SHON - Article

Orbital Period and Light Curve Changes in the Eclipsing Binary V839 Ophiuchi. Part 2.

5. Astrophysical model

Majority of authors who dealt with the computation of geometric and physical parameters of V839 Oph came to the conclusion that it is a contact system composed of larger, more massive primary component and smaller, less massive secondary component. The relative radii of components caclulated for bodies of spherical shape are within the range of 0.42 - 0.56 and 0.20 - 0.32, respectively. The value of inclination of orbital plane varies between 60 - 77 degrees, respective fractional luminosities of components are 0.75 - 0.87 and 0.13 - 0.25 (Niarchos, 1978, Jabbar, 1983, Lafta and Grainger, 1985, Niarchos, 1989, Al-Naimiy et al., 1989). Similar values were obtained with the use of the Roche model by Akalin and Derman (1997).

An exception represent Brancewicz and Dworak (1980) who state that it is a semi-detached system composed of two components with similar radii and luminosities, of which slightly larger and more massive primary component fills 89 per cent of its Roche lobe, while the secondary component outfills its Roche lobe (104 per cent of filling). All authors give the same absolute masses of components expressed in units of solar mass: 1.19 for the primary and 0.89 for the secondary. Spectrum of V839 Oph is clasified as F8 V (Hill et al., 1975), but also the type G0 V can be found in the literature. Effective temperature of the primary component is estimaed to be 5790 K, that of the secondary component 5620 K. A review of published data on geometric and physical parameters of V839 Oph is presented in Tables 3 and 4.

Table 3: Geometric parameters of V839 Oph

Notes to the table: R1 and R2 = relative radii of primary and secondary component, respectively (distance of the centers of components is equal to 1), k = R2/R1 (ratio of radii), i = inclination of orbital plane, L1 and L2 = fractional luminosities of primary and secondary component (total luminosity of both components is equal to 1).

Table 4: Physical parameters of V839 Oph

Notes to the table: A = distance of the centers of components in solar radii, R1 and R2 = radii of the primary and the secondary component, respectively, in solar radii, M1 and M2 = masses of components in units of solar mass, q = M2/M1 (mass ratio), L1 and L2 = luminositites of components in units of solar luminosity, T1 and T2 = effective temperatures of components, Mag1 and Mag2 = absolute bolometric magnitudes of components.

Within the Binnendijk's classification of W UMa-stars Cruddace and Dupree (1984) as well as Hughes and McLean (1984) put V839 Oph to the subtype A (primary minimum is a transit of secondary component) while Niarchos (1989) claims that it belongs to the subtype W (primary minimum is an occultation of secondary component). Some difference exists also in evaluation of the degree of eclipses: Niarchos (1978), Cruddace and Dupree (1984), Lafta and Grainger (1985) found eclipses of V839 Oph to be partial but Jabbar (1983) came to the conclusion that its eclipses are total.

Is V839 Oph contact or semi-detached binary? The answer can be found in the presence of the above mentioned depressions in the light curve that occur in phases before the beginning and after the end of eclipses. Such depression is well visible also in the mean light curve obtained by the discoverer of variability of this star, R. Rigollet, in 1945 (cf. Rigollet, 1947). Similar depressions are present in the light curves of binary stars which are reliably known to be semi-detached, e.g. beta Lyr (cf. Kondo et al., 1994) or UX UMa (cf. Kyurkchieva and Marchev, 1994). The cause of these depressions is not known with certainty but various authors agree that it is somehow connected with mass transfer between the components of binary systems (eclipse effect of gaseous stream flowing between components or eclipse effect of accretion disk). If V839 Oph shows signs of a gaseous stream between its components or an accretion disk around one component then it should be a semi-detached system.

However, one may say that gaseous streams can exist also in contact systems. It could be true but there is some other evidence that V839 Oph is a semi-detached binary. As mentioned above, the light curves of V839 Oph obtained in the years 1994 and 1995 exhibit large variations of the duration of interval of constant light in minimum (from zero to about 45 minutes). All eclipsing binary stars in which alternating occurrence of minima with and without flat bottom or rapid variations of "d" were observed in the past are semi-detached systems (e.g. RZ Cas, beta Lyr, U Cep, RW Tau).

The most famous of them is RZ Cas which has partial eclipses but some of its minima have flat bottom lasting up to half an hour (cf. Margrave, 1975; Olson, 1980b; Arganbright et al., 1988; Hegedüs, 1989; Nakamura et al., 1991). Another such star is beta Lyr (cf. Kreiner, 1978; Bahyl and Kreiner 1981; Bahyl, 1981). U Cep has normally total eclipses lasting 2.4 hours but sometimes the value of "d" substantially decreases (cf. Olson, 1980a) and in several cases complete disappearance of flat bottom have been observed (cf. Hall and Keel, 1977; Olson, 1986). Similar decreases of the duration of constant light in minimum are known in RW Tau (cf. Olson, 1980a).

Moreover, the presence of accretion disks has been demonstrated in all these stars. From this it can be deduced that also in V839 Oph is present an accretion disk. Overall behaviour of the light curve of V839 Oph in the years 1994 and 1995 gives strong support for this deduction. For example, the phase shift of the whole light curves shown in Figure 8 is explainable only by the presence of a body with eccentric shape that rotates around its axis coinciding here with the line marking the phase 0.5. Such eccentricity cannot be expected in normal stars but in accretion disks it is a common phenomenon.

From this it can be further deduced that the long-term increase in orbital period of V839 Oph is caused by mass transfer from the secondary component to the primary and not by mass loss from the system. Then, the abrupt decrease of orbital period of V839 Oph in 1994 could be explained by a single event during which the star had lost a part of the disk surrounding the primary component. The scatter of estimates of brightness inward the photoelectric light curve (see Figure 17) and the decrease of total duration of eclipses from more than two hours to mere one hour suggests that during 1994 and 1995 in comparison with the previous years something was missing in the system of V839 Oph. Evidence that this "something" is a part of an accretion disk would have far-reaching importance for understanding the structure of stars of the W UMa type. In the light of this explanation we can see that the above listed parameters of the binary (Tables 3 and 4) do not refer to two stars but to one star and one accretion disk (like in the system of beta Lyr).

The hypothesis of V839 Oph as a semi-detached system with an accretion disk around the primary component enables to explain the observed changes in orbital period and in the light curve of this star by the following course of events: During the second half of 1993 or at the beginning of 1994 a new episode of mass transfer from the secondary component to the accretion disk took place. It led to appreciable increase in the orbital period of the binary (see the second straight line in Figure 3). The large inflow of matter into the disk caused its instability and in the middle of May, 1994, due to an explosion or due to gravitational (tidal) effect of the secondary component, some part of the accretion disk outreached the Roche lobe of the primary component and has been expelled from the system. Just before this event, the shape of accretion disk was strongly eccentric which caused the displacement of the first visual minimum in 1994 by 20 minutes (see Figures 3 and 4). After the partial loss of the disk which resulted in the decrease of orbital period by more than 4.4 second the remaining part had very irregular shape and its orbiting around the primary component caused all the observed changes in the light curve. Figure 20 shows in a simplified form how the system of V839 Oph could look like after the partial disk loss.

Figure 20: The system of V839 Oph with partly destroyed accretion disk.

In this figure the primary component is a small, degenerated star which is only partially eclipsed by the secondary component. The remaining part of the disk, altough depicted here in a constant position in relation to the primary component, rotates around it with a period several times shorter than is the orbital period of the binary. It is understood that the disk residue does not rotate as a solid body but it maybe possible that a resonance with orbital period of the binary can prevent its rapid circularization and its eccentric shape may persist for more than one observing season. Luminosity of the disk represents a substantial share of the total luminosity of the system. Eclipsing effects (transits and occultations) of the disk are the major cause of light variation in this binary.

At the end of 1994 another episode of mass transfer from the secondary component to the disk caused the abrupt increase in orbital period. During 1995 the eccentricity of the disk was gradually increasing again which caused the increse in the scatter of O-C residuals. On August 24th (like in May of the previous year) a part of the eccentric disk has been somehow expelled from the system. At that time only outermost parts of the disk have been lost and no significant decrease of the orbital period followed. The disk gained approximately circular shape so there was no minima displacement and no scatter of O-C values during the following two months.

This model enables to explain the observed changes in values of "D" and "d", all the above described asymmetries in the light curve as well as the quasi-periodic displacements of times of minimum light. The most puzzling phenomena are the "anti-flares". Their cause is not clear but if we accept that they have eclipsing nature (as suggests their occurrence in certain orbital phases) we can assume that they could be caused by additional eclipses of higher latitudes of the primary component (which are normally uneclipsed all over the orbital cycle) with a clump of matter belonging to the accretion disk but orbiting temporarily well above the equatorial plane of the primary component. However, this assumption is questionable because it does not explain why the "anti-flares" occur only near orbital phases 0.0 and 0.5.

6. Discussion

The question can be raised whether V839 Oph is a unique object or a representative of a group of binary stars with similar properties. The long-term behaviour of its orbital period is typical of the whole class of W UMa-stars. (Great amount of W UMa-stars exhibit either long-term increase or long-term decrease of period; changes of periods are not continual but they are limited to short intervals among which the periods remain constant.) On the contrary, the sudden period decrease in V839 Oph by more than 4.4 second is the largest period change ever observed in a W UMa-star. It is by one order larger than are the values of usual period changes in these stars.

Rapid displacements of times of minimum light and the resulting scatter of O-C residuals reported here for V839 Oph have been observed photoelectrically in many stars of the W UMa-type. As a rule, for most time the scatter remains comparable to the error of determination of moment of minimum light and only temporarily it increases many times. Bearing in mind that the mean error of photoelectric determination of time of minimum light in short-period eclipsing binaries is about ± 0.001 day the following examples of increased scatter of O-C values have to be considered as real:

V676 Cen, year 1987, interval JD 24 46 965 - 47 008 (less than 150 epochs), scatter of secondary minima 0.004 day (Cerruti, 1997; Gomez and Lapasset, 1988),

W UMa, year 1983, interval JD 24 45 415 - 45 417 (6 epochs), scatter of primary minima 0.006 day (Davan, 1987),

TZ Boo, year 1972, interval JD 24 41 356 - 41 453, scatter of secondary minima 0.006 day (Gröbel, 1989; see also Güdür et al., 1976),

V566 Oph, year 1981, interval JD 24 44 750 - 44 827, scatter of primary minima 0.009 day, interval JD 44 781 - 44 795, scatter of secondary minima 0.008 day (Seeds and Dawson, 1985),

AG Vir, years 1952-1953, interval JD 24 34 086 - 34 455, scatter of primary minima 0.009 day (Michaels, 1988),

ER Vul, years 1984-1985, interval JD 24 45 900 - 46 256 (510 epochs), scatter of primary minima 0.012 day (Ibanoglu et al., 1985),

AB And, year 1974, interval of about 500 epochs, scatter of minima 0.018 day (Kalimeris et al., 1994),

BZ Eri, year 1980, interval JD 24 44 581 - 44 605 (36 epochs), scatter of primary minima 0.007 day, scatter of secondary minima 0.036 day, i.e. more than three quarters of an hour (Srivastava and Uddin, 1985).

In some cases an increased scatter of O-C residuals is obviously in connection with sudden period change. For example, in VW Cep after a period decrease in 1991 there was increased scatter of O-C values in the following years (1992-1993). Within four days of observation in 1992 (interval JD 24 48 604 - 48 607) the scatter of primary minima reached 0.004 day (Arai, 1994).

In 44 i Boo there are intervals of increased activity during which the light curve extremely varies, displacements of minima occur and also changes of period take place (Rovithis and Rovithis-Livaniou, 1989; Bergeat et al., 1972). In 1979 a period change was observed together with large displacements of minima. Within the interval around JD 24 43 600 the scatter of O-C values of primary minima increased to 0.011 day, while in earlier and later years it was only 0.002-0.003 day (Bergeat et al., 1984; see also Oprescu et al., 1996).

In V566 Oph a period change within the interval of years 1969-1971 was accompanied by very large scatter of minima timings (Scarfe and Barlow, 1978). During JD 24 41 119 - 41 165 the scatter of O-C values of primary minima was 0,013 day, in secondary minima it was 0.01 day. Within JD 24 42 203 - 42 257 the scatter of primary minima reached 0.019 day and the scatter of secondary minima 0.008 day. The scatter of primary minima within JD 24 42 590 - 42 617 was 0.011 day (Maddox and Bookmyer, 1981).

Three decades ago, Kwee (1966) studied periodic displacements of minima timings in VW Cep and explained them by the presence of a cloud of circumbinary matter. Large phase shifts of the light curves of AW UMa were observed in 1979 by Istomin et al. (1980) and in 1989-1990 by Derman et al. (1990). The later authors explained these changes by mass transfer between the components of this binary.

The sudden jump in O-C diagram of V839 Oph (when O-C residuals decreased by half an hour within 21 days) can be compared with similar jumps in other stars. For example, in RZ Cas (which does not belong to the W UMa class) there was an interval of 25 days (JD 24 38 286 - 38 311) in the year 1963 when its O-C residuals suddenly decreased by about 10 minutes and the scatter of minima temporarily reached up to 0.008 day. This jump coincided with an abrupt decrease of orbital period by 0.37 second (Robinson, 1966; Herczeg and Frieboes-Conde, 1974). If similar period change had occurred in V839 Oph in August, 1995, it would have been to small to be detectable within the following two months.

As concerns the above mentioned alternations of minima with and without flat bottom we can add here at least one star of the W UMa type: V523 Cas has normally sharp minima but Lavrov and Zhukov (1976) observed in this star a primary eclipse with an interval of constant light in minimum.

Depressions in phases preceding and following eclipses are another common feature of the light curves of W UMa-stars. They are found around primary as well as secondary minima. If they are present near phases 0.25 and 0.75 they cause light curve deformations that can be described as double (or splitted) maxima. The size, phase, and duration of these depressions are variable. As already mentioned, they are probably caused by eclipse effects of gaseous streams or gaseous envelopes that are present in these systems (see e.g. Breinhorst and Reinhardt, 1974; Gieseking, 1977; Zhukov, 1983; Al-Naimiy et al., 1984).

A good example of variability of the depressions provides 44 i Boo. Gieseking (1977) observed this star on two nights, February 6/7th and 9/10th, 1975, and he found substantial changes of depressions within the three days interval. According to Hopp et al. (1977) a depression in phase 0.87 was present in the light curve of this binary during the years 1975-1977. In 1977 within the interval of JD 24 43 232 - 43 295 they observed gradual shift of the depression from phase 0.87 to 0.73 accompanied by remarkable increase of its size. Other data on the variability of the depressions in 44 i Boo provide e.g. Genet et al. (1982), Al-Naimiy et al. (1986), Oprescu et al. (1989).

Another star with well-documented variability of depressions in its light curve is VW Cep (cf. Lunel et al., 1979; Abe, 1982). In W UMa itself Breinhorst and Reinhardt (1974) found four depressions in phases 0.125, 0.375, 0.625, and 0.875, two and two being symmetric to the primary and secondary minimum, respectively (cf. also Shenavrin and Zhukov, 1984).

From many other W UMa-stars which show the depressions in their light curves we can mention RV CVn (Hoffmann, 1981a), CC Com (Zhukov, 1983), DD Com (Hoffmann, 1981c), BV Dra (Hamdy et al., 1985), YY Eri (Budding, 1983), FG Hya (Mahdy et al., 1985), AM Leo (Derman et al., 1991), AP Leo (Zhang et al., 1989), V524 Mon (Hoffmann, 1981d), KV Peg (Robb, 1990), AW UMa (Mikolajewska and Mikolajewski, 1980), ER Vul (Al-Naimiy, 1978; Akan et al., 1987).

But this feature is not limited to the light curves of the W UMa-stars. We can find it also in other eclipsing binaries, e.g. in V1331 Aql (Leeuwen, 1975), R Ara (Banks, 1990), SV Cam (Akan et al., 1988), NN Cep (Barlow and Forbes,1980), W Cru (Kviz and Rufener, 1988), CG Cyg (Fuensalida et al., 1985), V836 Cyg (Bozkurt, 1982), MT Her (Budding and Murad, 1982), SU Ind (Budding et al., 1994), DD Mon (Shengbang et al., 1996), beta Per (Al-Naimiy et al., 1984), GR Tau (Fang et al., 1994), XY UMa (Li et al., 1989), BH Vir (Arevalo et al., 1987), FO Vir (Schmidt and Fernie, 1984).

Similar depressions are present also in the light curves of nova-like variables, e.g. UX UMa (cf. Warner and Nather, 1972; Kyurkchieva and Marchev, 1994), and UU Aqr (cf. Goldader and Garnavich, 1989).

The light curve asymmetry shown in Figure 14 which can be described as a hump or step at the beginning of the ascending branch has been observed also in several other eclipsing binaries. For example, Ogata and Hukusaku (1977) observed this asymmetry in secondary minimum of UW CMa, Glownia (1985) in primary minimum of AK Her, Gomez et al. (1988) in secondary minimum of RT Hyi. Hoffmann (1980) observed similar hump at the end of descending branch in the light curve of TZ Boo. None of these authors has attempted to explain the asymmetry. Zakirov (1993) observed the same asymmetry in primary minimum of FF Ori and tried to explain it by a flare activity.

The size of irregularities at flat bottom (see Figure 10) is close to the limit of detectability by visual observation so their reality may seem spurious. But photoelectric observations reveal such irregularities in other eclipsing binaries, e.g. in RZ Cas (Riazi et al., 1985), V442 Cas (Prokofyeva and Epishev, 1969), U Cep (Rafert et al., 1986), FG Hya (Mahdy et al., 1985), and W UMa (Dolzan, 1988). Two of these stars (RZ Cas and U Cep) are known to have accretion disks which according to some authors are responsible for the irregularities (cf. Rafert et al., 1986).

The photoelectric light curve of V839 Oph obtained in 1994 by Demircan et al. (1994) has normal shape and does not confirm the unusually flat maxima shown in Figure 18. But can we compare observations done within a few days with the mean light curve representing data from the whole observing season? The phenomenon of flat maxima in W UMa-stars has been described in the literature. Karetnikov (1977) and Rovithis and Rovithis-Livaniou (1983) observed such unusually flat maxima in V508 Oph. Later Lapasset and Funes (1985) and Lapasset (1985) found that the light curve of this star has returned to the normal shape with arched maxima. Yamasaki et al. (1988) observed flat maxima in BL Eri, later Liu et al. (1994) found them to be normal. Mahdy and Soliman (1982) reported rapid alternation of flat and normal maxima in VW Cep. Their observations covered an interval of six days (JD 24 44 821 - 44 826), on one night the depressed and asymmetric Max. II was very similar to Max. II shown in Figure 19.

As for the "anti-flares", they were observed photoelectrically at least in two other W UMa-stars: BX And (Castelaz, 1979), and V523 Cas (Bradstreet, 1981) but none of these authors gave some explanation of this phenomenon. It is interesting to note that in contrast to "flares" that were recorded in several W UMa-stars in phases around maximum light all "anti-flares" were observed in phases around minimum light.

The author of this paper recognizes that he is not the first who proposes the semi-detached model with an accretion disk for a W UMa-star. Pustylnik and Sorgsepp (1976) explained peculiarities of the light curve of VW Cep by the presence of such disk around the primary component of this binary and Bakos et al. (1991) came to the same model when studying AW UMa. It is not accidental that both these eclipsing binaries belong to the group of most active stars of the W UMa type. This group includes also 44 i Boo, TZ Boo and probably some other stars. Now it is apparent that V839 Oph belongs to this group too. In all these stars there are intervals of increased activity when changes of the light curve are so large and so rapid that they cannot be explained by the occurrence of star spots or by other "conventional" ways. The only plausible explanation seems to be an eccentric body (accretion disk) orbiting around one component star in these binary systems. At present we do not know to what extent we can generalize these findings. But as it was shown in this discussion, V839 Oph is not a unique object. Its properties are similar to several other well-studied W UMa-stars, especially to the members of the group of "active" W UMa's.

The conclusions concerning V839 Oph presented in this paper are to be considered only as a hypothesis. Nevertheless, this hypothesis is so interesting that it deserves attention of observers as well as theorists. The structure and evolution of eclipsing binaries of the W UMa type remain for decades a mystery. Maybe, V839 Oph is a clue to the resolution of this mystery.

Acknowledgements

The author of this paper is grateful to Mr. F. Agerer (Zweikirchen, Germany) for sending him (via Mgr. J. Silhan) the list of 114 times of minima of V839 Oph from the BAV Database. He would like to thank Dr. M. Zejda, Dr. P. Hajek and Mgr. J. Silhan of the Nicholas Copernicus Observatory and Planetarium in Brno for providing him access to the archives of B.R.N.O. - Variable Star Section of CAS. He wishes to thank Dr. M. Solc (Director), Dr. M. Wolf and Dr. A. Meszaros of the Astronomical Institute of Charles University in Prague for kind permission to use their library after usual office hours.

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