Thermal structure of the envelopes of rapidly rotating neutron stars

For a neutron star which rotates like the fastest millisecond pulsar we calculate the temperature profile within the envelope. We find a small decrease of the surface temperature and a considerable increase of the thickness of the liquid layer from the pole to the equator.     

Expanding envelopes of binary stars

In this paper which is a continuation of Neutsch and Schmidt (1985a) (later on referred to as part I) we shall investigate the structure of binary envelopes under the hypothesis that at least one of the companions produces a very intense radiation field. The limiting case in which the forces due to the system's rotation (i.e., Coriolis and centrifugal forces) as well as gas pressure can be neglected is solved analytically using a classical result of Euler. Beyond this the velocity and density distributions in the envelope are determined.Under the assumptions mentioned above, the results are exact and are, therefore, of value as a reference model for later more advanced considerations (see, e.g., Neutsch and Schmidt, 1985b). Although the present approach is quite simple, it nevertheless shows some typical features of the general case. Furthermore, it serves as a helpful guide in guessing the system parameters to be initially adopted in physically sounder. calculations (for example: the Monte-Carlo approximation which will be described in Part III).     

The frequency of circumstellar envelopes among TMSS stars

We investigated the distribution of Neugebauer and Leighton's two micron survey (TMSS) stars on the {K - [12];[12] - [25]} and {[12] - [25];[25] - [60]} planes. The frequency of circumstellar emission objects among B-K type stars is less than 5% while it reaches about 20% at M , 80% at stars with unknown spectra and 90% at carbon stars.     

Circumstellar envelopes and the spectroscopic classification of stars

Byurakan Astrophysical Observatory, Byruakan, Armenia. Translated from Astrofizika, Vol. 36, No. 4, pp. 535–542, October–December, 1993.     

Gas-grain processes in the envelopes of late-type stars

Some results from studies of the gas-grain chemistry in oxygen-rich circumstellar envelopes (CSEs) are presented.     

Physical conditions in the convective envelopes of stars

The physical conditions in the convective envelopes of Pop. II main-sequence stars are described with some detail. A particular care has been devoted to the effects due to the formation of hydrogen molecules and to the pressure ionization.

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Riassunto Vengono descritte in dettaglio le condizioni fisiche nelle zone convettive esterne delle stelle di sequenza principale di Popolazione II. In particolare vengono studiati gli effetti dovuti alla formazione della molecola dell'idrogeno ed alla ionizzazione per pressione.

     

The structure of rotating stars from an extension of the Kippenhahn-Thomas method

A method is presented whereby the structure of rotating stars may be determined from an initial guess at the geometry of equipotential surfaces. The method may be considered an extension of the work of Kippenhahn and Thomas in that a uniformly continuous geometry is defined in terms of the appropriate spherical model with Roche characteristics at the surface of the configuration and sphericity at the centre. A simple Cowling model in uniform rotation is employed to illustrate the technique and for comparison purposes with previous work.     

Models of dusty envelopes around Vega-like stars

Einfache Entwicklungsmodelle der Staubhüllen um Wega, die Sonne, β Pictoris und Ross 128 werden konstruiert. Die Infrarotspektren dieser Hüllen werden unter der Annahme berechnet, daß der Minimalradius der Teilchen durch den Poynting-Robertson-Effekt bestimmt wird, während ihr Wachstum bis zur maximalen Größe durch Koagulation erfolgt. Ein Vergleich mit den Beobachtungen der Staubhüllen durch IRAS zeigt, daß das Modell die Grundzüge der Entwicklung protoplanetarer Scheiben zufriedenstellend beschreibt.     

The circumstellar envelopes of Be stars: viscous disc dynamics

We have constructed models of axisymmetric, circumstellar envelopes for Be star discs by successfully combining two numerical codes: a non-local thermodynamic equilibrium (non-LTE) radiative transfer code which calculates the level populations and disc temperature distribution self-consistently, and a hydrodynamical code. The output of one code is used as input to the other code, and hence evolving density and thermal structures may be examined. The temperatures, disc density and velocity distribution are used to investigate the outflowing viscous disc model for Be stars. We find that these simulations place constraints on the power-law density decrease in the disc with increasing distance from the star. We find that the power-law index for the line-forming region of the disc lies between 3 and 3.5 with a small dispersion.     

Circumstellar envelopes and disks of pre-main sequence stars

The current state of knowledge about circumstellar matter of young stellar objects is briefly reviewed. It appears that some very young stars yet to accrete substantial amounts of mass may be seen through their dusty infalling envelopes even at optical wavelengths, because of the presence of holes or large departures from spherical symmetry in the envelopes. The evidence for this picture is summarized in the context of one wellstudied young star, HL Tau, indicating that much of the large-scale structure originally identified as a rotating disk is probably a flattened infalling envelope. Departures from spherical symmetry in protostellar clouds are likely to lead to quite flattened structures once collapse gets under way, further suggesting that infall in large-scale toroids may be a general feature of low-mass star formation. The best kinematic evidence for Keplerian disk rotation comes from optical and near-infrared high-resolution spectroscopy of the innermost regions of circumstellar disks. Disk masses are uncertain but are likely to be at least the order of minimum mass solar nebula models, if not much larger.     

Vibrational stability of rotating stars

The aim of the present paper will be to detail the explicit form of the equations which govern first-order oscillations of fast-rotating globes of self-gravitating fluids; with due account taken of the effects arising from the centrifugal as well as Coriolis force. As such configurations oscillate in general about distorted figures of equilibrium, the equations governing them can be conveniently expressed in terms of the Clairaut coordinates, associated with distorted spheroidal figures, and introduced in our previous paper (Kopal, 1980) for this purpose.In Section 2 which follows a brief outline of our problem, the equilibrium properties of fast-rotating configurations or arbitrary structure will be formulated. In Section 3 we shall carry out a separation of the variables in the equations of motion, and reduce the partial differential equations of the problem to an equivalent system of ordinary differential equations, by an expansion of expressions for the velocity componentsU, V, W in terms of tesseral harmonicsY _n ^m (, ). The explicit form of such a system, including the effects of all tesseral harmonics of orders up tom=n=4, will be specified in Section 3 for configurations whose equilibrium form is a sphere; while in Section 4 this latter condition will be relaxed to allow for the equilibrium configuration to become a rotational spheroid.In the concluding Section 5 we shall convert the complex form of our equations of motion into real terms, amenable to a solution-analytical or numerical-in terms of real variables; and shall establish the boundary conditions necessary for a specification of the characteristic frequencies of oscillation.     

Detection of detached dust envelopes around oxygen-rich AGB stars

Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period.     

Minimal models of cooling neutron stars with accreted envelopes

We study the 'minimal' cooling scenario of superfluid neutron stars with nucleon cores, where the direct Urca process is forbidden and enhanced cooling is produced by neutrino emission due to the Cooper pairing of neutrons. Extending our recent previous work, we include the effects of surface accreted envelopes of light elements. We employ the phenomenological density-dependent critical temperatures   T _cp(ρ)  and   T _cnt(ρ)  of singlet-state proton and triplet-state neutron pairing in a stellar core, as well as the critical temperature   T _cns(ρ)  of singlet-state neutron pairing in a stellar crust. We show that the presence of accreted envelopes simplifies the interpretation of observations of thermal radiation from isolated neutron stars in the scenario of our recent previous work and widens the class of models for nucleon superfluidity in neutron star interiors consistent with the observations.