V456 Ophiuchi and V490 Cygni: Systems with the shortest apsidal-motion periods
Key Words.:binaries: eclipsing – stars: fundamental parameters – stars: individual: V456 Oph, V490 Cyg
Our main aim is the first detailed analysis of the two eclipsing binaries V456 Oph and V490 Cyg. The system V456 Oph has been studied both photometrically via an analysis of its light curve observed by the INTEGRAL/OMC and by the period analysis of all available times of minima. V490 Cyg has been studied by means of a period analysis only. Many new times of minima for both systems have recently been observed and derived. This allows us for the first time to study in detail the processes that affect both binaries. The main result is the discovery that both systems have eccentric orbits. For V456 Oph we deal with the eccentric eclipsing binary system with the shortest orbital period known (about 1.016 day), while the apsidal motion period is about 23 years. V490 Cyg represents the eclipsing system with the shortest apsidal motion period (about 18.8 years only). The two components of V456 Oph are probably of spectral type F. We compare and discuss the V456 Oph results from the light curve and the period analysis, but a more detailed spectroscopy is needed to confirm the physical parameters of the components more precisely.
The eccentric eclipsing binaries (EEBs) provide a great opportunity for studying the stellar structure of the stars as well as testing the General Relativity outside the solar system. The diagram analysis, which investigates the revolution of the line of apsides in the system has been described elsewhere, e.g. Giménez & García-Pelayo (1983), Giménez & Bastero (1995). Nevertheless, new contributions to this topic with new systems are still welcome, especially for cases where the apsidal motion period is adequately short and a few periods are covered. This is the case for the two somewhat neglected systems V456 Oph and V490 Cyg.
1.1 V456 Oph
V456 Oph (= AN 108.1935 = SAO 123842, mag) has been discovered as a variable star by Hoffmeister (1935), with the remark that it is a ”short-periodic one, but probably not rapidly changing”. After than Guthnick & Prager (1936) incorrectly classified the star as a Cep one with a preliminary period of about 14.6 d. No such variation has been detected with the present data. The only spectral classification is that by Roman (1956), who indicated the spectral type A5, but with a remark that because of underexposed plates and uncertain ephemerides this classification is not very secure.
Although the first photoelectric light curve has been published by Demircan et al. (1988), there was no light curve analysis of the system performed until today. The same applies to the spectroscopic analysis, which has not yet been carried out, so the mass ratio of the pair in not known. Soydugan et al. (2006) included the binary in the catalogue of systems located in the instability strip, which means that is possibly contains a Scu component. However, no indication of pulsations in V456 Oph has been detected.
1.2 V490 Cyg
V490 Cyg (= AN 76.1939, mag) is an Algol-type eclipsing binary, even though the SIMBAD database lists V490 Cyg as a Lyrae one. The system has been discovered as a variable by Wachmann (1940), and its light curve coverage is too poor for any reliable analysis. Its spectral type was derived to be F8, while Svechnikov & Kuznetsova (1990) give an estimate of the spectral types F8+[G4]. Hegedüs (1988) included this system in his list of stars with possible apsidal motion, but since then it was not studied in detail. Other credible information about the physical properties of the components is missing because there is so little spectroscopy and photometry of this system.
2 The period analysis
2.1 V456 Oph
The set of published times of minima for V456 Oph is quite extensive, covering more than 70 years. Regrettably, the old minima are only photographic and their scatter is so large that one cannot use them for any reliable analysis. We used only the more precise photoelectric and CCD ones, which were published after 1970. These minima roughly follow the linear ephemerides given in GCVS, but there some variations are clearly visible.
We tried to collect all available minima times and also to derive some new ones. A few of the already published ones were recalculated once again and corrected for the final analysis. Besides these minima times, we also used the photometry of V456 Oph obtained with the robotized and automated telescopes working today. These are
Sixteen new minima times were derived from these surveys, and some of the published ones were computed again (see Table 1). Our new minima times were observed in the Ondřejov observatory with the 65-cm telescope. We used the Kwee & van Woerden (1956) method for all these minima. The mean linear light elements suitable for observations are
which were also used for deriving the proper epochs and types of the minima times written in Table 1.
If we plot these data points in the diagram, the difference between primary and secondary minima is clearly visible. Following the method of apsidal motion analysis as described in Giménez & García-Pelayo (1983), we tried the computation with orbital inclination for the first time. After that we used for the second attempt the inclination (see Sect. 3), which resulted in almost the same parameters (owing to the term , which is nearly 0 for our inclination). In Fig. 1 we plot the final fit with the apsidal motion hypothesis on all used data points. This leads to the parameters of the motion of apsides given in Table 2. All uncertainties of the parameters were calculated from the covariance matrix of the fit and from the uncertainty of the inclination. Evidently the apsidal period is very short, about only 23 yr, which places this system among one those with the shortest apsidal motion.
|HJD - 2400000||Error||Type||Filter||Observer/Reference|
|48113.4210||0.003||Pri||C||A.Paschke, BBSAG 96|
|48113.42288||0.00108||Pri||C||A.Paschke - recalculated|
|53089.7944||0.0003||Pri||V||P.Sobotka-OMC, IBVS 5809|
|53089.79189||0.0012||Pri||V||OMC - recalculated|
|53091.8267||0.0004||Pri||V||P.Sobotka-OMC, IBVS 5809|
|53091.82625||0.00037||Pri||V||OMC - recalculated|
|54232.79073||0.0009||Pri||C||Pi of the sky|
|54742.8314||0.0009||Pri||V||P.Zasche-OMC, IBVS 5931|
|54742.83119||0.00045||Pri||V||OMC - recalculated|
|54766.7134||0.0012||Sec||V||P.Zasche-OMC, IBVS 5931|
|54766.71352||0.00032||Sec||V||OMC - recalculated|
|54769.7640||0.0011||Sec||V||P.Zasche-OMC, IBVS 5931|
|54769.76438||0.00066||Sec||V||OMC - recalculated|
|Parameter||V456 Oph||V490 Cyg|
2.2 V490 Cyg
The system V490 Cyg has much lower published times of minima observations. The first rough times-of-minima estimates are those by Wachmann (1948) from his photometry in the 1930’s, but these have such a large scatter that they cannot be used for any reliable analysis. However, he also noticed that the secondary minimum is not symmetric with regards to the primary one. Nevertheless, a possible eccentricity and apsidal motion have never been studied since then. The more precise photoelectric and CCD observations have been measured since 1999, but there are only 12 published minima.
A few new CCD observations were obtained in the Ondřejov observatory with the same telescope as for V456 Oph, and two new minima times were also derived from the INTEGRAL/OMC data. The new measurements and the already published ones are presented in Table 3. The suitable linear ephemerides for observations are
|HJD - 2400000||Error||Type||Filter||Observer/Reference|
|51487.6184||0.0002||Sec||V||D.B.Caton - IBVS 5595|
|51491.5776||0.0004||Pri||V||D.B.Caton - IBVS 5745|
|51495.5994||0.0002||Sec||V||D.B.Caton - IBVS 5745|
|52613.024||0.002||Sec||V||P.Sobotka - IBVS 5809|
|52813.7080||0.0002||Sec||V||D.B.Caton - IBVS 5595|
|52841.6237||0.0002||Pri||V||D.B.Caton - IBVS 5595|
|53256.6755||0.0001||Pri||C||T.Krajci - IBVS 5690|
|53260.6752||0.0003||Sec||C||T.Krajci - IBVS 5690|
|53660.3244||0.0005||Pri||I||F.Agerer - IBVS 5731|
|53660.331||0.0005||Pri||V||R.Diethelm - IBVS 5713|
|54685.4130||0.0046||Pri||I||F.Agerer - IBVS 5889|
|55096.4362||0.0004||Sec||I||F.Agerer - IBVS 5941|
The minima times presented in Table 3 were used for the period analysis, which we did by applying the apsidal motion hypothesis. The only difference in analysis between V490 Cyg and V456 Oph was the assumption of an inclination for V490 Cyg because we had no light curve analysis. The difference between primary and secondary is clearly visible, reaching up to 47 minutes, which is surprisingly high for a binary with such a short orbital period. The analysis led to the parameters of the apsidal motion presented in Table 2. Obviously the resulting value of the apsidal motion period of about 18.8 years is even shorter than for V456 Oph; we are therefore dealing with the shortest apsidal motion period known among the EEBs today.
3 Light curve analysis
The whole light curve of V456 Oph was observed with the OMC camera onboard the INTEGRAL satellite, a description of which is given in Mas - Hesse et al. (2004). The standard filter was used, but the optical telescope has an aperture of only 5 cm in diameter. We obtained several hundred observations, of which we used 449 for the analysis.
The programme PHOEBE (ver. 0.29, Prša & Zwitter 2005), based on the Wilson-Devinney algorithm (Wilson & Devinney 1971) was used for the analysis. The ”detached binary” mode (in Wilson & Devinney mode 2) was used with several assumptions. First, the ephemerides ( and ) and the apsidal motion parameters (, , and ) were adopted from the period analysis, because the minima times cover a longer time span, therefore these quantities are derived with higher precision. Secondly, the mass ratio and temperature of the primary component were set and the other relevant parameters were adjusted for the best fit. We changed the and values in the wide range of values to obtain the best fit according to the RMS value and also the physical plausibility of the fit. This means during that the fitting process we scanned the parameter space in ranging from 0.1 to 1.2 and in from 15400 K to 6500 K.
We fitted the other light curve parameters, which are the luminosities and in the filer, the temperature of the secondary , the inclination , the Kopal’s modified potentials and , the synchronicity parameters and , the third light . The limb-darkening coefficients were automatically interpolated by the PHOEBE programme from van Hamme’s tables (see van Hamme 1993), using the linear cosine law for the values of and of both components resulting from the analysis. The values of the gravity brightening and bolometric albedo coefficients were set at their suggested values for convective atmospheres (see Lucy 1968), i.e. , .
|[K]||6840||(V)||59 4 %|
|[K]||6700 430||(V)||41 3 %|
|()||0.96 0.15||1.097 0.402|
|[deg]||87.88 0.81||0.843 0.356|
|5.09 0.32||0.505 0.021|
|5.08 0.29||0.504 0.019|
|0.249 0.05||0.247 0.04|
The best fit was achieved with the light curve parameters given in Table 4, and the figure with the final fit is plotted in Fig.3. Nonzero eccentricity is clearly visible from this plot, which is quite surprising in a binary with such a short orbital period. No other EEB with a shorter period is known today. The value of the third light is , which indicates that there is no other visible companion to the system in the filter (under the assumption that this component is also located on the main sequence). We made several attempts with nonzero values of the third light, but these did not lead to a satisfactory solution.
Because there is no spectroscopic analysis, the precise physical parameters cannot be computed directly, but need to be roughly estimated with the assumption that both components are located on the main sequence. We derived the following values: , , , . These are only very preliminary values, but lead to spectral types of about F1 + F2 for the two components. We obtained roughly the same result (F0+F1) with the standard mass-luminosity relation for the main sequence stars (e.g. Malkov 2007), applying the luminosity ratio derived from the light curve analysis.
The parameters are very different from what one could expect for a main sequence star of spectral type A5 (Roman 1956), and the masses are also different from those estimated by Brancewicz & Dworak (1980), but the presented values provided the best light curve fit, and the parameters of the apsidal motion also agree well with the theoretical values (see below). The spectral type presented by Roman (1956) was only estimated on the basis of poor photographic spectra. On the other hand, there is also the photometry in the NOMAD catalogue (Zacharias et al. 2004), from which mag and mag. These values indicate (Houdashelt et al. 2000) that the temperature of the system is about 6500 K, therefore of the spectral type about of F5.
For the light curve analysis of the system V456 Oph the ephemerides and the apsidal motion parameters were fixed, but another approach could be to compute these parameters directly also from the light curve. The problem is that the data coverage for the light curve is rather fairly in time (about only 1/5 of the apsidal period), and the data for the light curve have relatively high scatter as well.
The eclipsing binaries, V456 Oph and V490 Cyg, with their respective apsidal motion periods of about only 20 years place these systems among a few unique ones with apsidal periods below 30 years (see Table 5).
|System||Spectr.||P [d]||e||U [yr]||Reference|
|V490 Cyg||1.1402||0.045||18.8||This paper|
|V381 Cas||B3||1.7459||0.0253||19.74||Wolf et al. (2010)|
|U Oph||B5+B5||1.6773||0.00305||20.88||Vaz et al. (2007)|
|V456 Oph||?F1+F2?||1.0160||0.017||22.6||This paper|
|GL Car||B0+B1||2.4222||0.146||25.2||Wolf et al. (2008)|
|V478 Cyg||B0+B0||2.8809||0.0158||27.1||Wolf et al. (2006)|
Our next task was to derive the averaged internal structure constant as well and to compare it with the theoretical value. This task was done after subtraction of the relativistic term, which resulted for V456 Oph in the value , about only 2.5% of the total apsidal motion rate. Therefore, the internal structure constant is
The surprisingly high value of the uncertainty is mainly caused by the error of the relative radii from the light curve analysis. We can compare this value with the stellar evolution grids (e.g. by Claret 2004) and the theoretical values of . Using the value of (), we obtained the value of
for the main sequence star with an age between and yr. This could be interpreted as a rough estimation of maximum age for this system. No other eccentric eclipsing binary with such a late spectral type is known today. Therefore a detailed analysis of its spectra would be very welcome.
We performed the first detailed photometric and period analysis of the two eclipsing systems V456 Oph and V490 Cyg, which yielded the parameters of the apsidal motion with periods of about only 23 and 19 years. With the orbital period of V456 Oph of only about 1.016 days we are dealing with the shortest orbital period among the apsidal motion systems, while the period of apsidal motion of 18.8 years of V490 Cyg makes this system the shortest among the EEBs. However, because we lack a spectroscopic analysis, some of the physical parameters were only roughly estimated and apparently contradict each other. New times of minima observations as well as a detailed spectroscopic analysis are needed.
Acknowledgements.Based on data from the OMC Archive at LAEFF, pre-processed by ISDC. We thank the ”ASAS” team and also the ”Pi of the sky” team for making all of the observations easily public available. This work was supported by the Czech Science Foundation grant no. P209/10/0715 and also by the Research Programme MSM0021620860 of the Czech Ministry of Education. Mr. Anton Paschke is also acknowledged for sending us his photometric data and also Mr. Kamil Hornoch for the observational assistance. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and of NASA’s Astrophysics Data System Bibliographic Services.
- email: firstname.lastname@example.org
- email: email@example.com
- Brancewicz, H. K., & Dworak, T. Z. 1980, AcA, 30, 501
- Burd, A., et al. 2005, NewA, 10, 409
- Caton, D. B., & Smith, A. B. 2005, IBVS, 5595, 1
- Claret, A. 2004, A&A, 424, 919
- Demircan, O., Derman, E., & Selam, S. 1988, IBVS, 3237, 1
- Diethelm, R. 2006, IBVS, 5713, 1
- Giménez, A., & García-Pelayo, J. M. 1983, Ap&SS, 92, 203
- Giménez, A., & Bastero, M. 1995, Ap&SS, 226, 99
- Guthnick, P., & Prager, R. 1936, AN, 260, 393
- Hegedüs, T. 1988, BICDS, 35, 15
- Hoffmeister, C. 1935, AN, 255, 401
- Houdashelt, M. L., Bell, R. A., & Sweigart, A. V. 2000, AJ, 119, 1448
- Hübscher, J., Paschke, A., & Walter, F. 2006, IBVS, 5731, 1
- Hübscher, J., Steinbach, H.-M., & Walter, F. 2009, IBVS, 5889, 1
- Hübscher, J., Lehmann, P. B., Monninger, G., Steinbach, H.-M., & Walter, F. 2010, IBVS, 5941, 1
- Krajci, T. 2006, IBVS, 5690, 1
- Kwee, K.K., van Woerden, H. 1956, BAN, 12, 327
- Lucy, L. B. 1968, ApJ, 151, 1123
- Malkov, O. Y. 2007, MNRAS, 382, 1073
- Mas-Hesse, J. M., Giménez, A., Domingo, A., et al.& the OMC team 2004, 5th INTEGRAL Workshop on the INTEGRAL Universe, 552, 729
- Paschke, A. 1990, BBSAG, 96, 1
- Pojmanski, G. 2002, AcA, 52, 397
- Prša, A., & Zwitter, T. 2005, ApJ, 628, 426
- Roman, N. G. 1956, ApJ, 123, 246
- Smith, A. B., & Caton, D. B. 2007, IBVS, 5745, 1
- Sobotka, P. 2007, IBVS, 5809, 1
- Soydugan, E., Soydugan, F., Demircan, O., İbanoǧlu, C. 2006, MNRAS, 370, 2013
- Svechnikov, M. A., & Kuznetsova, E. F. 1990, Sverdlovsk : Izd-vo Ural’skogo universiteta, 1990
- Tassoul, J.-L., & Tassoul, M. 1992, ApJ, 395, 259
- van Hamme, W. 1993, AJ, 106, 2096
- Vaz, L. P. R., Andersen, J., & Claret, A. 2007, A&A, 469, 285
- Wachmann, A. A. 1940, ANZi, 22, 10
- Wachmann, A. A. 1948, AAAN, 11, 5
- Wilson, R. E., & Devinney, E. J. 1971, ApJ, 166, 605
- Wolf, M., Kučáková, H., Kolasa, M., et al. 2006, A&A, 456, 1077
- Wolf, M., Zejda, M., & de Villiers, S. N. 2008, MNRAS, 388, 1836
- Wolf, M., Šmelcer, L., Kučáková, H., et al. 2010, New A, 15, 530
- Zacharias, N., Monet, D. G., Levine, S. E., Urban, S. E., Gaume, R., & Wycoff, G. L. 2004, Bulletin of the American Astronomical Society, 36, 1418
- Zasche, P. 2010, IBVS, 5931, 1