Φωτομετρική μελέτη των σχεδόν σε επαφή εκλειπτικών διπλών αστέρων

Abstract

The main characteristics of the near-contact binary stars (NCBs) are: 1) Their orbital periods range from about 0.4 to 1.1 day. 2) There is tidal interaction between their components, which affects the shape of the light curves. 3) The facing surfaces of the two members are less than 0.1 orbital radius apart. However, they are not in contact, because the different Teff of the two components result in unequal minima. The NCBs are main sequence dwarfs or subgiants with spectral types A to F for the hotter member and F to K for the cooler. About 140 systems are presently known, 14 with double-line spectra available. We solved in a homogeneous manner the light curves of a sample of 16 bright NCBs as thus defined by Shaw (1990,1994) and from his tables (10 from published observations and 6 from our own observations, see Table 1) in order to determine the evolutionary state of this class of stars, especially in relation with the well-known W UMa - type eclipsing binaries, whose members are in extensive contact. Also, the subclass of these 16 systems according to Shaw (1994) was determined: V1010 Oph subclass or FO Vir subclass. Table 1: The 16 NCBs studied. The symbols for the subclass we determined are F for FO Vir subclass, V for V1010 Oph and C for contact systems. The spot symbols: c = cool, h = hot, P = on the primary, S = on the secondary. The preceding number: the no. of the spots specified. Names in boldface: studied from our own observations. NAME SPECTRAL TYPE PERIOD (d) SUBCLASS SPOTS BX And F2 V 0.61012 F - CX Aqr F5 + G9 0.55598 F - EE Aqr F2 + K 0.50900 V? - YY CMi F6 + G5 1.09379 C 1cS + 1hS VZ CVn F0 + F8 0.84246 Algol? 2cS DM Del A2 V + G8 0.84468 F 3cS + 1hP BL Eri B5 / G 0.41692 V 1cS + 1cP RS Ind F0 + K 0.62407 F - FT Lup F2 + K3 0.47008 F 1cS + 1cP GW Tau A3 0.64133 F? 2cS V1010 Oph A7 + F6 0.66143 V - RT Scl F2 + K 0.51156 V 1hS RU UMi F0 + K5 0.52493 F 1cS AG Vir A7 V 0.64265 C 2cS + 1cP BF Vir A + K1 0.64058 F - FO Vir A2 V + K2 0.77557 F - Moreover, new times of minima and (in a few cases) ephemerides from our own observations were calculated, using our own FORTRAN programme based on the Kwee - van Woerden method. The instruments used for our photometric observations were: 1) The 122-cm reflector at the Kryonerion Station of the National Observatory of Athens with a pulse-counting photoelectric photometer. 2) The 40-cm reflector of the Univ. of Athens Observatory and an SBIG-8 CCD camera. The Wilson-Devinney DC program and the Binary Maker 2.0 (Bradstreet 1993) have been used for the light curve analysis. The free parameters were: the conjunction phase φο , the inclination i, the Teff of the secondary component T2 , the fractional luminosity of the primary L1 and the potential Ω (according to the mode used), while the fixed ones were: the temperature of the primary T1 (from the spectral type), the gravity darkening coefficients g (0.32 for convective envelopes, 1 for radiative), the albedos A (0.5 for convective, 1 for radiative), and the limb darkening coefficients x (from tables). The border between convective and radiative envelopes was set at Teff = 7200 K, which was also our upper limit for the existence of cool spots, with the exception of AG Vir. The spectroscopic mass ratio q was a fixed parameter. Third light was zero in all cases. A preliminary "q-search" was conducted where no spectroscopic value for q was available (5 systems); in these cases q was a free parameter. The elements derived from the light curve analysis were combined with existing spectroscopic data to compute the absolute elements of the systems. These elements were then used to place the systems in the M-R, M-L and Hertzsprung-Russell diagrams in order to study their evolutionary status. The secondaries of NCBs appear more evolved than the primaries: The primaries cluster between ZAMS and TAMS, while the secondaries lie mostly beyond TAMS. The NCBs studied appear in general more evolved than the contact systems of W UMa type. Conclusions: It is difficult to separate between NCBs and systems in physical but not thermal contact. We found 1 overcontact system, 1 system in marginal contact, 13 semidetached (9 of the FO Vir subclass and 4 of the V1010 Oph) and 1 (VZ CVn) detached. Cool spots on secondaries tend to give insufficient photometric effect in the case of NCBs, since almost all the systemic light comes from the hot primary. In a few cases, hot spots can be assumed on the secondaries as a result of mass transfer. In the one case in which the filter I was used (GW Tau) there were indications for circumstellar matter. The evolutionary diagrams we compiled show that NCBs should not be considered as the precursors of overcontact systems, at least in their majority: their members tend to be clearly more evolved than the W UMa systems. We therefore propose the following evolutionary scheme: The latter evolved from the few observed relatively unevolved NCBs very fast at the start of stellar evolution, while the large majority of NCBs remain as such until the departure of one member from MS, when contact appears as a result of the evolution of the more massive star up to the giant branch.