bluuebunny/crossref_metadata_2025 huggingface dataset:
Follow-Up Detection in Project Phoenix
Authors: Cullers D. Kent , Stauduhar Richard P.
Published: July 2016

Effective detection of extraterrestrial radio signals requires elimination of terrestrial interference. However, the sensitivity of standard, two element interferometry, depends on the geometric mean of the antenna areas. This is very disadvantageous if the antennas are not the same size. Project Phoenix performs pseudointerferometry by transmitting detailed signal descriptions to its second site where they are used for confirmation. The primary site uses the larger antenna and square-law detectors. Use of matched filtering and a lower threshold makes detection with the second, much smaller, antenna possible. The pseudointerferometer can handle up to 12 signals having bandwidths of order 1 Hz in a 10 MHz band. In Australia, it allowed observing to complete in less than half the time that would have been required using a single site system.
On the Hydrodynamics of Stellar Pulsation and Stability
Authors: Gough D.O. , Ledoux Paul
Published: July 2016

Much of our knowledge of the hydrodynamics of stellar oscillations is summarized in the now-classical article by Ledoux & Walraven in volume 51 of the Handbuch der Physik. It is from that article that I, and many others, first learned the subject. Most of what we have learned since, aside, perhaps, from what we have learned from nonlinear studies, derives from that work. Even today, what is written in that work is hardly out of date. It is not possible to do justice to even the most outstanding contents of Ledoux & Walraven’s article in a single talk, so I highlight some aspects of further developments that have been of more recent interest, trying to set them into a context of current understanding.
Convective Overshooting: Physical Properties and Seismic Evidence
Authors: Zahn J.-P.
Published: July 2016

We review briefly the different prescriptions which have been proposed to predict the extent of convective penetration (or overshoot) in stellar interiors, and we confront them with the results of numerical simulations and with helioseismic data. It appears that the penetrative motions are structured in plumes, and that thermal diffusion plays an important role in controlling the temperature stratification in the stable domain. The most recent high-resolution simulations suggest that these plumes are less space-filling than thought before, and that they are therefore less efficient in establishing an adiabatic temperature profile. This property is compatible with the solar profiles obtained through acoustic sounding.
Djehuty: a Code for Modeling Whole Stars in Three Dimensions
Authors: Turcotte S. , Bazan G. , Castor J. , Cavallo R. , Cohl H. , Cook K. , Dearborn D.S.P. , Dossa D. , Eastman R. , Eggleton P.P. , Eltgroth P. , Keller S. , Murray S. , Taylor A.
Published: July 2016

The DJEHUTY project is an intensive effort at the Lawrence Livermore National Laboratory (LLNL) to produce a general purpose 3-D stellar structure and evolution code to study dynamic processes in whole stars.
Tidally Induced Oscillations in Close Binaries
Authors: Willems B. , Aerts C.
Published: July 2016

In this paper, we present the results of a theoretical investigation on tidally excited oscillation modes in close binaries. We apply our results to the slowly pulsating B star HD 177863 and present evidence for the resonant excitation of the second-degree -mode in this star.
Frequency Analysis of a Delta Scuti Type Component in the Eclipsing Binary Y Cam
Authors: Kim S.-L. , Lee J.-W. , Youn J.-H. , Kwon S.-G. , Kim Chulhee
Published: July 2016

We present CCD photometric results of the eclipsing binary Y Cam with a pulsating component. The dominant pulsation frequency of 15.0473 c d-1 has not changed over 39 years, but its amplitude decreased to about half. The pulsational properties of the second frequency showed great differences between Broglia & Conconi (1984)’s result and ours.
RR Lyrae Stars and Anomalous Cepheids in the Draco Dwarf Spheroidal Galaxy
Authors: Kinemuchi K. , Smith H.A. , LaCluyzé A.P. , Clark C.L. , Harris H.C. , Silbermann N. , Snyder L.A.
Published: July 2016

We present new results on RR Lyrae stars and anomalous Cepheids in the Draco dwarf spheroidal galaxy. We have increased the number of double-mode RR Lyrae stars and found three new anomalous Cepheids. With period-magnitude and period-amplitude diagrams, we discuss the Oosterhoff classification of Draco. Contradictory results were found in that Draco appears to contain both Oosterhoff I and II type RR Lyrae populations.
Frequency Analysis of β Cephei Stars in NGC 4755
Authors: Stankov A.
Published: July 2016

We present the results of the multi-color, two-site observing campaign of β Cephei stars in the young open cluster NGC 4755, from 1999 and 2000. More than 200 hours of time-series photometric U, B, V filter data of the variable stars were acquired. We show a table of the detected frequencies and discuss the results.
CCD Photometry of Variable Stars in the Galactic Open Cluster NGC 7419
Authors: Kołaczkowski Z. , Pigulski A. , Kopacki G.
Published: July 2016

We present results of a CCD variability search in the field of the young open cluster NGC 7419. This cluster contains a large number of Be stars and almost all of them turned out to be variable. They show mostly irregular variations on different time-scales with ranges up to 0.4 mag in the IC band. The most interesting result is the discovery of short-period variability in some of the Be stars.
Automated Classification of Variable Stars for ASAS Data
Authors: Eyer Laurent , Blake Cullen
Published: July 2016

With the advent of surveys generating multi-epoch photometry and their discoveries of large numbers of variable stars, the classification of the obtained time series has to be automated. We have developed a classification algorithm for the periodic variable stars using a Bayesian classifier on a Fourier decomposition of the light curve. This algorithm is applied to ASAS (AII Sky Automated Survey, Pojmanski, 2000). In ASAS 85% of the variables are red giants. A remarkable relation between their period and amplitude is found for a large fraction of those stars.
Tuning Pulsation Modes with Stellar Interferometry
Authors: Domiciano de Souza A. , Vakili F. , Jankov S. , Janot-Pacheco E.
Published: July 2016

Stellar interferometry represents a qualitative jump toward stellar surface imaging. We propose a method that tunes and filters out high degree non-radial pulsation (NRP) modes from the Fourier plane associated to the stellar brightness distribution. The proposed method can be applied to continuum flux, slow stellar rotation and high degree multi mode NRP stars.
Cycle to Cycle Irregularities in the Monoperiodic β Cephei Star BW Vulpeculae
Authors: Garnier D. , Nardetto N. , Mathias P. , Gillet D. , Fokin A.B.
Published: July 2016

High spectral and time resolution observations show that the monoperiodic β Cephei star BW Vul presents differences in both line profiles and velocities from one cycle to another, which may be due to the consequence of the passage of two strong shock waves.
Search for a Magnetic Field in the B2IV Star V 2052 Oph
Authors: Neiner C. , Henrichs H.F. , Tijani A. , Geers V. , Nichols J.S.
Published: July 2016

We have searched for magnetic signatures in the B2IV star V 2052 Oph, a β Cep star with stellar wind behavior typical for an oblique magnetic rotator, as β Cep itself. From UV spectroscopy we determined an accurate rotational period and v sin i and present the magnetic measurements as a function of rotational phase. The (small) class of magnetic pulsating early B stars are of great asteroseismological interest.
Photometric Modeling of Slowly Pulsating B Stars
Authors: Townsend R.H.D.
Published: July 2016

Highlights are presented from a theoretical study of the photometric characteristics of slowly pulsating B stars. One outstanding result is the discovery, for l ≥ 2 modes, of cancellation between the flux variations originating from surface temperature perturbations, and those arising from radius perturbations. In addition to reducing greatly the variability generated by such modes, the cancellation introduces significant phase differences between the flux changes in each passband. On the grounds that similarly-large phase differences are not seen in observational data, it is suggested that the light variations of slowly pulsating B stars might be due primarily to l = 1 modes.
Activity as the Main Explanation of Light Variations of o And — Towards a Rotational Modulation Model
Authors: Sareyan J.P. , Chauville J. , Alvarez M. , Guerrero G. , Ferro A. Arellano , Mathias P. , Peña J.
Published: July 2016

Observations carried out for 5 years on o And show that 65 to 85% of its light variations can be described by a double wave: A sin (2πt/P1 + ϕ1) + B sin (2πt/P2 + ϕ2) + C with P1 ≈ 1.6 d = 2P2. When determined independently, P1 and P2 are always found in a 2:1 ratio (within 1%), while they can vary together by as much as 4%. The peak to peak amplitudes of this double-wave fit lay between 40 and 140 mmag (and can even be reduced to less than 10 mmag - our 1987 observations). The rest of the light variations do not show any permanent period or behavior, although a ∼ 2.3 d. (i.e. ≈ 3P1/2) period is frequently detected. Sometimes a marginal ∼ 6 d. period or time constant has been detected.

In spite of the quality of our photometric data, the precision on the periods and amplitudes obtained over a few nights is never increased by longer observations: our phase diagrams show significant irregular displacements around the average double-wave analytical solution if we include longer data strings (Fig. 1). This phenomenon was already apparent in our 1992 study (Sareyan et al., 1998): the star shows real irregular behaviour superimposed onto its double-wave “mean” light curve; these changes may show up as a progressive, or sometimes abrupt, modification of the shape of the double-wave light curve (Fig. 1).
Unified NRP-Modelling of Be Stars
Authors: Rivinius Th. , Baade D. , Štefl S. , Maintz M.
Published: July 2016

Recently, the line profile variability (lpv) of two low-v sin i Be stars, μ Cen and ω (28) CMa was successfully modelled as nonradial pulsation (nrp) of rapidly rotating stars seen pole-on. In this work, it is shown that the lpv of low-v sin i early-type Be stars in general closely resembles these two cases, and is therefore explainable by the same mechanism. The lpv of intermediate to high-v sin i Be stars can be explained by the same model if the inclination angle of the model alone is increased. Consequently, early-type Be stars form a distinct, fairly homogeneous class of non-radial low-order g-mode pulsators.
Detailed Line-Profile Modeling of B Stars with Chemical Inhomogeneities
Authors: Briquet M. , Scuflaire R. , Aerts C. , De Cat P.
Published: July 2016

We report monoperiodic variability for the four B-type stars HD 131120, HD 105382, HD 138769 and HD 55522, which we explain by rotational modulation rather than by pulsation. Consequently these variables are removed from the list of candidate Slowly Pulsating B stars.
On the Combined Effects of a Magnetic Field and Rotation on Acoustic Modes
Authors: Bigot L. , Dziembowski W.A.
Published: July 2016

We present new results on the effects of a strong dipole magnetic field and rotation on p-mode oscillations. We take into account both Coriolis and centrifugal distortion as manifestations of rotation. We apply these improvements to the case of dipole (l = 1) modes. We show that these modes, during the pulsation cycle, move the stellar surface in a plane, which is inclined from both magnetic and rotation axes. In general, the displacement vector describes an ellipse in that plane. This work is devoted to the study of the roAp star pulsations.
δ Scuti Variables in the HR Diagram
Authors: Rodríguez E. , Breger M.
Published: July 2016

The location of the δ Scuti variables in the HR diagram is discussed on the basis of HIPPARCOS parallaxes and uvbyβ photometry together with the R00 catalogue (Rodríguez et al., 2000). In particular, the properties of the α Scuti-type pulsators with nonsolar surface abundances (SX Phe, λ Boo, ρ Pup, δ Del and classical Am stars subgroups) are examined.

The latter four classes of stars are spectroscopically defined subclasses with surface abundance anomalies. Although the stars ρ Pup and δ Del are also pulsators, the groups named after these stars should not be regarded as pulsation subclasses of δ Scuti stars. The abundance anomalies in these stars affect the pulsation properties, e.g., the classical Am stars are constant in light or show only small pulsation amplitudes (e.g., HD 1097). These four groups are only a small selection from the bewildering zoo of stars with unusual surface abundances in this temperature region and represent the groups with δ Scuti pulsation.
Close Frequency Pairs in Delta Scuti Stars
Authors: Bischof K.M. , Breger M.
Published: July 2016

The power spectra of several δ Scuti stars show close peaks with similar frequencies and amplitudes. Apart from possible observational problems, this can be interpreted in terms of two separate pulsation modes with similar, close frequencies or an artifact of amplitude variability of a single pulsation mode. If sufficient data are available, it is possible to distinguish between the two hypotheses on the basis of expected systematic phase changes associated with the amplitude variations of an assumed single frequency. This phase-shift test has been applied to modes found for BI CMi. In this paper we present the evidence for one of the close frequency pairs found in this star.
Oscillations in Young Stellar Objects
Authors: Pinheiro F.J.G. , Folha D.F.M. , Monteiro M.J.P.F.G. , Marconi M. , Ripepi V. , Palla F.
Published: July 2016

We report the discovery of periodicities in the light curve of the Herbig Ae star V346 Ori. We interpret these variations as the superposition of at least two signals with periods P1=42±6 min and P2=68±12 min resulting from stellar oscillations. The computation of linear non-adiabatic pulsation models for Pre-Main Sequence (PMS) stars reproduces these periods for a 1.5 M⊙ star with Teff= 7300 K and log L/L⊙= 0.74, pulsating in the fundamental and second overtones.
Chromatic Amplitudes and Line-of-Sight Velocity Variations for the Pulsating DB White Dwarf GD 358
Authors: Kotak R.
Published: July 2016

We report on preliminary results from time-resolved optical spectroscopy of the He-rich variable white dwarf GD 358.
Resonant Excitation of Nonradial Modes in RR Lyrae Stars
Authors: Nowakowski R.
Published: July 2016

A nonlinear development of radial pulsation instability to a resonant excitation of nonradial modes is studied. The theory covers the cases of axisymmetric (m = 0) modes as well as (m, −m) pairs. Adopting a simplified treatment of the mode coupling, it is found that multimode pulsation with constant amplitudes is a highly probable solution. An observable consequence of the m = 0 mode excitation is randomness of pulsation amplitude. The case of an l = 1 mode is the most important because of a small averaging effect and a large excitation probability. A significant amplitude and phase modulation is predicted in the case of excitation of an l = 1, m= ±1 pair. This may explain Blazhko-type modulation in RR Lyrae stars. If this model is correct, the modulation period is determined by the rotation rate and the Brunt-Väisälä frequency in the deepest part of the radiative envelope.
The Changing Blazhko Effect of XZ Cygni
Authors: LaCluyzé A. , Smith H.A. , Gill E.-M. , Hedden A. , Kinemuchi K. , Rosas A.M. , Pritzl B.J. , Sharpee B. , Robinson K.W. , Baldwin M. , Samolyk G.
Published: July 2016

An analysis of new and old photometry of the RR Lyrae star XZ Cyg indicates that both its primary period and its Blazhko period have undergone significant changes.
Interrelation of Modal Amplitudes of Double-Mode Pulsators
Authors: Szabados L.
Published: July 2016

During the study of the factors governing the modal amplitudes of the double-mode radial pulsators, it has been found that the R21 Fourier parameter deduced from the light curves is in a relationship with the ratio of the modal amplitudes, for beat Cepheids and RR Lyrae variables, as well. The form of this relation gives a natural explanation for the fact that double-mode pulsators with slightly excited second modes do not exist.
Tomography of the Atmosphere of the Mira Variable Z Oph
Authors: Jorissen A. , Dedecker M. , Plez B. , Alvarez R. , Gillet D. , Fokin A.B.
Published: July 2016

The new tomographic technique devised by Alvarez et al. (2000) to explore the velocity field across the atmosphere of long-period variable stars (LPVs) is applied to the Mira variable Z Oph. The method cross-correlates the optical spectrum with numerical masks constructed from synthetic spectra and probing layers of increasing depths. This technique reveals that the line doubling often observed in LPVs around maximum light is the signature of the shock wave propagating in the atmosphere of these pulsating stars.
Hydrodynamic Simulations of Convection
Authors: Asida S.M.
Published: July 2016

Convection is one of the most problematic components of almost every model of a pulsating star. RR Lyrae stars, in particular, have a narrow convection zone at the outer layers, and a model without convection cannot get the red edge of the instability strip (see for example Kollath et al., 2000).

In recent years, the common approach is to use non-local time-dependent extensions to the Mixing Length Theory of convection (Bono et al., 1997; Feuchtinger, 1999). These models have many free parameters that need to be calibrated. In this paper we report on a preliminary work in which a different approach is used: 2D hydro simulations similar to the red giant model in Asida (2000). This approach was used by Deupree (1975, 1977) but we used a much better numerical resolution and focused on comparison with MLT results.
Results of Spectrograph EMILIE with the AAA System on Solar-Like Oscillations
Authors: Bouchy F. , Schmitt J. , Bertaux J.-L. , Connes P.
Published: July 2016

The spectrograph EMILIE (Bouchy et al., 1999; Bouchy, 1999; Bouchy et al., 2000) coupled to the Absolute Astronomical Accelerometer (AAA, Connes, 1985; Schmitt, 1997) and implemented at the 152-cm Coudé telescope of the Observatoire de Haute Provence is dedicated to high-precision Doppler measurements. The AAA method uses a sliding reference spectrum constrained to track the stellar lines and to use the spectrograph as a null-checking device. The goal of AAA is to eliminate the calibration of the spectrograph as well as the displacement of the spectra across the CCD pixels due to the earth motion (and suspected to introduce a systematic error in the RV measurement). Here we report seismological results obtained with AAA on the Sun and the bright stars Procyon and ζ Her A that are expected to present solar-like oscillations.
Future Doppler Ground-Based Search for Solar-Like Oscillations with HARPS
Authors: Bouchy F.
Published: July 2016

Observations of solar-like oscillation frequencies allow us to constrain the theory of structure and evolution of stars with an outer convective zone. Recent improvements in radial velocity measurements have led to p-mode detection on Procyon (Martic et al., 1999) and β Hydri (Bedding et al., 2001; Carrier et al., 2001a). Very recently, characterization of the p-modes of α Cen A has been made with the spectrograph CORALIE mounted on the 1.2-m Swiss telescope at the ESO La Silla Observatory (Bouchy & Carrier, 2001, Carrier et al., these proceedings). The power of the radial velocity technique promises the development of ground-based search for solar-like oscillations on a large sample of stars especially with the future spectrograph HARPS.
The Link between Radiation-Driven Winds and Pulsation in Massive Stars
Authors: Owocki S.P. , Cranmer S.R.
Published: July 2016

Hot, luminous, massive stars have strong stellar winds driven by line-scattering of the star’s continuum radiation. They are also often observed to exhibit radial or non-radial pulsations. Such pulsations are possible candidates for providing the base perturbations that induce large-scale structure in the overlying wind, and as such they could help explain various observational manifestions of wind variability, e.g., absorption enhancemens or modulations in UV P-Cygni lines of OB stars, and perhaps even moving bumps in optical emission lines of Wolf-Rayet (WR) stars. Here we review the physics of line driving, with emphasis on how perturbations induce variations in a wind outflow. In particular, we present results of a time-dependent dynamical simulation of wind variations induced by the radial pulsation of the β Cep variable BW Vulpeculae, and show that observed variability in UV wind lines can be quite well reproduced by synthetic line profiles for this model. We conclude with a discussion of recent evidence that resonances among multiple modes of non-radial pulsation in Be stars play a role in inducing mass ejections that contribute to formation of a circumstellar disk.
Velocity Observations of Multiple Mode AGB Variable Stars
Authors: Lebzelter T. , Hinkle K.H. , Joyce R.R. , Fekel F.C.
Published: July 2016

Numerous infrared Spectroscopic observations were obtained of eight AGB field M giants that have multiple periods of light variability. For six of the eight giants we found radial-velocity periods that confirm the long-period light variability. Although we consider the possibility that the velocity variations result from orbital motion, we conclude that the long-period velocity changes in most, if not all of our sample stars, likely result from pulsation rather than duplicity.
Period-Gravity Relation for Semiregular Variable Stars
Authors: Szatmáry K. , Kiss L.L.
Published: July 2016

Red giant stars located on the AGB are intrinsically variable, which is usually associated with stellar pulsation. The complex light variation of semiregular variables (SRVs) has many alternative explanations. Besides the pulsation, other contributors to variability cannot be solidly excluded, such as time-dependent surface inhomogeneities, ellipsoidal deformation due to the presence of a close companion, or spots on a rotating star. The identification of ultra-short period M-giants with periods of a few days raised the possibility of very high-order overtone pulsation of these stars. Thus, it is very important to draw various constraints on the stellar pulsation in semiregular variables and to separate other physical causes. Here we present new empirical evidence for radial pulsation in SRVs through studying their period-gravity relation, following the ideas outlined by Fernie (1995, F95). F95 studied a log P versus log g relation for various types of radially pulsating variable stars. He found a well-defined linear relation, which might serve to test whether the pulsation is radial or nonradial and, in the former case, fundamental or overtone. The main goal of our study is to extend the relation into the domain of SRVs.
The COROT Mission and its Seismology Programme
Authors: Baglin A. , Auvergne M. , Catala C. , Michel E. , Goupil M.J. , Samadi R. , Popielsky B. ,   the COROT Team
Published: July 2016

The COROT project, developed in the framework of the CNES small satellite programme with a wide European cooperation, will be launched in 2004. It is dedicated to seismology and the detection of telluric planets. It will perform relative broad-band photometry in visible light, during very long (150 days) observing runs in the same direction. Both programmes are working simultaneously on the same region of the sky. The seismology programme aims at mapping the HR diagram and study in detail 50 targets brighter than 9th magnitude. Another 50 will also be observed with a lower accuracy. The exoplanet programme will search for telluric planets slightly larger than the Earth, in the habitable zone and closer. Its data will also address many seismology problems.
Preliminary Results from Observations with the Fourier Tachometer
Authors: Schou J. , Brown T.M. , Bachmann K.T.
Published: July 2016

Recently several interesting results concerning the time dependence of solar p-mode frequencies have appeared. In particular it has been shown that the mode frequencies change with solar activity (Libbrecht and Woodard 1991).

We present preliminary results from an analysis of two ≈ 3 month observation runs with the Fourier Tachometer (which was operated by HAO and NSO) from 1987 and 1989. The analysis was done with two different methods in order to test for systematic errors. We show that the mode frequencies change in a manner similar to that reported by Libbrecht and Woodard. We also present results for the frequency splittings caused by the solar rotation.
Seismic Evidence of Modulation of the Structure and Angular Velocity of the Sun Associated with the Solar Cycle
Authors: Gough D.O. , Kosovichev A.G. , Sekii T. , Libbrecht K.G. , Woodard M.F.
Published: July 2016

Solar p-mode multiplet oscillation frequencies and expansion coefficients for degeneracy splitting have been measured in the summers of 1986 and 1988–1990. Temporal variations not unlike the differences between 1988 and 1986 reported by Libbrecht & Woodard (1990) continue into 1990. These differences are possibly associated with the solar cycle. We report here on our first findings about the variation of the internal angular velocity and asphericity deduced from these frequency differences.
Progress in High S/N Spectroscopy and its Importance for Stellar Modelling
Authors: Nissen P.E.
Published: July 2016

Recent advances in high S/N spectroscopy are reviewed with particular emphasis on new data for the abundances of chemical elements of importance for the modelling of stellar interiors.

It seems well established that young, nearby B-type stars have abundances of CNO elements that are about a factor of two lower than in the Sun except for a small fraction of nitrogen-rich stars. The existence of such stars among main sequence stars remains to be explained.

Among normal A-type stars (excluding Am and Ap stars) large deviations from solar abundance ratios occur with interesting anticorrelations of the abundances of C and Si. This suggests that diffusion processes and/or gas-dust separation of the chemical elements in the protostellar cloud play an important rôle.
Effective Temperatures and Angular Diameters of A–G Main Sequence Stars
Authors: Glushneva I.N. , Voloshina I.B. , Knyazeva L.N.
Published: July 2016

For 9 stars of A5–G2 spectral type interval IV and V luminosity classes effective temperatures, bolometric corrections, radii, angular diameters and luminosities were obtained by means of the infrared flux method in the modification by Blackwell et al. (1980).
High S/N Na I D Line Profiles in Late-Type MS Stars and their Importance for Stellar Modelling
Authors: Covino E. , Gomez M.T. , Severino G. , Franchini M.
Published: July 2016

Progress in observational techniques (i.e. high S/N spectroscopy) and modelling (e.g. RE models including convection, nonLTE codes, accurate atomic data) make significant to carry to extremes the numerical fit of a portion of the stellar spectrum and try to interpret the residual differences between calculations and observations in terms of stellar characteristics new respect to the employed model.

We have started to apply this synthesis method to a set of high resolution, high S/N spectra of late-type dwarfs in the region of the Na I D lines, obtained at ESO with the 1.4m CAT on May 1989 (Franchini et al. 1992). The dwarf stars are modelled in radiative equilibrium (RE), and the Na I D line synthesis accounts for nonLTE and blend effects.
Line Formation in Stochastic Winds of T Tauri Stars
Authors: Mitskevich A.S. , Natta A. , Grinin V.P.
Published: July 2016

We discuss in this poster the problem of the formation of Hα lines in the stochastic winds of T Tauri stars.
Modelling Time-Dependent Diffusion in Am Star Envelopes
Authors: Alecian G.
Published: July 2016

We study the diffusion process occurring just below the superficial convection zone of Am stars, improving the methods used sofar. We are now able to compute, in a more realistic way, the evolution of the concentrations during the stay of the stars on the main sequence for a large number of elements. This allows to better constrain the different properties (mass loss, depth of the superficial convection zone, transition between convection and radiative zone) entering the modelling of Am stars in the framework of a diffusion-dominant description.
Stellar Masses and Radii as Constraints on Stellar Models
Authors: Andersen Johannes
Published: July 2016

The current status of empirical data on stellar masses and radii of sufficient accuracy to give constraints on stellar models is reviewed. Results from the best-studied eclipsing binaries can already trace the main-sequence evolution of 1-10 M⊙ stars in considerable detail and will be even more useful when supplemented by chemical abundance data.

Taking the deceptively simple question of the observed width of the main sequence as an example, it is shown how careful attention to the details of the data is required to reach robust conclusions about such features of modern stellar evolution models as opacity tables or convective overshooting. Only detailed modelling of specific systems with known masses, radii, and metal abundance constrain the theory strongly enough that a truly critical test is achieved. The same is true when using tidal interactions in binaries (apsidal motion, rotational synchronization, and orbital circularization) as another probe into stellar interiors.
The Braking Law of Solar Type Stars as Derived from Close Binary Dynamical Evolution
Authors: Maceroni Carla
Published: July 2016

The orbital period of a close binary system (P ~ a few days), formed by two solar type components, evolves under the influence of angular momentum loss (AML) by electromagnetic braking and of spin↔orbit angular momentum transfer (AMT) by tidal coupling. Because of AMT, which tends to reestablish the spin-orbit synchronization destroyed by AML, the loss of AM takes finally place at the expenses of the orbital reservoir and produces shrinking of the orbit and spinning-up of the components. The variation with time of wK and w, respectively the orbital and the rotational angular velocity, is expressed by a system of (stiff) ordinary differential equations and the results of the (numerical) integration strongly depend on the AML and AMT laws. A series of recent papers (Maceroni and Van ’t Veer 1991, 1992, Van’t Veer and Macerord 1992, hereafter MVI, MV2 and VM) presents the resulting orbital period evolution function (PEF) with a variety of choices. A typical result, in terms of orbital period, and for a binary formed by two identical G5 V components, is shown in fig. IA. The solution shown here corresponds to the braking law a) of fig. IB, to the AMT according to Zahn (1977) and to stars rotating as rigid bodies; different assumptions have been tried in MVland VM.
Time-Resolved Spectroscopy of Short Period RS CVn Systems
Authors: Lázaro C. , Arévalo M.J.
Published: July 2016

We have initiated a programme of spectroscopic observations of RS CVn short-period group, with orbital phase resolution. The systems of this group are all eclipsing binaries with both components at the Main Sequence, and most of them have similar spectral type components. The high rotational velocities and their short orbital periods (less than 1 day) hinder the spectroscopic study of these stars. We presents the first results yielded by Hα line observations of the systems XY UMa and WY Cnc. Both systems were observed during 1991 with reasonably wide orbital phase coverage. The analysis of the spectra is made by comparison with a binary model, constructed from the observed spectra of normal stars of the same spectral type as the RS CVn system components. The model accounts for the partial contribution of each component at any orbital phase within eclipses.
The Properties of Accretion Disks of Herbig AE/BE Stars and Their Influence on the Surface and the Inside
Authors: Thé P.S.
Published: July 2016

In this paper we review the observations which lead to the believe that intermediate mass pre-main sequence Herbig Ae/Be stars are surrounded by accretion disks. A discussion is given of their properties and their contributions to surface and inside phenomena of the central stars.
The Evolutionary Code Cesam: Numerical Techniques
Authors: Morel P.
Published: July 2016

CESAM is a consistent set of programs and routines designed for the calculations of stellar evolution. Untill now it allows the computation of the evolution from PMS or ZAMS to helium flash for stellar masses of some solar mass. It is constructed in such a way that all the physics works as external routines. The numerical techniques are based on the B-spline formalism. This formalism used both for the integration of the differential equations and for 1D and 2D interpolation schemes of various tables of physical data.
A Peculiar Effect of Core Overshooting on the Internal Structure of Low-Mass Stars
Authors: Mowlavi Nami
Published: July 2016

With the aid of detailed stellar model computations, we analyse the consequences of applying a mild overshooting to 1.0 M⊙ to 1.2 M⊙ stars with solar metallicity. It is shown that solar models with a small convective core can be constructed, resulting in a decrease in the 8B neutrino flux that can amount to 30% relative to the value obtained in models without convective core.
Convective Overshooting and The Observed Width of The Main-Sequence Band
Authors: Napiwotzki R. , Rieschick A. , Blöcker T. , Schönberner D. , Wenske V.
Published: July 2016

For many years we are witnessing a lively debate on the existence and extent of convective overshooting, mainly in the cores of main-sequence stars. This is an important issue, since even a small amount of overshooting increases considerably the mass of the finally hydrogen exhausted core and lenghthens the main-sequence lifetime correspondingly. The available evolutionary calculations assume either moderate overshooting, d/Hp = 0.25, (d = overshooting distance, Hp = pressure scale height; Maeder & Meynet 1988) or strong overshooting, d/Hp ≈ 0.50 (Bertelli et al. 1986). Presently theory is unable to quantify the exact amount of overshooting, and one has to resort to empirical determinations.

Recently, Stothers (1991) collected all available information from the literature on stellar parameters and evolutionary calculations and concluded that, within the errors, d/Hp = 0 is an acceptable result, with a conservative upper limit of d/Hp < 0.2. However, such an approach is hampered by observational errors (like distance or temperature uncertainties, rotation) that are difficult to quantify and that may mask any definitive result. Detailed investigations of detached binaries may help in this matter (Andersen et al. 1990) but the number of suitable binary systems is probably not very large.
Excitation Mechanisms of Oscillations in Stars
Authors: Osaki Yoji
Published: July 2016

Excitation mechanisms of oscillations in stars are discussed, in particular on the problems of the Beta Cephei pulsations and of the solar oscillations. A long-standing mystery about the excitation mechanism of the Beta Cephei pulsations seems to have finally been solved, which is due to availability of the new “OPAL” opacities. The very mechanism is the classical к-mechanism, however, due to enhanced heavy element opacities at temperature around 2 × 105K. By using the new opacities, three independent groups have found pulsational instabilities for the fundamental radial mode and a few low-degree non-radial modes in Beta Cephei models. Stochastic excitation of solar oscillations by turbulent convection is then discussed. It is shown that the noise generation by quadrupole radiation concentrated at the top of the convection zone may well explain the frequency dependence of the observed solar acoustic power pumped into individual p-modes.
The Subseismic Approximation For Low-Frequency Modes of the Earth Applied to Low-Degree, Low-Frequency G-Modes of Non-Rotating Stars
Authors: Boeck I.De , Hoolst T.Van , Smeyers P.
Published: July 2016

The subseismic approximation for low-frequency modes in the Earth is used in an asymptotic treatment of low-frequency g-modes belonging to low-degree spherical harmonics in a non-rotating star.
Initial Asteroseismic Inversions
Authors: Gough D.O. , Kosovichev A.G.
Published: July 2016

Accurate measurement of the frequencies of low-degree acoustic oscillations of sun-like stars is imminent. We report on our first calculations with proxy data, aimed at assessing the kind of physical information that is likely to be acquired from seismic analysis and the precision with which the frequencies must be measured in order to obtain that information. The results will have an important bearing on future observing strategies, for the duration of observation should be determined primarily by the precision required of the frequency measurements. Our inversions are of eigenfrequencies of modes of an evolved main-sequence star of mass 1.1M⊙. The modes are of degree 0, 1 and 2, with frequencies in the range 1-3 mHz. Thus, by analogy with solar oscillations, they are modes that one should expect to observe in stars similar to the sun. Figure Ia depicts an idealized spectrum of stellar acoustic oscillations as one might expect from intensity variations such as those that could be measured from the proposed ESA spacecraft PRISMA. We report on the extent to which we have found it possible to determine the mass and radius of the stars, and on the seismic evidence for evolution having taken place in the core.
A Procedure For Two-Dimensional Asymptotic Rotational-Splitting Inversion
Authors: Sekii T. , Gough D.O.
Published: July 2016

Rotational splitting Δω(n,l,m) of the eigenfrequencies of a star rotating with angular velocity Ω(r, θ) about a unique axis can be represented as a weighted integral of Ω over r and θ, (r, θ, φ) being spherical polar coordinates about the axis of rotation. For high-frequency acoustic modes, Δω/m collapses essentially to a function of ω — ω /(l+1/2) and M = m/(l + 1/2) alone, and the weighting kernel K(r,θ) becomes asymptotically degenerate, each factor being of essentially Abel type. Therefore, formally, the splitting integral can be inverted, once a procedure has been found for extending Δω over the domain of (ω, M) such that the turning points (rt,ωt), given by (c(rt)/rt, sinωt) = (ω, M) where c is sound speed, span the star. Obtaining that representation is the most difficult stage of the inversion. We report on a procedure that treats the inverted two-dimensional Abel integral as a repeated double integral, representing the data successively along a set of parallel lines M =constant. The method is illustrated by an inversion of artificial data which is compared with the angular velocity from which those data were computed.
Stellar Dynamos
Authors: Krause F.
Published: July 2016

Energy is transported from the central regions of a star to its surface. Generally this transport is in certain layers carried on by convective motions. Because of the structure, which these motions have due to the influence of the overall rotation, the star becomes electromagnetically unstable, i.e. a large magnetic field grows from small seed fields as a result of the dynamo process. The internal structure, especially the symmetries of a star, will be, at least to some extend, reflected by the spatial structure and the time behaviour of the excited magnetic field. In this sense observations of the magnetic field on a surface of a star and the related activity phenomena can provide insight in the internal structure of the star, since characteristic parameters like thickness of the convection zone, mixing length, turnover time, profile of the differential rotation, etc. strongly influence the dynamo process.

The actual magnetic field of a star is a product of a nonlinear process. Models elaborated on the kinematical (i.e. linear) level provide insight in the excitation conditions and the linear field modes. The marginal mode, i.e. the mode which is easiest to excite, reflects properties of the nonlinear solution in case the system operates not far from the margin to the dynamo unstable region. Here the solutions show symmetries with respect to the axis of rotation and the equatorial plane, properties which are, for example, to a large extend fulfilled for the solar average magnetic field. For systems operating far from this margin irregular or even chaotic behaviour has to be expected. From observations there is a strong indication that these theoretical possibilities find their realizations within the sample of late-type stars.
Evolution of Stellar Activity in Early Post-Main-Sequence Phases
Authors: Maggio A. , Sciortino S. , Bianchi L. , Harnden F.R. , Rosner R.
Published: July 2016

We present preliminary observational evidences on the variation of the activity level in late type stars, during the evolutionary phases on the main sequence and beyond. We have selected a sample of 51 stars (Fig. 1), lying mostly along evolutionary tracks between 1.3 and 1.7 solar masses, which have been observed in soft X-rays with the Einstein Observatory, and in UV with IUE (Maggio et al. 1990; Haisch et al.1990). Two ROSAT targets, and four new IUE observations are also included.

We find that the radiative emission from the outer atmospheres of stars with M > 1.6M⊙ seems to behave differently than for stars with lower mass.

On the main sequence, the X-ray luminosity of most stars with B-V < 0.42 (spectral type F3) is relatively low, at Lx ~ 3 × 1028erg s-1 (Fig. 2). In the early evolutionary phases beyond the main sequence, the X-ray luminosity of the higher mass stars tend to increase sistematically up to ~ 1030erg s-1, while the lower mass stars show an initial moderate increase followed by a drop, at B-V ~ 0.6, below our sensitivity threshold.
Energy Losses by Stellar Activity Phenomena
Authors: Katsova Maria
Published: July 2016

For active late-type dwarfs, the total energy losses including both chromospheric and coronal radiation as well as the energy losses caused by stellar flares reach 3·10−3 L001. To maintain such high energetic losses requires a high efficiency of the dynamo action in the convection zone. Mass loss by active late-type stars can probably approach the rate of 10−11 M⊙/yr if we take into account the large-scale structure of their coronae and transient events. The phase of activity can continue for up to 1% of the lifetime of these stars on the main sequence due to braking by such a stellar wind.
Search for Magnetic Stars at Early Stages of Evolution
Authors: Glagolevskij Yu. V.
Published: July 2016

Young stars, as a rule, are too faint for measurements of magnetic field either by photographic method with the use of Zeeman analizer, or photoelectrically from hydrogen lines. That is why it is necessary to look for indirect ways of magnetic field detection, for example, by measurement of polarization. Ae/Be Herbig stars without a magnetic field are surrounded by a gaseous envelope in the form of a globe or a spheroid, flattened along the rotational axes (as dependent on stellar rotation velocity), and also by a gaseous-dust accretion disc in the plane of equator. There are powerful flows in gaseous envelopes of stars, connected with mass loss and accretion. If a star is a magnetic oblique rotator (as a magnetic star of the Main Sequence), then the gaseous envelope may acquire the shape of alon-gated ellipsoid with the major axes coincident with that of dipole (Dolginov et al., 1979). From the poles there arises a jet flow controlled by a magnetic field, as in He-r and He-w stars, having already reached the Main Sequence (Barker et al., 1982). Calculations show (Dolginov et al., 1979), that maximum polarization in the extended envelope p ≈ 4% arises when the ratio of ellipsoid axes is ≈ 2.5b. The electric vector of the dominating oscillation of the light wave is perpendicular to the plane through the axis of symmetry of the ellipsoid and the line of sight. Naturally, the magnetosphere rotates together with the star, involving the gaseous envelope, resulting in the variation of the degree and direction of polarization. Additional polarization is created by the polar jets, where the direction of the dominating oscillations of the electric vector is perpendicular to the axis of the polar stream, and value of maximal polarization may reach 5% along the beam.
Prisma the Instruments
Authors: Jones Andrew
Published: July 2016

Our understanding of Solar structure has increased dramatically in the last couple of decades thanks mainly to the opening of new windows of observation providing high quality data to theoreticians with access to powerful computing facilities. Two of the new windows were UV and X-ray images of the Sun, allowing a detailed view of the upper solar atmosphere, and the development of very high resolution spectrometers allowing us to exploit the solar oscillations to probe the internal structure of the Sun. It is the goal of PRISMA to extend these techniques to other stars, which using the Sun as a calibration point will allow us to explore stellar structure and evolution in ways not possible now.

In this poster I will present a possible selection of instruments able to achieve this goal, and explain some of the rationale in their design. A more general overview is presented by T. Appouchaux also in these proceedings. It must be stressed that these are not the definitive instruments to be flown on PRISMA, but rather result from a study to show the feasibility of such a mission. Should PRISMA be chosen as the next ESA medium sized mission, an ‘Announcement of Opportunity’ wiH be issued by ESA and the responses of all people interested in constructing the instrument will be considered.
Comparison of Stellar Oscillations and Activity Data to Infer Internal Rotation
Authors: Belvedere Gaetano
Published: July 2016

Recently it has beeen suggested that the latitude distribution of the main surface features of solar activity is intimately related to the angular velocity profile inside the Sun through the working of a MHD dynamo in the boundary layer between the convective and the radiative zones (Belvedere et al. 1991).

Although the present observational capabilities are not very encouraging, here we want to point out, in the framework of the analogy to the Sun (solar-stellar connection), that space observations of surface distribution and latitudinal migration of active regions on stellar surfaces, which could be carried out in this decade with more sophisticated techniques, may conversely allow us to infer the rotation profile, and consequently the angular momentum distribution, in stellar interiors. This methodology may in principle be considered alternate or complementary to the classical one based on observation of acoustic oscillations.
Radial Pulsations In 63 Herculis
Authors: Uyaniker Bülent
Published: July 2016

Constructing a theoretical model of 1.88 M⊙ pulsation properties of 63 Her have been analyzed. For the model, In question, pulsation periods in the fundamental mode and first two overtones have been derived by solving the linearized pulsation equations. The model has been found to be vibrationally stable.