Nonlinear coupling between pulsation and convection in late type stars

A simple idealized nonlinear model applicable to long period variable stars has been formulated that assumes the convective envelope ofM giants is composed of giant convection cells, which are comparable in size to the stellar radius. The simplicity of this model essentially constitutes a physical analog to the strong dynamic coupling that occurs if the convective envelope of the star undergoes both modes of motion. As shown implicitly in the time scales associated with these motions, the coupling produces asymmetrical fluctuations of the entire star, the mean velocity of which is comparable to the escape velocity of the star at particular values of the ratio of the pulsation and convection time scales. It is suggested that this can account for the mass loss from late type stars, and the circumstellar dust shells that are associated extensively with long period variables.For critical values of the pulsation and convection time scales, the solutions correspond to the rapid expansion of the entire convective envelope, and is the basis of a new mechanism that simulates the manner in which pulsating stars ballistically accelerate their convective shells to form planetary nebulae.     

The role of stellar envelope convection in gravity-darkening and its relation to observational data

We study the role of convection in the surface heat flow of low mass stellar envelopes with the aid of Paczynski’s public domain program program GOB. This role is important, for example, for the analysis of light curves of close binary stars. We have considered atmospheric models for a range of masses similar to the components of contact or near-contact binaries between 0.4 and 1.1M_⊙ and examined the effects of changing luminosity, surface temperature and mixing length for stars of given mass. Our presentation includes checks of the modeling against published standard stellar envelopes. A slight program modification allowed gravity darkening coefficients for selected models to be calculated directly.If the procedure proposed by Lucy is followed, similar values of the index β (∼0.06–0.1) are obtained for a fairly wide range of masses, luminosities and effective temperatures of cool stars. There also appears no strong dependence on the mixing-length parameter α. There are, however, physical differences between the conditions that apply to this derivation of the index and those of a photosphere distorted by rotation and tides, but having net dynamical stability. Thus, the dependency of T_e on g was argued to come from the ratio of partial derivatives of the adiabatic constant K for the layer where convection starts; but a subconvective equipotential surface, where K would be constant, cannot coincide with such a layer. The adopted procedure is therefore inappropriate, and would involve different heat transfer regimes in different radial directions (say, polar or equatorial). A corresponding evaluation for the layer in which the convective flux becomes maximal shows less sensitivity to T_e. The corresponding index is then much closer to, or even greater than, the von Zeipel value.Other arguments are also considered, including those of Anderson and Shu [Anderson, L., Shu, F.H., 1977. ApJ 214, 798] about the independence of the convective flux to local gravity, as well as the role of superphotospheric circulation effects, which could reduce the gravity darkening index to less than its subphotospheric value.Observational evaluations of gravity darkening for close binary systems, in general, are still inconclusive, due to the strong correlation between β and other parameters characterizing the light curve shape, particularly in the scale photometric ‘ellipticity’ effects. It is possible that very precise light curves that may be obtained in the future will allow this situation to be improved. The general surface distribution of emergent flux in the far infra-red of Jupiter may be a pointer in the direction of future studies of the subject.     

Photometric Observations and Light Curve Studies of the Semi-Detached Eclipsing Binary R CMa

We present observations and light curve analysis of the eclipsing binary R CMa in the narrow band filters v and b. Observations were made during 1993 at Biruni Observatory and the light curves have been analyzed using the Wilson-Devinney light curve interpretation program. Assuming a semi-detached configuration for R CMa, the parameters i, Ω₁, L ₁, T ₂ and A ₂ were adjusted for the best fit between the synthesized light curves and observations. Both light curves were fitted well with a lower value of bolometric albedo than what would be expected for a normal cool star with a convective envelope. The masses of the primary and secondary components and the absolute dimensions of the stars have been calculated using the derived relative dimensions from Wilson-Devinney codes and the spectroscopic observations.     

Contact Binaries: A Study of the Proximity Effects and Gravity Darkening Based on Kopal’s Fourier Method

A new method for the determination of the proximity effects and gravity darkening exponents in contact binaries of W UMa type is presented. The method is based on Kopal’s method of Fourier analysis of the light changes of eclipsing variables in the Frequency Domain. The method was applied to 36 W UMa systems for which geometric and photometric elements have been derived by the most powerful techniques. The derived values are very close to those predicted by the existing theory of radiative transfer or convective equilibrium.     

Gravity and effective temperature distribution on the surface of contact binary systems

The aim of this work is to study the gravity and the effective temperature distribution on the outer equipotential surface of a contact binary. In particular, the lines of constant g, (the iso‐g curves), are computed and plotted on the outer surface C_s of contact configurations. Since the gravity distribution would also specify the effective temperature distribution on the binary's common photosphere through the gravity darkening effect, these lines correspond to isothermals too. Applications have been made to two contact binaries; namely, to AW UMa and OO Aql, two systems with very different mass ratios. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitation of Radial P-Modes in the Sun and Stars

P-mode oscillations in the Sun and stars are excited stochastically by Reynolds stress and entropy fluctuations produced by convection in their outer envelopes. The excitation rate of radial oscillations of stars near the main sequence from K to F and a subgiant K IV star have been calculated from numerical simulations of their surface convection zones. P-mode excitation increases with increasing effective temperature (until envelope convection ceases in the F stars) and also increases with decreasing gravity. The frequency of the maximum excitation decreases with decreasing surface gravity.     

Discussion of infrared measurements of T Tauri stars and related objects

It is at first reported that certain kinds of stars which have been classified as T Tauri stars or related objects are in reality not of this type. After the exclusion of those objects, the infrared measurements accessible in the literature permit to draw some astrophysical inferences. It is then possible to distinguish three classes of light variations. All T Tauri stars have an infrared excess. From the colour indices H – K and K – L it can be deduced that the infrared excess for more than one half of the objects is due to the thermal radiation of the circumstellar dust envelope; for the remaining stars also free-free radiation from the gas envelope can play an essential part. The largest infrared excesses E_H–K were found with the hotter stars (spectral type A) and the strongest emission lines with the cooler stars (spectral types G, K, M). This can finally be explained by the fact that the convection zone in cooler stars reaches far down into their interior than in hotter stars.     

Photometric investigation of the evolved contact binary system DN Aur

We present here the photometric light curve analyses of the eclipsing binary star DN Aur. The CCD photometry, performed at the Behlen observatory using the 0.76 m automated telescope gave 646 individual data points inV andR bandpass filters. From this data we have determined a new epoch and an orbital period of 0.6168891 days. The published spectral classification is F3.The Wilson-Devinney model was used to derive the photometric solutions. DN Aur is a W UMa type contact binary system. The mass ratio, (q=m ₂/m ₁0.210, where star 2 is eclipsed at the primary minimum) suggests that the system has A-type configuration. The computed light curve has a third light of about 22 percent and a total eclipse in the secondary minimum. A solution with a cool spot on the secondary component is also found. We recommend spectroscopic study of DN Aur even though the light curve analysis show it to be a single line spectroscopic system. Generally contact systems of spectral type F3 have periods ranging from 0.25 to 0.5 days. The longer period of DN Aur suggests that it is an evolved contact system with case A mass transfer.     

Population Synthesis of Binary Be Stars with Degenerate Companions in the Galaxy

Using the numerical code (`Scenario Machine') we study of number and physical properties of binary Be stars. Evolutionary tracks leading to a formation of the observational binary systems are presented. We conclude that synchronization must be taken into account when calculating binary Be star evolution and calculate the minimal orbital period for Be/evolved companion binary. The obtained distributions over orbital parameters are in good agreement with the observational lack of short-period Be/X-ray binaries. According to our calculations 70% of all Be stars must have a white dwarf. The white dwarfs in these systems should be hot enough with the surface temperature distribution peaking at 10000–20000 K. Their detection is possible during the period of the lack of Be star envelope by the detection of white dwarf extremely UV and soft X-ray emission. This method of registration appears to be particularly promising for `single' early-type Be stars because in these systems the white dwarfs must have a very high surface temperature. However, the loss of the Be disc-like envelope does not often occur and it is a rather rare event for many Be stars. The best possibility of white dwarf detection is given by the study of helium spectral lines found in emission from several Be stars. The ultraviolet continuum energy of these Be stars is found to be not enough to produce the observed helium emission. Besides, we also discuss the orbital properties of binary Be star systems with other evolved companions such as helium stars and neutron stars and give a possible explanation for the lack of Be/black hole binaries. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonradiative activity across the H-R diagram: Which types of stars are solar-like?

Major advances in our understanding of nonradiatively heated outer atmospheric layers (coronae, transition regions, and chromospheres) and other solar-like activity in stars has occurred in the past few years primarily as a result of ultraviolet spectroscopy from IUE, X-ray imaging from the Einstein Observatory, microwave detections by the VLA, and new optical observing techniques. I critically review the observational evidence and comment upon the trends with spectral type, gravity, age, and rotational velocity that are now becoming apparent. I define a solar-like star as one which has a turbulent magnetic field sufficiently strong to control the dynamics and energetics in its outer atmospheric regions. The best indicator of a solar-like star is the direct measurement of a strong, variable magnetic field and such data are now becoming available, but good indirect indicators include photometric variability on a rotational time scale indicating dark starspots and nonthermal microwave emission. X-rays and ultraviolet emission lines produced by plasma hotter than 10⁴ K imply nonradiative heating processes that are likely magnetic in character, except for the hot stars where the heating is likely by shocks in the wind resulting from radiative instabilities. I conclude that dwarf stars of spectral type G-M and rapidly rotating subgiants and giants of spectral type F-K in spectroscopic binary systems are definitely solar-like. Dwarf stars of spectral type A7-F7 are almost certainly solar-like, and T Tauri and other pre-Main-Sequence stars are probably solar-like. Slowly rotating single giants of spectral type F to early K are also probably solar-like, and the helium-strong hottest Bp stars are interesting candidates for being solar-like. The O and B stars exhibit some aspects of activity but probably have weak fields and are not solar-like. Finally, the A dwarfs and the cool giants and supergiants show no evidence of being solar-like.Staff Member, Quantum Physics Division, National Bureau of Standards.     

Microlensing of close binary stars

The gravity due to a multiple-mass system has a remarkable gravitational effect: the extreme magnification of background light sources along extended so-called caustic lines. This property has been the channel for some remarkable astrophysical discoveries over the past decade, including the detection and characterization of extrasolar planets, the routine analysis of limb darkening, and, in one case, limits set on the apparent shape of a star several kiloparsec distant. In this paper, we investigate the properties of the microlensing of close binary star systems. We show that in some cases it is possible to detect flux from the Roche lobes of close binary stars. Such observations could constrain models of close binary stellar systems.     

New classification and basic stellar parameter of SW Equulei

We review the classification of the variable star SW Equulei, formerly classified as a close binary of the W UMa type with a period of 0.83632 d and components of spectral types O–A or F–G and later. New observations indicate that the light curve is not that of an eclipsing binary but of a pulsating star with a period half of the old value. Further examination shows that SW Equulei is a double mode star of the RRd Lyrae type with a fundamental period of P = 0.560727 d and an overtone of P = 0.4181777 d at a distance of ≈ 7.1 kpc. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gravity-darkening in eclipsing binaries

A quantitative analysis of the ellipticity effect in close binary systems of the W Ursae Maioris type with spectroscopically known mass-ratios discloses that the photometric ellipticities of these variables are, in general, more than twice as large as the dynamical ellipticities (due to the geometrical distortion alone) computed for contact models of centrally condensed stars. If, moreover, this excess of the photometric over dynamical ellipticity is attributed to the phenomenon of gravity-darkening over distorted surfaces of the constituent components, this darkening must be present to a considerablylarger degree than that predicted by the theory of radiative transfer — a result difficult to reconcile with the existence of extensive sub-surface convection zones in late-type Main-Sequence stars. If, on the other hand, gravity-darkening is present in W UMa-type systems in the amount predicted by the theory (let alone in the presence of sub-surface convection), the only effective way of reconciling the theory with the observations would be to assume that the mean fractional radii of their constituent components are appreciablylarger than those appropriate for contact models in which the two components just fill the largestclosed Roche equipotentials capable of containing their mass. In other words, it would appear that the W UMa-type stars considered in this paper may constitute a single dumb-bell figure rather than two distinct components; and that the observed variations of light, colour or radial velocity are invoked by the axial rotation (and consequent variable cross-section) or this dumb-bell figure rather than to the contribution of individual stars.     

The influence of magnetic field and rotation on the onset of convection in the envelopes of neutron stars

The influence of magnetic field and rotation on the occurrence of convective instabilities in the liquid layer of neutron star envelopes has been investigated. The critical wavelength _c , which denotes the boundary between stable and unstable behaviour of convective disturbances, is calculated for a neutron star model as a function of magnetic field and rotation. It is shown that the strength of the magnetic fields of neutron stars strongly suppresses the onset of convection, whereas the limiting effect of rotation acts only if the magnetic field vanishes.     

Gravity Modes in Delta Scuti Stars

This paper examines the observational evidence for the detectionof gravity modes in Scuti stars, which are p-mode pulsators.Low-order gravity modes have also been found in at least one star (FG Vir).Some reports of gravity modes may be due to systematic errors in theabsolute magnitude calibrations for slowly rotating stars. Furthermore,many detected low frequencies are not high-order gravity modes, but linear combinations,f _i -f _j , of the main pulsation modes. Other low frequencies are caused bya close binary companion leading to tidal deformation as well as tidallyexcited gravity modes.     

Simultaneous solutions for photometric and spectroscopic observations of TX UMa

B andV light curves for one epoch and radial velocity curves of three different epochs have been analyzed to revise the solution of TX UMa. The solution has been adjusted simultaneously in the light curves and radial velocity curves by the method of Wilson and Devinney's differential correction. The primary star's surface rotation rate to synchronous rate is determined as 1.768 from one of the radial velocity curves. The absolute dimension of the system has been deduced based on the simultaneous solution. The primary star is well fitted to the evolutionary track for a single star while the secondary star, while filling its Roche lobe, is fitted to the evolutionary track for a close binary system.     

Photometric Investigation of the Eclipsing Binary Star KN Per

We used the Behlen observatory 0.76 m telescope and the CCD photometer to secure 689 observation of the eclipsing binary star KN Per. The observations were made on 8 nights during 1993 and 1994 with V and R bandpass filters. From 7 determinations of eclipse timings of minimum (V&R together), we have determined a new epoch and an orbital period of 0.8664604 days. The published spectral classification is A9. The 1993 version of the Wilson-Devinney model gave the photometric solutions. The adopted solution indicates that KN Per is a W UMa type contact binary. The mass ratio, q = (m₂/m₁, where star 1 eclipses at the primary minimum) = 0.23 suggests that KN Per is a W UMa system with A-type configuration. The secondary minimum shows a total eclipse. The asymmetry in the light curve is fitted with a cold spot on the secondary component of the system. The luminosity difference between the components is very large KN Per therefore, is most likely a single line spectroscopic binary. We recommend spectroscopic study of this system. Generally contact systems of spectral type A9 have periods ranging from 0.4 to 0.6 days. KN Per has considerably longer period and thus appears to be an evolved contact system with case B mass transfer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gravity darkening of the secondary component of Algol

An analysis of the infrared light curve of the secondary minimum of the Algol system — when the contact subgiant component of spectral class K0 undergoes eclipse by its nearly spherical and early-type mate — for the gravity-darkening of the distorted contact star has disclosed that the monochromatic coefficient of this darkening at =1.6 is some three to four timeslarger than that resulting from von Zeipel's theory. This is in agreement with recent photometric studies of gravity darkening in WUMa-type systems, but in flagrant contrast with the consequences of theories requiring deep sub-surface convection zones for subgiants of advanced spectral types. The origin of this discrepancy remains unresolved, and underlines the need of further observations of the secondary minima in semi-detached eclipsing systems in the infrared.     

Accretion-Effekt bei exzentrischen galaktischen Sternbahnen

In this paper the accretion effect of metal-poor stars on elliptic orbits is considered. Stars moving on eccentric galactic orbits and passing through spiral arms take up material from the interstellar medium. The stars pick up the more material by accretion, the more their orbits are osculating the spiral arms. If this material is mixed with the original matter of the star only in the outer convection envelope and if the star is old enough, than the ultraviolet excess of the star can be changed. But the accretion effect alone cannot explain the observed correlation between the ultraviolet excess and the orbital eccentricity.