tion will be studied with another set of exposures. How far these bands agree with those observed by IRAS at different times and wavelengths remains to be seen, but it is at least possible by earthbound ob- servations to see structures within the zodiacal light and to measure them. In particular, it will also be possible to make long-term observations which will throw more light on the constancy of this phenomenon. Other, still open questions are the inclination of the bands to the ecliptic, their intensity and how far they correlate with the distribu- tion of the asteroids. Here we only demonstrated that with a quite modest equipment new results can be obtained in this field of as- tronomy. Surely, the excellent condi- Figure 2: Contrast-enhanced picture of the same photograph as Figure 1. Two bright tions on La Silla contributed to this. have been marked to facilitate the orientation. Especially west of the gegenschein a separation of the light bridge is noticeable. 4. Acknowledgements antisolar point was at high galactic within the light bridge were marked, tak- I am indebted to Prof. W. Schlosser, latitudes and the Milky Way did not in- ing care that they did not influence the Bochum, for his suggestion to embark terfere too much. Because of this, the judgement of the pictures themselves. upon this subject and for his continuing zodiacal light bridge could be observed This procedure was repeated with three support. Thanks are also due to the over a large extension along the ecliptic. different pictures with various scales to Director General of ESO, Prof. H. van Unfortunately, at the time of our obser- improve the accuracy of measurement. der Laan, and also to Prof. L. Woltjer vations (March 14, 1989) there was a Furthermore, the degree of error could and Dr. R. West for permission to realize strong airglow due to high solar activity be determined. the observing campaign from La Silla. I close to its maximum, and the areas of also want to thank Prof. H. Debrunner, the Ecliptic near the horizon were over- Bern, who gave me the opportunity to make observations from the Jungfrau- exposed and the zodiacal light bridge 3. Results had a low contrast. joch observatory. My gratitude goes The negatives obtained were either All measurements over an ecliptical also to P. Riepe, Bochum, for the critical revision of the manuscript. digitized and processed to false-colour length of 70" in the region 140" 5 h 5 images (Fig. 1) or contact-copied to in- 210" showed a separation of 6" k lo crease the contrast (Fig. 2). Further of the bands within the zodiacal light References techniques, like the correction of in- bridge; this could also be established for 1. Cassini, G.D.: Decouverte de la lumiere homogeneous illumination by the optics the field of the gegenschein. Within the celeste qui paroist dans le Zodiaque. or subtraction of the stars, has not yet accuracy the bands are parallel to each Mem. Ac. ad. Roy. Sci. Tom Vlll (1666 - 1699). Paris: Comp. Libraires 1730, p. been performed. Especially the subtrac- other although one has the impression 119-209. tion of stars would have been a good that they might diverge in the direction 2. Winkler, C., Schmidt-Kaler, T., Schlosser, means to avoid any deceiving of the eye of increasing h. The Southern band W.: 1985, Astron. Astrophys. 143, by chains. In another step the scale coincides with the Ecliptic and is more 194-200. of the picture was determined and the prominent, probably because it contains 3. Sykes, M.V.: 1988, Astrophysical Journal positions of the two bands appearing more dust. The deviation of the inclina- (Letters), 334, L 55.

Fundamental Stellar Quantities of Early-type Stars H. DRECHSEL, R. L ORENZ, Dr. Remeis Observatory, Bamberg, Germany P. MA YER, Department of Astronomy and Astrophysics, Charles University, Prague, CSFR

1. Background and Motivation able data at all are available for stars core overshooting and continuous stel- Absolute dimensions (masses, radii, with M > 40 Ma. However, especially lar wind mass loss with important impli- ) of massive stars are well for massive stars improvements in the cations for the stellar structure and tem- known only for a few early-type stars, theoretical treatment of the internal poral evolution of single and double which comprise about 30 OB binaries of structure and have stars. spectral types earlier than B5 and less been reported during the last few . There is an urgent need for an in- than 10 0-type systems, while no reli- Such new findings include convective crease in the amount of high-precision The Wilson-Devinney approach based on the Roche model can handle tidal and rotational distortion, reflection effect as well as limb and gravity dark- ening in a satisfactory physical way. For hot luminous stars radiation pressure has to be considered as an additional important factor, which modifies the shape of the potential field. We further- more use the nonlinear simplex parame- ter optimization procedure (Kallrath and Linnell, 1987), which is superior to the conventional differential corrections method in several respects. The so-called simplex is a geometri- cal figure defined in the n-dimensional space of adjustable parameters, which can perform various operations like re- flection, contraction, etc. The light-func- 0 1 .2 .3 .4 .5 .6 .7 .o .9 1.0 tion values are calculated for each of the V1331Aq l PHASE (n+l) vertices of the simplex, and the Figure 1 : UBV light curves of V1331 Aql obtained with the ESO 0.50-m telescope between movement through the free parameter June 15-29, 1990; individual measurements (U top, B middle, V bottom) are differential space is determined by a comparison of magnitudes in the sense: comparison (HD 173003) minus V1331 Aql. Ephemeris used is: he/. the corresponding sums of squared re- JD 24426 10.0581 + 1964 1953 E.

absolute parameters of early-type stars / Normallzed light 1 to provide a statistically significant basis for a comparison of the actually ob- served properties with the revised evolutionary scenario. However, the ab- solute number of OB stars is relatively small, and there is an even greater de- ficit of "clean", undisturbed OB binary systems, which are by far the most im- portant source of our knowledge of ab- solute dimensions. Most of these dou- ble-lined eclipsing binaries are close systems exhibiting a variety of interac- tion processes like tidal deformation, mass transfer and mass loss with im- mediate effects on the eclipse light curves and radial velocities.

Hence a programme has been initi- I L ~.~8i~!8t~J!i.LlLl3~lfltltnt~,,,

ated to analyse also complex systems - 3 2 - 1 0 1 2 3 5 6 Phase 7 with the main aim to derive fundamental stellar quantities and orbital elements as precisely as possible, and to enlarge the data base to be compared with stellar structure and evolution theory for mas- sive stars.

2. Light Curve Solutions of Interacting Close Binaries The various interaction processes in close early-type systems cause appre- ciable complications of the light curves. Frequently also a finite amount of third light (in triple or multiple systems and 06 associations) introduces additional parameter correlations, which usually prevent convergent and unique solu- tions with classical methods. Therefore, flexible solution procedures are re- quired, which take these complicating Figure 2: V curve solution for V1331 Aql (top); dots are normal points, for which residuals are effects into account and make use of shown at bottom; solid line is mean value of residuals (very nearly zero), dashed lines give In refined numerical methods. and 30 belts. q = 0.63 fi, = 3.429 r)i,4rit= 3.1145

n, = 3.480 r) ,,,,i, = 2.7491

Figure 3: Meridional intersection of the close detached system V1331 Aql; shown are both components together with inner and outer critical Roche surfaces.

siduals. The simplex will eventually con- More than 500 individual measurements tract towards a locus in the parameter in each filter are shown in Figure 1. space at which best coincidence be- Preliminary solutions converge to tween observed and calculated light several possible values of the mass ratio curves is achieved. The simplex al- q (= M,/M,), ranging between about 0.6 gorithm exhibits a particularly good to 0.8, with nearly equally good fit quali- convergence behaviour, even for cases ty. In all cases the system configuration where the start parameter set is far apart is detached. The of the sec- from the final solution, and for systems ondary is about three times lower than with strong parameter correlations due that of the primary, and its temperature to light-curve distortions and the pres- turns out to be around 19,000 K (assum- ence of third light. ing 25,400 K for the primary). Both com- Figure 4: Aspects of the massive contact Our experience shows that the appli- ponents are apparently not far from system LY Aur (09.5111 + 09.5111, P = 44' 003) cation of this modified Wilson-Devinney ZAMS. According to the spectral types, at orbital phases 0, 0.1, 0.2, 0.3, 0.4, and 0.5 approach combined with a careful treat- the masses amount to about 13 and 8 (top to bottom). ment of the spectroscopic information Ma, and the value of q should be close and other limiting boundary conditions to 0.6. A very similar value of q = 0.63 can yield consistent system solutions resulted from our simultaneous solution figuration are shown in Figures 2 and 3. and sufficiently accurate stellar parame- of the UBV light curves. The V curve Third light was a free parameter in all ters in many hitherto contradictory or solution, together with residuals of nor- solutions, but converged towards ne- unexplored cases. mal points, and the derived system con- glectable amount. The final decision

3. A Few Recent Solutions Table 1: System parameters of early-type binaries Table 1 summarizes photometric so- lutions and absolute dimensions of four I I V1331 Aql I AH Cep I IU Aur I LY Aur I recently analysed OB-type binaries, which should all be counted as difficult cases to complex light curves or third- light contributions. i wed 70.3 70.5' 88.3' 89.0

Separation A (Rg) 19.8 18.7 38.7 V133 1 Aquilae RI @a)2 0.37 A 6.4 6.5 17.9 This close eclipsing binary is of type RP @a)2 0.28 A 6.0 6.5 14.7 BlV and has an of 1.364 MI (Ma) 13:3 17.7 15.9 30.0 days. Only scarce photoelectric mea- M2 (Ma) 8:3 15.6 10.8 18.6 surements have been reported, and were solved with the classical Russell- log (LAQ) 4.2:3 4.58 4.62 5.51 Merrill method and Wood's WINK code log (L2/La) 3.4:3 4.52 4.40 5.32 in only one case. New UBV light curves 13 0% 5% 20 ?'a 10 % were obtained with the ESO 0.50-m te- I ' for 1984; mean Roche rad~us; ZAMS values. lescope between June 15-29, 1990. I about the mass ratio must be delayed ical third component is not only proved early stars can be enhanced by inclu- until a spectroscopic determination of q by the light-time effect or third light, but sion of interacting close binaries with will be available. It is planned to collect also by a time change of the orbital hitherto unexplored or uncertain system the necessary rv data in July 1991 at the inclination (Drechsel et al., 1989). parameters. The current programme not ESO 1.52-m telescope (with ECHELEC). only aims at the measurement and solu- It should be noted that in a review of lU Aurigae tion of eclipse light curves, but will also early-type binaries by Hilditch and Bell complement radial-velocity data (1987) two objects with very similar pa- IU Aurigae (BOVp + B0.5, P = Id. 81 1) necessary for independent spectro- rameters appear: V Puppis (BIV, P = is a semi-detached system with the sec- scopic determinations of mass ratios, Id. 495) and TX Aurigae (BIV, P = ondary filling its critical Roche volume. which are necessary for reliable photo- 1910); these are semi-detached sys- As in the case of AH Cephei, IU Aurigae metric results. An essential subject of tems, with secondaries filling their is a rare case for which the presence of future investigation will be an adequate Roche lobes. The detached system a third body can be confirmed beyond treatment of radiation pressure effects, V1331 Aquilae therefore seems to be an any doubt by light-time effect, third light which have already been shown (Drech- important addition to the sample of the of about 20 per cent, and in addition by sel et al., 1991) to be of major impor- earliest binaries. In the following, a few the time variation of the orbital inclina- tance for the shape and configuration of more complex interacting binaries are tion due to the precessional motion of early-type close binary stars. briefly described, for which recent solu- the binary triggered by the third Even at the end of the pre-VLT era tions were derived in the scope of this body (Mayer and Drechsel, 1987). and certainly also in the future, bright programme. stars are far from being exhaustively explored. For good reasons, large in- L Y Aurigae struments are not available for such ob- LY Aurigae (09.5111 + 09.5111, P = jects, while medium- to small-size tele- AH Cephei 4d. 003) is a massive contact system. A scopes are still able to provide a wealth AH Cephei (BOV + B0.5V, P = 1 d. 775) close visual field star with an angular of important new data - especially if is an early-type close detached system. separation of 0.5 arcseconds con- they are located at such marvellous Light curves from two widely separated tributes appreciable third light to the sites like La Silla. epochs (1930 and 1984) with clearly dif- total flux. This might be one reason for ferent depths of eclipse minima were partly contradictory previous results analysed. The derived time variation of concerning the mass ratio and system the suggests the pres- configuration. The recently obtained ence of a third body in the system, light-curve solution yields a third-light References which manifests itself not only by light- contribution of 10 per cent and a mass Drechsel, H., Lorenz, R., Mayer, P.: 1989, time effect, but also by the precession ratio of about 0.6, which is compatible Astron. Astrophys. 221, 49. of the orbital plane of the eclipsing bi- with the spectroscopic value (Drechsel Drechsel, H., Gayler, S., Lorenz, R., Mayer, P.: 1991, AG Abstract Ser. 6, 74. nary. This finding is further confirmed by et al., 1989). Hilditch, R.W., Bell, S.A.: 1987, Monthly Not. a fraction of third light of about 5 per Roy. Astr. Soc. 229, 529. cent, as is evident from the solution of 4. Future Prospects Kallrath, J., Linnell, A.P.: 1987, Astrophys. J. the light curves. Besides IU Aurigae, AH 313, 346. Cephei is a unique example for a triple We have shown that the sample of Mayer, P., Drechsel, H.: 1987, Astron. Astro- system, where the presence of a phys- absolute dimensions known for very phys. 183, 61 .

Hunting the Brown Dwarf J, -M. MAR1077-I, DESPA, Observatoire de Paris, France, and C. PERRIER, Observatoire de Lyon, France

and ends as a "failed star", faintly shin- achieved in astronomical instrumenta- 1. What are "Brown Dwarfs"? ing in the infrared due to the release of tion, specifically at infrared wave- During the seven last years there has gravitational energy associated with its lengths, which opened the door to the been a considerable interest developing progressive contraction. Observations possible direct detection of at least the in view of the discovery and observa- of some members of this new class of brightest, i.e. the youngest, brown tions of presumed sub-stellar objects, celestial bodies would of course be of dwarfs: high efficiency infrared arrays also named "brown dwarfs". This illus- the highest importance for the theory of and diffraction-limited imaging tech- trative term denotes a class of objects very low mass star formation, and mod- niques in the IR are prime weapons in that appears naturally in the theory of els have been proposed which aim at this hunting. star formation: recent models predict predicting the photometric and spec- trophotometric characteristics of brown that the collapse and fragmentation of a 2. First Attempts molecular cloud should produce clumps dwarfs and their evolution with respect down to 0.02 solar masses. Between to their birth mass and age. The first observers to spot something this lower limit and 0.07 solar masses, Another reason for the revival of this were McCarthy et al. (1985). They used the fragment is not massive enough to observational activity is of course to infrared speckle interferometry to detect ignite nuclear reactions inside its core be found in several breakthroughs brown dwarf companions possibly or-