searches for exoplanets with radial velocity techniques are increasingly
sensitive to stellar activity. It is therefore crucial to characterize how this
activity influences radial velocity measurements in their study of the
detectability of planets in these conditions. In a previous work we simulated
the impact of spots and plages on the radial velocity of the Sun. Our objective
is to compare this simulation with the observed radial velocity of the Sun for
the same period. We use Dopplergrams and magnetograms obtained by MDI/SOHO over
one solar cycle to reconstruct the solar integrated radial velocity in the Ni
line 6768 \AA. We also characterize the relation between the velocity and the
local magnetic field to interpret our results. We obtain a stronger redshift in
places where the local magnetic field is larger (and as a consequence for
larger magnetic structures): hence we find a higher attenuation of the
convective blueshift in plages than in the network. Our results are compatible
with an attenuation of this blueshift by about 50% when averaged over plages
and network. We obtain an integrated radial velocity with an amplitude over the
solar cycle of about 8 m/s, with small-scale variations similar to the results
of the simulation, once they are scaled to the Ni line. The observed solar
integrated radial velocity agrees with the result of the simulation made in our
previous work within 30%, which validates this simulation. The observed
amplitude confirms that the impact of the convective blueshift attenuation in
magnetic regions will be critical to detect Earth-mass planets in the habitable
zone around solar-like stars.Comment: 17 pages, 11 figures, accepted in Astronomy and Astrophysic
Evolution of the rates of mass wasting and fluvial sediment transfer from the epicentral area of the 1999, Mw 7.6 earthquake
The 1999 Chichi earthquake (Mw=7.6) triggered more than 20,000 landslides in the epicentral area in central west Taiwan, and subsequent typhoons have caused an even larger number of slope failures. As a result, the suspended sediment load of the epi- central Choshui River has increased dramatically. Measurements of suspended sedi- ment at a downstream gauging station indicate that the unit sediment concentration increased about six times due to the earthquake, and decreased exponentially due to flushing by subsequent typhoons. The e-folding time scale of the seismic perturbation of sediment transfer in the Choshui River is 3-5 years. Based on this estimate of the de- cay of the erosional response to the earthquake, a mass balance can be calculated for the earthquake, including co-seismic uplift and subsidence, post-seismic relaxation, and erosion. This mass balance shows that the Chi-Chi earthquake has acted to build ridge topography in the hanging wall of the fault, but in the far field, some destruc- tion of topography has occurred. However, our estimate of seismically-driven erosion may be incomplete. A detailed analysis of landsliding in the Chenyoulan tributary of the Choshui River indicates that most co-and post seismic landslide debris remains on hillslopes within the catchment. Recent typhoons have continued to cause high rates of landsliding high in the landscape, but rates of mass wasting near the stream net- work have decreased. The full geomorphic response to the Chi-Chi earthquake may be much larger, and more protracted than indicated by river gauging data
Variability of stellar granulation and convective blueshift with spectral type and magnetic activity. I. K and G main sequence stars
In solar-type stars, the attenuation of convective blueshift by stellar
magnetic activity dominates the RV variations over the low amplitude signal
induced by low mass planets. Models of stars that differ from the Sun will
require a good knowledge of the attenuation of the convective blueshift to
estimate its impact on the variations. It is therefore crucial to precisely
determine not only the amplitude of the convective blueshift for different
types of stars, but also the dependence of this convective blueshift on
magnetic activity, as these are key factors in our model producing the RV. We
studied a sample of main sequence stars with spectral types from G0 to K2 and
focused on their temporally averaged properties: the activity level and a
criterion allowing to characterise the amplitude of the convective blueshift.
We find the differential velocity shifts of spectral lines due to convection to
depend on the spectral type, the wavelength (this dependence is correlated with
the Teff and activity level), and on the activity level. This allows us to
quantify the dependence of granulation properties on magnetic activity for
stars other than the Sun. The attenuation factor of the convective blueshift
appears to be constant over the considered range of spectral types. We derive a
convective blueshift which decreases towards lower temperatures, with a trend
in close agreement with models for Teff lower than 5800 K, but with a
significantly larger global amplitude. We finally compare the observed RV
variation amplitudes with those that could be derived from our convective
blueshift using a simple law and find a general agreement on the amplitude. Our
results are consistent with previous results and provide, for the first time,
an estimation of the convective blueshift as a function of Teff, magnetic
activity, and wavelength, over a large sample of G and K main sequence stars
Alien Registration- Meunier, Lea M. (Augusta, Kennebec County)
https://digitalmaine.com/alien_docs/18996/thumbnail.jp
Efficient C-Phase gate for single-spin qubits in quantum dots
Two-qubit interactions are at the heart of quantum information processing.
For single-spin qubits in semiconductor quantum dots, the exchange gate has
always been considered the natural two-qubit gate. The recent integration of
magnetic field or g-factor gradients in coupled quantum dot systems allows for
a one-step, robust realization of the controlled phase (C-Phase) gate instead.
We analyze the C-Phase gate durations and fidelities that can be obtained under
realistic conditions, including the effects of charge and nuclear field
fluctuations, and find gate error probabilities of below 10-4, possibly
allowing fault-tolerant quantum computation.Comment: 5 pages, 3 figure
Three-dimensional instabilities of a stratified cylinder wake
International audienceThis paper describes experimentally, numerically and theoretically how the three-dimensional instabilities of a cylinder wake are modified by the presence of a linear density stratification. The first part is focused on the case of a cylinder with a small tilt angle between the cylinder's axis and the vertical. The classical mode A well-known for a homogeneous fluid is still present. It is more unstable for moderate stratifications but it is stabilized by a strong stratification. The second part treats the case of a moderate tilt angle. For moderate stratifications, a new unstable mode appears, mode S, characterized by undulated layers of strong density gradients and axial flow. These structures correspond to Kelvin–Helmholtz billows created by the strong shear present in the critical layer of each tilted von Kármán vortex. The last two parts deal with the case of a strongly tilted cylinder. For a weak stratification, an instability (mode RT) appears far from the cylinder, due to the overturning of the isopycnals by the von Kármán vortices. For a strong stratification, a short wavelength unstable mode (mode L) appears, even in the absence of von Kármán vortices. It is probably due to the strong shear created by the lee waves upstream of a secondary recirculation bubble. A map of the four different unstable modes is established in terms of the three parameters of the study: the Reynolds number, the Froude number (characterizing the stratification) and the tilt angle
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