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Register a new userA Speckle-Imaging Search for Close Triple Companions of Cataclysmic Binaries
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The orbital periods of most eclipsing cataclysmic binaries are not undergoing linear secular decreases of order a few parts per billion as expected from simple theory. Instead, they show several parts per million increases and decreases on timescales of years to decades, ascribed to magnetic effects in their donors, triple companions, or both. To directly test the triple companion hypothesis, we carried out a speckle imaging survey of six of the nearest and brightest cataclysmic variables. We found no main sequence companions earlier than spectral types M4V in the separation range 0.02 - 1.2, corresponding to projected linear separations of 2 - 100 AU, and periods of 3 - 1000 years. We conclude that main sequence triple companions to CVs are not very common, but cannot rule out the presence of the faintest M dwarfs or close brown dwarf companions.
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Gravitationally bound companions to stars enable determinations of their masses, and offer clues to their formation, evolution and dynamical histories. So motivated, we have carried out a speckle imaging survey of eight of the nearest and brightest Wolf-Rayet (WR) stars to directly measure the frequency of their resolvable companions, and to search for much fainter companions than hitherto possible. We found one new, close companion to each of WR 113, WR 115 and WR 120 in the separation range 0.2 - 1.2. Our results provide more evidence that similar-brightness, close companions to WR stars are common. More remarkably, they also demonstrate that the predicted, but much fainter and thus elusive companions to WR stars are now within reach of modern speckle cameras on 8m class telescopes by finding the first example. The new companion to WR 113 is just 1.16 distant from it, and is 8 magnitudes fainter than the WR star. The empirical probability of a chance line-of-sight of the faint companion at the position of WR 113 is < 0.5%, though we cannot yet prove or disprove if the two stars are gravitationally bound. If these three new detections are physical companions we suggest, based on their narrowband magnitudes, colors, reddenings and GAIA distances that the companions to WR113, WR 115 and WR 120 are an F-type dwarf, an early B-type dwarf, and a WNE-type WR star, respectively.
We propose a formation mechanism for twin blue stragglers (BSs) in compact binaries that involves mass transfer from an evolved outer tertiary companion on to the inner binary via a circumbinary disk. We apply this scenario to the observed double BS system Binary 7782 in the old open cluster NGC 188, and show that its observed properties are naturally reproduced within the context of the proposed model. We predict the following properties for twin BSs: (1) For the outer tertiary orbit, the initial orbital period should lie between 220 days $lesssim$ P$_{rm out}$ $lesssim$ 1100 days, assuming initial masses for the inner binary components of $m_{rm 1} = 1.1$ M$_{odot}$ and $m_{rm 2} =$ 0.9 M$_{odot}$ and an outer tertiary mass of $m_{rm 3} = 1.4$ M$_{odot}$. After Roche-lobe overflow, the outer star turns into a white dwarf (WD) of mass 0.43 to 0.54,MSun. There is a correlation between the mass of this WD and the outer orbital period: more massive WDs will be on wider orbits. (3) The rotational axes of both BSs will be aligned with each other and the orbital plane of the outer tertiary WD. (4) The BSs will have roughly equal masses, independent of their initial masses (since the lower mass star accretes the most). The dominant accretor should, therefore, be more enriched by the accreted material. Hence, one of the BSs will appear to be more enriched by either He, C and O or by s-process elements, if the donor started Roche lobe overflow on, respectively, the red giant or asymptotic giant branch. (5) Relative to old clusters, twin BSs in close binaries formed from the proposed mechanism should be more frequent in the Galactic field and open clusters with ages $lesssim$ 4-6 Gyr, since then the donor will have a radiative envelope. (6) The orbit of the binary BS will have a small semi-major axis (typically $aplt 0.3$,au) and be close to circular ($e aplt 0.2$).
A total of 28 young nearby stars (ages $leq 60$,Myr) have been observed in the K$_{rm s}$-band with the adaptive optics imager Naos-Conica of the Very Large Telescope at the Paranal Observatory in Chile. Among the targets are ten visual binaries and one triple system at distances between 10 and 130 pc, all previously known. During a first observing epoch a total of 20 faint stellar or sub-stellar companion-candidates were detected around seven of the targets. These fields, as well as most of the stellar binaries, were re-observed with the same instrument during a second epoch, about one year later. We present the astrometric observations of all binaries. Their analysis revealed that all stellar binaries are co-moving. In two cases (HD 119022 AB and FG Aqr B/C) indications for significant orbital motions were found. However, all sub-stellar companion-candidates turned out to be non-moving background objects except PZ Tel which is part of this project but whose results were published elsewhere. Detection limits were determined for all targets, and limiting masses were derived adopting three different age values; they turn out to be less than 10 Jupiter masses in most cases, well below the brown dwarf mass range. The fraction of stellar multiplicity and of the sub-stellar companion occurrence in the star forming regions in Chamaeleon are compared to the statistics of our search, and possible reasons for the observed differences are discussed.
(Abbreviated) Kepler planet candidates require both spectroscopic and imaging follow-up observations to rule out false positives and detect blended stars. [...] In this paper, we examine a sample of 11 Kepler host stars with companions detected by two techniques -- near-infrared adaptive optics and/or optical speckle interferometry imaging, and a new spectroscopic deblending method. We compare the companion Teff and flux ratios (F_B/F_A, where A is the primary and B is the companion) derived from each technique, and find no cases where both companion parameters agree within 1sigma errors. In 3/11 cases the companion Teff values agree within 1sigma errors, and in 2/11 cases the companion F_B/F_A values agree within 1sigma errors. Examining each Kepler system individually considering multiple avenues (isochrone mapping, contrast curves, probability of being bound), we suggest two cases for which the techniques most likely agree in their companion detections (detect the same companion star). Overall, our results support the advantage the spectroscopic deblending technique has for finding very close-in companions ($theta lesssim$0.02-0.05) that are not easily detectable with imaging. However, we also specifically show how high-contrast AO and speckle imaging observations detect companions at larger separations ($theta geq$0.02-0.05) that are missed by the spectroscopic technique, provide additional information for characterizing the companion and its potential contamination (e.g., PA, separation, $Delta$m), and cover a wider range of primary star effective temperatures. The investigation presented here illustrates the utility of combining the two techniques to reveal higher-order multiples in known planet-hosting systems.
Ground-based optical long-baseline interferometry has the power to measure the orbits of close binary systems at ~10 micro-arcsecond precision. This precision makes it possible to detect wobbles in the binary motion due to the gravitational pull from additional short period companions. We started the ARrangement for Micro-Arcsecond Differential Astrometry (ARMADA) survey with the MIRC-X instrument at the CHARA array for the purpose of detecting giant planets and stellar companions orbiting individual stars in binary systems. We describe our observations for the survey, and introduce the wavelength calibration scheme that delivers precision at the tens of micro-arcseconds level for <0.2 arcsecond binaries. We test our instrument performance on a known triple system kappa Peg, and show that our survey is delivering a factor of 10 better precision than previous similar surveys. We present astrometric detections of tertiary components to two B-type binaries: a 30-day companion to alpha Del, and a 50-day companion to nu Gem. We also collected radial velocity data for alpha Del with the Tennessee State University Automated Spectroscopic Telescope at Fairborn Observatory. We are able to measure the orbits and masses of all three components in these systems. We find that the previously published RV orbit for the inner pair of nu Gem is not consistent with our visual orbit. The precision achieved for these orbits suggests that our ARMADA survey will be successful at discovering new compact triple systems to A/B-type binary systems, leading to better statistics of hierarchical system architectures and formation history.
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