Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program II. Activity and radial velocity

Due to their low mass and luminosity, M dwarfs are ideal targets if one hopes to find low-mass planets similar to Earth by using the radial velocity (RV) method. However, stellar magnetic cycles could add noise or even mimic the RV signal of a long-period companion. Following our previous work that studied the correlation between activity cycles and long-term RV variations for K dwarfs we now expand that research to the lower-end of the main sequence. Our objective is to detect any correlations between long-term activity variations and the observed RV of a sample of M dwarfs. We used a sample of 27 M-dwarfs with a median observational timespan of 5.9 years. The cross-correlation function (CCF) with its parameters RV, bisector inverse slope (BIS), full-width-at-half- maximum (FWHM) and contrast have been computed from the HARPS spectrum. The activity index have been derived using the Na I D doublet. These parameters were compared with the activity level of the stars to search for correlations. We detected RV variations up to ~5 m/s that we can attribute to activity cycle effects. However, only 36% of the stars with long-term activity variability appear to have their RV affected by magnetic cycles, on the typical timescale of ~6 years. Therefore, we suggest a careful analysis of activity data when searching for extrasolar planets using long-timespan RV data.Comment: 20 pages, 12 figures, 3 tables, accepted for publication in Astronomy and Astophysic

Bound States and Critical Behavior of the Yukawa Potential

We investigate the bound states of the Yukawa potential $V(r)=-\lambda \exp(-\alpha r)/ r$, using different algorithms: solving the Schr\"odinger equation numerically and our Monte Carlo Hamiltonian approach. There is a critical $\alpha=\alpha_C$, above which no bound state exists. We study the relation between $\alpha_C$ and $\lambda$ for various angular momentum quantum number $l$, and find in atomic units, $\alpha_{C}(l)= \lambda [A_{1} \exp(-l/ B_{1})+ A_{2} \exp(-l/ B_{2})]$, with $A_1=1.020(18)$, $B_1=0.443(14)$, $A_2=0.170(17)$, and $B_2=2.490(180)$.Comment: 15 pages, 12 figures, 5 tables. Version to appear in Sciences in China

Scalar-Kinetic Branes

This work tries to find out thick brane solutions in braneworld scenarios described by a real scalar field in the presence of a scalar-kinetic term $F(X,\phi)=X\phi^m$ with a single extra dimension, where $X=\frac12\nabla_M\phi\nabla^M\phi$ stands for the standard kinetic term and $m=0,1,2\cdots$. We mainly consider bent branes, namely de Sitter and Anti-de Sitter four-dimensional slices. The solutions of a flat brane are obtained when taking the four-dimensional cosmological constant $\Lambda_4\rightarrow 0$. When the parameter $m=0$, these solutions turn to those of the standard scenario. The localization and spectrum of graviton on these branes are also analyzed.Comment: 10 pages, no figures, accepted by EP

Generalized Hermite-Gauss decomposition of the two-photon state produced by spontaneous parametric down-conversion

We provide a general decomposition of the two-photon state produced by spontaneous parametric down-conversion in Hermite-Gaussian modes, in the case that the pump beam is described by a Hermite-Gaussian beam of any order. We show that the spatial correlations depend explicitly on the order of the pump beam, as well as other experimental parameters. We use the decomposition to demonstrate a few interesting cases. Our results are applicable to the engineering of two-photon spatial entanglement, in particular for non-Gaussian states.Comment: 14 page draft, 5 figure

InAs avalanche photodiodes as X-ray detectors

We designed and demonstrated an InAs avalanche photodiode (APD) for X-ray detection, combining narrow band gap semiconductor materials and avalanche gain from APDs. The InAs APD (cooled by liquid nitrogen) was tested with a 55Fe X-ray source. Full width at half maximum (FWHM) from the spectra decreases rapidly with reverse bias, rising again for higher voltages, resulting in a minimum FWHM value of 401 eV at 5.9 keV. This minimum value was achieved at 10 V reverse bias, which corresponds to an avalanche gain of 11. The dependence of FWHM on reverse bias observed is explained by the competition between various factors, such as leakage current, capacitance and avalanche gain from the APD, as well as measurement system noise. The minimum FWHM achieved is largely dominated by the measurement system noise and APD leakage current

Dual Spaces of Resonance In Thick p−p-Branes

In this work we consider $q-$form fields in a $p-$brane embedded in a $D=(p+2)$ space-time. The membrane is generated by a domain wall in a Randall-Sundrum-like scenario. We study conditions for localization of zero modes of these fields. The expression agrees and generalizes the one found for the zero, one, two and three-forms in a $3-$brane. By a generalization we mean that our expression is valid for any form in an arbitrary dimension with codimension one. We also point out that, even without the dilaton coupling, some form fields are localized in the membrane. The massive modes are considered and the resonances are calculated using a numerical method. We find that different spaces have identical resonance structures, which we call dual spaces of resonances(DSR).Comment: 15 page

Fermion localization on asymmetric two-field thick branes

In this paper we investigate the localization of fermions on asymmetric thick branes generated by two scalars $\phi$ and $\chi$. In order to trap fermions on the asymmetric branes with kink-like warp factors, the couplings with the background scalars $\eta\bar{\Psi}F(\chi,\phi)\Psi$ are introduced, where $F(\chi,\phi)$ is a function of $\phi$ and $\chi$. We find that the coupling $\eta\bar{\Psi}\chi\phi\Psi$ do not support the localization of 4-dimensional fermions on the branes. While, for the case $\eta\bar{\Psi}\chi\Psi+\eta'\bar{\Psi}\phi\Psi$, which is the kink-fermion coupling corresponding to one-scalar-generated brane scenarios, the zero mode of left-handed fermions could be trapped on the branes under some conditions.Comment: v2: 11 pages, 4 figures, accepted by CQ

Flexible piezoresistive pressure sensors for smart textiles

The development of smart textiles relies in many applications on of textile-integrated sensors. Flexible piezoresistive pressure sensors have many potential applications, for instance in sports science. In this study, flexible pressure sensors are built using piezoresistive polymer film and conductive fabric. Tests using a universal testing machine show that the sensors are functional, accurate, although showing some hysteresis. However, methods for joining the electrode and piezoresistive layers are necessary to assure mechanical stability of the sensor, without affecting electrical contact between layers. Several methods were tested and results are reported here. The use of thermofusible bonding nets and webs has been found to be an interesting solution.This work is financed by Project “Deus ex Machina”, NORTE-01-0145-FEDER-000026, funded by CCDRN, through Sistema de Apoio à Investigação Cientifica e Tecnológica (Projetos Estruturados I&D&I) of Programa Operacional Regional do Norte, from Portugal 2020 and by FEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT – Foundation for Science and Technology within the scope of the project POCI-01-0145-FEDER-007136. Derya Tama thanks FCT for fellowship 2C2T-BPD-08-2017.info:eu-repo/semantics/publishedVersio