High-resolution spectroscopic observations of the W UMa-type binary CrA obtained as a time-monitoring sequence on four full and four partial nights within two weeks have been used to derive orbital ...elements of the system and discuss the validity of the Lucy model for description of the radial-velocity data. The observations had more extensive temporal coverage and better quality than similar time-sequence observations of the contact binary AW UMa. The two binaries share several physical properties and show very similar deviations from the Lucy model: the primary component is a rapidly rotating star almost unaffected by the presence of the secondary component, while the latter is embedded in a complex gas flow and appears to have its own rotation-velocity field, in contradiction to the model. The spectroscopic mass ratio is found to be larger than the one derived from the light-curve analysis, as in many other W UMa-type binaries, but the discrepancy for CrA is relatively minor, much smaller than for AW UMa. The presence of the complex velocity flows contradicting the assumption of solid-body rotation suggests a necessity of modification to the Lucy model, possibly along the lines outlined by Stȩpie in his concept of energy transfer between the binary components.
High-resolution spectroscopic observations of AW UMa, obtained on three consecutive nights with a median time resolution of 2.1 minutes, have been analyzed using the broadening function method in the ...spectral window of 22.75 nm around the 518 nm Mg I triplet region. Doppler images of the system reveal the presence of vigorous mass motions within the binary system; their presence puts into question the solid-body rotation assumption of the contact binary model. AW UMa appears to be a very tight, semi-detached binary; the mass transfer takes place from the more massive to the less massive component. It should be studied why photometry and spectroscopy give such discrepant results and whether AW UMa is an unusual object or if only very high-quality spectroscopy can reveal the true nature of W UMa-type binaries.
Abstract
Tycho-
Gaia
Astrometric Solution (TGAS) parallax data are used to determine absolute magnitudes
M
V
for 318 W UMa-type (EW) contact binary stars. A very steep (slope ≃ −9), single-parameter ...(
), linear calibration can be used to predict
M
V
to about 0.1–0.3 mag over the whole range of accessible orbital period,
days. A similar calibration for the most common systems with
days predicts
M
V
values to about 0.06–0.16 mag. For orbital period values both shorter and longer than the central range, the period dependence is respectively steeper and shallower, i.e., the binaries are fainter in
M
V
than predicted by the whole-range linear law. The steepness of the relation for short-period systems implies important consequences for the detectability of the faintest binaries, defining the short-period cut-off of the period distribution. Although the scatter around the linear
-fit is fairly large (0.2–0.4 mag), the current data do not support the inclusion of a
color term in the calibration.
The search volume-corrected period distribution of contact binaries of the W UMa type appears to reflect primarily the constant number ratio of ≃1/500 to the number of stars along the main sequence; ...there exist no evidence for angular momentum evolution. The maximum in contact binary numbers is located at shorter periods than estimated before, P≃ 0.27 d. The drop in numbers towards the cut-off at P≃ 0.215–0.22 d still suffers from the small number statistics while the cut-off itself remains unexplained. Only one out of seven short-period All-Sky Automated Survey variables with P < 0.22 d have been retained in the sample considered here within 8 < V < 13; this short-period field-sky record holder at P= 0.2178 d should be studied.
We present results of the Canada-France-Hawaii Telescope adaptive optics (AO) search for companions of a homogeneous group of contact binary stars, as a contribution to our attempts to prove the ...hypothesis that these binaries require a third star to become as close as observed. In addition to directly discovering companions at separations of >=1'', we introduced a new method of AO image analysis utilizing distortions of the AO diffraction ring pattern at separations of 0.07''-1''. Very close companions, with separations in the latter range, were discovered in the systems HV Aqr, OO Aql, CK Boo, XY Leo, BE Scl, and RZ Tau. More distant companions were detected in V402 Aur, AO Cam, and V2082 Cyg. Our results provide a contribution to the mounting evidence that the presence of close companions is a very common phenomenon for very close binaries with orbital periods <1 day.
This study is the first attempt to determine the metallicities of W UMa-type binary stars using spectroscopy. We analyzed about 4500 spectra collected at the David Dunlap Observatory. To circumvent ...problems caused by the extreme spectral line broadening and blending and by the relatively low quality of the data, all spectra were subject to the same broadening function (BF) processing to determine the combined line strength in the spectral window centered on the Mg I triplet between 5080 A and 5285 A. All individual integrated BFs were subsequently orbital-phase averaged to derive a single line-strength indicator for each star. The best defined results were obtained for an F-type sub-sample (0.32 < (B - V) sub(0) < 0.62) of 52 binaries for which integrated BF strengths could be interpolated in the model atmosphere predictions.
Abstract
This is the final photometric study of TW Hya based on new MOST satellite observations. During 2014 and 2017, the light curves showed stable 3.75- and 3.69-d quasi-periodic oscillations, ...respectively. Both values appear to be closely related to the stellar rotation period, as they might be created by changing visibility of a hotspot formed near the magnetic pole directed towards the observer. These major light variations were superimposed on a chaotic, flaring-type activity caused by hotspots resulting from unstable accretion – a situation reminiscent of that in 2011, when TW Hya showed signs of a moderately stable accretion state. In 2015, only drifting quasi-periods were observed, similar to those present in 2008–2009 data and typical for magnetized stars accreting in a strongly unstable regime. A rich set of multicolour data was obtained during 2013–2017 with the primary aim of characterizing the basic spectral properties of the mysterious occultations in TW Hya. Although several possible occultation-like events were identified, they are not as well defined as in the 2011 MOST data. The new ground-based and MOST data show a dozen previously unnoticed flares, as well as small-amplitude 11 min–3 h brightness variations, associated with ‘accretion bursts’. We cannot exclude the possibility that the shortest 11–15 min variations could also be caused by thermal instability oscillations in an accretion shock.
The luminosity function for contact binary stars of the W UMa type is evaluated on the basis of the All Sky Automated Survey (ASAS) photometric project covering all stars south of δ=+ 28° within a ...magnitude range 8 < V < 13. Lack of colour indices enforced a limitation to 3374 systems with P < 0.562 d (i.e. 73 per cent of all systems with P < 1 d) where a simplified MV(log P) calibration could be used. The spatial density relative to the main-sequence FGK stars of 0.2 per cent, as established previously from the Hipparcos sample to V= 7.5, is confirmed. While the numbers of contact binaries in the ASAS are large and thus the statistical uncertainties small, derivation of the luminosity function required a correction for missed systems with small amplitudes and with orbital periods longer than 0.562 d; the correction, by a factor of 3, carries an uncertainty of about 30 per cent.
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