COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2560 Konkoly Observatory Budapest 25 July 1984 HU ISSN 0374-0676 AMPLITUDE-WAVELENGTH DEPENDENCE FOR W UMa-SYSTEMS It is well known that the amplitude of light change of a W UMa type system in blue region is larger than in yellow. Indeed, the first IR-observations of these systems (Jameson and Akinci, 1979) confirmed the existence of this regularity up to lambda = 2.2 microm. The dependence of the amplitude difference (Av - Alambda) on lambda is shown in Figure 1. IR-data for this figure are given in Table I. Our observations of V 523 Cas were used for the optical range. [FIGURE 1] Figure 1 The observed dependence of the difference of the amplitudes (Av - Alambda) on lambda for the W-systems (min 1 is shown by dots, min 2 - by crosses). The theoretical curves (solid lines) for systems of spectral type K0V, filled up their Roche lobes, are shown for three values of beta. In spite of the scattering of points a general character of the dependence may be revealed by the diagram: a sharp decrease of the difference of amplitude in the range up to 1 microm and more smooth change of the (Av - Alambda)-value at lambda>1 microm. The observations of W UMa by Philips et al. (1980) give larger Table I System Min AV AH AK AV-AH AV-AK IR-data from: W UMa 1 0m73\ 0m43 0m43 0m25 0m25 Philips et al.(1980) 2 0.64/ 1 0.73 0.59 - 0.14 - Shenavrin and 2 0.64 0.55 - 0.09 - Zhukov (1984) AB And 1 0.82 - 0.68 - 0.14 Jameson and 2 0.67 - 0.66 - 0.01 Akinci (1979) SW Lac 1 0.78 - 0.66 - 0.12 " 2 0.72 - 0.63 - 0.08 VW Cep* 1 0.48 0.32 0.33 0.16 0.15 Shenavrin and 2 0.38 0.31 - 0.07 - Zhukov (1984) 1,2 - - 0.28 - 0.20 Lunel et al.(1982) 44i Boo* 1 0.61 - 0.47 - 0.14 Jameson and 2 0.49 - 0.44 - 0.05 Akinci (1979) 1 0.61 - 0.41 - 0.20 Bergeat et al.(1981) 2 0.49 - 0.39 - 0.10 * - the third light is taken into account values of (AV-Alambda) for W UMa in comparison with other systems. The reason of this may be explained by using the optical observations made non-simultaneously with IR ones and by a comparatively low precision of early IR-observations. In any case, our observations of W UMa in H-band do not show such deviation (Shenavrin and Zhukov, 1984). Zhukov and Chruzina (1983) computed theoretical light curves for W UMa-systems permitting to obtain the theoretical dependence (Av-Alambda) which is shown in Figure 1 by solid lines (due to special features of the computer this dependence is suitable for interpretation of data only for primary minima of W-systems). As it is seen from Figure 1, theoretical dependence satisfactorily imitates the observations assuming that primaries fill up their Roche lobes and the gravitation darkening coefficient is beta >= 0.25. (Av-Alambda)2 -dependence for secondary minima has a less steep slope. It is well seen from Figure 2 which represents the dependence of (A1-A2) amplitude difference on lambda in primary and secondary minima. Different behaviour of the dependences (AV-Alambda) for min 1 and min 2 in infrared range can be explained by different temperatures of the components, But in this case, the observed dependence (AV-Alambda)2 must have quite different behaviour even if Delta T = 100-200 K. Our calculations show that the dependence (AV-Alambda) becomes weaker if the components do not fill up their Roche lobes. Thus the different degree of the accretion of the lobes of the components may be a cause of the decrease of (A1-A2)-value in the IR-range. The last suggestion, however, needs further confirmation. The difference in the depths of minima which reaches maximum value. in optical range again decreases in UV-range (UV light curves of W UMa by Eaton [FIGURE 2] Figure 2. The observed dependence of the magnitude difference in primary and secondary minima (A1 - A2) on lambda. et al. (1980) were used by us). One can. explain this neither by different temperatures nor different geometry of the components. However, if UV-excess in some W UMa-systems will be related mainly to the inner atmosphere of primary, one can try to explain the observed decrease of the amplitude differences of minima in UV-range with an eclipse of regions, causing this excess (it happens in secondary minima). Conclusions 1. In all investigated systems (W-type only) the amplitude of the light change decreases with the increase of lambda. 2. This dependence (AV-Alambda) essentially differs for primary and for secondary minima. 3. The observed dependence (AV-Alambda) as a whole can be explained with different geometry of the components, assumed beta >= 0.25 in addition. With beta =0.08 the necessity appears to include some additional factor which decreases the amplitude of the variability of W UMa-stars in IR-region. Accurate observations at lambda > 2 microm are needed to resolve this problem. Then the cause of IR-excess in W UMa-systems would be clear. 4. The magnetic activity of W-system's primaries which is connected with a considerable UV-excess is a probable reason of (A1-A2)-value decrease in UV-range. O.V. ZHUKOV Kazan University Department of Astronomy Kazan, U.S.S.R. References: Bergeat J., Van't Veer F., Lunel M., Garnier R., Sibille F. and Roux S., 1981, Astron. and Astrophys., 94, 350 [BIBCODE 1981A&A....94..350B ] Eaton, J.A., Wu C.-C. and Rucinski S.M., 1980 Astrophys. J., 239, 919 [BIBCODE 1980ApJ...239..919E ] Jameson R.F. and Akinci R., 1979, Mon.Not.Roy.Astron.Soc., 188, 421 [BIBCODE 1979MNRAS.188..421J ] Lunel M., Garnier R., Bergeat J. and Van't Veer F., 1982, I.B.V.S. No. 2176 Philips J.B., Selby M.J., Wade R. and Sanchez Margo, 1980, Mon.Not.Roy.Astron. Soc., 190, 337 [BIBCODE 1980MNRAS.190..337P ] Shenavrin V.I. and Zhukov G.V., 1984, I.B.V.S. No. 2541 Zhukov G.V. and Chruzina T.S., 1983, Astron.Circ., No. 1266, 3 [BIBCODE 1983ATsir1266....3Z ]