X The Combination of Stellar Influences

2009

Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal Xray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yet-undiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of short-period active stars, e.g. of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital period of the inner pair drops to a few days, fast surface rotation of the companions is driven by tidal forces, boosting activity by a few orders of magnitude. If the period drops still further, a merger may take place leaving a rapidly-rotating active dwarf with only a distant companion.

Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal X-ray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yet-undiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of short-period active stars, e.g. of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital peri...

Astronomy & Astrophysics, 2011

In this paper we use the Reynolds stress models (RSM) to derive algebraic expressions for the following variables: a) heat fluxes; b) μ fluxes; and c) momentum fluxes. These relations, which are fully 3D, include: 1) stable and unstable stratification, represented by the Brunt-Väisäla frequency, N 2 = −gH −1 p (∇ − ∇ ad)(1 − R μ); 2) double diffusion, salt-fingers, and semi-convection, represented by the density ratio R μ = ∇ μ (∇ − ∇ ad) −1 ; 3) shear (differential rotation), represented by the mean squared shear Σ 2 or by the Richardson number, Ri = N 2 Σ −2 ; 4) radiative losses represented by a Peclet number, Pe; 5) a complete analytical solution of the 1D version of the model. In general, the model requires the solution of two differential equations for the eddy kinetic energy K and its rate of dissipation, ε. In the local and stationary cases, when production equals dissipation, the model equations are all algebraic.

2010

We discuss the incidence of multiplity, particularly among the bright and therefore relatively thoroughly examined stars, and note certain types of (a) binary stars that might be expected to merge, and (b) single stars with characteristics that suggest they may be former binaries that merged. Some Be stars and rapidly rotating red giants seem like possible merger products; and perhaps even some magnetic peculiar stars that are rapidly rotating.

2005

Given the emphasis at this meeting on stellar models that treat diffusive and deep-mixing processes, as well as on recent computations for the AGB phase, a panel of experts was organized to further discuss the comparisons between theory and observations, the limitations of current models, and anticipated future advances. G. Michaud provided further insights on which stars are expected to show the largest abundance anomalies due to gravitational settling and radiative accelerations, and on the role of turbulent mixing. A. Weiss emphasized the importance of fully understanding diffusive processes and of investigating the still largely untested consequences of such processes (and deep mixing) for the late evolutionary phases of stars. S. Vauclair reported on the possible interaction between meridional circulation currents and diffusion, which could potentially explain why the observed abundance anomalies are less than those predicted by diffusive models. P. Denissenkov briefly reviewed several of the main problems that need to be solved in order to achieve an understanding of the observed abundances in giant stars, including the identification of the physical mechanism of deep mixing and an understanding of how the rotation profile in stars evolves. F. D'Antona outlined the reasons why she believes that massive AGB stars are the source of the material that produces the observed abundances in globular cluster stars (via pollution), including, in particular, the constraints provided by the observed Li abundances. Finally, J. Lattanzio commented that deep mixing is likely restricted to the upper giant branch and that some kind of primordial mechanism must be found to explain the abundance anomalies in less evolved stars: he also reminded us of some difficulties with the AGB scenario. Short contributions from each of the above are included in this paper.

Research in Astronomy and Astrophysics, 2019

X-ray emission is an important indicator of stellar activity. In this paper, we study stellar X-ray activity using the XMM-Newton and LAMOST data for different types of stars. We provide a sample including 1259 X-ray-emitting stars, of which 1090 have accurate stellar parameter estimations. Our sample size is much larger than those used in previous works. We find a bimodal distribution of the X-ray to optical flux ratio (log(f X/fV )) for G and K stars. We interpret that this bimodality is due to two subpopulations with different coronal heating rates. Furthermore, using the full widths at half maxima calculated from Hα and Hβ lines, we show that these stars in the inactive peaks have smaller rotational velocities. This is consistent with the magnetic dynamo theory that presumes stars with low rotational velocities have low levels of stellar activity. We also examine the correlation between log(f X/fV ) and luminosity of the excess emission in the Hα line, and find a tight relation ...

Astronomy & Astrophysics, 2012

Context. From correlations between orbital parameters, several new coherent groups of stars were recently identified in the Galactic disc and suggested to correspond to remnants of disrupted satellites. To reconstruct their origin at least three main observational parameters -kinematics, chemical composition and age -must be known. Aims. We determine detailed elemental abundances in stars belonging to the so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical composition with Galactic thin-and thick-disc stars, as well as with the Arcturus and AF06 streams (Arifyanto & Fuchs 2006, A&A, 449, 533). The aim is to search for chemical signatures that might give information about the formation history of this kinematic group of stars. Methods. High-resolution spectra were obtained with the FIES spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. Results. The average value of [Fe/H] for the 32 stars of Group 2 is −0.42 ± 0.10 dex. The investigated group consists mainly of two 8-and 12-Gyr-old stellar populations. Abundances of oxygen, α-elements, and r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has similar characteristics as that of the Galactic thick-disc. Conclusions. The similarity in chemical composition of stars in Group 2 with that in stars of the thick-disc might suggest that their formation histories are linked. The chemical composition together with the kinematic properties and ages of stars in the stars investigated provides evidence of their common origin and possible relation to an ancient merging event. A gas-rich satellite merger scenario is proposed as the most likely origin. Groups 2 and 3 of the Geneva-Copenhagen Survey might have originated in the same merging event.

Proceedings of the International Astronomical Union, 2006

We consider the multiplicity of stellar systems with (combined) magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4555 such bright systems, and the frequencies of multiplicities 1, 2, . . , 7 are found to be 2722, 1412, 299, 86, 22, 12 and 2. We also consider the distributions of periods of orbits and sub-orbits. For the even more restricted set of 474 systems with VH ≤ 4.00 the proportions of higher multiples up to sextuple are progressively larger, suggesting incompleteness in even the relatively well-studied larger sample.We construct a Monte-Carlo algorithm that will generate systems with roughly the observed multiplicities and orbital parameters, taking account of selection effects.

Arxiv preprint arXiv: …, 2011

Astronomy and Astrophysics, 2007

Context. The comparison of observed counts in a given sky direction with predictions by Galactic models yields constraints on the spatial distribution and the stellar birthrate of young stellar populations. The XMM-Newton Bright Serendipitous Survey (XBSS) is an unbiased survey that includes a total of 58 stellar sources selected in the 0.5-4.5 keV energy band, having a limiting sensitivity of 10 −2 cnt s −1 and covering an area of 28.10 sq deg. Aims. We present the results of analysing the stellar content of the XBSS so as to understand the recent star formation history of the Galaxy in the vicinity of the Sun. Methods. We compared the observations with the predictions obtained with XCOUNT, a model of the stellar X-ray content of the Galaxy. The model predicts the number and properties of the stars to be observed in terms of magnitude, colour, population and f x / f v ratio distributions of the coronal sources detected with a given instrument and sensitivity in a specific sky direction. Results. As in other shallow surveys, we observe an excess of stars not predicted by our Galaxy model. Comparing the colours of the identified infrared counterparts with the model predictions, we observe that this excess is produced by yellow (G+K) stars. The study of the X-ray spectrum of each source reveals a main population of stars with the coronal temperature stratification typical of intermediate-age stars. As no assumptions have been made for the selection of the sample, our results must be representative of the entire solar neighbourhood. Some stars show infrared excess due to circumstellar absorption, which is indicative of youth.