UMa Stars Research Articles

CURious Variables Experiment (CURVE): CCD photometry of active dwarf nova DI Ursae Majoris

Context. We report an analysis of photometric behaviour of DI UMa, an extremely active dwarf nova. The observational campaign (completed in 2007) covers five superoutbursts and four normal outbursts. Aims. We examined principal parameters of the system to understand peculiarities of DI UMa, and other active cataclysmic variables. Methods. Based on precise photometric measurements, temporal light curve behaviour, O−C analysis, and power spectrum analysis, we investigated physical parameters of the system. Results. We found that the period of the supercycle now equals 31.45 ± 0.3 days. Observations during superoutbursts infer that the period of superhumps equals Psh = 0.055318(11) days (79.66 ± 0.02 min). During quiescence, the light curve reveals a modulation of period Porb = 0.054579(6) days (78.59 ± 0.01 min), which we interpret as the orbital period of the binary system. The values obtained allowed us to determine a fractional period excess of 1.35% ± 0.02%, which is surprisingly small compared to the usual value for dwarf novae (2%–5%). A detailed O−C analysis was performed for two superoutbursts with the most comprehensive coverage. In both cases, we detected an increase in the superhump period with a mean rate of u P/Psh = 4.4(1.0) × 10 −5 . Conclusions. Based on these measurements, we confirm that DI UMa is probably a period bouncer, an old system that reached its period minimum a long time ago, has a secondary that became a degenerate brown dwarf, the entire system evolving now toward longer periods. DI UMa is an extremely interesting object because we know only one more active ER UMa star with similar characteristics (IX Dra).

New absolute magnitude calibrations for WUrsa Majoris type binaries

AbstractParallaxes of W UMa stars in the Hipparcos catalogue have been analyzed. 31 W UMa stars, which have the most accurate parallaxes (σπ /π < 0.15) which are neither associated with a photometric tertiary nor with evidence of a visual companion, were selected for re‐calibrating the Period‐Luminosity‐Color (PLC) relation of W UMa stars. Using the Lutz‐Kelker (LK) bias corrected (most probable) parallaxes, periods (0.26 < P < 0.87, P in days), and colors (0.04 < (B – V)0 < 1.28) of the 31 selected W UMa, the PLC relation have been revised and re‐calibrated. The difference between the old (revised but not bias corrected) and the new (LK bias corrected) relations are almost negligible in predicting the distances of W UMa stars up to about 100 pc. But, it increases and may become intolerable as distances of stars increase. Additionally, using (J – H)0 and (H – Ks)0 colors from 2MASS (TwoMicron All Sky Survey) data, a PLC relation working with infrared data was derived. It can be used with infrared colors in the range –0.01 < (J – H)0 < 0.58, and –0.10 < (H – Ks)0 < 0.18. Despite of the fact that the 2MASS data refer to single epoch observations which are not guaranteed to be taken at maximum brightness of theWUMa stars, the established relation has been found surprisingly consistent and reliable in predicting LK corrected distances of W UMa stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photometric Studies of New Southern SU UMa-Type Dwarf Novae, FL Trianguli Australis and CTCV J0549–4921

We report on the time-resolved optical CCD photometry of newly discovered SU UMa-type dwarf novae, FL TrA and CTCVJ0549$-$4921. During a 2006 August outburst, we detected superhumps with a period of 0.59897(11) days for FL TrA, clarifying the SU UMa nature of the system. On the first night of our observations of FL TrA, the object showed no superhumps. This implies that it takes a few days for the full development of superhumps. The superhump period variation diagram of FL TrA was similar to that observed in some WZ Sge stars and short-period SU UMa-type stars. This indicates that the system is closely related to WZ Sge stars and SU UMa stars having short orbital periods. For CTCV J0549$-$4921, the candidates of the mean superhump period are 0.083249(10) days and 0.084257(8) days, respectively. Due to a lack of observations, we cannot determine the true superhump period, but the latter period is favorable. Using the ASAS-3 archive, it turned out that the system showed only four outbursts over the past 6 years. The outburst amplitude of CTCV J0549$-$4921 was relatively small, with about 4.5 mag. One possibility is that mass evaporation may play a role during quiescence.

Is the Evolutionary Outcome of Algol-Type Systems T Leonis Minoris and RW Monocerotis the W UMa Systems?

Abstract The variations of the orbital periods of two Algol-type eclipsing binaries, T LMi and RW Mon, are presented through a detailed analysis of $(O-C)$ diagrams. It is found that, apart from a secular decrease, several sudden jumps have occurred in the orbital periods of both binary systems. The period decrease rate was derived to be $-1.12\times10^{-8}$d/cycle and $-$1.456$\times$10$^{-9}$d/cycle for T LMi and RW Mon, respectively. Such a large rate is impossibly explained by angular momentum loss and mass loss due to magnetic stellar winds. The sudden jumps in the orbital periods of both stars might be caused by a rapid mass exchange between the circumstellar matter and binaries. At last, the evolutionary trend of the two Algol-type systems, whose periods decreased continuously, is discussed. Their evolutionary outcome might be W UMa stars.

Infrared Spectroscopy of Short-Period Cataclysmic Variables

We present near-infrared spectra for five dwarf novae with short orbital periods. We performed fittings of the spectral energy distributions (SEDs) by assuming a power-law distribution for the accretion disk and using template spectra of late-type dwarfs for the secondary star. Two are WZ Sge–or WX Cet-type stars, whose spectra show water absorption features, and one of them shows K$\;$ I and Na$\;$ I absorption lines in the $J$-band. We find that the secondary contributions to the overall SED are larger in WZ Sge stars than in ER UMa stars. The SED fitting suggests M9 and L1 type dwarf companions for our WZ Sge systems, and an M2 dwarf for our ER UMa star. We discuss the direct relationship between the high temperatures of secondary stars and high mass-transfer rates in ER UMa stars. SDSS J013701.06–091234.9 is the only object showing a spectrum dominated by the secondary component. The spectrum strongly suggests that the evolutionary path of SDSS J013701.06–091234.9 is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.

Discovery of a bright eclipsing cataclysmic variable

We report on the discovery of J0644+3344, a bright deeply eclipsing cataclysmic variable (CV) binary. Spectral signatures of both binary components and an accretion disk can be seen at optical wavelengths. The optical spectrum shows broad H I, He I, and He II accretion disk emission lines with deep narrow absorption components from H I, He I, Mg II and Ca II. The absorption lines are seen throughout the orbital period, disappearing only during primary eclipse. These absorption lines are either the the result of an optically-thick inner accretion disk or from the photosphere of the primary star. Radial velocity measurements show that the H I, He I, and Mg II absorption lines phase with the the primary star, while weak absorption features in the continuum phase with the secondary star. Radial velocity solutions give a 150+/-4 km/s semi-amplitude for the primary star and 192.8+/-5.6 km/s for the secondary. The individual stellar masses are 0.63-0.69 Mdot for the primary and 0.49-0.54 Mdot for the secondary. The bright eclipsing nature of this binary has helped provide masses for both components with an accuracy rarely achieved for CVs. This binary most closely resembles a nova-like UX UMa or SW Sex type of CV. J0644+3344, however, has a longer orbital period than most UX UMa or SW Sex stars. Assuming an evolution toward shorter orbital periods, J0644+3344 is therefore likely to be a young interacting binary. The secondary star is consistent with the size and spectral type of a K8 star, but has an M0 mass.

Photometry of VS 0329+1250: A New Short‐Period SU Ursae Majoris Star

ABSTRACTWe present time‐resolved CCD photometry of the recently discovered (V≃15 at maximum light) eruptive variable star in Taurus, which we dub VS 0329+1250. A total of ∼20 hr of data obtained over six nights reveal superhumps in the light curves, confirming the star as a member of the SU UMa class of dwarf novae. The superhumps recur with a mean period of 0.053394(7) days (76.89 minutes), which represents the shortest known superhump period in a classical SU UMa star. A quadratic fit to the timings of superhump maxima reveals that the superhump period was increasing at a rate given by P˙sh≃(2.1 ± 0.8) × 10-5 over the course of our observations. An empirical relation between orbital period and the absolute visual magnitude of dwarf novae at maximum light suggests that VS 0329+1250 lies at a distance of ∼ 1.2 ± 0.2 kpc.

WX Ceti: a closer look at its behaviour in quiescence and outburst

Context. WX Cet is a dwarf nova with rare outbursts of large amplitude. Aims. We compile the available data of WX Cet, compare the results with other SU UMa stars, and discuss our findings in the context of current theories of superhumps and superoutbursts to progress with our understanding of SU UMa stars. Methods. We analyse all recorded outbursts of WX Cet, based on the AAVSO archive and other published sources, and present new CCD photometry during two recent superoutbursts, including the determination of the corresponding periodicities. We perform numerical disc instability model calculations and compare its predictions with the observations. Results. WX Cet is a SU UMa type dwarf nova with a superoutburst cycle of 880 days on average, and short eruptions every 200 days. It seems that the outburst cycle length increased by nearly a factor of 2 during the past 70 years. According to our numerical simulations, this can be explained in the context of the disc instability model by assuming enhanced mass transfer during outburst and a decreasing mean mass transfer rate during the last decades. Using the data available, we refine the orbital period of WX Cet to 0.0582610 ± 0.0000002 days and interpret the orbital hump found in quiescence as emission from the hot spot. During two recent superoutbursts in July 2001 and December 2004 we observed superhumps, with a rather large positive period derivative of u Ps/Ps = 1.6 × 10 −4 , present only during the first 9 days of a superoutburst. Afterwards and during decline from the “plateau” phase, a constant superhump period of about 0.05922 days was observed. Late superhumps are present for at least 12 days after the decline from the “plateau”, with a period of 0.05927 days. We find this phenomenology difficult to interpret in the context of the standard explanation for superhumps, i.e. the thermal tidal instability model. Conclusions. We interpret the long-term light curve of WX Cet as the result of a significantly decreasing mean mass transfer rate. Highlighting the complexity of the observed superhump light curves, we emphasise the importance of WX Cet for a proper understanding of the SU UMa star outburst physics and the evolution of ultra-short period cataclysmic variables.

Dynamical evolution of active detached binaries on the log Jo–log M diagram and contact binary formation

Orbital angular momentum (OAM, J o ), systemic mass (M) and orbital period (P) distributions of chromospherically active binaries (CAB) and W Ursae Majoris (W UMa) systems were investigated. The diagrams of log J o -logP, logM-logP and log J o -log M were formed from 119 CAB and 102 W UMa stars. The log J o -log M diagram is found to be most meaningful in demonstrating dynamical evolution of binary star orbits. A slightly curved borderline (contact border) separating the detached and the contact systems was discovered on the log J o -log M diagram. Since the orbital size (a) and period (P) of binaries are determined by their current J o , M and mass ratio, q, the rates of OAM loss (d log J o /dt) and mass loss (d log M/dt) are primary parameters to determine the direction and the speed of the dynamical evolution. A detached system becomes a eontact system if its own dynamical evolution enables it to pass the contact border on the log J o -log M diagram. The evolution of q for a mass-losing detached system is unknown unless the mass-loss rate for each component is known. Assuming q is constant in the first approximation and using the mean decreasing rates of Jo and M from the kinematical ages of CAB stars, it has been predicted that 11, 23 and 39 per cent of current CAB stars would transform to W UMa systems if their nuclear evolution permits them to live 2, 4 and 6 Gyr, respectively.

Orbital Period of the Dwarf Nova RXS J053234.9+62475511Based on observations obtained at the MDM Observatory, operated by Dartmouth College, Columbia University, Ohio State University, and the University of Michigan.

ABSTRACTWe report spectroscopy of the newly discovered SU Ursae Majoris dwarf nova identified with the X‐ray source RXS J053234.9+624755. Radial velocities of the Hα emission line in the quiescent state give an orbital period of 0.05620(4) days (80.93 minutes), which is among the shortest for SU UMa stars with determined periods. We also report UBVI magnitudes of the quiescent dwarf nova and surrounding stars. Using a previous measurement of the superhump period, we find the fractional superhump excess ϵ to be 0.016(4), which is not atypical of dwarf novae in this period range.

The 2005 July Superoutburst of the Dwarf Nova 2QZ J021927.9−304545: the SU UMa Nature Confirmed

Abstract We report on the time-resolved photometry of the 2005 July superoutburst of a dwarf nova, 2QZ J021927.9$-$304545. The resultant light curves showed conspicuous superhumps with a period of 0.081113(19) days, confirming the SU UMa nature of the object. Although we missed the maximum phase of the outburst, the amplitude of the superoutburst well exceeded 5 mag. This value is slightly larger than that of typical SU UMa-type dwarf novae. The superhump period decreased as the time elapsed, as can be seen in most SU UMa-type dwarf novae. Based on the archive of ASAS-3, the recurrence time of a superoutburst of the variable turned out to be about 400 days. This value is typical of well-known SU UMa stars. The distance to this system was roughly estimated as $370\,(+20,-60) \,\mathrm{pc}$ using an empirical relation.

Eclipsing binaries in the All Sky Automated Survey catalogue

Abstract The All Sky Automated Survey (ASAS) is a long-term project to monitor bright variable stars over the whole sky. It has discovered 50 099 variables brighter than V < 14 mag south of declination +28°, and among them 11 076 eclipsing binaries. We present a preliminary analysis of 5384 contact, 2949 semi-detached, and 2743 detached systems. The statistics of the distribution provides a qualitative confirmation of decades old idea of Flannery and Lucy that the W UMa-type binaries evolve through a series of relaxation oscillations: the ASAS finds comparable number of contact and semi-detached systems. The most surprising result is a very small number of detached eclipsing binaries with periods P < 1 d, the systems believed to be the progenitors of the W UMa stars. As many (perhaps all) contact binaries have companions, there is a possibility that some were formed in a Kozai cycle, as suggested by Eggleton and his associates.

W Ursae Majoris Contact Binary Variables as X-Ray Sources

We present cross-identification of archived ROSAT X-ray point sources with W UMa variable stars found in the All-Sky Automated Survey. A total of 34 W UMa stars have been found associated with X-ray emission. We compute the distances of these W UMa systems and hence their X-ray luminosities. Our data support the supersaturation phenomenon seen in these fast rotators, namely that the faster a W UMa star rotates, the weaker its X-ray luminosity.

Photometric Properties of the Variable Star LZ Herculis

ABSTRACTWe present BV photometric results of the variable star LZ Her. Frolov & Irkaev classified it as a field SX Phe–type pulsating star, but our observations clearly show that LZ Her is not an SX Phe star, but rather a W UMa–type eclipsing binary, as reported by Garrido et al. The orbital period is derived to be 0.33173646 days by combining our data with that in the literature. The eclipsing light‐curve solution from the Wilson‐Devinney code shows that LZ Her belongs to the W subgroup of W UMa stars; the primary component is less massive and hotter, with a spectral type of F9–G0, and the secondary is more massive and cooler, with a spectral type G5–G6. The spectral types were estimated from the photometric light‐curve solution. Asymmetric light curves could be explained by assuming a cool spot on the secondary star. In the observing field of LZ Her, we found a new eclipsing binary, 2MASS 17500367+2928020, with an orbital period of 0.3056 days.

Indirect evidence for short period magnetic cycles in W UMa stars

Complex period variations of five W UMa type binaries (AB And, OO Aql, DK Cyg, V566 Oph, U Peg) were investigated by analyzing their O--C diagrams, and several common features were found. Four of the five systems show secular period variations at a constant rate on the order of |dP / dt 1/P | ~ 10E-7 1/yr. In the case of AB And, OO Aql, and U Peg a high-amplitude, nearly one-century long quasi-sinusoidal pattern was also found. It might be explained as light-time effect, or by some magnetic phenomena, although the mathematical, and consequently the physical, parameters of these fits are very problematic, as the obtained periods are very close to the length of the total data range. The most interesting feature of the studied O--C diagrams is a low amplitude (2-4 x 10E-3 d) modulation with a period around 18--20 yr in four of the five cases. This phenomenon might be indirect evidence of some magnetic cycle in late-type overcontact binaries as an analog to the observed activity cycles in RS CVn systems.

Light Curve Analysis of Two New W Uma Stars in M15

Light Curve Analysis of Two New W Uma Stars in M15

Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824

We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in quiescence () reveals a double-humped light curve from which we derive an orbital period of 86.2 min. Additional photometry obtained during a superoutburst reaching clearly shows superhumps with a period of 89.05 min. The optical spectrum contains double-peaked Balmer and He I emission lines from the accretion disc as well as broad absorption troughs of , , and from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of , a distance of , and suggests that the spectral type of the donor star is later than M 5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star.

The disk instability model for dwarf nova outbursts

The development and the present state of the disk instability model for outbursts of dwarf nova are reviewed. Two intrinsic instabilities are known in dwarf nova accretion disks, i.e., the thermal instability and the tidal instability. The thermal-tidal instability model (abbreviated the TTI model) that combines these two intrinsic instabilities was first proposed in 1989 by Osaki to explain the superoutburst phenomenon of SU UMa stars. In this paper a complete account of the TTI model is presented. We first explain the basic concept of the model and how it works for the superoutburst phenomenon. We then discuss recent refinements of the model, in particular on the start and the end of superoutbursts, by which we are now able to explain wide varieties in superoutburst light curves of different stars and of different superoutbursts within one and the same star. We also discuss some exceptional cases that apparently seem to contradict the theory, such as the 1985 superoutburst of U Gem itself. It is argued that the TTI model can after all explain most of the observed phenomena related to superoutbursts and superhumps in dwarf novae. (Communicated by Yoshihide KOZAI, M. J. A.)

Structure and evolution of low-mass W Ursae Majoris type systems - II. With angular momentum loss

In a preceding paper, using Eggleton's evolution code we have discussed the structure and evolution of low-mass W UMa type contact binaries without angular momentum loss (AML). The models exhibit cyclic behavior about a state of marginal contact on a thermal time-scale. Part of the time of each cycle is spent in contact and part in a semi-detached state. According to observations, W UMa systems suffer AML. We present the models of low-mass contact binaries with AML due to gravitational wave radiation (GR) or magnetic stellar wind (MSW) are presented. We find that gravitational radiation cannot prevent the cyclic evolution of W UMa systems, and the effect of gravitational radiation on the cyclic behavior of contact binary evolution is almost negligible. We also find that the most likely AML mechanism for W UMa systems is magnetic braking, and that magnetic braking effects can increase the period of the cyclic evolution, and shorten the fraction of the time spent in the poor thermal contact state. If W UMa stars do not undergo cyclic evolution, and their angular momentum loss is caused simultaneously by MSW of both components, we find that the value of the parameter, $\lambda$, should be taken a larger value in comparison with those derived from observations of single stars. This indicates that the AML efficiency in W UMa systems may be lowered in comparison with non-contact stars because of the less mass contained in the convective envelopes of the components in W UMa systems. If W UMa systems lose their angular momentum at a constant rate. An angular momentum rate of $\frac{{\rm dln}J}{{\rm d}t}\approx 1.6\times 10^{-9} {\rm yr^{-1}}$ can prevent the cyclic behaviour of the model, and the model can keep in good contact with an essentially constant depth of contact.

Variable stars in the field of the old open cluster NGC 188

We report the results of a new time-series charge-coupled device (CCD) photometry survey of variable stars in the field of the old open cluster NGC 188. In a 1 ◦ field covering the cluster, a total number of 27 variable stars, including eight new variables, were detected during this survey. The periods, the classifications as well as the membership of the newly discovered variables are discussed. Among those, two are definite member stars and one is a probable member star of the cluster. They are all W UMa binaries. Thus the total number of W UMa stars belonging to the cluster is increased to 10. A peculiar variable (V26 = GSC 4619−450) is found from the field. It is very likely a new large-amplitude β Cephei-type variable with a pulsating period of about 0.1331 d. Revised basic data are given for the known variables in the programme field. The period of V8 is refined as 5.2096 d. The variability of V11 is confirmed by this observation. It is suggested to be a very probable FK Com-type variable with a period of about 1.2433 d. Finally, the relative frequency of the occurrences of W UMa stars and the distribution in the cluster are discussed. The result supports the hypothesis of mass segregation in NGC 188. Ke yw ords: binaries: eclipsing ‐ stars: variables: other ‐ open clusters and associations: individual: NGC 188.