bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evolution of Am and Ap Stars
Authors: Richard O. , Michaud G. , Richer J. 
Published: July 2016

This work is a continuation of the Montréal stellar evolutionary model development (Richard et al., 2001 and references therein) where the effects of atomic diffusion are taken into account for 28 elements (see Turcotte, these proceedings), along with an approximate treatment of turbulent transport (Richer et al., 2000).We used the CEFF equation of state and the Bahcall nuclear energy generation routine. The Rosseland opacity is computed at each time step, from the OPAL monochromatic opacities for 21 elements (to which we added locally computed spectrum tables for Li, Be, and B) for each mesh point in the model for the current local chemical composition. Convection and semi-convection are taken into account in every species’ diffusion velocity and in the determination of the thermal gradient (Richard et al., 2001). The models also take into account gravitational settling, thermal diffusion, radiative accelerations and turbulent transport. The radiative accelerations are from Richer et al. (1998). The turbulent transport is modeled as a diffusion process (Richer et al., 2000) with diffusion coefficient: DT = ωD(He)0 (ρ/ρ0)n where D(He)0 is the Ηe atomic diffusion coefficient at density ρ0.
Time-Resolved High-Resolution Spectroscopy of roAp Stars
Authors: Kochukhov O. , Ryabchikova T. 
Published: July 2016

AbstractWe report results of Spectroscopic monitoring of the roAp stars γ Equ, α Cir and HR 3831 with the ESO 3.6-meter telescope. Series of very high-resolution and high S/N spectra allowed to resolve changes of line profiles due to the pulsations. We found that pulsational behaviour of all three roAp stars is dominated by the variations of the doubly ionized rare-earth lines. Detailed analysis of the pulsational changes of Nd III and Pr III spectral features allowed us to identify the pulsational mode of γ Equ and to study rotational modulation of the pulsational pattern in the spectra of α Cir and HR 3831.
Radial Velocity Studies of roAp stars: Rotational Modulation in HR 1217
Authors: Hatzes A.P. , Mkrtichian D. , Kanaan A. 
Published: July 2016

AbstractWe present the preliminary results of our analysis of the precise radial velocity (RV) measurements of the roAp star HR1217. We have obtained over 2000 spectra of this star (54 hours of coverage) covering a complete rotation period using the 2-d coude echelle spectrograph of the McDonald Observatory’s 2.7m telescope. Our RV data show rotational modulation in both the amplitude and phase of the pulsations. The broad-band (covering 110 Å) RV semi-amplitudes of the f2 mode (=2.65 mHz) varied in the range of 25 m s–1 to 200 m s–1.
GSC 4778-00324, a New Small-Amplitude Multiperiodic δ Scuti Star
Authors: Lampens P. , Van Cauteren P. , Niarchos P. , Gazeas K. , Manimanis V. , Arentoft T. , Wils P. , Bruch A. , Garrido R. , Shobbrook R. 
Published: July 2016

A careful analysis of the light curves of comparison stars to the intermediate amplitude δ Scuti star V1162 Ori observed in 1999/2000 within an international campaign (Arentoft et al., 2001) revealed two new variable stars: GSC 4778-00324, a small-amplitude δ Scuti star, and GSC 4778-00001, an eclipsing variable (see http://www.lunarpages.com/bho for a map of the field). Here, we will concentrate on the first of these.Since the colour indices (Handler, 2000) put GSC 4778-00324 right in the middle of the δ Scuti instability strip, we decided to monitor its variability (Van Cauteren & Lampens, 2000) taking as much advantage as possible from the international campaign on V1162 Ori and adopting the same comparison stars. The new observations were performed between October 1999 and March 2001, mainly from the Greek and Belgian sites, making use of a standard V filter. We collected and used 2866 data points for GSC 4778-00324 (V = 10.26; b − y = 0.20).
Asteroseismic Analysis of Pulsating Subdwarf B Stars
Authors: Charpinet S. , Fontaine G. , Brassard P. 
Published: July 2016

AbstractWe briefly present a method to perform detailed asteroseismological analyses of pulsating subdwarf B (sdB) stars (also referred to as the EC14026 stars). We use a forward approach based on the computation of large grids of sdB stellar models (each model being analyzed with a linear nonadiabatic pulsation code) to derive, in an objective way, the most appropriate set of model parameters that can best reproduce the observed period spectrum of a given sdB pulsator. We discuss an illustrative application of this method to the pulsating sdB star PG 1047+003, for which we derive the fundamental parameters Teff, logg, Mtot, and Menv.
Nonlinear Convective Pulsation Models of Second Overtone SMC Cepheids
Authors: Marconi M. , Bono G. , Caputo F. 
Published: July 2016

AbstractNonlinear convective pulsation models representative of SMC Cepheids pulsating in the second overtone mode are presented. Model predictions are compared with observations and implications are derived for second overtone selection criteria. The role of overtone Cepheids as distance indicators is finally investigated and the derived distance scale is at odds with the so called short distance scale.
Pulsation in Cool Hydrogen Deficient Stars
Authors: Pikall Holger 
Published: July 2016

AbstractModels for hydrogen deficient stars are presented for different L/M-ratios and temperatures typical for RCB and HdC. Initially, we searched for unstable pulsation modes and identified their character using a simple linear code based on matrix methods. In a second step, we employed our fully-implicit Radiation-Hydro-Dynamics code to obtain amplitudes, radial velocities and lightcurves. For small L/M-ratios, nonlinear periods are close to the linear ones. Cooler models show strong shock-waves, which are suggested to play a key role in the dust formation of RCB stars which will be our ultimate goal.
Non-Linear Radial Pulsation Models for the Early-Type Helium Stars V652 Her and BX Cir
Authors: Montañés Rodríguez P. , Jeffery C.S. 
Published: July 2016

AbstractHydrogen-deficient early-type stars have an extremely high surface helium abundance (> 98% by number) and have represented a considerable challenge to stellar evolution theory. Recent work supports the view that they are the product of a merger between two white dwarfs.All very luminous stars (log L/M > 4) are believed to pulsate, but at effective temperatures around 20 000 K, helium stars with lower luminosities (log-L/M < 3) are also known to pulsate due to iron-group bump instability. The two known cases are V652 Her and BX Cir. Their observed periods are in good agreement with linear theory. Recently high-resolution Spectroscopic observations were used to measure the stellar dimensions and radial velocity curves with high precision.A hydrodynamic code including recent OPAL opacity data has been used to construct non-linear models of the pulsations of these two stars. The results impose additional constraints on those stellar dimensions, including mass, which remain poorly determined by observation.
A Model of Time-Dependent and Non-Local Convection Included in a Stellar Evolution Code
Authors: Flaskamp Martin 
Published: July 2016

AbstractThe Kuhfuß theory of convection is derived from the exact hydrodynamic equations by means of anelastic approximation and diffusion approximation. In the local and time-independent limit the result is identical to the MLT case. In the more general case the theory includes overshooting and convective mixing as well as time-dependence. We present test calculations in a simple box model and first steps toward the implementation in a stellar code.
Swing Coupling between Stellar Pulsations and Torsional Alfvén Oscillations
Authors: Zaqarashvili T. , Roberts B. 
Published: July 2016

AbstractWe suggest a mechanism of energy transmission from stellar radial pulsations into torsional oscillations in a dipole-like magnetic field. We show that the stellar radial pulsations cause a periodical variation of the Alfén speed which leads to the Mathieu equation for torsional Alfén waves. Harmonics with half the frequency of the pulsations have exponentially growing amplitudes. Then the energy supporting the various type of pulsations can be transformed into purely electromagnetic oscillations of the star.
A Study of Convection with δ Scuti Stars in Open Clusters: the Pleiades
Authors: Suárez J.-C. , Michel E. , Houdek G. , Lebreton Y. , Pérez Hernández F. 
Published: July 2016

AbstractIn this work we propose a preliminary seismic investigation of δ Scuti stars in the Pleiades cluster, focusing on potential diagnostics of convection and core-overshooting. Taking into account the effect of fast rotation in the modelling, we compare observed frequencies for 4 δ Scuti stars with radial linear instability predictions. A satisfying agreement is reached between the predicted ranges of unstable modes and those derived from observations for “low-mass” stars (∼ 1.55M⊙). However, a strong disagreement is found for “high-mass” stars (∼ 1.77M⊙), whatever the mixing length (α) value. These results are compared with previous ones obtained for Praesepe.
Dynamical Stellar Dust Shells: Different Perspectives
Authors: Sedlmayr E. 
Published: July 2016

In the last decades considerable progress has been achieved in the physical understanding and modelling of dusty stellar atmospheres and surroundings, as represented by stationary dust driven winds, pulsating shells of Miras and LPVs, episodic phenomena, like occultations of RCrB stars, the atmospheres of brown dwarfs, or even by “hot” objects like dust forming Wolf-Rayet stars.For all these systems the notions “understanding” and “modelling” depend on the level of the physical approach defined by specific perspectives chosen by a basic “window of perception” for each class of particular objects and its appropriate description within this frame. This is defined by the fundamental assumptions with regard to the global and local appearance and the properties of the considered object, its focused characteristic length and time scales of the various determining processes taken into account, and, of course, by an appropriate body of physical equations necessary for an adequate description within the context of the adopted perspective. In this view, any real approach concentrates on special aspects (or a combination of it), like radiative transfer, wind generation, spectral appearance, dynamical behaviour, global and local stability, cluster nucleation, grain growth and processing, chemistry, interplay of physical and chemical processes, etc. Consequently, any approach projects only particular properties of the considered object, each constituting important ingredients necessary for a final consistent quantitative modelling and a comprehensive physical understanding.
The Pulsation of M-type Miras: Multi-Epoch ISO-SWS Observations and Dynamical Models
Authors: Aringer B. , Jørgensen U.G. , Kerschbaum F. , Hron J. , Höfner S. 
Published: July 2016

AbstractWe present time series of observed and synthetic ISO-SWS spectra of oxygen-rich Mira variables covering the wavelength range between 2.36 and 7.75 μm. The calculations are based on new dynamical models, which have been computed with a non-grey radiative transfer taking into account all relevant molecular opacities. It turns out that many features in the ISO spectra of cool long period variables which could not be reproduced within the framework of classical hydrostatic model atmospheres nor with grey dynamical calculations can now be understood without any additional assumptions. This is especially true for the water bands, which dominate the opacity in the infrared range of M-type Miras.
Requirements for Future Photometric and Spectroscopic Observations for Asteroseismology: One Theorist’s Perspective
Authors: Kawaler Steven D. 
Published: July 2016

AbstractThe interplay between theory and observation has been particularly close in asteroseismology, ranging from white dwarfs through roAp stars. This trend should continue as the field expands to include new classes such as solar-type stars. This synergistic relationship relies on communication between theorists and observers, but also on healthy skepticism in both directions. Here, I outline a (somewhat personal) view of the theoretical side - what kinds of observations are most helpful for optimizing utility of theoretical research efforts ? Where might theory be of closer guidance as observing programs are planned ?
The Delta Scuti Network: Steps Towards Successful Asteroseismology of Delta Scuti Stars
Authors: Zima W. , Breger M. , Bischof K. , Rodler F. , Stankov A. , Pamyatnykh A. A. , Antoci V. , Lorenz D. , Sperl M. , Zechner R. , Garrido R. , Wood M. , Handler G. , Shobbrook R.R. 
Published: July 2016

AbstractThe Delta Scuti Network (DSN) is a collaboration of astronomers all around the globe who study and observe short-period variables. The field of scientific research includes high-precision photometric and Spectroscopic global campaigns, mode identification techniques, and asteroseismological pulsation modeling. We present results for three stars that are receiving the most attention at the moment: 4 CVn, BI CMi, and 44 Tau. Our results demonstrate that a large number of simultaneously excited nonradial oscillations in stars on and above the main sequence can be detected by conventional means.
Test of a New Theory for Stellar Convection using Helioseismology
Authors: Paternó L. , Ventura R. , Canuto V.M. , Mazzitelli I. 
Published: July 2016

AbstractTwo evolutionary models of the Sun have been tested using helioseismological data. The two models use the same input micro-physics (nuclear reaction rates, opacity, equation of state) and the same numerical evolutionary code, but differ in the treatment of turbulent convection. The first model employs the standard mixing - length theory of convection, while the second one employs a new turbulent convection model which overcomes some basic inconsistencies of the standard theory of convection.The test rests on the calculation of p-mode eigenfrequencies and on the comparison with the helioseismological data.The comparison shows an overall improvement of the eigenfrequencies calculated with the new model with respect to those calculated with the standard model, although it appears that both models still suffer from inaccuracies especially in the treatment of the surface layers.
An Improved Equation of State for Hydrogen
Authors: Saumon D. , Chabrier G. 
Published: July 2016

An improved theory of fluid hydrogen at high density, based on a detailed treatment of inter-particle correlations and a self-consistent treatment of pressure ionization, has become available recently (Chabrier 1990, Saumon and Chabrier 1991, 1992). We present a preliminary comparison between this new EOS (hereatfer SC) and equations of state frequently used in astrophysical contexts, namely: Fontaine, Graboske and Van Horn 1977 (FGVH), Däppen et al. 1988 (MHD) and Magni and Mazzitelli 1979 (MM).The SC theory predicts a first-order phase transition in the region of pressure-ionization (the so-called Plasma Phase Transition, or PPT), between an essentially neutral mixture of atoms and molecules (xe– < 10−2), and a partially ionized plasma (xe– ≈ 50 %), with a critical point located at Pc = 0.614 Mbar, Tc = 15300K and pc = 0.35 g/cm3.
The Non-Explosive Modes of Hydrogen Burning: A Brief Review
Authors: Arnould Marcel , Mowlavi Nami 
Published: July 2016

AbstractThe present status of our theoretical and experimental knowledge of the rates of hydrogen-burning reactions in hydrostatic stellar conditions is very briefly reviewed. Special emphasis is put on reactions of the proton-proton chain, CNO cycles, NeNa cycle and MgAl chain whose rates are still affected by uncertainties that can have an impact on predictions regarding astrophysical issues of interest. This concerns in particular the solar neutrino problem, or the nucleosynthesis accompanying the CNO, NeNa and MgAl chains of reactions, as it is revealed in particular by the first dredge-up episode. A brief discussion of the possibility of reducing the present rate uncertainties is presented.
The Pre - Main Sequence
Authors: D’Antona Francesca 
Published: July 2016

AbstractThe Pre-Main-Sequence (pre-MS) is defined as the phase following the protostellar stage and ending with the ignition of hydrogen in the stellar core. Models in hydrostatic and thermal equilibrium such as typical “Hayashi track” models are basically adequate to describe the internal structure of pre-MS stars, although for a fraction of pre-MS objects there can be phases of accretion also at late stages, which may influence the surface abundances of light elements.The interior evolution of pre-MS structures is focused in the two main stages’ of Deuterium and Lithium burning for the low mass (M ≤ 1.5M⊙) stars. The location in the HR diagram of theoretical tracks is still subject to large uncertainty, even more apparent today that new opacities and treatment of turbulent convection are available, and the results can be internally compared. Uncertainties amount to a factor ~ two for mass and age determination of individual objects, and affect the absolute location for both the D-burning and the Li-burning regions. Qualitative constraints, such as that there can not be Li-depletion at L ≥ L⊙, remain valid. Abundances of7Li at the surface of pre-MS stars, if measured at a stage which can definitely be considered previous to the possible occurrence of nuclear burning, and, consequently, previous to other depletion mechanism which may occur later on, provide constraints on galactic evolution of this element of cosmological interest.
Main Sequence Structures Including The Overadiabatic CM Convection Model
Authors: Mazzitelli Italo , D’Antona Francesca 
Published: July 2016

AbstractWe computed the main sequence structure of masses from 2 to 0.7 M⊙ for two choices of helium (Y) and metal (Z) mass fractions. Models are given for solar composition (Y=0.285, Z=0.018) and for a composition appropriate to a younger population I (Y=0.32, Z=0.03). We took particular care both in the choice of the most recent opacities presently available and in the treatment of the input physics and numerics (see also D’Antona et al, 1992).Opacities from Rogers and Iglesias (1992) were adopted where available, complemented with those by Kurucz (1991) at T≤10000 K and, elsewhere, by the Los Alamos opacities (Huebner et al. 1977). The equation of state by Mihalas et al. (1988) was used at ρ ≤ 0.01, and elsewhere the relation P = P(ρ,T) was taken from Magni and Mazzitelli (1979), while the adiabatic gradient was obtained from a Saha type e.o.s..
Determination of Age and Original Chemical Composition of Binary Stars
Authors: Ferluga S. , Antillon M. 
Published: July 2016

AbstractA numerical method is presented, which is able to determine quantitatively the age t and the initial abundances of helium and metals (Y0, Z0) of a given binary system. Input data are simply the values of mass m, temperature Teff and surface gravity g, of both stars. The procedure works for detached main-sequence binaries, and it does not require any preliminary hypothesis about surface abundances or age. It is assumed that the two stars evolve parallely with no interaction.
Modelling The ζ Herculis System
Authors: Lebreton Y. , Auvergne M. , Morel P.J. , Baglin A. 
Published: July 2016

AbstractThe ζ Herculis system is one of the closest well-kown binaries. A tentative modelling shows that though the brighter star is easily described with a mass compatible with the observed one and an age of approximately 3.4 billion years, difficulties arise for the secondary, which plea for more precise observations.
The Blue Straggler Stars of Galactic Clusters
Authors: SchÖNberner D. , Napiwotzki R. 
Published: July 2016

AbstractThe origin and evolution of blue straggler stars (BS) in clusters is still an unsolved problem for our understanding of stellar evolution. Several rivalling theories exist, of which that of Saio & Wheeler (1980) is the most interesting one since it assumes additional mixing of fuel in the interior of BS. This theory predicts larger luminosity-to-mass ratios for a given position in the HR diagram compared to the standard evolution theory. This theory has been taylored for the old galactic cluster NGC 7789 which contains many bona fide BS, but it can also be used for other clusters of similar ages. The other theories rely on the standard theory of stellar evolution and assume (1) binary mass exchange or even merging, or (2) delayed star formation. In either case follows a “normal” luminosity-to-mass ratio. We report here on a careful spectroscopic investigation in order to discrimate between these rivalling theories.
Pulsation of Rotating and Magnetic Stars
Authors: Vorontsov Sergei V. 
Published: July 2016

AbstractRotation and a magnetic field break the spherical symmetry of a star viewed as a pulsating system, lifting the degeneracy of oscillation frequencies, and leading to (sometimes prominent) observational consequences. Theoretical studies of rotational and magnetic effects in pulsating stars are reviewed, starting with simple configurations with slow rotation and weak magnetic fields.
The Asteroseismic Calibration of Solar-Type Stars
Authors: Gough D.O. , Novotny E. 
Published: July 2016

AbstractThe addition of seismic parameters to stellar-model calibrations substantially increases the constraints one can place on the properties of stars. We present some preliminary calculations to assess the accuracy with which certain stellar parameters can be inferred. For simplicity we use just two of the three most basic seismic parameters characterizing the low-degree p modes that might be measured from intensity variations by instruments such as photometers planned for the ESA spacecraft PRISMA. We ascertain the accuracy of a calibration of an isolated star and of a cluster of N solar-type stars.
Activity Tracers
Authors: Strassmeier Klaus G. 
Published: July 2016

AbstractThe vast variety of solar-like phenomena on other late-type stars, so-called activity tracers, provide an important tool for studying the structure of active stellar atmospheres and their connection to the stellar interior via strong magnetic fields. These “chromospherically active” stars include single and binary stars as well as pre- and post-main sequence objects and have rapid rotation and deep convective layers in common. They serve as astrophysical laboratories to study the vast phenomenology of activity tracers: starspots, plages, flares, prominences, which might be seen as enhanced analogs of solar activity and could be spatially resolved due to rotationally modulated indicators. In this paper we review the current observational material and discuss its impact on our knowledge of “active” atmospheres, especially in the context of stellar rotation.
Photospheric Activity among Early F-Type Stars
Authors: Mantegazza L. , Poretti E. , Antonello E. , Zerbi F. 
Published: July 2016

In the last decade stars showing variability of unclear origin have been detected among early F-type stars mainly as secondary results in the context of observational programmes devoted to the study of δ-Scuti and CP stars.For this reason only a small number of these objects has been thoroughly studied: their positions in HR diagram is shown in fig.l.Figure 1: HR diagram of the observed stars.The stars in NGC 2516 have been singled out in a survey work by Antonello and Mantegazza (1986), HD 164615 has been studied by Abt et al.(1983), the variability in 9 Aur has been discovered by Krischinas et al.(1990,1991) and HD 23375 in Pleiades has been pointed out by Breger (1972). The solid line in Fig. 1 is the ZAMS and the dashed lines are the borders of the i-Scuti stars instability strip.