Search results for "Astrophysics::Solar and Stellar Astrophysics"
showing 10 items of 835 documents
Electromagnetically induced transparency in Doppler-broadened three-level systems with resonant standing wave drive
2005
Summary form only given. Electromagnetically induced transparency (EIT) and related phenomena such as lasing without inversion (LWI) are topics of increasing interest in quantum optics. In EIT, an otherwise absorbing medium is made transparent to a weak coherent field on resonance with a certain atomic transition by applying an intense coherent "driving" field to an adjacent transition. There has been a large number of theoretical papers and EIT was demonstrated experimentally. Up to now, most theoretical papers dealing with EIT in gas media have considered only the case of a traveling (TW) driving field. In contrast, we study EIT in Doppler broadened three-level media with standing wave (S…
A VLBI study of the wind-wind collision region in the massive multiple HD 167971
2019
Context. Colliding winds in massive binaries are able to accelerate particles up to relativistic speeds as the result of the interaction between the winds of the different stellar components. HD 167971 exhibits this phenomenon which makes it a strong radio source. Aims. We aim at characterizing the morphology of the radio emission and its dependence on the orbital motion, traced independently by near-infrared (NIR) interferometry of both the spectroscopic binary and the tertiary component comprising HD 167971. Methods. We analyze 2006 and 2016 very long baseline interferometric data at C and X bands. We complement our analysis with a geometrical model of the wind-wind collision region and a…
X-ray emitting MHD accretion shocks in classical T Tauri stars. Case for moderate to high plasma-beta values
2009
AIMS. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of beta >= 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. METHODS. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere, by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). RESULTS. The dynamics and stability of the accretion shock strongly depends on the plasma beta. In the case of shocks with…
Conformations of Star-Branched Polyelectrolytes
1996
Scaling theory describing the conformations of weakly charged star-branched polyelectrolytes in dilute and semi-dilute salt-free solutions is developed. The dependence of the star size on the number of branches as well as on the solution concentration is analyzed. It is shown that the star size increases with an increase in the number of branches, f, at small f and tends to a constant value at large f. An increase in the concentration of stars in a solution results in a decrease in the star size R according to the power low : R ∼ c -1/3 in the range of a moderate concentration and R ∼ c -1/4 at larger concentration. For stars with a small number of branches the behavior R ∼ c -1/2 in a cert…
The HADES RV Programme with HARPS-N@TNG. III. Flux-flux and activity-rotation relationships of early-M dwarfs
2016
(Abridged) Understanding stellar activity in M dwarfs is crucial for the physics of stellar atmospheres as well as for ongoing radial velocity exoplanet programmes. Despite the increasing interest in M dwarfs, our knowledge of the chromospheres of these stars is far from being complete. We aim to test whether the relations between activity, rotation, and stellar parameters and flux-flux relationships also hold for early-M dwarfs on the main-sequence. We analyse in an homogeneous and coherent way a well defined sample of 71 late-K/early-M dwarfs that are currently being observed in the framework of the HArps-n red Dwarf Exoplanet Survey (HADES). Rotational velocities are derived using the cr…
The ALHAMBRA survey: tight dependence of the optical mass-to-light ratio on galaxy colour up to z = 1.5
2018
[Aims] Our goal is to characterise the dependence of the optical mass-to-light ratio on galaxy colour up to z = 1.5, expanding the redshift range explored in previous work. [Methods] From the redshifts, stellar masses, and rest-frame luminosities of the ALHAMBRA multi-filter survey, we derive the mass-to-light ratio versus colour relation for quiescent and for star-forming galaxies. The intrinsic relation and its physical dispersion are derived with a Bayesian inference model. [Results] The rest-frame i-band mass-to-light ratio of quiescent and star-forming galaxies presents a tight correlation with the rest-frame (g - i) colour up to z = 1.5. The mass-to-light ratio versus colour relation …
Simultaneous Kepler/K2 and XMM-Newton observations of superflares in the Pleiades
2019
Flares are powerful events ignited by a sudden release of magnetic energy. With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. Our objective is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. We selected bright X-ray/optical flares occurred in 12 known members of the Pleiades fr…
The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga
2007
The Taurus-Auriga star-forming complex hosts the only population of T Tauri stars in which an anticorrelation of X-ray activity and rotation period has been observed. We have used XMM-Newton's European Photon Imaging Cameras to perform the most sensitive survey to date of X-ray emission (0.3-10 keV) from young stars in Taurus-Auriga and investigate the dependences of X-ray activity measures -- X-ray luminosity, Lx, its ratio with the stellar luminosity, Lx/Lstar, and the surface-averaged X-ray flux, Fxs -- on rotation period. We tested for differences in the distributions of Lx/Lstar of fast and slow rotators, accretors and non-accretors, and compared the dependence of Lx/Lstar on the ratio…
Pulsations in the late-type Be star HD 50 209 detected by CoRoT
2009
The presence of pulsations in late-type Be stars is still a matter of controversy. It constitutes an important issue to establish the relationship between non-radial pulsations and the mass-loss mechanism in Be stars. To contribute to this discussion, we analyse the photometric time series of the B8IVe star HD 50209 observed by the CoRoT mission in the seismology field. We use standard Fourier techniques and linear and non-linear least squares fitting methods to analyse the CoRoT light curve. In addition, we applied detailed modelling of high-resolution spectra to obtain the fundamental physical parameters of the star. We have found four frequencies which correspond to gravity modes with az…
HADES RV Programme with HARPS-N at TNG. IV. Time resolved analysis of the Ca II H&K and Hα chromospheric emission of low-activity early-type M dwarfs
2016
Context. M dwarfs are prime targets for current and future planet search programs, particularly those focused on the detection and characterization of rocky planets in the habitable zone. In this context, understanding their magnetic activity is important for two main reasons: it affects our ability to detect small planets and it plays a key role in the characterization of the stellar environment. Aims: We analyze observations of the Ca II H&K and Hα lines as diagnostics of chromospheric activity for low-activity early-type M dwarfs. Methods: We analyze the time series of spectra of 71 early-type M dwarfs collected in the framework of the HADES project for planet search purposes. The HARPS-…