Search results for "Stars"
showing 10 items of 798 documents
Colloquium: Laboratory experiments on hydromagnetic dynamos
2002
Cosmic magnetic fields, including the fields of planets, stars, and galaxies, are believed to be caused by dynamo action in moving electrically conducting fluids. While the theory and numerics of hydromagnetic dynamos have flourished during recent decades, an experimental validation of the effect was missing until recently. We sketch the long history towards a working laboratory dynamo. We report on the first successful experiments at the sodium facilities in Riga and Karlsruhe, and on other experiments which are carried out or planned at various places in the world.
The photospheric abundances of active binaries II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems
2003
Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been derived for 13 chemical species, including several key elements such as Li, Mg, and Ca. Two methods have been used. The effective temperatures, surface gravities and microturbulent velocities were first derived from a fully self-consistent analysis of the spectra, whereby the temperature is determined from the excitation equilibrium of the Fe I lines. The second approach relies on temperatures derived from the (B-V) colour index. These two methods give broadly consistent results for the stars in …
The age evolution of Ca II emission in late type stars
2008
It has been widely demonstrated that chromospheric emission is dependent on rotation rate (Hartmann and Noyes, 1987, Catalano, 1990, for recent reviews). Since the rotation decays with time, the chromospheric activity of single main sequence stars is also dependent on age. Catalano and Marilli (1983) suggested that the chromospheric emission of solar mass stars decays exponentially with the square root of the time, as recently confirmed by Barry (1988). Magnetic activity depends also on the convection parameters, i.e. on the mass. Since the evolution of surface rotation is faster for lower mass stars (Catalano et al., 1988), the internal rotation distribution, and therefore the dynamo actio…
The Astrophysical r-process
2002
In 1957, Burbidge, Burbidge, Fowler and Hoyle (B2FH) provided a basis for forty years of research in various aspects of nucleosynthesis in stars. We will focus in this paper on progress in r-process nucleosynthesis, with emphasis on the most recent developments in nuclear physics. In 1986, the first experimental data on two crucial, neutron-magic “waiting-point” nuclei provided valuable clues to the astrophysical conditions and the nature of the r-process site. Beginning in the 1990’s, our group (FK2L) presented considerably improved r-abundance calculations, which were for the first time based on a modern, internally consistent nuclear-theory input. The phenomenon of shell-quenching far fr…
Coulomb dissociation of 27P: A reaction of astrophysical interest
2011
The ground-state decay of 26Al(0+) (T 1/2=1.05× 106) has a shorter life-time than the Universe. The presence of this element in the Galaxy was measured via g-ray spectroscopy, showing that the nucleosynthesis of this element is an ongoing process in stars. The proton-capture reaction 26Si(p,γ) 27P competes with the production of 26Al(0+) by β-decay. Coulomb dissociation of 27P has been suggested as an indirect method to measure radiative-proton capture when the direct reaction is not feasible. Such an experiment was performed at GSI with a secondary 27P beam produced by fragmenting a 36Ar primary beam at 500 A MeV. Two main observables are preliminarily presented in this work: the reaction …
Nuclei Far from Stability and the R-Process Waiting-Point Concept
1992
The nucleosynthesis process by rapid neutron captures (the r-process) is responsible for the formation of about half of the nuclear species in nature beyond Fe. While the astrophysical site for the r-process is not yet unambiguously identified, its association with the cores of low-mass stars undergoing type II supernova (SN) events is strongly suggested (see, e.g., Refs.1,2).
Boson stars in Palatini f(R) gravity
2021
Combination of 3 Different Measurements: Branching Fractions, Radiative Lifetimes, and Absorption Oscillator Strengths- a Good Opportunity for the An…
2021
We present the method and setup for determination and self-testing of atomic radiative constants: transition probabilities, oscillator strengths and natural lifetimes. We describe how it works in simple 4 level scheme, present our experimental setup, and preliminary results for As I branching fractions, and discuss some aspects of future development. This method can be valuable for analyzing the results from different experiments performed by various authors and methods without directly re-measuring them. For example, in astrophysics is very important to select correct oscillator strengths for evaluation element abundances in stars and interstellar media since inaccurate data lead to uncert…
Erratum: On the existence of a luminosity threshold of GRB jets in massive stars
2019
Short-term variations in Be stars observed by the CoRoT and Kepler space missions
2010
AbstractThe corot and kepler space missions are collecting very high-precision long-duration photometric data of many Be stars, allowing us to better understand the origin of their short-term variability and the link between these variations and the Be phenomenon. In this paper, we present a brief summary of the results obtained in the analysis of several Be stars observed with corot in terms of pulsations. In addition, we show that variations of the Be star HD 175869 can be explained as two active regions separated by 150 degrees or as unstable pulsating modes in a star with an extensive mixing in radiative layers corresponding to a core overshooting of 0.35Hp. A preliminary study of the p…