Kinematics of the ultracompact helium accretor AM Canum Venaticorum

We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03−015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio   q = 0.18 ± 0.01  for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna ( LISA ).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to   e = 0.04 ± 0.01  , which is smaller than previous models indicated.     

The multiperiodic δ Scuti star 4 Canum Venaticorum: amplitude variability

Recent multisite campaigns of the Delta Scuti Network have revealed 34 frequencies of pulsation for the star 4 CVn. Our present knowledge of the frequencies makes it possible to reanalyse the shorter data sets in the literature, photometric observations from 1966 to 1997.
4 CVn shows strong amplitude variability with time-scales of ten years or longer, although for neighbouring years the amplitudes usually are similar. Seven of the eight dominant modes show annual variability of ∼12 per cent. The variability increases to ∼40 per cent over a decade. The formally derived time-scale of variation of 30 years can only be a rough estimate, since this is also the length of the available data span. The variability is compared with that of FG Vir, which shows lower amplitude variability.
The cyclic behaviour of the amplitude variations excludes an evolutionary origin. There exists some evidence that a mode at 6.12 d⁻¹, which appeared during 1996 and 1997, may have been present with small amplitudes in the 1976–1978 time period.
The pulsation mode at 7.375 d⁻¹ exhibited the most rapid decrease found so far: the V amplitude dropped from the highest known value of 15 mmag in 1974 to 4 mmag in 1976 and 1 mmag in 1977. After that the mode has been increasing in amplitude. There exists a phase jump between 1976 and 1977, suggesting the growth of a new mode. It is interesting to note that this mode also has the strongest coupling with other modes with combination frequencies, f _i ± f _j . The amplitudes of these combination frequencies are also strongly variable from year to year. We speculate that power is transferred between the modes through mode-coupling.     

Superhumps, magnetic fields and the mass ratio in AM Canum Venaticorum

We show that the observed K velocities and periodicities of AM CVn can be reconciled given a mass ratio   q ≈ 0.22  and a secondary star with a modest magnetic field of surface strength   B ∼ 1 T  . We see that the new mass ratio implies that the secondary is most likely semidegenerate. The effect of the field on the accretion disc structure is examined. The theory of precessing discs and resonant orbits is generalized to encompass higher order resonances than 3 : 2 and shown to retain consistency with the new mass ratio.     

A photometric study of the contact binary AZ Virginis

New light curves and photometric solutions of the contact binary AZ Vir are presented in this paper. The light curves appear to exhibit a typical O'Connell effect, with Maximum I being 0.021 mag (V) and 0.023 mag (B) brighter than Maximum II, respectively. From the observations, six times of minimum light were determined and from the present times of minimum light and those collected from the references, the light elements of the system were improved. The light curves were analyzed by means of the Wilson‐Devinney program. The results suggest that AZ Vir is a W‐subtype contact binary with a mass ratio of q = 0.623(2). The asymmetry of the light curves is explained by star spot models. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Revision of the photometric parameters of BS Cassiopeiae

Seven charge-coupled device(CCD)photometric times of light minimum of the overcontact binary BS Cas which were obtained from 2007 August to November and one CCD light curve in the R band which was observed on 2007 September 24 and October 15,are presented.It is found that the light curve of BS Cas has characteristics like a typical EW-type light variation.The light curve obtained by us is symmetric and shows total eclipses,which is very useful for determining photometric parameters with high precision.Photometric solutions were derived by using the 2003 version of the Wilson-Devinney code.It shows that BS Cas is a W-subtype overcontact binary(f = 27.5% ± 0.4%)with a mass ratio of q = 2.7188 ± 0.0040.The temperature difference between the two components is 190 K.Analysis of the O-C curve suggests that the period of AE Phe shows a long-term continuous decrease at a rate of dP/dt=-2.45 × 10-7 dyr-1.The long-time period decrease can be explained by mass transfer from the primary to the secondary.     

VZ CVn: a close binary system with γ Doradus-like variations

We present new photometric two-colour observations of the double-lined close binary star VZ CVn. Combining two data sets obtained in 1971–72 and in 2006, the physical parameters of both components were derived. After removing the light variations due to the eclipses and proximity effects, a periodic variation in the more massive component with a dominant period of 1.068 76 d could be detected in the first photometric data set. In accordance with its position on the Hertzsprung–Russell diagram and both its pulsation period and pulsation constant of 0.62 d, the primary component of VZ CVn should be an excellent γ Doradus candidate. The less massive secondary component seems to have smaller radius with respect to its mass. Both components appear to have lower luminosities with respect to their masses; hence, their radiative properties seem to be different. The evolutionary status of the components is also discussed.     

A study of the distortion wave in the RS CVn eclipsing binary SV Camelopardalis

The light outside the eclipses of the totally eclipsing RS CVn binary SV Camelopardalis (SV Cam) is Fourier analysed and the amplitudes of the distortion waves have been derived. The distribution of the percentage contributions of these amplitudes inV, B andU colours with respect to the luminosities of the binary components indicates that the hotter component is the source of the distortion waves. These distortion waves, attributed to star spots, are modelled according to Budding (1977) and spot parameters like longitude, latitude, temperature and size are obtained. From this study it is noticed that while symmetric waves with two minima could be fitted satisfactorily, asymmetric waves with more than two minima could not be fitted well. From the longitudes of the minima of the best fitted curves, migration periods of four spot groups are determined. Assuming synchronism between rotation and orbital periods, the rotation periods of the four spot groups are derived from their migration periods. The period of rotation of one of the spot groups having direct motion is found to be 0^d.5934209 while the periods of the other three spot groups having retrograde motion are 0^d.5926588, 0^d.592607 and 0^d.5924688. As the latitudes of these spots are known from modelling parameters, the latitude having a rotation period equal to that of the orbital period (co-rotating latitude) is found to be about 30°     

New photometric investigation of the low-mass-ratio contact binary star V1853 Orionis

Four-color charge-coupled device(CCD) light curves in the B, V, Rc and I c bands of the totaleclipsing binary system V1853 Orionis(V1853 Ori) are presented. By comparing our light curves with those published by previous investigators, it is determined that the O'Connell effect on the light curves has disappeared. By analyzing those multi-color light curves with the Wilson-Devinney code(W-D code),it is discovered that V1853 Ori is an A-type intermediate-contact binary with a degree of contact factor of f = 33.3%(3.7%) and a mass ratio of q = 0.1896(0.0013). Combining our 10 newly determined times of light minima together with others published in the literature, the period changes of the system are investigated. We found that the general trend of the observed minus calculated(O-C) curve shows a downward parabolic variation that corresponds to a long-term decrease in the orbital period with a rate of d P/dt =-1.96(0.46)×10^-7 d yr^-1. The long-term period decrease could be explained by mass transfer from the more-massive component to the less-massive one. By combining our photometric solutions with data from Gaia DR_2, absolute parameters were derived as M_1 = 1.20 M⊙, M_2 = 0.23 M⊙, R_1 = 1.36 R⊙and R_2 = 0.66 R⊙. The long-term period decrease and intermediate-contact configuration suggest that V1853 Ori will evolve into a high fill-out overcontact binary.     

Observations of active-chromosphere stars – V. A photometric study of the RS CVn system CF Tuc (HD 5303)

The Wilson–Devinney program is used to model 27 light curves (our own and others) for CF Tuc. We find new parameters for the binary system, and estimate the longitudes and radii of the spots on the cooler secondary star. We also find a strong tendency for spots on the cooler star to appear in a limited range of longitudes, rather than to migrate fairly rapidly as for other RS CVn systems. There is evidence that the mean light level of the cooler star is varying cyclically.
The orbital period clearly changes discontinuously. We discuss this, and the apparently cyclic variations in mean light level, in relation to the model proposed by Applegate.     

New observations,photometric study and preliminary elements of the W UMa system DF CVn

CCD (V) light curve of the EW‐type eclipsing variable DF CVn was obtained during seven nights in April–May, 2004. With our data we were able to determine 4 new times of minimum light. The light curve appears to exhibit a typical O'Connell effect, with Maximum I brighter than Maximum II by 0.013 mag. in V. TwoWilson‐Devinney (WD) code working sessions, using the V light curve, were done with and without spots. The analysis of the results shows that the best fit was obtained with the spotted solution and indicates contact geometry. The photometric mass ratio of the system is found to be q = 0.347 and its inclination i = 72°, the primary minimum shows a transit. The star may be classified as an A‐type W Uma system. Assuming a reasonable value for the mass of the primary component an estimate of the absolute elements of DF CVn has been made, with the assumption that the primary has a mass corresponding to its spectral type according to Straizys and Kuriliene (1981). (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A 2007 photometric study and UV spectral analysis of the Wolf‐Rayet binary V444 Cyg

Photometric and spectroscopic characteristics of the WN5+O6 binary system, V444 Cyg, were studied. The Wilson‐Devinney (WD) analysis, using new BV observations carried out at the Ankara University Observatory, revealed the masses, radii, and temperatures of the components of the system as M_WR = 10.64 M_⊙, M_O = 24.68 M_⊙, R_WR = 7.19 R_⊙, R_O = 6.85 R_⊙, T_WR = 31 000 K, and T_O = 40000 K, respectively. It was found that both components had a full spherical geometry, whereas the circumstellar envelope of the WR component had an asymmetric structure. The O – C analysis of the system revealed a period lengthening of 0.139 ± 0.018 syr^–1, implying a mass loss rate of (6.76 ± 0.39) ×10^–6 M_⊙ yr^–1 for the WR component. Moreover, 106 IUE‐NEWSIPS spectra were obtained from NASA's IUE archive for line identification and determination of line profile variability with phase, wind velocities and variability in continuum fluxes. The integrated continuum flux level (between 1200–2000 Å) showed a mild and regular increase from orbital phase 0.00 up to 0.50 and then a decrease in the same way back to phase 0.00. This is evaluated as the O component making a constant and regular contribution to the system's UV light as the dominant source. The C IV line, originating in the circumstellar envelope, had the highest velocity while N IV line, originating in deeper layers of the envelope, had the lowest velocity. The average radial velocity calculated by using the C IV line (wind velocity) was found as 2326 km s^–1 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A photometric study of the contact binaries V523 Cas and TY UMa

Broad-band V and I photometry of the short-period W UMa-type contact binaries V523 Cas and TY UMa are reported. From the light curves, the system parameters have been determined and evidence is presented for extensive spot activity in both systems, with V523 Cas being the more active system. A compilation of all the available photoelectric and CCD-based timings of eclipse minima is made; the O–C diagrams show evidence for an increasing period in the case of V523 Cas and a decreasing period in TY UMa. In addition we derive a more accurate astrometric position for V523 Cas from our CCD images.     

Orbital elements of the RS CVn eclipsing binary,SV Camelopardalis

The extensiveUBV observations of SV Camelopardalis by Patkos (1982) have been analysed to derive the orbital elements of the system. The data were corrected for the effect of third body (Sarma, Sarma & Abhyankar 1985) and for the ‘RS CVn’ distortion wave (Sarma, Vivekanandarao & Sarma 1988). The cleaned data were used to obtain a preliminary solution by a modified version of Wellmann method (Sarma & Abhyankar 1979) from which we concluded that the primary eclipse is a transit. The final orbital elements of SV Cam were obtained by the modified version (Sarma 1988; Sarmaet al. 1987) of WINK program by Wood (1972). The colour and median brightness variation are discussed. From the spectroscopic mass functionf(m) = 0.118 M_⊙ (Hiltner 1953), the absolute dimensions of the components are found to be 0.826 M_bd & 0.592 M_⊙ and 1.236 R_⊙ & 0.778 R_⊙ for the primary and secondary components, respectively. The age of the binary system is estimated to be 6.0 ± 1.0 × 10⁸ years     

A new light‐time effect study of AR Aurigae

A period study of the young binary AR Aur based on the extensive series of published photoelectric/ccd minima times indicates the cyclic (O – C) variation for the system. This continuous oscillatory variation covers almost three cycles, about 6000 orbital periods, by the present observational data. It can be attributed to the light‐time effect due to a third body with a period of 23.68 ± 0.17 years in the system. The analysis yields a light‐time semi‐amplitude of 0.0084 ± 0.0002 day and an orbital eccentricity of 0.20 ± 0.04. Adopting the total mass of AR Aur, the mass of the third body assumed in the co‐planar orbit with the binary is M₃ = 0.54 ± 0.03 M_⊙ and the semimajor axis of its orbit is a₃ = 13.0 + 0.2 AU. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)