This topical meeting took place during the AAS meeting in Winston-Salem,
North Carolina, on **Tuesday, June 10, 1997 **. The program lasted
half a day, from 08:30 to 12;30. The well-attended session concluded with a
poster review by Walter Van Hamme and a Round-Table Discussion about the
philosophy and future of light-curve modeling codes.

An important outcome was the creation of a community list-server for binary
star modeling by Stefan Mochnacki of the David Dunlap Observatory, Univ.
of Toronto. This community bulletin board would be a way for modelers to
interact with the binary star community and be made aware of the needs of the
wider astronomical community. As eclipsing binary star programmes become
ever more sophisticated, it may be useful to colleagues who have to deal with
binary stars to be aware of modeling tools that are forthcoming, as well as
those already available. So here is your chance to find out and contribute
your ideas and needs to the modeling group.
Contact Stefan (stefan@crux.astro.utoronto.ca) to join this list, or look here:

list-server subscription info .

To receive further information about the session, or copies of the proceedings, please contact:

Abbreviated texts of the full papers will be available at this Web Site as
they are received, The full texts will be gathered together in a hard-bound
spiral bound publication, and made available for $10US, to cover costs,
including mailing. See below for more details about this, or hit this switch:

proceedings copy.

Date: Tuesday morning, June 10, 1997; 8:30-10:00; 10:45-12:30.

Format: Only invited 10-20 min. papers are listed.

-- all times exclusive of 5-min. question periods --

- Enlightening Light Curves or Modeling for the Masses:
Opening Remarks
[E.F. Milone, UC] {~10]

- Extending the Applications of Binary Star Models
[R. E. Wilson, U. of Florida] [30]

- A Damped Least Squares Wilson-Devinney Based Program for Binary
Star Analysis
[J. Kallrath, BASF & UBonn] [12]

- Tools for the Analysis of Binary Star Data Using Desktop
Workstations [Dirk Terrell, UF] [12] {read by Title only}
- Radial Velocities Weighted with line Strength
[W. Van Hamme, FIU] [12]

--- Break --- [45]

- Radiation Pressure Effects on Binary Star Geometry -- with
Application to Modeling Binary Stars with Hot Components
[H. Drechsel, Remeis Observatory Bamberg] [20]

- Combined Spectrum and Light Synthesis Analysis of Eclipsing
Binary Stars
[A. Linnell, MSU] [15]

- Simultaneous Fitting of Orbital Elements and Doppler Imaging of
Spots Using Photometric and Spectroscopic Data
[S. Mochnacki and P. Hendry, UToronto] [15]

- Poster session review by Walter Van Hamme
- Round-Table Discussion. Moderated by E.F. Milone

- Spot Modeling of the Chromospherically Active Binatu SV Cam
[J. I. Ordway, San Diego S. U.; P. A. Heckert, W. Carolina U]
- The Modeling of the Halpha Spectral Features of V471 Tauri
[T. Vaccaro, UFL and Arthur Young, SDSU]

- Photometric Investigation of the Total Eclipsing, Very Short Period
Active W UMa System, V743 Sagittarii
[R. G. Samec, M. W. Looi, Bob Jones U.; B. Carrigan, Millikin U.]

## Meeting Justification

The advances in modeling eclipsing binary light curves (generically including polarimetric, spectrophotometric, radial velocity curves, spectral line profiles, spectral indices, and other observables) were last reviewed in 1991 at the IAU meeting in Argentina (

cf. Milone 1993 for these proceedings).The field continues to develop, and since the light curve models and codes are depended on to produce the most reliable radii and masses among other fundamental properties of stars, it is appropriate for another meeting to be held to review the improvements of the models and of the programs which implement them. The new model improvements include more realistic spot simulations, radiative transfer treatment of eclipses of atmospheric clouds, improved radiation interactions between the components, non-linear limb-darkening, and non-solar metallicities. Code improvements include improved integration and convergence techniques; modularization of light curve analysis programs. The discovery of large numbers of eclipsing variable stars in the galactic bulge from gravitational lensing projects means that new techniques may be required to further increase the efficiency of the analysis process, hence the recent interest in the possible usefulness of neural nets.

Finally, a number of issues raised in Argentina have not been resolved. The philosophy of model development and questions of responsibility for model and technique implementation and the codes themselves need further discussion in as open a forum as the community can provide. Indeed this concern is much broader than the light curve modeling area, and will be of interest to a still wider audience.

All listed speakers have now agreed to participate; titles are subject to change. All papers will emphasize results of the applications to binary star studies and to other areas of astronomy.

Reference:

- Milone, E. F., 'Light Curve Modeling of Eclipsing Binary Stars'.
(New York: Springer-Verlag), 1993.

## Abstracts

### Enlightening Light Curves or Modeling for the Masses: Opening Remarks

--- E. F. Milone, RAO, The University of Calgary

The advances in the modeling of eclipsing binary light curves (generically including polarimetric, spectrophotometric, and radial velocity curves, spectral line profiles, spectral indices, and other observables) were last reviewed comprehensively in 1991 at IAU sessions in Argentina. The field has continued to develop, and since light curve models and codes are depended on to produce reliable radii and masses, among other fundamental stellar properties, it is appropriate to review what is new in both models and the programs to implement them.

The new model improvements include more realistic spot simulations, radiative transfer treatment of eclipses by translucent atmospheric clouds, improved radiation interactions between the components, non-linear limb-darkening, and non-solar metallicities. Codes are now available with improved integration and convergence techniques, and are modularized to permit added operations. The session is designed to illustrate these advances and the benefits for binary star research.

This special session is made possible by grants to EFM from NSERC of Canada, the University of Calgary's Research Grants Committee, Research Services, Faculty of Science, and the Department of Physics /& Astronomy, all of which are appreciated.

### Extending the Applications of Binary Star Models

--- R. E. Wilson, Astronomy Department, University of Florida

Light curve model work has increasingly been extending into general observables. It now includes (or soon will) polarimetry, line profiles, pulse arrival times, and interferometric observations in addition to brightness and radial velocity. The inclusion criterion for the above-listed observation types is not that every one require a physical binary star model, but only that any combinations that are fitted simultaneously involve a physical model. Thus ordinary pulse arrival time analysis involves only the geometry of orbiting point sources, but can involve physical models when combined with light curves and radial velocities. Solution of combined light and velocity observations, although not yet common, is becoming more frequent. Models now have circumstellar disks, stellar atmospheres, and attenuation by circumstellar gas and dust in addition to basic external physical attributes such as tides, reflection, orbital eccentricity, and non-synchronous rotation. Some aspects of the pulse arrival time, line profile, and circumstellar attenuation problems will be discussed here. The pulse discussion centers on an algorithm through which differential corrections solutions can be carried out, including designation of the independent variable and formation of partial derivatives of arrival time with respect to model parameters. The profile discussion focusses on generalization of a binning procedure so as to include various broadening mechanisms and on computational strategies to improve accuracy. The attenuation discussion concerns efficient computation of attenuation in matter of arbitrary optical thickness with effective inclusion of several scattering and absorption mechanisms, arbitrary spatial distribution, and proper wavelength dependence.

### A New Damped Least Squares Version of a Wilson-Devinney-Based Program for Binary Star Analysis

---- Josef Kallrath, BASF-AG, Ludwigshafen, Germany; E. F. Milone, RAO, Univ. of Calgary; Dirk Terrell, Univ. of Florida; A. T. Young, San Diego State Univ.We summarize and report recent improvements to a version of the Wilson-Devinney program. This program is widely used for the analysis of eclipsing binary data and is open to extensions and improvements.

WD95 is such an extension and contains our version of the Wilson-Devinney code. It supports the use of the Kurucz atmosphere models; it provides options to use multiple epoch data and multi-wavelength synoptic passbands. The WD95 program contains an improved I/O interface, simplex algorithms for initial searches and tests, and versions of Wilson-Devinney DC and LC programs and options to switch to automatic differential corrections or to a damped least-squares solver using normal equations that are modified as per Levenberg-Marquardt. The damped least-squares solver is tested with simulated data and used to demonstrate the features and performance of the code. The algorithm proves to be less dependent on the quality of guessed initial parameters and thus may be efficiently used to reduce the time involved in light curve analysis.This work has been supported by GKSS (Germany; to JK and EFM), NSERC (Canada), and URGC grants (Univ. of Calgary) (to EFM).

### Radiation Pressure Effects on Binary Star Geometry -- with Application to Modeling Binary Stars with Hot Components

--- H. Drechsel, Remeis Observatory Bamberg

A method is presented to include radiation pressure effects in the modeling of close binaries. Radiative interaction is particularly important for hot OB-type binary stars in close configurations. The potential field has to be calculated by a numerical approach, since no analytical representation is possible if the Roche potential is accordingly modified. Radiative forces exerted by luminous, hot stars may cause geometric deformations of stellar surfaces and influence the binary configuration and evolution. Principal effects are demonstrated by model calculations accounting for radiation pressure, which is parametrized using the ratio of radiative relative to gravitational forces.

There are several important implications for the binary structure: the geometry of the stellar surfaces is modified; the Lagrangean points are shifted, and the shape and extent of the Roche lobes are changed; the tendency to take up inner contact is counteracted by radiative forces, while

outer contact components (with surfaces incorporating L2 or L3) may result above some critical radiation pressure strength; obvious consequences for the evolution of systems with hot and luminous components like Wolf-Rayet or X-ray binaries exist.The modified Roche potential is used as an improved binary model for the computation of eclipse light curves, based on the Wilson-Devinney technique. The inverse problem is solved by application of the non-linear simplex parameter optimization algorithm. The feasibility of the new method is demonstrated by photometric solutions of a few OB systems. The implementation of radiation pressure effects yields improved light curve representations compared with conventational methods, even in the case of weak radiative forces typical for late O and early B-type stars.

### Combined Spectrum Synthesis and Light Synthesis Analysis of Eclipsing Binary Stars

--- A.P. Linnell, Michigan State University

A new software package provides a self-consistent, physically realistic procedure for calculating both synthetic spectra and synthetic light curves of eclipsing binary stars. A system synthetic spectrum calculated for each orbital phase of spectral observation, together with a solution of eclipse light curves, determines a system model, including effective temperatures of both components. Use of the synthetic spectra in a synthetic photometry mode produces consistent system parameters in analysis of UBV photometry. An application to MR Cygni illustrates application of the procedure

Addition of an accretion disk presents additional complexity. The program uses a visibility key attached to each grid point on the star photospheres, the accretion disk visible face, and the accretion disk rim to determine the contribution of the associated surface element to the system spectrum. Separate output spectra are available for each stellar component, the accretion disk face, and the accretion disk rim. The calculated output includes a composite system spectrum. The program provides an option to model a single bright spot on the rim, with an assignable longitude, azimuthal width, and effective temperature. Application of the program to the cataclysmic variable PG 0859+415 illustrates program performance.

### Simultaneous Fitting of Orbital Elements and Doppler Imaging of Spots Using Photometric and Spectroscopic Data

S.W.Mochnacki, and P.D.Hendry, DDO

We briefly review previous work on Doppler and photometric imaging of starspot distributions on close binary stars, particularly contact binaries. Difficulties involving standard spherical and cylindrical surface element distributions for imaging are discussed. The method of geodesic distribution binary synthesis (GDDSYN) is faster and more accurate for such work.

The Maximum Entropy regularisation method has been extended to include simultaneous photometry and spectroscopy. A further extension allows auxiliary parameters such as mass ratio and fill-out to be determined, as well as the radial velocity amplitude. The spectrum of third light contamination is also obtained, using theoretical spectrum synthesis for the binary as input. Accurate distances act as a powerful constraint on the fitting process.

The procedure has been applied by PDH to the system VW Cephei. The development of spots over a two year period has been studied. The fraction of the surface covered by spots appears to be be very large.

Future developments will be discussed. This type of approach is essential to obtain good models for cool, heavily-spotted binary systems.

Proceedings Cost and Availability The texts of the full papers will be available at this Web Site following the meeting and will be gathered together in a hard-bound spiral bound publication, available at cost, as noted above: $10 US, payable to the University of Calgary. Prepaid requests may be sent to:

E. F. Milone

Department of Physics & Astronomy

The University of Calgary

250 University Dr., NW

Calgary, AB T2N 1N4

Canada

More to come!.

Number of visitors to this site to date:

- Milone, E. F., 'Light Curve Modeling of Eclipsing Binary Stars'.
(New York: Springer-Verlag), 1993.