CVn Systems Research Articles

Superhumps in the helium dwarf nova KL Draconis

We report on the discovery of a 25.5-min superhump period for the suspected helium dwarf nova system KL Draconis in a high state. The presence of superhumps combined with the previously observed helium spectrum and large-amplitude photometric variations confirm that KL Dra is an AM CVn system similar to CR Bootis, V803 Cen and CP Eridani. We also find a low-state photometric period at 25.0 min that we suggest may be the orbital period. With this assumption, we estimate q=0.075, M1=0.76 M⊙ and M2=0.057 M⊙.

Optical Identification of Multiple Faint X‐Ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables1

We report on the Chandra ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect 6 X-ray sources within the 10.5 core radius and 13 more within the 115 half-mass radius down to a limiting luminosity of Lx approx 10^{30} erg/s for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), 1-3 likely RS CVn or BY Dra systems, and 1 or 2 possible background objects. Of the 7 sources for which no optical identifications were made, we expect that ~2-4 are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

Long-term study of the starspot activity on the eclipsing short-period RS canum venaticorum star UV piscium

We present optical photometry of the short period eclipsing RS CVn system, UV Piscium for the years 1966–1984. After removing the spot effects from the light curves of Vivekananda Rao and Sarma (1981), we analyzed the cleaned data to obtain system parameters. For each light curve, we model the distortion waves in order to study the behaviour of star spots in this system.

Period Changes in Four Short-Period Spotted Binaries: UV Piscium, YY Geminorum, CG Cygni, and XY Ursae Majoris

Recently acquired times of minimum for three RS CVn systems and one BY Draconis binary, all with periods under a day, are presented. Times of minimum for UV Psc, YY Gem, CG Cyg, and XY UMa have been measured for decades. These observations have been collected, and the O-C curves for each system have been examined for period changes. The O-C curves for UV Psc and YY Gem showed a linear deviation, and improved periods have been computed. CG Cyg and XY UMa, though, appear to have experienced true changes in their periods. An additional 11.0 yr periodic variation has been found in the O-C data of XY UMa. The mechanism for the period variations remains uncertain.

The Quiescent Spectrum of the AM Canum Venaticorum Star CP Eridani

We used the 6.5 m Multiple Mirror Telescope to obtain a spectrum of the AM Canum Venaticorum star CP Eridani in quiescence. The spectrum is dominated by He I emission lines, which are clearly double-peaked with a peak-to-peak separation of ~1900 km s-1. The spectrum is similar to that of the longer period AM CVn systems GP Comae Berenices and CE 315, linking the short- and long-period AM CVn systems. In contrast to GP Com and CE 315, the spectrum of CP Eri does not show a central spike in the line profiles, but it does show lines of Si II in emission. The presence of these lines indicates that the material being transferred is of higher metallicity than in GP Com and CE 315, which, combined with the low proper motion of the system, probably excludes a halo origin of the progenitor of CP Eri. We constrain the primary mass to M1 > 0.27 M☉ and the orbital inclination to 33° < i 80°. The presence of the He I lines in emission opens up the possibility for phase-resolved spectroscopic studies, which will allow us to determine the system parameters and to study in detail helium accretion disks under highly varying circumstances.

A light and period study of SV Camelopardalis

New BVR light curves and times of minimum light for the short period RS CVn system SV Cam were analysed to derive the physical parameters of the system and the parameters of the third body orbit. The light curves obtained at the TÜBİTAK National Observatory during two nights in 2000 show considerable asymmetry and night-to-night variations. The analysis of the light curves is made using Djurasevic's inverse problem method. The Roche model with spotted areas on the hotter primary component yields a good fit to observations. The extensive series of published photoelectric minima times indicate that the eclipsing pair orbits around the common mass center of the triple system with a period of 41.32 yr.

NLTE accretion disc models for the AM Canum Venaticorum systems

We present models of the optical spectra of two types of accretion disc atmospheres: a pure helium model and a H-He model, in a low mass ratio, helium transferring, interacting binary white dwarf system. The computations are restricted to stationary discs in LTE and NLTE cases. We also investigate the influence on the spectra of some of the disc parameters such as the mass accretion rate, the angle of inclination, the H/He mass ratio, and the inner and outer radius of the disc. Departures from LTE are also investigated in order to assess the necessity of computing more complicated and more time consuming models. We found that dierences in predicted equivalent widths of helium line computed in LTE and NLTE models range between 10 and 40 percent. Finally we compare our disc models with 4 of the AM CVn systems observed with the Nordic Optical Telescope: AM CVn, HP Lib, V803 Cen and CR Boo to determine their parameters as mass transfer rates and inclinations of their discs.

The radio corona of AR Lacertae

We present multifrequency VLA and VLBA observations at 8.4 GHz of the RS CVn system AR Lac, that were performed in autumn 1997 simultaneously with X-ray observations obtained from Rodon o et al. (1999). Our VLBA data indicate a resolved source with dimension close to the system separation, while the study of the flux density curve reveals a small amplitude outside of the eclipse variability. The derived ve-frequencies spectra, combined with the size information from VLBA data, are compared with gyrosynchrotron emission from a two component structured source. A comparison with the results of the X-ray observations allow us to exclude the possibility that a thermal gyrosynchrotron is responsible for the radio emission, but it is compatible with the hypothesis of co-spatial X-ray and radio emitting sources.

Population synthesis for double white dwarfs

We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ~ 50% and that the lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of 0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time.

The XMM-Newton view of stellar coronae: Flare heating in the coronae of HR 1099

The RS CVn system HR 1099 was observed by XMM-Newton for a time span of 25 days during the commissioning phase. Rotational modulation in the RGS light curve has been detected with maximum flux when the active K1 IV star is in front. Following a quiescent phase of emission, the rise and peak of a large flare were observed. Time-dependent spectroscopy has been performed to derive elemental abundances and their variations during the various stages of the flare. Emission measure distributions have been reconstructed. Two different components are detected: a very hot plasma (up to 100 MK) that evolves rapidly, and a stable quiescent plasma. Low first-ionization-potential elemental (such as Fe and Si) abundances increase significantly during the flare, while the abundance of the high-FIP element Ne stays constant at the quiescent value. We report direct detection of a flux increase in the Fexxiv X-ray lines during a stellar flare.

Caii K Emission‐Line Asymmetry among Red Giants Detected by the ROSAT Satellite

ABSTRACTSpectra of the Ca ii H and K lines are reported for a number of field giants from which soft X‐ray emission was detected by the ROSAT satellite. Several of these stars are RS CVn systems and exhibit very strong Ca ii emission. The majority of the noninteracting giants in the sample have MV> - 2.0, as determined from Hipparcos parallaxes, and spectral types earlier than K3. The Ca ii K emission‐line profile for these stars is most often double‐peaked and asymmetric, with the short‐wavelength peak being stronger than the longward peak. This asymmetry is in the same sense as for the integrated disk of the Sun. The X‐ray and Ca ii K‐line data indicate that giants of spectral types G and early K have coronae and chromospheres seemingly analogous to those of the Sun. Four M giants that were detected by ROSAT were also observed. Their Ca ii emission spectra show asymmetries in which the violet wing is weaker than the red wing, a phenomenon that is typical of M giants in general and indicative of mass outflows in their chromospheres. The majority of these M giants, but not all, are known to be in binary systems, so it is possible that the X‐ray emission for at least some of them may come from a companion. Alternatively, some or all of these M giants may be examples of hybrid stars.

Star-spots activity on BH Virginis

Virginis is a short-period RS CVn system. From our observations and archival sources we have gained an ephemeris by means of the times of minima. The light curves and the photoelectric observations in B and V obtained by Koch (1967) have been analyzed by using the Wilson-Devinney program. The principal results are as follows: (1) The mass ratio obtained from the photoelectric light curves is q = m(2)/m(1) = 0.967, close to the spectroscopic value 0.968 obtained by Zhai et al. (1990). (2) The photospheric temperature of the primary, T-1, at two epochs (1963-1964 and 1991) was about; 5945 K - 6010 K. (3) Several spotted active regions are revealed on both components in 1963-1964. data sets, while only one small spotted active region appears on the secondary star in 1991.

Doppler Imaging of VW Cephei: Distribution and Evolution of Starspots on a Contact Binary

We present maximum entropy Doppler images of the contact binary VW Cephei, produced from seven data sets of simultaneous spectroscopic and photometric data at epochs from 1991 March to 1993 May. The geometric and orbital parameters of the contact binary were fitted, along with third-light characteristics, at the same time as spots, using both the spectroscopic and photometric data to constrain the models. The Doppler images indicated the presence of large polar spots on both components. The polar spot on the primary was about 50° in diameter and slightly off-center, similar to polar spots on other unevolved systems, while the polar spot on the secondary was 30° in diameter. A number of lower latitude features were also present. Spots were found to migrate around the primary component in the same direction as the orbital motion of the system. Slow differential rotation of the primary component was observed, with spots at higher latitudes moving at higher angular rates. The spot distribution on the secondary appeared to be quite stable, with spots congregating at active longitudes, and with no organized pattern of migration. The spot coverage on both components was extremely high, though not unrealistic compared to spot coverages of some RS CVn systems. Spots covered 66% of the surface area of the primary and 55% of the secondary. The spot coverage was much greater than that suggested by asymmetry of the light curves. Our Doppler images were modeled in agreement with the Mullan starspot model. However, they also explained the success of the hot secondary model, since the bolometric flux-weighted mean surface temperature of the primary was less than that of the secondary, owing to the large numbers of nonblack spots. The models indicated the presence of a great number of unresolved spots. We discuss the implications of this to the field of Doppler imaging and argue in favor of the use of two-temperature photosphere models for the Doppler imaging of all rapidly rotating systems. We detected three flares at Hα and estimate that one such detectable flare occurs on VW Cep every 35 ± 20 hr. We also noted that the distribution of chromospheric emission at Hα over the primary of VW Cep may vary from epoch to epoch.

VLBA Imaging of Quiescent Radio Emission from UX Arietis

Using the Very Long Baseline Array (VLBA) and the Very Large Array (VLA), we have observed the quiescent coronal microwave emission from the active RS CVn system UX Ari. We complement the data with X-ray information obtained from the X-ray satellite ASCA. The 8.4 GHz continuum radio source is resolved on linear scales comparable to the projected separation of the binary components, indicating the existence of large magnetic structures. Circular polarization is detected, and a polarization gradient across the radio source is observed, suggesting the existence of interacting magnetic fields between the components or a large magnetic loop anchored to one component. The VLBA lower limit to the brightness temperature (~26 MK) is close to the broad temperature peak in the contemporaneous emission measure distribution seen by ASCA; we examine the possibility that the quiescent radio flux may be gyrosynchrotron emission from a thermal electron energy distribution. We find that no simple thermal model can reproduce the observed spectral index of the quiescent emission.

Physical Requirements for Flares in Stars

AbstractA wide range of stellar analogs of solar X-ray flares has been observed. During the maximum of the solar activity cycle, one or two Mclass flares peaking at Lx ~ 1026 erg s−1 in the GOES 1−8 Å passband take place on average every day on the Sun. Such run-of-the-mill events have been measured by ASCA on our nearest neighbor, Proxima Centauri, a dM5.5e flare star (Haisch, Antunes &amp; Schmitt 1995). At the other extreme, RS CVn systems and T Tauri stars have been observed to flare with peak luminosities of Lx ~ ×1032 ergs s−1 (Haisch &amp; Schmitt 1996 and references therein). Current wisdom has it that this wide range of flares spanning at least six order of magnitude (and another factor of 10–100 if one counts even lower level solar flares) on the young T Tauri stars, main sequence G, K and M stars, and in evolved RS CVn subgiants and giants can all be understood as versions of solar flares originating ultimately in a convectively-driven magnetic dynamo. There is evidence pointing to two distinct types of dynamo perhaps even in the Sun (Durney, Young and Roxburgh 1993): the α − ω dynamo generated in a rather thin boundary layer near — in some interpretations just below — the interface between a star’s radiative core and convective envelope; and a turbulent distributed dynamo operative more or less throughout a convection zone. But what is one to make of the evidence for flares on Be stars (Smith, Robinson and Corbet 1998) where there is no subsurface convection to drive a dynamo? (Convective envelopes are thought to begin among the late-A spectral type stars, and steadily deepen for cooler stars.) This paper does not attempt to provide an answer, but it does attempt to outline the characteristics and requirements of magnetically-powered flaring.

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.

Photometry and spectroscopy of the RS CVn system σ Geminorum

We present new UBV photoelectric photometry and high-resolution H spectroscopy of the RS CVn system σ Gem made in 1993-1994. A brief photometric analysis for the new light curves is carried out using the Wilson-Devinney method with spot approximation based on the Roche model. The result suggests the probable presence of two cool spots on the K1 III primary around the phases of and . The spot activity is discussed in connection with the chromospheric H emission. The enhanced H core emission is found to vary with phase and shows distinct anti-correlation with the spot regions. It may be an indication for the existence of plage regions on σ Gem.

Coordinated multiwavelength observations of the RS CVn system CF Tuc

The relatively bright eclipsing RS CVn binary CF Tuc was observed in both optical (photometry and spectroscopy) and microwave ranges in a 'multisite, multiwavelength' campaign in 1996. The microwave data, gathered using the Australia Telescope Compact Array,covered slightly more than one complete orbital cycle at 4.8 and 8.64 GHz in one continuous nm. There was also limited coverage at 1.38 and 2.38 GHz. High-dispersion spectroscopy was obtained using the McLellan 1-m telescope and echelle spectrograph at Mt John University Observatory (New Zealand). Supporting photometry came from smaller scale facilities associated with the Royal Astronomical Society of New Zealand (RASNZ), The data show the correlation between microwave signal enhancement and photometric flux diminution (maculation effect) noted previously in similar studies. The spectroscopy also points to the phenomena being related to a very enhanced active region on the secondary star located close to the maculation region. Cross-correlation of the microwave emission data between the 4.8- and 8.64-GHz ranges shows a strong positive correlation, with a significant lag of the 4.8-GHz behind the 8.64-GHz variations. This indicates energized waves propagating through the corona of the K4 subgiant at the electron sound speed. The emitting region may have been eclipsed bye primary star, but such eclipse effects are of marginal significance for the present analysis. An attempt is made to put all the evidence into one coherent picture of the active region.

Extreme Ultraviolet ExplorerPhotometry of RS Canum Venaticorum Systems: Four Flaring Megaseconds

We present an analysis of 12.2 Ms of Extreme Ultraviolet Explorer (EUVE) photometry of 16 RS CVn systems (including four flaring megaseconds). Our study attempts a systematic categorization of stellar coronal emission in RS CVn binary systems. The temporal resolution of the EUVE satellite and the opportunity for long observations enables us to study separately the quiescent and flaring states of RS CVn systems. Thirty of 31 observations are statistically variable. We examine light curves, characterizing both phase-dependent variations and large- and small-scale flaring. There is evidence for small-scale stochastic variability on the one short-period (Porb 0.5Porb and thus cannot originate from compact emission regions close to the stellar surfaces. Many flares have unusual flare morphologies, with rise times comparable to or greater than the decay time, and emission plateaus. In addition, eight flares have decay phases that are fit better by a broken power law than a single power law. The decay times for the second power law are statistically different than the single decay times and are correlated with the flare rise times. This observed connection points to the potential importance of the change in decay rate as a means of examining the physical processes operating during the flare. The distribution of orbital phases of flare onsets on V711 Tau (HR 1099), the best-studied RS CVn in our sample, is consistent with a Poisson distribution that is random in time. In two systems that are partially eclipsing (CF Tuc and ER Vul), we find no evidence for an eclipse in the light curve, indicating that the emission region is large compared to the stellar radius. We parameterize the distribution of flare energies as a power law, finding a cut-off energy of ≈1033 ergs and a slope of -0.6. Flare energy increases with flare duration as E∝Δt1.42, confirming the long-duration nature of high-energy flares on RS CVn systems. The integrated flare luminosity depends on the quiescent luminosity as Lflare∝L1.05quiescent. Flare frequencies range from 0.1 day-1 to 1.5 day-1. For observations not affected by the dead spot on the Deep Survey detector, 40% of the observed time was spent in a flaring state, indicating that flaring is a normal state for the coronae of these systems. We note the presence of a quasi-periodicity in EUVE photometric data on the order of 1 day, which is not corrected for in the standard reduction and analysis software.

Magnetic cycles of HR 1099 and LQ Hydrae

We present in this paper a 6-yr time series of magnetic (and brightness) surface images of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri) and of the young K0 dwarf LQ Hydrae, reconstructed (with the help of a dedicated maximum entropy image reconstruction software) from Zeeman--Doppler imaging observations collected at the Anglo-Australian Telescope. All the stellar magnetic images that we reconstruct host at least one high-contrast feature in which the field is predominantly azimuthal, thus confirming that such surface structures (already detected in previous similar studies) are indeed real. We take this as strong evidence that dynamo processes of late-type rapid rotators operate throughout the whole stellar convective envelope, rather than being confined to an interface layer between the convective and radiative zones as in the Sun. The latitudinal polarity pattern of azimuthal and radial fields that we observe at the surface of both stars suggests that these magnetic regions respectively reveal the toroidal and poloidal components of the large-scale dynamo field. The spatial structure of these two magnetic field components becomes increasingly more complex (with higher axisymmetric spherical harmonic degrees) for larger rotation rates and deeper convective zones. The strength of the toroidal and poloidal components is typically a few hundred G, more than two orders of magnitude stronger than in the Sun. Long-term evolution of the toroidal and poloidal components of the large-scale field is clearly detected in our time series. We also report the detection of small fluctuations in the orbital period of HR 1099, with a peak-to-peak amplitude of about 36±1 s (i.e. 0.015 per cent) and a period of about 18±2 yr (assuming sinusoidal variations around the nominal value). The most plausible way of explaining such fluctuations is that the quadrupole moment of the K1 subgiant is varying with time, and that this modulation is driven by the magnetic activity cycle of the primary star itself (through a periodic exchange between kinetic and magnetic energy within the convective zone). It provides in particular an independent confirmation that a dynamo operates within the whole convective zone, and suggests that the average azimuthal field in the convective envelope is of the order of 6 kG.